Geological records of the Lhasa-Qiangtang and Indo-Asian collisions
in the Nima area of central Tibet
Paul Kapp†
Peter G. DeCelles
George E. Gehrels
Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA
Matthew Heizler
New Mexico Geochronological Research Laboratory, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA
Lin Ding
Institutes of Tibetan Plateau Research and Geology and Geophysics, Chinese Academy of Sciences, Beijing 100085, People’s Republic
of China
ABSTRACT
A geological and geochronologic investigation of the Nima area along the Jurassic–
Early Cretaceous Bangong suture of central
Tibet (~32°N, ~87°E) provides well-dated
records of contractional deformation and
sedimentation during mid-Cretaceous and
mid-Tertiary time. Jurassic to Lower Cretaceous (≤125 Ma) marine sedimentary rocks
were transposed, intruded by granitoids,
and uplifted above sea level by ca. 118 Ma,
the age of the oldest nonmarine strata documented. Younger nonmarine Cretaceous
rocks include ca. 110–106 Ma volcanic-bearing strata and Cenomanian red beds and
conglomerates. The Jurassic–Cretaceous
rocks are unconformably overlain by up to
4000 m of Upper Oligocene to Lower Miocene lacustrine, nearshore lacustrine, and
fluvial red-bed deposits. Paleocurrent directions, growth stratal relationships, and a
structural restoration of the basin show that
Cretaceous–Tertiary nonmarine deposition
was coeval with mainly S-directed thrusting
in the northern part of the Nima area and Ndirected thrusting along the southern margin
of the basin. The structural restoration suggests >58 km (>47%) of N-S shortening following Early Cretaceous ocean closure and
~25 km shortening (~28%) of Nima basin
strata since 26 Ma. Cretaceous magmatism
and syncontractional basin development are
attributed to northward low-angle subduction of the Neotethyan oceanic lithosphere
and Lhasa-Qiangtang continental collision,
†
E-mail: pkapp@geo.arizona.edu.
respectively. Tertiary syncontractional basin
development in the Nima area was coeval
with that along the Bangong suture in westernmost Tibet and the Indus-Yarlung suture
in southern Tibet, suggesting simultaneous,
renewed contraction along these sutures during the Oligocene-Miocene. This suture-zone
reactivation immediately predated major
displacement within the Himalayan Main
Central thrust system shear zone, raising the
possibility that Tertiary shortening in Tibet
and the Himalayas may be interpretable in
the context of a mechanically linked, composite orogenic system.
Keywords: Tibet, plateau, thrust belt, Indo-Asian
collision, suture zone, basin development.
INTRODUCTION
The vast, internally drained region of the
Tibetan Plateau interior (Fig. 1A) is the focus
of some of the most provocative concepts in
continental tectonics today, yet our understanding of its geological evolution and uplift history
remains poor. Numerous popular models of
Tibetan Plateau formation assume that the thick
crust and high elevation of Tibet are mainly consequences of India’s collision with Asia since the
Eocene. There is growing documentation that
challenges this assumption; evidence shows that
large parts of southern Asia underwent major
pre–Indo-Asian collision crustal shortening and
thickening, including the Karakoram-Pamirs in
the west (Fraser et al., 2001; Hildebrand et al.,
2001; Robinson et al., 2004), ranges bordering the northern margin of the Tibetan Plateau
(e.g., Sobel, 1995; Sobel et al., 2001; Ritts and
Biffi, 2000, 2001; Jolivet et al., 2001; Robinson
et al., 2003), the Lhasa and Qiangtang terranes
in Tibet (Fig. 1B) (Murphy et al., 1997; Yin and
Harrison, 2000; Ding and Lai, 2003; Kapp et al.,
2003, 2005; Guynn et al., 2006), and the Longmen Shan along the eastern plateau margin (e.g.,
Arne et al., 1997; Wallis et al., 2003).
Another common assumption is that the
Tibetan Plateau interior was formed by mechanisms that are proposed to be acting along
the actively growing margins of the plateau
or predicted from geodynamic models. These
include (1) northward underthrusting/insertion
of Indian lithosphere (Argand, 1924; Powell
and Conaghan, 1973; Ni and Barazangi, 1984;
Zhao and Morgan, 1987; DeCelles et al., 2002),
(2) homogeneous lithospheric shortening and
thickening (Dewey and Burke, 1973; England
and Houseman, 1986; Dewey et al., 1988)
and subsequent removal of mantle lithosphere
(England and Houseman, 1989; Molnar et al.,
1993), (3) upper-crustal shortening coupled
with passive infilling of intermontane basins
and oblique intracontinental subduction along
reactivated suture zones (Mattauer, 1986; Meyer
et al., 1998; Roger et al., 2000; Tapponnier et
al., 2001), and (4) thickening and flow of weak
middle crust away from the India-Asia collision
zone, driven by topographic gradients (Bird,
1991; Royden, 1996; Royden et al., 1997; Clark
and Royden, 2000; Beaumont et al., 2001, 2004;
Shen et al., 2001).
Some necessary pieces of information fundamental to advancing models of plateau formation are quantitative constraints on the timing
and magnitude of pre–Indo-Asian collision
versus post–Indo-Asian collision shortening in
Tibet, and ultimately, changes in paleoelevation.
GSA Bulletin; July/August 2007; v. 119; no. 7/8; p. 917–932; doi: 10.1130/B26033.1; 9 figures; 1 table; 1 insert; Data Repository item 2007166.
For permission to copy, contact editing@geosociety.org
© 2007 Geological Society of America
917
Kapp et al.
80°E
90°E
35°N
35°N
JS
Ka
ra
ko
ra
m
Fig. 2
fau
lt
BS
Nima
Jiali
Him
30°N
A
ala
fault
ya
30°N
nT
MFT
80°E
hru
st
B
elt
0
Quaternary basin
80°E
200 km
IYS
Region of internal drainage
84°E
B
Lhasa
90°E
88°E
92°E
Songpan Ganzi
Jinsha suture
Fenghu
o Shan
Qiang
34°N
anticli tang ?
noriu
m
Ban
gon
g su
?
ture
-Hoh X
il
Qiangtang
terrane
?
?
Amdo
Shiquanhe
32°N
SGAT
Gaize
Lunpola
GST
Nima
H
im
al
ay
an
Coqin
Th
ru
st
30°N
Tarim
Pamir
Qa
ida
m
Sichuan
0
Early Cretaceous granite
suture zone
Lhasa terrane
Siling Co
Duba
GCT
Bel
t
Tibet
India
Fig. 2
Lhasa
200 km
Late Cretaceous - early Tertiary
Gangdese batholith
thrust fault
Indus-Yarlung suture
GT
normal fault
65-40 Ma volcanic rocks
42-30 Ma volcanic rocks
Tertiary strata
strike-slip fault
Figure 1. (A) Map showing the major sutures and distribution of late Cenozoic deformation and basins in southern and central Tibet. The
modern internally drained region of the Tibetan plateau is shown in gray. JS—Jinsha suture. (B) Tectonic map showing the major sutures
and distribution of Tertiary thrust faults and associated nonmarine basins in southern and central Tibet. The southern margin of Tibet
is defined geologically by the Indus-Yarlung suture zone (IYS), which was modified by the Oligocene Gangdese thrust (GT) and Miocene
Great Counter thrust (GCT). The Bangong suture zone (BS) was modified by the mid-Tertiary N-dipping Shiquanhe-Gaize-Amdo thrust
system (SGAT) and S-dipping Gaize–Siling Co thrust (GST). Figure modified from Kapp et al. (2005).
918
Geological Society of America Bulletin, July/August 2007
Geological records of the Lhasa-Qiangtang and Indo-Asian collisions
With this goal in mind, we conducted geological mapping and integrated geochronologic and
detailed stratigraphic-sedimentologic studies of
the Nima basin area (~32°N, ~87°E) along the
Jurassic–Early Cretaceous Bangong suture in
central Tibet (Fig. 1). Detailed measured sections and provenance studies of Nima basin
strata (DeCelles et al., 2007a) and oxygen and
carbon isotopic studies indicating high regional
paleoelevation (>4.6 km) and arid conditions
during the late Oligocene (DeCelles et al.,
2007b) are presented elsewhere. The purpose
of this contribution is to present our data and
interpretations pertaining to the geological setting, age, and structural evolution of Cretaceous
to Quaternary rocks in the Nima area and their
broader tectonic implications.
GEOGRAPHIC SETTING
The town of Nima is located along the
Mochang River, which flows north-northeastward through Nima and then to the east before
draining into Dagze Lake (Fig. 21). The modern
Nima basin is a part of a larger belt of discontinuous, elongate ~E-W–trending internally drained
basins, which are generally <20 km wide and
at ~4500 m elevation and are located along the
approximate surface trace of the Bangong suture
(Fig. 1A). The southern margin of the Nima basin
is bounded by a series of E-W–trending ranges
that exhibit a general increase in both width and
relief toward the south, where the southernmost
range has a width of up to 5 km and maximum
elevations of ~5400 m (Fig. 2). This region is
referred to as the southern Nima area. The Nima
basin is bounded along its western margins by
~N-trending, moderate-relief (~300 m) ranges,
~4 km wide and up to 10 km long. The westernmost extent of the Nima basin is the Puzuo Lake
subbasin (Fig. 2), and the surrounding geology
is referred to as the Puzuo Lake area. North
of Dagze Lake, the Nima basin is bounded by
an up to 20-km-wide series of E-W–trending
ranges that exhibit a general decrease in width
and total relief to the north toward the southern
flank of the Muggar Range (Fig. 2). This region
is referred to as the northern Nima area. The ~15km-wide Muggar Range is locally dissected by
active ~N-trending rifts (Fig. 2) but is regionally
E-W–trending and contains glaciated peaks with
elevations in excess of 6100 m.
MAP UNITS
Map units in the Nima area range in age
from Jurassic to Quaternary. The oldest rocks
1
Figures 2 and 8 are on a separate sheet accompanying this issue.
are exposed in the Muggar Range and consist
of banded argillite interbedded (or tectonically
interlayered) with an ~50-m-thick massive
white limestone and thinly interbedded shale,
siltstone, turbiditic sandstone, fossiliferous
limestone, and metavolcanic rocks. The primary stratigraphy is unknown because this unit
has been greatly deformed and locally exhibits tight upright to overturned folds, penetrative cleavage, and transposed bedding. These
rocks are assigned a Jurassic age (mapped as Jr,
Fig. 2) and appropriately described as sedimentary-matrix mélanges on regional geological
maps (Cheng and Xu, 1986; Pan et al., 2004).
