geological records of the lhasa-qiangtang and indo-asian collisions in the nima area of central...

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917

ABSTRACT

A geological and geochronologic investi-gation 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 Creta-ceous (≤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 doc-umented. Younger nonmarine Cretaceous rocks include ca. 110–106 Ma volcanic-bear-ing strata and Cenomanian red beds and conglomerates. The Jurassic–Cretaceous rocks are unconformably overlain by up to 4000 m of Upper Oligocene to Lower Mio-cene lacustrine, nearshore lacustrine, and fl uvial red-bed deposits. Paleocurrent direc-tions, 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 N-directed thrusting along the southern margin of the basin. The structural restoration sug-gests >58 km (>47%) of N-S shortening fol-lowing 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 subduc-tion of the Neotethyan oceanic lithosphere and Lhasa-Qiangtang continental collision,

respectively. Tertiary syncontractional basin development in the Nima area was coeval with that along the Bangong suture in west-ernmost Tibet and the Indus-Yarlung suture in southern Tibet, suggesting simultaneous, renewed contraction along these sutures dur-ing 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, compos-ite 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 understand-ing 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 con-sequences 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 border-ing 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 Long-men 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 mech-anisms 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 infi lling 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 fl ow 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 funda-mental to advancing models of plateau forma-tion 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.

Geological records of the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of central Tibet

Paul Kapp†

Peter G. DeCellesGeorge E. GehrelsDepartment of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA

Matthew HeizlerNew Mexico Geochronological Research Laboratory, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA

Lin DingInstitutes of Tibetan Plateau Research and Geology and Geophysics, Chinese Academy of Sciences, Beijing 100085, People’s Republic of China

†E-mail: [email protected].

GSA Bulletin; July/August 2007; v. 119; no. 7/8; p. 917–932; doi: 10.1130/B26033.1; 9 fi gures; 1 table; 1 insert; Data Repository item 2007166.

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Kapp et al.

918 Geological Society of America Bulletin, July/August 2007

Indus-Yarlung suture

34°N

32 N°

30 N°

92 E°80 E° 84 E° 88 E°

Lhasa

ShiquanheGaize

NimaLhasa terrane

Qiangtangterrane

Songpan Ganzi

Coqin

200 km0

Lunpola

??

Fenghuo Shan-Hoh Xil

?

Jinsha suture

?Bangong suture

Amdo

GCT

GT

SGAT

GST

Siling CoDuba

65-40 Ma volcanic rocksTertiary strata

Tibet

India

42-30 Ma volcanic rocksLate Cretaceous - early TertiaryGangdese batholith

Early Cretaceous granite

Fig. 2

Qiangtanganticlinorium

TarimPamir

Qaidam

Sichuan

Himalayan

Thrust

Belt

34°N

suture zone thrust fault normal fault strike-slip fault

B

Lhasa

Nima

Fig. 2

200 km0

80 E°

35°N

30°N

80°E 90°E

30°N

35°N

90°E

Karakoramfault

Jiali faultHimalayan Thrust BeltMFT

Quaternary basin Region of internal drainage

IYS

BS

JS

A

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 defi ned geologically by the Indus-Yarlung suture zone (IYS), which was modifi ed by the Oligocene Gangdese thrust (GT) and Miocene Great Counter thrust (GCT). The Bangong suture zone (BS) was modifi ed by the mid-Tertiary N-dipping Shiquanhe-Gaize-Amdo thrust system (SGAT) and S-dipping Gaize–Siling Co thrust (GST). Figure modifi ed from Kapp et al. (2005).



Geological records of the Lhasa-Qiangtang and Indo-Asian collisions

Geological Society of America Bulletin, July/August 2007 919

With this goal in mind, we conducted geologi-cal 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 sec-tions 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 set-ting, 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 fl ows north-northeast-ward 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 discontinu-ous, 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 western-most 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 fl ank of the Muggar Range (Fig. 2). This region is referred to as the northern Nima area. The ~15-km-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

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 pri-mary stratigraphy is unknown because this unit has been greatly deformed and locally exhib-its tight upright to overturned folds, penetra-tive cleavage, and transposed bedding. These rocks are assigned a Jurassic age (mapped as Jr, Fig. 2) and appropriately described as sedimen-tary-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 metaba-sites. Regional geological maps show contrast-ing 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 inter-val 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, reef-facies 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 subse-quent 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 briefl y here, and detailed descriptions and measured sections are provided in DeCelles et al. (2007a).

