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TECTONICS, VOL. 29, TC1004, doi:10.1029/2008TC002390, 2010
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Basin formation in the High Himalaya by arc-parallel extension
and tectonic damming: Zhada basin, southwestern Tibet
Joel Saylor,1,2 Peter DeCelles,1 George Gehrels,1 Michael Murphy,3
Ran Zhang,3 and Paul Kapp1
Received 3 September 2008; revised 25 August 2009; accepted 23 September 2009; published 27 February 2010.
[1] The late Miocene–Pleistocene Zhada basin in
southwestern Tibet provides a record of subsidence
and basin formation within an active collisional thrust
belt. The >800 m thick basin fill is undeformed and
was deposited along an angular unconformity on top
of Tethyan strata that were previously shortened in
the Himalayan fold-thrust belt. Modal sandstone
petrographic data, conglomerate clast count data, and
detrital zircon U-Pb age spectra indicate a transition
from detritus dominated by a distal, northern source to a
local, southern source. This transition was accompanied
by a change in paleocurrent directions from uniformly
northwestward to basin-centric. At the same time the
depositional environment in the Zhada basin changed
from a large, braided river to a closed-basin lake.
Sedimentation in the Zhada basin was synchronous
with displacement on the Qusum and Gurla Mandhata
detachment faults, which root beneath the basin and
exhume midcrustal rocks along the northwestern and
southeastern flanks of the basin, respectively. These
observations indicate that accommodation for Zhada
basin fill was produced by a combination of tectonic
subsidence and damming, as midcrustal rocks were
evacuated from beneath the Zhada basin in response
to arc-parallel slip on crustal-scale detachment faults.
Citation: Saylor, J., P. DeCelles, G. Gehrels, M. Murphy, R. Zhang,
and P. Kapp (2010), Basin formation in the High Himalaya by arcparallel extension and tectonic damming: Zhada basin, southwestern
Tibet, Tectonics, 29, TC1004, doi:10.1029/2008TC002390.
1. Introduction
[2] Since they were first recognized [Molnar and
Tapponnier, 1978; Ni and York, 1978], Miocene normal
fault-bounded basins in the High Himalaya and Tibetan
Plateau have fueled speculation on their origins and have
been used as fundamental arguments in various tectonic
models. These include basin formation and arc-parallel
1
Department of Geosciences, University of Arizona, Tucson, Arizona,
USA.
2
Now at Department of Geological Sciences, Jackson School of
Geosciences, University of Texas at Austin, Austin, Texas, USA.
3
Department of Earth and Atmospheric Sciences, University of
Houston, Houston, Texas, USA.
Copyright 2010 by the American Geophysical Union.
0278-7407/10/2008TC002390$12.00
extension resulting from oblique convergence between India
and Asia [McCaffrey and Nabelek, 1998; Seeber and Pecher,
1998], eastward extrusion of Asian lithosphere [Armijo et al.,
1986], attainment of a limiting crustal thickness and subsequent gravitational collapse of the upper crust [Molnar
and Tapponnier, 1978; Molnar and Lyon-Caen, 1988; Kapp
and Guynn, 2004], removal of dense mantle lithosphere
[England and Houseman, 1989], oroclinal bending of the
Himalayan arc [Ratschbacher et al., 1994], and formation
as a result of arc-normal extension [Hurtado et al., 2001].
Moreover, these basins provide a record of tectonic and
climatic processes essential for discriminating among competing hypotheses about the evolution of the HimalayanTibetan orogenic system.
[3] The largest (>150 km long and >60 km wide, with an
outcrop extent of >9,000 km2) of these enigmatic basins
is the Miocene-Pleistocene Zhada basin in the western portion
of the Himalayan arc (Figure 1) [Zhang et al., 1981; Zhu et
al., 2004, 2007; Meng et al., 2008; Wang et al., 2008a, 2008b;
Saylor et al., 2009]. The Zhada basin trends NW – SE and is
located just north of the High Himalayan ridge crest in the
western part of the orogen (!32!N, 82!E). The basin occupies a region that is more than 1000 m lower in elevation than
anywhere else along strike in the High Himalaya and it
dwarfs other late Cenozoic sedimentary basins in southern
Tibet and the northern Himalaya [Ganser, 1964]. Stable
isotope data indicate that the Zhada region may have lost
up to 1.5 km of elevation since the late Miocene [Saylor et
al., 2009], suggesting a direct link between basin-forming
processes and regional elevation.
[4] The published literature on Zhada basin presents conflicting information and interpretations [Ni and Barazangi,
1985; Zhang et al., 2000; Murphy et al., 2002; Thiede et al.,
2006; Valli et al., 2007; Wang et al., 2008a]. For example,
the basin-filling Zhada Formation has been reported as
both an upward fining fluviolacustrine sequence [Zhang et
al., 1981; Zhou et al., 2000; Li and Zhou, 2001] and a
boulder conglomerate-capped succession [Zhu et al., 2004,
2007]. The basin has been interpreted to be the result of
movement on the South Tibetan Detachment system due to
arc-perpendicular extension [Wang et al., 2004], uplift
caused by arc-perpendicular compression [Zhou et al.,
2000; Zhu et al., 2004], and arc-perpendicular rifting [Wang
et al., 2008a].
[5] In this paper we examine the development of the
Zhada basin and its relationship to basin-bounding faults as
informed by new sedimentological and provenance data. Our
results show that the basin evolved from a throughgoing
fluvial system to an internally drained depocenter concom-
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Figure 1. (a) Elevation, shaded relief, and generalized tectonic map of the Himalayan-Tibetan orogenic
system showing the location of the Zhada basin relative to several other normal fault bounded basins.
Image from the UNAVCO Jules Verne Voyager and the Generic Mapping Tools (GMT). (b) Generalized
geologic map of the Zhada region [Chen and Xu, 1987; Murphy et al., 2000, 2002; M. Murphy, unpublished
mapping, 2005, 2006, 2007]. Locations of measured sections presented in Figure 6 are indicated by solid
black lines. Location of Figure 2 is outlined.
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itant with exhumation of midcrustal rocks to the northwest
and southeast in the footwalls of major arc-parallel extensional systems. By combining our results with regional
structural studies, we present a tectonic model that explains
Zhada basin development through a combination of sill uplift
and basin subsidence due to arc-parallel extension on crustalscale detachment faults.
2. Geologic Setting
[6] Major structural features near the margins of the
Zhada basin are the South Tibetan Detachment system
(STDS) to the southwest, the Great Counter thrust and right
slip Karakoram fault to the northeast, and the Leo Pargil and
Gurla Mandhata gneiss domes to the northwest and southeast, respectively (Figure 1b). The STDS is a series of north
dipping, low-angle, top-to-the-north normal faults that place
Paleozoic-Mesozoic low-grade metasedimentary rocks of
the Tethyan sequence on high-grade gneisses and granitoids
of the Greater Himalayan sequence [Hodges et al., 1992].