Similarly deformed rocks are widely exposed
south of the Muggar Range in the Nima area.
Where studied, they consist largely of cleaved
shale, siltstone, and turbiditic sandstone with
subordinate metasedimentary-matrix mélange
interlayered with greenschist-facies metabasites. Regional geological maps show contrasting Mesozoic age assignments for these rocks.
We assign a Jurassic to Early Cretaceous age
for clastic deposition and mélange formation
(labeled as J-K, Fig. 2) because this time interval spans that over which oceanic subduction
was occurring along the Bangong suture (e.g.,
Dewey et al., 1988; Yin and Harrison, 2000)
and our geochronologic results presented in the
subsequent section demonstrate that this unit is
locally as young as ca. 125 Ma. Jurassic strata
of the Muggar Range are intruded by a biotite
granite exposed north and west of Xiabie Lake
(Xiabie granite, Fig. 2), whereas the J-K unit is
intruded by a biotite granite west of Puzuo Lake
(Puzuo granite, Fig. 2).
The youngest marine rocks in the map area
consist of Aptian-Albian, shallow-marine, reeffacies limestone of the Langshan Formation
(e.g., Leeder et al., 1988), which are exposed
in the southernmost range of the southern
Nima area (Kl, Fig. 2). These marine rocks are
exclusively restricted to the hanging wall of the
S-dipping Gaize–Siling Co thrust fault (GST,
Fig. 2; Kapp et al., 2005). To the north of this
range, all map units younger than the J-K unit
are nonmarine. The deformed nonmarine strata
have been previously inferred to be Triassic
(Cheng and Xu, 1986), Cretaceous (Pan et al.,
2004), or Tertiary (Schneider et al., 2003). Our
geochronologic results, presented in the subsequent section, show that the nonmarine strata
can be divided into Lower Cretaceous, Upper
Cretaceous, Upper Cretaceous to Paleocene,
and mid-Tertiary intervals. These strata are
described briefly here, and detailed descriptions
and measured sections are provided in DeCelles
et al. (2007a).
In the northern Nima area, the oldest nonmarine strata lie unconformably on transposed
marine rocks of the J-K unit and consist of a
>400-m-thick Lower Cretaceous succession
of volcaniclastic conglomerate, sandstone, and
siltstone with tuffaceous and paleosol horizons
in the lower part (Kvc; Figs. 2 and 3). There is
a low-angle unconformity between the Kvc unit
and the overlying Lower Muggar unit (Kml;
Figs. 2 and 3), which consists of >400 m of
Upper Cretaceous to Paleocene (based on palynomorphs; DeCelles et al., 2007a) red beds, fluvial and eolian sandstones, and conglomerates,
with the fluvial deposits showing southward
paleocurrent indicators (Fig. 3). The Lower Muggar unit is in thrust-fault contact with Eocene to
Miocene (based on palynomorphs; DeCelles et
al., 2007a) siltstone, marl, evaporite, conglomerate, and sandstone of the Upper Muggar unit
to the north (Tmu; Figs. 2 and 3). Structural disruption and lateral variability in lithology make
it difficult to determine the relative ages of the
different measured sections in the Upper Muggar unit. However, we infer a general northward
decrease in the age of exposed Tertiary strata
across the northern Nima area.
In the Puzuo Lake and southern Nima areas,
the oldest nonmarine strata unconformably overlie the J-K unit and consist of Lower Cretaceous
volcanic flows, tuffs, and breccias interbedded
with volcaniclastic conglomerate and sandstone (Kv; Figs. 2 and 4). Upper Cretaceous red
beds (Kr) with volcaniclastic conglomerates lie
unconformably on both the Kv and J-K units in
the southern Nima area (Figs. 2 and 4). These
are in turn conformably overlain by two conglomeratic successions: a lower, ~660-m-thick
volcaniclastic conglomerate (Kcv) and an upper,
>730-m-thick conglomerate with dominantly
Aptian-Albian limestone clasts (Kcl) (Figs. 2
and 4). The transition from volcanic to limestone clast composition is abrupt, occurs over a
~20 m interval of mixed clast composition, and
is marked by a reversal in paleocurrent direction
(from southward to northward; Fig. 4). MidTertiary strata in the southern Nima area (Tr)
are best exposed in the frontal ranges south of
Dagze Lake and along the Mochang River west
and southwest of the town of Nima (Fig. 2).
They consist of a >1000-m-thick succession of
lacustrine marl, mudstone, and sandstone with
stacked conglomeratic fan-delta parasequences
overlain by a >3000-m-thick succession of red
clastic rocks and thin marl beds (Fig. 4). The
lacustrine marls in the lower part of the Tertiary
succession contain biotitic sandy tephra layers.
The entire Tertiary succession in the southern
Nima area shows generally northward to northeastward paleocurrent directions (Fig. 4).
The mid-Tertiary units are overlain by generally poorly exposed younger units that were
not studied in detail. Gently dipping (~10–20°)
Geological Society of America Bulletin, July/August 2007
919
Kapp et al.
Xiabie granite. Southern Nima area conglomerates were likely derived from the nearby range
of Aptian-Albian limestone to the south (Fig. 2)
because their clasts consist almost entirely of
this limestone. More gently dipping (<10°)
Neogene to Quaternary(?) deposits are also
locally exposed (N-Q, Fig. 2). Near the town
alluvial fan conglomerates of inferred Neogene
age occur in both the northern and southern
Nima areas (Nf, Fig. 2) and lie unconformably
on Tertiary and older strata. The compositions
of clasts in the northern Nima area conglomerates are similar to those of rocks exposed within
the Muggar Range to the north, including the
Northern Nima Area
Tmu
>800 m
Legend
conglomerate
eolianite
sandstone
paleocurrent
marl
direction
silt/sandstone
mud/siltstone
Kml
>400 m
thrust fault contact
Kvc
>400 m
angular unconformity (<10°)
volcaniclastic rocks
paleosols
ca. 118 Ma tuffs; ( 4MK28, 175)
volcanic-clast conglomerate
J-K Unit (Jurassic - Lower
Cretaceous transposed rocks)
silt/clay
sand conglomerate
Figure 3. Schematic Cretaceous–Tertiary stratigraphic column of the northern Nima area
based on measured sections (1MK-6MK; Fig. 2). See DeCelles et al. (2007a) for detailed
measured sections, descriptions, and interpretations of the stratigraphy.
TABLE 1. SUMMARY OF GEOCHRONOLOGIC RESULTS
Sample name
Latitude Longitude
Description
Interpreted age
(°N)
(°E)
(Ma)
7-14-98-2
31.90
87.17
Puzuo granite
124 ± 4
7-19-98-2
32.22
87.22
Xiabie granite
118 ± 4
7-11-05-2
31.76
87.52
Sandstone
≤125 ± 5
4MK28
32.07
87.44
Tuff
118 ± 3
4MK175
32.07
87.44
Tuff
117 ± 2
6-11-04-4
31.75
87.52
Sandstone
≤106 ± 2
3MC13
31.72
87.52
Tuff
99 ± 2
6-11-04-2
31.71
87.53
Sandstone
≤97 ± 2
8-6-03-1
31.77
87.10
Reworked tuff
26.0 ± 0.2
8-6-03-2
31.77
87.10
Reworked tuff
26.1 ± 0.2
2NM170
31.75
87.10
Reworked tuff
25.3 ± 0.2
1DC82
31.82
87.67
Reworked tuff
25.8 ± 0.4
1DC367
31.81
87.67
Reworked tuff
24.9 ± 0.2
8-13-03-1
31.81
87.47
Reworked tuff
23.5 ± 0.2
920
of Nima, these deposits consist of gray fluvial
conglomerate with clasts of foliated granite,
and they show eastward paleocurrent indicators
(Fig. 4). Neogene to Quaternary deposits in the
northern Nima area consist of alluvial fan conglomerates along the southern flank of the Muggar Range, fluvial conglomerates exposed near
the N-trending valley between Xiabie Lake and
Dagze Lake, and green lacustrine(?) deposits
standing at elevations as high as 4900 m north
of Dagze Lake (N-Q, Fig. 2). Quaternary alluvium is divided into deposits that show evidence
of incision and lake shoreline development and
those that are actively being reworked (Q1 and
Q2, respectively, Fig. 2).
GEOCHRONOLOGY
U-Pb Methods
U-Pb spot analyses were conducted on single
zircons separated from samples of two intrusive
rocks, three volcanic tuffs, and three sandstones
in the Nima area using a Micromass Isoprobe
multicollector inductively coupled plasma–mass
spectrometer (MC-ICP-MS) at the Arizona
Laserchron Center. The zircons were ablated
using an Excimer laser with a spot diameter of
35 µm. Elemental fractionation between U and
Pb was monitored by analyzing fragments of a
large Sri Lankan zircon with a concordant isotope-dilution thermal ionization mass spectrometry (ID-TIMS) age of 564 ± 4 Ma in between
sets of 3–5 analyses on unknown grains. Common Pb was corrected for using measured
204
Pb and assuming an initial Pb composition
from the model of Stacey and Kramers (1975).
The majority of the zircon grains analyzed in
this study were young (Cretaceous) and hence
yielded low concentrations of 207Pb relative to
206
Pb. Consequently, 206Pb*/207Pb* and 207Pb*/
235
U ages have significantly higher uncertainties than 206Pb*/238U ages and are excluded from
consideration in our age interpretations. Our
approach in dating the igneous rocks was to analyze a relatively large number of zircon grains
from each sample (>25) and interpret the mean
age of the youngest population of zircon ages
as the crystallization age. The uncertainties in
the mean ages are cited at the 2σ level, include
all known analytical and systematic errors, and
are in the range of 2%–4%. For the sandstone
samples, we use the youngest population of
zircon ages, rather than the youngest individual
zircon age determined, to provide constraints
on maximum depositional ages. Interpreted
crystallization and maximum depositional ages
for igneous and sandstone samples, respectively, are summarized in Table 1. Additional
details of the analytical methods and a complete
Geological Society of America Bulletin, July/August 2007
Geological records of the Lhasa-Qiangtang and Indo-Asian collisions
tabulation of the U-Pb data are available in the
GSA Data Repository.2
Southern Nima Area
N-Q
gray granite-clast conglomerate
U-Pb Results
2
GSA Data Repository item 2007166, U-Pb and
Ar/ 39Ar methodology and results, is available at
http://www.geosociety.org/pubs/ft2007.htm, or on
request from editing@geosociety.org.