In the northern Nima area, the oldest non-marine 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 paly-nomorphs; DeCelles et al., 2007a) red beds, fl u-vial and eolian sandstones, and conglomerates, with the fl uvial deposits showing southward paleocurrent indicators (Fig. 3). The Lower Mug-gar unit is in thrust-fault contact with Eocene to Miocene (based on palynomorphs; DeCelles et al., 2007a) siltstone, marl, evaporite, conglom-erate, and sandstone of the Upper Muggar unit to the north (Tmu; Figs. 2 and 3). Structural dis-ruption and lateral variability in lithology make it diffi cult to determine the relative ages of the different measured sections in the Upper Mug-gar 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 over-lie the J-K unit and consist of Lower Cretaceous volcanic fl ows, tuffs, and breccias interbedded with volcaniclastic conglomerate and sand-stone (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 con-glomeratic 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 lime-stone 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). Mid-Tertiary 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 north-eastward paleocurrent directions (Fig. 4).

The mid-Tertiary units are overlain by gen-erally poorly exposed younger units that were not studied in detail. Gently dipping (~10–20°)

1Figures 2 and 8 are on a separate sheet accompa-nying this issue.



Kapp et al.


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 conglomer-ates are similar to those of rocks exposed within the Muggar Range to the north, including the

Xiabie granite. Southern Nima area conglomer-ates 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

of Nima, these deposits consist of gray fl uvial 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 con-glomerates along the southern fl ank of the Mug-gar Range, fl uvial 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 allu-vium 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 iso-tope-dilution thermal ionization mass spectrom-etry (ID-TIMS) age of 564 ± 4 Ma in between sets of 3–5 analyses on unknown grains. Com-mon Pb was corrected for using measured 204Pb 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 206Pb. Consequently, 206Pb*/207Pb* and 207Pb*/235U ages have signifi cantly higher uncertain-ties than 206Pb*/238U ages and are excluded from consideration in our age interpretations. Our approach in dating the igneous rocks was to ana-lyze 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, respec-tively, are summarized in Table 1. Additional details of the analytical methods and a complete

silt/clay sand conglomerate

Tm

u>

800

mK

vc>

400

m volcaniclastic rockspaleosols

tuffs; (volcanic-clast conglomerateca. 118 Ma 4MK28, 175)

Northern Nima Area

angular unconformity (<10°)

J-K Unit (Jurassic - Lower Cretaceous transposed rocks)

thrust fault contact

Km

l>

400

m

mud/siltstonesilt/sandstonemarlsandstonesandstoneeolianiteconglomerate

paleocurrentdirection

Legend

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

(°N)Longitude

(°E)Description Interpreted age

(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





tabulation of the U-Pb data are available in the GSA Data Repository.2

U-Pb Results

Intrusive RocksThe Puzuo granite intrudes the J-K unit and

is unconformably overlain by volcanic fl ows and tuffs of the Kv unit (Fig. 2). Ages of 26 zir-con 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).

J-K UnitA 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 distribu-tion of detrital zircon ages that range from Creta-ceous to Archean (Fig. 5C). The provenance sig-nifi cance 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 popu-lation of ages (125 ± 5 Ma) is more relevant to this study because it provides a maximum depo-sitional age for this sample and the unconform-ably overlying Kv unit. This is the fi rst robust documentation that marine deposition persisted along the central Bangong suture north of the Gaize–Siling Co thrust until at least ca. 125 Ma.

Kvc UnitTwo 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 defi ne a dominant popula-tion 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 sta-tistically indistinguishable mean ages provided

Kr

~18

0 m

Kv

>20

0 m

Kcv

~66

0 m

Kcl

>73

0 m

Tr

~40

00 m

100 - 115 Ma6-11-04-4DZ);

, detrital zircons( volcanic flows, tuffs,

breccias, sandstone

90 - 105 Ma6-11-04-2DZ)

, detrital zircons(

redbedsvolcaniclastic conglomerate, rippled sandstone; tuff, (ca. 99 Ma 3MC13)

volcaniclastic conglomerate

limestone-clast conglomerate

stacked sequences of conglomerate, marl, and

siltstone; lacustrine fan-delta deposits

red siltstone and rippled sandstone

lacustrine marl, rippled sandstone and siltstone

red beds

fluvial red beds

gray granite-clast conglomerateN-Q

syncontractional growth

syncontractional growth

Southern Nima Area

Legend

volcanic rocksvolcanic brecciaconglomeratesandstonemarlsilt/sandstonemud/siltstone

paleocurrentdirection

angular unconformity



marine turbiditic sandstone; phyllitic; youngest detrital zircons ca. 125 Ma (7-11-05-2)J-

K

silt/clay sand conglomerate

2GSA Data Repository item 2007166, U-Pb and 40Ar/ 39Ar methodology and results, is available at http://www.geosociety.org/pubs/ft2007.htm, or on request from [email protected].

