THE STRUCTURE, STRATIGRAPHY, AND EVOLUTION OF THE LESSER HIMALAYA OF
CENTRAL NEPAL
by
Edward A. Cross III
A Prepublication Manuscript Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements
for the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
2014
1
The structure, stratigraphy, and evolution of the Lesser Himalayan thrust belt of central Nepal
Edward A. Cross III
0. Abstract
Central Nepal has been the focus of a wide range of studies seeking to understand the
history of the Himalaya and the dynamics of mountain ranges. However, the Lesser Himalaya of
the area remains understudied, despite the potential utility of improved stratigraphic age control
and structural detail. Regional mapping, detrital zircon geochronology, neodymium isotopic
analysis, and zircon (U-Th)/He thermochronology provide the basis for a structural restoration of
the Lesser Himalaya of central Nepal. Three orogenic-scale thrust faults with over 90 km
displacement crop out on either side of the Gorkha-Pokhara anticlinorium: (1) the Main Central
Thrust (MCT), which places Greater Himalayan Sequence rocks on Lower Lesser Himalayan
Sequence (LHS) rocks, (2) the Ramgarh Thrust (RT), which places Lower LHS rocks on top of
Upper LHS rocks, and (3) the Trishuli Thrust (TT), which carries the entire Proterozoic Lesser
Himalayan Sequence. These thrust faults were active in succession until ca. 10-12 Ma, when the
fold-thrust belt began deforming internally in the form of the Lesser Himalayan Duplex, which
by growing created the Gorkha-Pokhara anticlinorium and the broad synclinorium that contains
the Kathmandu and Damauli klippen. We present a map of the Lesser Himalaya to the west of
the closure of the Kathmandu klippe, including a range of possible interpretations of geologic
units in the area. An updated compilation of detrital zircon U-Pb ages for the LHS is presented,
as well as an updated compilation of ɛNd (0) values for the different tectonostratigraphic zones
of the Himalaya. A line-length balanced cross-section from the MCT to the MFT yields
minimum shortening of 304 km. The MCT, RT, and TT are discrete thrust faults with different
histories and should not be considered equivalents. We date the Syangja Formation at ca. 1750
2
Ma, and detrital zircon ages reveal roughly continuous deposition throughout the LHS until the
unconformity at the base of the Gondwanan Sisne Formation. This suggests that the uppermost
part of the LHS of Nepal does not correlate to that of India or Bhutan.
1. Introduction
The Himalayan fold-thrust belt, which comprises the world’s highest mountain range,
formed as a result of India colliding into Asia [Argand, 1924; Gansser, 1964; Dewey et al.,
1989]. Much research has focused on central Nepal, due to ease of access from the capital city of
Kathmandu and because extreme gradients in elevation and monsoonal precipitation provide a
natural laboratory to test tectonic concepts. The portion of Nepal stretching from the Annapurna
Range and the city of Pokhara in the west to Kathmandu and convenient exposures of the
Langtang range in the east has been the site of significant studies on the geology of the Himalaya
[Gansser, 1964; Stöcklin and Bhattarai, 1977; Colchen et al., 1980; Stöcklin, 1980; Pearson,
2002; Pearson and DeCelles, 2005; Khanal and Robinson, 2013], interrogations of the linkage
between climate and tectonics [Burbank et al., 2003; Wobus et al., 2005; Robert et al., 2009;
Herman et al., 2010; Paudel, 2011; Nadin and Martin, 2012; Godard et al., 2014] important
studies on metamorphism [Harrison et al., 1997; Catlos et al., 2001; Johnson et al., 2001;
Beyssac et al., 2004; Kohn, 2008; Corrie and Kohn, 2011], structural studies of major thrust
faults [Martin et al., 2005; Pearson and DeCelles, 2005; Searle et al., 2008], and studies of the
early Paleozoic Greater Himalayan Orogeny [Gehrels et al., 2003; 2006a]. Additional modellingoriented studies have used thermochronologic data to study the kinematics of the range,
attempting to infer driving forces and factors in the evolution of the fold-thrust belt [Avouac,
2003; Bollinger et al., 2004; Hodges et al., 2004; Bollinger et al., 2006; Wobus et al., 2006;
Herman et al., 2010; Robert et al., 2011], but these have lacked the detailed geologic mapping
3
and nuanced stratigraphic knowledge required for a synthetic understanding of the Himalaya.
Despite this volume of research, large questions remain about the evolution of the
Himalaya in central Nepal. These range from debates about Quaternary out-of-sequence
thrusting at the base of the high mountains [Wobus et al., 2006; Herman et al., 2010]; to major
discrepancies in the locations and definitions of the two largest Miocene thrust faults, the Main
Central Thrust (MCT) and the Ramgarh Thrust (RT) [Pearson and DeCelles, 2005; Searle et al.,
2008]; to disagreements about whether the Lesser Himalayan Sequence was deposited in a
passive-margin, rift, or arc setting [Kohn et al., 2010; Sakai et al., 2013b]. These questions
inhibit our ability to determine the history of the India-Asia collision and understand the
processes by which mountain ranges develop. Unraveling these major disagreements requires
improved understanding of the stratigraphy, structure, and exhumational history of central Nepal.
This study doubles the number of U-Pb age dates published on the Lesser Himalayan Sequence,
providing improved age constraints and provenance analysis; we employ neodymium isotopic
analysis on the black slates and phyllites beneath the Damauli klippe, synthesizing these data
with observations from the higher-grade rocks exposed in the high mountains. These data,
combined with geologic mapping, zircon (U-Th)/He thermochronology, and balanced structural
restoration, provide an opportunity to understand the history of the Lesser Himalaya, from
Proterozoic depositional setting to Cenozoic shortening.
2. Tectonic, stratigraphic, and morphologic setting
2.1. Tectonic setting
The Himalayan thrust belt initiated around 60-55 Ma to accommodate convergence due to
the India-Asia collision [Powell and Conaghan, 1973; Ni and Barazangi, 1984; Mattauer, 1986;
Garzanti et al., 1987; Searle, 1991; Rowley, 1996; Hodges, 2000; Yin and Harrison, 2000; Leech
4
et al., 2005; Green et al., 2008; Najman et al., 2010; Wang et al., 2011; Hu et al., 2012;
DeCelles et al., 2014]. As India has moved north into Asia, pieces of the sedimentary margin of
India have been clipped off and stacked in the form of generally southwest-vergent thrust sheets.
To the north, the Himalaya is bounded by the Indus-Tsangpo suture zone. Researchers have
divided the thrust belt into four tectonostratigraphic sequences [Gansser, 1964; Hodges, 2000].
From north to south, these are the Tethyan (Tibetan) Himalayan Zone, Greater (Higher)
Himalayan Zone, Lesser Himalayan Zone, and Sub-Himalayan Zone.
2.2. Tectonostratigraphy
2.2.1. The Tethyan Himalayan Zone
The Tethyan Himalaya consists of the Tethyan Himalayan Sequence (THS), a series of
unmetamorphosed and very slightly metamorphosed sedimentary rocks of Cambrian to Eocene
age. Chiefly, these are phyllites, limestones, and quartzose sandstones in a fold-thrust belt
[Gansser, 1964; Searle, 1986; Ratschbacher et al., 1994; Renne et al., 1998; Murphy and Yin,
2003]. The South Tibetan Detachment system (STDS) separates the Tethyan Himalaya from the
Greater Himalaya [Burchfiel et al., 1992]. THS rocks sit in depositional contact on the Greater
Himalayan Sequence rocks of the Kathmandu klippe [Gehrels et al., 2006a] and overlap the
Lesser Himalayan Sequence southward, more proximal to cratonic India [Sakai, 1983; DeCelles
et al., 2002].
2.2.2. The Greater Himalayan Zone
The Greater Himalayan Zone contains upper Proterozoic-lower Paleozoic(?) upper
greenschist to amphibolite facies metamorphic rocks, Cambrian-Ordovician granites and
orthogneisses, and Miocene leucogranites [Le Fort, 1975; Hodges et al., 1996; Searle and Godin,
2003; Gehrels et al., 2011]. The Greater Himalayan Sequence (GHS) rocks are exposed in the
5
high mountains and also in isolated klippen, erosional remnants of a large thrust sheet that
extended continuously along the front of the range and still extends nearly to the Main Boundary
Thrust (MBT) in eastern Nepal, Sikkim, and Bhutan. In central Nepal, these klippen are the
Kathmandu klippe and the Damauli klippe.
GHS rocks in the high mountains consist of
paragneiss, schist, migmatite, marble, and orthogneiss. The less-metamorphosed equivalents in
the klippen consist of schist, marble, phyllite, slate, quartzite, and granite. The Greater
Himalayan Zone is bounded to the south by the Main Central Thrust system (MCT) and, in
central Nepal, by the Mahabarat Thrust (MT). The GHS rocks were metamorphosed during
burial from near the beginning of the collision to around 25 Ma, with leucogranite emplacement
at 23-12 Ma during exhumation along the MCT and STDS at 25-15 Ma [Harrison et al., 1992;
Hodges et al., 1996; Searle, 1996], though more recent reactivation has been discussed
[Harrison et al., 1997; Robinson et al., 2003; Robert et al., 2009]. The GHS rocks were also
subject to early Paleozoic metamorphism during the Greater Himalayan Orogeny [Argles et al.,
1999; DeCelles et al., 2000; Marquer et al., 2000; Gehrels et al., 2003; 2006a].
From east to west, the klippen of GHS rocks (also sometimes referred to as “nappes,” even
though they generally lack regional-scale recumbent folds) are named the Kathmandu, Damauli,
Jajarkot, and Dadeldhura klippen. In central Nepal, the Kathmandu klippe has been the focus of
much work dating back to the foundational maps of Hagen [1969] and Stöcklin [1980].
Researchers consider the Damauli klippe, also known as the Kahun klippe, to be the western
continuation of the Kathmandu klippe [Beyssac et al., 2004; Bollinger et al., 2004; Paudyal et
al., 2012b], and these workers have utilized preexisting stratigraphic and metamorphic models
from the Kathmandu klippe as a frame for understanding the Damauli klippe. With a width of
~15 km, the DK is much smaller than the Kathmandu klippe (KK) which extends in strike length
6
for over 100 km. The stratigraphy of the Kathmandu klippe consists of the Bhimpedi and
Phulchauki Groups. The Bhimpedi Group consists of schist, marble, and quartzite, intruded by
Cambrian-Ordovician granites [Stöcklin, 1980]. The Bhimpedi Group is considered equivalent to
the Greater Himalayan Sequence of the high mountains; these rocks contain similar detrital
zircon age signatures [Gehrels et al., 2011] and ɛNd isotopic values [Parrish and Hodges, 1996;
Pearson, 2002]. The units of the Bhimpedi Group are the Raduwa Formation, a garnet mica
schist with micaceous quartzite; the Bhainsedobhan Formation, a coarse, crystalline marble; and
the Kalitar Formation, a garnet-mica schist with micaceous quartzite (nomenclature varies
locally; see Gehrels et al., [2006a]). Above the Bhimpedi Group lies the Phulchauki Group,
considered to be equivalent to the Tethyan Himalayan Sequence. Rocks of the Kathmandu klippe
were first deformed during the Greater Himalayan Orogeny [Gehrels et al., 2006a].
2.2.3 The Lesser Himalayan Zone
The Lesser Himalayan Zone, so named because of the lower relief and elevation of the
mountains there, contains Paleo- and Mesoproterozoic metasedimentary rocks up to greenschist
facies, ~1.8 Ga felsic intrusives (generally orthogneisses), likely Neoproterozoic sedimentary
rocks, and Permian through Miocene sedimentary rocks [Upreti, 1999; DeCelles et al., 2000;
Kohn et al., 2010; Martin et al., 2011]. Lesser Himalayan Sequence (LHS) rocks consist of
schist, phyllite, quartzite, marble, and unmetamorphosed dolostone, limestone, sandstone, and
shale. The LHS is bounded below and to the south by the Main Boundary thrust (MBT); the
upper structural boundary of the LHS is the MCT both to the north in the high mountains and
beneath isolated klippen of the GHS.
Stratigraphy of the Lesser Himalayan Sequence has been the subject of much debate, both
within Nepal and elsewhere in the Himalaya [see summaries in Martin et al., 2010 and Kohn et
7
al., 2011]. Table 1 contains the nomenclature used in this study to refer to LHS rocks. The basal
unit of the Lesser Himalayan Sequence in central Nepal is the Kuncha Formation, a green
quartzose mica schist and green chloritic phyllite. The depositional age of the Kuncha Formation
is bracked between ~1856 and 1900 Ma with detrital zircons and dates of the intrusive Ulleri
gneiss [Martin et al., 2011]. The Ulleri gneiss is a felsic augen gneiss that intrudes the Kuncha
Formation and lateral equivalents across the Himalaya [Kohn et al., 2010]. Precise age
determination by U-Pb dating of zircons from the Ulleri gneiss has been difficult because of the
presence of zircon rims less than 10 µm in thickness, inherited cores, and multiple intrusive
events [Célérier et al., 2009; Kohn et al., 2010; Martin et al., 2011]. Martin et al. [2011], dating
the type location of the Ulleri gneiss and compiling the results from previous studies, determined
that the Ulleri gneiss crystallized at 1780 +/- 29 Ma; an earlier lithologically similar felsic
intrusion into the Kuncha Formation was dated at 1878 +/- 22 Ma. Although Martin et al. [2011]
recommended using a different name for the augen gneisses crystallized ca. 1780 and ca. 1880,
we retain the term Ulleri gneiss for all of the felsic augen gneisses intruding the Kuncha
Formation for the sake simplicity.
Significant debate exists about the protolith and depositional setting of the Kuncha
Formation, with some arguing that it is primarily volcanogenic, deposited adjacent to an arc that
stretched along the Paleoproterozoic margin of northern India [Kohn et al., 2010], and others
arguing in favor of turbidites deposited in a passive margin setting [Brookfield, 1993; Upreti,
1999; Myrow et al., 2003], which may have begun as a rift basin [Sakai et al., 2013b]. In
discriminating between these models, the ages of intrusions are important, with Kohn et al.
[2010] using the continuity of ages across the Himalaya as evidence for an arc, and Sakai et al.
[2013b] correlating ages and chemistries of intrusions with what they suggest is the conjugate of
8
the rifted margin, the Coronation Supergroup of northwest Canada [Hoffman, 1980; Hoffman and
Bowring, 1984].
Above the Kuncha Formation lies the Fagfog Formation, a cliff-forming white quartzite
with abundant ripples and m-scale cross-stratification. Martin et al. [2011] suggest that the
Fagfog Formation and the Kushma Formation, also a cliff-forming white quartzite with abundant
ripples, are upper and lower members of the same formation, raising questions for the basal
white quartzite mapped as Kushma Formation in western Nepal.
Above the Fagfog Formation lies the Galyang Formation, known locally in central Nepal
as the Dandagaon Formation or Dandagaon phyllites [Stöcklin and Bhattarai, 1977]. The
Galyang Formation is composed mainly of a thick (>1 km) succession of brown phyllite, with
local creamy green phyllite with isolated dolostone beds known as Baitadi carbonates. Above the
Galyang Formation is the Syangja Formation, known locally as the Nourpul Formation. The
Syangja Formation is composed of maroon and white quartzite, variegated slate, sandy slate, and
dolostone. Sedimentary structures indicative of evaporitive conditions and shallow water wave
activity are abundant [Sakai, 1983; DeCelles et al., 2001a]. Paudyal et al. [2012], working in the
same area as this study, broke the Syangja/Nourpul into four members, but we map it as one unit.
The Syangja Formation is in depositional contact with the overlying Lakharpata Group. The base
of the Lakharpata Group is the Dhading Formation, a siliceous dolostone containing
stromatolites. Above the Dhading Formation is the Benighat Formation composed of black slate
with increasing graphite and carbonate content upsection. Thickness of the Benighat appears to
vary significantly under the Kathmandu klippe from 1 km under the north limb to 3 km under the
western closure [Stöcklin, 1980; Beyssac et al., 2004; Bollinger et al., 2004; Khanal and
Robinson, 2013], possibly as a result of thickening in internal thrusts [Paudyal et al., 2012a].
9
Above the Dhading Formation is the Malekhu Formation, a dolomitic limestone.
The age of the upper portion of the Lesser Himalayan Sequence, which lacks fossils and
dateable volcanics, has been subject to debate, with workers in India and Bhutan favoring
Neoproterozoic- Paleozoic ages [McKenzie et al., 2011; Long et al., 2011a]; and workers in
Nepal favoring Meso- to Neoproterozoic ages [Martin et al., 2011].
Stratigraphically above the Lakharpata Group is the Gondwanan Unit, consisting of upper
Paleozoic to Mesozoic sedimentary rocks, upon which were deposited early foreland basin
deposits of the Eocene Bhainskati and Miocene Dumri Formations [Sakai, 1983; DeCelles et al.,
1998b; 2001a].
2.2.4 The Subhimalayan Zone
To the south of the MBT is the Subhimalayan Zone, where Miocene and Pliocene foreland
basin sedimentary rocks of the Siwalik Group are exposed. These sedimentary rocks were
deposited in a subaerial basin analogous to the modern Indo-Gangetic foreland [DeCelles et al.,
1998a]. Though we map the LHS-Siwalik contact (MBT) in places, the Siwalik Group
sedimentary rocks will not be discussed int his paper.
2.3 Major Structures and exhumational history of central Nepal
The northernmost and oldest structure in the study area is the Main Central Thrust (MCT),
which was active roughly from 25-15 Ma [Hodges et al., 1996; Searle, 1996]. A major obstacle
to determining the history of the Himalaya has been a lack of agreement about the location of the
Main Central Thrust (MCT). Traditionally, the Main Central Thrust has been mapped at the
thrust contact between the Greater and the Lesser Himalayan Sequences [Gansser, 1964;
Colchen et al., 1980; Valdiya, 1980]. Deformation is widely distributed both above and below
10
this boundary, and the contact can be difficult to determine in the field because similar
lithologies are present in the fault zone. Some argue that the Main Central Thrust is a broad zone,
with no discrete boundary [Le Fort, 1975; Pêcher, 1977; Brunel, 1986; Hubbard and Harrison,
1989; Macfarlane et al., 1992; Vannay and Hodges, 1996]. Building on this tradition, Searle et
al. [2008] suggested the following definition for the Main Central Thrust: “the base of the largescale zone of high strain and ductile deformation, commonly coinciding with the base of the zone
of inverted metamorphic isograds, which places Tertiary metamorphic rocks of the GHS over
unmetamorphosed or low-grade rocks of the Lesser Himalaya.” This definition posits the
Ramgarh Thrust to be equivalent to the MCT, despite structural, metamorphic, and stratigraphic
evidence supporting two separate thrust faults [Martin et al., 2005; Pearson and DeCelles, 2005;
Kohn, 2008]. A proper understanding of the location of the MCT—knowing which rocks were
moving at which times—is essential for correct interpretation of metamorphic data [e.g., Kohn,
2008] and structural restorations [e.g., Robinson and McQuarrie, 2012]. These types of studies
attempt no less than to understand the causes of and mechanisms by which mountain ranges
grow and deform through time.
Debate about the MCT is not limited to its definition. In the high mountains of central
Nepal, a sharp physiographic transition and young (ca. 1 Ma) apatite fission track ages have led
some to interpret a Quaternary out-of-sequence thrust fault in the vicinity of the MCT [Burbank
et al., 2003; Wobus et al., 2005], though others find that duplex formation and uplift along a midcrustal ramp better explain the data [Robinson et al., 2003; Herman et al., 2010; Nadin and
Martin, 2012]. Though this debate essentially reduces to a disagreement over the exact
mechanism for maintenance of steep topography at the physiographic transition, it has relevance
for “channel-flow” models which posit the Greater Himalayan Sequence was excreted in a hot,
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ductile channel from beneath the Tibetan Plateau, with the MCT and STD acting as the base and
top of this channel, respectively [Nelson et al., 1996; Beaumont et al., 2001; Hodges et al., 2001;
Grujic et al., 2002]. This model requires significant erosion to advect material from the front of
the channel and to maintain a steep slope and significant gravitational potential energy, and
workers favoring the out-of-sequence model see it as a possible modern analogue of Miocene
channel flow [Wobus et al., 2005].
Many researchers consider the fault at the base of the Kathmandu klippe, referred to as the
Mahabharat Thrust (MT), to be continuous with the MCT [Upreti and Le Fort, 1999; Johnson et
al., 2001; Gehrels et al., 2003; Bollinger et al., 2004; Pearson and DeCelles, 2005; Robinson et
al., 2006; Gehrels et al., 2006a] though some consider it a separate fault [Webb et al., 2011].
Structural mapping supports MT-MCT continuity [Rai et al., 1998; Upreti and Le Fort, 1999].
Structurally beneath the MCT and MT lies the Ramgarh Thrust (RT), an orogen-scale
thrust fault carrying lower LHS rocks locally termed Kuncha, Ranimata, Robang, and/or Dunga
Formations [Pearson and DeCelles, 2005]. The RT cuts the Dumri Formation in western Nepal
[DeCelles et al., 2001; Sakai et al., 2013a]. The age of the Dumri Formation is ca. 20-15 Ma
[DeCelles et al., 2001; Ojha et al., 2009], so the RT must have been active after ~15 Ma.
Estimating the cessation of RT slip is more difficult. Muscovite
40
Ar/39Ar (MAr) cooling ages
from rocks in the RT range from ca. 20 to 7 Ma in both western and central Nepal [Robinson et
al., 2006; Herman et al., 2010], though the younger cooling can be explained by exhumation due
to the growth of the Lesser Himalayan Duplex [Herman et al., 2010].
Structurally beneath the RT lies the Lesser Himalayan Duplex (LHD), consisting of several
horses and present across much of the Himalaya [Srivastava and Mitra, 1994; DeCelles et al.,
2001a; McQuarrie et al., 2008; Mitra et al., 2010]. In central Nepal, the uppermost horse of the
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LHD is carried on the Trishuli Thrust (TT), a thrust fault with nearly the same displacement as
the RT and MCT [Pearson, 2002; Khanal and Robinson, 2013]. Muscovite
40
Ar/39Ar cooling
ages of 12.4 Ma from far-Western Nepal [Robinson et al., 2006] provide an estimate for the
beginning of LHD activity, with the sedimentology of Siwalik Group foreland basin deposits
supporting an influx of LHS material ~10 Ma [DeCelles et al., 1998]. The Lesser Himalayan
zone is bounded to the south by Main Boundary Thrust (MBT), which activated after about 5 Ma
[Huyghe et al., 2001; DeCelles et al., 2001a].
To the south of the MBT is an imbricate belt of thrust horses carrying Siwalik Group
sedimentary rocks, bounded to the south by the currently active Main Frontal Thrust (MFT),
which, along with the MBT and MCT, roots into the common Main Himalayan Thrust (MHT)
detachment [Hirn et al., 1984; Makovsky et al., 1996; Avouac, 2003; Schulte-Pelkum et al., 2005;
Nábělek et al., 2009; Caldwell et al., 2013; Zhao et al., n.d.]. The MFT is presently locked in
central Nepal, and shortening is accumulating by creep in a broad zone centered on the Lesser
Himalayan duplex [Bilham et al., 1997; Bettinelli et al., 2006; Ader et al., 2012]. To the south of
the MFT is the modern Gangetic foreland basin, which laps onto the Indian craton [Lyon-Caen
and Molnar, 1985].