No ages for movement on the STDS in this area have been
published, but the timing of displacement along strike is
bracketed between 21 and 12 Ma [Hodges et al., 1992,
1996; Searle et al., 1997; Murphy and Harrison, 1999;
Hodges, 2000; Murphy and Yin, 2003; Searle et al., 2003;
Yin, 2006; Cottle et al., 2007]. In the Zhada region the south
dipping, top-to-the-north Oligocene-Miocene Great Counter
thrust system cuts the Indus suture [e.g., Ganser, 1964; Yin
et al., 1999; Murphy and Yin, 2003]. The region north of the
Indus suture is dominated by Mesozoic and Cenozoic
igneous rocks of the Gangdese magmatic arc [Yin et al.,
1999; Aitchison et al., 2002; Murphy et al., 2002; Murphy
and Yin, 2003]. Exhumation of migmatites and paragneisses
in the Leo Pargil gneiss dome by normal faulting initiated at
either !15 Ma or !9 Ma, as indicated by 40Ar/39Ar and
AFT cooling ages, respectively [Zhang et al., 2000; Thiede
et al., 2006]. Exhumation of Greater Himalayan and Lesser
Himalayan rocks in the Gurla Mandhata gneiss dome began
at !9 Ma [Murphy et al., 2002; Murphy, 2007]. Faulted
Quaternary basin fill indicates that gneiss dome development continues today [Murphy et al., 2002]. Although
previous studies have linked development of the Zhada
basin with movement on faults to the northeast and southwest of the basin [Zhou et al., 2000; Wang et al., 2004; Zhu
et al., 2004; Wang et al., 2008a], our mapping (Figure 2)
reveals that only in its northwest corner is the basin fill cut
by a basin-bounding fault. Elsewhere in the basin, the
undeformed Zhada Formation onlaps paleotopography in
the hanging walls and footwalls of the Great Counter thrust
and the STDS, indicating that it entirely postdates slip on
these major fault systems (Figure 2).
[7] The Zhada basin is filled by the Zhada Formation,
which consists of >800 m of fluvial, lacustrine, eolian, and
alluvial fan deposits (Figures 2 and 3). The basin fill is largely
structurally undisturbed and sits above shortened Tethyan
sequence strata along an angular or buttress unconformity
(Figures 4a – 4c). The Zhada Formation is capped by a
geomorphic surface that is correlative across the basin
(Figures 4b and 4d). The geomorphic surface is interpreted
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as a paleodepositional plain that marks the maximum extent
of sedimentation prior to integration of the modern Sutlej
River drainage network. After deposition, the basin was
incised to basement by the Sutlej River, exposing the entire
basin fill in spectacular canyons and cliffs. The best estimate
for the onset of sedimentation in the Zhada basin is 9.2 Ma
based on magnetostratigraphy (Figure 5) [Wang et al.,
2008b; Saylor et al., 2009]. Correlations to the geomagnetic
polarity timescale [Lourens et al., 2004] were constrained by
Miocene-Pliocene mammal fossils and by an observed shift
from pure C3 to mixed C3 and C4 vegetation in the Zhada
basin. The correlation presented in Figure 5 accounts for all
of our normal and reversed intervals; places the C3-C4
transition at !7 Ma, which is consistent with its age elsewhere in the Himalaya [France-Lanord and Derry, 1994;
Quade et al., 1995; Garzione et al., 2000; Ojha et al., 2000;
Wang et al., 2006]; and yields relatively constant and
reasonable sedimentation rates. For an in-depth discussion
of alternative correlations see Saylor et al. [2009].
3. Sedimentology of the Zhada Formation
[8] We measured 14 stratigraphic sections spanning the
basin from the Zhada county seat in the southeast to the Leo
Pargil/Qusum Range front in the northwest (Figures 1b, 2
and 3). Here, we define the Zhada Formation as the entire
basin fill extending from the basal unconformity to the paleodepositional surface. Sections were measured at centimeter
scale and correlated by tracing major stratigraphic units
and the paleodepositional surface that caps most sections
(Figures 3, 4b, and 4d). The 14 lithofacies associations in
Table 1 are defined on the basis of lithology, texture and
sedimentary structures, and are grouped into five depositional environment associations. Only abbreviated descriptions and interpretations are presented here; for details see
Saylor [2008]. Lithofacies codes following the lithofacies
associations in Table 1 are based on the work by Miall [1978]
and DeCelles et al. [1991] and described in Table S1.1 Unless
otherwise indicated, all deposits are laterally continuous
for hundreds of meters to several kilometers.
3.1. Zhada Formation Members
[9] We identified five members of the Zhada Formation
based on lithofacies assemblages (Figures 2 and 6). The
lower conglomerate member (Nlc) consists of lithofacies
associations F1, F2, and rare F3 (Figures 7a, 7b and 7e).
Where present, it extends from the base of the section to the
top of the last F1 interval >1 m thick. Above this, the lower
fluvial member (Nlf) consists of lithofacies associations F2,
F3, S2 and S3 and extends from the top of the lower
conglomerate member to the base of the lacustrine claystone. The lacustrine member (Nl) includes primarily lithofacies associations P1, P2, L1, L2, S1 and minor S2, S3
and F2 (Figures 7d and 7f). The lacustrine member extends
from the first occurrence of lacustrine claystone to the first
thick (>1 m) interval displaying soft-sediment deformation.
1
Auxiliary material data sets are available at ftp://ftp.agu.org/apend/tc/
2008tc002390. Other auxiliary material files are in the HTML.
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Figure 2. Geologic map of the Zhada region showing the traces of measured sections and the
relationship between the Zhada Formation and major faults in the region.
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Figure 3. Measured sections, paleocurrent measurements, clast compositions, and lithologic correlations from the Zhada
basin. See Figure 2 for locations.
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Figure 3. (continued)
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Figure 4. The Zhada Formation is undeformed and lies in buttress unconformity upon deformed
Tethyan sequence strata. (a) The unconformity relationship in the basin center. (b) The unconformity
relationship at the basin margins. (c) Deformation in the central Zhada basin is limited to small-offset
faults. The minor offset, lack of basement involvement, and the fact that deformation is limited to a thin
stratigraphic interval indicates that this deformation is related to syndepositional slumping rather than
significant tectonic activity. (d) The geomorphic surface which caps the Zhada Formation
The upper fluvial member (Nuf) is similar to the lower
fluvial member with the exception that it is coarser and
contains more abundant soft-sediment deformation. This
member is composed primarily of sand- to boulder-sized
material in contrast to the lower fluvial member which is
dominantly silt- to sand-sized material. The upper fluvial
member extends from the lacustrine member to the first
significant (>10 m thick) conglomeratic interval. The upper
alluvial fan member (Nal) includes lithofacies associations
A1 – A4, F3, P1 and P2 (Figure 7c). It extends from the first
significant conglomeratic interval above the lacustrine member to the top of the Zhada Formation.
3.2. Paleocurrent Measurements
[10] Paleocurrent data consist of measurements of 298
limbs of trough cross strata (method I of DeCelles et al.
[1983]) at 29 sites and measurements of 568 long-axis
transverse imbricated clasts at 47 sites in measured sections
throughout Zhada basin (Figure 8).
[11] Paleocurrent measurements fall into two groups: those
that show a northwestward paleoflow direction and those
directed toward the basin center. The first group comes from
the lower two members of the Zhada Formation (Nlc and
Nlf). The average paleocurrent azimuth from 8 measured
sections in the lower members ranges between 270! and 349!
(Figure 8a and Table S2). The second group of measurements
comes from the two upper members (Nuf and Nal) or
anomalously coarse-grained tongues (cobble-boulder) within
lower members. These paleocurrent indicators are less uniform but generally indicate flow toward the center of the
basin in seven of the nine measured sections (Figure 8b and
Table S2). In the northwest end of the basin there is a trend
from uniformly northwestward directed paleoflow (Qusum
section; Figure 8a) to a more complex, but average northwestward directed paleoflow (1NWZ and 2NWZ sections)
and finally a complete reversal to an eastward average
paleoflow direction (3NWZ section; Figure 8b).
4. Provenance Analysis
4.1. Methods
4.1.1. Sandstone Petrography and Conglomerate
Clast Counts
[12] Modal sandstone composition data were collected
from 35 standard petrographic thin sections from 9 measured
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Figure 5. Correlation of the composite magnetostratigraphic section with the GPTS of Lourens et al. [2004]. The
composite magnetostratigraphic section was constructed from
two magnetostratigraphic sections, each of which spanned
the entire thickness of the Zhada Formation [Saylor et al.,
2009]. The stratigraphic location of the switch from plants
utilizing exclusively the C3 carbon fixation pathway to a mix
of C3 and C4 plants is indicated by the gray box (between 170
and 250 m in the South Zhada measured section). The
location of the first Hipparion fossils is at !240 m in the
South Zhada measured section.