Figure 4. Schematic Creta-ceous–Tertiary stratigraphic column of the southern Nima area based on mea-sured sections (1MC-3MC, 1DC-4DC, and 1NM-2NM; Fig. 2). See DeCelles et al. (2007a) for detailed mea-sured sections, descriptions, and interpretations of the stratigraphy.



Kapp et al.


0

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

80 100 120 140 160

0 400 800 1200 1600 2000

70 90 110 130 150 170

145

135

125

115

105

95

140

130

120

110

100

90

150

140

130

120

110

100

90

7-14-98-2Puzuo granite

n = 26

7-14-98-2 (granitoid)124 ± 4 Ma, MSWD = 1.4, n = 26

0

1

2

3

4

5

80 100 120 140 160

4MK28tuff

n = 27

4MK28 (tuff)118 ± 3 Ma, MSWD = 1.9, n = 18

4MK175tuff

n = 25

4MK175 (tuff)117 ± 2 Ma, MSWD = 1.3, n = 25

Age

(Ma)

Age

(Ma)

Age

(Ma)

Age

(Ma)

mean of main population

7-19-98-2 (granitoid)118 ± 3 Ma, MSWD = 2.3, n = 26

140

120

110

100

80

90

130

0

2

4

6

8

10

70 90 170110 130 150

7-19-98-2Xiabie granite

n = 27

0 500 1000 1500 2000 2500 3000

0

1

2

3

4

70 100 130 160 190

n = 127-11-05-2sandstone

n = 99

180

140

120

100

80

160

7-11-05-2 mean of youngestpopulation: 125 ± 5 MaMSWD = 1.3, n = 12

n = 18

Age (Ma)

Age

(Ma)

A

B

C

D

E





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 sta-tistically indistinguishable from the interpreted crystallization age of the Xiabie granite, sug-gesting that intrusive and extrusive rocks of the northern Nima area may be related to a single magmatic episode.

Kv UnitFour 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 alter-native 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 defi ne a single population, tailing off slightly to older ages, and give a mean age of 106 ± 2 Ma (Fig. 5F). A con-servative interpretation is that this age provides a maximum depositional age for the volcanicla-stic 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.

Kr and Kcv UnitsZircon 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

with a mean age of 99 ± 2 Ma (Fig. 5G). This age is consistent with stratigraphic relations indicat-ing that the Kr unit is younger than ca. 106 Ma volcanic rocks of the Kv unit. A sample of sand-stone from the upper part of the conformably overlying Kcv unit (6-11-04-2; Fig. 2) yields 43 detrital zircon ages that defi ne 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, consider-ing the stratigraphic relations with the underly-ing ca. 99 Ma Kr unit, is interpreted to closely approximate the depositional age.

40Ar/ 39Ar Methods

The 40Ar/39Ar studies were conducted on seven biotite separates from tuffaceous horizons in

0

1

2

3

4

5

6

7

80 90 100 110 120 130

1 16

1 12108104

10096928884

3MC13tuff

n = 32

3MC13 (tuff)99 ± 2 Ma, MSWD = 2.4, n = 32

Age

(Ma)

Age (Ma)0

2

4

6

8

10

12

70 80 90 100 120 130110 140

6-11-04-2sandstone

n = 43

86

90

94

102

98

106

1 10A

ge(M

a)

6-11-04-297 ± 2 Ma, MSWD = 2.8, n = 33

0123456789

70 90 110 130 150

150

135

120

105

90

75

6-11-04-4volcaniclastic

sandstonen = 40

6-11-04-4, mean of youngest population106 ± 2 Ma, MSWD = 1.5, n = 34

Age

(Ma)

G

H

F

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.



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 step-heating 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 diffu-sion (MDD) model (Lovera et al., 1989, 1997). Details of the analytical methods and complete data tables and fi gures 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).

Laser single-crystal and step-heating age results are summarized in Figure 6. The preci-sion of the single-crystal laser fusion ages is signifi cantly lower than that for ages determined from step-heating analyses, primarily because of the small argon signals provided by the indi-vidual 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/36Ar

o 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 sensitiv-ity to systematic errors for the very small argon signals. In general, the step-heating age spectra are characterized by initial steps that yield rela-tively young ages compared to the majority of the higher-temperature steps, which give appar-ent 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 distribu-tion (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 consis-tent (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 uncertain-ties cited at the 1σ level; Table 1). Singe-crys-tal 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 strati-graphic 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) sec-tions 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–earli-est Miocene time (between 26.0 and 23.5 Ma). We assign a Miocene age to the ~3000-m-thick succession of fl uvial red beds near the town of Nima (Figs. 2 and 4) because they conformably

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.





overlie lacustrine strata that are lithologically similar to those in sections 2NM and 1DC.