Thermochronology has been used to study the Himalaya for over twenty years [e.g.
Copeland et al., 1991; MacFarlane, 1993; Catlos et al., 2001; Bollinger et al., 2004; Thiede et
al., 2004; Vannay et al., 2004; Grujic et al., 2006; Blythe et al. 2007; Herman et al., 2010;
Robert et al., 2011]. Central Nepal has been the focus of many studies due to the exceptionally
young low-T ages in the high mountains, with apatite fission track (AFT) ages as young as ~1
Ma [Blythe et al., 2007; Wobus et al., 2008; Robert et al., 2009; Herman et al., 2010; Nadin and
Martin, 2012]. AFT ages are roughly 1-3 Ma in the MCTZ/physiographic transition and increase
13
linearly to the south, reaching 5-9 Ma near the MBT. In addition to AFT, researchers have
employed muscovite 40Ar/39Ar (MAr) [MacFarlane, 1993; Catlos et al., 2001; Robinson et al.,
2006; Wobus et al., 2006; Herman et al., 2010]. 40Ar/39Ar muscovite ages of the high mountains
range from ~4-10 Ma in the physiographic transition and increase to ~18-20 at the front of the
Kathmandu klippe; detrital muscovite 40Ar/39Ar from small catchments in the Lesser Himalayan
Sequence to the south of the physiographic transition in the Budhi Gandaki River – Trishuli river
area to the NW of the Kathmandu klippe give unreset Proterozoic ages, indicating that LHS
rocks beneath the RT, at least in central Nepal, were not buried deeply enough to reset the
muscovite 40Ar/39Ar system [Wobus et al., 2006].
In contrast to the large volume of AFT and Mar ages from central Nepal, the ZHe system
remains understudied. Herman et al. [2010] provide five ZHe ages of 7-9 Ma from the front of
the Kathmandu klippe, and this is the extent of the ZHe work in Nepal. Additional work from the
portion of the thrust belt between the high mountains and the mountain front will provide
valuable information on timing of Lesser Himalayan structures and inform future modeling
studies. ZHe data will also allow comparison to the Himalaya of Bhutan and NW India, where
ZHe has proven an accurate and robust thermochronomter and has been employed along with
AFT and MAr in transects parallel to balanced cross-sections [Deeken et al., 2011; Long et al.,
2012; McQuarrie et al., 2014].
2.4 Shortening estimates Estimating shortening is essential to understanding various processes in thrust belts.
Shortening is connected to flexural wave migration, continental underthrusting, and orogenic
wedge propagation [DeCelles and DeCelles, 2001]. Previous studies have looked at minimum
shortening estimates along strike in the Himalaya for the entire thrust belt [DeCelles et al., 2002;
14
Long et al., 2011b] or just the frontal Siwaliks belt [Hirschmiller et al., 2014] to test various
hypotheses about orogenic development, for instance whether the east-west increase in
precipitation will result in an increase in shortening to the east, [Grujic et al., 2006], or whether
the geometry of the collision is dominant, and greatest shortening should occur in the middle of
the fold-thrust belt [Elliott, 1976]. Line-length balanced cross-sections have been constructed to
the east of the field area beneath the Kathmandu klippe [Pearson, 2002; Khanal and Robinson,
2013], and in western Nepal [DeCelles et al., 2001a; Robinson et al., 2006].
Excluding
shortening within the GHS and THS, minimum shortening in these cross-sections is ~400 km
and ~450 km, respectively.
Balanced cross-sections are also used in the Himalaya to study variation in shortening rates
through time [Robinson and McQuarrie, 2012; Long et al., 2012a; McQuarrie et al., 2014].
These studies combine shortening estimates from balanced cross-sections along with timing
information from thermochronologic and sedimentologic data to estimate rates of shortening
through time, and have generally found a decrease in shortening rate in the Himalaya through
time, at least in western Nepal and Bhutan,.
3. Methods
3.1 Geologic mapping
Geologic mapping was conducted on topographic sheets at 1:25,000 scales, primarily
comprised of a series of transects along roads and rivers roughly extending from the Kali
Gandaki River in the south to the hinge of the Gorkha-Pokhara anticlinorium in the North.
Numerous samples were collected for petrographic analysis. Selected quartzose lithologies were
sampled for detrital zircon U-Pb geochronology, and selected phyllitic lithologies were collected
15
for whole rock neodymium isotopic analysis. Table 2 contains information from all the samples
presented in this study. Mapping was guided and reinforced by selected portions of preexisting
maps, principally Nepal Department of Mines and Geology maps.
3.2 Detrital zircon U-Pb analysis
Detrital zircon U-Pb analysis provides crystallization ages on zircons contained within a
sedimentary rock or modern-day sediments. Approximately 3-kg samples of sandstones and
quartzites were collected in the field (Table 2). Zircon crystals were extracted from samples by
hand crushing followed by separation with hand panning, heavy liquids, and a Frantz magnetic
separator. Zircons were mounted in a 1” epoxy mount together with fragments of the Sri Lanka
standard zircon. The mounts were sanded down by about 20 microns polished, imaged, and
cleaned prior to isotopic analysis.
U-Pb geochronology of zircons was conducted by laser ablation multicollector
inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the Arizona LaserChron
Center [Gehrels et al., 2006c; 2008]. The analyses involve ablation of zircon with a Photon
Machines Analyte G2 excimer laser using a spot diameter of 30 microns. The ablated material is
carried in helium into the plasma source of a Nu HR ICPMS, which is equipped with a flight
tube of sufficient width that U, Th, and Pb isotopes are measured simultaneously. All
measurements are made in static mode, using Faraday detectors with 3x1011 ohm resistors for
238
U,
232
Th,
208
Pb-206Pb, and discrete dynode ion counters for
204
Pb and
202
Hg. Ion yields are
~0.8 mv per ppm. Each analysis consists of one 15-second integration on peaks with the laser
off (for backgrounds), 15 one-second integrations with the laser firing, and a 30 second delay to
purge the previous sample and prepare for the next analysis. The ablation pit is ~15 microns
deep.
16
For each analysis, the errors in determining
206
Pb/238U and
206
Pb/204Pb result in a
measurement error of ~1-2% (at 2-sigma level) in the 206Pb/238U age. The errors in measurement
of 206Pb/207Pb and 206Pb/204Pb also result in ~1-2% (at 2-sigma level) uncertainty in age for grains
that are >1.0 Ga, but are substantially larger for younger grains due to low intensity of the 207Pb
signal. For most analyses, the cross-over in precision of
206
Pb/238U and
206
Pb/207Pb ages occurs
at ~1.0 Ga.
204
Hg interference with
ablation and subtraction of
204
204
Pb is accounted for by measurement of
Hg according to the natural
202
202
Hg during laser
Hg/204Hg of 4.35. This Hg
correction is not significant for most analyses because our Hg backgrounds are low (generally
~150 cps at mass 204).
Common Pb correction is accomplished by using the Hg-corrected 204Pb and assuming an
initial Pb composition from Stacey and Kramers [1975]. Uncertainties of 1.5 (unitless ratio) for
206
Pb/204Pb and 0.3 for
207
Pb/204Pb are applied to these compositional values based on the
variation in Pb isotopic composition in modern crystal rocks.
Inter-element fractionation of Pb/U is generally ~5%, whereas apparent fractionation of
Pb isotopes is generally <0.2%. In-run analysis of fragments of a large zircon crystal (generally
every sixth measurement) with known age of 563.5 ± 3.2 Ma (2-sigma error) is used to correct
for this fractionation. The uncertainty resulting from the calibration correction is generally 1-2%
(2-sigma) for both
206
Pb/207Pb and
206
Pb/238U ages. Concentrations of U and Th are calibrated
relative to the Sri Lanka zircon, which contains ~518 ppm of U and 68 ppm Th. The analytical
data are reported in Supplementary Table 1. Uncertainties shown in these tables are at the 1sigma level, and include only measurement errors. Analyses that are >20% discordant (by
comparison of
206
Pb/238U and
206
Pb/207Pb ages) or >5% reverse discordant (in italics or red in
17
Supplementary Table 1) are not considered further. Composite age probability plots (see
LaserChron Analysis Tools for program link) are normalized according to the number of
constituent analyses, such that each curve contains the same area.
3.3
Neodymium isotopic analysis
Whole-rock neodymium isotopes have proven useful for distinguishing Greater and
Lesser Himalayan rocks, which have different Nd isotopic ratios [Parrish and Hodges, 1996;
Robinson et al., 2001], and this study employs Nd to determine the tectonostratigraphic affinity
of ambiguous rocks in the Damauli area (Figure 2). Conventionally, measured 143Nd/144Nd ratios
are normalized to CHUR values and reported using epsilon notation [McCulloch and
Wasserburg, 1978]. Generally, rocks that were extracted from the mantle earlier in Earth’s
history will have more negative ɛNd values, reflecting the accumulation of 143Nd by the decay of
147
Sm through time [McCulloch and Wasserburg, 1978; Allègre and Othman, 1980; DePaolo,
1981]. Because Nd isotopes are largely unaffected by crutal processes [McCulloch and
Wasserburg, 1978], the Nd isotopic ratio of sedimentary rocks reflects that of their source
material [McLennan et al., 1989]. LHS rocks sourced from the Archean Indian craton have more
negative ɛNd(0) values than GHS and THS rocks which contain more juvenile material
[Robinson et al., 2001].
Eight 0.5 kg samples of slate and phyllite were collected in the field (Table 2). The
samples were crushed by hand, and the material was placed in a porcelain grinding bowl and
ground to a powder. The bowl was precontaminated each time with a portion of the sample about
to be used. The resulting powder was then placed in a Savillex vial and dissolved in a hot HFNO3 mixture. The Nd was then separated in an anion column with LN Spec resin and 0.1-2.5 N
HCl. Separated Nd was placed on a Re filament with resin beads. Mass spectrometry work was
18
performed at the University of Arizona on an automated VG Sector multicollector with
adjustable Faraday collectors and a Daly photomultiplier [Ducea and Saleeby, 1998]. La Jolla
Nd standard was analyzed during study and yielded 143Nd/144Nd =0.5118460. Following previous
workers in the Himalaya, results are reported in terms of ɛNd(0), which is the measured ɛNd
value, because the age of the rocks is subject to significant uncertainty, hindering the ability to
extrapolate to a model age.
3.4 Zircon (U-Th)/He analysis
The zircon (U-Th)/He (ZHe) thermochronometer provides information on the timing of
cooling through the roughly 180-200 °C temperature window [Reiners et al., 2004]. Thus the
method provides information that can be used to interpret the upper crustal exhumation history of
a region. Previous workers in the Himalaya have employed ZHe to study the exhumation of the
fold-thrust belt, both to time exhumation due to specific structures [McQuarrie et al., 2014] and
to provide an estimation of exhumation rate [Long et al., 2012]. Others have used ZHe in
conjunction with apatite fission track and muscovite
40
Ar/39Ar to test the viability of various
deformation mechanisms and thrust fault geometries [Herman et al., 2010; Robert et al., 2011].
Zircons from quartzites and igneous rocks (Table 1) were separated according to the
procedure outlined in Section 3.2. Zircon (U-Th)/He analysis was perofmed at the Arizona
Radiogenic Helium Dating Laboratory (ARHDL) at the University of Arizona.
Selection of grains from mineral separates was done with a Leica MZ16 stereozoom
microscope. Samples were examined, picked, and checked for inclusions and fractures under
either cross-polarization with a rotating stage at 160x, or in plane-polarized darkfield
illumination at 240x. For zircon, inclusions are a minor problem given relative U-Th
19
concentrations and more aggressive dissolution procedures. For each sample, five grains were
individually analyzed.
Grain dimensions were measured from digital photomicrographs taken in at least two
different orientations, usually perpendicular to the c-axis. Imaging software that is manually
calibrated and recorded using a stage micrometer at each operator sitting was used to measure
several features of the grain. Fractures and other features were noted and, if appropriate,
accounted for by decreasing the surface-area-to-volume ratio of the grain by a geometric factor.
Alpha-ejection correction factors were calculated using surface-area-to-volume ratios and the
approach of Hourigan et al., [2005].
Because direct lasing of minerals volatiilizes parent nuclides, grains are wrapped in
metallic foil “microfurnaces” for laser heating [House et al., 2000]. Zircon was placed in 1-mm
Nb foil envelopes.
Approximately 30 crystal-bearing foil packets were placed in a Cu or SS planchet, under
a KBr coverslip, inside a ~7-cm laser cell pumped to <10-9 torr. With Nd:YAG laser degassing
we used a sapphire window. Samples were heated for ~15 minutes by a focused beam of a 1-2 W
Nd:YAG laser. Most importantly, routine reheating and analyses (“re-extracts”) were performed
on all zircons (sometimes multiple times), to confirm that 4He has been quantitatively extracted,
and zircons yield less than 1-2% in subsequent re-extracts.
Gas released from heated samples was then spiked with 0.1-0.2 pmol 3He, and condensed
onto activated charcoal in the cold head of a cryogenic trap at 16 K. Helium was then released
from the cold head at 37 K into a small volume (~50 cc) with an activated Zr-Ti alloy getter and
the source of a Balzers quadrupole mass spectrometer (QMS) with a Channeltron electron
multiplier. Peak-centered masses at approximately m/z (“masses,” though the presence of
20
multiply charged species is recognized) of 1, 3, 4, and 5.2 are measured. Mass 5.2 established
background, and mass 1 was used to correct mass 3 for HD and H3+ . Experiments relating
masses 1, 2, and 3 with no He in the system generally showed better correlations between masses
1 and 3 (non- 3He-mass 3 = 0.005 ± 0.002 times mass 1, though this breaks down at higher gas
pressures) than between masses 2 and 3, possibly because of other, multiply charged, species at
mass 2. Corrected ratios of masses 4 to 3 were regressed through ten measurement cycles over
~15 s to derive an intercept value, which has an uncertainty of 0.05-0.5% over a 4/3 range of
~10^3) and compared with the mean corrected ratio to check for significant anomalous changes
in the ratio during analysis.
Helium contents of unknown samples were calculated by first subtracting the average
mass-1-corrected 4/3 measured on multiple procedural blanks analyzed by the same method (a
“hotblank”), from the mass-1-corrected 4/3 measured on the unknown. This is then ratioed to the
the mass-1-corrected 4/3 measured on a shot of an online reference 4He standard analyzed with
the same procedure [minus the mass-1-corrected 4/3 measured on a 3He-only spike shot analyzed
using the same procedure as the reference 4He standard (a “lineblank”)]. The resulting ratio of
measured 4/3's is then multiplied by the moles of 4He delivered in the reference shot.
This procedure assumes linearity between measured 4/3 and 4He pressure, which has
been confirmed over the the vast majority of the range of 4He contents analyzed at the Arizona
Radiogenic Helium Dating Laboratory by performing multiple replicate analyses of known-age
standards with masses and therefore 4He yields ranging over three orders of magnitude. This
procedure also relies on the accuracy of the 4He delivery from the reference standard and the
precision of its measurement with the 3He spiking procedure. The delivery and its depletion with
time are calibrated by multiple capacitance manometry measurements of the volumes of the
21
reference tank and pipette, and the final filling pressure of the tank. One of the two He lines has a
4
He tank and pipette with volumes of 15920 ± 7.8 cc (0.05%, 1s ) and 0.9439 ± 0.0016 cc
(0.16%, 1s ), respectively; uncertainties reported as standard errors on multiple (n=6)
manometric volume determinations. The other He line has tank and pipette volumes of 3655 ±
8.2 cc (0.2%, 1s ) and 0.09675 ± 0.0021 cc (2%, 1s ). The ARHDL also has a similarly calibrated
detachable portable tank that can be moved between lines for cross-calibration.
Between ~2-6 (depending on the number of unknowns) 4/3 measurements of spiked 4He
reference standards was made each measurement day. Although the long-term, day-to-day
change in these measurements can be large due to drift of the QMS, in a single measurement day
the corrected 4/3 measurements on reference standards vary by less than 0.5% (1s ). This
uncertainty can be reduced for reducing unknown data by monitoring intraday secular trends.
Average measured 4/3 of lineblanks ( 3He spike only) are nearly indistinguishable from that
predicted by the purity of the 3He spike (99.75% 3He). Hotblanks, or procedural blanks measured
by lasing/heating empty Pt or Nb foil packets are typically 0.05-0.1 fmol 4He.
Following He measurement, foil packets were retrieved, transferred to Teflon vials, and
spiked with a 50 ml shot of a mixed spiked containing 7.55 ± 0.10 ng/ml 233 U and 12.3 ± 0.10
ng/ml 229Th. Zircons were dissolved using high-pressure digestion vessels for dissolution of the
entire grain-bearing Nb foil packet. This process ensures quantitative recovery of the grain, has
low U-Th blanks, and has been proven analytically reliable.
Natural-to-spike isotope ratios were measured on a high-resolution (single-collector)
Element2 ICP-MS with all-PFA Teflon sample introduction equipment and sample
preparation/analytical equipment. Careful monitoring of procedural blanks and spike and normal
concentrations (including evaporative effects) and isotopic compositions is essential for He
22
dating. Routine U-Th procedural blanks at the ARHDL for zircon are 2.6 ± 0.5 pg U and 5.5 ±
1.0 pg Th . Routine blanks for zircons are orders of magnitude lower than U-Th contents of even
extremely small zircons with very anomalously low U-Th concentrations, given the much higher
U-Th concentrations in typical zircons. Long-term (~4-yr) reproducibility of ARHDL spiked
normal solutions (used to calibrate spike concentrations) is 0.8-1.0%, and new, fresh normals are
periodically purchased to mitigate against evaporative effects. Zircons yield precision on
measured U-Th ratios better than 0.5%.
Propagated analytical uncertainties for most typical zircon samples leads to an estimated
analytical uncertainty on (U-Th)/He ages of approximately 1-3% (1s ). In some cases,
reproducibility of multiple aliquots approaches analytical uncertainty. Eight analyses of single
crystal zircons from a xenolith erupted from a basaltic vent average 157 ka with one standard
deviation of 4 ka (2.6%) [Blondes et al., 2007], and fourteen chips of large gem quality crystals
of Sri Lanka zircon average 442 Ma with one standard deviation of 10 Ma (2.3%) [Nasdala et
al., 2004]. In general, however, reproducibility of repeat analyses of (U-Th)/He ages is
significantly worse than analytical precision. Cooling ages of zircon from igneous rocks typically
show scatter on the order of one standard deviation of at least 6%, and in many cases more than
10%. This has several possible origins, including variable He diffusion characteristics among
grains, unidentified intracrystalline inclusions that prevent complete U-Th-Sm recovery
following degassing, or petrographic siting effects such as He implantation from adjacent highU-Th phases or varying He retention due to varying diffusivity or partitioning of surrounding
phases. As discussed above, however, it is likely that a major origin of the observed poor
reproducibility comes through uncertainty in the alpha-ejection correction [Farley et al., 1996],
not from uncertainty in actual dimensions of dated grains, but uncertainties in relating observed
23
grain boundaries of dated aliquots to original boundaries in the host rock, implantation from
other phases, and inhomogeneous distribution of parent nuclides in dated grains [Farley et al.,
1996; Reiners et al., 2004; Hourigan et al., 2005]. It is extremely difficult to estimate, a priori,
the expected magnitude of error arising from any of these potential sources. Thus He ages
typically show a much greater scatter and higher MSWD than expected based on analytical
precision alone. In any case, an important take-home message of this is that multiple replicate
analyses of (U-Th)/He ages on several aliquots is necessary for confidence in a particular sample
age.
Fish Canyon Tuff (FCT) zircons were picked and analyzed as a standard along with
sample unknowns. Whole FCT zircon crystals are a commonly used volcanic age standard for a
variety of systems, including U/Pb and
40
Ar/39Ar. [Schmitz and Bowring, 2001] obtained high
precision U/Pb zircon ages on the FCT of 28.48 ± 0.06 (2s ) Ma, and other high precision zircon
U/Pb studies yield similar results [Bachmann et al., 2007]. There is evidence suggesting that
some zircon growth significantly preceded eruption, however, and Bachman et al. [2007] suggest
an eruption age of 28.04 ± 0.18 (2s ) Ma, based on sanidine 40Ar/39 Ar ages [Renne et al., 1998],
based on the fact that this system would remain open at magmatic, pre-eruption temperatures.
One hundred fourteen single grain (U-Th)/He zircon ages average 28.3 Ma, with two standard
deviations of 2.8 Ma, and mean age and errors of 28.29 ± 0.26 Ma (95%; 2s external error of 2.6
Ma or 9.3%, MSWD = 20). The observed external error probably has a component in errors
associated with the effects of heterogeneous intracrystalline U-Th distribution on the a -ejection
correction. Laser ablation depth profiling of FCT zircons show a range of U-Th zonation styles
and extents that should lead to an approximately 10% scatter in ages from this effect alone
[Hourigan et al., 2005].
24
4. Results
4.1 Detrital and igneous zircon U-Pb age dating
We report detrital zircon results from a total of fourteen samples: six from the Kuncha
Formation, two from the Fagfog Formation, five from the Syangja Formation, and one from the
Greater Himalayan Sequence. We also report U-Pb ages from three igneous intrusions into the
Kuncha Formation and one volcanic deposit in the Syangja Formation. Sample information is
located in Table 2, and Supplementary Table 1 contains analytical information. Probability
distribution functions for detrital zircon samples, along with comparison compilation curves
from the different tectonostratigraphic zones [Gehrels et al., 2011] are given in Figure 4.
4.1.1 Kuncha Formation
Five samples from the Kuncha Formation were collected along a transect from the south
limb to the hinge of the Gorkha-Pokhara anticlinorium, covering approximately 3-4 km of
stratigraphic thickness (Figure 2). Figure 4 shows probability distribution functions of samples
from the Kuncha Formation. These spectra are dominated by grains with ages between 1855 and
1900 Ma, with 239 of 455 total grains falling in that age range. Samples also contain minor
peaks at ~2000 Ma, ~2190 Ma, ~2500 Ma. The youngest peak age provides a maximum
depositional age for the sample [Gehrels et al., 2008]. The AgePick program is used to determine
an average minimum age from the youngest group of least three grains, given uncertainties in
individual measurements and systematic error. Though the rocks are heavily deformed, possibly
including cryptic internal thrusts, the center of the anticlinorium should contain the oldest portion
of the Kuncha Formation. With increasing proximity to the hinge of the anticlinorium, the
samples show youngest peak ages of 1865 (n=21), 1878 (n=36), 1864 (n=20), 1881 (n=39), and
1860 (n=13) Ma. The lack of a trend in ages is probably due to insignificant sample size, as the
25
compiled set of five samples yields a youngest peak age of 1880 (n=187).
Sample 12_30_06_2 was collected in mid-western Nepal (Figure 1) from a quartzite unit in
the Ranimata Formation, which is generally regarded as the lateral equivalent to the Kuncha
Formation. The sample yielded a youngest peak age of 1903 Ma (n=43), with minor peaks at
~2190 and ~2500 Ma (Figure 5).
