Geological Records of the Lhasa-Qiangtang and Indo-Asian Collisions in the Nima Area of Central Tibet

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 respectively. Tertiary syncontractional basin Biffi , 2000, 2001; Jolivet et al., 2001; Robinson development in the Nima area was coeval et al., 2003), the Lhasa and Qiangtang terranes A geological and geochronologic investi- with that along the Bangong suture in west- in Tibet (Fig. 1B) (Murphy et al., 1997; Yin and gation of the Nima area along the Jurassic– ernmost Tibet and the Indus-Yarlung suture Harrison, 2000; Ding and Lai, 2003; Kapp et al., Early Cretaceous Bangong suture of central in southern Tibet, suggesting simultaneous, 2003, 2005; Guynn et al., 2006), and the Long- Tibet (~32°N, ~87°E) provides well-dated renewed contraction along these sutures dur- men Shan along the eastern plateau margin (e.g., records of contractional deformation and ing the Oligocene-Miocene. This suture-zone Arne et al., 1997; Wallis et al., 2003). sedimentation during mid-Cretaceous and reactivation immediately predated major Another common assumption is that the mid-Tertiary time. Jurassic to Lower Creta- displacement within the Himalayan Main Tibetan Plateau interior was formed by mech- ceous (≤125 Ma) marine sedimentary rocks Central thrust system shear zone, raising the anisms that are proposed to be acting along were transposed, intruded by granitoids, possibility that Tertiary shortening in Tibet the actively growing margins of the plateau and uplifted above sea level by ca. 118 Ma, and the Himalayas may be interpretable in or predicted from geodynamic models. These the age of the oldest nonmarine strata doc- the context of a mechanically linked, compos- include (1) northward underthrusting/insertion umented. Younger nonmarine Cretaceous ite orogenic system. of Indian lithosphere (Argand, 1924; Powell rocks include ca. 110–106 Ma volcanic-bear- and Conaghan, 1973; Ni and Barazangi, 1984; ing strata and Cenomanian red beds and Keywords: Tibet, plateau, thrust belt, Indo-Asian Zhao and Morgan, 1987; DeCelles et al., 2002), conglomerates. The Jurassic–Cretaceous collision, suture zone, basin development. (2) homogeneous lithospheric shortening and rocks are unconformably overlain by up to thickening (Dewey and Burke, 1973; England 4000 m of Upper Oligocene to Lower Mio- INTRODUCTION and Houseman, 1986; Dewey et al., 1988) cene lacustrine, nearshore lacustrine, and and subsequent removal of mantle lithosphere fl uvial red-bed deposits. Paleocurrent direc- The vast, internally drained region of the (England and Houseman, 1989; Molnar et al., tions, growth stratal relationships, and a Tibetan Plateau interior (Fig. 1A) is the focus 1993), (3) upper-crustal shortening coupled structural restoration of the basin show that of some of the most provocative concepts in with passive infi lling of intermontane basins Cretaceous–Tertiary nonmarine deposition continental tectonics today, yet our understand- and oblique intracontinental subduction along was coeval with mainly S-directed thrusting ing of its geological evolution and uplift history reactivated suture zones (Mattauer, 1986; Meyer in the northern part of the Nima area and N- remains poor. Numerous popular models of et al., 1998; Roger et al., 2000; Tapponnier et directed thrusting along the southern margin Tibetan Plateau formation assume that the thick al., 2001), and (4) thickening and fl ow of weak of the basin. The structural restoration sug- crust and high elevation of Tibet are mainly con- middle crust away from the India-Asia collision gests >58 km (>47%) of N-S shortening fol- sequences of India’s collision with Asia since the zone, driven by topographic gradients (Bird, lowing Early Cretaceous ocean closure and Eocene. There is growing documentation that 1991; Royden, 1996; Royden et al., 1997; Clark ~25 km shortening (~28%) of Nima basin challenges this assumption; evidence shows that and Royden, 2000; Beaumont et al., 2001, 2004; strata since 26 Ma. Cretaceous magmatism large parts of southern Asia underwent major Shen et al., 2001). and syncontractional basin development are pre–Indo-Asian collision crustal shortening and Some necessary pieces of information funda- attributed to northward low-angle subduc- thickening, including the Karakoram-Pamirs in mental to advancing models of plateau formation of the Neotethyan oceanic lithosphere the west (Fraser et al., 2001; Hildebrand et al., tion are quantitative constraints on the timing and Lhasa-Qiangtang continental collision, 2001; Robinson et al., 2004), ranges border- and magnitude of pre–Indo-Asian collision ing the northern margin of the Tibetan Plateau versus post–Indo-Asian collision shortening in †E-mail: [email protected]. (e.g., Sobel, 1995; Sobel et al., 2001; Ritts and 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 ﬁ gures; 1 table; 1 insert; Data Repository item 2007166.

80° E 90°E

35°N 35°N JS

Karakoram fault

Fig. 2

BS Nima

Jiali fault Hima layan 30°N 30°N Thrus MFT t Belt Lhasa 80°E 0 200 km A 90°E IYS Quaternary basin Region of internal drainage 80°84E E °88E° 92° E B Jinsha suture Songpan Ganzi Fenghuo Shan-Hoh Xil

Qiang 34°N tang anticlin ? Bangong sutureorium ? ? Qiangtang ? terrane

Amdo 32° N Shiquanhe SGAT Gaize GST Lunpola H i Nima m Fig. 2 Lhasa terrane al ay Siling Co an Duba Coqin Th ru 30° N st Tarim Qai Pamir dam GCT B Lhasa Tibet elt

India Sichuan 0 200 km GT Indus-Yarlung suture Early Cretaceous granite Late Cretaceous - early Tertiary 65-40 Ma volcanic rocks 42-30 Ma volcanic rocks Gangdese batholith Tertiary strata suture zone thrust fault normal fault 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 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).