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intervals from within individual beds. Bed load counts
involved identification of !100 pebble-sized or larger clasts
that were dredged from the streambed.
4.1.2. U-Pb Detrital Zircon Geochronology
[14] U-Pb geochronology was conducted on zircons
separated from nine sandstone samples and one paragneiss
sample from the Zhada basin and the footwall of the Qusum
Detachment, respectively, and were processed for detrital
zircon analysis using procedures described by Gehrels
[2000] and Gehrels et al. [2008]. Analyses were conducted
using the laser ablation multicollector inductively coupled
plasma mass spectrometer (LA-MC-ICPMS) at the Arizona
LaserChron Center. Approximately 100 individual zircon
grains were analyzed from each sample (with the exception
of samples 2EZ88 and 2EZ60.7A). These were selected
randomly from all sizes and shapes, although grains with
obvious cracks or inclusions were avoided. In-run analysis of
fragments of a large zircon crystal (generally every sixth
measurement) with known age of 564 ± 4 Ma (2-sigma
error) was used to correct for interelement and intraelement
fractionation. The uncertainty resulting from the calibration
correction is generally 1%–2% (2 sigma) for both 206Pb/207Pb
and 206Pb/238U ages. Common Pb was corrected by using the
measured 204Pb and assuming an initial Pb composition from
Stacey and Kramers [1975] (with uncertainties of 1.0 for
206
Pb/204Pb and 0.3 for 207Pb/204Pb).The analytical data are
reported in Data Set S2. Details of the operating conditions
and analytical procedures can be found in the work by Gehrels
et al. [2008].
[15] For each analysis, the errors in determining 206Pb/238U
and 206Pb/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
crossover in precision of 206Pb/238U and 206Pb/207Pb ages
occurs at 0.8–1.0 Ga. Interpreted ages are based on 206Pb/238U
for <!300 Ma grains and on 206Pb/207Pb for >!300 Ma
grains. Though 206Pb/238U ages are more precise below 1 Ga,
the occurrence of numerous zircon grains with lead loss
due to Cenozoic metamorphism necessitated a younger
cutoff. Analyses with >20% uncertainty, that are >30%
discordant (by comparison of 206Pb/238U and 206Pb/207Pb
ages) or >5% reverse discordant are omitted from interpretation (Figure S1).
4.2. Results
sections across the basin. Thin sections were stained for
potassium and calcium feldspar and point counted (only
grains larger than silt sized, 403 –744 counts per slide) using
the Gazzi-Dickinson method [Ingersoll et al., 1984]. The
petrographic counting parameters are listed in Table S3, and
recalculated modal data are given in Data Set S1.
[13] Conglomerate composition data consist of 3447
identified clasts from 32 sites throughout Zhada basin,
and bed loads of several modern streams and rivers.
Conglomerate clast counts involved identification of clast
lithology of !100 pebble-sized or larger clasts at regular
4.2.1. Sandstone and Conglomerate Compositions
[16] Quartz is present in monocrystalline, polycrystalline
(commonly in granitoid lithic grains), and foliated polycrystalline (commonly in mylonitic grains) grains, and also
monocrystalline grains within sandstone or quartzite grains
(Figure S2). Other lithic grains include phyllite, schist,
carbonate, chert, and mudstone. Volcanic grains are common,
particularly in the lowest part of the Zhada Formation. The
most abundant are felsic volcanic grains. This group includes
porphyritic, highly altered quartzofeldspathic grains, quartzrich grains with disseminated plagioclase, and volcanic
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Tabular units 0.1 – 5 m thick of horizontally or cross-stratified sandstone;
abundant load casts and ball-and-pillow structures and rare channel forms
Massive, organic-rich, strongly calcareous siltstones and sandstones
0.1 – 4 m thick featuring soft-sediment deformation but no
pedogenesis; contains ostracods, root traces, and small
planorbid gastropods (Gyraulus sp?)
Tabular, upward coarsening sandstone to pebble conglomerate 0.25 to
6 m thick; erosional bases and abrupt tops; horizontal stratification,
climbing, trough, and planar cross stratification; channelized
deposits up to 0.5 m thick are locally present; abundant,
well-preserved, robust gastropod shells
Low-angle laminae up to 3 cm thick without internal structure; developed
in well-sorted, fine-grained sand; root traces and soft-sediment
deformation are common
Stacked units (0.5 – 1.5 m thick) of steeply inclined, upward coarsening
laminations; developed in well-sorted, fine-grained sandstone;
root traces and soft-sediment deformation are common
Papery, laminated siltstone; abundant fossil grasses, fragmentary shell material
and occasional fish fossils
Upward coarsening sandstone deposits 0.5 – 3 m thick; gradational bases
and erosional tops; climbing ripple and wave ripple cross stratification
and mud drapes
Upward coarsening massive or horizontally laminated silty claystone;
up to 10 m thick, includes dispersed, plant, ostracod, and fish fossils
2 m thick stacks of upward fining laminae up to 3 cm thick; basal surfaces
of individual laminae are abrupt and the upward transition to siltstone
is gradational, lower portion is massive and grades upward
to horizontally laminated
Poorly sorted, very angular to subrounded, pebble to boulder conglomerate;
0.5 – 20 m thick;
erosive bases, horizontal stratification and long-axis transverse imbrication
Massive, unorganized, matrix-supported boulder conglomerate 0.5 – 3 m thick;
no erosive bases
Imbricated, lenticular, clast-supported, pebble-boulder conglomerate bodies;
erosive bases and horizontal and cross stratification
Chaotic matrix- and clast-supported boulder conglomerate deposits;
abundant load casts and ball and pillow structures (up to 2 m
of relief on some structures)
F2: St, Sp, Sh, Sc
F3: Sm, Sc, Mm, Mc, Ml, Mh
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High-concentration flood or debris flow deposits into a water-saturated
environment [Blair, 2000; Miall, 2000; Nemec and Steel, 1984;
Pivnik, 1990; Postma, 1983]
Deposited by streams in alluvial fan channels [DeCelles et al., 1991]
High-concentration, unconfined sheetflood deposits [Blair and McPherson,
1994b; Blair, 2000; DeCelles et al., 1991; Nemec and Steel, 1984;
Pierson, 1980, 1981]
Debris flow deposits [Blair and McPherson, 1994a; Pierson, 1980]
Stacked deposits of turbidity currents (primarily Bouma A and B)
[Bouma, 1962; Giovanoli, 1990; Lowe, 1982; Mutti et al., 2003]
Deposits of a grass-rich, shallowly submerged, low-gradient, lake margin
environment [Allen and Collinson, 1986; Talbot and Allen, 1996]
Deposits of a terminal lobe on a prograding delta front
[Allen and Collinson, 1986; Dam and Surlyk, 1993; Jopling,
1965; Saez et al., 2007; Talbot and Allen, 1996]
Profundal lacustrine deposition (below fair-weather wave base)
Lacustrine coastal plain dune deposits
Vegetated eolian sand flat [Hunter, 1977]
Product of sedimentation within a delta complex [Allen and Collinson, 1986;
Saez et al., 2007]
Deposits of migrating channels, bars, bed load sheets, and gravel dunes of a
braided river within a low-gradient fluvial setting [Bristow and Best, 1993;
Cant and Walker, 1978; Collinson, 1996; Heinz and Aigner, 2003;
Lunt et al., 2004]
Sheetflood deposits on medial to distal fluvial fans in a marshy floodplain
or marginal lacustrine setting [Hampton and Horton, 2007;
Saez et al., 2007]
Marshy floodplain or lake margin wetland deposits [Allen and Collinson, 1986]
Interpretation
SAYLOR ET AL.: ZHADA EVOLUTION ARC-PARALLEL EXTENSION
a
We identify 14 lithofacies associations and 5 depositional environments. Depositional environments are as follows: F, fluvial; S, supralittoral; L, littoral; P, profundal; and A, alluvial fan. Modern analogs to
these environments occur both at Kungyu Co (N30!360, E82!110) and to the west of Zhongba (N26!, E84!) in southern Tibet.