4.1.2 Ulleri Gneiss
In the study area of Figure 2, we collected a sample from a felsic augen gneiss intrusion
near the town of Gorkha (MG-16). The age-determination routine TuffZirc dated the sample at
1877 +2.72 –4.34, based on a coherent population of 11 grains. This age is within error of the
maximum depositional age (1880 Ma) of the Kuncha Formation as determined by youngest peak
age. Sample MG-28 from another intrusive body south of the town of Pokhara (Figure 3) yielded
a TuffZirc age of 1893 +7.32 – 1.65, from a coherent group of 14 grains. This intrusion is from
near the top of the Kuncha Formation, whereas the intrusion from which sample MG-16 was
collected is near the base. Sample DL-12B, from an Ulleri intrusion north of the town of
Birendranagar (Figure 1) produced a TuffZirc age of 1886 +/- 7.15 Ma.
4.1.3 Fagfog Formation
We report detrital zircon U-Pb ages from three samples of the Fagfog Formation. Sample
MG-2, from near the town of Muglin (Figure 2) has a youngest peak age of 1772 Ma (n=14),
with additional peaks at ~1880, ~2500 Ma, and ~3000 Ma (Figure 4). This sample was collected
from the belt of Fagfog Formation that crops out around the entire Gorkha-Pokhara anticlinorium
[Colchen et al., 1980; Shrestha et al., 1987a; Amatya et al., 1994]. Sample MG-29 is from the
same outcrop belt, collected south of Pokhara and east of the town of Syangja. MG-29 has a
youngest peak age of 1792 Ma (n=6). Like sample MG-2, it contains additional peaks at ~1880
26
and ~2500 Ma.
Sample DL9, originally mapped as basal Kushma Formation, was collected in midwestern Nepal north of the town of Birendranagar (Figure 1). The U-Pb age spectrum (Figure 5)
has a youngest peak age of 1863 (n=33). This age is younger than the maximum age of the
Ranimata Formation from the same area, as constrained by the dated Ulleri Gneiss (sample
DL12B) of age 1886 +/- 7.15 Ma; therefore the sampled quartzite is probably from the Fagfog
Formation rather than the Kushma Formation.
4.1.4 Syangja Formation
Six samples from the Syangja Formation were collected in the study area in central Nepal
(Figure 2). All six samples are dominated by a peak in the 1730-1780 Ma range (Figure 4).
Sample MG-3, from a lithic-rich placer deposit within a pink and white sandstone, contains a
youngest peak at 1741 Ma (n=58), with another statistically significant peak at 1757 Ma (n=65).
These ages dominate the sample, with only one grain older than 1800 Ma. Sample MG-8, from a
gray, micaceous, lithic-rich sandstone, has a youngest peak age of 1767 Ma (n=43), with minor
peaks at ~1880 and ~2500. Sample MG-20, from a rippled, maroon quartzite, has a youngest
peak age of 1734 Ma (n=17), a larger peak at 1766 Ma (n=31), and minor peaks at ~1890 and
~2500. Sample MG-23, from a variegated green, pink, and white mica-rich sandstone, contains a
youngest peak age of 1732 Ma (n=32), with additional peaks at ~1752 and 1766 Ma. The sample
produced only 3 grains older than 1800 Ma. Sample KT-14A, from an outcrop of green, white,
and pink quartzite, contains a youngest peak age of 1735 Ma (n=39), with a larger peak at 1757
Ma (n=70). Grains contributing to these peaks dominate the sample, with only one grain dated at
older than 1800 Ma.
Sample KT-16 is from a fine-grained green- and white-banded phyllitic quartzite. The
27
sample has a youngest peak age of 1646 Ma (n=5), with additional peaks at 1733 Ma (n=34),
1753 Ma (n= 26), and minor peaks at ~1885 and ~2500 Ma. We tentatively assign this sample to
the Syangja Formation on the basis of the similarity of its detrital zircon spectrum to those from
samples known to be from the Syangja Formation; however, the sample could be from a
quartzose bed within the Lakharpata Group.
A compilation of the five samples definitively identified as Syangja Formation (MG-3,
MG-8, MG-20, MG-23, and KT-14) yielded a youngest peak age of 1757 Ma. This age, with its
larger sample size, is a conservative maximum depositional age for the Syangja Formation.
We also present results from a sample of intermediate volcanic rock, PK2, found within the
Syangja Formation along the Tansen-Pokhara Road (Figure 3). The sample yielded a TuffZirc
age of 1772 +/- 5 Ma. Excluding a cluster of 15 old ages, >1810 Ma, the sample has a weighted
mean age of 1754 +/- 8 Ma (2s). This is slightly older than the maximum depositional age from
several of our samples, but is nearly identical to the youngest peak age of 1757 Ma determined
from the five samples analyzed together.
The six detrital samples yielded, in total, six zircon U-Pb ages of <1000 Ma. Sample
MG-8 contained one grain dated at 478 +/- 12 Ma; MG-23 contained grains dated at 556 +/- 3
Ma and 571 +/- 9.4 Ma; and KT-14A contained grains dated at 556 +/- 4 Ma and 862 +/- 13 Ma.
Sample KT-16 contained one grain dated at 572 +/- 5 Ma.
These ages are concordant, the grains are morphologically similar to those from the rest
of the sample, and the grains do not have anomalous U/Th ratios. Samples KT-16 and MG-23
were repolished and the anomalously young grains were reanalyzed with multiple shots. The
three young grains, upon reanalysis, yielded ages of 1742 +/- 6.6 Ma, 1459 +/- 12.0 Ma, and
1732 +/- 10 Ma. These results suggest that the young ages from the first analysis were
28
metamorphic rim ages. As such, the anomalously young ages from these two samples are
excluded from the PDFs in Figure 4.
4.1.5 Probable Greater Himalayan Sequence sample
Sample KT-11 was collected from a quartzite along the east-west trending ridge to the
north of the Kali Gandaki river (Figure 2). The zircon age spectrum (Figure 4) is dominated by
grains in the 800-1000 Ma range (n=46), along with smaller peaks at ca. 1600, 1900, and 2500
Ma. One grain has an age of 560.7 +/- 4.5 Ma, and this is the only grain younger than 800 Ma.
The 800-1100 Ma ages are typical of Greater and Tethyan Himalayan Sequence rocks
4.2 Neodymium isotopic analysis
Neodymium isotopic analysis has been applied in the Himalaya many times over a number
of years, with workers establishing that Greater, Lesser, and Tethyan Himalayan Sequence rocks
have distinct ɛNd(0) signatures [Parrish and Hodges, 1996; Ahmad et al., 2000; Robinson et al.,
2001]. Figure 6 shows a compilation of ɛNd(0) values from across the Himalaya. These values
are conventionally shown with the modern-day ɛNd value rather than calculating the ɛNd value
at time of deposition due to the often-ambiguous ages of Himalayan strata. Studies have taken
advantage of the difference in ɛNd(0) values between tectonostratigraphic zones to differentiate
units where distinctive lithologies are not present and/or where the rocks are extremely deformed
and metamorphosed [Martin et al., 2005; Imayama and Arita, 2008]. [McKenzie et al., 2011]
disagree with the use of neodymium isotopes to differentiate tectonostratigraphic zones on the
basis of Lesser Himalayan samples yielding ɛNd (0) values of -22 to -16; however their
argument is based on a meager four analyses as opposed to the 293 compiled in Figure 6, and the
stratigraphic divisions of Northern India may not apply to Nepal, possibly due to “upper Lesser
Himalayan sequence” being Neoproterozoic in India but Paleo- to Meso-Proterozoic in Nepal
29
[Kohn et al., 2010]. Even if the values determined by McKenzie et al. [2011] are from the
stratigraphic equivalent of the Lesser Himalayan Sequence of Nepal, they only suggest that high
values of -18 to -16 may not be useful for distinguishing tectonostratigraphic units, a fact known
to previous workers [e.g., Martin et al., 2005]. Extremely negative values have been found only
in the Lesser Himalayan Sequence. Tobgay et al. [2010] also suggest that ɛNd values may not
serve as a useful tectonostratigraphic affinity tool on the basis of combined ɛNd analysis and
detrital zircon U-Pb age dating on the same quartzites of the Paro Formation of Bhutan.
However, ɛNd analyses in the Himalaya are conventionally performed on rocks with mud-rich
protoliths, and modern sediments can have up to a 7 ɛNd unit discrepancy between sand and mud
fractions [McLennan et al., 1989]. Even if the data from Tobgay et al. [2010] and McKenzie et
al. [2011] are taken at face value,
the
vast majority of ɛNd analyses show clearly
distinguishable tectonostratigraphic zones (Figure 6).
Neodymium analysis was conducted on eight samples (Table 2). Three samples from a
graphitic calc-schist unit (mapped as Benighat Formation in Figure 2) gave ɛNd (0) values of 24.4, -23.6, and -19.3. These values fall within the range of the LHS and show that the unit is not
part of the GHS, which has not given values more negative than -19 (Figure 6). Samples KT-7
and 1.18.13a of the Galyang Formation, from two separate thrust sheets both collected to the
west of the Damauli klippe, gave values of -20.2 and -21.3. Sample MG-22 of a talc-schist
located just to the north of the Seti River gave a value of -23.8, and sample KT-12 of the
Kuncha Formation in the Ramgarh Thrust sheet gave a value of -20.4. All of these values are in
the range of LHS rocks (Figure 6).
4.3 Zircon (U-Th)/He Thermochronology
Zircon (U-Th)/He (ZHe) was conducted on six samples from central Nepal (Figure 2) and
30
twelve samples from mid-western and western Nepal. Samples were collected from quartzites,
quartzose phyllites, and one igneous rock (Table 2). For each sample, five individual zircon
grains were dated, and we report individual zircon ages and weighted means for each sample.
Supplementary Table 2 contains individual grain analyses for each sample. Weights used for the
weighted mean were calculated with equation 1, with reported standard deviations based on
equations 2 and 3, where wi is the weight of the i-th grain, σi is the standard deviation of the age
of the i-th grain, xi is the age of the ith-grain, and µ is the weighted mean.
(1) wi =
1
.
σ i2
n
(2)
2
σ weighted
=
∑ w (x − µ )
i
2
i
i=1
V
n
(3) V = ∑ wi
i=1
For every sample except MG-20, grains fall within ~1.5 Ma or less of each other, which is
reflected in the low standard deviations of their weighted means of 0.56 Ma or less. Zircon grains
of MG-20 show an inverse age-effective Uranium concentration, which may be the result of
inherited cores [Guenthner et al., 2013]. Ages increase from 3.1 +/- 0.4 Ma in the axis of the
anticlinorium to 6.9 +/- 0.6 Ma in the Damauli klippe.
Figure 7 contains ZHe results from four regions of Nepal. The weighted means of each
sample are plotted versus distance from the MCT. In addition to the results from the Damauli
area of central Nepal, we also present four samples from west-central Nepal located roughly
along the Kali Gandaki River, three samples from the Daleikh-Birendranagar area of midwestern Nepal, and five samples from the Dadeldhura region of far-western Nepal. Samples from
31
west-central Nepal also increase in age away from the MCT and towards the MFT, from 4.5 Ma
to 10 Ma. The samples from mid-western Nepal are all from the southern side of the Karnali
klippe and decrease away from the MCT from 14.2 +/- 1.4 Ma to 11.1 +/- 1.2 Ma. The samples
from far-western Nepal are all from the Dadeldhura klippe and increase in age away from the
MCT from 10.1 +/- 0.8 to 14.3 +/- 0.7 Ma.
5. Interpretation
5.1. Formation Ages
5.1.1 Age of Lower Nawakot Group Formations
Martin et al. [2011] provided detrital zircon age spectra from samples from type localities
of the Kushma, Fagfog, and Kuncha Formations. An igneous intrusion in the Kuncha Formation
constrained it as older than 1878 +/- 22 Ma (2s). Using a weighted mean of the three youngest
grains, Martin et al. [2011] determined a maximum depositional age of 1770 Ma for the Lower
Fagfog Formation. These grains came from a group of zircons with a peak age of 1800 Ma,
which is a more conservative and statistically robust estimate of maximum depositional age. We
stress that maximum depositional age is a maximum age, and the depositional age of the sample
may be considerably younger than the zircons that it contains. We provide more conclusive
evidence that the Fagfog/Kushma Formation is younger than the Kuncha Formation, with three
pairs of Fagfog-Kuncha Formation dates. Near the town of Muglin, sample MG-2, collected in
close proximity to a dated Kuncha Formation sample, contains a peak at 1772 Ma, made of 14
grains. This peak age represents a maximum depositional age of the Fagfog Formation and is 28
Ma younger than the youngest peak found by Martin et al. [2011]. MG-2 was collected ~20 km
from an Ulleri gneiss intrusion into the Kuncha, dated at 1877 +/- 4 Ma. Thus, in the Muglin area
the Fagfog Formation is at least 100 Ma younger than the Kuncha Formation.
32
To the west, south of Pokhara, sample MG-29 contains a youngest peak age of 1792 Ma,
which is ~100 Ma younger than an Ulleri Gneiss intrusion (sample MG-28) into the Kuncha
from 6 km away, dated at 1893 Ma +/- 7 Ma. These dates come from the belt of Fagfog
Formation which crops out continuously along the south side of the Gorkha-Pokhara
anticlinorium and continues west to the type locality of the Kushma Formation [Colchen et al.,
1980; Shrestha et al., 1987a]. These results confirm the work of Martin et al. [2011].
A third pair of Fagfog Formation – Kuncha Formation ages, from mid-western Nepal
north of the town of Birendranagar (Figure 1) is not as conclusive as the data from central Nepal.
U-Pb spectra from three samples are presented in Figure 5. Sample 12_30_06_2 was collected
from the Ranimata Formation, the lateral equivalent to the Kuncha Formation (Table 1). The
sample yielded a youngest peak age of 1903 Ma. Sample DL-12B, from an Ulleri intrusion into
the Kuncha Formation, gave a TuffZirc age of 1886 +/- 8 Ma, constraining the minimum age of
the Kuncha/Ranimata Formation in mid-western Nepal. Sample DL9, originally mapped as
Kushma Formation, has a youngest peak age of 1863 Ma. Though the sample from Kushma
(Fagfog) Formation has a youngest peak age younger than that of the Ulleri Gneiss in the area,
the ages are within error of each other, in contrast to that of central Nepal.
We date the Syangja Formation at 1754 +/- 8 Ma, based on a weighted mean of the
volcanic sample PK2 from along the Tansen-Pokhara road. This age aligns with the maximum
depositional age of our compiled five Syangja samples at 1757 Ma. The age range of 1730-1780
Ma is not a dominant portion of the LHS spectrum from previous samples [Gehrels et al., 2011],
and also not a common age observed in the Paleoproterozoic igneous bodies of the Himalaya or
the Indian craton [Kohn et al., 2010]. Several bodies of the Ulleri Gneiss have been dated ca.
1780 Ma [Kohn et al., 2010; Martin et al., 2011], so the zircons from the Syangja Formation may
33
have been derived from volcanic eruptions related to this igneous activity. The overlap between
the volcanic age and maximum depositional age suggests a largely volcanogenic source for the
zircons. The ~1755 Ma ages may come from a volcanic event with no corresponding intrusive
body exposed or yet discovered in Nepal. Alternatively, the zircons could be sourced from
magmatism that emplaced the felsic intrusions of the far-eastern Himalaya of Arunachal, dated at
1743 ± 4 and 1747 ± 7 Ma [Yin et al., 2009], requiring ~1000 km of sediment transport. Notably,
the Syangja Formation samples contain very few (<10%) grains from the ~1880-1900 peak that
dominates the Kuncha Formation samples (Figure 4). A large amount of volcanism ca. 1755 Ma
may have been enough to drown out any remaining igneous sources with ~1880 Ma ages, or the
~1880 Ma sources may have been eroded away at this point. Limited other detrital zircon
samples from the Syangja Formation [Gehrels et al., 2011] are not dominated by a ca. 1750 Ma
peak, so these peculiar spectra may simply reflect a limited, local source. Importantly, the
strength of this peak suggests extension of Lesser Himalayan igneous activity beyond the ca.
1880 peak that was the source of the Kuncha Formation zircons.
The young metamorphic rim ages from the Syangja Formation samples (section 4.1.4) are
roughly contemporaneous with the Greater Himalayan orogeny [DeCelles et al., 2000; Gehrels
et al., 2003], the likely source of Paleozoic metamorphism in the Himalaya capable of growing
metamorphic zircon. This event has been recognized in rocks of the Greater Himalayan
Sequence of the Kathmandu and Jajarkot klippen [Gehrels et al., 2003; 2006b; 2006a], though
evidence has been lacking for an effect on the Lesser Himalayan Sequence. Perhaps most
importantly, these data suggest that Greater and Lesser Himalayan rocks had to be structurally
juxtaposed during the Greater Himalayan orogeny. These metamorphic rim ages are not enough
to definitively show the effect of the Greater Himalayan orogeny on the Lesser Himalayan
34
Sequence. However, future studies might look to dating Lesser Himalayan Sequence zircon rims
to search for a possible signature of the Greater Himalayan Orogeny.
5.1.2 Age of Upper Nawakot Group Formations
This study reinforces previous work from Martin et al. [2011] and others that the upper
Nawakot Group, the upper portion of the Lesser Himalayan Sequence in Nepal, cannot be
correlated with the Neoproterozoic and Paleozoic Lesser Himalayan units from India and Bhutan
[Hagen, 1959; Frank and Fuchs, 1970; Dhital et al., 2002; Myrow et al., 2003; McQuarrie et al.,
2008]. Detrital zircon youngest peak ages steadily become younger upsection from ~1880 Ma in
the Kuncha Formation, to ~1775 Ma for the Fagfog Formation, to ~1755 Ma for the Syangja
Formation. In multiple locations in central Nepal, we observed a gradational contact between the
Syangja Formation and the basal dolostones of the Lakharpata Group. Two detrital zircon
samples from the Lakharpata Group [Gehrels et al., 2011] show a broad distribution of ages
from 1600-1800 Ma. Though there are not dateable igneous bodies within the Lakharpata Group
to constrain minimal depositional age, carbon isotopes from the uppermost member of the
Lakharpata Group, the Malekhu Formation, show an extremely limited range clustered around 1‰ (VPDB), requiring deposition prior to ~1300 Ma [Martin et al., 2011]. Deposition closer to
the maximum depositional age of the Lakharpata Group at ~1600 Ma would align with age
intepretation based on carbon isotopes observed by Martin et al., [2011]. In any case, a profound
unconformity separates the Malekhu Formation from the basal member of the Gondwanan Unit,
the Carboniferous-Permian Sisne Formation [Sakai, 1989; Upreti, 1999].
5.2 Interpretation of Field Area Tectonostratigraphy
In Figures 11, 12, and 13, we present different interpretations for the rock units of the
Damauli area. We hope that by presenting a range of options for three unit identities, readers will
35
have a more complete understanding of the uncertainty in the area, which propagates in a certain
manner to some of the broader discussions presented here; and future workers will be able to
target specific locations for field examination and sampling for analytic work.
5.2.1 Identity of coal-bearing unit near Bandipur
In Figure 11, we present two interpretations for the geology of units near the town of
Bandipur. While in the field, we mapped many of these rocks as part of the Gondwanan unit,
because the micaceous- and lithic-rich sandstones exposed in the area match most closely to
Gondwanan Unit sandstones [Sakai, 1983]. Furthermore, organic-rich, sulfurous deposits, and at
one outcrop, a lignite deposit, provided further evidence in favor of the Gondwanan
interpretation.
However, three samples collected for detrital zircon analysis (MG-1, MG-3, and MG-28)
from what we originally mapped as one continuous Gondwanan unit yielded age spectra
consistent with Lesser Himalayan Sequence rocks of the Kuncha and Syangja Formations
(spectra in Figure 4, sample locations in Figure 2). These results show that micaceous- and lithicrich sandstones can belong to the Syangja Formation, not just the Gondwanan Unit. Additionally,
sample MG-1, which yielded a zircon age spectrum similar to that from the Kuncha Formation,
was from an outcrop with organic-rich, sulfurous layers. Field stations 221 and 278 contain what
we thought were unequivocal coal layers, including one which had been mined by locals. We
present two interpretations in Figure 11: one which recognizes the coal-bearing outcrops as part
of the Gondwanan unit (Figure 11a) and one which lumps those outcrops with the Syangja
Formation, following the detrital zircon evidence. Given the lack of any analytical support for
the Gondwanan Unit, our preferred interpretation maps all of these outcrops as Syangja
Formation. The Syangja Formation contains stromatolites and other indicators of deposition in a
36
tidal-flat setting [Sakai, 1983], and these “coals” could simply be extremely organic-rich deposits
in such a setting.
Future workers should return to the coal-bearing outcrops and examine the coals for
macrofossils indicative of land plants, which would rule out the 1.75 Ga Syangja Formation. If
no fossils are present, detrital zircon or neodymium isotopic samples would differentiate the
Syangja Formation and Gondwanan Unit, which have distinct signatures [Gehrels et al., 2011;
this study].
The structural discrepancies between the two map patterns presented in Figure 11 are not
large. A window of Gondwanan Unit from a lower thrust sheet would have only minimal impacts
on the shortening estimate, and the map pattern could be created by lengthening the thrust sheet
just above the MBT (Figure 4) and putting a ramp into the Trishuli Thrust sheet which contains
the Syangja Formation exposed here.
5.2.2 Identity of Units Along the Seti River
In Figures 11b and 12, we present three interpretations for the geology of units on either
side of the Seti River south of the towns of Damauli and Bandipur. The lithological pattern is a
band of green and silvery phyllite with quartz augen and veins that crops out to the north of the
Seti River along with minor white quartzites, followed by a 40-100 m thick, cliff-forming
silicified dolostone, which sits underneath organic-rich black slates and phyllites. Green and
silvery phyllite usually belongs to the Ranimata Formation (probable Kuncha Formation
equivalent), which, in combination with the high degree of deformation, led us to the
interpretation presented in Figure 12a, with the black slates and marble belonging to the GHS,
thrust over the Ramgarh thrust sheet, which is in turn thrust over the LHS.
37
However, three isotopic analyses from the unit containing the organic-rich black slates
and phyllites yielded ɛNd(0) values of -24.4, -23.6, -19.3, values that rule out this unit belonging
to the GHS (see section 4.2).
A map pattern that would allow the green phyllite with quartz sweats to belong to the
Ranimata Formation is presented in Figure 12b. In this model, the Ranimata Formation rocks sit
beneath Fagfog Formation, which pinches out laterally, and the silicified dolostone is assigned to
the Galyang Formation, which sometimes contains at its base carbonate beds known as the
“Baitadi” carbonates. The organic-rich slates and phyllites would belong to the Galyang
Formation, which does sometime contain organic-rich black slates [DeCelles et al., 2001a].
In a third model (preferred, shown in Figures 3 and 11b), the rocks belong to a straight
section, with the green phyllites belonging to the Syangja Formation, followed by the Dhading
Formation and the Benighat Formation. The Syangja/Nourpul Formation does contain phyllitic
members and exhibits significant lithological variability both in the area [Paudyal et al., 2012a]
and to the west [Sakai, 1985]. Observations from just south of Khairenitar 20 km to the west
support this interpretation. First, the carbonate exposed at the bridge across the Seti river sits in
depositional contact over quartzites of the Syangja Formation, supporting the notion that it
belongs to the Lakharpata Group. This carbonate is followed by black slates exposed at the Seti
River. The Syangja Formation quartzite – carbonate – black slate pattern is also seen to the east
south of Damauli and Bandipur, suggesting the Lakharpata Group interpretation is best there.