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 geologi- are exposed in the Muggar Range and consist marine rocks of the J-K unit and consist of a cal mapping and integrated geochronologic and of banded argillite interbedded (or tectonically >400-m-thick Lower Cretaceous succession detailed stratigraphic-sedimentologic studies of interlayered) with an ~50-m-thick massive of volcaniclastic conglomerate, sandstone, and the Nima basin area (~32°N, ~87°E) along the white limestone and thinly interbedded shale, siltstone with tuffaceous and paleosol horizons Jurassic–Early Cretaceous Bangong suture in siltstone, turbiditic sandstone, fossiliferous in the lower part (Kvc; Figs. 2 and 3). There is central Tibet (Fig. 1). Detailed measured sec- limestone, and metavolcanic rocks. The pri- a low-angle unconformity between the Kvc unit tions and provenance studies of Nima basin mary stratigraphy is unknown because this unit and the overlying Lower Muggar unit (Kml; strata (DeCelles et al., 2007a) and oxygen and has been greatly deformed and locally exhib- Figs. 2 and 3), which consists of >400 m of carbon isotopic studies indicating high regional its tight upright to overturned folds, penetra- Upper Cretaceous to Paleocene (based on paly- paleoelevation (>4.6 km) and arid conditions tive cleavage, and transposed bedding. These nomorphs; DeCelles et al., 2007a) red beds, fl u- during the late Oligocene (DeCelles et al., rocks are assigned a Jurassic age (mapped as Jr, vial and eolian sandstones, and conglomerates, 2007b) are presented elsewhere. The purpose Fig. 2) and appropriately described as sedimen- with the fl uvial deposits showing southward of this contribution is to present our data and tary-matrix mélanges on regional geological paleocurrent indicators (Fig. 3). The Lower Mug- interpretations pertaining to the geological set- maps (Cheng and Xu, 1986; Pan et al., 2004). gar unit is in thrust-fault contact with Eocene to ting, age, and structural evolution of Cretaceous Similarly deformed rocks are widely exposed Miocene (based on palynomorphs; DeCelles et to Quaternary rocks in the Nima area and their south of the Muggar Range in the Nima area. al., 2007a) siltstone, marl, evaporite, conglom- broader tectonic implications. Where studied, they consist largely of cleaved erate, and sandstone of the Upper Muggar unit shale, siltstone, and turbiditic sandstone with to the north (Tmu; Figs. 2 and 3). Structural dis- GEOGRAPHIC SETTING subordinate metasedimentary-matrix mélange ruption and lateral variability in lithology make interlayered with greenschist-facies metaba- it diffi cult to determine the relative ages of the The town of Nima is located along the sites. Regional geological maps show contrast- different measured sections in the Upper Mug- Mochang River, which fl ows north-northeast- ing Mesozoic age assignments for these rocks. gar unit. However, we infer a general northward ward through Nima and then to the east before We assign a Jurassic to Early Cretaceous age decrease in the age of exposed Tertiary strata draining into Dagze Lake (Fig. 21). The modern for clastic deposition and mélange formation across the northern Nima area. Nima basin is a part of a larger belt of discontinu- (labeled as J-K, Fig. 2) because this time inter- In the Puzuo Lake and southern Nima areas, ous, elongate ~E-W–trending internally drained val spans that over which oceanic subduction the oldest nonmarine strata unconformably over- basins, which are generally <20 km wide and was occurring along the Bangong suture (e.g., lie the J-K unit and consist of Lower Cretaceous at ~4500 m elevation and are located along the Dewey et al., 1988; Yin and Harrison, 2000) volcanic fl ows, tuffs, and breccias interbedded approximate surface trace of the Bangong suture and our geochronologic results presented in the with volcaniclastic conglomerate and sand- (Fig. 1A). The southern margin of the Nima basin subsequent section demonstrate that this unit is stone (Kv; Figs. 2 and 4). Upper Cretaceous red is bounded by a series of E-W–trending ranges locally as young as ca. 125 Ma. Jurassic strata beds (Kr) with volcaniclastic conglomerates lie that exhibit a general increase in both width and of the Muggar Range are intruded by a biotite unconformably on both the Kv and J-K units in relief toward the south, where the southernmost granite exposed north and west of Xiabie Lake the southern Nima area (Figs. 2 and 4). These range has a width of up to 5 km and maximum (Xiabie granite, Fig. 2), whereas the J-K unit is are in turn conformably overlain by two con- elevations of ~5400 m (Fig. 2). This region is intruded by a biotite granite west of Puzuo Lake glomeratic successions: a lower, ~660-m-thick referred to as the southern Nima area. The Nima (Puzuo granite, Fig. 2). volcaniclastic conglomerate (Kcv) and an upper, basin is bounded along its western margins by The youngest marine rocks in the map area >730-m-thick conglomerate with dominantly ~N-trending, moderate-relief (~300 m) ranges, consist of Aptian-Albian, shallow-marine, reef- Aptian-Albian limestone clasts (Kcl) (Figs. 2 ~4 km wide and up to 10 km long. The western- facies limestone of the Langshan Formation and 4). The transition from volcanic to lime- most extent of the Nima basin is the Puzuo Lake (e.g., Leeder et al., 1988), which are exposed stone clast composition is abrupt, occurs over a subbasin (Fig. 2), and the surrounding geology in the southernmost range of the southern ~20 m interval of mixed clast composition, and is referred to as the Puzuo Lake area. North Nima area (Kl, Fig. 2). These marine rocks are is marked by a reversal in paleocurrent direction of Dagze Lake, the Nima basin is bounded by exclusively restricted to the hanging wall of the (from southward to northward; Fig. 4). Mid- an up to 20-km-wide series of E-W–trending S-dipping Gaize–Siling Co thrust fault (GST, Tertiary strata in the southern Nima area (Tr) ranges that exhibit a general decrease in width Fig. 2; Kapp et al., 2005). To the north of this are best exposed in the frontal ranges south of and total relief to the north toward the southern range, all map units younger than the J-K unit Dagze Lake and along the Mochang River west fl ank of the Muggar Range (Fig. 2). This region are nonmarine. The deformed nonmarine strata and southwest of the town of Nima (Fig. 2). is referred to as the northern Nima area. The ~15- have been previously inferred to be Triassic They consist of a >1000-m-thick succession of km-wide Muggar Range is locally dissected by (Cheng and Xu, 1986), Cretaceous (Pan et al., lacustrine marl, mudstone, and sandstone with active ~N-trending rifts (Fig. 2) but is regionally 2004), or Tertiary (Schneider et al., 2003). Our stacked conglomeratic fan-delta parasequences E-W–trending and contains glaciated peaks with geochronologic results, presented in the subse- overlain by a >3000-m-thick succession of red elevations in excess of 6100 m. quent section, show that the nonmarine strata clastic rocks and thin marl beds (Fig. 4). The can be divided into Lower Cretaceous, Upper lacustrine marls in the lower part of the Tertiary MAP UNITS Cretaceous, Upper Cretaceous to Paleocene, succession contain biotitic sandy tephra layers. and mid-Tertiary intervals. These strata are The entire Tertiary succession in the southern Map units in the Nima area range in age described briefl y here, and detailed descriptions Nima area shows generally northward to north- from Jurassic to Quaternary. The oldest rocks and measured sections are provided in DeCelles eastward paleocurrent directions (Fig. 4). et al. (2007a). The mid-Tertiary units are overlain by gen- 1Figures 2 and 8 are on a separate sheet accompa- In the northern Nima area, the oldest non- erally poorly exposed younger units that were nying this issue. marine strata lie unconformably on transposed not studied in detail. Gently dipping (~10–20°)

Geological Society of America Bulletin, July/August 2007 919 Kapp et al. alluvial fan conglomerates of inferred Neogene Xiabie granite. Southern Nima area conglomer- of Nima, these deposits consist of gray fl uvial age occur in both the northern and southern ates were likely derived from the nearby range conglomerate with clasts of foliated granite, Nima areas (Nf, Fig. 2) and lie unconformably of Aptian-Albian limestone to the south (Fig. 2) and they show eastward paleocurrent indicators on Tertiary and older strata. The compositions because their clasts consist almost entirely of (Fig. 4). Neogene to Quaternary deposits in the of clasts in the northern Nima area conglomer- this limestone. More gently dipping (<10°) northern Nima area consist of alluvial fan con- ates are similar to those of rocks exposed within Neogene to Quaternary(?) deposits are also glomerates along the southern fl ank of the Mug- the Muggar Range to the north, including the locally exposed (N-Q, Fig. 2). Near the town 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 Northern Nima Area of Dagze Lake (N-Q, Fig. 2). Quaternary allu- vium is divided into deposits that show evidence Legend of incision and lake shoreline development and conglomerate those that are actively being reworked (Q1 and eolianite Q2, respectively, Fig. 2). sandstone paleocurrent GEOCHRONOLOGY mu marl

T direction

>800 m silt/sandstone mud/siltstone U-Pb Methods U-Pb spot analyses were conducted on single thrust fault contact 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

ml spectrometer (MC-ICP-MS) at the Arizona

K Laserchron Center. The zircons were ablated >400 m using an Excimer laser with a spot diameter of 35 µm. Elemental fractionation between U and angular unconformity (<10°) Pb was monitored by analyzing fragments of a large Sri Lankan zircon with a concordant isotope-dilution thermal ionization mass spectrom- volcaniclastic rocks etry (ID-TIMS) age of 564 ± 4 Ma in between

vc paleosols sets of 3–5 analyses on unknown grains. Com- K mon Pb was corrected for using measured >400 m ca. 118 Ma tuffs; ( 4MK28, 175) volcanic-clast conglomerate 204Pb and assuming an initial Pb composition J-K Unit (Jurassic - Lower from the model of Stacey and Kramers (1975). Cretaceous transposed rocks) The majority of the zircon grains analyzed in this study were young (Cretaceous) and hence silt/clay sand conglomerate yielded low concentrations of 207Pb relative to 206Pb. Consequently, 206Pb*/207Pb* and 207Pb*/ Figure 3. Schematic Cretaceous–Tertiary stratigraphic column of the northern Nima area 235U ages have signiﬁ cantly higher uncertain- based on measured sections (1MK-6MK; Fig. 2). See DeCelles et al. (2007a) for detailed ties than 206Pb*/238U ages and are excluded from measured sections, descriptions, and interpretations of the stratigraphy. 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 TABLE 1. SUMMARY OF GEOCHRONOLOGIC RESULTS age of the youngest population of zircon ages Sample name Latitude Longitude Description Interpreted age (°N) (°E) (Ma) as the crystallization age. The uncertainties in 7-14-98-2 31.90 87.17 Puzuo granite 124 ± 4 the mean ages are cited at the 2σ level, include 7-19-98-2 32.22 87.22 Xiabie granite 118 ± 4 all known analytical and systematic errors, and 7-11-05-2 31.76 87.52 Sandstone ≤125 ± 5 4MK28 32.07 87.44 Tuff 118 ± 3 are in the range of 2%–4%. For the sandstone 4MK175 32.07 87.44 Tuff 117 ± 2 samples, we use the youngest population of 6-11-04-4 31.75 87.52 Sandstone ≤106 ± 2 zircon ages, rather than the youngest individual 3MC13 31.72 87.52 Tuff 99 ± 2 6-11-04-2 31.71 87.53 Sandstone ≤97 ± 2 zircon age determined, to provide constraints 8-6-03-1 31.77 87.10 Reworked tuff 26.0 ± 0.2 on maximum depositional ages. Interpreted 8-6-03-2 31.77 87.10 Reworked tuff 26.1 ± 0.2 crystallization and maximum depositional ages 2NM170 31.75 87.10 Reworked tuff 25.3 ± 0.2 1DC82 31.82 87.67 Reworked tuff 25.8 ± 0.4 for igneous and sandstone samples, respec- 1DC367 31.81 87.67 Reworked tuff 24.9 ± 0.2 tively, are summarized in Table 1. Additional 8-13-03-1 31.81 87.47 Reworked tuff 23.5 ± 0.2 details of the analytical methods and a complete