A4: Gx
A3:Gcmi,Gch, Gcf, Gt
A2: Gmm, Gcm
A1: Gcm, Gch, Gcmi
P2: Sh, Sm, Mh
P1: Mh, Mm
L2: Mr, Sr, Srw, Sp, Sh, Sm
L1: Ml
S3: Sf
S2: Sh
S1: Gct, Gcmi, Sp, St, Sh, Sm, Sr
Amalgamated channel forms and tabular units of moderately sorted,
clast-supported pebble conglomerate featuring abundant traction
sedimentary structures; deposits are 2 – 10 m thick; erosive bases.
Description
F1: Gcmi, Gch, Gt, Gcf
Lithofacies Association
Table 1. Lithofacies Association Codes, Descriptions, and Interpretationsa
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Figure 6. Stratigraphic cross section across the Zhada basin showing the relationship between Zhada
Formation members. Cross-section line is indicated in Figure 3. Vertical exaggeration is 10X.
grains that include exotic lithic fragments. Lath work volcanic
grains contain lath-shaped plagioclase crystals and are typically felsic in composition. Vitric volcanic grains are those
with pseudoisotropic textures. These commonly resemble
chert, but, where stained, have taken a pink plagioclase stain.
In most thin sections plagioclase is slightly more abundant
than potassium feldspar. Both biotite and muscovite are
present in most samples with muscovite typically being more
common. Accessory minerals include zircon, tourmaline,
garnet, olivine, serpentine, chlorite, and cordierite. The most
abundant cement is carbonate with secondary occurrences of
gypsum cementation.
[17] Sandstones from the lower conglomeratic and fluvial
members of the Zhada Formation consist of poorly lithified
carbonate- or gypsum-cemented litharenites. Subrounded
to subangular lithic grains, including polycrystalline quartz,
are the dominant constituents with lesser amounts of monocrystalline quartz grains (Figures 9b, 9e, and 9h). The lithic
fraction is dominated by felsic and vitric volcanic grains
(Figures 9c and 9i). Feldspar constitutes !10%– 20% of the
total compositions (by comparison with either Qt and L or
Qm and Lt) with plagioclase being slightly more abundant.
Whereas volcanic grains are the most abundant lithic grains
in lower member sandstones, in some samples up to 66%
of the lithic portion is composed of either metamorphic or
sedimentary lithic grains (Figure 9c samples 0SZ1 and
0.2SZ30).
[18] As in the lower members, sandstones from the upper
members consist of poorly lithified carbonate- or gypsumcemented litharenites. However, starting in the middle of the
lower fluvial member the composition of sandstones changes
abruptly. This is particularly evident in the lithic fraction,
which changes from being dominated by volcanic grains to
subequal amounts of sedimentary and metamorphic grains
(Figures 9c and 9i) or entirely metamorphic grains (Figure 9f).
Feldspar constitutes a larger component of upper member
sandstones (10% – 55% of the total), with the proportion of
feldspar increasing up section. Plagioclase dominates over
alkali feldspars. In contrast to lower member sandstones, volcanic lithic grains are a minor constituent of upper member
sandstones.
[19] Conglomerate clast count data are consistent with
modal sandstone analyses. Sedimentary and volcanic clasts
dominate the lower members of the Zhada Formation;
metamorphic clasts become more abundant up section,
and volcanic clasts less so (Figure 3).
4.2.2. U-Pb Geochronologic Analyses of Detrital Zircons
[20] A total of 840 new zircon ages from 10 samples are
reported here. The preferred ages are shown on relative ageprobability diagrams [from Ludwig, 2003]. These diagrams
show each age and its uncertainty (for measurement error
only) as a normal distribution and sum all ages from a
sample into a single curve (Figure 10).
[21] Detrital zircon samples fall broadly into two groups:
those with significant peaks >100 Ma and those with almost
exclusively <100 Ma detrital zircon ages (Figure 10d). No
correlation was found between the degree of zircon grain
shape and age. The youngest zircon age population is 23 Ma
(n = 8, LwrCgm58.15) though a small cluster (n = 2) are as
young as 17 Ma (2EZ66.5; Figure 10 and Data Set S2). The
>100 Ma age population has prominent peaks at 450– 550,
800 – 1200, and 2500 Ma and minor peaks at 550 – 650,
1300– 1450 and 2100 Ma. The <100 Ma population has
major peaks at !35, 45– 50, and !55 Ma and minor peaks at
17– 23, 60– 65 and 90– 100 Ma.
4.3. Interpretation
4.3.1. Sandstone and Conglomerate Provenance
[22] Detrital modes of sandstones show an up-section
shift from an arc-orogen source to a recycled orogen source.
The lowermost samples are from sandstones within the
fluvial Nlc and Nlf members. Their lithic fractions fall largely
within the arc-orogen source field in Qp-Lv-Ls ternary
diagrams and straddle the border between the arc-orogen
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Figure 7. (a) Gravel foresets of lithofacies F1. Foresets are 3 – 4 m tall. (b) Large-scale trough cross
stratification in F2 sandstones. (c) Coarse progradational sequences at the top of the Zhada Formation.
Visible in this image is an organic-poor example of lithofacies P1 and lithofacies A4 and A1. TS,
transgressive surface; P, upward coarsening trends inferred to represent progradation. (d) Horizontally
stratified and planar and low-angle climbing ripple cross stratification of lithofacies S1. (e) F2 (now
contorted) overlying marshy wetland deposits (F3). Staff is !50 cm long. (f) Upward fining laminae of
lithofacies P2 that are interpreted as turbidites.
and recycled orogen fields in Qm-F-Lt and Qt-F-L diagrams
(Figure 9) [Dickinson, 1985]. Sandstone compositions from
the top of the Nlf member are more scattered but trend toward
the quartzose, recycled orogen fields in all ternary diagrams
(Figure 9). Samples from the upper members of the Zhada
Formation (Nl, Nuf, Nal) continue the trend toward more
compositionally mature, recycled orogen sources, even
extending into the continental block field in Qm-F-Lt and
Qt-F-L diagrams (Figures 9g and 9h). These trends are also
present in samples from the northwestern part of Zhada
basin (Figures 9e and 9f).
[23] The primary source of arc-orogen-derived volcanic
detritus for samples from the fluvial interval of the lower
member of the Zhada Formation is interpreted to be the
Mount Kailash region to the northeast of Zhada basin (north
of the Karakoram fault; Figures 1 and 8). The region south
of the Zhada basin is dominated by high-grade metamorphic
rocks and the only local sources of volcanic detritus are
relatively small zones of generally mafic ophiolitic rocks in
the Tethyan Himalaya. In contrast, the Mount Kailash
region is dominated by both volcanic and plutonic rocks
associated with the Cretaceous-Tertiary Gangdese magmatic
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Figure 8. Paleocurrent data for (a) the lower members of the Zhada Formation and (b) the upper
members of the Zhada Formation superimposed on topography. Paleocurrent data show an up-section
evolution from uniformly northwestward directed to basin-centric paleocurrent indicators. Numbers in
rose diagrams identify measured sections in Table S2. The outline of Zhada basin sediments is shown in
Figure 8a. For comparison, the modern drainage network is shown in Figure 8b. The modern Sutlej River
rises in the region of Rakkas tal and Manasarovar (near Mount Kailash) and exits the Zhada basin near
the Leo Pargil Dome.
arc and younger clastic rocks derived from these igneous
rocks [Yin et al., 1999; Aitchison et al., 2002; Murphy et al.,
2002; Murphy and Yin, 2003]. This area is also the headwaters of the modern Sutlej River (Figure 8b). Probable
sources of the recycled orogen detritus which dominates
the upper members of the Zhada Formation include rocks
in the Ayi Shan Mountains northeast of the basin, the Tethyan
sequence strata which underlie the basin, and the Greater
Himalayan and Tethyan sequence rocks southwest of the
basin. Greater Himalayan rocks are also exposed in the Gurla
Mandhata dome to the southeast of Zhada basin [Murphy,
2007].