Furthermore, the unit containing black slates has increasing carbonate content upsection,
including some beds that are dominantly carbonate, which suggests that it belongs to the
Benighat Formation of the Lakharpata Group rather than the Galyang Formation.
38
Structural context also supports the straight section model. Organic-rich black slates and
schists are recognized in the footwall of the RT along strike underneath the western side of the
Kathmandu klippe [Colchen et al., 1980; Stöcklin, 1980; Beyssac et al., 2004; Bollinger et al.,
2004], in the high mountains of central Nepal [Ohta et al., 1973; Pêcher, 1978; Bollinger et al.,
2004]; and in the Okhaldunga window to the east of the Kathmandu klippe [Schelling, 1992].
Underneath the Kathmandu klippe, these rocks are recognized as part of the Benighat Formation,
the middle member of the Lakharpata Group, and they are exposed as part of a straight section
containing the entire Nawakot Unit [Stöcklin, 1980; Upreti, 1999; Khanal and Robinson, 2013].
Though some caution should be maintained as this straight section has proven to be incorrectly
interpreted in the past, with the Robang Formation revealed to be not the capping unit of the LHS
but instead Kuncha Formation equivalent carried in the Ramgarh Thrust [Pearson and DeCelles,
2005].
To the north of the study area in the north limb of the Gorkha-Pokhara anticlinorium,
Martin et al. [2005] provided the only definitive age control on these graphite-bearing schists,
with a detrital zircon sample from a quartzite interbedded within graphitic schists yielding a peak
age of ~1700 Ma. Because the Galyang Formation is older than the Syangja Formation, which
we have dated to ~1750 Ma, the graphitic schists of the high mountains must belong to the
Lakharpata Group, reinforcing the equivalence of the organic-rich rocks of our field area to the
Benighat Formation underneath the Kathmandu klippe.
Differentiating among these different possibilities for the Seti River units is critical to
understanding the location of major thrust faults and interpreting the structure of the range.
Locations of the MCT and RT could change 4-5 km, as could the units carried by each. We have
already discussed that neodymium isotopic evidence rules out option one, the “large klippe”
39
option. The two best models involve calling the black schists either Galyang or Benighat
Formation. Though we favor the Benighat Formation interpretation, we note that workers in the
Himalaya often disagree over assigning rocks to one of these two formations. Along the TansenPokhara road, some workers see the Galyang Formation [Shrestha et al., 1987a], and others
interpret the Benighat [Sakai, 1985; Paudel and Arita, 2006]. Underneath the Kathmandu klippe,
there are also disagreements, with Pearson and DeCelles [2005] interpreting Galyang Formation
where Khanal and Robinson [2013] interpret Benighat Formation.
In our study area, differentiating between the Benighat and Galyang Formations would
require acquiring detrital zircons from the unit in question. We collected multiple samples for
detrital zircon analysis, but all contained a small fraction of quartz relative to carbonate and none
yielded zircons. Future sampling will require careful searching for rocks with sufficient sand
fraction to expect zircons and acquisition of large-volume samples to compensate for the mixed
carbonate-clastic system. We also suggest that future work on the unit may be best focused on
the south side of the Seti River between the town of Damauli and the confluence with the
Trishuli River. At the time of our field work, no road bridges crossed the Seti River east of the
town of Khairenitar. Unless a car bridge has been built in Damauli, these rocks must be accessed
via foot bridges from a road on the north side of the river or by looping around from a road
leaving Khairenitar to the south.
5.2.3 Identity of structurally highest quartzite unit
Detrital zircon sample KT-11 is from a mylonitized quartzite located structurally highest in
the field area (Figure 13). This quartzite is located above a marble, which in its eastern exposure
forms a prominent cliff. Both the detrital zircon spectrum and the geologic context of this sample
allow for it to belong to either the Gondwanan Unit or to the Greater Himalayan Sequence.
40
The detrital zircon spectrum (Figure 4) is dominated by ages in the 800-1000 Ma range.
These ages are typical of GHS and THS rocks, though THS rocks in the Himalaya are
distinguished by a large population of grains at ~500-600 Ma (Figure 4), probably sourced from
granites emplaced during the Greater Himalayan Orogeny, and a small number of grains <400
Ma. With only a single grain in the 500-600 Ma range that significantly contributes to THS
rocks, KT-11 is probably not part of the THS, though a larger number of detrital zircon dates
would improve statistical confidence in this conclusion.
The quartzite is from a similar structural position to Greater Himalayan Sequence rocks of
the Kathmandu klippe and was originally mapped by the Nepal Department of Mines and
Geology as Tistung Formation, though detrital zircons spectra of the Kathmandu Complex have
their strongest peak at ~1100 Ma [Gehrels et al., 2003], and lack the strong peak at ~850 Ma
found in sample KT-11, suggesting this rock either does not correlate to the sedimentary rocks of
the Kathmandu klippe or detrital zircon provenance varied significantly on the ~100 km scale.
Even if the detrital zircons do not suggest a direct correlation to the GHS rocks of the
Kathmandu klippe, the age distribution of KT-11 is roughly similar to a sample of GHS
Formation I (402092A) analyzed by Martin et al. [2005], collected in the Marsyangdi River
Valley ~50 km to the north of sample KT-11. Their sample 402092A was found just a few
hundred meters above the MCT, a similar distance from the MCT equivalent for KT-11 as
presented in our map. Both KT-11 and 402092A have a large proportion of ages in the 800-1000
Ma range. However, Gondwana Unit sample SLY-15 has peaks which very closely align with
those of KT-11 (Figure 4). Similarity analysis measures the relative proportions of the different
age populations and provides a score of 0.0 (no similarity) to 1.0 (perfect overlap and identical
proportions) [Gehrels, 2000]. KT-11 has a similarity score of 0.418 with “Tistung 2” [Gehrels et
41
al., 2006a], 0.671 with GHS Formation I sample 402092A [Martin et al., 2005] and 0.744 with
Gondwana quartzite sample SLY-15 (this study). The highest similarity score with the
Gondwanan Unit Quartzite suggests that the quartzite may be part of the Gondwanan Unit rather
than the GHS.
With our current data, we cannot definitively assign the quartzite of sample KT-11 to
either the GHS or the Gondwanan unit. Due to poor exposure, we were not able to observe the
contact between KT-11 and the cliff-forming marble to the east or the graphitic calc-schists to
the west along the prominent east-west ridge north of the Kali Gandaki. We must rely on
inference based on stratigraphy to produce a most likely structural interpretation. If the graphitebearing unit containing black slates exposed at the Seti River is indeed Benighat Formation (see
section 5.2.2), the two possible formation sequences are Benighat Formation (with green
phyllites at the top) – Malekhu Formation – Gondwanan Unit (straight section model in Figure 3)
or Benighat Formation – Ramgarh Thrust – Kuncha Formation – MCT
- Bhainsedobhan
Formation – GHS quartzite.
For our preferred interpretation, we assign the quartzite of KT-11 to the GHS. The cliffforming marble is found only on the east side of the quartzite, suggesting a fault contact with a
lateral ramp in the hanging wall, though lateral stratigraphic variability is also possible.
Additionally, the basal member of the Gondwanan Unit, the Sisne Formation, is composed of
diamictites [Sakai, 1983]. These lithologies were not observed in our study area, though in
West-central Nepal, to the east of the Dadeldhura klippe, Amile Formation quartzite was found
sitting on Lakharpata group [Sakai et al., 1999]. This pattern of lithologies would fit that
observed in our study area, but Amile Formation zircon U-Pb age spectra contain Cretaceous
42
ages and a more prominent Paleoproterozoic peak [DeCelles et al., 2004, 2014], quite different
than that of sample KT-11.
Proper identification of this uppermost quartzite is important for workers wishing to
study lateral variation of the MCT and the geology of the GHS as exposed in the klippen.
However, even if the quartzite belongs to the Gondwanan Unit, the eroded GHS thrust sheet
cannot have been very far above the current uppermost rocks, as the inverted metamorphic
pattern and absolute temperatures are similar beneath the “Damauli klippe” and the Kathmandu
klippe [Bollinger et al., 2004].
5.3 Exhumation of central Nepal
We here analyze all of our zircon (U-Th)/He (ZHe) data, along with studies from Bhutan
[Long et al., 2012b; McQuarrie et al., 2014]. A ZHe age represents cooling through the
temperature window of roughly 180-200 °C and is a function of time, rate of cooling, and
helium diffusivity [Reiners et al., 2004]. Helium diffusivity can have large effects on ZHe ages
if the zircon remains in the partial retention zone, where some helium is retained, for a
significant amount of time [Guenthner et al., 2013], though our data mostly lack age-effective
uranium correlations that signal large He diffusivity effects (Table S2).
Cooling of a rock can be accomplished by several means. Lateral heat transfer occurs
when a block is uplifted by a fault, resulting in heat advection and geothermal gradient changes
even absent erosion [ter Voorde et al., 2004]. Cooling also occurs due to exhumation, which can
be tectonic (e.g., normal faulting removing material overlying a rock), or erosive in response to
topography built by thrust or reverse faulting [Reiners and Brandon, 2006]. Workers in the
Himalaya generally assume topography and geothermal gradients to be roughly steady-state, and
interpret mineral cooling ages as the result of uplift due to thrust faulting [Avouac, 2003;
43
Bollinger et al., 2004; Thiede et al., 2004; Vannay et al., 2004; Brewer and Burbank, 2006;
Herman et al., 2010].
Figure 7 shows ZHe ages from four areas of Nepal (this study). These samples (Table 1)
were all collected roughly along arc-normal transects. Significant variation exists both in terms
of absolute age, which is as high as 15 Ma and as low as 3 Ma, and in the trend these ages show
with respect to distance from the MCT.
Figure 8a shows ZHe ages from Nepal and Bhutan plotted versus arc-normal distance from
the front of the GHS sheet, which is either a klippe (most of Nepal) or a continuous sheet (most
of Bhutan). Generally, ages increase towards the GHS front before decreasing towards the
foreland. However, there is again wide variation in maximum ZHe age for a transect, and the
slopes of ages also widely differ. Figure 8b shows the maximum ZHe age from each of these
transects; in almost every instance the maximum age occurs near the front of the GHS sheet.
ZHe ages decrease from ~14 Ma in western Nepal to ~7 Ma in central Nepal and then increase
again to ~12 Ma in western Bhutan. The younger ages in Nepal can be partially explained by the
abnormally low elevations in the region, with erosion exposing rocks which more recently
cooled through the closure temperature of the ZHe system, but with the age-elevation R2=0.3,
elevation does not explain the majority of the age-longitude trend.
Two other possibilities exist for the young ZHe ages in central Nepal (1) slower cooling
and (2) greater deformation and exhumation post-RT. Quantifying cooling rates requires
additional thermochronometers [Reiners and Brandon, 2006], but two lines of evidence suggest
the importance of heat transfer to this problem. First, Raman spectroscopy of carbonaceous
material (RSCM) shows inverted metamorphism of Lesser Himalayan rocks beneath the klippen
of central Nepal, indicating heat transfer from a hot upper thrust sheet to cool footwall rocks
44
[Beyssac et al., 2004; Bollinger et al., 2004], with peak metamorphic temperatures ranging from
~500 °C in the hanging wall rocks to ~350 °C in the footwall rocks. Second, zircon fission track
ages also show ages younging upwards from the footwall to the hanging wall of the klippen in
mid-western Nepal, indicating cooling due to heat transfer [Sakai et al., 1999; 2013a]. If the
rocks in central Nepal cooled more slowly than those from western Nepal or Bhutan, they would
have younger ZHe ages. Possible explanations for slower cooling include a hotter fault footwall,
a thicker overburden, or slower erosion rates, but additional thermochronometers should first be
explored to establish cooling rates. A second possibility for the young ages is that central Nepal
has been subject to greater recent deformation and erosion. Before considering this possibility in
more detail, we will examine spatial trends in ZHe ages.
ZHe ages from both Bhutan and Nepal align coherently when plotted versus distance from
the MCT (Figure 8c). Ages get older away from the MCT until younging again at roughly the
front of the GHS sheet. Previous workers have taken the slopes of these ages (in Ma/km) and
converted them to exhumational rates (in km/Ma). Assuming that the rocks were exhumed as a
block through the closure termperature, workers have interpreted these as a type of speedometer
for the shortening rate in the thrust belt [Long et al., 2012] or as a measure of the “overthrusting”
rate [Avouac, 2003; Bollinger et al., 2004; Herman et al., 2010].
Figure 9 shows a cartoon representation of the overthrusting/underthrusting model. In this
model, the overriding plate, underthrusting plate, topographic surface, and basal thrust fault are
in a steady-state configuration [Avouac, 2003]. In contrast to a balanced cross-section, which
does not differentiate under- and overthrusting, the reference frame for this model is the
topographic surface. Only overthrusting, which is held equal to erosion, results in exhumation of
45
rocks. Underthrusting fluxes material beneath the wedge with no upper-plate response,
accommodating convergence with no exhumational or thermochronological response.
Though it is an extreme simplification of the complicated dynamics of a fold-thrust belt, this
representation of the Himalayan wedge is employed in modeling studies [Brewer and Burbank,
2006; Herman et al., 2010; Robert et al., 2011]. These studies invert or forward model
thermochronologic data to test tectonic processes (e.g., out-of-sequence thrust faulting vs.
duplexing as the cause of the central Nepal physiographic transition, see Section 2.3) and either
use or calculate a parameter which sets the relative contributions of overthrusting and
underthrusting.
Brewer and Burbank [2006] and Herman et al. [2010], in modeling studies of detrital and
bedrock thermochronologic data, respectively, found that data from central Nepal were best
explained with ~25% of the convergence partitioned into overthrusting. Their results suggest that
the gradient of thermochronologic ages cannot be used as a speedometer for shortening rate (e.g.,
Long et al., [2012]) without an estimate of the relative contributions of over- and underthrusting.
However,
comparing
the
data
from
this
study
and
Bhutan
suggest
that
the
overthrusting/underthrusting model cannot be used to interpret ZHe ages. First, the rates do not
follow predictions. Following the assumption that the mountain range is a self-similar wedge that
maintains constant taper [Davis et al., 1983], the portions of the range which have the highest
overthrusting rate should also be exhumed to the deepest levels. Following this logic, Herman et
al. [2010] predicted that as the depth of exhumaiton increases and the rocks of the Lesser
Himalayan Duplex dominate the surface exposure, the overthrusting rate should increase. In
terms of age-distance plots (Figure 8c), this means that as depth of exhumation increases, the
slope should decrease (become flatter). However, the highest rates from the six transects
46
compared in this study occur in western Bhutan, where the rate is 14.4 km/Ma [McQuarrie et al.,
2014]. In central and far-western Nepal, where our highest-quality data are from, the rates are 8.9
and 5.5 km/Ma, respectively. Western Bhutan is dominated by the GHS sheet, with only a small
window (the Paro window) into the Lesser Himalayan sequence [McQuarrie et al., 2008];
whereas central and western Nepal are dominated by LHS units, with the GHS exposed only in
isolated klippen. Thus, as a first-order test, the connection between ZHe age gradients,
overthrusting, and exhumation fails.
The overthrusting/underthrusting model also fails when incorporating higher-temperature
data. MAr and retrograde monazite ages from western Bhutan are nearly flat across the thrust
belt [McQuarrie et al., 2014], resulting in “overthrusting” rates of ~50 and ~30 km/Ma,
respectively, far higher than the MAr “overthrusting” rate of 4-5 km/Ma from more deeplyexhumed central Nepal [Herman et al., 2010].
A second line of evidence rebuts the notion that these ZHe ages can be interpreted as a
shortening or overthrusting speedometer. If the rocks were cooled through the closure
temperature as a result of movement over a ramp, a central tenet of these models, then the ages
would unimodally increase away from the MCT. The MCT in this scenario would not be the
structure responsible for exhuming ages but simply a useful proxy for the mid-crustal ramp
responsible for uplifting these rocks. However, ages decrease at the front of the GHS sheet
(Figure 8a), suggesting that these rocks sat at a depth below the ZHe closure isotherm until
exhumation via the MBT system. Thus, an explanation other than variation in overthrusting rates
is required to explain the steeper gradient in ages from Nepal compared to Bhutan (Figure 8c).
As previously discussed, the young ages in central Nepal may possibly be explained by
slower cooling. However, the depth of exhumation is higher in Nepal than it is in Bhutan, with
47
the LHD contributing to a higher portion of the rocks exposed at the surface. Setting aside rocks
in front of the klippen exhumed via the MBT system, the youngest ZHe ages from Nepal and
Bhutan occur in central Nepal (Figure 8a) over the LHD (Figure 2). Thus, we propose that the
young ages are due to extended growth of the LHD. Following this logic, the GHS klippen from
central Nepal show younger ages due to the narrowness of the thrust belt and increased
proximity to the LHD as compared to western Nepal or Bhutan. With ZHe ages along a transect
(Figure 8c), more duplexing produces steeper age profiles (Nepal), and less duplexing produces
flatter age profiles (Bhutan).
5.4 Shortening estimates and kinematic evolution.
Figure 14 shows a line-length balanced cross-section and retrodeformed state of the study
area located in Figure 2. Total minimum shortening is 304 km. Sources of uncertainty for this
shortening estimate include location of eroded hanging wall cutoffs, which are drawn as quickly
as possible to minimize shortening. Small-scale brittle folds and thrusts, which are pervasive in
much of the area between the MBT and the first appearance of the Kuncha Formation, would
also increase total shortening if included; ductile deformation, including mineral stretching
lineations, is observed in the Kuncha Formation and proximal to the MCT both in the klippe and
the high mountains. The cross-section is constructed with the assumption that the footwall is
rigid and pinned, but footwall cutoffs may have been thrust beneath their original depositional
locations, which would also increase the shortening estimate.
This shortening estimate does not include thrusts within the GHS, many of which
probably belong to the Paleozoic Greater Himalayan Orogeny [Gehrels et al., 2003; 2006a], and
does not include thrusts within the Tethyan Himalaya. Uncertainties associated with ambiguous
geologic units discussed in Section 5.2 would leave major structures relatively intact, though
48
details would change. The major structures which accomodated the largest amount of shortening,
the MCT, RT, and TT would stay approximately the same. A duplex would still be required
underneath the Gorkha-Pokhara anticlinorium. If the klippe was thicker than shown (see section
5.2.2) or not present (5.2.3), its location would change by +/- 3 km vertically, which would have
a significant impact on the interpretation of field data but leaving the overall structure intact.
However, similarity of this cross-section to previous work from the Kathmandu klippe to the east
[Pearson, 2002; Khanal and Robinson, 2013] and metamorphic studies [Beyssac et al., 2004;
Bollinger et al., 2004] supports this interpretation.
6. Discussion
6.1 Nawakot Group ages
Martin et al. [2011] recognized that the fact that the Kushma Formation of central Nepal
was younger than the Kuncha Formation had ramifications for the age of the quartzite mapped as
“Kushma Formation” in western Nepal [Shrestha et al., 1987b; DeCelles et al., 1998b; 2001a;
Pearson and DeCelles, 2005; Robinson et al., 2006; Robinson, 2008]. This quartzite is
considered the basal LHS unit in western Nepal. Four possibilities exist: (1) all of the quartzite
mapped in western Nepal as Kushma Formation correlates to the Fagfog Formation of central
Nepal, and thus belongs above the Ranimata Formation (Kuncha Formation equivalent). This
would alter locations of thrust faults in western Nepal. (2) None of the quartzite mapped in
western Nepal as Kushma Formation correlates to the Fagfog Formation of central Nepal.
Previous workers [DeCelles et al., 2001a] broke out as a separate unit the Sangram Formation,
which they placed above the Ranimata Formation and correlated to the Fagfog Formation of
central Nepal. This possibility would leave unchanged previous geologic maps and shortening
estimates, though a different name for the basal quartzite should be used, as recommended by
49
Martin et al. [2011]. (3) Thick white quartzites occur above, below, and possibly within the
Ranimata Formation of western Nepal. In this scenario, some of the quartzite mapped in western
Nepal as Kushma Formation correlates to the Fagfog Formation of central Nepal. This would
significantly decrease some shortening estimates, as duplex-forming thrust faults have been
inferred due to repetition of a thick white quartzite previously thought to only occur at the base
of the Ranimata Formation [Robinson et al., 2003]. (4) The Ranimata Formation of western
Nepal is younger than the Kuncha Formation. This possibility would allow the Kushma
Formation of western Nepal to be equivalent in age to the Fagfog Formation of central Nepal and
would leave most thrust fault locations intact. Discriminating between these possibilities is
important for shortening estimates, metamorphic studies, strain analyses, and a more precise
understanding of the history of the range.
Results from mid-western Nepal reflect the difficulties in distinguishing between these
issues. The age relationships from detrital and igneous zircons from the Birendranagar area
suggest, though do not definitively prove, that the quartzite there is younger than the Kuncha
Formation. However, the quartzite belt from which sample DL-9 is taken is in thrust contact with
tertiary foreland basin sedimentary rocks and sits below, in seeming stratigraphic continuity,
with Kuncha/Ranimata Formation phyllites. This relationship suggests that either zircon results
are incorrect and the Ranimata Formation is younger, or the Ranimata Formation is in thrust
contact with the Fagfog Formation quartzite. Adding to the ambiguity, an outcrop located 35 km
to the northwest of sample DL9, from a separate body of cliff-forming, rippled white quartzite
mapped as “Kushma” Formation [Shrestha et al., 1987b] shows the quartzite in depositional
contact with the Ranimata Formation below [Figure 10].
The work of Martin et al. [2011] and this study shows that the Fagfog and Kushma
50
Formations of central Nepal are younger than the Kuncha Formation; therefore, the presence of
cliff-forming white quartzites above green phyllites cannot be indiscriminantly used to infer the
presence of the RT. However, previous studies have observed the actual RT fault surface at the
bottom of quartzite beds in western and central Nepal and have found Ranimata Formation in
depositional contact above quartzites [DeCelles et al., 2001a; Pearson and DeCelles, 2005]. This
leaves the most likely identities of western Nepal quartzites as options 3 or 4 discussed above:
thick white quartzites may occur above, below, and possibly within the Ranimata Formation of
western Nepal; and the Ranimata Formation of western Nepal may not correlate to the Kuncha
Formation of central Nepal, lying above, rather than below, the Kushma Formation. Future work
must take into account the ambiguity of green phyllites and white quartzites in Nepal.
Martin et al. [2011] divided the Fagfog Formation into Upper and Lower members, taking
the type locality at the town of Kushma as the lower member, and the type locality of the Fagfog,
located toward the eastern end of the Gorkha-Pokhara anticlinorium, as the upper member.