40
Tr
~4000 m
volcanic breccia
volcanic rocks
lacustrine marl, rippled sandstone
and siltstone
stacked sequences of
conglomerate, marl, and
siltstone; lacustrine fan-delta
deposits
red siltstone and rippled sandstone
angular unconformity
Figure 4. Schematic Cretaceous–Tertiary stratigraphic
column of the southern
Nima area based on measured sections (1MC-3MC,
1DC-4DC, and 1NM-2NM;
Fig. 2). See DeCelles et al.
(2007a) for detailed measured sections, descriptions,
and interpretations of the
stratigraphy.
Kcl
>730 m
syncontractional growth
limestone-clast
conglomerate
Kcv
~660 m
90 - 105 Ma, detrital zircons
(6-11-04-2DZ)
Kr
~180 m
Kvc Unit
Two tuffs were sampled from a measured
section of the Kvc unit in the northern Nima
area (4MK; Fig. 3) at stratigraphic heights
of 28 m and 175 m (4MK28 and 4MK175,
respectively). Of the 27 zircon grains analyzed
from 4MK28, 18 define a dominant population between 105 Ma and 125 Ma, with a mean
age of 118 ± 3 Ma (Fig. 5D). A component of
inheritance is indicated by the presence of eight
older grains, ranging in age from Jurassic to
Proterozoic (Fig. 5D). Zircon ages (n = 25) from
sample 4MK175 show a single population with
a mean age of 117 ± 2 Ma (Fig. 5E). The statistically indistinguishable mean ages provided
Legend
mud/siltstone
silt/sandstone
marl
sandstone paleocurrent
conglomerate direction
red beds
Kv
>200 m
J-K Unit
A sample of marine turbiditic sandstone from
the transposed J-K unit in the southern Nima
area (7-11-05-2; Fig. 2) yields a broad distribution of detrital zircon ages that range from Cretaceous to Archean (Fig. 5C). The provenance significance of the detrital zircon age populations
between ca. 1.8 Ga and ca. 1.9 Ga and between
ca. 220 Ma and ca. 280 Ma will be discussed
elsewhere. The mean age of the youngest population of ages (125 ± 5 Ma) is more relevant to
this study because it provides a maximum depositional age for this sample and the unconformably overlying Kv unit. This is the first robust
documentation that marine deposition persisted
along the central Bangong suture north of the
Gaize–Siling Co thrust until at least ca. 125 Ma.
fluvial red beds
J-K
Intrusive Rocks
The Puzuo granite intrudes the J-K unit and
is unconformably overlain by volcanic flows
and tuffs of the Kv unit (Fig. 2). Ages of 26 zircon grains from a sample of the Puzuo granite
(7-14-98-2; Fig. 2) yield a single population
with a mean age of 124 ± 4 Ma (Fig. 5A). This
result provides a minimum age for the J-K unit
in the Puzuo Lake area and a maximum age for
the overlying volcanic rocks. Zircon grains (n =
27) from a sample of Xiabie granite (7-19-98-2;
Fig. 2) yield a population of ages between
110 Ma and 125 Ma, with one older age of
ca. 140 Ma. We interpret the latter age to be
inherited and the mean age of the population as
the crystallization age (118 ± 3 Ma; Fig. 5B).
syncontractional
growth
volcaniclastic conglomerate
redbeds
volcaniclastic conglomerate, rippled
sandstone; ca. 99 Ma tuff, (3MC13)
angular unconformity
100 - 115 Ma, detrital zircons
(6-11-04-4DZ); volcanic flows, tuffs,
breccias, sandstone
angular unconformity
marine turbiditic sandstone; phyllitic; youngest
detrital zircons ca. 125 Ma (7-11-05-2)
silt/clay sand conglomerate
Geological Society of America Bulletin, July/August 2007
921
Kapp et al.
7
5
4
7-14-98-2
Puzuo granite
n = 26
Age (Ma)
6
A
3
2
6
100
120
140
B
7-19-98-2
Xiabie granite
n = 27
4
2
0
70
90
110
130
150
C
Age (Ma)
1
100
130
1000
1500
2000
190
2500
800
120
1200
140
1600
160
100
160
140
120
Age (Ma)
4MK175
tuff
n = 25
2
120
110
130
150
170
4MK28 (tuff)
118 ± 3 Ma, MSWD = 1.9, n = 18
150
140
130
120
110
100
1
110
7-11-05-2 mean of youngest
population: 125 ± 5 Ma
MSWD = 1.3, n = 12
140
90
E
90
7-19-98-2 (granitoid)
118 ± 3 Ma, MSWD = 2.3, n = 26
180
100
2000
3
0
70
110
130
6
4
120
80
3000
n = 18
100
mean of main population
130
100
D
5
4
3
2
1
0
80
400
160
Age (Ma)
500
0
70
4MK28
tuff
n = 27
5
140
7-14-98-2 (granitoid)
124 ± 4 Ma, MSWD = 1.4, n = 26
80
170
n = 12
7-11-05-2 3
sandstone
n = 99 2
0
7
115
90
4
0
125
95
160
Age (Ma)
8
135
105
1
0
80
10
145
90
4MK175 (tuff)
117 ± 2 Ma, MSWD = 1.3, n = 25
Age (Ma)
922
Geological Society of America Bulletin, July/August 2007
Geological records of the Lhasa-Qiangtang and Indo-Asian collisions
5
4
110
130
G
2
1
8
100
110
120
130
H
6-11-04-2
sandstone
n = 43
Age (Ma)
10
6
4
2
0
70
120
105
75
150
3
90
1 16
1 12
108
104
100
96
92
88
84
90
100
110
Age (Ma)
120
130
140
3MC13 (tuff)
99 ± 2 Ma, MSWD = 2.4, n = 32
1 10
106
102
98
94
90
80
6-11-04-4, mean of youngest population
106 ± 2 Ma, MSWD = 1.5, n = 34
90
3MC13
tuff
n = 32
0
80
12
150
135
Age (Ma)
6
F
Age (Ma)
9
8
6-11-04-4
7 volcaniclastic
sandstone
6
n = 40
5
4
3
2
1
0
70
90
7
86
6-11-04-2
97 ± 2 Ma, MSWD = 2.8, n = 33
Figure 5 (on this and previous page). Relative probability and histogram plots of U-Pb zircon ages (left column) and weighted mean ages
of the youngest zircon populations (right column). See Figure 2 for sample locations. Plots were made using Isoplot 3.00 of Ludwig (2003).
MSWD—mean square of weighted deviates.
by the two tuff samples are reassuring that our
approach in assigning crystallization ages yields
accurate results, as we would not expect the
ages to differ in age by more than the obtainable
precision given their stratigraphic separation of
only 147 m. Furthermore, the tuff ages are statistically indistinguishable from the interpreted
crystallization age of the Xiabie granite, suggesting that intrusive and extrusive rocks of the
northern Nima area may be related to a single
magmatic episode.
Kv Unit
Four samples of volcanic rocks from the Kv
unit in the southern Nima area were processed
for heavy mineral separation but did not yield
enough zircons to justify analysis. As an alternative to constraining the age of this unit, we
analyzed a sample of volcaniclastic sandstone
interbedded with the volcanic rocks that was
comparatively enriched in zircon (6-11-04-4;
Fig. 2). Ages of 40 zircon grains define a single
population, tailing off slightly to older ages, and
give a mean age of 106 ± 2 Ma (Fig. 5F). A conservative interpretation is that this age provides
a maximum depositional age for the volcaniclastic sandstone. However, considering that the
source of the zircon grains is most likely from
nearby volcanic rocks, we infer that the Kv unit
includes volcanic rocks with crystallization ages
of ca. 106 Ma.
with a mean age of 99 ± 2 Ma (Fig. 5G). This age
is consistent with stratigraphic relations indicating that the Kr unit is younger than ca. 106 Ma
volcanic rocks of the Kv unit. A sample of sandstone from the upper part of the conformably
overlying Kcv unit (6-11-04-2; Fig. 2) yields
43 detrital zircon ages that define a population
between 90 and 105 Ma, with a mean age of 97
± 2 Ma (Fig. 5H). This age provides a maximum
depositional age for the Kcv unit and, considering the stratigraphic relations with the underlying ca. 99 Ma Kr unit, is interpreted to closely
approximate the depositional age.
Kr and Kcv Units
Zircon grains (n = 32) from a tuff sampled from
near the base of the Kr unit in the southern Nima
area (3MC13; Fig. 2) yield a single population
40
Ar/ 39Ar Methods
The 40Ar/ 39Ar studies were conducted on seven
biotite separates from tuffaceous horizons in
Geological Society of America Bulletin, July/August 2007
923
Kapp et al.
Tertiary strata from the southern Nima area at
the New Mexico Geochronological Research
Laboratory. All samples were analyzed by the
single-crystal laser fusion method, and six of
the seven biotite separates were also analyzed
as bulk samples using the incremental stepheating method. In addition, a step-heating
experiment was conducted on K-feldspar from
the Xiabie granite with the aim of calculating
a thermal history using the multidomain diffusion (MDD) model (Lovera et al., 1989, 1997).
Details of the analytical methods and complete
data tables and figures summarizing the results
are available in the GSA Data Repository (see
footnote 2).
Biotite 40Ar/39Ar Age Results
Biotite was analyzed from seven tuffaceous
horizons collected from three different measured
sections in the southern Nima area. There are
two samples from a stratigraphic height of 70 m
(8-6-03-1 and 8-6-03-2) from section 2NM in the
far west (Fig. 2) and one sample from a height of
170 m (2NM170). Three samples from the 1DC
section located in the eastern part of the map area
(Fig. 2) are from stratigraphic heights of 77 m,
82 m, and 367 m (1DC77, 1DC82, and 1DC367,
respectively). One sample (8-13-03-1) is from
the northernmost measured section in the central
part of the southern Nima area (5DC; Fig. 2).
Figure 6. Summary of 40Ar/39Ar dating results. Samples are arranged in stratigraphic order.
All errors are shown as 1σ. WMA—weighted mean age; SC—single-crystal age; TGA—
total gas age.