920 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 Southern Nima Area GSA Data Repository.2 N-Q gray granite-clast conglomerate

U-Pb Results syncontractional growth Intrusive Rocks The Puzuo granite intrudes the J-K unit and is unconformably overlain by volcanic ﬂ ows fluvial red beds 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 Legend in the Puzuo Lake area and a maximum age for mud/siltstone the overlying volcanic rocks. Zircon grains (n = silt/sandstone 27) from a sample of Xiabie granite (7-19-98-2; marl paleocurrent Fig. 2) yield a population of ages between sandstone conglomerate direction 110 Ma and 125 Ma, with one older age of

r volcanic breccia ca. 140 Ma. We interpret the latter age to be T volcanic rocks inherited and the mean age of the population as ~4000 m the crystallization age (118 ± 3 Ma; Fig. 5B). red beds J-K Unit A sample of marine turbiditic sandstone from the transposed J-K unit in the southern Nima lacustrine marl, rippled sandstone Figure 4. Schematic Creta- area (7-11-05-2; Fig. 2) yields a broad distribu- and siltstone ceous–Tertiary stratigraphic tion of detrital zircon ages that range from Creta- column of the southern ceous to Archean (Fig. 5C). The provenance sig- Nima area based on mea- niﬁ cance of the detrital zircon age populations sured sections (1MC-3MC, between ca. 1.8 Ga and ca. 1.9 Ga and between stacked sequences of conglomerate, marl, and 1DC-4DC, and 1NM-2NM; ca. 220 Ma and ca. 280 Ma will be discussed siltstone; lacustrine fan-delta Fig. 2). See DeCelles et al. elsewhere. The mean age of the youngest popu- deposits (2007a) for detailed mea- lation of ages (125 ± 5 Ma) is more relevant to sured sections, descriptions, this study because it provides a maximum depo- red siltstone and rippled sandstone and interpretations of the sitional age for this sample and the unconform- stratigraphy. ably overlying Kv unit. This is the ﬁ rst robust angular unconformity documentation that marine deposition persisted syncontractional growth along the central Bangong suture north of the Gaize–Siling Co thrust until at least ca. 125 Ma. cl K

Kvc Unit >730 m limestone-clast Two tuffs were sampled from a measured conglomerate section of the Kvc unit in the northern Nima 90 - 105 Ma, detrital zircons area (4MK; Fig. 3) at stratigraphic heights (6-11-04-2DZ) of 28 m and 175 m (4MK28 and 4MK175, respectively). Of the 27 zircon grains analyzed cv volcaniclastic conglomerate K from 4MK28, 18 deﬁ ne a dominant popula- ~660 m tion between 105 Ma and 125 Ma, with a mean age of 118 ± 3 Ma (Fig. 5D). A component of redbeds inheritance is indicated by the presence of eight r volcaniclastic conglomerate, rippled K sandstone; ca. 99 Ma tuff, ( 3MC13) older grains, ranging in age from Jurassic to ~180 m Proterozoic (Fig. 5D). Zircon ages (n = 25) from sample 4MK175 show a single population with angular unconformity

v 100 - 115 Ma, detrital zircons

a mean age of 117 ± 2 Ma (Fig. 5E). The sta- K (6-11-04-4DZ); volcanic flows, tuffs, tistically indistinguishable mean ages provided >200 m breccias, sandstone angular unconformity 2GSA Data Repository item 2007166, U-Pb and marine turbiditic sandstone; phyllitic; youngest 40 39 Ar/ Ar methodology and results, is available at J-K detrital zircons ca. 125 Ma (7-11-05-2) http://www.geosociety.org/pubs/ft2007.htm, or on request from [email protected]. silt/clay sand conglomerate

Geological Society of America Bulletin, July/August 2007 921 Kapp et al.

7 145 6 A 7-14-98-2 135 5 Puzuo granite 4 n = 26 125 3 115 2 105 1 7-14-98-2 (granitoid) 124 ± 4 Ma, MSWD = 1.4, n = 26 0 95 80 100 120 140 160 10 140 B mean of main population 8 130 7-19-98-2 Xiabie granite 120 6 n = 27 110 4 100 2 90 7-19-98-2 (granitoid) 118 ± 3 Ma, MSWD = 2.3, n = 26 0 80 70 90110 130 150 170 180 4 n = 12 C 7-11-05-2 3 160 sandstone n = 99 2 140

1 120

0 Age (Ma) 7-11-05-2 mean of youngest 70 100 130 160 190 100 population: 125 ± 5 Ma MSWD = 1.3, n = 12 80 0 500 1000 1500 2000 2500 3000 140 5 D 130 4 n = 18 4MK28 3 tuff 120 n = 27 2 1 110 0 80 100 120 140 160 100 4MK28 (tuff) 118 ± 3 Ma, MSWD = 1.9, n = 18 90 0 400 800 1200 1600 2000 7 150 6 E 140 4MK175 5 130 4 tuff n = 25 120 3 110 2 Age (Ma) Age (Ma) Age (Ma) Age (Ma) 1 100 4MK175 (tuff) 117 ± 2 Ma, MSWD = 1.3, n = 25 0 90 70 90 110 130 150 170 Age (Ma)

922 Geological Society of America Bulletin, July/August 2007 Geological records of the Lhasa-Qiangtang and Indo-Asian collisions

9 150 F 6-11-04-4, mean of youngest population 8 6-11-04-4 106 ± 2 Ma, MSWD = 1.5, n = 34 7 volcaniclastic 135 6 sandstone 5 n = 40 120 4 105 3 Age (Ma) 2 90 1 0 75 70 90 110 130 150 7 116 G 6 3MC13 112 5 tuff 108 n = 32 104 4 100 3 96 2 Age (Ma) 92 3MC13 (tuff) 1 88 99 ± 2 Ma, MSWD = 2.4, n = 32 0 84 80 90 100 110 120 130 12 110 H 10 6-11-04-2 106 sandstone 8 n = 43 102 6 98 4 94 Age (Ma) 2 90 6-11-04-2 97 ± 2 Ma, MSWD = 2.8, n = 33 0 86 70 80 90 100110 120 130 140 Age (Ma)

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 analyzed a sample of volcaniclastic sandstone with a mean age of 99 ± 2 Ma (Fig. 5G). This age approach in assigning crystallization ages yields interbedded with the volcanic rocks that was is consistent with stratigraphic relations indicat- accurate results, as we would not expect the comparatively enriched in zircon (6-11-04-4; ing that the Kr unit is younger than ca. 106 Ma ages to differ in age by more than the obtainable Fig. 2). Ages of 40 zircon grains deﬁ ne a single volcanic rocks of the Kv unit. A sample of sand- precision given their stratigraphic separation of population, tailing off slightly to older ages, and stone from the upper part of the conformably only 147 m. Furthermore, the tuff ages are sta- give a mean age of 106 ± 2 Ma (Fig. 5F). A con- overlying Kcv unit (6-11-04-2; Fig. 2) yields tistically indistinguishable from the interpreted servative interpretation is that this age provides 43 detrital zircon ages that deﬁ ne a population crystallization age of the Xiabie granite, sug- a maximum depositional age for the volcanicla- between 90 and 105 Ma, with a mean age of 97 gesting that intrusive and extrusive rocks of the stic sandstone. However, considering that the ± 2 Ma (Fig. 5H). This age provides a maximum northern Nima area may be related to a single source of the zircon grains is most likely from depositional age for the Kcv unit and, consider- magmatic episode. nearby volcanic rocks, we infer that the Kv unit ing the stratigraphic relations with the underly- includes volcanic rocks with crystallization ages ing ca. 99 Ma Kr unit, is interpreted to closely Kv Unit of ca. 106 Ma. approximate the depositional age. Four samples of volcanic rocks from the Kv unit in the southern Nima area were processed Kr and Kcv Units 40Ar/ 39Ar Methods for heavy mineral separation but did not yield Zircon grains (n = 32) from a tuff sampled from enough zircons to justify analysis. As an alter- near the base of the Kr unit in the southern Nima The 40Ar/ 39Ar studies were conducted on seven native to constraining the age of this unit, we area (3MC13; Fig. 2) yield a single population 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 Biotite 40Ar/39Ar Age Results Laser single-crystal and step-heating age the New Mexico Geochronological Research results are summarized in Figure 6. The preci- Laboratory. All samples were analyzed by the Biotite was analyzed from seven tuffaceous sion of the single-crystal laser fusion ages is single-crystal laser fusion method, and six of horizons collected from three different measured signifi cantly lower than that for ages determined the seven biotite separates were also analyzed sections in the southern Nima area. There are from step-heating analyses, primarily because as bulk samples using the incremental step- two samples from a stratigraphic height of 70 m of the small argon signals provided by the indi- heating method. In addition, a step-heating (8-6-03-1 and 8-6-03-2) from section 2NM in the vidual grains. One problem with some of the experiment was conducted on K-feldspar from far west (Fig. 2) and one sample from a height of single-crystal data (1DC77, 1DC82, 1DC367, the Xiabie granite with the aim of calculating 170 m (2NM170). Three samples from the 1DC 8-13-03-1) is isochron ages that are older than a thermal history using the multidomain diffu- section located in the eastern part of the map area their corresponding weighted mean ages. This sion (MDD) model (Lovera et al., 1989, 1997). (Fig. 2) are from stratigraphic heights of 77 m, problem stems from isochron regressions that 40 36 Details of the analytical methods and complete 82 m, and 367 m (1DC77, 1DC82, and 1DC367, project to Ar/ Aro values that are less than data tables and fi gures summarizing the results respectively). One sample (8-13-03-1) is from atmosphere, which are probably related to minor are available in the GSA Data Repository (see the northernmost measured section in the central argon loss from some grains and/or inaccurate footnote 2). part of the southern Nima area (5DC; Fig. 2). 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 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). Figure 6. Summary of 40Ar/39Ar dating results. Samples are arranged in stratigraphic order. We assign a Miocene age to the ~3000-m-thick All errors are shown as 1σ. WMA—weighted mean age; SC—single-crystal age; TGA— succession of fl uvial red beds near the town of total gas age. Nima (Figs. 2 and 4) because they conformably