Though Martin et al. [2011] were unable to distinguish between Upper and Lower Fagfog
Formation on the basis of lithological characteristics in the field, they suggested the detrital
zircons were different, with lower Fagfog Formation containing a younger population with a
peak at ~1800 Ma. Sample MG-2 (this study) contains a peak of ages at 1772 Ma. This sample
was taken from an outcrop near the town of Muglin, closer to the type “Upper Fagfog” than the
type “Lower Fagfog” Formation, suggesting that the spatial distribution of the “Upper Fagfog”
might be extremely limited. With no distinguishing lithological characteristics between the two
members and no detrital zircon evidence from one single location showing a difference between
Upper and Lower members, we suggest that the differences in detrital zircon U-Pb ages from the
type Upper and Lower localities are due simply to intra-formational variability. This variability
51
might be studied to establish sediment provenance changes along strike or through time.
However, we do not see any utility at this time in breaking the Fagfog Formation into Upper and
Lower members for mapping purposes.
Figure 4 includes an updated compilation of Lesser Himalayan Sequence zircon ages. This
includes all formations of the Nawakot Complex and excludes those of the Gondwanan Unit.
This compilation contains zircon analyses compiled by Gehrels et al. [2011], with additional
zircons from this study. We do not include recent results of McKenzie et al. [2011] and Tobgay
et al. [2010] that come from rocks of uncertain affinity.
The updated curve shows two peaks in the Paleoproterozoic, one at ~1880 Ma, followed by
a lull centered around ~1810 Ma, and a second peak at ~1750 Ma. These two peaks may
represent two-stage magmatism in the source for Lesser Himalayan Sequence zircons or a shift
in provenance. Samples from detrital zircon analysis have not been taken systematically up the
metasedimentary section and thus a compilation is not an unbiased representation of magmatic
activity in the source region through time. However, we do note that samples from the Syangja
Formation show detrital zircons in two peaks roughly centered on the two described above. The
peak at ~1750 Ma is probably overrepresented on this curve relative to the overall magmatic flux
in the source region, as the Syangja Formation and Kuncha Formation contribute equal numbers
of grains to the curve, despite their thicknesses being 500-1000 m and >4 km, respectively.
This study, by providing a Paleoproterozoic volcanic age of the Syangja Formation and
drastically increasing the number of detrital zircon ages for formations located above the Kuncha
Formation, reinforces the discrepancy in age estimates of the upper Lesser Himalayan Sequence
between Nepal, India and Bhutan, with the rocks of India and Bhutan believed to be
Neoproterozoic to early Paleozoic [McKenzie et al., 2011; McQuarrie et al., 2008].
52
Two possible explanations for the inability to correlate the upper LHS of Nepal with that of
India and Bhutan exist: (1) late Proterozoic-early Paleozoic Lesser Himalayan Sequence rocks
are present in India and Bhutan but absent in Nepal due to non-deposition or erosion, and (2) late
Proterozoic-early Paleozoic LHS rocks of India and Bhutan correlate to the Bhimphedi Group of
the Kathmandu klippe and are actually in fault contact with the LHS. The debate might only be
resolved when workers from each country, with the attendant geologic vocabularies and models,
examine the geology of the countries along-strike.
The improved detrital zircon and volcanic record provided by this study for the Lesser
Himalayan Sequence allows us to test the model of [Sakai et al., 2013b] correlating the
sediments of the Proterozoic margin of northern India (Nawakot Complex) with those from the
margin of northwest Canada (Coronation Supergroup). In this model, the Syangja, Dhading, and
Benighat Formations of Nepal correlate with the Odjick, Rocknest, and Fontano Formations of
northwest Canada. This study dates the Syangja Formation at ~1750 Ma, constraining the
maximum age of the Dhading and Benighat Formations. [Bowring and Grotzinger, 1992] date an
ash bed from the Fontano Formation at 1882 +/-4 Ma, indicating that these rocks were not
deposited at the same time and do not correlate. These results do not disprove the Canada-India
connection, but they do require that the sedimentary overlap across the rifted margins end at the
time of Kuncha Formation deposition ca. 1880 Ma.
6.2 Stratigraphy and definition of the Main Central Thrust and Ramgarh Thrust
In central Nepal, beneath the Main Central Thrust lie rocks of the lower LHS, alternately
called Robang Formation, Dunga Quartzites, or Kuncha Formation [Stöcklin, 1980; Upreti,
1999]. Pearson and DeCelles [2005] showed that these rocks are in a thin thrust sheet, bounded
below by the Ramgarh Thrust and above by the Main Central Thrust. These rocks crop out on the
53
south side of the Kathmandu klippe and can be followed all the way around the western end of
the klippe, and from there northeastward and then northward into the high mountains around
Langtang [see also Kohn, 2008; Khanal and Robinson, 2013]. These rocks belong to the lower
portion of the Lesser Himalayan Sequence. Martin et al. [2005], in five separate transects across
the MCT in central Nepal, found that the location of highest strain coincides with the
stratigraphic boundary between the Lesser and Greater Himalayan Sequences, which was also a
boundary separating rocks with distinct detrital zircon U-Pb ages [see Section 2.2] and Nd
isotopic signatures [see Section 4.2]. Imayama and Arita [2008] extended this work to eastern
and western Nepal and found the same major discontinuity in epsilon Nd values at the location of
the “Upper” MCT, which we would map as the MCT. Girault et al. [2012] used effective
Radium (ECRa) measurements to show a discontinuity across the MCT, and they also found a
discontinuity across the RT. The ease and low cost of measuring ECRa enables analysis of a large
number of samples (n=622 in their study), more than twice as many as the ɛNd measurements
compiled here, providing greater spatial density and a larger sample size. ECRa varies with age,
deformation, and protolith, with age of the rock probably the dominant control in the Himalaya.
In six sample transects across Nepal, Girault et al. found a ratio in ECRa values of LHS samples
to GHS samples of 7.8 ± 1.6, close to an order of magnitude difference. Mapping the MCT at the
boundary between the GHS and LHS, using the maps of Colchen [1980] as guide, they found a
sharp decrease in ECRa at the MCT, corresponding to the boundary between LHS and GHS
rocks. Their results correlate with ɛNd values determined by previous workers in the same
location, providing yet another quantitative stratigraphic confirmation of the difference between
Lesser and Greater Himalayan rocks. Importantly, Girault et al. [2012] found higher values of
ECRa in the hanging wall than in the footwall of the RT, probably due to older age of the Kuncha
54
and Ulleri Formations relative to the Lakharpata Group.
Underneath the Kathmandu klippe, rocks of the Ramgarh sheet are thrust over Lakharpata
Group carbonates [Colchen et al., 1980; Amatya et al., 1994; Pearson and DeCelles, 2005;
Khanal and Robinson, 2013]. Deciphering the stratigraphy underneath the RT in the high
mountains has been more difficult due to higher grade metamorphism, but several lines of
evidence suggest that the RT footwall contains upper LHS rocks of the Lakharpata Group. First,
large-scale mapping efforts have found a continuous sequence of LHS rocks cropping out around
the Gorkha-Pokhara anticlinorium, with key marker lithologies of quartzite of the Fagfog
Formation and organic-rich black slates of the Benighat Formation or Ghan Pokhara Formation
[Colchen, 1980; Shrestha et al., 1987; Amatya et al., 1984]. Others have observed these black
slates in the footwall of the RT and MCT in central Nepal, including Ohta et al. [1973], who
observed black garnet-biotite schists; Pêcher [1978], terming them the Barpak graphitic schists);
Colchen [1980] who mapped these as continuous with the Benighat Formation beneath the
Kathmandu klippe; Beyssac et al. and Bollinger et al. [2004], who correlated black slates beneath
the MCT of the high mountains of central Nepal, beneath the Kathmandu and Damauli klippen,
and beneath the MCT in the Okhaldunga window. Other recent studies in central Nepal have
identified these graphite-rich rocks in the Langtang Mountains [Kohn et al., 2008]. Schelling
[1992] observed black slates of the Khare Phyllite at a similar structural location in the
Okhaldunga window of eastern Nepal, though his pioneering study was prior to widespread
recognition of the RT. Martin et al. [2005] provided detrital zircon U-Pb evidence for presence
of upper Nawakot Group rocks, with a sample from a quartzite within graphitic schists yielding
ages with a peak at ~1700 Ma, consistent with the age of the Lakharpata Group.
The GHS klippen provide an opportunity to study the rocks on either side of the MCT and
55
RT at a lower-grade setting. We observe black slates of the Benighat Formation beneath the
MCT at the base of the Damauli klippe (Figures 2 and 13). Nd isotopic analysis on the black
slates [see section 4.2.2], detrital zircons from the underlying Syangja Formation [see section
4.1.4], and stratigraphic observations confirm that these black slates are part of the LHS. Section
4.1.5 contains our discussion of the necessity of a fault contact beneath the GHS of the Damauli
klippe. We also map the RT, which has been mapped extensively around the Kathmandu klippe
[Pearson and DeCelles, 2005; Khanal and Robinson, 2013].
These observations give us
confidence that the Ramgarh Thrust, even in the high mountains with higher-grade rocks, places
lower Nawakot Group rocks on Upper Nawakot Group rocks, indicating over 3 km of
stratigraphic separation.
Mapping stratrigraphic repetition is essential for location of thrust faults in thrust belts, and
has been used extensively all over the world. We have provided evidence that below the RT in
central Nepal is a straight section of Kuncha through Upper Nawakot, and in some places
Cenozoic foreland basin sedimentary rocks [DeCelles et al., 2001a; Martin et al., 2011; Sakai et
al., 2013a]. Above the Ramgarh thrust, the Kuncha Formation is again present, though the upper
portion of the Lesser Himalayan Sequence, the Galyang Formation through Lakharpata Group, is
absent, also necessitating a fault. This stratigraphic sequence is the same stratigraphy and fault
location that we observe in our field area and is observed beneath the Kathmandu klippe
[Colchen, 1980; Stöcklin,1980; Upreti, 1999; Khanal and Robinson, 2013]. The sequence of
faulting, with the MCT cutting through the Kuncha Formation/Ulleri Augen Gneiss with the later
Ramgarh thrust cutting up-section into the upper portion of the Lesser Himalayan Sequence, is to
be expected in thrust belts.
Searle et al. [2008] would dismiss lithologic repetition as unimportant because a shear
56
zone occurs below the repeated rocks in the Ramgarh Thrust sheet. However, this ignores the
lithologic repetition as an important strain marker; the work of Pearson and DeCelles [2005] and
Khanal and Robinson [2013] shows that rocks in the hanging wall of the Ramgarh Thrust have
been moved at least 100 km relative to rocks in the footwall. The RT and MCT have discrete
histories, with a minimum of 100 km of slip occurring along the Main Central Thrust, and then a
few Ma later, 100+ km of slip again happening on the Ramgarh Thrust. The fact that both thrusts
have ductile character in the high mountains does not mean that they are the same thrust fault; it
simply means that they were both active at depths required for ductile deformation.
Searle et al. [2008] prefer to consider the RT as the MCT so that the inverted metamorphic
isograds all occur in the hanging wall of the MCT and can be thus explained by channel flow
and/or overturning of a regional scale fold limb; however, these inverted isograds occur in
central Nepal in rocks that are unequivocally LHS [Bollinger et al., 2004], and in western Nepal,
inverted metamorphism has even been observed in rocks of the Miocene foreland basin deposits,
the Dumri Formation [Sakai et al., 2013a].
A separate history for the MCT and RT is suggested also by the sedimentology, with the
Dumri Formation foreland basin sediments containing detrital zircons of Greater Himalayan
Sequence provenance, rather than Kuncha Formation or other portions of the Lesser Himalayan
Sequence [DeCelles et al., 2004; Sakai et al., 2013a]. The Siwalik Group contains more detrital
zircons consistent with the LHS. The MCT and RT are not equivalents any more than the MCT
is equivalent to the MFT: they both root to the basal Main Himalayan detachment, but they have
distinct histories that can be determined with careful geologic observation.
6.3 Evolution of the thrust belt
Field work and structural restoration reveal the kinematic history of the thrust belt in
57
central Nepal. First, thrusting along the MCT placed Greater Himalayan Sequence rocks on
Lesser Himalayan Sequence rocks of the Kuncha Formation. These LHS rocks were then thrust
along the Ramgarh thrust on top of ULHS rocks of the Lakharpata Group. ZHe data with
maximum ages of 14-15 Ma are roughly synchronous with RT slip, which began around 15 Ma
as constrained by 15-16 Ma Dumri Formation in its footwall [DeCelles et al., 2001; Ojha et al.,
2009]. Another orogenic scale (>90 km) thrust fault, the Trishuli Thrust, carried the entire LHS,
which is now exposed in the Gorkha-Pokhara Anticlinorium. The thrust belt then switched to
greater internal deformation, with the Lesser Himalayan Duplex growing since the emplacement
of the Trishuli Thrust, with ZHe data showing significant uplift occuring in the area from ~6 Ma
to present. These ages are younger than those from Bhutan and reflect a greater amount of
duplexing and exhumation in Nepal.
From 5 Ma to present, the frontal thrust belt of Siwaliks Group sedimentary rocks has
developed [Huyghe et al., 2001; DeCelles et al., 2001a], though thrusting in the MFT system has
been accompanied by duplex development in the hinterland, as evidenced by geophysical studies
observing creep in this area [Bilham et al., 1997; Bettinelli et al., 2006; Ader et al., 2012] and
thermochronological studies showing exhumation in central Nepal continuing to the present [this
study; Herman et al., 2010]. This modern-day shortening hidden in thrust faults not exposed in
the duplex must be accounted for in studies which attempt to evaluate long-term variations in
shortening. The reason for abnormally high duplexing and exhumation in central and western
Nepal is unclear. With geologic and thermochronologic data requiring duplexing in central
Nepal, the burden of proof lies upon advocates of out-of-sequence thrusting to show the
necessity of such a process to produce the physiographic transition in central Nepal.
Total minimum shortening between the MCT and MFT is 304 km, with the majority of
58
shortening occurring on the MCT, RT, and TT. 304 km is lower than previous estimates to the
east beneath the Kathmandu klippe [Khanal and Robinson, 2013] and in western Nepal
[DeCelles et al., 2001b], which found at least 400 km of shortening. The lower amount of
shortening could possibly be due to the uncertainties inherent in creating a line-length balanced
cross-section (see section 5.4). Interestingly, this lower shortening estimate comes from a crosssection close to the MBT re-rentrant (Figure 1), where the MBT-MCT distance is smallest in
Nepal. The intriguing coincidence of low shortening, large duplexing, a narrow thrust belt, and
young thermochronologic ages implies a feedback mechanism, perhaps between erosion and
internal deformation of the thrust wedge. Future work will have to establish the cause which
nucleated this feedback loop.
7. Conclusions
1. The Lesser Himalayan Sequence was deposited roughly continuously from the Kuncha
Formation through the end of the Lakharpata Group, with no unconformity >200 Ma. The
Kuncha Formation is dated at ~1880 Ma, the Fagfog Formation was deposited sometime
between 1775 and 1755 Ma, and the Syangja Formation is dated at ~1755 Ma. With detrital
zircons from within the Benighat Formation dated at ~1600-1700 and carbonates showing the
Malekhu Formation >1300 Ma [Martin et al., 2011], deposition was continuous until the
unconformity at the base of the Gondwanan Unit. This means the upper Lesser Himalayan
Sequence of Nepal does not completely correlate with that of India or Bhutan.
2. The Fagfog and Kushma Formations are younger than the Kuncha Formation, and some of the
quartzite from western Nepal mapped as basal is actually younger than the Kuncha Formation.
Future work must recognize that green phyllites and thick white quartzites may be ambiguous
lithologies in Nepal. The Kuncha and Ranimata Formations may not be equivalent.
59
3. Lesser Himalayan Sequence detrital zircons show two large peaks of zircon crystallization at
~1880 and ~1750 Ma. Minimal recycling or sourcing of zircons from earlier magmatic events
occurred as time went on. If northern India rifted away from northwest Canada, the sedimentary
overlap must have ended after deposition of the Kuncha Formation.
4. RT slip began around 15 Ma, and central Nepal has seen significant exhumation due to
duplexing from 15 Ma to present. The young ZHe ages of the Damauli klippe are probably due
to increased proximity to the Lesser Himalayan Duplex.
5. The overthrusting/underthrusting model of exhumation fails when comparing data from Nepal
and Bhutan. Future modeling work may have to include additional geologic detail.
6. The Main Central Thrust, Ramgarh Thrust, and Trishuli Thrust are discrete faults with >90 km
of slip each. The MCT and RT are not equivalents and their individual histories must be
recognized.
7. Total shortening in this portion of the thrust belt is a minimum 304 km, which is lower than
along-strike estimates.
Acknowledgments
The National Science Foundation and University of Arizona Dept. of Geosciences contributed to
research funding. We thank D. Hoffman and M. Ducea for neodymium isotopic analyses, thank
the Arizona LaserChron staff for assistance, and thank the Arizona Radiogenic Helium Dating
Laboratory staff for their help. The author would like to thank P.G. DeCelles for all of his help,
advisement, and accents, and B. Carrapa, and G. Gehrels for serving on the committee for this
manuscript and providing valuable assistance. Thanks to T.P. Ojha for field support and helping
with many of the ideas in this manuscript.
60
8. References
Ader, T., J. Avouac, J. Liu-Zeng, and H. Lyon-Caen (2012), Convergence rate across the Nepal
Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic
hazard, Journal of Geophysical Research, 117, doi:10.1029/2011JB009071.
Ahmad, T., N. Harris, and M. Bickle (2000), Isotopic constraints on the structural relationships
between the lesser Himalayan series and the high Himalayan crystalline series, Garhwal
Himalaya, Geological Society of America Bulletin, 112(3), 467–477.
Allègre, C. J., and D. B. Othman (1980), Nd–Sr isotopic relationship in granitoid rocks and
continental crust development: a chemical approach to orogenesis, Nature, 286(5771), 335–
342, doi:10.1038/286335a0.
Amatya, K. M., B. M. Jnawali, P. L. Shrestha, and N. D. O. M. A. Geology (1994), Geological
Map of Nepal: Kathmandu, 1994: Scale: 1: 1 000 000,
Argand, E. (1924), La tectonique de l'Asie. Conférence faite á Bruxelles, le 10 août 1922,
Compte-rendu du XIII° Congrès géologique international (XIIIe session)-Belgique 1922,
171–372.
Argles, T. W., C. I. Prince, G. L. Foster, and D. Vance (1999), New garnets for old? Cautionary
tales from young mountain belts, Earth and Planetary Science Letters, 172(3), 301–309.
Avouac, J.-P. (2003), Mountain building, erosion, and the seismic cycle in the Nepal Himalaya,
Advances in Geophysics, 46, 1–80.
Bachmann, O., F. Oberli, M. A. Dungan, M. Meier, R. Mundil, and H. Fischer (2007), 40 Ar/ 39
Ar and U–Pb dating of the Fish Canyon magmatic system, San Juan Volcanic field,
Colorado: Evidence for an extended crystallization history, Chemical Geology, 236(1), 134–
166.
Beaumont, C., R. Jamieson, M. Nguyen, and B. Lee (2001), Himalayan tectonics explained by
extrusion of a low-viscosity crustal channel coupled to focused surface denudation, Nature,
1–0.
Bettinelli, P. et al. (2006), Plate Motion of India and Interseismic Strain in the Nepal Himalaya
from GPS and DORIS Measurements, Journal of Geodesy, 80(8-11), 567–589,
doi:10.1007/s00190-006-0030-3.
Beyssac, O., Beyssac, O., L. Bollinger, L. Bollinger, J.-P. Avouac, J.-P. Avouac, B. Goffé, and
B. Goffé (2004), Thermal metamorphism in the lesser Himalaya of Nepal determined from
Raman spectroscopy of carbonaceous material, Earth and Planetary Science Letters, 225(12), 233–241, doi:10.1016/j.epsl.2004.05.023.
Bilham, R., K. Larson, J. Freymueller, and F. Jouanne (1997), GPS measurements of present-day
convergence across the Nepal Himalaya, Nature.
61
Blondes, M. S., P. W. Reiners, B. R. Edwards, and A. Biscontini (2007), Dating young basalt
eruptions by (U-Th)/He on xenolithic zircons, Geology, 35(1), 17, doi:10.1130/G22956A.1.
Blythe, A., D. Burbank, A. CARTER, and K. Schmidt (2007), Plio‐Quaternary exhumation
history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite [U‐
Th]/He analyses, Tectonics.
Bollinger, L. et al. (2004), Thermal structure and exhumation history of the Lesser Himalaya in
central Nepal, Tectonics, 23(5).
Bollinger, L., P. Henry, and J. P. Avouac (2006), Mountain building in the Nepal Himalaya:
Thermal and kinematic model, Earth and Planetary Science Letters, 244(1), 58–71.
Bowring, S. A., and J. P. Grotzinger (1992), Implications of new chronostratigraphy for tectonic
evolution of Wopmay Orogen, northwest Canadian Shield, American Journal of Science,
292(1), 1–20.
Brewer, I. D., and D. W. Burbank (2006), Thermal and kinematic modeling of bedrock and
detrital cooling ages in the central Himalaya, Journal of Geophysical Research: Solid Earth
(1978–2012), 111(B9), B09409.
Brookfield, M. E. (1993), The Himalayan passive margin from Precambrian to Cretaceous times,
Sedimentary Geology, 84(1–4), 1–35.
Brunel, M. (1986), Ductile thrusting in the Himalayas: shear sense criteria and stretching
lineations, Tectonics, 5(2), 247–265.
Burbank, D., A. Blythe, J. Putkonen, B. Pratt-Sitaula, E. Gabet, M. Oskin, A. Barros, and T.
Ojha (2003), Decoupling of erosion and precipitation in the Himalayas, Nature, 426(6967),
652–655.
Burchfiel, B. C. et al. (1992), The South Tibetan detachment system, Himalayan orogen:
Extension contemporaneous with and parallel to shortening in a collisional mountain belt,
Geological Society of America Special Paper, 269, 51 pp.
Caldwell, W. B., S. L. Klemperer, J. F. Lawrence, and S. S. Rai (2013), Characterizing the Main
Himalayan Thrust in the Garhwal Himalaya, India with receiver function CCP stacking,
Earth and Planetary Science Letters, 367, 15–27.
Catlos, E. J., T. M. Harrison, M. J. Kohn, M. Grove, F. J. Ryerson, C. E. Manning, and B. N.
Upreti (2001), Geochronologic and thermobarometric constraints on the evolution of the
Main Central Thrust, central Nepal Himalaya, Journal of Geophysical Research: Solid Earth
(1978–2012), 106(B8), 16177–16204.
Célérier, J., T. M. Harrison, A. A. G. Webb, and A. Yin (2009), The Kumaun and Garwhal
Lesser Himalaya, India: Part 1. Structure and stratigraphy, Geological Society of America
Bulletin, 121(9-10), 1262–1280.
62
Colchen, M., P. Fort, and A. Pêcher (1980), Carte Geologique Annapurna-Manaslu-Ganesh,
Himalaya du Nepal, Echelle, 1, 200000.
Corrie, S. L., and M. J. Kohn (2011), Metamorphic history of the central Himalaya, Annapurna
region, Nepal, and implications for tectonic models, Geological Society of America Bulletin,
123(9-10), 1863–1879, doi:10.1130/B30376.1.