K-Feldspar Thermochronologic Results

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 inter-mediate 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 dis-play a characteristic oscillating age pattern that is indicative of excess argon hosted in fl uid 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 refl ect 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

is also recorded by nearly 80% of the spectrum that does not involve the age hump.

STRUCTURAL GEOLOGY

Early Cretaceous Unconformity

Cretaceous nonmarine strata lie unconform-ably 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 fi ndings demonstrate that this portion of the Bangong suture underwent signifi cant deformation, erosion, and a tran-sition from marine to nonmarine conditions between ca. 125 Ma and ca. 118 Ma. Further-more, 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 Aptian-Albian 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 Sil-ing 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 decreas-ing bedding attitudes away from the fold axis, which are interpreted to indicate syndeposi-tional growth of the syncline during N-directed slip along the Gaize–Siling Co thrust. This sug-gests 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–Sil-ing 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

0 20 40 60 80 100Cumulative % 39Ar released

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Figure 7. (A) K-feldspar 40Ar/39Ar apparent age spectrum. (B) Thermal history calculated from multidomain diffusion modeling. The dark-gray fi eld shows the mean of at least 20 solutions, and the gray fi eld represents a 90% confi dence window.



Kapp et al.


in the proximal footwall of the thrust, suggest-ing that the thrust is south vergent. The Creta-ceous volcanic rocks in the hanging wall are the most likely source for the northerly derived (Fig. 4) and proximal volcaniclastic conglomer-ates 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 Queri-Malai 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). Analy-sis 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, juxtapos-ing 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 demon-strate 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 neo-tectonic 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 hang-ing 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 syn-cline (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).

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 con-tinued 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 demon-strably 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, col-lectively 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 character-ized by widely distributed strike-slip and nor-mal faults with relatively small strike lengths (generally <6 km; Fig. 2). A detailed kine-matic 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 strik-ing faults, which is taken to indicate a domi-nantly 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 Xia-bie granite in the Nima area, place fi rst-order constraints on the Cretaceous–Tertiary his-tory of upper-crustal deformation and basin development. This is illustrated in the form of a cross section and its sequential restora-tion shown in Figures 8A–8C (see footnote 1). Whereas the cross section is poorly constrained at depth, specifi cally with respect to the style and depth of deformation within the penetra-tively deformed J-K unit, it provides a useful estimate of upper-crustal shortening following fi nal 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.





Cross-Section Assumptions

The cross section was constructed using strati-graphic thicknesses for Cretaceous and Tertiary nonmarine strata determined by projecting sur-face bedding measurements to depth and from detailed measured sections (DeCelles et al., 2007a). Thicknesses were kept constant at depth unless geological relations required lateral varia-tions. The thickness of the Aptian-Albian lime-stone unit is ≤1000 m in the northern Lhasa ter-rane (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 exten-sive 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 con-structing the cross section are as follows: (1) The regional unconformity beneath Creta-ceous units had minimal initial relief and can therefore be restored to horizontal. The valid-ity of this assumption is diffi cult to assess, but we observed no evidence in the sedimentary record for major intrabasinal relief (such as but-tress 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 downwarp-ing of the Cretaceous unconformity below its initial regional elevation). This assumption is appropriate for typical continental lithosphere with suffi cient strength to support short-wave-length loads (<100 km; e.g., Turcotte and Schubert, 1982) but may not be for an anoma-lous 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 Cre-taceous, coeval with slip along the Gaize–Sil-ing 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 Mug-gar 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 construct-ing the cross section are itemized in Figure 8.