924 Geological Society of America Bulletin, July/August 2007 Geological records of the Lhasa-Qiangtang and Indo-Asian collisions

115 400 AB 350 110

300 105 (°C)

Measured ture 250 100 Model 200

Tempera

Apparent Age (Ma) 95 7-19-98-2 kspar Xiabie granite 150

90 100 0 20406080100 90 95 100 105 110 115 Cumulative % 39Ar released Age (Ma) 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. overlie lacustrine strata that are lithologically is also recorded by nearly 80% of the spectrum from at least the Gaize area in the west to Sil- similar to those in sections 2NM and 1DC. that does not involve the age hump. ing Co, ~600 km to the east (Fig. 1B; Kapp et al., 2005). Conglomerates of the Kcl unit are K-Feldspar Thermochronologic Results STRUCTURAL GEOLOGY deformed into a northward-verging overturned syncline in the proximal footwall of the Gaize– The K-feldspar age spectrum from the Xiabie Early Cretaceous Unconformity Siling Co thrust (Fig. 2) and are derived almost granite displays an age gradient from ca. 105 Ma entirely from the hanging-wall Aptian-Albian to 113 Ma (Fig. 7A). The overall monotonically Cretaceous nonmarine strata lie unconform- limestone. Both the northern and southern limbs increasing age pattern is disrupted by an inter- ably on the more strongly deformed, marine of the syncline exhibit progressively decreas- mediate age hump recorded between ~5% and J-K unit in the Nima area (Fig. 2), which was ing bedding attitudes away from the fold axis, 15% 39Ar released. The origin of the age hump locally deposited after ca. 125 Ma. The oldest which are interpreted to indicate syndeposi- is uncertain, but it is a fairly common feature nonmarine strata deposited in the Nima area tional growth of the syncline during N-directed in many K-feldspar age spectra (Lovera et al., is ca. 118 Ma. These fi ndings demonstrate that slip along the Gaize–Siling Co thrust. This sug- 2002). In addition to this age hump, the initial this portion of the Bangong suture underwent gests that the Gaize–Siling Co thrust was active isothermal duplicate steps of the spectrum dis- signifi cant deformation, erosion, and a tran- after ca. 99 Ma, the depositional age of the Kr play a characteristic oscillating age pattern that sition from marine to nonmarine conditions unit, and probably at ca. 97 Ma, the inferred is indicative of excess argon hosted in fl uid between ca. 125 Ma and ca. 118 Ma. Further- depositional age of the Kcl unit (Fig. 4). Slip inclusions (Harrison et al., 1994). As detailed more, the Cretaceous nonmarine strata range in along the Gaize–Siling Co thrust was also active in the GSA Data Repository (see footnote 2), age from ca. 118 Ma to ca. 99 Ma and overlap during the Tertiary, since it cuts 26–25 Ma strata these excess argon–affected ages are corrected in age with the Aptian-Albian shallow-marine of the 2NM section along strike to the west in its to younger values based on a correlation with limestone exposed in the southernmost part of footwall (Fig. 2). Neogene fan conglomerates the degassing behavior of chlorine. the Nima area (in the hanging wall of the Gaize– locally onlap the Gaize–Siling Co thrust and The age gradient is interpreted to refl ect Siling Co thrust), which is regionally extensive provide an upper age bound for fault motion. an overall protracted thermal history that has across the entire Lhasa terrane (e.g., Leeder et resulted in variable degrees of argon loss from al., 1988; Yin et al., 1988). These relations show Queri-Malai Thrust multiple diffusion domains (MDD). We used the that the incursion of marine waters into southern MDD model of Lovera et al. (1989) to extract Tibet during the Aptian-Albian did not extend Approximately 4 km north of the Gaize–Sil- the thermal history that is shown in Figure 7B. northward into the northernmost Lhasa terrane ing Co thrust is a N-dipping thrust fault that The intermediate age hump returned by the directly south of the Bangong suture. extends across the southern Nima area from at measured spectrum cannot be matched with the least the Queri Range in the west to Malai Peak MDD model spectra, and thus there is a degree Gaize–Siling Co Thrust in the east (Fig. 2), and it is here named the of uncertainty in the accuracy of the thermal Queri-Malai thrust. Near Malai Peak (Fig. 2), history. However, the fairly slow cooling from The Gaize–Siling Co thrust juxtaposes Aptian- the Queri-Malai thrust juxtaposes the Kv unit ca. 112 to 108 Ma that transitions into more Albian limestone in the hanging wall against in the hanging wall against ca. 99 Ma red beds rapid cooling from ~300 °C to 200 °C between Cretaceous and Tertiary nonmarine strata in the and older strata in the footwall. The red beds are 108 Ma and 105 Ma is considered robust since it footwall (Fig. 2) and can be traced continuously 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, suggest- The Nima thrust tips out along strike to the Muggar Thrust ing that the thrust is south vergent. The Creta- west within Tertiary strata ~6 km southeast of ceous volcanic rocks in the hanging wall are the town of Nima (Fig. 2). In contrast, the axial The northernmost thrust fault in the Nima the most likely source for the northerly derived trace of the Nima syncline extends westward area is the inferred N-dipping Muggar thrust, (Fig. 4) and proximal volcaniclastic conglomer- across the map area. Near the town of Nima, which is buried beneath the Neogene and ates of the Kcv unit in the footwall (Fig. 2). We the upper 900 m of the ~3000-m-thick Miocene Quaternary deposits that separate exposures infer that deposition of the Kcv unit between red bed sequence along the southern limb of the of Jurassic rocks in the Muggar Range in the ca. 99 Ma and ca. 97 Ma records denudation of syncline exhibits a northward decrease in dip north from Tertiary strata in the south (Fig. 2). hanging-wall volcanic rocks during slip on the angles toward the axis of the syncline (Fig. 2) This structure is required to structurally bury Queri-Malai thrust. Hence, activity on the Queri- and bed thickness variations indicative of con- the Tertiary strata and is well exposed ~15 km Malai thrust appears to have shortly predated or tinued growth of the syncline during red bed along strike to the west of the Nima area, overlapped slip along the Gaize–Siling Co thrust deposition. where it places Jurassic rocks directly against to the south. Located between the Queri-Malai red beds of uncertain Cretaceous or Tertiary and Gaize–Siling Co thrusts is an E-plunging Puzuo Thrust Faults age. This thrust fault, along with the Puzuo anticline cored by the J-K unit (Fig. 2). Analy- thrust faults to the south, is part of a regional sis of the unconformable relationships between West of Puzuo Lake the J-K unit is repeated system of N-dipping thrust faults along the the J-K unit and the Kr and Kv units, as well in the hanging walls of two N-dipping thrust length of the Bangong suture that has been as between the Kv and Kcl units along the fold faults (Fig. 2). The Northern Puzuo thrust cuts named the Shiquanhe-Gaize-Amdo thrust limbs, indicates that anticline growth must have volcanic rocks of the Kv unit in the footwall, system (Fig. 1B; Yin and Harrison, 2000; initiated prior to Kr deposition and continued which lies unconformably on the ca. 128 Ma Kapp et al., 2005). during deposition of the Kcl unit. Along strike to Puzuo granite. Along the Southern Puzuo thrust, the west, the Queri-Malai thrust cuts upsection there is a fault-bounded sliver of volcanic rocks Late Cenozoic Faults in both its hanging wall and footwall, juxtapos- (Fig. 2) that yielded an imprecise but demon- ing the Kcl unit and unconformably overlying strably Cretaceous 40Ar/39Ar whole-rock age of None of the contractional structures in Tertiary strata southward against Tertiary strata, ca. 110 Ma (Kapp et al., 2005), indicating that the Nima area deforms Quaternary deposits. including the 26–25 Ma 2NM section in the thrusting was active subsequent to ca. 110 Ma. Rather, Quaternary deformation is character- footwall (Fig. 2). These relationships demon- The footwall Kr unit includes conglomerates ized by widely distributed strike-slip and nor- strate that the Queri-Malai thrust was reactivated with clasts of volcanic rocks and is inferred mal faults with relatively small strike lengths during the Tertiary, similar to the Gaize–Siling to have been deposited coeval with slip on the (generally <6 km; Fig. 2). A detailed kine- Co thrust. Neogene alluvial fan conglomerates Puzuo thrust faults. Tertiary activity on the matic study of these faults was not undertaken. locally bury the trace of the Queri-Malai thrust, Puzuo thrust faults cannot be demonstrated but However, where sense of slip is discernible proving that the fault has not experienced neo- is likely, considering the absence of Tertiary on satellite imagery, it appears that easterly to tectonic activity. strata in the hanging walls (presumably missing southeasterly striking faults are right lateral, due to hanging-wall denudation during Tertiary whereas northeasterly striking faults are left Nima Thrust thrusting). The footwall Kr unit is deformed into lateral. A distinguishable generation of relief is an E-W–trending anticline with a wavelength of only associated with the more northerly strik- The northernmost thrust fault in the southern ~6 km. The northern limb exhibits generally ing faults, which is taken to indicate a domi- Nima area is exposed east of the town of Nima steeper dips than the southern limb, indicating nantly normal sense of motion for these faults. and is named the Nima thrust (Fig. 2). This fault a northward vergence for the fold. The anticline dips southward and places Cretaceous volcanic must have grown during or following deposition HISTORY OF DEFORMATION AND rocks and the underlying J-K unit in the hang- of the Tertiary strata to explain the structural BASIN DEVELOPMENT ing wall against Tertiary strata in the footwall. relief of the Kr unit relative to the mid-Tertiary The Tertiary footwall strata are deformed into strata to the south. Our new data pertaining to the geological an E-W–trending syncline with a wavelength relations, age, and provenance of Cretaceous– of ~6 km, which is referred to as the Nima syn- Zanggenong Thrusts Tertiary units, and cooling history of the Xia- cline (Fig. 2). In the east, the northern limb of bie granite in the Nima area, place fi rst-order the syncline dips gently to the south, whereas In the northern Nima area, Upper Cretaceous– constraints on the Cretaceous–Tertiary his- the southern limb of the syncline is moderately Tertiary strata are disrupted by three closely tory of upper-crustal deformation and basin to steeply north-dipping and locally overturned, spaced (< 1 km) S-dipping reverse faults, col- development. This is illustrated in the form indicating a northward vergence for the fold. lectively referred to as the Zanggenong thrusts of a cross section and its sequential restora- Tertiary strata of the southern limb locally (Fig. 2). Bedding attitudes of hanging-wall and tion shown in Figures 8A–8C (see footnote 1). exhibit fanning growth geometry, where steeply footwall strata suggest a style of deformation Whereas the cross section is poorly constrained dipping or overturned strata are cut by the Nima consistent with N-directed fault propagation at depth, specifi cally with respect to the style thrust and more moderately north-dipping strata folding. The faults likely merge at relatively and depth of deformation within the penetra- overlap the Nima thrust (Fig. 2). These relations shallow structural depths because there is a tively deformed J-K unit, it provides a useful indicate N-directed displacement along the single S-dipping thrust fault juxtaposing the J-K estimate of upper-crustal shortening following Nima thrust during deposition of Tertiary strata, unit in the hanging wall against Tertiary strata fi nal ocean closure along the Bangong suture the age of which is constrained to be 26–25 Ma in the footwall (Fig. 2) along strike to the west and accurately depicts the importance of both based on the ages of tuffs in the 1DC section and structurally elevated in the footwall of a late Cretaceous and mid-Tertiary shortening and (Fig. 2; Table 1). Cenozoic E-dipping normal fault. the wedge-top style of basin development.