[24] The modal petrography points to an up-section shift
from a northeasterly source to a source located southwest of
the Karakoram fault. The enrichment in recycled orogen
detritus is also evident in the northwestern Zhada basin.
However, being farther from the inferred source of volcanic
detritus, the initial volcanic fraction is reduced and so the
evolution from an arc-orogen dominated source to a more
quartzose source is not as dramatic as in central Zhada
basin.
[25] Like the petrographic data, conglomerate clast count
data show an up-section decrease in the abundance of
volcanic detritus and an increase in the relative abundance
of high-grade metamorphic and plutonic clasts (Figure 3).
Particularly in the southern margin of the basin, where
paleocurrent indicators from the upper members of the Zhada
Formation show northward paleoflow directions, the only
source of high-grade metamorphic clasts is the Greater
Himalayan sequence to the south of Zhada basin. However,
low-grade metasedimentary clasts dominate the clast counts
throughout the basin, indicating that the local Tethyan
basement was a significant source of detritus. In the northern
and northwestern parts of the basin, paleocurrent data indicate that high-grade metamorphic and plutonic rocks were
derived from the footwall of the Qusum detachment. Consistent with greater distance from the source of volcanic
rocks (the Kailash Range), the ratio of volcanic clasts to
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Figure 9. Ternary diagrams showing modal petrographic data from this study. Samples show an
up-section trend from a northern Gangdese Arc dominated source to a southern Himalayan fold-thrust belt
dominated source. The change in source terrane coincides with the reorientation of paleocurrent indicators
(Figure 8). Fields are from Dickinson [1985].
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other clasts is lower in the northwestern Zhada basin than
in the central Zhada basin. The conglomerate clast count data
indicate that the source of sediments in the Zhada basin
changed from a distal, northeastern source, with local input,
to a predominantly proximal southwestern source.
4.3.2. U-Pb Detrital Zircon Geochronology
[26] Samples with significant detrital zircon ages <100 Ma
come from the bottom of the Zhada Formation and those
with almost exclusively >100 Ma ages come from the top
(Figure 10d). Possible sources for detrital zircons include
the Cretaceous-Tertiary igneous rocks related to Gangdese
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(Kailash) arc magmatism, Paleozoic-Mesozoic metasedimentary rocks of the Tethyan Sedimentary sequence and
late Proterozoic – early Paleozoic rocks of the Greater
Himalayan sequence.
[27] Good correlation exists between most of the <100 Ma
age peaks with zircon age spectra from regions north of
Zhada basin (compare the left plots of Figures 10a, 10b and
10c). Specifically, age peaks at 17– 23 Ma, !55 Ma and
78 – 80 Ma correlate with igneous or detrital zircon age
spectra from the Mount Kailash area (Figure 10a). The only
documented source for !55 Ma zircons is from the Gangdese
arc (Figure 10a). However, as there are no good correlations
from this region for peaks from 35 to 50 Ma, these zircons
must have been derived from the Ayi Shan. Ayi Shan zircons
also may contribute to the 78– 80 Ma peak in the Zhada basin
(Figure 10a).
[28] Ages of detrital zircons from the top of the Zhada
Formation (e.g., 4SZ14, 3NWZ45.9 and 3NWZ77) are
dominantly Paleozoic-Proterozoic; these cannot have been
derived from the Mount Kailash region. The Neogene
Kailash Formation has a greater proportion of <100 Ma
zircons than do the samples mentioned above (Figure 10a,
Kailash). Hence, the Kailash Formation may be a minor
source, but the major source of detritus must lie elsewhere.
Likely sources include inherited zircons from Ayi Shan
paragneisses or leucogranites which have Tethyan sequence
protoliths, the Tethyan sequence basement, or Greater
Himalayan sequence rocks (compare the right plots of
Figures 10a, 10b and 10c) [DeCelles et al., 2000, 2004;
Martin et al., 2005; Gehrels et al., 2006a, 2006b, 2008].
Consistent with the petrographic data, the detrital zircon data
show a significant up-section shift from a source in the Mount
Kailash area to a local basement or southwestern source.
[29] There is an apparent disconnect between the relative
abundance of detritus derived from northeast of Zhada basin
Figure 10. U-Pb relative age-probability density diagrams
for (a) possible source terranes for Zhada basin sediment
and (b, c, and d) detrital zircon samples from this study.
Figure 10b shows detrital zircon samples from the northwest
of Zhada basin with age populations split into <100 Ma
grains in the left plots and >100 Ma grains in the right plots.
Figure 10c shows detrital zircon samples from central Zhada
basin with age populations split into <100 Ma grains in the
left plots and >100 Ma grains in the right plots. Figure 10d
shows detrital zircon samples from this study plotted without
splitting young and old age populations. Within Figures 10b,
10c, and 10d, samples are plotted stratigraphically up section
from bottom to top. Sample names correlate to measured
sections and heights within those measured sections. Samples
show an up-section change from a Kailash or Ayi Shan
source to a local (Tethyan sequence) or southerly (Greater
Himalayan sequence) source that is coincident with the
reorientation of paleocurrent indicators (Figure 8). Stars in
Figure 10a indicate igneous ages, and colors correlate with
text color. Bin sizes for histograms associated with relative
age-probability diagrams are 5 Myr for ages between 0 and
100 Ma and 50 Myr for ages between 100 Ma and 3500 Ma.
Source region data from Gehrels et al. [2008].
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Figure 11. (a) Decompacted and tectonic subsidence curves for the Zhada basin. (b) Tectonic
subsidence curve for the Zhada basin compared to tectonic subsidence curves for a variety of tectonic
settings [from Allen and Allen, 1990; Angevine et al., 1990].
indicated by the detrital zircon data and the conglomerate
clast count data. The conglomerate clast counts indicate a
dominance of low-grade metamorphic rocks (i.e., Tethyan
Sequence) over volcanic rocks (i.e., Gangdese source) in the
lower members of the Zhada Formation while the detrital
zircon data show the opposite. One possible reason for this
is downstream grain-size sorting. Unstable volcanic and
igneous clasts were trapped or disaggregated close to their
source north of the Zhada basin while sand-sized igneous
zircon grains were transported to and deposited together with
locally derived low-grade metasedimentary clasts in the
Zhada basin. This explanation is consistent with a more
distal northeastern source and a more proximal local or
southwestern source.
5. Subsidence Curve
[30] A tectonic subsidence curve for the Zhada basin was
produced from the South Zhada section by decompacting
the sediments using standard backstripping techniques
(Figure 11a, decompacted subsidence curve) [Vanhinte,
1978; Dickinson et al., 1987; Allen and Allen, 1990] and
then removing the load of the sediment assuming Airy
isostacy (Figure 11a, tectonic subsidence curve). Age control
is based on magnetostratigraphic tie points (Figure 5) [Saylor
et al., 2009]. Bathymetry was not a major contributor to
sediment accommodation because the lake was never more
than a few tens of meters deep based on the thickest observed
upward coarsening parasequence within the Zhada Formation. The essentially linear Zhada basin curve best matches
typical subsidence curves from strike-slip or rift basins
(Figure 11b), though the subsidence rate is low: 0.09 mm/yr
and 0.06 mm/yr for the decompacted and tectonic subsidence
curves, respectively. In addition, Zhada basin fill is thinner
than classic rift and strike-slip basin fills, probably as a result
of both low subsidence rates and the brief duration of the
basin. Though exceptions are admitted rift basins are typically longer lived than supradetachment basins [Friedmann
and Burbank, 1995].