Shortening Estimates

The restored cross section shows the unde-formed N-S length of the Cretaceous unconfor-mity to be 123 km (Fig. 8C). Since the present-day 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 hanging-wall Cretaceous marine limestone and the foot-wall Cretaceous nonmarine rocks is unknown. The original separation was likely signifi cant (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 Queri-Malai, 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 excep-tion 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 Xia-bie granite from a minimum depth of ~10 km (the granite was at temperatures >300° prior to the mid-Cretaceous based on thermochrono-logic 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 fl attens with depth or exhibits ramp-fl at geometries within the upper crust. Distinguishing the relative mag-nitude of Cretaceous versus Tertiary shortening is diffi cult 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 sedimen-tary rocks in the northern Nima area were pen-etratively 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 Xia-bie 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). Fold-ing 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 signifi cantly along the Muggar thrust and propagated southward to the Puzuo thrust fault. We infer that the ca. 99 Ma Creta-ceous 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 struc-ture, and similar deposits may also have been shed northward. Initiation of the Gaize–Siling Co thrust shortly postdated slip along the Queri-Malai 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 hang-ing wall of the Nima thrust is inferred to have uniformly elevated overlying Cretaceous rocks.

Mid-Tertiary History

Geologic relations provide no evidence for sig-nifi cant deformation in the Nima area subsequent



Kapp et al.


to Cenomanian time and prior to onset of non-marine sedimentation during the late Oligocene. It appears that all of the major structures that were active during Cretaceous time were reac-tivated 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 Ter-tiary strata at the latitude of the town of Nima fi lled 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 north-ern Nima area are inferred to have been folded and structurally elevated due to internal short-ening 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 sedi-ment accumulation rates of ~1 mm/yr are typi-cal of those in foredeep depozones of fl exural 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 contraction-related deformation in the area, is ca. 21 Ma.

Late Cenozoic History

Late Cenozoic, approximately N-striking nor-mal 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 displace-ment south of the town of Nima is accommo-dated 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

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-strik-ing 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 infl uence on the pattern of Quaternary sedimen-tation (Fig. 2). The Xiabie Lake basin is a gra-ben 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-the-west tilting of the Dagze Lake basin toward E-dipping 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 deforma-tion 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 con-trast to the Nima area, however, these deposits are entirely conformable. The clastic deposits consist of marginal marine to fl uvial facies, show mainly southward paleocurrent indicators, and are interpreted to have been deposited in a foreland basin related to the S-directed thrust-ing associated with Lhasa-Qiangtang collision in the north (Leeder et al., 1988; Zhang et al., 2004; Leier, 2005). The Lower Cretaceous clas-tic deposits of the Lhasa terrane are conform-ably overlain by Aptian-Albian shallow-marine limestones. Whereas the cause of the Aptian-Ablian 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 demon-strate 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 Creta-ceous 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 magma-tism 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 thick-ening during Lhasa-Qiangtang collision (Xu et al., 1985) or mantle thinning following Lhasa-Qiangtang 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 fl at-slab subduction of Neotethyan oceanic lithosphere along the Indus-Yarlung suture (Fig. 9A). In this hypothesis, the N-directed mid-Cretaceous Gaize–Siling Co and Nima thrusts are back thrusts, and the associ-ated nonmarine strata represent wedge-top deposits trapped in a triangle zone structural setting (Fig. 9B). Dominantly S-directed, thin-skinned shortening related to ongoing, postcol-lisional convergence in the Bangong suture zone propagated southward into the northern Lhasa terrane during the Late Cretaceous to Paleo-cene (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 signifi cant crustal thickening and elevation gain prior to the Indo-Asian collision. Paleoelevation studies indicate that the late Oli-gocene 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 Indo-Asian collision.

Magnitude of Cretaceous versus Mid-Tertiary Shortening

The magnitude of mid-Tertiary shortening esti-mated for the Nima area is ~25 km over the pres-ent-day N-S width of ~65 km (Fig. 8). Although this estimate is a minimum, it is unlikely to be signifi cantly greater. The magnitude of Tertiary





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Figure 9. Schematic cross-sectional diagrams illustrating the Cretaceous to mid-Tertiary tectonic evolution of the Himalayan-Tibetan oro-gen. (A) Early Cretaceous northward continental underthrusting of the Lhasa terrane beneath the Qiangtang terrane along the Bangong suture (BS), driven by northward fl at-slab subduction of the Neotethyan oceanic lithosphere to the south. (B) Between 100 and 50 Ma, S-directed 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 suffi cient to have produced signifi cant crustal thickening and elevation gain in Tibet prior to Indo-Asian colli-sion. (C) During the early Tertiary, signifi cant 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 suffi cient 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.



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 Mug-gar 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 large-magnitude 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; Tap-ponnier et al., 2001). Based on this study and pre-vious 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 signifi cant 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). E-W–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 fi ll of reportedly Paleocene to Oligo-cene 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 succes-sion 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-Creta-ceous and late Oligocene. Similarly, Tertiary contraction and basin development in the Shi-quanhe area along the Bangong suture zone in far western Tibet (Fig. 1) are restricted to the late Oligocene–early Miocene (Kapp et al., 2003).