924
Laser single-crystal and step-heating age
results are summarized in Figure 6. The precision of the single-crystal laser fusion ages is
significantly lower than that for ages determined
from step-heating analyses, primarily because
of the small argon signals provided by the individual grains. One problem with some of the
single-crystal data (1DC77, 1DC82, 1DC367,
8-13-03-1) is isochron ages that are older than
their corresponding weighted mean ages. This
problem stems from isochron regressions that
project to 40Ar/ 36Aro values that are less than
atmosphere, which are probably related to minor
argon loss from some grains and/or inaccurate
regressions related to a high degree of sensitivity to systematic errors for the very small argon
signals. In general, the step-heating age spectra
are characterized by initial steps that yield relatively young ages compared to the majority of
the higher-temperature steps, which give apparent ages between 24 Ma and 27 Ma (see GSA
Data Repository; footnote 2). Isochron data for
the step-heated samples yield ages either within
error of the weighted mean spectra ages or
younger. For several of the samples (8-6-03-1,
8-6-03-2, 8-13-03-1, 2NM170), the isochron
results suggest excess argon contamination
because 40Ar/36Ar trapped values are greater than
atmosphere (295.5). The minor complexity of the
age spectra may be related to 39Ar recoil distribution (e.g., Lo and Onstott, 1989). In this case,
the total gas ages may be more accurate than the
weighted mean ages. We note that all of the total
gas ages determined are stratigraphically consistent (Fig. 6) and interpret them in such a way as
to provide the best means of assigning an age to
each sample. These are as follows: 26.0 ± 0.1 Ma
for 8-6-03-1, 26.1 ± 0.1 Ma for 8-6-03-2, 25.3
± 0.1 Ma for 2NM170, 25.8 ± 0.2 Ma for
1DC82, 24.9 ± 0.1 Ma for 1DC367, and 23.5
± 0.1 Ma for 8-13-03-1 (analytical uncertainties cited at the 1σ level; Table 1). Singe-crystal laser fusion analyses on sample 1DC77, for
which step-heating analyses were not conducted,
yielded discordant weighted mean and isochron
ages that both differed from the total gas age for
sample 1DC82 collected from a similar stratigraphic height; therefore, this sample was not
assigned an age. A more detailed discussion of
the 40Ar/39Ar age results for individual samples is
provided in the GSA Data Repository.
The 40Ar/ 39Ar age results indicate that (1) sections 2NM and 1DC are age correlative, (2) the
youngest section measured is 5DC, and (3) the
lacustrine strata in the southern Nima area
were deposited during late Oligocene–earliest Miocene time (between 26.0 and 23.5 Ma).
We assign a Miocene age to the ~3000-m-thick
succession of fluvial red beds near the town of
Nima (Figs. 2 and 4) because they conformably
Geological Society of America Bulletin, July/August 2007
Geological records of the Lhasa-Qiangtang and Indo-Asian collisions
115
400
A
B
350
Temperature (°C)
Apparent Age (Ma)
110
105
Measured
100
95
90
0
Model
7-19-98-2 kspar
Xiabie granite
300
250
200
150
20
40
60
80
Cumulative % 39Ar released
100
100
90
95
100
105
Age (Ma)
110
115
Figure 7. (A) K-feldspar 40Ar/39Ar apparent age spectrum. (B) Thermal history calculated from multidomain diffusion modeling. The darkgray field shows the mean of at least 20 solutions, and the gray field represents a 90% confidence window.
overlie lacustrine strata that are lithologically
similar to those in sections 2NM and 1DC.
is also recorded by nearly 80% of the spectrum
that does not involve the age hump.
K-Feldspar Thermochronologic Results
STRUCTURAL GEOLOGY
The K-feldspar age spectrum from the Xiabie
granite displays an age gradient from ca. 105 Ma
to 113 Ma (Fig. 7A). The overall monotonically
increasing age pattern is disrupted by an intermediate age hump recorded between ~5% and
15% 39Ar released. The origin of the age hump
is uncertain, but it is a fairly common feature
in many K-feldspar age spectra (Lovera et al.,
2002). In addition to this age hump, the initial
isothermal duplicate steps of the spectrum display a characteristic oscillating age pattern that
is indicative of excess argon hosted in fluid
inclusions (Harrison et al., 1994). As detailed
in the GSA Data Repository (see footnote 2),
these excess argon–affected ages are corrected
to younger values based on a correlation with
the degassing behavior of chlorine.
The age gradient is interpreted to reflect
an overall protracted thermal history that has
resulted in variable degrees of argon loss from
multiple diffusion domains (MDD). We used the
MDD model of Lovera et al. (1989) to extract
the thermal history that is shown in Figure 7B.
The intermediate age hump returned by the
measured spectrum cannot be matched with the
MDD model spectra, and thus there is a degree
of uncertainty in the accuracy of the thermal
history. However, the fairly slow cooling from
ca. 112 to 108 Ma that transitions into more
rapid cooling from ~300 °C to 200 °C between
108 Ma and 105 Ma is considered robust since it
Early Cretaceous Unconformity
Cretaceous nonmarine strata lie unconformably on the more strongly deformed, marine
J-K unit in the Nima area (Fig. 2), which was
locally deposited after ca. 125 Ma. The oldest
nonmarine strata deposited in the Nima area
is ca. 118 Ma. These findings demonstrate that
this portion of the Bangong suture underwent
significant deformation, erosion, and a transition from marine to nonmarine conditions
between ca. 125 Ma and ca. 118 Ma. Furthermore, the Cretaceous nonmarine strata range in
age from ca. 118 Ma to ca. 99 Ma and overlap
in age with the Aptian-Albian shallow-marine
limestone exposed in the southernmost part of
the Nima area (in the hanging wall of the Gaize–
Siling Co thrust), which is regionally extensive
across the entire Lhasa terrane (e.g., Leeder et
al., 1988; Yin et al., 1988). These relations show
that the incursion of marine waters into southern
Tibet during the Aptian-Albian did not extend
northward into the northernmost Lhasa terrane
directly south of the Bangong suture.
Gaize–Siling Co Thrust
The Gaize–Siling Co thrust juxtaposes AptianAlbian limestone in the hanging wall against
Cretaceous and Tertiary nonmarine strata in the
footwall (Fig. 2) and can be traced continuously
from at least the Gaize area in the west to Siling Co, ~600 km to the east (Fig. 1B; Kapp et
al., 2005). Conglomerates of the Kcl unit are
deformed into a northward-verging overturned
syncline in the proximal footwall of the Gaize–
Siling Co thrust (Fig. 2) and are derived almost
entirely from the hanging-wall Aptian-Albian
limestone. Both the northern and southern limbs
of the syncline exhibit progressively decreasing bedding attitudes away from the fold axis,
which are interpreted to indicate syndepositional growth of the syncline during N-directed
slip along the Gaize–Siling Co thrust. This suggests that the Gaize–Siling Co thrust was active
after ca. 99 Ma, the depositional age of the Kr
unit, and probably at ca. 97 Ma, the inferred
depositional age of the Kcl unit (Fig. 4). Slip
along the Gaize–Siling Co thrust was also active
during the Tertiary, since it cuts 26–25 Ma strata
of the 2NM section along strike to the west in its
footwall (Fig. 2). Neogene fan conglomerates
locally onlap the Gaize–Siling Co thrust and
provide an upper age bound for fault motion.
Queri-Malai Thrust
Approximately 4 km north of the Gaize–Siling Co thrust is a N-dipping thrust fault that
extends across the southern Nima area from at
least the Queri Range in the west to Malai Peak
in the east (Fig. 2), and it is here named the
Queri-Malai thrust. Near Malai Peak (Fig. 2),
the Queri-Malai thrust juxtaposes the Kv unit
in the hanging wall against ca. 99 Ma red beds
and older strata in the footwall. The red beds are
deformed into a S-facing overturned syncline
Geological Society of America Bulletin, July/August 2007
925
Kapp et al.
in the proximal footwall of the thrust, suggesting that the thrust is south vergent. The Cretaceous volcanic rocks in the hanging wall are
the most likely source for the northerly derived
(Fig. 4) and proximal volcaniclastic conglomerates of the Kcv unit in the footwall (Fig. 2). We
infer that deposition of the Kcv unit between
ca. 99 Ma and ca. 97 Ma records denudation of
hanging-wall volcanic rocks during slip on the
Queri-Malai thrust. Hence, activity on the QueriMalai thrust appears to have shortly predated or
overlapped slip along the Gaize–Siling Co thrust
to the south. Located between the Queri-Malai
and Gaize–Siling Co thrusts is an E-plunging
anticline cored by the J-K unit (Fig. 2). Analysis of the unconformable relationships between
the J-K unit and the Kr and Kv units, as well
as between the Kv and Kcl units along the fold
limbs, indicates that anticline growth must have
initiated prior to Kr deposition and continued
during deposition of the Kcl unit. Along strike to
the west, the Queri-Malai thrust cuts upsection
in both its hanging wall and footwall, juxtaposing the Kcl unit and unconformably overlying
Tertiary strata southward against Tertiary strata,
including the 26–25 Ma 2NM section in the
footwall (Fig. 2). These relationships demonstrate that the Queri-Malai thrust was reactivated
during the Tertiary, similar to the Gaize–Siling
Co thrust. Neogene alluvial fan conglomerates
locally bury the trace of the Queri-Malai thrust,
proving that the fault has not experienced neotectonic activity.
Nima Thrust
The northernmost thrust fault in the southern
Nima area is exposed east of the town of Nima
and is named the Nima thrust (Fig. 2). This fault
dips southward and places Cretaceous volcanic
rocks and the underlying J-K unit in the hanging wall against Tertiary strata in the footwall.
The Tertiary footwall strata are deformed into
an E-W–trending syncline with a wavelength
of ~6 km, which is referred to as the Nima syncline (Fig. 2). In the east, the northern limb of
the syncline dips gently to the south, whereas
the southern limb of the syncline is moderately
to steeply north-dipping and locally overturned,
indicating a northward vergence for the fold.
Tertiary strata of the southern limb locally
exhibit fanning growth geometry, where steeply
dipping or overturned strata are cut by the Nima
thrust and more moderately north-dipping strata
overlap the Nima thrust (Fig. 2). These relations
indicate N-directed displacement along the
Nima thrust during deposition of Tertiary strata,
the age of which is constrained to be 26–25 Ma
based on the ages of tuffs in the 1DC section
(Fig. 2; Table 1).
926
The Nima thrust tips out along strike to the
west within Tertiary strata ~6 km southeast of
the town of Nima (Fig. 2). In contrast, the axial
trace of the Nima syncline extends westward
across the map area. Near the town of Nima,
the upper 900 m of the ~3000-m-thick Miocene
red bed sequence along the southern limb of the
syncline exhibits a northward decrease in dip
angles toward the axis of the syncline (Fig. 2)
and bed thickness variations indicative of continued growth of the syncline during red bed
deposition.