6. Discussion
6.1. Zhada Basin Evolution
[31] Sedimentology and paleocurrent data indicate that a
large, northwestward flowing, low-gradient river deposited
the lower members of the Zhada Formation. Whereas this
may seem to contradict the argument for a northern source
for the volcanic detritus based on petrography, clast composition, and detrital zircon analyses, we note that the modern
Sutlej River has its headwaters in the Mount Kailash region,
flows southward into, and then northwestward along the
Zhada basin and currently exits the basin to the southwest
(Figure 8b). The presence of volcanic detritus in the lower
member of the Zhada Formation indicates that the paleoSutlej River followed a similar path into the Zhada basin and
flowed northwestward along the basin axis. Paleocurrent
indicators at the base of the Zhada Formation are oriented
northwestward even within 10 km of the modern Qusum
range front, indicating that, at the time that the sediments
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Figure 12. Growth structure in Zhada Formation in the proximal hanging wall of the Qusum
detachment fault. This is the only significant deformation observed in the Zhada basin. Telephone poles
are 8 m tall. View is toward the northeast. The normal fault shown is antithetic to the top-to-the-east master
Qusum detachment fault (not visible, to the west). See text for additional discussion of the growth-strata
geometry.
were deposited, the Leo Pargil/Qusum Range presented no
obstacle to northwestward flow.
[32] Up section, paleocurrent indicators, provenance data,
and sedimentology indicate that the Zhada basin became
fully or partially closed, and that detritus was derived from
local highlands surrounding the basin. Northwestward
paleoflow suggests that the cause of ponding within Zhada
basin is to be found to the northwest. Deformation and
growth structures (Figure 12) in the Zhada Formation in the
northwestern part of the basin, near the Leo Pargil/Qusum
Range, indicate that movement on the Qusum detachment
and exhumation of the range were synchronous with deposition of the Zhada Formation. It is unlikely that the Zhada
basin was entirely hydrologically closed because a large
river, such as the Sutlej, would have quickly filled and
overflowed a closed basin. Moreover, the Zhada Formation
contains no significant thicknesses of evaporites, which
would be expected in a closed basin. Some outflow probably
continued either via a small outlet in the southwest of the
basin or groundwater flow which would not be detected by
paleocurrent data [Dühnforth et al., 2006]. However, paleocurrent indicators and provenance data show that major
surface flow during the Pliocene-Pleistocene was basincentric and stable isotope data indicate that the lake underwent significant evaporation [Saylor et al., 2009].
[33] The presence of Zhada basin sediments in buttress
unconformity on top of deformed Tethyan sequence strata
indicates that there was some paleotopography on the
basement prior to deposition of the Zhada Formation. In
measured sections located near Tethyan basement rocks,
alluvial fan facies interfinger with fluvial facies. Paleocurrent
indicators from alluvial fan facies are commonly at high
angles to those from axial fluvial facies. This was observed
particularly in the northwestern end of the basin and near the
Guga section and is the cause of the anomalous northward
oriented petals on the 2NWZ, 3NWZ and 1NZ rose diagrams
and east or southward oriented petals on the Guga rose
diagrams (Figure 8). These data add detail to environmental
reconstructions, but do not change the overall basin evolution
picture.
6.2. Tectonic Origins of the Zhada Basin
[34] Any model for Zhada basin must explain (1) the
transition from nondeposition to deposition at !9.2 Ma,
(2) the relationship of the Zhada Formation to preexisting
topography in the Tethyan fold-thrust belt, (3) the variety
and stacking patterns of depositional environments in the
Zhada Formation, (4) the lack of widespread deformation
in the Zhada Formation, (5) the lack of cyclic incision and
infilling or significant hiatuses in sedimentation in the
Zhada Formation, and (6) the transition from deposition to
nondeposition and incision at <1 Ma.
[35] All evidence indicates that closure of the Zhada
basin was the result of uplift of the Leo Pargil and possibly
Gurla Mandhata domes, which created tectonic dams or sills
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and caused basin subsidence. Exhumation of both the Leo
Pargil/Qusum Range and the Gurla Mandhata dome was
synchronous with sedimentation in the Zhada basin. The
only basement-involved deformation observed in the Zhada
Formation is a growth structure at the northwestern end of the
basin, !10 km from the Qusum detachment fault (Figure 12).
The observed structure is adjacent to the Qusum measured
section (Figures 2 and 3) which we correlate to the base of the
Zhada Formation (!9 – 4.5 Ma). The growth-stratal geometry
is best explained by progressive sedimentation and rotation
of basin sediments in the proximal hanging wall above a
blind, upward propagating normal fault [Gupta et al., 1999;
Sharp et al., 2000; Gawthorpe and Hardy, 2002]. Paleocurrent indicators from the base of the Zhada Formation are
uniformly northwestward, even in the Qusum measured
section (Figure 2 and 8a), indicating that prior to uplift of
the Leo Pargil/Qusum Range, the paleo-Sutlej River flowed
unobstructed toward the northwest. The linkage between
basin formation and exhumation is less direct for the Gurla
Mandhata detachment than for the Qusum detachment.
However, several lines of evidence suggest that exhumation
on the Gurla Mandhata detachment also drove basin subsidence. (1) The temporal coincidence between exhumation
of the Gurla Mandhata dome and basin development allows
a causal link. (2) The Gurla Mandhata detachment accommodates between 35 and 66 km of displacement and
exhumation [Murphy et al., 2002] but the basin in its
immediate hanging wall (Pulan basin) is less than 15 km
wide. (3) Rollover structures in the Pulan basin fill show
that the Gurla Mandhata detachment flattens at an extremely
shallow depth. The implication is that exhumation-related
subsidence was not focused in the proximal hanging wall and
but rather was accommodated in the distal hanging wall.
[36] Basin subsidence in the hanging wall of the Qusum
(and possibly Gurla Mandhata) detachment was spatially
variable. Correlation of magnetostratigraphy between measured sections shows that there has been more subsidence
in the basin center (the South Zhada and Guga sections)
than toward the basin margins (East Zhada or Southeast
Zhada sections) (Figure 3) [Saylor et al., 2009]. Significant
differential subsidence and sediment accumulation are evident between the South Zhada and Southeast Zhada sections between 9.2 Ma and 7 Ma (Figure 3a). Differential
subsidence between the South Zhada and East Zhada sections provides evidence for ongoing subsidence post-7 Ma
(Figure 3b). Additional evidence for basin subsidence is
provided by the change in paleoslope indicated by Figure 3a.
Paleocurrent analysis indicates that the paleoslope at the time
of deposition of the lower Zhada Formation was northwestward (to the right in Figure 3a). This would have been
impossible if the basin was in its current configuration
because both the southeast (Southeast Zhada section) and
northwest portions of the basin (1Northwest Zhada and
Qusum sections) are higher than the center of the basin. This
could result from subsidence of the basin center or from
uplift of the northwestern margin of the basin. Regardless,
it provides evidence for removal of footwall rocks from
beneath the center of the basin toward the northwest, and
possibly southeast, of the basin.
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[37] Migmatites and paragneisses were exhumed from
midcrustal levels in both the Leo Pargil/Qusum Range and
the Gurla Mandhata dome [Zhang et al., 2000; Murphy et
al., 2002; Thiede et al., 2006]. Kinematic analysis indicates
top-to-the-east sense of shear on the Qusum detachment
[Zhang et al., 2000] and top-to-the-west sense of shear on
the Gurla Mandhata detachment [Murphy et al., 2002]. In
both cases, the midcrustal footwall rocks have been exhumed
from beneath the Zhada basin synchronously with sedimentation in the hanging wall. The thin basin fill, short-lived
extension and low subsidence rate are characteristic of supradetachment basins [Friedmann and Burbank, 1995], which is
generally consistent with the regional tectonic setting of
Zhada basin in the hanging walls of the Qusum and Gurla
Mandhata detachments.