926 Geological Society of America Bulletin, July/August 2007 Geological records of the Lhasa-Qiangtang and Indo-Asian collisions

Cross-Section Assumptions systems, and the Muggar thrust is inferred to root been Tertiary in order to produce the folding in deeply to minimize estimated shortening along Tertiary strata and to structurally bury Tertiary The cross section was constructed using strati- it. The Cretaceous unconformity is line-length strata in the footwalls of the Gaize–Siling Co graphic thicknesses for Cretaceous and Tertiary balanced, whereas internal thickening within the and Muggar thrusts. nonmarine strata determined by projecting sur- Aptian-Albian limestone and J-K units is area face bedding measurements to depth and from balanced. Additional assumptions in construct- Cretaceous History detailed measured sections (DeCelles et al., ing the cross section are itemized in Figure 8. 2007a). Thicknesses were kept constant at depth Jurassic–Lower Cretaceous marine sedimen- unless geological relations required lateral varia- Shortening Estimates tary rocks in the northern Nima area were pen- tions. The thickness of the Aptian-Albian lime- etratively deformed and uplifted above sea level stone unit is ≤1000 m in the northern Lhasa ter- The restored cross section shows the unde- by ca. 118 Ma, the age of the oldest nonmarine rane (Leier, 2005). However, this unit is thickened formed N-S length of the Cretaceous unconfor- strata (Kvc unit) above the Cretaceous angular internally by folding, and we assume a structural mity to be 123 km (Fig. 8C). Since the present- unconformity (Fig. 8C). At ca. 118 Ma, the Xia- thickness of 2000 m, the minimum required to day N-S width of the cross-section is 65 km, this bie granite was intruded into the middle crust. bury Cretaceous nonmarine rocks in the footwall corresponds to 58 km (47%) of shortening since Onset of rapid cooling of the Xiabie granite at of the Gaize–Siling Co thrust without exposing the Early Cretaceous. An identical estimate of ca. 108 Ma (Fig. 7B) is attributed to initial slip rocks older than Aptian-Albian in the hanging percent shortening was determined along strike along the Muggar thrust at this time (Fig. 8B). wall. The stratigraphic thickness of the J-K unit to the west in the Gaize region (Fig. 1; Kapp et Marine rocks of the J-K unit are as young as is unknown. However, given the regionally exten- al., 2005). If the assumptions that were made ca. 125 Ma in the southern Nima area (Fig. 5C). sive exposure and transposed nature of this unit, in order to construct the Nima cross section These marine rocks were penetratively deformed and the fact that older rocks are rarely exposed are valid, then our estimate is a minimum for and uplifted above sea level prior to deposition along the length of the central Bangong suture several reasons. Whereas internal thickening of the unconformably overlying ca. 110–106 Ma zone (Kapp et al., 2005), a structural thickness of of the Aptian-Albian limestone unit is restored volcanic-bearing strata. >5000 m as shown is reasonable. by area balancing, slip along the Gaize–Siling In the eastern part of the southern Nima area, Additional assumptions that went into con- Co thrust is not considered. This is because the the oldest deformation that affected Cretaceous structing the cross section are as follows: original lateral separation between the hanging- nonmarine strata is growth of the anticline (1) The regional unconformity beneath Creta- wall Cretaceous marine limestone and the foot- south of the Queri-Malai thrust and north of the ceous units had minimal initial relief and can wall Cretaceous nonmarine rocks is unknown. Gaize–Siling Co thrust (Figs. 2 and 8C). Fold- therefore be restored to horizontal. The valid- The original separation was likely signifi cant ing must have initiated prior to the ca. 99 Ma ity of this assumption is diffi cult to assess, but (more than tens of km), because no intercalated red beds of the Kr unit to locally erode the Kv we observed no evidence in the sedimentary marine and nonmarine or marginal marine strata unit along the limbs of the anticline. This fold record for major intrabasinal relief (such as but- of Aptian-Albian age have been documented in is interpreted to have formed above a footwall tress unconformities, megabreccias, or landslide the hanging wall or footwall of the Gaize–Siling ramp in the Nima thrust (Fig. 8C) and to mark blocks) at the onset of Cretaceous nonmarine Co thrust; all Aptian-Albian strata documented onset of N-directed thrusting by ca. 99 Ma. Also deposition. (2) Any subsidence in response north of the Gaize–Siling Co thrust are entirely by ca. 99 Ma, S-directed thrusting may have to topographic or sediment loading occurred nonmarine. Hanging-wall cut-offs for the Queri- ceased or slowed signifi cantly along the Muggar at a wavelength greater than the width of the Malai, Nima, Puzuo, and Zanggenong thrusts thrust and propagated southward to the Puzuo major thrust-bounded ranges in the Nima area have been eroded. For these thrusts, only the thrust fault. We infer that the ca. 99 Ma Creta- (<20 km) and can therefore be considered to minimum magnitude of slip needed to erode the ceous red beds to the south were largely derived have been uniform (no localized downwarp- hanging-wall cut-offs is shown. The only excep- from sources elevated in the hanging wall of the ing of the Cretaceous unconformity below its tion is the northern Puzuo thrust, where we Puzuo thrust (Fig. 8B). The Queri-Malai thrust initial regional elevation). This assumption is include an additional ~6 km of slip to illustrate is interpreted as a back thrust that branched from appropriate for typical continental lithosphere that the thrust sheet in the hanging wall of the the Nima thrust and resulted in the development with suffi cient strength to support short-wave- Southern Puzuo thrust could be interpreted to be of a synclinal pop-up structure in its hanging length loads (<100 km; e.g., Turcotte and a horse within a duplex. The slip shown for the wall. Northerly derived conglomerates of the Schubert, 1982) but may not be for an anoma- Muggar thrust is that needed to exhume the Xia- Kcv unit record the growth of this pop-up struc- lous lithospheric structure like that of modern bie granite from a minimum depth of ~10 km ture, and similar deposits may also have been Tibet where isostatic compensation is probably (the granite was at temperatures >300° prior to shed northward. Initiation of the Gaize–Siling occurring within the ductile middle crust (e.g., the mid-Cretaceous based on thermochrono- Co thrust shortly postdated slip along the Queri- Bird, 1991; Masek et al., 1994; Royden, 1996). logic results; Fig. 7B) to the surface along a fault Malai thrust, as indicated by syncontractional (3) The Nima thrust was active during the Cre- with a northward dip of 45°. The magnitude of deposition of the southerly derived Kcl unit on taceous, coeval with slip along the Gaize–Sil- slip would be greater if the fault fl attens with top of northerly derived conglomerates of the ing Co and Queri-Malai thrusts. This allows the depth or exhibits ramp-fl at geometries within Kcv unit. Duplexing of the J-K unit in the hang- major structures of the southern Nima area to be the upper crust. Distinguishing the relative mag- ing wall of the Nima thrust is inferred to have simply interpreted as a dominantly N-directed nitude of Cretaceous versus Tertiary shortening uniformly elevated overlying Cretaceous rocks. thrust system. (4) Rapid cooling of the Xiabie is diffi cult because the major thrust faults in the granite during the Early Cretaceous is attributed Nima area are shown or inferred to have been Mid-Tertiary History to exhumation in response to slip on the Mug- active during both Cretaceous and Tertiary time. gar thrust. (5) The cross section is pinned in the However, a minimum of 25 km of the estimated Geologic relations provide no evidence for sig- undeformed footwall