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 colli-sion. Alternatively, if the Lunpola basin is an along-strike chronostratigraphic equivalent of the Nima-Shiquanhe basins, then its fi ll (which oxygen isotope studies indicate was deposited at a paleoelevation of >4 km; Rowley and Currie, 2006) may be substantially younger than pres-ently assumed.

Although the possibility of early Tertiary con-traction along portions of the Bangong suture remains to be tested, the available timing con-straints 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 non-marine strata of late Oligocene–early Miocene age (Kailas or Gangrinboche conglomerate; Yin et al., 1999; Aitchison et al., 2002), and it was modifi ed by the S-directed Gangdese thrust sys-tem between 30 and 23 Ma (Fig. 1; Yin et al., 1994, 1999; Ratschbacher et al., 1994; Harri-son 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 ter-ranes, 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 observa-tion 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 poten-tial 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 suffi cient to inhibit upper-crustal shortening in this area and to result in contractional deformation in lower-ele-vation regions in north-central Tibet (Cyr et al., 2005) and to the south in the Tethyan Himalaya during the early Tertiary (Fig. 9C). The feasi-bility 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 short-ening 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 thrust-ing in Tibet was mechanically linked with the Himalayan thrust belt, perhaps representing hin-terland 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 deforma-tion and basin development along the Bangong suture in central Tibet. Jurassic to Early Creta-ceous marine sedimentary rocks were deformed and uplifted above sea level prior to the onset of nonmarine deposition at ca. 118 Ma. Cre-taceous 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 activ-ity 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 suc-cessions of nonmarine strata (locally ~4000 m thick) accumulated in wedge-top basins and at high elevation (>4.6 km). Mid-Tertiary thrust-ing 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 asso-ciated 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 deforma-tion is characterized by widely distributed but relatively small displacement on approximately N-striking normal and more easterly striking strike-slip faults, which together are accom-modating 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 pre-dating 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





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 Exxon-Mobil. 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 fi eld and J. Fox, F. Guerrero, and A. Pullen for their assistance with mineral separa-tion and U-Pb analysis. We thank Brad Ritts and an anonymous reviewer for critical reviews, and sugges-tions by Paul Heller and Karl Karlstrom helped us improve this paper.

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MANUSCRIPT RECEIVED 4 MAY 2006REVISED MANUSCRIPT RECEIVED 18 DECEMBER 2006MANUSCRIPT ACCEPTED 19 JANUARY 2007

Printed in the USA



4700

4900

4700

4600

4800

4600

4600

4700

4500

4800

5000

6081

4600

4500

4600

5000

4900

4500

Q1

Q2

Q1

Q1

Q1

Q2

Q1

7-13-98-1

6-10-04-1

5469

6-11-04-1

7-13-98-1

7-13-98-2

7-13-98-3

Puzuogranite

7-14-98-2124 ± 4 Ma

7-14-98-1ca. 110 Ma

7-14-98-3

7-22-98-2

7-22-98-1

7-22-98-3

7-22-98-4 7-22-98-5

7-19-98-3

7-16-98-17-16-98-2

7-19-98-2118 ± 4 Ma

7-19-98-1

7-14-98-2b

6-10-04-1

2

3 6-11-04-4~106 Ma5

6-11-04-1

6-11-04-2~97 Ma

7-11-05-2<125 Ma

46

46

33

5835

30

67

42

8283

72

204659

35

57

13

50

12 20

2620

65 6043

85

70

57

22

54

14

2730

24

7062

20

75

75

22

51

52

60

8845

3072

52

30

10

30 35 23 20

35 58

65

36

40

69

76

7572 80

6752

4220

4233

6240

2321

4121

41

56

74

22

22

356527

35

48

46

46

5946 20

3533

1764

32

43

42

33 32

5064

5572

4638

70

69

8042

66

5216

4033 27

60

67

75

82

84

89

16

20

34

1315

385760

47

8

7545

2356

7766

37

32

35

12

20

3220

3758

75

75

65

22

46

72

51

45 88

60

5243

30

52

76

6858

40 8 4236

78

3050 30

183750425555

70 65

70

60

42

4385

1836

607867

40 4554

69

57

3422

54 60

65

40

52

6845

3085

68

45

21

50

20

30

75 75

25

29

30

52

30

29

7882

8372

18

12 20

2620

3920

56 46

82

1810

1020

30

33

14

2730

24

7062

58

20

83

80

75

7045

1MC

2MC

76

86

58

7985

85

A

A'A'