[38] The elongate shape of the Zhada basin reflects the
spatial extent of subsidence caused by exhumation. The
Gurla Mandhata detachment is kinematically linked to and
spatially limited on the north side by the Karakoram fault
[Murphy et al., 2000, 2002; Murphy and Burgess, 2006].
Although a similar relationship has not been documented
for the Leo Pargil/Qusum system, the abrupt northward
termination of arc-parallel extension as both detachment
faults approach the Karakoram fault, the strain compatibility
between the Leo Pargil and Qusum detachments and the
Karakoram fault, and the synchronicity in deformation
between the Leo Pargil/Qusum Range and the Karakoram
fault indicate that all three systems, Karakoram, Leo Pargil/
Qusum, and Gurla Mandhata, may be kinematically linked
[Zhang et al., 2000; Phillips et al., 2004; Thiede et al.,
2006; Valli et al., 2007], and that the Zhada basin is
therefore a hybrid supradetachment/strike-slip basin. One
attractive aspect of this interpretation is that these structures
can accommodate arc-parallel extension and be the primary
controls on Zhada basin evolution in the absence of a large
strike-slip faulting on the southwestern margin of the basin
(see section 7).
[39] The onset of sedimentation in Zhada basin at !9.2 Ma
is within error of the oldest AFT ages determined by Thiede
et al. [2006] from the west side of the Leo Pargil Dome. The
increase in exhumation rate indicated by the AFT data was
also synchronous with an increase in tectonic subsidence
rate at 5.23 – 2.58 Ma and with the onset of lacustrine
sedimentation in the Zhada basin [Thiede et al., 2006]. This
further implicates exhumation of the Qusum/Leo Pargil
Range as a controlling factor in Zhada basin development.
The synchronicity of movement on the Karakoram fault in
the Zhada region (10 Ma [Yin et al., 1999; Murphy et al.,
2000]) and exhumation of Gurla Mandhata (9 Ma [Murphy
et al., 2002]) with the 9 – 10 Ma AFT ages from Leo Pargil
and the 9.2 Ma onset of sedimentation in Zhada basin
points to the late Miocene as the time of major structural
reorganization in the area.
[40] The presence of !15 Ma 40Ar/39Ar cooling ages,
from both the west side of the Leo Pargil Dome and the east
side of the Leo Pargil/Qusum Range, has been used to argue
for a mid-Miocene onset of exhumation [Zhang et al., 2000;
Thiede et al., 2006]. If true, the fact that sedimentation in
Zhada basin did not begin until the onset of exhumation of
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Figure 13. (a) LandSat image of the northwestern corner of the Zhada basin showing a wind gap,
possibly representing the course of the paleo-Sutlej River. (b) A view to the southeast from within the
valley in Figure 13a. Inset box in Figure 13a is enlarged in Figure 13b. Note the relict surface that has
been abandoned and incised by the modern river which occupies this valley. The cliff face indicated by
‘‘a’’ is !20 m tall.
the Gurla Mandhata dome, despite apparent ongoing exhumation of the Leo Pargil/Qusum Range, indicates that the
combination of sill uplift and basin subsidence was necessary
for basin formation in this region. However, in an alternate
scenario, mid-Miocene 40Ar/39Ar cooling ages may reflect
cooling during slip on the Main Central Thrust [Robinson et
al., 2003, 2006]. This is suggested by the abundance of early
to mid-Miocene 40Ar/39Ar cooling ages from the Greater
Himalayan and Tethyan sequences on the south flank of the
Himalaya [Metcalfe, 1993; Vannay et al., 2004; Thiede et al.,
2005; Robinson et al., 2006]. Thus, the mid-Miocene cooling
ages obtained from the Leo Pargil/Qusum Range may be
inherited from tectonic events related to arc-normal thrusting
rather than arc-parallel extension. Determining which scenario is accurate, and hence whether exhumation of Gurla
Mandhata played a role in subsidence and sedimentation in
Zhada basin, must await additional research.
[41] Finally we point to an anomalous valley, possibly a
wind gap, striking orthogonally across the Leo Pargil/Qusum
Range (32.33!N, 79!E; Figure 13) as the possible outlet of
the paleo-Sutlej River. This is the only significant valley in
an otherwise unbroken mountain range. The valley extends
across the entire width of the range and yet is currently
occupied by an underfit stream <1 m deep and <5 m wide.
This stream originates at a glacier midway through the valley
and thus cannot account for a valley transecting the entire
range. The valley is mantled by terrace deposits that have
recently been incised (Figure 13b). We tentatively interpret
this valley as the course of the paleo-Sutlej River prior to
exhumation of the Leo Pargil/Qusum Range. Whereas this
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interpretation remains speculative pending further investigation, if true, the paleo-Sutlej may have been the headwaters of
either the Indus or the Chenab Rivers as suggested by
Brookfield [1998].
[42] Several explanations for the existence of the Zhada
basin can be ruled out based on the variety and stacking
patterns of lithofacies assemblages in the Zhada Formation.
The basin exhibits none of the lithologic, stratigraphic or
structural features of flexural or rift basins. Specifically,
there are no thrust or high-angle normal faults adjacent to the
basin that were active during the mid to late Miocene. The
tectonic subsidence curve does not match the classic upward
convex flexural basin subsidence curve, and the uniformly
upward coarsening textural trend and strong asymmetry
typical of flexural basins are absent (Figures 3 and 6). Unlike
rift basins, where subsidence is maximized adjacent to the
master fault [Gawthorpe and Leeder, 2000], the greatest
subsidence took place in the middle of the basin; far removed
from any structures. The basin lacks the wedge-shaped or
steep-sided cross-sectional geometry which might indicate
the presence of a half-graben or graben. Whereas the subsidence curve superficially matches that of other rift basins,
the Zhada Formation is much thinner than typical rift basin
fill [Friedmann and Burbank, 1995] and mapping of the
northeastern and southwestern margins of the basin revealed
no geometric relationships other than onlap of basin sediments onto the basin bounding highlands. In no place along
these margins is the basin fill faulted.
[43] The duration of the basin and the lack of incision or
hiatuses in sedimentation indicate that climate change was
not the direct cause of sedimentation. It is difficult to envision
a scenario in which climate change, acting alone, would
initiate sediment accumulation and basin development, produce the variety of sedimentary environments archived in
the Zhada Formation, drive continuous sedimentation for
!9 Myr, and then abruptly terminate deposition and return
the region to an erosional, incising state. The basin could also
be the result of damming by landslides, glaciers or tectonic
activity. However, the duration of the basin and the lack of
glacial deposits argue against the first two causes, leaving
only structural damming as a viable possibility.
[44] The South Tibetan detachment system (STDS) had
little to no effect on the Zhada basin. The Zhada Formation
on the southwestern margin of the basin is undeformed and
onlaps preexisting topography on the southwestern margin
of the basin as far southward as the trace of the STDS.
Where documented elsewhere in the Himalaya, the youngest slip on the STDS predates the onset of sedimentation in
Zhada basin [Hodges et al., 1992, 1996; Searle et al., 1997;
Murphy and Harrison, 1999; Hodges, 2000; Murphy and
Yin, 2003; Searle et al., 2003; Yin, 2006; Cottle et al., 2007].
In contrast to quiescence on the STDS, kinematic analyses
of the Gurla Mandhata and Qusum detachments indicate
that these faults accommodate arc-parallel extension and are
actively exhuming midcrustal rocks from beneath the Zhada
basin [Zhang et al., 2000; Murphy et al., 2002]. Structural
and isotopic analysis also indicates that the Gurla Mandhata
detachment cuts the STDS, indicating that slip on the STDS
TC1004
had ceased prior to 9 Ma [Murphy et al., 2002; Murphy and
Copeland, 2005; Murphy, 2007].