of the basin-margin thrust total minimum shortening of 58 km must have nifi cant 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 non- Cenozoic deformation in the Nima area mimics Early Cretaceous igneous rocks are widely marine sedimentation during the late Oligocene. that of the Bangong suture zone at the regional distributed in the northern Lhasa terrane (e.g., It appears that all of the major structures that scale (Fig. 1A), where conjugate strike-slip Xu et al., 1985; Coulon et al., 1986; Harris were active during Cretaceous time were reac- faults (sinistral in the north and dextral in the et al., 1990). The ca. 124 Ma Puzuo granite, tivated during the mid-Tertiary. Along strike to south) are linked with approximately N-strik- ca. 118 Ma Xiabie granite, and Lower Creta- the west of the line of the cross section, Tertiary ing normal fault systems and accommodate ceous volcanic rocks in the Nima area show strata were deposited and cut in the footwalls distributed eastward extrusion of wedge-shaped that igneous activity of this age extended as far of the Gaize–Siling Co and Queri-Malai thrusts crustal fragments (Taylor et al., 2003). Despite north as the Bangong suture. Given the scarcity (Fig. 2). The >4000-m-thick succession of Ter- their relatively small slip magnitudes, the late of documented igneous rocks of this age in the tiary strata at the latitude of the town of Nima Cenozoic faults in the Nima area exert a strong southern Lhasa terrane, this inboard magma- fi lled accommodation space within a triangle infl uence on the pattern of Quaternary sedimen- tism is interpreted to be the consequence of zone, bounded by the S-dipping Nima thrust in tation (Fig. 2). The Xiabie Lake basin is a gra- northward low-angle subduction of Neotethyan the south and the N-dipping South Puzuo thrust ben between E-dipping normal faults in the west oceanic lithosphere beneath the southern margin in the north (Fig. 8C). Folding of these Tertiary and a W-dipping normal fault system in the east. of Asia at this time (Coulon et al., 1986; Kapp strata is inferred to have been related to their The Puzuo Lake basin is a half-graben basin, et al., 2005). This contrasts with the alternative northward displacement over a footwall ramp in bounded to the west by an E-dipping normal interpretations that this magmatic belt is related the décollement to the N-directed thrust system. fault. Dagze Lake shorelines are best preserved to crustal anatexis in response to crustal thick- Slip along this décollement may have been fed east of the lake, and deposition of postshoreline ening during Lhasa-Qiangtang collision (Xu et to the surface by a combination of back thrusting deposits (Q2) is localized in the west (Fig. 2). al., 1985) or mantle thinning following Lhasa- along the Southern Puzuo thrust and N-directed Although not demonstrative, these relations Qiangtang collision (Harris et al., 1990). displacement along the Zanggenong thrusts to could be explained by regional down-to-the- Collectively, the Early to mid-Cretaceous the north (Fig. 8A). Tertiary strata in the northwest tilting of the Dagze Lake basin toward E- geology of Tibet is consistent with the model ern Nima area are inferred to have been folded dipping normal faults in the Puzuo Lake area. of Kapp et al. (2005) of northward continental and structurally elevated due to internal short- underthrusting of the Lhasa terrane beneath the ening and thickening of the underlying J-K DISCUSSION Qiangtang terrane at this time, driven by the unit (Fig. 8A). This shortening was balanced at northward fl at-slab subduction of Neotethyan deeper structural levels by southward slip along Cretaceous Lhasa-Qiangtang Collision oceanic lithosphere along the Indus-Yarlung the Southern Puzuo thrust. suture (Fig. 9A). In this hypothesis, the N- The youngest contractional deformation in Our mapping and geochronologic results directed mid-Cretaceous Gaize–Siling Co and the Nima area is recorded by growth stratal demonstrate that the Nima area underwent Nima thrusts are back thrusts, and the associ- relations in the uppermost part of the red bed major deformation and denudation and was ated nonmarine strata represent wedge-top sequence near the town of Nima (Fig. 2; also uplifted above sea level between ca. 125 Ma and deposits trapped in a triangle zone structural see Figure 7A of Kapp et al., 2005), suggesting ca. 118 Ma. Major Early Cretaceous deforma- setting (Fig. 9B). Dominantly S-directed, thin- continued growth of the Nima syncline during tion and exhumation have also been documented skinned shortening related to ongoing, postcol- their deposition. The youngest dated strata that along strike to the west and east in the Gaize lisional convergence in the Bangong suture zone are inferred to conformably underlie the red-bed and Amdo areas (Fig. 1), respectively, as well as propagated southward into the northern Lhasa sequence are ca. 24 Ma. Relatively high sedi- in the Qiangtang terrane to north (Kapp et al., terrane during the Late Cretaceous to Paleo- ment accumulation rates of ~1 mm/yr are typi- 2005; Guynn et al., 2006). Farther south in the cene (Fig. 9B; Kapp et al., 2003), which could cal of those in foredeep depozones of fl exural Lhasa terrane, Lower Cretaceous (pre-Aptian) explain why there is no evidence for major foreland basins (e.g., Angevine et al., 1990; clastic deposits are regionally extensive. In con- contraction in the Nima area during this time DeCelles and Giles, 1996) as well those within trast to the Nima area, however, these deposits interval. This model predicts that central Tibet the Qaidam basin during late Cenozoic time are entirely conformable. The clastic deposits underwent signifi cant crustal thickening and (e.g., Metivier et al., 1998). Assuming a similar consist of marginal marine to fl uvial facies, elevation gain prior to the Indo-Asian collision. deposition rate for the ~3000-m-thick Nima red show mainly southward paleocurrent indicators, Paleoelevation studies indicate that the late Oli- bed sequence, a conservative estimate for the and are interpreted to have been deposited in a gocene Nima basin (DeCelles et al., 2007) and maximum age of the top of the sequence, and foreland basin related to the S-directed thrust- Eocene(?) to Miocene Lunpola basin to the east hence the youngest documented contraction- ing associated with Lhasa-Qiangtang collision (Fig. 1B; Rowley and Currie, 2006) developed related deformation in the area, is ca. 21 Ma. in the north (Leeder et al., 1988; Zhang et al., at high elevations (>4 km). Additional studies 2004; Leier, 2005). The Lower Cretaceous clas- on older deposits are necessary to quantify how Late Cenozoic History tic deposits of the Lhasa terrane are conform- much elevation gain occurred prior to the Indo- ably overlain by Aptian-Albian shallow-marine Asian collision. Late Cenozoic, approximately N-striking nor- limestones. Whereas the cause of the Aptian- mal faults in the Xiabie and Puzuo Lake areas Ablian marine incursion has been attributed to Magnitude of Cretaceous versus Mid- appear to be kinematically linked by a system of back-arc extension (Zhang, 2000; Zhang et al., Tertiary Shortening approximately NE-striking sinistral strike-slip 2004), previous studies near Gaize (Kapp et al., faults (Fig. 2). In contrast, eastward displace- 2005) and this study in the Nima area demon- The magnitude of mid-Tertiary shortening esti- ment south of the town of Nima is accommo- strate that the marine incursion did not extend mated for the Nima area is ~25 km over the pres- dated in large part along an ESE-striking dextral north of the Lhasa terrane in central Tibet and ent-day N-S width of ~65 km (Fig. 8). Although strike-slip fault located ~1–2 km north of the that it was coeval with syncontractional basin this estimate is a minimum, it is unlikely to be Queri-Malai thrust (Fig. 2). This pattern of late development along the Bangong suture. signifi cantly greater. The magnitude of Tertiary