Davis, D., J. Suppe, and F. Dahlen (1983), Mechanics of fold‐and‐thrust belts and accretionary
wedges, Journal of Geophysical.
DeCelles, P. G., and P. C. DeCelles (2001), Rates of shortening, propagation, underthrusting,
and flexural wave migration in continental orogenic systems, Geology, 29(2), 135–138,
doi:10.1130/0091-7613(2001)029&lt;0135:ROSPUA&gt;2.0.CO;2.
DeCelles, P. G., D. M. Robinson, and G. Zandt (2002), Implications of shortening in the
Himalayan fold‐thrust belt for uplift of the Tibetan Plateau, Tectonics, 21(6), 12–1–12–25.
DeCelles, P. G., D. M. Robinson, J. Quade, T. P. Ojha, C. N. Garzione, P. Copeland, and B. N.
Upreti (2001a), Stratigraphy, structure, and tectonic evolution of the Himalayan fold‐thrust
belt in western Nepal, Tectonics, 20(4), 487–509.
DeCelles, P. G., G. E. Gehrels, J. Quade, B. LaReau, and M. Spurlin (2000), Tectonic
Implications of U-Pb Zircon Ages of the Himalayan Orogenic Belt in Nepal, Science,
288(5465), 497–499, doi:10.1126/science.288.5465.497.
DeCelles, P. G., G. E. Gehrels, J. quade, T. P. Ojha, P. A. Kapp, and B. N. Upreti (1998a),
Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the
Himalayan fold-thrust belt, western Nepal, Geological Society of America Bulletin, 110(1),
2–21.
DeCelles, P. G., G. E. Gehrels, Y. Najman, A. J. Martin, A. CARTER, and E. Garzanti (2004),
Detrital geochronology and geochemistry of Cretaceous–Early Miocene strata of Nepal:
implications for timing and diachroneity of initial Himalayan orogenesis, Earth and
Planetary Science Letters, 227(3), 313–330.
DeCelles, P. G., P. Kapp, G. E. Gehrels, and L. Ding (2014), Paleocene-Eocene foreland basin
evolution in the Himalaya of southern Tibet and Nepal: Implications for the age of initial
India-Asia collision, Tectonics, 33(5), 2014TC003522.
DeCelles, P., D. Robinson, J. Quade, and T. Ojha (2001b), Stratigraphy, structure, and tectonic
evolution of the Himalayan fold-thrust belt in western Nepal, Tectonics, 1–0.
DeCelles, P., G. Gehrels, J. Quade, and T. Ojha (1998b), Eocene-early Miocene foreland basin
development and the history of Himalayan thrusting, western, Tectonics, 17(5), 741–765.
Deeken, A., R. C. Thiede, E. Sobel, J. Hourigan, and M. Strecker (2011), Exhumational
variability within the Himalaya of northwest India, Earth and Planetary Science Letters,
63
305(1), 103–114.
DePaolo, D. J. (1981), Neodymium isotopes in the Colorado Front Range and crust–mantle
evolution in the Proterozoic, Nature, 291(5812), 193–196, doi:10.1038/291193a0.
Dewey, J., Dewey, J., S. Cande, S. Cande, W. Pitman, and W. Pitman (1989), Tectonic evolution
of the India/Eurasia collision zone, Eclogae Geologicae Helvetiae, 82(3), 717–734.
Dhital, M. R., P. B. Thapa, and H. Ando (2002), Geology of the inner Lesser Himalaya between
Kusma and Syangja in western Nepal, Bulletin of the Department of Geology, 9, 1–60.
Ducea, M. N., and J. B. Saleeby (1998), The age and origin of a thick mafic–ultramafic keel
from beneath the Sierra Nevada batholith, Contrib Mineral Petrol, 133(1-2), 169–185,
doi:10.1007/s004100050445.
Elliott, D. (1976), The motion of thrust sheets, Journal of Geophysical Research, 81(5), 949–
963, doi:10.1029/JB081i005p00949.
Farley, K. A., R. A. Wolf, and L. T. Silver (1996), The effects of long alpha-stopping distances
on (U-Th)/He ages, Geochimica et Cosmochimica Acta, 60(21), 4223–4229.
Frank, W., and G. Fuchs (1970), Geological investigations in west Nepal and their significance
for the geology of the Himalayas, Geologische Rundschau.
Gansser, A. (1964), Geology of the Himalayas, Interscience Publishers, 289 pp.
Garzanti, E., A. Baud, and G. Mascle (1987), Sedimentary record of the northward flight of India
and its collision with Eurasia (Ladakh Himalaya, India), Geodinamica Acta, 1(4-5), 297–
312.
Gehrels, G. E. (2000), Introduction to detrital zircon studies of Paleozoic and Triassic strata in
western Nevada and northern California, Geological Society of America Special Papers, 347,
1–17.
Gehrels, G. E., P. G. DeCelles, A. Martin, T. P. Ojha, G. Pinhassi, and B. N. Upreti (2003),
Initiation of the Himalayan orogen as an early Paleozoic thin-skinned thrust belt, GSA
TODAY, 13(9), 4–9.
Gehrels, G. E., V. A. Valencia, and J. Ruiz (2008), Enhanced precision, accuracy, efficiency, and
spatial resolution of U‐Pb ages by laser ablation–multicollector–inductively coupled plasma–
mass spectrometry, Geochemistry Geophysics Geosystems, 9(3).
Gehrels, G. et al. (2011), Detrital zircon geochronology of pre-Tertiary strata in the TibetanHimalayan orogen, Tectonics, 30(5), n/a–n/a, doi:10.1029/2011TC002868.
Gehrels, G., P. DeCelles, T. Ojha, and B. Upreti (2006a), Geologic and U-Th-Pb geochronologic
evidence for early Paleozoic tectonism in the Kathmandu thrust sheet, central Nepal
Himalaya, Geological Society of America Bulletin, 118(1-2), 185–198.
64
Gehrels, G., P. DeCelles, T. Ojha, and B. Upreti (2006b), Geologic and U–Pb geochronologic
evidence for early Paleozoic tectonism in the Dadeldhura thrust sheet, far-west Nepal
Himalaya, Journal of Asian Earth Sciences, 28(4), 385–408.
Gehrels, G., V. Valencia, and A. Pullen (2006c), Detrital zircon geochronology by laser-ablation
multicollector ICPMS at the Arizona Laserchron Center, Paleontological Society Papers, 12,
67.
Godard, V., D. L. Bourlès, F. Spinabella, D. W. Burbank, B. Bookhagen, G. B. Fisher, A.
Moulin, and L. Léanni (2014), Dominance of tectonics over climate in Himalayan
denudation, Geology, 42(3), 243–246.
Green, O. R., M. P. Searle, R. I. Corfield, and R. M. Corfield (2008), Cretaceous-Tertiary
carbonate platform evolution and the age of the India-Asia collision along the Ladakh
Himalaya (Northwest India), The Journal of Geology, 116(4), 331–353.
Grujic, D., I. Coutand, B. Bookhagen, S. Bonnet, A. Blythe, and C. Duncan (2006), Climatic
forcing of erosion, landscape, and tectonics in the Bhutan Himalayas, Geology, 34(10), 801,
doi:10.1130/G22648.1.
Grujic, D., L. S. Hollister, and R. R. Parrish (2002), Himalayan metamorphic sequence as an
orogenic channel: insight from Bhutan, Earth and Planetary Science Letters, 198(1-2), 177–
191, doi:10.1016/S0012-821X(02)00482-X.
Guenthner, W. R., P. W. Reiners, R. A. Ketcham, L. Nasdala, and G. Giester (2013), Helium
diffusion in natural zircon: Radiation damage, anisotropy, and the interpretation of zircon
(U-Th)/He thermochronology, American Journal of Science, 313(3), 145–198.
Hagen, T., 1969, Report on the geological survey of Nepal preliminary reconnaissance: Zürich,
Mémoires de la soc. Helvétique des sci. naturelles, 185.
Hagen, T. (1959), Über den geologischen Bau des Nepal-Himalaya,
Harrison, T. M. et al. (1997), A late Miocene-Pliocene origin for the central Himalayan inverted
metamorphism, Earth and Planetary Science Letters, 146(1), E1–E7.
Harrison, T. M., P. Copeland, W. Kidd, and A. Yin (1992), Raising Tibet, Science, 255(5052),
1663–1670.
Herman, F. et al. (2010), Exhumation, crustal deformation, and thermal structure of the Nepal
Himalaya derived from the inversion of thermochronological and thermobarometric data and
modeling of the topography, Journal of Geophysical Research, 115(B6), B06407,
doi:10.1029/2008JB006126.
Hetényi, G., R. Cattin, J. Vergne, and J. L. Nábělek (2006), The effective elastic thickness of the
India Plate from receiver function imaging, gravity anomalies and thermomechanical
modelling, Geophysical Journal International, 167(3), 1106–1118.
65
Hirn, A., Hirn, A., M. Sapin, and M. Sapin (1984), The Himalayan zone of crustal interaction:
suggestions from explosion seismology,, 2(2), 123–130.
Hirschmiller, J., D. Grujic, B. Bookhagen, I. Coutand, P. Huyghe, J. L. Mugnier, and T. Ojha
(2014), What controls the growth of the Himalayan foreland fold-and-thrust belt? Geology,
42(3), 247–250, doi:10.1130/G35057.1.
Hodges, K. V. (2000), Tectonics of the Himalaya and southern Tibet from two perspectives,
Geological Society of America Bulletin, 112(3), 324–350.
Hodges, K. V., C. Wobus, K. Ruhl, T. Schildgen, and K. Whipple (2004), Quaternary
deformation, river steepening, and heavy precipitation at the front of the Higher Himalayan
ranges, Earth and Planetary Science Letters, 220(3), 379–389.
Hodges, K. V., J. M. Hurtado, and K. X. Whipple (2001), Southward extrusion of Tibetan crust
and
its
effect
on
Himalayan
tectonics,
Tectonics,
20(6),
799–809,
doi:10.1029/2001TC001281.
Hodges, K., R. Parrish, and M. Searle (1996), Tectonic evolution of the central Annapurna range,
Nepalese Himalayas, Tectonics, 15(6), 1264–1291.
Hoffman, P. F. (1980), Wopmay Orogen: a Wilson cycle of Early Proterozoic age in the
northwest of the Canadian Shield, The continental crust and its mineral deposits: Geological
Association of Canada Special Paper, 20, 523–549.
Hoffman, P. F., and S. A. Bowring (1984), Short-lived 1.9 Ga continental margin and its
destruction, Wopmay orogen, northwest Canada, Geology, 12(2), 68–72.
Hourigan, J. K., P. W. Reiners, and M. T. Brandon (2005), U-Th zonation-dependent alphaejection in (U-Th)/He chronometry, Geochimica et Cosmochimica Acta, 69(13), 3349–3365.
House, M. A., K. A. Farley, and D. Stockli (2000), Helium chronometry of apatite and titanite
using Nd-YAG laser heating, Earth and Planetary Science Letters, 183(3), 365–368.
Hu, X., H. D. Sinclair, J. Wang, H. Jiang, and F. Wu (2012), Late Cretaceous‐Palaeogene
stratigraphic and basin evolution in the Zhepure Mountain of southern Tibet: implications for
the timing of India‐Asia initial collision, Basin Research, 24(5), 520–543.
Hubbard, M. S., and T. M. Harrison (1989), 40Ar/39Ar age constraints on deformation and
metamorphism in the Main Central Thrust zone and Tibetan Slab, eastern Nepal Himalaya,
Tectonics, 8(4), 865–880.
Huyghe, P., A. Galy, J. L. Mugnier, and C. France-Lanord (2001), Propagation of the thrust
system and erosion in the Lesser Himalaya: Geochemical and sedimentological evidence,
Geology, 29(11), 1007–1010.
Imayama, T., and K. Arita (2008), Nd isotopic data reveal the material and tectonic nature of the
Main Central Thrust zone in Nepal Himalaya, Tectonophysics, 451(1-4), 265–281,
66
doi:10.1016/j.tecto.2007.11.051.
Johnson, M., G. Oliver, R. Parrish, and S. Johnson (2001), Synthrusting metamorphism, cooling,
and erosion of the Himalayan Kathmandu Complex, Nepal, Tectonics, 20(3), 394–415.
Khanal, S., and D. M. Robinson (2013), Upper crustal shortening and forward modeling of the
Himalayan thrust belt along the Budhi-Gandaki River, central Nepal, International Journal
of Earth Sciences, 102(7), 1871–1891.
Kimura, K. (1999), Diachronous evolution of sub‐Himalayan piggyback basins, Nepal, Island
Arc, 8(1), 99–113.
Kohn, M. J. (2008), P-T-t data from central Nepal support critical taper and repudiate large-scale
channel flow of the Greater Himalayan Sequence, Geological Society of America Bulletin,
120(3-4), 1–15, doi:10.1130/B26252.1.
Kohn, M. J., S. K. Paul, and S. L. Corrie (2010), The lower Lesser Himalayan sequence: A
Paleoproterozoic arc on the northern margin of the Indian plate, Geological Society of
America Bulletin, 122(3-4), 323–335, doi:10.1130/B26587.1.
Le Fort, P. (1975), Himalayas: the collided range. Present knowledge of the continental arc,
American Journal of Science, 275(1), 1–44.
Leech, M. L., S. Singh, A. K. Jain, S. L. Klemperer, and R. M. Manickavasagam (2005), The
onset of India–Asia continental collision: early, steep subduction required by the timing of
UHP metamorphism in the western Himalaya, Earth and Planetary Science Letters, 234(1),
83–97.
Long, S. P. et al. (2012a), Variable shortening rates in the eastern Himalayan thrust belt, Bhutan:
Insights from multiple thermochronologic and geochronologic data sets tied to kinematic
reconstructions, Tectonics, 31(5), doi:10.1029/2012TC003155.
Long, S., N. McQuarrie, and T. Tobgay (2011a), Tectonostratigraphy of the Lesser Himalaya of
Bhutan: Implications for the along-strike stratigraphic continuity of the northern Indian
margin, Society of America, 123(7/8), 1406–1426, doi:10.1130/B30202.
Long, S., N. McQuarrie, T. Tobgay, and D. Grujic (2011b), Geometry and crustal shortening of
the Himalayan fold-thrust belt, eastern and central Bhutan, Geological Society of America
Bulletin, 123(7-8), 1427–1447, doi:10.1130/B30203.1.
Long, S., N. McQuarrie, T. Tobgay, and I. Coutand (2012b), Variable shortening rates in the
eastern Himalayan thrust belt, Bhutan: Insights from multiple thermochronologic and
geochronologic data sets tied to kinematic reconstructions, Tectonics, 31,
doi:10.1029/2012TC003155.
Lyon-Caen, H., and P. Molnar (1985), Gravity anomalies, flexure of the Indian plate, and the
structure, support and evolution of the Himalaya and Ganga basin, Tectonics, 4(6), 513–538,
doi:10.1029/TC004i006p00513.
67
Macfarlane, A. M., K. V. Hodges, and D. Lux (1992), A structural analysis of the Main Central
thrust zone, Langtang National Park, central Nepal Himalaya, Geological Society of America
Bulletin, 104(11), 1389–1402.
Makovsky, Y., S. L. Klemperer, L. Huang, and D. Lu (1996), Structural elements of the southern
Tethyan Himalaya crust from wide‐angle seismic data, Tectonics, 15(5), 997–1005.
Marquer, D., H. S. Chawla, and N. Challandes (2000), Pre-alpine high-grade metamorphism in
High Himalaya crystalline sequences: Evidence from Lower Palaeozoic Kinnaur Kailas
granite and surrounding rocks in the Sutlej Valley (Himachal Pradesch, India), Eclogae
Geologicae Helvetiae, 93(2), 207–220.
Martin, A. J., K. D. Burgy, A. J. Kaufman, and G. E. Gehrels (2011), Stratigraphic and tectonic
implications of field and isotopic constraints on depositional ages of Proterozoic Lesser
Himalayan rocks in central Nepal, Precambrian Research, 185(1-2), 1–17,
doi:10.1016/j.precamres.2010.11.003.
Martin, A. J., P. G. DeCelles, G. E. Gehrels, P. J. Patchett, and C. Isachsen (2005), Isotopic and
structural constraints on the location of the Main Central thrust in the Annapurna Range,
central Nepal Himalaya, Geological Society of America Bulletin, 117(7), 926,
doi:10.1130/B25646.1.
Mattauer, M. (1986), Intracontinental subduction, crust-mantle décollement and crustal-stacking
wedge in the Himalayas and other collision belts, Geological Society, London, Special
Publications, 19(1), 37–50.
McCulloch, M. T., and G. J. Wasserburg (1978), Sm-Nd and Rb-Sr Chronology of Continental
Crust Formation, Science, 200(4345), 1003–1011, doi:10.1126/science.200.4345.1003.
McKenzie, N. R., N. C. Hughes, P. M. Myrow, S. Xiao, and M. Sharma (2011), Correlation of
Precambrian–Cambrian sedimentary successions across northern India and the utility of
isotopic signatures of Himalayan lithotectonic zones, Earth and Planetary Science Letters,
312(3), 471–483, doi:10.1016/j.epsl.2011.10.027.
McLennan, S. M., M. T. McCulloch, S. R. Taylor, and J. B. Maynard (1989), Effects of
sedimentary sorting on neodymium isotopes in deep-sea turbidites, Nature, 337(6207), 547–
549, doi:10.1038/337547a0.
McQuarrie, N., D. Robinson, S. Long, T. Tobgay, D. Grujic, G. Gehrels, and M. Ducea (2008),
Preliminary stratigraphic and structural architecture of Bhutan: Implications for the along
strike architecture of the Himalayan system, Earth and Planetary Science Letters, 272(1-2),
105–117, doi:10.1016/j.epsl.2008.04.030.
McQuarrie, N., T. Tobgay, S. P. Long, P. W. Reiners, and M. A. Cosca (2014), Variable
exhumation rates and variable displacement rates: Documenting recent slowing of
Himalayan shortening in western Bhutan, Earth and Planetary Science Letters, 386, 161–
174.
68
Meigs, A. J., D. W. Burbank, and R. A. Beck (1995), Middle-late Miocene (> 10 Ma) formation
of the Main Boundary thrust in the western Himalaya, Geology, 23(5), 423–426,
doi:10.1130/0091-7613(1995)023&lt;0423:MLMMFO&gt;2.3.CO;2.
Mitra, G., K. Bhattacharyya, and M. Mukul (2010), The lesser Himalayan Duplex in Sikkim:
Implications for variations in Himalayan shortening, Journal of the Geological Society of
India, 75(1), 289–301, doi:10.1007/s12594-010-0016-x.
Murphy, M. A., and A. Yin (2003), Structural evolution and sequence of thrusting in the Tethyan
fold-thrust belt and Indus-Yalu suture zone, southwest Tibet, Geological Society of America
Bulletin, 115(1), 21–34.
Myrow, P. M., N. C. Hughes, T. S. Paulsen, I. S. Williams, S. K. Parcha, K. R. Thompson, S. A.
Bowring, S. C. Peng, and A. D. Ahluwalia (2003), Integrated tectonostratigraphic analysis of
the Himalaya and implications for its tectonic reconstruction, Earth and Planetary Science
Letters, 212(3-4), 433–441, doi:10.1016/S0012-821X(03)00280-2.
Nadin, E. S., and A. J. Martin (2012), Apatite thermochronometry within a knickzone near the
Higher Himalaya front, central Nepal: No resolvable fault motion in the past one million
years, Tectonics, 31(2), doi:10.1029/2011TC003000.
Najman, Y. et al. (2010), Timing of India‐Asia collision: Geological, biostratigraphic, and
palaeomagnetic constraints, Journal of Geophysical Research: Solid Earth (1978–2012),
115(B12).
Nasdala, L., P. W. Reiners, J. I. Garver, A. K. Kennedy, R. A. Stern, E. Balan, and R. Wirth
(2004), Incomplete retention of radiation damage in zircon from Sri Lanka, American
Mineralogist, 89(1), 219–231.
Nábělek, J., G. Hetényi, J. Vergne, S. Sapkota, B. Kafle, M. Jiang, H. Su, J. Chen, and B.-S.
Huang (2009), Underplating in the Himalaya-Tibet collision zone revealed by the HiCLIMB experiment, Science, 325(5946), 1371–1374.
Nelson, K. D., W. Zhao, L. D. Brown, J. Kuo, J. Che, X. Liu, S. L. Klemperer, Y. Makovsky, R.
Meissner, and J. Mechie (1996), Partially molten middle crust beneath southern Tibet:
synthesis of project INDEPTH results, Science, 274(5293), 1684–1688.
Ni, J., and M. Barazangi (1984), Seismotectonics of the Himalayan collision zone: Geometry of
the underthrusting Indian plate beneath the Himalaya, Journal of Geophysical Research:
Solid Earth (1978–2012), 89(B2), 1147–1163.
Ohta, Y., C. Akiba, K. Arita, and Y. Maruo (1973), Pokhra-Gurka region, Geology of the Nepal
Himalayas, 159–188.
Parrish, R., and V. Hodges (1996), Isotopic constraints on the age and provenance of the Lesser
and Greater Himalayan sequences, Nepalese Himalaya, Geological Society of America
Bulletin, 108(7), 904–911.
69
Paudel, L. (2011), K-Ar Dating of White Mica from the Lesser Himalaya, Tansen-Pokhara
Section, Central Nepal: Implications for the Timing of Metamorphism, Nepal Journal of
Science and Technology.
Paudel, L. P., and K. Arita (2006), Thermal evolution of the Lesser Himalaya, central Nepal:
Insights from K-white micas compositional variation, Gondwana Research, 9(4), 409–425.
Paudyal, K., L. Adhikari, and N. Maharjan (2012a), Geological setting and lithostratigraphy of
Bandipur-Gondrang area of Lesser Himalaya, central Nepal, Bulletin of the Department of
Geology, 15, 49–62, doi:10.3126/bdg.v15i0.7417.
Paudyal, K., R. Koirala, and L. Paudel (2012b), The Kahun Crystalline Klippe, Tanahu, central
Nepal: Westward continuation of the Kathmandu Nappe? The 27th Himalaya-KarakoramTibet Workshop (HKT).
Pearson, O. (2002), Structural evolution of the central Nepal fold-thrust belt and regional
tectonic and structural significance of the Ramgarh thrust,
Pearson, O. N., and P. G. DeCelles (2005), Structural geology and regional tectonic significance
of the Ramgarh thrust, Himalayan fold-thrust belt of Nepal, Tectonics, 24(4),
doi:10.1029/2003TC001617.
Pêcher, A. (1977), Geology of the Nepal Himalaya: deformation and petrography in the Main
Central Thrust zone, Ecologie et Géologie de l’Himalaya, 268, 301–318.
Pêcher, A. (1978), Déformations et métamorphisme associés à une zone de cisaillement.
Exemple du grand Chevauchement Central Himalayen (MCT), transversale des Anapurnas
et du Manaslu (Népal),
Powell, C. M., and P. J. Conaghan (1973), Plate tectonics and the Himalayas, Earth and
Planetary Science Letters, 20(1), 1–12.
Rai, S. M., S. Guillot, P. Le Fort, and B. N. Upreti (1998), Pressure–temperature evolution in the
Kathmandu and Gosainkund regions, Central Nepal, Journal of Asian Earth Sciences, 16(2),
283–298.