Puzuo Thrust Faults
West of Puzuo Lake the J-K unit is repeated
in the hanging walls of two N-dipping thrust
faults (Fig. 2). The Northern Puzuo thrust cuts
volcanic rocks of the Kv unit in the footwall,
which lies unconformably on the ca. 128 Ma
Puzuo granite. Along the Southern Puzuo thrust,
there is a fault-bounded sliver of volcanic rocks
(Fig. 2) that yielded an imprecise but demonstrably Cretaceous 40Ar/39Ar whole-rock age of
ca. 110 Ma (Kapp et al., 2005), indicating that
thrusting was active subsequent to ca. 110 Ma.
The footwall Kr unit includes conglomerates
with clasts of volcanic rocks and is inferred
to have been deposited coeval with slip on the
Puzuo thrust faults. Tertiary activity on the
Puzuo thrust faults cannot be demonstrated but
is likely, considering the absence of Tertiary
strata in the hanging walls (presumably missing
due to hanging-wall denudation during Tertiary
thrusting). The footwall Kr unit is deformed into
an E-W–trending anticline with a wavelength of
~6 km. The northern limb exhibits generally
steeper dips than the southern limb, indicating
a northward vergence for the fold. The anticline
must have grown during or following deposition
of the Tertiary strata to explain the structural
relief of the Kr unit relative to the mid-Tertiary
strata to the south.
Zanggenong Thrusts
In the northern Nima area, Upper Cretaceous–
Tertiary strata are disrupted by three closely
spaced (< 1 km) S-dipping reverse faults, collectively referred to as the Zanggenong thrusts
(Fig. 2). Bedding attitudes of hanging-wall and
footwall strata suggest a style of deformation
consistent with N-directed fault propagation
folding. The faults likely merge at relatively
shallow structural depths because there is a
single S-dipping thrust fault juxtaposing the J-K
unit in the hanging wall against Tertiary strata
in the footwall (Fig. 2) along strike to the west
and structurally elevated in the footwall of a late
Cenozoic E-dipping normal fault.
Muggar Thrust
The northernmost thrust fault in the Nima
area is the inferred N-dipping Muggar thrust,
which is buried beneath the Neogene and
Quaternary deposits that separate exposures
of Jurassic rocks in the Muggar Range in the
north from Tertiary strata in the south (Fig. 2).
This structure is required to structurally bury
the Tertiary strata and is well exposed ~15 km
along strike to the west of the Nima area,
where it places Jurassic rocks directly against
red beds of uncertain Cretaceous or Tertiary
age. This thrust fault, along with the Puzuo
thrust faults to the south, is part of a regional
system of N-dipping thrust faults along the
length of the Bangong suture that has been
named the Shiquanhe-Gaize-Amdo thrust
system (Fig. 1B; Yin and Harrison, 2000;
Kapp et al., 2005).
Late Cenozoic Faults
None of the contractional structures in
the Nima area deforms Quaternary deposits.
Rather, Quaternary deformation is characterized by widely distributed strike-slip and normal faults with relatively small strike lengths
(generally <6 km; Fig. 2). A detailed kinematic study of these faults was not undertaken.
However, where sense of slip is discernible
on satellite imagery, it appears that easterly to
southeasterly striking faults are right lateral,
whereas northeasterly striking faults are left
lateral. A distinguishable generation of relief is
only associated with the more northerly striking faults, which is taken to indicate a dominantly normal sense of motion for these faults.
HISTORY OF DEFORMATION AND
BASIN DEVELOPMENT
Our new data pertaining to the geological
relations, age, and provenance of Cretaceous–
Tertiary units, and cooling history of the Xiabie granite in the Nima area, place first-order
constraints on the Cretaceous–Tertiary history of upper-crustal deformation and basin
development. This is illustrated in the form
of a cross section and its sequential restoration shown in Figures 8A–8C (see footnote 1).
Whereas the cross section is poorly constrained
at depth, specifically with respect to the style
and depth of deformation within the penetratively deformed J-K unit, it provides a useful
estimate of upper-crustal shortening following
final ocean closure along the Bangong suture
and accurately depicts the importance of both
Cretaceous and mid-Tertiary shortening and
the wedge-top style of basin development.
Geological Society of America Bulletin, July/August 2007
Geological records of the Lhasa-Qiangtang and Indo-Asian collisions
Cross-Section Assumptions
The cross section was constructed using stratigraphic thicknesses for Cretaceous and Tertiary
nonmarine strata determined by projecting surface bedding measurements to depth and from
detailed measured sections (DeCelles et al.,
2007a). Thicknesses were kept constant at depth
unless geological relations required lateral variations. The thickness of the Aptian-Albian limestone unit is ≤1000 m in the northern Lhasa terrane (Leier, 2005). However, this unit is thickened
internally by folding, and we assume a structural
thickness of 2000 m, the minimum required to
bury Cretaceous nonmarine rocks in the footwall
of the Gaize–Siling Co thrust without exposing
rocks older than Aptian-Albian in the hanging
wall. The stratigraphic thickness of the J-K unit
is unknown. However, given the regionally extensive exposure and transposed nature of this unit,
and the fact that older rocks are rarely exposed
along the length of the central Bangong suture
zone (Kapp et al., 2005), a structural thickness of
>5000 m as shown is reasonable.
Additional assumptions that went into constructing the cross section are as follows:
(1) The regional unconformity beneath Cretaceous units had minimal initial relief and can
therefore be restored to horizontal. The validity of this assumption is difficult to assess, but
we observed no evidence in the sedimentary
record for major intrabasinal relief (such as buttress unconformities, megabreccias, or landslide
blocks) at the onset of Cretaceous nonmarine
deposition. (2) Any subsidence in response
to topographic or sediment loading occurred
at a wavelength greater than the width of the
major thrust-bounded ranges in the Nima area
(<20 km) and can therefore be considered to
have been uniform (no localized downwarping of the Cretaceous unconformity below its
initial regional elevation). This assumption is
appropriate for typical continental lithosphere
with sufficient strength to support short-wavelength loads (<100 km; e.g., Turcotte and
Schubert, 1982) but may not be for an anomalous lithospheric structure like that of modern
Tibet where isostatic compensation is probably
occurring within the ductile middle crust (e.g.,
Bird, 1991; Masek et al., 1994; Royden, 1996).
(3) The Nima thrust was active during the Cretaceous, coeval with slip along the Gaize–Siling Co and Queri-Malai thrusts. This allows the
major structures of the southern Nima area to be
simply interpreted as a dominantly N-directed
thrust system. (4) Rapid cooling of the Xiabie
granite during the Early Cretaceous is attributed
to exhumation in response to slip on the Muggar thrust. (5) The cross section is pinned in the
undeformed footwall of the basin-margin thrust
systems, and the Muggar thrust is inferred to root
deeply to minimize estimated shortening along
it. The Cretaceous unconformity is line-length
balanced, whereas internal thickening within the
Aptian-Albian limestone and J-K units is area
balanced. Additional assumptions in constructing the cross section are itemized in Figure 8.
Shortening Estimates
The restored cross section shows the undeformed N-S length of the Cretaceous unconformity to be 123 km (Fig. 8C). Since the presentday N-S width of the cross-section is 65 km, this
corresponds to 58 km (47%) of shortening since
the Early Cretaceous. An identical estimate of
percent shortening was determined along strike
to the west in the Gaize region (Fig. 1; Kapp et
al., 2005). If the assumptions that were made
in order to construct the Nima cross section
are valid, then our estimate is a minimum for
several reasons. Whereas internal thickening
of the Aptian-Albian limestone unit is restored
by area balancing, slip along the Gaize–Siling
Co thrust is not considered. This is because the
original lateral separation between the hangingwall Cretaceous marine limestone and the footwall Cretaceous nonmarine rocks is unknown.
The original separation was likely significant
(more than tens of km), because no intercalated
marine and nonmarine or marginal marine strata
of Aptian-Albian age have been documented in
the hanging wall or footwall of the Gaize–Siling
Co thrust; all Aptian-Albian strata documented
north of the Gaize–Siling Co thrust are entirely
nonmarine. Hanging-wall cut-offs for the QueriMalai, Nima, Puzuo, and Zanggenong thrusts
have been eroded. For these thrusts, only the
minimum magnitude of slip needed to erode the
hanging-wall cut-offs is shown. The only exception is the northern Puzuo thrust, where we
include an additional ~6 km of slip to illustrate
that the thrust sheet in the hanging wall of the
Southern Puzuo thrust could be interpreted to be
a horse within a duplex. The slip shown for the
Muggar thrust is that needed to exhume the Xiabie granite from a minimum depth of ~10 km
(the granite was at temperatures >300° prior to
the mid-Cretaceous based on thermochronologic results; Fig. 7B) to the surface along a fault
with a northward dip of 45°. The magnitude of
slip would be greater if the fault flattens with
depth or exhibits ramp-flat geometries within
the upper crust. Distinguishing the relative magnitude of Cretaceous versus Tertiary shortening
is difficult because the major thrust faults in the
Nima area are shown or inferred to have been
active during both Cretaceous and Tertiary time.
However, a minimum of 25 km of the estimated
total minimum shortening of 58 km must have
been Tertiary in order to produce the folding in
Tertiary strata and to structurally bury Tertiary
strata in the footwalls of the Gaize–Siling Co
and Muggar thrusts.
Cretaceous History
Jurassic–Lower Cretaceous marine sedimentary rocks in the northern Nima area were penetratively deformed and uplifted above sea level
by ca. 118 Ma, the age of the oldest nonmarine
strata (Kvc unit) above the Cretaceous angular
unconformity (Fig. 8C). At ca. 118 Ma, the Xiabie granite was intruded into the middle crust.
Onset of rapid cooling of the Xiabie granite at
ca. 108 Ma (Fig. 7B) is attributed to initial slip
along the Muggar thrust at this time (Fig. 8B).
Marine rocks of the J-K unit are as young as
ca. 125 Ma in the southern Nima area (Fig. 5C).
These marine rocks were penetratively deformed
and uplifted above sea level prior to deposition
of the unconformably overlying ca. 110–106 Ma
volcanic-bearing strata.