A'' A''

A'''

Muggar thrust

Nima thrust

Pazuo thrust

Queri - Malai thrust


Malai peak

Queri range

Gaize - SilingCo thrust


Kcv

Kr

area unmapped

N-Q

Kl

Nima

Kl

Kl

Nf

Jr

KvKv

gr

grgr

Jr

32°00'N32°00'N

87°30'E

Q2

Q1

geysers

Qglacial

Dagze Lake

Muggar Range

Kv

31°40'N

N-Q

gr

Jr

Jr

Jr

snowand ice

Kl

Kl

Kl

Kl

Kl

Kr

Kr

Kvc

Kcv


Kv

Kv

Kv

KvKv

Kv

Kv

Tr

Tmu

J-K

Nf

Nf

Nf

NfNf

Nf

Nf

Nf

N-Q

N-Q

N-Q

N-Q

N-Q

N-Q

N-Q

N-Q

Nf

Nf

N-Q

N-Q

N-Q

area unmapped area unmappedarea unmappedarea unmappedarea unmapped

area unmappedarea unmapped

Xiabie Lake

PuzuoLake

Kr

Kr

Kr

Kr

Kr ?Kcl

Kcl

Kcl

Kcv

KcvKcv

Kcv

KclKcl

Kcl

KclKv

J-K

J-K

J-K

J-KJ-K

J-K

J-K

J-K

J-K

J-K

J-K

J-KJ-K

J-K

J-K

J-K

J-K

J-K

J-K

J-K

J-K

J-K

J-K

Kvc

Jr

Mochang R.

Q1

Q2

Q2

Q1

Q1

Q2Q1

shor

elin

es

shorelines

shorelines

Q1Q1

Q2

Moc

hang

Riv

er

Q1

Q1

Q1

Q2

Q2

Q2

Q1

Q1

Q1

Q1

Q1

Q2

Q2

Q2

Q1

Q2 Q2

Q2

Q2

Q2

Q1

Q1

Q1Q2

Q2

Q2

Q2

Q2

Q2




Queri Range

Malai Peak



Nima synclineNima syncline

Puzuothrust

Puzuo thrust

SouthPuzuothrust

Zanggenongthrusts

Nima thrust

Nima thrust

Muggar thrust

Muggar thrust

Xiabiegranite

Tr

Tr

Tr

Tr

Tr

Tr

Tr Tr

Tr

Tr

Tr

TrTr

Tr Tr

TrTr

Tr Tr

Tr

Tr

Tr

KmlKml

Kml

Tmu

Tmu

Tmu

Tmu

Tmu

Tmu

Tmu

Tmu

Tmu

TmuTmu

contour interval = 100 m

0 2 4 6 8 10 km

Geological Map of the Nima AreaCentral TibetN

J-K

gr

Kl

Kv

Tr

lake

Geologic SymbolsContacts

Thrust fault; arrow showsdip; diamond shows trend

of striations

High-angle normal fault;bar-ball on hanging wall

FoldsSolid where well located, dashed where approximately located or

inferred, dotted where buried; arrow shows direction of plunge

AnticlineSyncline

Strike and dipTransposed

bedding Cleavage

Trend and plunge ofsmall-scale fold axis

Other

Marker-bed6-2-98-2

Sample locality

Quaternary deposits; incised

Albian volcanic flows, tuffs, breccias; volcaniclastic sandstone and conglomerate

Cretaceous granite and granodiorite

Map Units

Lithologic

river

Jr

Kr

Kcv

Kcl

Neogene fan conglomeratesNf

N-Q Neogene - Quaternary (?) conglomerate

Q1

Horizontal Inclined Vertical Overturned

3MC Start ofmeasured section

Strike-slip fault

Jurassic: argillite, shale, siltstone, limestone, turbiditic sandstone, metavolcanic rocks

Jurassic - Cretaceous: shale, siltstone, turbiditic sandstone, metasedimentary-matrix mélange

Aptian-Albian massive reef-facies foraminiferal- and rudist-bearing limestone

Kvc Aptian volcaniclastic conglomerate, sandstone, siltstone; paleosols, tuffs

Albian-Cenomanian red beds; volcanic-clast sandstone and conglomerate

Cenomanian conglomerate with mainly volcanic clasts

Cenomanian conglomerate with mainly Aptian-Albian limestone clasts

Tertiary red beds of the S. Nima Area (T) and Upper Muggar Unit (T) of the N. Nima Area rmu

Q2 Quaternary deposits; youngest

shorelinesGlacier or year-

round snow

faults in red showneotectonic activity;mapped largely from

satellite imagery

Solid where well located, dashed where approximately located or inferred, dottedwhere concealed and inferred

muT

Kml Upper Cretaceous to Paleocene Lower Muggar Unit

2NM (26-25 Ma)3DC

2DC

1MK

1MC

2MC

1MK

3MK

2MK

5MK

4MK118 ± 3Ma

1NM

2DC

3DC

5DC23.5 Ma

4DC

1DC26-25 Ma

1MC

2MC

2NM (26-25 Ma)

3MC~99 Ma

Figure 2. Kapp et al.