7. Synthesis
[45] We divide the evolution of the Zhada basin into
5 stages (Figure 14). Before 9.2 Ma the Zhada basin was
occupied by a large, northwestward flowing, braided river
which had its headwaters in the Mount Kailash region
(Figure 14a). Exhumation of the Leo Pargil/Qusum Range
and the Gurla Mandhata gneiss dome along a system of
northeast striking detachment faults beginning at !10–
9.2 Ma decreased the fluvial gradient and caused ponding and
upstream sediment accumulation. Before being completely
defeated, the paleo-Sutlej River cut and flowed through the
valley identified above as a wind gap (Figure 14b). The
combination of sill uplift and basin subsidence (owing to
simultaneous footwall uplift and hanging wall subsidence
during slip on the detachments) resulted in basin closure
and lacustrine and alluvial fan sedimentation upstream of
the dam between !5.6 Ma and <1 Ma. The Zhada basin
became a holding pond for detritus derived from local highlands (Figure 14c). This situation continued until the sill was
breached at the current outlet in the southwestern corner of
the basin at <1 Ma (Figure 14d), probably by either headward erosion or overtopping. Following breach of the sill the
modern Sutlej River system began to reequilibrate to the new
base level resulting in upstream migration of a knickpoint
and incision of the Zhada Formation (Figure 14e). A similar
geomorphological evolution characterized the Thakkhola
graben in central Nepal [Garzione et al., 2003]. Integration
of the modern Sutlej drainage network and enhanced erosion
associated with upstream knickpoint migration also provide
a mechanism to explain the post-5 Ma reorientation of transHimalayan rivers and increase in Himalayan, as opposed to
suture zone, detritus delivered to the Indus fan [Clift and
Blusztajn, 2005].
[46] Arc-parallel extension is clearly accommodated by
extension across the Leo Pargil/Qusum Range and Gurla
Mandhata, and by dextral slip on the Karakoram fault [Zhang
et al., 2000; Murphy et al., 2002; Phillips et al., 2004; Thiede
et al., 2006]. It is unclear which structures accommodate
extension on the southwestern side of Zhada basin. No largescale structure comparable to the Karakoram fault is present
on this side of the basin. It is possible that extension is
taken up by distributed faulting directly south of the basin
(Figure 15b). Another possibility is that slip on the Leo Pargil
and Qusum detachments is transferred westward or northwestward, a process similar to that described by Murphy
and Copeland [2005] for the Gurla Mandhata detachment
(Figure 15b).
[47] The arc-parallel extension process provides a mechanism to explain the loss of elevation suggested by Saylor et
al. [2009]. They inferred a 0.8– 1.5 km loss of elevation
based on oxygen isotope paleoaltimetry. Assuming Airy
isostacy, that loss of elevation would require between 4.5
and 8 km of crustal thinning. This could be achieved by two
interacting detachment fault systems below the Zhada basin
19 of 24
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SAYLOR ET AL.: ZHADA EVOLUTION ARC-PARALLEL EXTENSION
TC1004
Figure 14. Perspective diagram showing the interpreted basin evolution of the Zhada basin from a
throughgoing river to a closed-basin lake before final breaching of a new sill resulting in integration of
the modern Sutlej River.
(Figure 15d). The Gurla Mandhata detachment fault excavates structurally deeper rocks [Murphy, 2007] and thus is
inferred to cut the Qusum detachment fault. The zone of
interaction of these two fault zones is inferred to be below
the zone of maximum subsidence (the depocenter) of the
Zhada Basin. A flat in the Gurla Mandhata detachment is
inferred based on a zone of minimal subsidence (and nondeposition) between the Pulan and Zhada basins. Flattening
20 of 24
SAYLOR ET AL.: ZHADA EVOLUTION ARC-PARALLEL EXTENSION
TC1004
TC1004
Figure 15. Paleogeographic maps showing the influence of arc-parallel extension on the Sutlej River
and Zhada basin. (a) Prior to 9.2 Ma the paleo-Sutlej River drained regions north of the Indus suture and
flowed to the northwest. (b) With the onset of arc-parallel extension and exhumation of the Gurla
Mandhata and Leo Pargil/Qusum core complexes the Sutlej River begins to pond, eventually forming the
large Zhada paleolake. Arc-parallel extension is accommodated on the southwestern side of the basin
either by distributed shortening, shown schematically by circle a, or by transferring extension to the west
or northwest (circle b). (c) In the Pleistocene a new sill is breached in the west of the basin, resulting in
the integration of the modern Sutlej River system. (d) NW – SE cross section showing the relationship
between extension on the Qusum and Gurla Mandhata detachment faults and sedimentation in the Zhada
and Pulan basins. The cross-section line is indicated in Figure 15c.
of the Gurla Mandhata detachment is also indicated by
rollover structures in the Pulan basin fill. In Figure 15d,
the preextension depth of the STDS is inferred from
DeCelles et al. [2001]. Crustal thinning and the associated
elevation loss also explain the anomalously low elevations in
the Zhada region.
8. Conclusions
[48] The sedimentary facies in the Zhada formation show
that depositional environments in the Zhada basin included
braided fluvial, marshy wetland, lacustrine, dune field, sand
flat, lacustrine fan delta, and alluvial fan systems. Throughout the basin’s history the climate was sufficiently arid to
produce dune fields, evaporative marshes, evaporative sand
flats, and bedded gypsum. These environmental conclusions
are consistent with high d18O values from lacustrine gastropods [Saylor et al., 2009].
[49] Paleocurrent data indicate that the Zhada basin
evolved from a northwestward draining river valley to a
broad depositional plain and finally to a largely closed basin.
Closure of the basin resulted in lacustrine and lake margin
21 of 24
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SAYLOR ET AL.: ZHADA EVOLUTION ARC-PARALLEL EXTENSION
alluvial fan sedimentation until the drainage sill was
breached in the southwest corner of the basin. The change
in sedimentation style was accompanied by a change in
sediment provenance, from a volcanic-rich source in the
Mount Kailash region, to more local metamorphic/clastic
sources, mainly in the Tethyan and Greater Himalayan
sequence rocks evidenced by modal petrography and detrital
zircon U-Pb frequency spectra.
[50] Sedimentation in the basin was due to a combination
of topographic damming by sill uplift and tectonic subsidence. Integrating this basin history with existing regional
structural models suggests that the Zhada basin is the result
of arc-parallel extension which was accommodated on
crustal-scale detachment faults at the northwest and possibly
TC1004
southeast extremities of the basin (Figure 15). Subsidence
likely took place in response to vertical thinning and
exhumation of the midcrust during late Miocene detachment
faulting.
[51] Acknowledgments. The authors would like to thank Richard
Heermance for his contribution to this paper. Thanks also for field
assistance from Jay Quade, Cai Fulong, Jeannette Saylor, and Scott
McBride. We are thankful for thorough reviews by two anonymous reviewers
and Tectonics Associate Editor Eric Kirby which helped to improve this
manuscript. U-Pb geochronological analyses were conducted with support
from the National Science Foundation grants EAR-0443387 and 0732436
with lab help from Victor Valencia. Additional support was provided by the
National Science Foundation Tectonics Program, ExxonMobil, ChevronTexaco, and the Galileo Circle of the University of Arizona.
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""P.""
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DeCelles, G. Gehrels, and P. Kapp, Department
of Geosciences, University of Arizona, Tucson, AZ
85721, USA.
M. Murphy and R. Zhang, Department of Earth and
Atmospheric Sciences, University of Houston, Houston,
TX 77204, USA.
J. Saylor, Department of Geological Sciences, Jackson
School of Geosciences, University of Texas at Austin,
Austin, TX 78712, USA. (jsaylor@mail.utexas.edu)