928 Geological Society of America Bulletin, July/August 2007 Geological records of the Lhasa-Qiangtang and Indo-Asian collisions

region of upper-crustal shortening

A ca. 130-100 Ma Nima growth of South area Bangong Qiangtang North km Xigaze foreland basin suture culmination forearc 0 10 130-120 Ma Qiangtang Lhasa 20 magmatism 120-100 Ma 30 magmatism Songpan Ganzi Moho Moho 40 Moho 50 60 Asian mantle lithosphere 70 Neotethyan oceanic lithosphere 80

Nima 0 100 200 300 km Linzizong area B ca. 100-50 Ma Xigaze km forearc volcanics 0 10 India Gangdese Qiangtang 20 retroarc Songpan Ganzi 30 Moho Lhasa Moho thrust belt 40 Moho 50 Gangdese arc 60 Moho 70 80 Indus- Yarlung Tethyan Himalaya suture C ca. 50-30 Ma Qiangtang thrusts Fenghuo Shan km thrust belt (~25% shortening) (~50% shortening) 0 minimal upper crustal shortening 10 20 Lhasa Qiangtang 30 40 India Songpan Ganzi 50 Moho 60 Moho Moho 70 80

Kailas Gangdese Nima area ca. 30-23 Ma km conglomerate thrust D 0 10 Qiangtang Songpan 20 Ganzi 30 MCT sheet Lhasa 40 50 India 60 Moho Moho 70 Moho 80 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 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 collision. (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.

Geological Society of America Bulletin, July/August 2007 929 Kapp et al. slip along the Nima thrust is limited by the If the inferred Paleocene-Oligocene age of the Hubbard and Harrison, 1989; Hodges et al., preservation of Cretaceous strata in its hanging Lunpola basin is correct, this would suggest 1996) and initial erosion of Greater Himalayan wall. Thermochronologic results on the Xiabie that portions of the Bangong suture underwent metamorphic rocks in its hanging wall (early granite suggest that it cooled to ~150 °C during localized contraction and basin development Miocene; e.g., DeCelles et al., 2001). This the Early Cretaceous. Under the assumption of at variable times during the Indo-Asian colli- observation raises the possibility that thrust- a 25 °C/km geothermal gradient, this suggests a sion. Alternatively, if the Lunpola basin is an ing in Tibet was mechanically linked with the maximum Tertiary throw of ~6 km for the Mug- along-strike chronostratigraphic equivalent of Himalayan thrust belt, perhaps representing hin- gar thrust. Unlike the Muggar and Gaize–Siling the Nima-Shiquanhe basins, then its fi ll (which terland out-of-sequence deformation that helped Co thrusts, the Puzuo thrust faults cannot be oxygen isotope studies indicate was deposited at build the orogenic wedge taper (or gravitational traced continuously along the Bangong suture a paleoelevation of >4 km; Rowley and Currie, potential energy) necessary to drive subsequent for distances >100 km, implying to us that these 2006) may be substantially younger than pres- southward emplacement/extrusion of the Main thrust faults are subordinate in slip magnitude to ently assumed. Central thrust sheet (Fig. 9D). those bounding the Nima basin. Although large- Although the possibility of early Tertiary con- magnitude Tertiary slip along the Gaize–Siling traction along portions of the Bangong suture CONCLUSIONS Co thrust is permissible, at this time there is no remains to be tested, the available timing con- strong evidence for large-magnitude (~50%) straints point to an important episode of suture The geology of the Nima area provides a rich shortening along the Bangong suture during the zone reactivation during the late Oligocene– record of Cretaceous to Quaternary deforma- Indo-Asian collision as suggested (or predicted) early Miocene. Interestingly, the Indus-Yarlung tion and basin development along the Bangong in some models of Tibetan Plateau formation suture to the south (Fig. 1) also exposes non- suture in central Tibet. Jurassic to Early Creta- (e.g., England and Houseman, 1986, 1989; Tap- marine strata of late Oligocene–early Miocene ceous marine sedimentary rocks were deformed ponnier et al., 2001). Based on this study and pre- age (Kailas or Gangrinboche conglomerate; Yin and uplifted above sea level prior to the onset vious studies in central Tibet (Kapp et al., 2003, et al., 1999; Aitchison et al., 2002), and it was of nonmarine deposition at ca. 118 Ma. Cre- 2005), there is no evidence for early Miocene or modifi ed by the S-directed Gangdese thrust sys- taceous nonmarine strata range in age from older strike-slip deformation along the Bangong tem between 30 and 23 Ma (Fig. 1; Yin et al., Aptian to Cenomanian and were deposited suture. This challenges models invoking oblique 1994, 1999; Ratschbacher et al., 1994; Harri- coeval with S-directed thrusting along the subduction of continental lithosphere along the son et al., 2000). Simultaneous reactivation of northern margin of the Nima basin and mainly Bangong suture (Tapponnier et al., 2001) and the Bangong and Indus-Yarlung sutures during N-directed thrusting along the southern margin any signifi cant eastward extrusion of central the Oligocene-Miocene (Fig. 9D) postdated of the Nima basin. Cretaceous tectonic activ- Tibet relative to southern Tibet (Tapponnier et Tertiary contraction and basin development in ity in the Nima area is attributed to continued al., 1982) prior to late Cenozoic time. the northern Qiangtang and Songpan-Ganzi ter- shortening after the initial collision between ranes, which initiated during the earliest stages the Lhasa and Qiangtang terranes. No evidence Mid-Tertiary Reactivation of the Bangong of Indo-Asian collision and had largely ceased exists for subsequent contraction until the late Suture by ca. 30 Ma (Fig. 9C; Coward et al., 1988; Liu Oligocene–early Miocene, when the Cretaceous and Wang, 2001; Liu et al., 2001, 2003; Horton thrust faults were reactivated and thick suc- Minimal post–50 Ma shortening and basin et al., 2002; Spurlin et al., 2005). This observa- cessions of nonmarine strata (locally ~4000 m development have been documented within tion begs the question of why contraction was thick) accumulated in wedge-top basins and at the interior of the Lhasa terrane, in contrast to localized in north-central Tibet during the early high elevation (>4.6 km). Mid-Tertiary thrust- its bounding sutures (Figs. 1B and 9C–9D). E- stages of Indo-Asian collision and then jumped ing along the Bangong suture zone was coeval W–trending, thrust-bounded Tertiary nonmarine southward to the Bangong and Indus-Yarlung with shortening along the Indus-Yarlung suture clastic basins are widespread along the Bangong sutures during the mid-Tertiary. zone, suggesting simultaneous reactivation of suture zone (Fig. 1A). These basins have been We speculate that the gravitational poten- these suture zones bounding the Lhasa terrane. widely mapped and cited to be early Tertiary, tial energy associated with a preexisting thick We speculate that mid-Tertiary shortening asso- although no robust age data have been published crust in southern Tibet due to Cretaceous–early ciated with regional suture zone reactivation in the international literature. The largest of these Eocene orogenesis (Fig. 9B; see Kapp et al., may have driven the Himalayan thrust belt into is the Lunpola-Duba basin system, ~200 km east 2005, for summary) was suffi cient to inhibit a supercritical state, leading to the subsequent of the Nima area (Fig. 1B), which includes up to upper-crustal shortening in this area and to southward emplacement/extrusion of the Main 5000 m of fi ll of reportedly Paleocene to Oligo- result in contractional deformation in lower-ele- Central thrust sheet. Late Cenozoic deforma- cene age (e.g., Ai et al., 1998; Guo et al., 2002; vation regions in north-central Tibet (Cyr et al., tion is characterized by widely distributed but Pananont et al., 2002). In contrast, our results 2005) and to the south in the Tethyan Himalaya relatively small displacement on approximately show that the >4000-m-thick Tertiary succes- during the early Tertiary (Fig. 9C). The feasi- N-striking normal and more easterly striking sion in the southern Nima area is restricted in bility of this mechanical explanation has been strike-slip faults, which together are accom- age to the late Oligocene–early Miocene. The demonstrated in thin-viscous-sheet models of modating distributed N-S shortening and E-W disconformable relationships between Tertiary Cenozoic deformation with the initial condition extension. It is estimated that the upper crust of and Cretaceous strata in the Nima area suggest of preexisting thick crust and high elevation in the Nima area underwent >59 km (>47%) of minimal deformation between the mid-Creta- southern Tibet (England and Searle, 1986; Kong shortening during Cretaceous to mid-Tertiary ceous and late Oligocene. Similarly, Tertiary et al., 1997). Mid-Tertiary reactivation of short- time, with more than half of this shortening pre- contraction and basin development in the Shi- ening along the Bangong and Indus-Yarlung dating Indo-Asian collision. Our results suggest quanhe area along the Bangong suture zone in sutures immediately predated the oldest dated that the thick crust and high elevation of central far western Tibet (Fig. 1) are restricted to the late shear zone activity within the Main Central Tibet were achieved in large part by shortening Oligocene–early Miocene (Kapp et al., 2003). thrust system in the Himalaya (23–20 Ma; e.g., of the Tibetan crust over a protracted period of