In the eastern part of the southern Nima area,
the oldest deformation that affected Cretaceous
nonmarine strata is growth of the anticline
south of the Queri-Malai thrust and north of the
Gaize–Siling Co thrust (Figs. 2 and 8C). Folding must have initiated prior to the ca. 99 Ma
red beds of the Kr unit to locally erode the Kv
unit along the limbs of the anticline. This fold
is interpreted to have formed above a footwall
ramp in the Nima thrust (Fig. 8C) and to mark
onset of N-directed thrusting by ca. 99 Ma. Also
by ca. 99 Ma, S-directed thrusting may have
ceased or slowed significantly along the Muggar
thrust and propagated southward to the Puzuo
thrust fault. We infer that the ca. 99 Ma Cretaceous red beds to the south were largely derived
from sources elevated in the hanging wall of the
Puzuo thrust (Fig. 8B). The Queri-Malai thrust
is interpreted as a back thrust that branched from
the Nima thrust and resulted in the development
of a synclinal pop-up structure in its hanging
wall. Northerly derived conglomerates of the
Kcv unit record the growth of this pop-up structure, and similar deposits may also have been
shed northward. Initiation of the Gaize–Siling
Co thrust shortly postdated slip along the QueriMalai thrust, as indicated by syncontractional
deposition of the southerly derived Kcl unit on
top of northerly derived conglomerates of the
Kcv unit. Duplexing of the J-K unit in the hanging wall of the Nima thrust is inferred to have
uniformly elevated overlying Cretaceous rocks.
Mid-Tertiary History
Geologic relations provide no evidence for significant deformation in the Nima area subsequent
Geological Society of America Bulletin, July/August 2007
927
Kapp et al.
to Cenomanian time and prior to onset of nonmarine sedimentation during the late Oligocene.
It appears that all of the major structures that
were active during Cretaceous time were reactivated during the mid-Tertiary. Along strike to
the west of the line of the cross section, Tertiary
strata were deposited and cut in the footwalls
of the Gaize–Siling Co and Queri-Malai thrusts
(Fig. 2). The >4000-m-thick succession of Tertiary strata at the latitude of the town of Nima
filled accommodation space within a triangle
zone, bounded by the S-dipping Nima thrust in
the south and the N-dipping South Puzuo thrust
in the north (Fig. 8C). Folding of these Tertiary
strata is inferred to have been related to their
northward displacement over a footwall ramp in
the décollement to the N-directed thrust system.
Slip along this décollement may have been fed
to the surface by a combination of back thrusting
along the Southern Puzuo thrust and N-directed
displacement along the Zanggenong thrusts to
the north (Fig. 8A). Tertiary strata in the northern Nima area are inferred to have been folded
and structurally elevated due to internal shortening and thickening of the underlying J-K
unit (Fig. 8A). This shortening was balanced at
deeper structural levels by southward slip along
the Southern Puzuo thrust.
The youngest contractional deformation in
the Nima area is recorded by growth stratal
relations in the uppermost part of the red bed
sequence near the town of Nima (Fig. 2; also
see Figure 7A of Kapp et al., 2005), suggesting
continued growth of the Nima syncline during
their deposition. The youngest dated strata that
are inferred to conformably underlie the red-bed
sequence are ca. 24 Ma. Relatively high sediment accumulation rates of ~1 mm/yr are typical of those in foredeep depozones of flexural
foreland basins (e.g., Angevine et al., 1990;
DeCelles and Giles, 1996) as well those within
the Qaidam basin during late Cenozoic time
(e.g., Metivier et al., 1998). Assuming a similar
deposition rate for the ~3000-m-thick Nima red
bed sequence, a conservative estimate for the
maximum age of the top of the sequence, and
hence the youngest documented contractionrelated deformation in the area, is ca. 21 Ma.
Late Cenozoic History
Late Cenozoic, approximately N-striking normal faults in the Xiabie and Puzuo Lake areas
appear to be kinematically linked by a system of
approximately NE-striking sinistral strike-slip
faults (Fig. 2). In contrast, eastward displacement south of the town of Nima is accommodated in large part along an ESE-striking dextral
strike-slip fault located ~1–2 km north of the
Queri-Malai thrust (Fig. 2). This pattern of late
928
Cenozoic deformation in the Nima area mimics
that of the Bangong suture zone at the regional
scale (Fig. 1A), where conjugate strike-slip
faults (sinistral in the north and dextral in the
south) are linked with approximately N-striking normal fault systems and accommodate
distributed eastward extrusion of wedge-shaped
crustal fragments (Taylor et al., 2003). Despite
their relatively small slip magnitudes, the late
Cenozoic faults in the Nima area exert a strong
influence on the pattern of Quaternary sedimentation (Fig. 2). The Xiabie Lake basin is a graben between E-dipping normal faults in the west
and a W-dipping normal fault system in the east.
The Puzuo Lake basin is a half-graben basin,
bounded to the west by an E-dipping normal
fault. Dagze Lake shorelines are best preserved
east of the lake, and deposition of postshoreline
deposits (Q2) is localized in the west (Fig. 2).
Although not demonstrative, these relations
could be explained by regional down-to-thewest tilting of the Dagze Lake basin toward Edipping normal faults in the Puzuo Lake area.
DISCUSSION
Cretaceous Lhasa-Qiangtang Collision
Our mapping and geochronologic results
demonstrate that the Nima area underwent
major deformation and denudation and was
uplifted above sea level between ca. 125 Ma and
ca. 118 Ma. Major Early Cretaceous deformation and exhumation have also been documented
along strike to the west and east in the Gaize
and Amdo areas (Fig. 1), respectively, as well as
in the Qiangtang terrane to north (Kapp et al.,
2005; Guynn et al., 2006). Farther south in the
Lhasa terrane, Lower Cretaceous (pre-Aptian)
clastic deposits are regionally extensive. In contrast to the Nima area, however, these deposits
are entirely conformable. The clastic deposits
consist of marginal marine to fluvial facies,
show mainly southward paleocurrent indicators,
and are interpreted to have been deposited in a
foreland basin related to the S-directed thrusting associated with Lhasa-Qiangtang collision
in the north (Leeder et al., 1988; Zhang et al.,
2004; Leier, 2005). The Lower Cretaceous clastic deposits of the Lhasa terrane are conformably overlain by Aptian-Albian shallow-marine
limestones. Whereas the cause of the AptianAblian marine incursion has been attributed to
back-arc extension (Zhang, 2000; Zhang et al.,
2004), previous studies near Gaize (Kapp et al.,
2005) and this study in the Nima area demonstrate that the marine incursion did not extend
north of the Lhasa terrane in central Tibet and
that it was coeval with syncontractional basin
development along the Bangong suture.
Early Cretaceous igneous rocks are widely
distributed in the northern Lhasa terrane (e.g.,
Xu et al., 1985; Coulon et al., 1986; Harris
et al., 1990). The ca. 124 Ma Puzuo granite,
ca. 118 Ma Xiabie granite, and Lower Cretaceous volcanic rocks in the Nima area show
that igneous activity of this age extended as far
north as the Bangong suture. Given the scarcity
of documented igneous rocks of this age in the
southern Lhasa terrane, this inboard magmatism is interpreted to be the consequence of
northward low-angle subduction of Neotethyan
oceanic lithosphere beneath the southern margin
of Asia at this time (Coulon et al., 1986; Kapp
et al., 2005). This contrasts with the alternative
interpretations that this magmatic belt is related
to crustal anatexis in response to crustal thickening during Lhasa-Qiangtang collision (Xu et
al., 1985) or mantle thinning following LhasaQiangtang collision (Harris et al., 1990).
Collectively, the Early to mid-Cretaceous
geology of Tibet is consistent with the model
of Kapp et al. (2005) of northward continental
underthrusting of the Lhasa terrane beneath the
Qiangtang terrane at this time, driven by the
northward flat-slab subduction of Neotethyan
oceanic lithosphere along the Indus-Yarlung
suture (Fig. 9A). In this hypothesis, the Ndirected mid-Cretaceous Gaize–Siling Co and
Nima thrusts are back thrusts, and the associated nonmarine strata represent wedge-top
deposits trapped in a triangle zone structural
setting (Fig. 9B). Dominantly S-directed, thinskinned shortening related to ongoing, postcollisional convergence in the Bangong suture zone
propagated southward into the northern Lhasa
terrane during the Late Cretaceous to Paleocene (Fig. 9B; Kapp et al., 2003), which could
explain why there is no evidence for major
contraction in the Nima area during this time
interval. This model predicts that central Tibet
underwent significant crustal thickening and
elevation gain prior to the Indo-Asian collision.
Paleoelevation studies indicate that the late Oligocene Nima basin (DeCelles et al., 2007) and
Eocene(?) to Miocene Lunpola basin to the east
(Fig. 1B; Rowley and Currie, 2006) developed
at high elevations (>4 km). Additional studies
on older deposits are necessary to quantify how
much elevation gain occurred prior to the IndoAsian collision.
Magnitude of Cretaceous versus MidTertiary Shortening
The magnitude of mid-Tertiary shortening estimated for the Nima area is ~25 km over the present-day N-S width of ~65 km (Fig. 8). Although
this estimate is a minimum, it is unlikely to be
significantly greater. The magnitude of Tertiary
Geological Society of America Bulletin, July/August 2007
Geological records of the Lhasa-Qiangtang and Indo-Asian collisions
region of upper-crustal shortening
South
km
0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
60
70
80
km
0
10
20
30
40
50
60
70
80
130-120 Ma
magmatism
Lhasa
growth of
Qiangtang
culmination
Nima
area Bangong
suture
foreland basin
120-100 Ma
magmatism
North
Qiangtang
Songpan Ganzi
o
Moh
Moho
Asian mantle lithosphere
Neotethyan oceanic lithosphere
B
ca. 100-50 Ma
Xigaze
forearc
India
0
100
200
300 km
Qiangtang
Gangdese
retroarc
thrust belt
Moho
Tethyan Himalaya
thrust belt
Nima
area
Linzizong
volcanics
Songpan Ganzi
Moho
Lhasa
Gangdese arc
o
km
Xigaze
forearc
Moh
0
10
20
30
40
50
60
70
80
ca. 130-100 Ma
o
oh
M
km
A
Moho
IndusYarlung
suture
C
Qiangtang thrusts
Fenghuo Shan
(~25% shortening) (~50% shortening)
ca. 50-30 Ma
minimal upper crustal shortening
Qiangtang
Lhasa
India
Songpan Ganzi
Moho
Moho
Kailas
Gangdese
conglomerate
thrust
Moho
D
Nima area
ca. 30-23 Ma
Qiangtang
Lhasa
MCT sheet
Moho
Songpan
Ganzi
India
Moho
Moho
Figure 9. Schematic cross-sectional diagrams illustrating the Cretaceous to mid-Tertiary tectonic evolution of the Himalayan-Tibetan orogen. (A) Early Cretaceous northward continental underthrusting of the Lhasa terrane beneath the Qiangtang terrane along the Bangong
suture (BS), driven by northward flat-slab subduction of the Neotethyan oceanic lithosphere to the south. (B) Between 100 and 50 Ma, Sdirected thrusting related to continued Lhasa-Qiangtang collision propagated southward into the northern Lhasa terrane, and the southern
Lhasa terrane was characterized by a major N-directed retroarc fold-and-thrust belt (Leier et al., 2007; Kapp et al., 2007). Shortening
during this time period was sufficient to have produced significant crustal thickening and elevation gain in Tibet prior to Indo-Asian collision. (C) During the early Tertiary, significant upper-crustal shortening was localized in north-central Tibet and in the Tethyan Himalaya.