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.

3210

km

45

-1-2-3-4-5

3210

km

45

-1-2-3-4-5-6-6

Gaize -Siling Co

thrust

Nimathrust

Queri - Malaithrust

SouthernPuzuothrust

Puzuothrust Muggar

thrustZanggenong

thrusts

A A' A'' A'''South North

Kl

Kcl

Kcv Kv

J-K

J-K

J-K

J-K

J-K

Kr

KrKv Kv

Jr

Xiabiegranite

Kvc

Xiabiegranite

108-104 Marapid cooling

KvcKr Kr

Kv

Kv

Kcv

J-K

J-K

J-KJ-K

Jr

Kcv

KclKl Kcv

118 Ma tuffsca. 110 Mavolcanicsca. 123 Ma

granitoid

ca. 106 Mavolcanics

Kv

Kv

ca. 99 Matuff

folding between106 and 99 Ma

Assumption: No initial reliefon unconformity beneath Cretaceous;

restored to horizontal

stratigraphic pinch-out requiredbut location uncertain due to erosion

future location ofTertiary Nima basin future location of

Tertiary Northern Nima basin

0 2 64 8 10 km

no vertical exaggeration

originalseparationunknown

Aptian-Albianmarine

carbonate

Kl

Xiabie granite (ca. 118 Ma)>10 km below surface

Kvc

Cross section of the Nima area

Cretaceous shortening and basin development

C Restored cross section

1.

2.

3.

4. KK

5. J-KJ-K

6.J-K K

J-K7. J-K K

J-K

8.

Geometry of Tertiary Nima basin constrained from bedding attitudes; thickness of >4km for southern Nima basin confirmed from measured sections.

Nima thrust is assumed to be active during Cretaceous deposition. Space betweenTertiary Nima basin and underlying Cretaceous volcanic rocks is filled by wedges ofsyncontractional deposits derived from the hanging wall of the Nima thrust.

The elevation of the Cretaceous unconformity beneath the Tertiary Nima basin istaken to mark the initial regional elevation of the unconformity.

Gaize - Siling Co thrust soles out at base of , as no older strata are observed alongits entire strike length. is shown with a structural thickness of ~2 km, likely obtainedby folding/imbrication of a limestone unit <500 m in stratigraphic thickness.

Thickening of unit is required to explain the regional structural elevation of rocksin the hanging wall of the Nima thrust; accomplished here by emplacing a horse of .

The location of this footwall ramp is constrained from the regional geometry of thecontact between and . The structural relief on the ramp is the minimumrequired to erode post- strata between the ramp and the Puzuo thrust.

Duplexing in the unit is inferred to elevate the vc unit above its initial regionalelevation (see 3 above). Area balance of the unit is maintained by slip on thePuzuo thrust system.

The geometry of the Muggar thrust at depth is uncertain, but it is taken to root deeplyto minimize estimated shortening.

ll

vc

1

Cretaceous unconformity

Cretaceous unconformity

J-K2

3

4

5

6

7

Shortening Estimates

Present-day length = 64.6 km

Restored length = 123.0 km

Minimum Tertiary shortening = 25 km(28%)

Minimum total shortening (post-mid-Cretaceous) = 58 km (47%)

Position of undeformedfootwall is pinned

1

Figure 8. Kapp et al.

Tmu

Kml

Tr

A

B

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 TibetPaul Kapp, Peter G. DeCelles, George E. Gehrels, Matthew Heizler, and Lin Ding

Figures 2 and 8Supplement to: Geological Society of America Bulletin, v. 119, no. 7/8, doi: 10.1130/B26033.S1.

© Copyright 2007 Geological Society of America



Geological Society of America Bulletin

doi: 10.1130/B26033.1 2007;119, no. 7-8;917-933Geological Society of America Bulletin

Paul Kapp, Peter G. DeCelles, George E. Gehrels, Matthew Heizler and Lin Ding Nima area of central TibetGeological records of the Lhasa-Qiangtang and Indo-Asian collisions in the

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