Ratschbacher, L., W. Frisch, G. Liu, and C. Chen (1994), Distributed deformation in southern
and western Tibet during and after the India‐Asia collision, Journal of Geophysical
Research: Solid Earth (1978–2012), 99(B10), 19917–19945.
Reiners, P. W., and M. T. Brandon (2006), Using thermochronology to understand orogenic
erosion, Annual Review of Earth and Planetary Sciences, 34(1), 1–50,
doi:10.1146/annurev.earth.34.031405.125202.
Reiners, P. W., T. L. Spell, S. Nicolescu, and K. A. Zanetti (2004), Zircon (U-Th)/He
thermochronometry: He diffusion and comparisons with 40 Ar/ 39 Ar dating, Geochimica et
Cosmochimica Acta, 68(8), 1857–1887, doi:10.1016/j.gca.2003.10.021.
70
Renne, P. R., C. C. Swisher, A. L. Deino, D. B. Karner, T. L. Owens, and D. J. DePaolo (1998),
Intercalibration of standards, absolute ages and uncertainties in 40 Ar/ 39 Ar dating,
Chemical Geology, 145(1), 117–152.
Robert, X., P. van der Beek, J. Braun, C. Perry, and J.-L. Mugnier (2011), Control of detachment
geometry on lateral variations in exhumation rates in the Himalaya: Insights from low‐
temperature thermochronology and numerical modeling, Journal of Geophysical Research:
Solid Earth (1978–2012), 116(B5), doi:10.1029/2010JB007893.
Robert, X., P. van der Beek, J. Braun, C. Perry, M. Dubille, and J.-L. Mugnier (2009), Assessing
Quaternary reactivation of the Main Central thrust zone (central Nepal Himalaya): New
thermochronologic data and numerical modeling, Geology, 37(8), 731–734.
Robinson, D. M. (2008), Forward modeling the kinematic sequence of the central Himalayan
thrust belt, western Nepal, Geosphere, 4(5), 785–801, doi:10.1130/GES00163.1.
Robinson, D. M., P. G. DeCelles, and P. Copeland (2006), Tectonic evolution of the Himalayan
thrust belt in western Nepal: Implications for channel flow models, Geological Society of
America Bulletin, 118(7-8), 865–885.
Robinson, D. M., P. G. DeCelles, C. N. Garzione, O. N. Pearson, T. M. Harrison, and E. J.
Catlos (2003), Kinematic model for the Main Central thrust in Nepal, Geology, 31(4), 359–
362.
Robinson, D. M., P. G. DeCelles, P. J. Patchett, and C. N. Garzione (2001), The kinematic
evolution of the Nepalese Himalaya interpreted from Nd isotopes, Earth and Planetary
Science Letters, 192(4), 507–521.
Robinson, D., and N. McQuarrie (2012), Pulsed deformation and variable slip rates within the
central Himalayan thrust belt, Lithosphere, 4(5), 449–464.
Rowley, D. B. (1996), Age of initiation of collision between India and Asia: A review of
stratigraphic data, Earth and Planetary Science Letters, 145(1), 1–13.
Sakai, H. (1983), Geology of the Tansen group of the Lesser Himalaya in Nepal, Mem. Fac. Sci.
Kyushu University, 25, 27–74.
Sakai, H. (1985), Geology of the Kali Gandaki Supergroup of the Lesser Himalayas in Nepal,
Mem. Fac. Sci., Kyushu University, Ser. D, Geol., 25, 337–397.
Sakai, H. (1989), Rifting of the Gondwanaland and uplifting of the Himalayas recorded in
Mesozoic and Tertiary fluvial sediments in the Nepal Himalayas, Sedimentary Facies in the
Active Plate Margin, 723–732.
Sakai, H., H. Iwano, T. Danhara, T. Hirata, and Y. Takigami (2013a), Emplacement of hot
Lesser Himalayan nappes from 15 to 10  Ma in the Jumla-Surkhet region, western Nepal, and
their thermal imprint on the underlying Early Miocene fluvial Dumri Formation, Island Arc,
22(3), 361–381, doi:10.1111/iar.12030.
71
Sakai, H., H. Iwano, T. Danhara, Y. Takigami, S. M. Rai, B. N. Upreti, and T. Hirata (2013b),
Rift‐related origin of the Paleoproterozoic Kuncha Formation, and cooling history of the
Kuncha nappe and Taplejung granites, eastern Nepal Lesser Himalaya: a multichronological
approach, Island Arc, 22(3), 338–360.
Sakai, H., Y. Takigami, Y. Nakamuta, and H. Nomura (1999), Inverted metamorphism in the
Pre-Siwalik foreland basin sediments beneath the crystalline nappe, western Nepal
Himalaya, Journal of Asian Earth Sciences, 17(5), 727–739.
Sastri, V. V., L. L. Bhandari, A. Raju, and A. K. Datta (1971), Tectonic framework and
subsurface stratigraphy of Ganga basin, Journal of the Geological Society of India, 12(3),
222–&.
Schelling, D. (1992), The tectonostratigraphy and structure of the eastern Nepal Himalaya,
Tectonics, 11(5), 925–943.
Schmitz, M. D., and S. A. Bowring (2001), U-Pb zircon and titanite systematics of the Fish
Canyon Tuff: an assessment of high-precision U-Pb geochronology and its application to
young volcanic rocks, Geochimica et Cosmochimica Acta, 65(15), 2571–2587.
Schulte-Pelkum, V., G. Monsalve, A. Sheehan, M. R. Pandey, S. Sapkota, R. Bilham, and F. Wu
(2005), Imaging the Indian subcontinent beneath the Himalaya, Nature, 435(7046), 1222–
1225.
Searle, M. (1996), Cooling history, erosion, exhumation, and kinematics of the HimalayaKarakoram-Tibet orogenic belt, World and Regional Geology, 110–137.
Searle, M. P. (1986), Structural evolution and sequence of thrusting in the High Himalayan,
Tibetan—Tethys and Indus suture zones of Zanskar and Ladakh, Western Himalaya, Journal
of Structural Geology, 8(8), 923–936.
Searle, M. P. (1991), Geology and tectonics of the Karakoram Mountains,, 357.
Searle, M. P., and L. Godin (2003), The south Tibetan detachment and the Manaslu
Leucogranite: A structural reinterpretation and restoration of the Annapurna‐Manaslu
Himalaya, Nepal, The Journal of Geology, 111(5), 505–523.
Searle, M. P., R. D. Law, L. Godin, K. P. Larson, M. J. Streule, J. M. Cottle, and M. J. Jessup
(2008), Defining the Himalayan main central thrust in Nepal, Journal of the Geological
Society, 165(2), 523–534, doi:10.1144/0016-76492007-081.
Shrestha, S. B., J. N. Shrestha, S. R. Sharma, J. M. Tater, and V. K. Neupane (1987a),
Geological map of central Nepal,
Shrestha, S. B., J. N. Shrestha, S. R. Sharma, J. M. Tater, and V. K. Neupane (1987b),
Geological Map of Far Western Nepal 1: 250,000,
Stacey, J. T., and 1. Kramers (1975), Approximation of terrestrial lead isotope evolution by a
72
two-stage model, Earth and Planetary Science Letters, 26(2), 207–221.
Stöcklin, J. (1980), Geology of Nepal and its regional frame Thirty-third William Smith Lecture,
Journal of the Geological Society, 137(1), 1–34.
Stöcklin, J., and K. D. Bhattarai (1977), Geology of the Kathmandu area and central Mahabharat
range, Nepal: Nepal Department of Mines and Geology, Himalayan Report.
Thiede, R. C., B. Bookhagen, J. R. Arrowsmith, E. R. Sobel, and M. R. Strecker (2004), Climatic
control on rapid exhumation along the Southern Himalayan Front, Earth and Planetary
Science Letters, 222(3-4), 791–806, doi:10.1016/j.epsl.2004.03.015.
Upreti, and P. Le Fort (1999), Lesser Himalayan crystalline nappes of Nepal: Problems of their
origin, GSA Special Paper, (328), 225–237.
Upreti, B. N. (1999), An overview of the stratigraphy and tectonics of the Nepal Himalaya,
Journal of Asian Earth Sciences, 17(5-6), 577–606, doi:10.1016/S1367-9120(99)00047-4.
Valdiya, K. S. (1980), The two intracrustal boundary thrusts of the Himalaya, Tectonophysics,
66(4), 323–348.
Vannay, J. C., and K. V. Hodges (1996), Tectonometamorphic evolution of the Himalayan
metamorphic core between the Annapurna and Dhaulagiri, central Nepal, Journal of
Metamorphic Geology, 14(5), 635–656.
Vannay, J., Vannay, J., B. Grasemann, B. Grasemann, M. Rahn, M. Rahn, W. Frank, W. Frank,
A. CARTER, and A. CARTER (2004), Miocene to Holocene exhumation of metamorphic
crustal wedges in the NW Himalaya: Evidence for tectonic extrusion coupled to fluvial
erosion, Tectonics.
Voorde, Ter, M., C. H. De Bruijne, S. A. P. L. Cloetingh, and P. A. M. Andriessen (2004),
Thermal consequences of thrust faulting: simultaneous versus successive fault activation and
exhumation,
Earth
and
Planetary
Science
Letters,
223(3–4),
395–413,
doi:10.1016/j.epsl.2004.04.026.
Wang, J., X. Hu, L. Jansa, and Z. Huang (2011), Provenance of the Upper Cretaceous–Eocene
deep-water sandstones in Sangdanlin, southern Tibet: Constraints on the timing of initial
India-Asia collision, The Journal of Geology, 119(3), 293–309.
Webb, A. A. G., A. K. Schmitt, D. He, and E. L. Weigand (2011), Structural and
geochronological evidence for the leading edge of the Greater Himalayan Crystalline
complex in the central Nepal Himalaya, Earth and Planetary Science Letters, 304(3), 483–
495.
Wobus, C. W., K. X. Whipple, and K. V. Hodges (2006), Neotectonics of the central Nepalese
Himalaya: Constraints from geomorphology, detrital 40Ar/ 39Ar thermochronology, and
thermal modeling, Tectonics, 25(4), n/a–n/a, doi:10.1029/2005TC001935.
73
Wobus, C., A. Heimsath, K. Whipple, and K. Hodges (2005), Active out-of-sequence thrust
faulting in the central Nepalese Himalaya, Nature, 434(7036), 1008–1011.
Wobus, C., M. Pringle, K. Whipple, and K. Hodges (2008), A Late Miocene acceleration of
exhumation in the Himalayan crystalline core, Earth and Planetary Science Letters, 269(12), 1–10, doi:10.1016/j.epsl.2008.02.019.
Yin, A., and T. M. Harrison (2000), Geologic evolution of the Himalayan-Tibetan orogen,
Annual Review of Earth and Planetary Sciences, 28(1), 211–280.
Yin, A., C. S. Dubey, T. K. Kelty, A. Webb, T. M. Harrison, C. Y. Chou, and J. Célérier (2009),
Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology,
geochronology, and tectonic evolution of the Eastern Himalaya, Geological Society of
America Bulletin, B26461. 1.
Zhao, W., K. D. Nelson, J. Che, J. Quo, D. Lu, C. Wu, and X. Liu (n.d.), Deep seismic reflection
evidence for continental underthrusting beneath southern Tibet, Nature, 366(6455), 557–
559, doi:10.1038/366557a0.
74
List of figures
1. Nepal/Himalayas regional map
2. Damauli Area
3. TP & BP area
4. Detrital Zircon U-Pb PDFs
5. Detrital Zircons from Birendranagar
6. Neodymium compilation/results
7. ZHe results –Nepal
8. ZHe results –Nepal and Bhutan, vs. klippen front (a), (b) ages, (c) vs. MCT
9. Overthrusting/Underthrusting model
10. Birendranagar Kuncha/Fagfog depo contact (a,b,c,d)
11. Map: Gd window (a,b)
12. Map: Seti river alternatives (a,b)
13. Map: klippe qtzite (a,b)
14. Cross-section, balanced and restored (a,b)
List of tables:
1. Stratigraphic nomenclature
2. List of samples presented
S1. Zircon U-Pb datatable
S2. Zircon (U-Th)/He datatable
75
Figure captions
Figure 1. Geologic map of Nepal modified from Martin et al., [2005].
Labeled points indicate locations of DZ samples not in field area of Figures 3 and 4.
Thick line through field area box indicates location of balanced cross-section (Figure 14).
MFT= Main Frontal Thrust, MBT = Main Boundary Thrust, RT = Ramgarh Thrust
MCT = Main Central Thrust, STDS= South Tibetan Detachment System.
Figure 2. Geologic map of Damauli area based on field work conducted December 2012 and
January 2013. Some contacts inferred with assistance from maps from Nepal Department of
Mines and Geology and Paudyal et al. [2012]. Towns labeled with single uppercase letters. P=
Pokhara, K = Khairenitar, D = Damauli, B=Bandipur. Includes structural data from Bollinger et
al. 2004. DTM basemap from GeoMapApp, www.geomapapp.org.
Figure 3. Geologic map of Tansen-Pokhara road and Tansen-Mityal area. Some contacts inferred
with assistance from maps from Sakai [1983].
Figure 4. Probability distribution functions of detrital zircon ages. (n=X) refers to number of
grains contributing ages to curve. 1: compilation curves from Gehrels et al., [2011]. 2: data from
sample 402092A from Martin et al., [2005]. All other data from this study. Samples are colored
according to formation. Updated LLHS compilation curve includes data from Gehrels et al.,
[2011] and this study.
76
Figure 5. Probability distribution functions of detrital zircon ages from Birendranagar area.
(n=X) refers to number of grains contributing ages to curve. Samples colored according to
formation.
Figure 6. Histogram of Himalayan Neodymium isotopic analyses from literature and this study.
See supplementary information for data sources. Red boxes refer to data from the unit exposed
along the Seti River containing black slates and phyllites which grades upwards to graphitic calcschists.
Figure 7. Zircon (U-Th)/He results from Nepal. Ages are plotted versus arc-normal distance from
the MCT. Error bars are two standard deviations of weighted means (see section 4.3). The ages
increase for each transect away from the MCT with the exception of the data from mid-western
Nepal, all of which is from in front of the Karnali klippe. The furthest datapoint from westcentral Nepal is located in front of the Jajarkot klippe.
Figure 8. Zircon (U-Th)/He results from Nepal (this study) and Bhutan [Long et al., 2012;
McQuarrie et al., 2014]. (a) Weighted mean ages plotted versus arc-normal distance from the
front of the GHS sheet. Ages increase until just before the front of the sheet, after which they
decrease. (b) Maximum ZHe ages of GHS sheet plotted versus longitude. Dadeldhura, Karnali,
Jajarkot, and Damauli klippen data from this study; additional data from northwest India [Deeken
et al., 2011], the Kathmandu klippe [Herman et al., 2010], and western Bhutan [McQuarrie et
al., 2014]. Error bar on NW India data point are equal to 10% of the age, as reported in Deeken
et al. [2011]. Errror bars from data points from this study are equal to one standard deviation of
77
the weighted mean (see section 4.3). The datum from the Kathmandu klippe is a weighted mean
of ages from 5 samples collected in a vertial profile, with standard deviation of weighted mean
smaller than the symbol. The age shown from western Bhutan [McQuarrie et al., 2014] is the
average of two weighted means from one anomalously high sample and the nearest other sample,
with error bars showing the range between those two samples plus reported uncertainties from
each of the two samples. Data points are colored according to elevation color bar shown.
Figure 9. Overthrusting/underthrusting model of Himalayan wedge development, modified from
Avouac, [2003]. In this model, the topographic surface stays fixed and movement of the
overriding plate over the Main Himalayan Thrust (MHT) is equal to erosion. Stars are location
markers and arrows indicate motion. In (a) and (b), the Indian foreland lithosphere stays fixed
and convergence is accommodated by fluxing Asian lithosphere of the overriding plate over the
MHT and through the topographic surface. In (c) and (d), the Asian lithosphere stays fixed, and
the Indian foreland lithosphere fluxes beneath the overriding wedge. In this model, the plates are
still converging, but none of this convergence results in exhumation of the overriding wedge.
Figure 10. Example from mid-western Nepal of Fagfog Formation sitting in depositional contact
with Ranimata Formation. (a) Photograph of depositional contact. (b) Quartzite has thickness of
(c) Fagfog Formation (yellow) sitting in depositional contact on Ranimata Formation (green),
with uneven, channelized base (d) closeup of channel deposit. Tops indicators not shown in
photo but present in outcrop.
78
Figure 11. Two alternatives for the identity of the coal-bearing unit near the town of Bandipur.
(a) The coal-bearing unit belongs to the Permian-to-Cretaceous Gondwanan Unit and is exposed
in a tectonic window. (b) The coal-bearing unit belongs to the Proterozoic Syangja Formation.
Legend to come.
Figure 12. Two alternatives for the identities of the units on either side of the Seti River. (a) The
organic-rich black slate and phyllite along the Seti River belong to the GHS and is thrust over
green phyllites of the Kuncha Formation in the hanging wall of the Ramgarh Thrust. (b) The
organic-rich black slate and phyllite along the Seti River belong to the Galyang Formation, and
the cliff-forming carbonate at its base is the Baitadi Carbonate. The preferred interpretation is
presented in the map of Figure 2: the organic-rich black slate and phyllite belong to the Benighat
Formation, and the green phyllite belongs to the Syangja Formation.
Figure 13. Two alternatives for the identity of the uppermost unit in the section, a mylonitized
quartzite. (a) The quartzite belongs to the Gondwanan Unit and is sitting as part of a straight
section above the Malekhu Formation (uppermost Lakharpata Group). (b) The quartzite belongs
to the Greater Himalayan Sequence and is an isolated erosional remnant of the MCT sheet, the
lateral equivalent to the Kathmandu klippe.
Figure 14. Line-length balanced cross-section of Damauli area (Figure 2). (a) Current state of the
thrust belt. (b) Restored state. MFT= Main Frontal Thrust, MBT= Main Boundary Thrust. RT =
Ramgarh Thrust. MCT = Main Central Thrust. Horses are labeled with letters on cross-section
and retrodeformed section. Notes:
79
1) Structure of frontal Siwaliks Group groups incorporates mapping ~20 km to the east by
Nepal Source and Seal Study (DMG/Petro-Canada International assistance corporation
collaboration, 1993), which included a cross-section based on seismic data and field
mapping. This reconstruction uses elements from their cross-section along with
accompanying geologic map across this cross-section line, though precise dip data were
unavailable
2) Depth of basal thrust of ~5-6 km from seismic data [DMG/Petro-Canada International,
1993] matches data from the Raxual deep well [Sastri et al., 1971] and gravity studies
[Hetényi et al., 2006].
3) Dips and units from [Kimura, 1999].
4) MBT extrapolated from mapping along Chitwan-Muglin road to the east [Figure 2], with
assistance from geologic map by DMG/Petro-Canada International [1993] and Google
Earth
5) Lakharpata Group and Gondwanan Unit with Syangja Formation above in thrust contact
outcrop in the hanging wall of the MBT to the east along Chitwan-Muglin Road [Figure
2] and to the west [Figure 3; Sakai, 1985].
6) Ramp located at distance required by minimum shortening in Siwaliks Group frontal
thrusts.
7) Total thickness of Kuncha Formation is not known, and its basal contact has not been
observed.
8) Hanging wall ramp of Syangja Formation included to create local anticline above.
9) Horses inferred to fill space underneath the Kuncha Formation, which sits well above its
regional elevation. The horses to not breach the surface beneath the anticlinorium but
using horses made of the entire Nawakot Group minimizes shortening relative to horses
similar to the most forelandward LHS horse containing just Lakharpata Group and
Gondwanan Unit.
10) Metamorphism and deformation mask the identity of units in footwall of RT. Khanal and
Robinson [2013] to the east map the full Lesser Himalayan Sequence. Studies of central
Nepal have variably mapped LLHS and ULHS [Martin et al., 2005; Nadin and Martin,
2013]. We favor presence of the full LHS in the high mountains (see section 6.2).
11) Dips and location of RT and MCT from Martin et al. [2005].
80
Lesser Himalayan Sequence
~1300 Ma1
1481 Ma2
Meso-proterozoic (?)
Paleo-proterozoic
1750 Ma3
1772 Ma3
1880 Ma3
Formations from Upreti,
[1999]
Siwaliks
Dumri
Gondwanas
Formations from
Tater et al., [1983]
Siwaliks?