The gravitational potential energy related to a preexisting thick crust in the Lhasa terrane and along the Bangong suture may have been
sufficient to inhibit upper-crustal shortening in these areas during this time period. (D) Mid-Tertiary reactivation of shortening and basin
development along the Bangong and Indus-Yarlung sutures immediately predated southward emplacement/extrusion of the Main Central
thrust (MCT) sheet in the Himalaya. These timing relations suggest that mid-Tertiary deformation in Tibet may have been mechanically
linked with the Himalayan fold-and-thrust belt.
Geological Society of America Bulletin, July/August 2007
929
Kapp et al.
slip along the Nima thrust is limited by the
preservation of Cretaceous strata in its hanging
wall. Thermochronologic results on the Xiabie
granite suggest that it cooled to ~150 °C during
the Early Cretaceous. Under the assumption of
a 25 °C/km geothermal gradient, this suggests a
maximum Tertiary throw of ~6 km for the Muggar thrust. Unlike the Muggar and Gaize–Siling
Co thrusts, the Puzuo thrust faults cannot be
traced continuously along the Bangong suture
for distances >100 km, implying to us that these
thrust faults are subordinate in slip magnitude to
those bounding the Nima basin. Although largemagnitude Tertiary slip along the Gaize–Siling
Co thrust is permissible, at this time there is no
strong evidence for large-magnitude (~50%)
shortening along the Bangong suture during the
Indo-Asian collision as suggested (or predicted)
in some models of Tibetan Plateau formation
(e.g., England and Houseman, 1986, 1989; Tapponnier et al., 2001). Based on this study and previous studies in central Tibet (Kapp et al., 2003,
2005), there is no evidence for early Miocene or
older strike-slip deformation along the Bangong
suture. This challenges models invoking oblique
subduction of continental lithosphere along the
Bangong suture (Tapponnier et al., 2001) and
any significant eastward extrusion of central
Tibet relative to southern Tibet (Tapponnier et
al., 1982) prior to late Cenozoic time.
Mid-Tertiary Reactivation of the Bangong
Suture
Minimal post–50 Ma shortening and basin
development have been documented within
the interior of the Lhasa terrane, in contrast to
its bounding sutures (Figs. 1B and 9C–9D). EW–trending, thrust-bounded Tertiary nonmarine
clastic basins are widespread along the Bangong
suture zone (Fig. 1A). These basins have been
widely mapped and cited to be early Tertiary,
although no robust age data have been published
in the international literature. The largest of these
is the Lunpola-Duba basin system, ~200 km east
of the Nima area (Fig. 1B), which includes up to
5000 m of fill of reportedly Paleocene to Oligocene age (e.g., Ai et al., 1998; Guo et al., 2002;
Pananont et al., 2002). In contrast, our results
show that the >4000-m-thick Tertiary succession in the southern Nima area is restricted in
age to the late Oligocene–early Miocene. The
disconformable relationships between Tertiary
and Cretaceous strata in the Nima area suggest
minimal deformation between the mid-Cretaceous and late Oligocene. Similarly, Tertiary
contraction and basin development in the Shiquanhe area along the Bangong suture zone in
far western Tibet (Fig. 1) are restricted to the late
Oligocene–early Miocene (Kapp et al., 2003).
930
If the inferred Paleocene-Oligocene age of the
Lunpola basin is correct, this would suggest
that portions of the Bangong suture underwent
localized contraction and basin development
at variable times during the Indo-Asian collision. Alternatively, if the Lunpola basin is an
along-strike chronostratigraphic equivalent of
the Nima-Shiquanhe basins, then its fill (which
oxygen isotope studies indicate was deposited at
a paleoelevation of >4 km; Rowley and Currie,
2006) may be substantially younger than presently assumed.
Although the possibility of early Tertiary contraction along portions of the Bangong suture
remains to be tested, the available timing constraints point to an important episode of suture
zone reactivation during the late Oligocene–
early Miocene. Interestingly, the Indus-Yarlung
suture to the south (Fig. 1) also exposes nonmarine strata of late Oligocene–early Miocene
age (Kailas or Gangrinboche conglomerate; Yin
et al., 1999; Aitchison et al., 2002), and it was
modified by the S-directed Gangdese thrust system between 30 and 23 Ma (Fig. 1; Yin et al.,
1994, 1999; Ratschbacher et al., 1994; Harrison et al., 2000). Simultaneous reactivation of
the Bangong and Indus-Yarlung sutures during
the Oligocene-Miocene (Fig. 9D) postdated
Tertiary contraction and basin development in
the northern Qiangtang and Songpan-Ganzi terranes, which initiated during the earliest stages
of Indo-Asian collision and had largely ceased
by ca. 30 Ma (Fig. 9C; Coward et al., 1988; Liu
and Wang, 2001; Liu et al., 2001, 2003; Horton
et al., 2002; Spurlin et al., 2005). This observation begs the question of why contraction was
localized in north-central Tibet during the early
stages of Indo-Asian collision and then jumped
southward to the Bangong and Indus-Yarlung
sutures during the mid-Tertiary.
We speculate that the gravitational potential energy associated with a preexisting thick
crust in southern Tibet due to Cretaceous–early
Eocene orogenesis (Fig. 9B; see Kapp et al.,
2005, for summary) was sufficient to inhibit
upper-crustal shortening in this area and to
result in contractional deformation in lower-elevation regions in north-central Tibet (Cyr et al.,
2005) and to the south in the Tethyan Himalaya
during the early Tertiary (Fig. 9C). The feasibility of this mechanical explanation has been
demonstrated in thin-viscous-sheet models of
Cenozoic deformation with the initial condition
of preexisting thick crust and high elevation in
southern Tibet (England and Searle, 1986; Kong
et al., 1997). Mid-Tertiary reactivation of shortening along the Bangong and Indus-Yarlung
sutures immediately predated the oldest dated
shear zone activity within the Main Central
thrust system in the Himalaya (23–20 Ma; e.g.,
Hubbard and Harrison, 1989; Hodges et al.,
1996) and initial erosion of Greater Himalayan
metamorphic rocks in its hanging wall (early
Miocene; e.g., DeCelles et al., 2001). This
observation raises the possibility that thrusting in Tibet was mechanically linked with the
Himalayan thrust belt, perhaps representing hinterland out-of-sequence deformation that helped
build the orogenic wedge taper (or gravitational
potential energy) necessary to drive subsequent
southward emplacement/extrusion of the Main
Central thrust sheet (Fig. 9D).
CONCLUSIONS
The geology of the Nima area provides a rich
record of Cretaceous to Quaternary deformation and basin development along the Bangong
suture in central Tibet. Jurassic to Early Cretaceous marine sedimentary rocks were deformed
and uplifted above sea level prior to the onset
of nonmarine deposition at ca. 118 Ma. Cretaceous nonmarine strata range in age from
Aptian to Cenomanian and were deposited
coeval with S-directed thrusting along the
northern margin of the Nima basin and mainly
N-directed thrusting along the southern margin
of the Nima basin. Cretaceous tectonic activity in the Nima area is attributed to continued
shortening after the initial collision between
the Lhasa and Qiangtang terranes. No evidence
exists for subsequent contraction until the late
Oligocene–early Miocene, when the Cretaceous
thrust faults were reactivated and thick successions of nonmarine strata (locally ~4000 m
thick) accumulated in wedge-top basins and at
high elevation (>4.6 km). Mid-Tertiary thrusting along the Bangong suture zone was coeval
with shortening along the Indus-Yarlung suture
zone, suggesting simultaneous reactivation of
these suture zones bounding the Lhasa terrane.
We speculate that mid-Tertiary shortening associated with regional suture zone reactivation
may have driven the Himalayan thrust belt into
a supercritical state, leading to the subsequent
southward emplacement/extrusion of the Main
Central thrust sheet. Late Cenozoic deformation is characterized by widely distributed but
relatively small displacement on approximately
N-striking normal and more easterly striking
strike-slip faults, which together are accommodating distributed N-S shortening and E-W
extension. It is estimated that the upper crust of
the Nima area underwent >59 km (>47%) of
shortening during Cretaceous to mid-Tertiary
time, with more than half of this shortening predating Indo-Asian collision. Our results suggest
that the thick crust and high elevation of central
Tibet were achieved in large part by shortening
of the Tibetan crust over a protracted period of
Geological Society of America Bulletin, July/August 2007
Geological records of the Lhasa-Qiangtang and Indo-Asian collisions
time, beginning during the Early Cretaceous and
continuing until at least the early Miocene.
ACKNOWLEDGMENTS
This research was supported by the U.S. National
Science Foundation grant EAR-0309844 and ExxonMobil. Acknowledgment is also given to the donors of
the American Chemical Society Petroleum Research
Fund for partial support of this research (ACS PRF#
39376-G8). We thank Duo Jie and Zhou Ma for their
assistance in the field and J. Fox, F. Guerrero, and
A. Pullen for their assistance with mineral separation and U-Pb analysis. We thank Brad Ritts and an
anonymous reviewer for critical reviews, and suggestions by Paul Heller and Karl Karlstrom helped us
improve this paper.
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MANUSCRIPT RECEIVED 4 MAY 2006
REVISED MANUSCRIPT RECEIVED 18 DECEMBER 2006
MANUSCRIPT ACCEPTED 19 JANUARY 2007
Printed in the USA
Geological Society of America Bulletin, July/August 2007
Figure 2. Kapp et al.
gr
gr
gr
Xiabie
granite
68
Jr
37
58
7-19-98-1
20
Q2
Q2
Nf
Muggar
Tmu
Tmu
60
46
N-Q
Nf
65
Q1
Quaternary deposits; incised
N-Q
Neogene - Quaternary (?) conglomerate
Nf
Neogene fan conglomerates
20
22
Muggar thrust
Nf
12
Q2
Q1
52
Q2
Upper Cretaceous to Paleocene Lower Muggar Unit
Kcl
Cenomanian conglomerate with mainly Aptian-Albian limestone clasts
Kcv
Cenomanian conglomerate with mainly volcanic clasts
Q2
500
0
Tmu 5MK
20
Q2
35
Q2
32
Tmu
30
2MK
Tmu
47
Q2
Tmu
Tmu
Zanggenong
thrusts
Tmu
56
46
50
20
39
21
N-Q
Aptian volcaniclastic conglomerate, sandstone, siltstone; paleosols, tuffs
gr
Cretaceous granite and granodiorite
J-K
Jurassic - Cretaceous: shale, siltstone, turbiditic sandstone, metasedimentary-matrix mélange
Jr
33
66
77
1MK
J-K
shorelines
river
N-Q
Kml
Geologic Symbols
Contacts
Solid where well located, dashed where approximately located or inferred, dotted
where concealed and inferred
25
Lithologic
Kvc
Q1
84
J-K
Q2
N-Q
N-Q
00
J-K
46
Trend and plunge of
small-scale fold axis
Syncline
Strike and dip
Transposed
Cleavage
bedding
Vertical Overturned
Anticline
87°30'E
89
Strike-slip fault
Horizontal Inclined
Other
J-K
6-2-98-2
Sample locality
Marker-bed
J-K
46
00
faults in red show
neotectonic activity;
mapped largely from
satellite imagery
Folds
Solid where well located, dashed where approximately located or
inferred, dotted where buried; arrow shows direction of plunge
J-K
N-Q
High-angle normal fault;
bar-ball on hanging wall
Thrust fault; arrow shows
dip; diamond shows trend
of striations
Q1
Q2
Glacier or yearround snow
lake
3MK
4MK
29
118 ± 3Ma
16
J-K
Jurassic: argillite, shale, siltstone, limestone, turbiditic sandstone, metavolcanic rocks
Tmu
23
45 82 75 7-19-98-3
75
75
47
60
57
38
Kml
13
15
20
Kvc
8 34
10
10
18
20
Kml
J-K
Albian volcanic flows, tuffs, breccias; volcaniclastic sandstone and conglomerate
37
30
30
20 56
00
Kv
Aptian-Albian massive reef-facies foraminiferal- and rudist-bearing limestone
30
Tmu
Albian-Cenomanian red beds; volcanic-clast sandstone and conglomerate
Kl
Q2
Q1
Tmu
Kr
Kvc
Q1
10 km
Tertiary red beds of the S. Nima Area (T) and Upper Muggar Unit (T) of the N. Nima Area rmu
Kml
Q1
00
48
Tmu
Tmu
Tr
Nf
Nf
Q2
8
Map Units
Quaternary deposits; youngest
Jr
45 88
75
thrust
6
Q2
52
51
Nf
Q2
43
30
72
75
4
contour interval = 100 m
32
76
Nf
2
52
7-16-98-2 7-16-98-1
Xiabie Lake
geysers
Q2
Nf
0
Jr
58
Geological Map of the Nima Area
N
Central Tibet
snow
and ice 6081
Muggar Range
0
460
7-19-98-2
118 ± 4 Ma
Jr
A'''
Qglacial
Jr
3MC Start of
measured section
J-K
32°00'N
Q1
32°00'N
82
J-K
Q1
00
46
Q1
J-K
45
00
J-K
Q1
J-K
Q2
eli
ne
s
J-K
sh
0
J-K
or
J-K
450
Q2
J-K
45
00
Puzuo
Lake
A''
Q1
Jr
Puzuo
thrust
A''
Pazuo thrust
Puzuo
7-14-98-3
0
64 17
Puzuo
granite
7-14-98-2
124 ± 4 Ma
20
46
59
46
46
J-K
South
Puzuo
thrust
Dagze Lake
32
Q1
7-14-98-1
ca. 110 Ma
48
Kv
Kv
J-K
shorelines
gr
460
33 35
Kv
7-14-98-2b
Q2
Moc
hang
R.
35
Q2
27 65
Kr
Q2
35
Q1
ive
r
22
an
gR
22
Mo
62
40
41
21
41
Nima
N-Q
74
52
Tr
30
52
42
?
1NM
ri -
20
23
10
Ma
lai
7-13-98-1
Kl
35
69
N-Q
Nf
Q1
Q1
Tr
thru
st
58
83
42
66
7-22-98-4
70
Tr
38
50
64
55
72
52
32
7-22-98-1
Queri - Malai thrust
43
Tr
J-K
Q1
Kcv
Kl
Kcv
Kcv
65
45
68
Kv
4DC
68
58
85
J-K
Kv
40
Peak
peak
J-K
~99 Ma
78
67
45
69
30
7-11-05-2
78
<125 Ma
18
30 3
37
6-11-04-4
50
50
30
~106 Ma 6-10-04-1
5
42
55
55 70
Kv
2
29
60
70 65
Malai
43
Kr
85
Kr
42
3MC
18
36
60
Kv
Kcl
Kr
Q1
54
Kl
Kcv
57
6214
70
22
27
34
1MC 30
24
54
Kcl
Kl
42
36
6-11-04-2
~97 Ma
2MC
Kcv
Tr
52
8
6-11-04-1 40
Kr
45
3DC
Tr
J-K
00
Nf
J-K
Q2
49
4700
40
00
48
Kv
36
Tr
40
Kv
Kv
42
46
58
Q1
7-22-98-3
Kr
Tr
16
7-22-98-2
Gaize - Siling
Co thrust
27
40
33
85
79
7-22-98-5
60
33
Kcl
Q1
67
Nima thrust
Kv
Kcl
75
Tr
Tr
Tr
86
Q1
Tr
82
72 80
76
Nf
2DC78
Tr
Tr
Tr
Nima thrust
85
Tr
Tr
A'
Tr
Kcl
5000
60
Kl
Kl
area unmapped
area unmapped
4900
Q1
Gaize - Siling
Co thrust
area unmapped
A
area unmapped
Figure 2. Geological map of the Nima area (see Fig. 1), including and significantly expanding on the preliminary mapping in this region by Kapp et al. (2005). Chinese topographic maps at 1:100,000 scale and satellite images (NASA Landsat 7, ca. 2000) were used for base maps.
Figure 8. Kapp et al.
A Cross section of the Nima area
South Gaize - Queri - Malai Nima
A Siling Co
thrust
thrust A'
Southern
Puzuo
thrust
thrust
km
Kcl
5
Kl
4
Kcv
Kv
3 4
J-K
2
1
J-K
5
0
-1
-2
-3
-4
-5
-6
Shortening Estimates
Present-day length = 64.6 km
Restored length = 123.0 km
Minimum Tertiary shortening = 25 km
(28%)
Minimum total shortening (post-midCretaceous) = 58 km (47%)
Kl
Kcv
2
Puzuo A''
thrust
0
2
4
6
8
10 km
Kv
1
Kr
Kr
Kv
J-K
7
6
J-K
Kv
Cretaceous unconformity
J-K
Cretaceous unconformity
ca. 99 Ma
Kcl tuff
Kcv
km
5
4
3
Xiabie 2
granite 1
0
-1
-2
-3
-4
-5
-6
Kvc
J-K
J-K
3
1. Geometry of Tertiary Nima basin constrained from bedding attitudes; thickness of >4
Jr
Tmu 1
Tr
Tertiary Nima basin and underlying Cretaceous volcanic rocks is filled by wedges of
syncontractional deposits derived from the hanging wall of the Nima thrust.
3. The elevation of the Cretaceous unconformity beneath the Tertiary Nima basin is
taken to mark the initial regional elevation of the unconformity.
4. Gaize - Siling Co thrust soles out at base of Kl, as no older strata are observed along
its entire strike length. Kl is shown with a structural thickness of ~2 km, likely obtained
by folding/imbrication of a limestone unit <500 m in stratigraphic thickness.
5. Thickening of J-K unit is required to explain the regional structural elevation of rocks
in the hanging wall of the Nima thrust; accomplished here by emplacing a horse of J-K.
6. The location of this footwall ramp is constrained from the regional geometry of the
contact between J-K and Kvc. The structural relief on the ramp is the minimum
required to erode post-J-K strata between the ramp and the Puzuo thrust.
7. Duplexing in the J-K unit is inferred to elevate the Kvc unit above its initial regional
elevation (see 3 above). Area balance of the J-K unit is maintained by slip on the
Puzuo thrust system.
8. The geometry of the Muggar thrust at depth is uncertain, but it is taken to root deeply
to minimize estimated shortening.
B Cretaceous shortening and basin development
Kcv future location of
Tertiary Nima basin
Kvc
Kv
J-K
Jr
J-K
Xiabie
granite
J-K
original
folding between
Aptian-Albian separation
106 and 99 Ma ca. 106 Ma
marine
unknown
volcanics
Kv
carbonate
Kl
108-104 Ma
rapid cooling
future location of
Tertiary Northern Nima basin
Kr
Kr
31°40'N
North km for southern Nima basin confirmed from measured sections.
A'''
2. Nima thrust is assumed to be active during Cretaceous deposition. Space between
Muggar
thrust
Zanggenong
thrusts
Kml
no vertical exaggeration
Gaize - Siling
Co thrust
65 Q2
Kcl
Kl
20
26
Nima syncline
Q1
Tr
80
30
7-13-98-2
5DC
23.5 Ma
N-Q
69
7-13-98-3
Que
20
Tr
Nima
20
76
Tr
35
ine
72
75
2NM (26-25 Ma)
83
80
syncl
67
4700
75
Q1
N-Q
A'
Q
Quueerr
ii R
raanngg Kr
45
ee
18
12
33
42
Tr
70
shorelines
Q2
21
N-Q
56
Kcl
1DC
26-25 Ma
Tr
23
N-Q
Q2
ch
Q2
Q1
ca. 123 Ma
granitoid
ca. 110 Ma
volcanics
stratigraphic pinch-out required
but location uncertain due to erosion
118 Ma tuffs
C Restored cross section
Kvc
Kv
Position of undeformed
footwall is pinned
Assumption: No initial relief
on unconformity beneath Cretaceous;
restored to horizontal
Xiabie granite (ca. 118 Ma)
>10 km below surface
Figure 8. (A) Cross section of the Nima area. (B) Cross section of the Nima area with the minimum magnitude of mid-Tertiary shortening restored to show distribution of deformation and basin development during the Early to mid-Cretaceous. (C) Completely restored cross section of the Nima area, excluding slip on the Gaize–Siling Co thrust.
Geological records of the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of central Tibet
Paul Kapp, Peter G. DeCelles, George E. Gehrels, Matthew Heizler, and Lin Ding
Figures 2 and 8
Supplement to: Geological Society of America Bulletin, v. 119, no. 7/8, doi: 10.1130/B26033.S1.
© Copyright 2007 Geological Society of America