Suntar
Not Present
DeCelles et al.,
[2001] formations
Siwaliks
Dumri
Gondwanan Unit
(not present)
Robang
(not present)
(not present)
Malekhu
Benighat
Dhading
Syangja
Malekhu
Benighat
Dhading
Nourpul
Lakharpata
Lakharpata group
Syangja
Syangja
Galyang
Dandagon
Galyang
Galyang
Fagfog
Fagfog
Sangram
Kuncha
(including metagranite and
meta-diorite)
Kuncha
(including
meta-granite
and Ulleri
meta-granite)
Naudanda
Kushma
Ranimatta and
Ulleri
Tansen
Unit
Miocene-Quaternary
Miocene
CarboniferousPaleocene
Formations this
study
Siwaliks
Dumri
Gondwanan Unit
Upper Nawakot
Group
LHS units
Max Depo Age
Lower Nawakot
Group
Depositional Age
Ranimata
(including Ulleri
Gneiss)
Kushma
Table 1. Stratigraphic nomenclature for Lesser Himalayan Sequence Units and maximum depositional ages. 1: Age based on carbon isotopes [Martin et al., 2011]. 2: Age based on detrital zircons from Lakharpata Group, undivided [Gehrels et
al., 2011]. 3: Age based on detrital zircons, this study
81
Sample Unit Lithology 1.18.13.A KT-­‐7 Galyang Fm. Galyang Fm. Central Nepal samples olive and gray slate olive slate KT-­‐11 KT-­‐12 GHS? Kuncha Fm.? lineated micaceous, lithic-­‐rich quartzite silvery mica schist KT-­‐13 Raduwa Fm.? mica calc-­‐schist KT-­‐14A Syangja Fm. green and pink quartzite KT-­‐16 Syangja Fm.? MG-­‐1 MG-­‐2 Kuncha Fm. Fagfog Fm. white and green quartzite interbedded with green phyllite with quartz sweats gray micaceous quartzite rippled white and tan quartzite MG-­‐3 Syangja Fm. MG-­‐6 MG-­‐8 Zircon (U-­‐Th)/He weighted mean age U-­‐Pb youngest peak age or crystallization age (*) ɛNd (0) Station Latitude (°N) Longitude (°E) Elevation (meters above sea level) -­‐21.3 -­‐20.18 475 431 27.950190 27.880090 83.685200 83.936410 784 356 -­‐20.4 437 441 27.919660 27.857280 84.180490 84.303700 942 442 27.858260 84.294880 838 1735 Ma 489 28.005850 84.087210 471 1646 Ma 513 27.884360 84.137090 570 1865 Ma 1772 Ma 209 211 27.865450 27.886767 84.547500 84.539433 242 306 5.5 +/-­‐ 0.1 Ma 1741 Ma 242 27.940691 84.420116 1042 Benighat Fm.? placer deposit within white and pink lithic-­‐rich sandstone quartzose gray phyllite 259 27.917333 84.305467 451 Syangja Fm. gray, micaceous, lithic-­‐rich sandstone 261 27.977433 84.372500 578 MG-­‐10 Benighat Fm.? crenulated, gray phyllite -­‐23.63 275 27.947283 84.096624 775 MG-­‐11 Benighat Fm.? gray and green phyllite with quartzose layers -­‐19.27 277 27.925200 84.096367 886 MG-­‐12 Kuncha Fm. gray and green quartzose schist 279 27.919000 84.549417 311 MG-­‐13 Kuncha Fm. green quartzose schist 281 27.933500 84.556900 347 MG-­‐14 Kuncha Fm. gray and green quartzose schist 4.7 +/-­‐ 0.4 Ma 1864 Ma 282 27.943517 84.555083 347 MG-­‐16 Ulleri Gneiss foliated biotite and muscovite-­‐bearing granitoid 3.1 +/-­‐ 0.4 Ma 288 28.074467 84.541333 1185 MG-­‐17 Kuncha Fm. green mica schist 1877 +2.7 -­‐4.3 Ma* 1860 Ma 292 28.057950 84.523683 1214 MG-­‐18 Kuncha Fm. gray and green quartzose schist 3.1 +/-­‐ 0.4 Ma 1881 Ma 294 28.023467 84.458800 430 MG-­‐20 Syangja Fm. rippled red and purple variegated quartzite 5.9 +/-­‐ 2.6 Ma 1734 Ma 302 27.821450 84.471667 218 MG-­‐21 Benighat Fm.? dark gray mica schist -­‐24.35 449 27.835590 84.452990 223 MG-­‐22 Syangja Fm.? silvery gray chloritic phyllite with quartz sweats -­‐23.78 453 27.863995 84.395635 621 MG-­‐23 Syangja Fm. 454 27.870410 84.392980 743 MG-­‐24 Syangja Fm.? coarse white lithic-­‐rich sandstone interbedded with pink and green micaceous sandstone green chloritic phyllite 455 27.868580 84.393950 710 MG-­‐28 Ulleri Gneiss mylonitized granitoid 465 28.130100 83.971890 922 MG-­‐29 Fagfog Fm. coarse tan and white quartzite 471 28.114970 83.913770 1045 MG-­‐30a Galyang Fm. dark gray compisitionally banded phyllite -­‐23.28 472 28.104540 83.889640 875 6.9 +/-­‐ 0.6 Ma 826 Ma 1767 Ma 1878 Ma 1732 Ma 1893 +7.3 -­‐1.7 Ma* 1792 Ma 82
MK-­‐2 Ulleri Gneiss myloinitized augen gneiss Other Samples 499 28.060260 84.237720 367 DR-­‐7 GHS Igneous mylonitic granitoid 12.3 +/-­‐ 0.6 Ma 315 29.148350 80.592033 1865 DR-­‐8 mylonitic biotite-­‐bearing augen gneiss 14.3 +/-­‐ 0.7 316 29.182450 80.608983 2042 DR-­‐9 Budiganga Gneiss GHS Igneous mylonitic granitoid 14.0 +/-­‐ 1.5 Ma 317 29.215483 80.623083 2076 DR-­‐11 Kalitar Fm. biotite muscovite schist 11.6 +/-­‐ 1.7 Ma 319 29.264667 80.972483 1085 DR-­‐12 mica schist 10.1 +/-­‐ 0.8 Ma 320 29.250700 81.219883 616 SK2 Budiganga Gneiss? GHS 10.3 +/-­‐ 0.3 Ma 2075 28.199350 83.054733 SK3 GHS 10.5 +/-­‐ 0.5 Ma 2146 28.248803 82.796283 KG1 Syangja Fm. 8.3 +/-­‐ 2.7 Ma 543 27.951000 83.455267 BG1 Syangja Fm. 4.9 +/-­‐ 0.7 Ma 1491 28.228817 83.481667 DL9 "Kushma" Fm. 14.2 +/-­‐ 1.4 Ma 1863 Ma 62 ('06) 28.874450 81.736483 DL12B Ulleri Gneiss 12.3 +/-­‐ 0.7 Ma 76 ('06) 28.76703 81.67182 12_30_06_2 Ranimata Fm. 1886 +7.2 -­‐8.5 Ma* 1903 Ma 39 ('06) 28.649933 81.628700 DL2 "Kushma" Fm. 1890 Ma 47 ('06) 28.706070 81.628980 PK2 Syangja Fm. 1754 +/-­‐ 8 Ma* 381 ('07) 28.091700 83.860270 SL9 "Kushma" Fm. 1828 Ma 404 ('08) 28.618660 82.453730 PK1 Fagfog Fm. 1872 Ma 370 ('07) 28.138980 83.861650 BH2 Syangja Fm. SyangjaDZ1 Syangja Fm. SL-­‐15 Gondwanan Unit volcanic deposit quartzite 11.1 +/-­‐ 1.2 Ma 1874 Ma 86 ('07) 28.449080 81.942020 1887 Ma 22 ('06) 28.046700 82.783690 732 Ma 462 ('08) 28.333680 82.191840 Table 2. List of samples with new data presented in this study.
83
Figure 1
80˚ E
A
30˚ N
84˚ E
Kushma
88˚ E
Miocene Leucogranite
N
Tethyan Himalayan Rocks
RT
RT
M
CT
SL9
Greater Himalayan Rocks
Lesser Himalayan Rocks
Sub-Himalayan Rocks
Figure 3
STD
S
TIBET
Budar
DL9
DL12B
12_30_06_2
26˚ N
80˚ E
MCT
MBT
INDIA
Figure 4
0
100
200
kilometers
84˚ E
MFT
INDIA
ARDZ5
26˚ N
88˚ E
28
30
46
34
10
57
pCf
75
r
ive
75 70
pCb
47
-23.6
MG-10
42
23
-19.3
MCT
33
MG-11
20
23
34
GHSq
0 km
15
5 km
10 km
A
84° 10’ E
28
Bedding orientation with dip value
Foliation orientation with dip value
Thrust fault. Dashed
where inferred
Contact. Dashed
where inferred
25
60
40
60
pCkn
53
MG-8
MG-14
15
20
27
pCd
20 40
45
58
25
86
Se
ti R
40
45
37
30
20
10 B
11
41
22 30
20
20
45
30
MG-3
55
60
Ma
Riv rsyan
er
gd
i
MG-12
pCf
63
10
25
30
71 51
20
pCg
74
45
10
36
10
70
50
pCsy
42
MG-23
35
MG-22
GHSb 39 32 KT-12
40
25 MG-1
-23.8 43
45
-20.4
75
35
pCd
pCb
pCkn
MG-21 32
76
32
-24.4
43
Not Mapped
MG-20
28 45
Trishuli River 30 To Kathmandu
55
(65 km)
60
B
MG-16
GHSr
ive 83
r
MBT
Ns
84° 20’ E
GHSb Bhainsedoban Fm.
GHSr
Raduwa Fm.
84° 30’ E
Lower LHS
Upper LHS
GHS
Siwalik Group (Neogene) GHSq GHS Quartzite
River
pCul
MG-18
pCsy
Subhimalayan Zone
Ns
37 5
38
64
30
60 30
65
25
65
KT-11 Detrital Zircon sample
-19.3 Nd analysis and εNd(0) value
MG-11
KT-11 Zircon (U-Th)/He sample
39
25
20
pCkn
Kali Gandaki
27° 50’ N
60
10
RT
35
50
55
KT-16
Not Mapped
55 0
45
pCb
KT-11
25
55 52
D
Seti River
MG-16
pCg
45 75
30
15
25
15
60
di
ti R
Se
28° N
KT-14
22
17 33
pCf
MG-3
43 43
pCd
42
15
33
pCsy
40
25
15
pCul
69
85
pCg
K
a ng
rsy
Ma r
e
Riv
pCkn
25
22
A’
40
Figure 2
d
Ma
28° 8’ N
32
ver
i Ri
50
34
23
PKg
pCl
pCb
pCf
Fagfog Fm.
pCsy Syangja Fm.
pCul
Ulleri gneiss
Galyang Fm.
pCkn
Kuncha Fm.
Gondwanan Unit pCd
Lakharpata Gr.
(undiff )
Benighat Fm.
pCg
Dhading Fm.
Figure 3
28° 10’ N
49
14 88
PK1
MG-29
25
65
65
58
20
20
Not Mapped
30
MG-28
44
85 32
44
60
30
35
45
PK2
pCsy
20
28
25
29
27
pCsy
20
Tansen
20
19
Td
Tb
Kali Gandaki
?
17
5437 46
31
65
64
37
45
75
48
70 60
40 62
53
28
Foliation orientation with dip value
Thrust fault. Dashed
where inferred
Contact. Dashed
where inferred
45
pCsy
Siwalik Group
River
22
82
50
Td
45
47
Mityal
83° 55’ E
Tansen Unit
Subhimalayan Zone
Ns
25 15
27° 50’ N
61
83
30
KT-7
57
78
83° 45’ E
Bedding orientation with dip value
-20.2
Rampur
45
55
39
MBT
39
10 km
10 18
34
47 3670
pCl
83° 36’ E
5 km
KT-11 Detrital Zircon sample
-19.3 Nd analysis and εNd(0) value
MG-11
Ramdi
fault
33
26 46
55
153546
55
PKg
79
31
0 km
32 3660
Tb
36
28° N
25
28 44
pCsy
43 30
60
90
19
15
pCsy
55
Not Mapped
Upper LHS
Lakharpata Gr.
(undiff )
Lower LHS
pCf
Fagfog Fm.
pCsy Syangja Fm.
pCul
Ulleri gneiss
Galyang Fm.
pCkn
Kuncha Fm.
Td
Dumri Fm.
pCl
Tb
Bhainskati Fm.
PKg
Gondwanan Unit
pCg
Figure 4
(n=91)
KT-11 (Damauli Klippe Qtzite)
(n=63)
402092A (GHS Fm. 1)2
(n=83)
SLY-15 (Gondwana Unit)
Peak height reduced by 1.6x
KT-16 (Syangja Fm.?)
(n=72)
Peak height reduced by 1.6x
MG-8 (Syangja Fm.)
(n=97)
Peak height reduced by 1.6x
KT-14A (Syangja Fm.)
(n=83)
Peak height reduced by 1.6x
MG-23 (Syangja Fm.)
(n=94)
Peak height reduced by 1.6x
MG-3 (Syangja Fm.)
(n=84)
Peak height reduced by 1.6x
MG-20 (Syangja Fm.)
Relative probability
(n=97)
(n=79)
MG-2 (Fagfog Fm.)
(n=67)
MG-29 (Fagfog Fm.)
(n=92)
MG-1 (Kuncha Fm.)
(n=91)
MG-12 (Kuncha Fm.)
(n=94)
MG-14 (Kuncha Fm.)
(n=87)
MG-18 (Kuncha Fm.)
(n=91)
MG-17 (Kuncha Fm.)
(n=3895)
THS + ULHS Compilation1
(n=1400)
GHS Compilation1
(n=3397)
Updated LLHS Compilation
(n=1666)
0
200
LLHS Compilation1
400
600
800
1000
1200 1400 1600 1800 2000 2200 2400 2600
Age (Ma)
2800 3000 3200
3400 3600
Figure 5
Relative probability
(n=98)
(n=98)
DL-9 (Fagfog Fm.)
DL-12B (Ulleri Gneiss)
12_30_06_2 (Ranimata Fm.)
(n=91)
1500 1700 1900 2100 2300 2500 2700 2900
Age (Ma)
Figure 6
35
30
20
15
10
5
0
-30
-28
-26
-24
-22
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
Number
25
εNd (0 Ma)
Organic-rich Seti River Unit
GHS
THS
ULHS
LLHS
Figure 7
15
Far-western Nepal
ZHe age (Ma)
12
Dadheldhura Road transect
9
y = 0.1152x + 3.6774
R² = 0.6002
8.7 mm/y
6
3
0
15
ZHe age (Ma)
12
20
40
60
80
100
Arc-normal distance from MCT (km)
Mid-western Nepal
Dailekh - Birendranagar
9
y = -0.0606x + 18.574
R² = 0.6643
-16.5 mm/y
6
3
0
20
40
60
80
100
Arc-normal distance from MCT (km)
15
West-central Nepal
ZHe age (Ma)
12
9
6
Baglung and Kali Gandaki
y = 0.147x + 1.3914
R² = 0.5372
6.8 mm/y
3
0
20
40
60
80
100
Arc-normal distance from MCT (km)
15
ZHe age (Ma)
12
Central Nepal
Damauli Klippe to Gorkha
9
y = 0.1126x + 1.715
R² = 0.9396
8.9 mm/y
6
3
0
20
40
60
80
Arc-normal distance from MCT (km)
100
Figure 8
A
ZHe age vs. distance from frontal MCT (Klippen front)
16
14
ZHe age (Ma)
12
10
Central Nepal
8
Western Nepal
West-central Nepal
6
Mid-western Nepal
4
Far eastern Bhutan
Eastern Bhutan (Trashigang)
2
-60
-40
Elevation (km)
Damauli
6
4
C
0
74
1
2
76
3
78
4
Bhutan
Sikkim, India
West Bhutan3
8
Nepal
10
Kathmandu2
NW India1
12
Dadeldhura
14
0
60
India
Nepal
16
2
40
Maximum GHS ZHe ages versus longitude
18
Max ZHe age (Ma)
-20
0
20
Distance from GHS front (km)
Jajarkot
B
-80
Karnali
0
Western Bhutan
5
80
82
Longitude (Degrees)
84
86
88
90
ZHe age vs. distance from high mountains MCT
16
14
ZHe age (Ma)
12
10
8
6
Nepal
4
Bhutan3,4
2
0
0
20
40
60
80
Distance from MCT in high mountains (km)
100
120
Figure 9
(A) -- 100% overthrusting, T1
Fixed topographic surface
MHT
Fixed topographic surface
(B) -- 100% overthrusting, T2
MHT
Fixed topographic surface
(C) -- 100% underthrusting T1
MHT
Fixed topographic surface
(D) -- 100% underthrusting T2
MHT
Figure 10
N
A
S
S
B
N
S
N
D
dd
ing
C
N
S
g
din
ed
x. b
o
r
p
.
rox
p
p
ap
pr
ox
.
be
ap
dd
ing
pr
ox
.
be
ap
a
g
din
d
be
Figure 11
A
MG-18
43 43
75 70
40
45 75
pCd
47
55 0
45
pCb
2034
23
50KT-11
34
KT-16
Gandaki
55 52
53
64
D
Seti River
3
50
60 60
er
Riv 60
di55
a
20
M
45
40
40
28
25
10
25
30
20
GHSq
39
10
15
57 65
45
MG-8
pCs
30
60 30
65
15
27
37
30
20
10 B
58
25
11
41
86
Se
ti R
63
pCs
KT-16
MG-18
25
38
40
55 52
53
64
D
55 0
45
50
KT-11
60
55
25
10
60
40
65
15
20
27
58
25
20 40
45
86
Se
ti R
pCkn
37
30
20
10 B
11
41
i
63
pCs
36
MG-21 32
55
GHSq GHS Quartzite
PKg
GHSb Bhainsedoban Fm.
pCl
Raduwa Fm.
MG-20
28 45
Trishuli River
pCb
pCf
Fagfog Fm.
pCsy Syangja Fm.
pCul
Ulleri gneiss
Galyang Fm.
pCkn
Kuncha Fm.
Gondwanan Unit pCd
Lakharpata Gr.
(undiff )
Benighat Fm.
Syangja Fm. or Kuncha Fm. DZ sample
30
Lower LHS
Upper LHS
GHS
GHSr
32
pCg
Dhading Fm.
10
25
30
To Kathmandu
(65 km)
45
35
25
-24.4
Not Mapped
60
Ma
Riv rsyan
er
gd
MG-3
55
30
22 30
71 51
20
20
74
ive 83
10 20
r
10
70
50
42
MG-23
MG-22
32 KT-12
40
-23.8
-20.4
43
75
35
pCb
39
45
15
60
25
45
MG-8
30
60 30
65
20
GHSq
Gandaki
di
47
33
34
MG-16
76
75 70
Seti River
20
23
43
37 5
30
60
MG-1
45
33
60
MG-3
45 75
pCb
ang
rsy
Ma r
e
Riv
Cd
15
32
MG-20
28 45
Trishuli River
55
B
43 43
pCg
To Kathmandu
(65 km)
45
36
35
25
-24.4
Not Mapped
10
25
30
i
MG-21 32
69
85
60
Ma
Riv rsyan
er
gd
MG-3
55
PKg
30
22 30
71 51
20
20
74
ive 83
10 20
r
10
70
50
42
MG-23
MG-22
32 KT-12
40
-23.8
-20.4
43
75
35
pCb
45
pCkn
20
40
75
B
pCs
25
38
MG-3
43
MG-1
45
76
Figure 12
r
MG-18
40
32
15
15
42
22
23
25
Seti River
-23.6
15
55 52
47
MG-10
55 0
45
GHSu
23
33
-19.3
34
20
23
MG-11
50
KT-11
34
25
KT-16
35
46
34
Kali Gandaki
25
53
64
MG-8
30
60 30
65
45
15
20
30
20
37
ti R
Se
Ma
Riv rsyan
27
RT
er 50
v
er
di
i
60 60
10
MG-3
di R
MCT
58
60
Ma 55
55
20 40
40
11
25
45
30
28
41
30
40
86
22
71 51
25
Se
30
20
ti R
20
74
25
ive 83
10 20
r
20
10
70
50
42
MG-23
10
MG-22
40
39 32
-20.4 GHSu
57
-23.8 43
15
75
35
65
MG-21 32
75
32
-24.4
Not Mapped
MG-20
28 45
69
Trishuli River
55
85
33
ive
r
Not Mapped
25
38
MG-3
42
KT-14
22
33
17
25 30
B
60
33
ive
A
ti R
Se
69
85
32
40
42
?
42
22
Seti River
-23.6
15
23
55 52
47
MG-10
-19.3
15
3060
38
15
KT-14
22
17 33
25 30
15
35
MG-18
60
B
MG-3
36
Baitadi
33
MG-11
20
23
34
KT-11
35
Kali Gandaki
40
65
15
27
39
30
20
10
58
25
20 40
45
25
45
20
60
20
15
MG-8
30
60 30
65
60
60
25
KT-16
RT
50
55
MCT
25
55 0
45
53
64
11
41
37
Ma
Riv rsyan
er
di
MG-3
55
30
22 30
71 51
20
20
74
ive 83
10 20
r
10
70
50
42
Baitadi
MG-23
MG-22
32
40
-20.4
-23.8 43
75
35
86
Se
ti R
36
35
MG-21 32
-24.4
Not Mapped
55
GHS
Not Mapped
Nd analysis and εNd(0) value
GHSq GHS Quartzite
B Upper LHS
GHSb Bhainsedoban Fm.
pCd
Detrital zircon sample
MCT= Main Central Thrust
RT= Ramgarh Thrust
GHSr
Raduwa Fm.
GHSu GHS Undivided
32
MG-20
28 45
Trishuli River
60
Lower LHS
pCf
Fagfog Fm.
pCsy Syangja Fm.
pCul
Ulleri gneiss
Galyang Fm.
with Baitadi
carbonates
pCkn
Kuncha Fm.
pCg
Dhading Fm.
30
25
Figure 13
69
85
ti R
Se
r
32
40
pCd
pCb
55 52
47
Seti River
15
15
55 0
45
34
20
23
23
50KT-11
34
25
KT-16
35
46
34
pCk
53
64
45
MG-8
30
60 30
65
15
37
30
20
20
Kali Gandaki
25
pCm
75
32
Not Mapped
ti R
Se
69
85
r
32
40
42
KT-14
22
33
17
25 30
pCg
pCd
pCb
22
55 52
47
Seti River
20
23
23
34
25
KT-16
35
55 0
45
pCk
33
42
35
25
15
374
3060
38
pCs
15
15
15
MG-3
4
36
MG-18
60
B
63
32
MG-20
28 45
Trishuli River
33
55
ive
NotBMapped
6
Ma
Riv rsyan
27
r 50
e
pCd
er
di
v
i
60 60
10
MG-3
di R
58
60
Ma 55
55
20 40
11
40
25
45
30
41
28
40
86
22 30
71 51
25
pCb Seti
20
30
pCs
20
Riv 83
74
25
10 20
er
20
10
70
50
42
PKg
MG-23
10
32
40
39
57
15
43
75
35
65
33
23
42
22
MG-18
38
MG-3
pCs
15
KT-14
22
17 33
25 30
25
25
33
pCg
42
37
60
ive
A
15
75
50
KT-11
60
55
MCT
RT
25
60
40
GHSq
65
Kali Gandaki
15
15
58
25
pCb
86
Se
ti R
37
30
20
27
60
39
45
20
20 40
45
25
20
MG-8
30
60 30
65
pCk
53
64
10
11
41
71 51
20
74
ive 83
10
r
10
70
50
42
32
35
22 30
20
20
30
Ma
Riv rsyan
er
di
MG-3
55
6
pCs
36
MG-23
40
43
75
4
35
25
MG-21 32
Not Mapped
Not MappedMCT=
Main Central Thrust
RT= Ramgarh Thrust
Detrital zircon sample
Upper LHS
GHS
GHSq GHS Quartzite
PKg
GHSb Bhainsedoban Fm.
pCl
GHSr
Raduwa Fm.
55
pCb
30
Lower LHS
pCf
Fagfog Fm.
pCsy Syangja Fm.
pCul
Ulleri gneiss
Galyang Fm.
pCkn
Kuncha Fm.
B Unit pCd
Gondwanan
Lakharpata Gr.
(undiff )
Benighat Fm.
60
Dhading Fm.
32
MG-20
28 45
Trishuli River
pCg
4
Figure 14
S
5 km
0 km
3
1
0 km
50 km
7
4
A
MBT
MFT
A
Nsu
Nsm
Nsl
B
-5 km
C
D
pCl
5
E
N
120 km
G
pCl
Gd
pCl
2
11
RT
RT MCT
-10 km
-15 km
A’
100 km
10
F
MCT
5 km
GH
S
0 km
H
I
-5 km
-10 km
-15 km
pCkn
pCkn
-20 km
-20 km
6
-25 km
8
9
-30 km
TT
-25 km
-30 km
Nsu
Nsm
Nsl
A
B
C
D
pCl
E
pCl
F
G
pCkn
TT
H
I
GHS
Subhimalayan Zone
MFT= Main Frontal Thrust
MBT= Main Boundary Thrust
RT= Ramgarh Thrust
MCT= Main Central Thrust
TT= Trishuli Thrust
Nsu
Upper Siwalik Group
Nsm
Middle Siwalik Group
Nsl
Lower Siwalik Group
pCkn
GHS
GHS Undivided
pCkn
GHS
Tansen Unit
Tb
PKg
Dumri and
Bhainskati Fm.
Gondwanan Unit
Upper LHS
Lower LHS
Lakharpata Gr. pCf
(undiff )
pCul
pCsy Syangja Fm.
pCl
pCg
Galyang Fm.
pCkn
Fagfog Fm.
Ulleri gneiss
Kuncha Fm.