930 Geological Society of America Bulletin, July/August 2007 Geological records of the Lhasa-Qiangtang and Indo-Asian collisions

DeCelles, P.G., and Giles, K.A., 1996, Foreland basin sys- Hodges, K.V., Parrish, R.R., and Searle, M.P., 1996, Tec- time, beginning during the Early Cretaceous and tems: Basin Research, v. 8, p. 105–123, doi: 10.1046/ tonic evolution of the central Annapurna Range: Nep- continuing until at least the early Miocene. j.1365-2117.1996.01491.x. alese Himalayas: Tectonics, v. 15, p. 1264–1291, doi: DeCelles, P.G., Robinson, D.M., Quade, J., Ojha, T.P., Gar- 10.1029/96TC01791. ACKNOWLEDGMENTS zione, C.N., Copeland, P., and Upreti, B.N., 2001, Horton, B.K., Yin, A., Spurlin, M.S., Zhou, J., and Wang, J., Stratigraphy, structure, and tectonic evolution of the 2002, Paleocene-Eocene syncontractional sedimenta- Himalayan fold-thrust belt in western Nepal: Tecton- tion in narrow, lacustrine-dominated basins of east- This research was supported by the U.S. National ics, v. 20, p. 487–509, doi: 10.1029/2000TC001226. central Tibet: Geological Society of America Bul- Science Foundation grant EAR-0309844 and Exxon- DeCelles, P.G., Robinson, D.M., and Zandt, G., 2002, Impli- letin, v. 114, p. 771–786, doi: 10.1130/0016-7606 Mobil. Acknowledgment is also given to the donors of cations of shortening in the Himalayan fold-thrust (2002)114<0771:PESSIN>2.0.CO;2. the American Chemical Society Petroleum Research belt for uplift of the Tibetan Plateau: Tectonics, v. 21, Hubbard, M.S., and Harrison, T.M., 1989, 40Ar/39Ar age con- Fund for partial support of this research (ACS PRF# p. 1062, doi: 10.1029/2001TC001322. straints on deformation and metamorphism in the Main DeCelles, P.G., Kapp, P., Ding, L., and Gehrels, G.E., 2007a, Central thrust zone and Tibetan slab, eastern Nepal 39376-G8). We thank Duo Jie and Zhou Ma for their Late Cretaceous to mid-Tertiary basin evolution in the cen- Himalaya: Tectonics, v. 8, p. 865–880. assistance in the fi eld and J. Fox, F. Guerrero, and tral Tibetan Plateau: Changing environments in response Jolivet, M., Brunel, M., Seward, D., Xu, Z., Yang, J., Roger, A. Pullen for their assistance with mineral separa- to tectonic partitioning, aridifi cation, and regional eleva- F., Tapponnier, P., Malavieille, J., Arnaud, N., and Wu, tion and U-Pb analysis. We thank Brad Ritts and an tion gain: Geological Society of America Bulletin (in C., 2001, Mesozoic and Cenozoic tectonics of the anonymous reviewer for critical reviews, and sugges- press), v. 119, no. 5/6, doi: 10.1130/B26074.1. northern edge of the Tibetan Plateau: Fission-track tions by Paul Heller and Karl Karlstrom helped us DeCelles, P.G., Quade, J., Kapp, P., Fan, M., Dettman, D.L., constraints: Tectonophysics, v. 343, p. 111–134, doi: improve this paper. and Ding, L., 2007b, High and dry in central Tibet during 10.1016/S0040-1951(01)00196-2. the late Oligocene: Earth and Planetary Science Letters, Kapp, P., Murphy, M.A., Yin, A., Harrison, T.M., Ding, L., v. 253, p. 389–401, doi: 10.1016/j.epsl.2006.11.001. and Guo, J., 2003, Mesozoic and Cenozoic tectonic REFERENCES CITED Dewey, J.F., and Burke, K.C.A., 1973, Tibetan, Variscan evolution of the Shiquanhe area of western Tibet: Tec- and Precambrian basement reactivation: Products of tonics, v. 22, p. 1029, doi: 10.1029/2001TC001332. 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932 Geological Society of America Bulletin, July/August 2007 Figure 2. Kapp et al. A''' gr Qglacial Jr Geological Map of the Nima Area gr snow gr Xiabie N Central Tibet granite Muggar Range and ice 6081 4600 0 2 4 6 8 10 km 7-19-98-2 Jr 118 ± 4 Ma Jr 52 contour interval = 100 m 7-16-98-2 7-16-98-1

Xiabie Lake 32 Jr 58 geysers 20 43 Q2 Quaternary deposits; youngest Map Units 68 37 52 7-19-98-1 58 30 72 60 Q2 Jr Q1 Quaternary deposits; incised 76 45 88 Nf 46 75 N-Q Neogene - Quaternary (?) conglomerate 51 Nf Q2 Nf Neogene fan conglomerates Q2 75 N-Q Nf Nf Tmu Tr Tertiary red beds of the S. Nima Area (T) and Upper Muggar Unit (T) of the N. Nima Area rmu Nf Muggar 65 Nf Kml Upper Cretaceous to Paleocene Lower Muggar Unit thrust Nf Q2 Tmu Tmu Kcl Cenomanian conglomerate with mainly Aptian-Albian limestone clasts Q2 20 22 Nf Muggar thrust 12 Kcv Cenomanian conglomerate with mainly volcanic clasts 4800 Tmu Q1 Kr Albian-Cenomanian red beds; volcanic-clast sandstone and conglomerate Q2 Q1 5000 Q2 52 Kv Albian volcanic flows, tuffs, breccias; volcaniclastic sandstone and conglomerate Q2 Tmu 5MK 20 Kvc Aptian volcaniclastic conglomerate, sandstone, siltstone; paleosols, tuffs Q1 Kl Aptian-Albian massive reef-facies foraminiferal- and rudist-bearing limestone Q2 35 Q2 32 Q2 gr Cretaceous granite and granodiorite Q1 J-K Jurassic - Cretaceous: shale, siltstone, turbiditic sandstone, metasedimentary-matrix mélange

Tmu Jr Jurassic: argillite, shale, siltstone, limestone, turbiditic sandstone, metavolcanic rocks 30 2MK 37 Tmu 30 Tmu Tmu 33 30 shorelines 66 Tmu Glacier or year- 4700 river Q2 30 77 lake round snow 56 3MK 20 56 46 23 Tmu Tmu 45 82 7-19-98-3 Zanggenong Kml 75 Kml 75 Geologic Symbols 50 47 75 Tmu thrusts 60 Contacts 20 57 39 38 21 Solid where well located, dashed where approximately located or inferred, dotted 13 Kml 15 where concealed and inferred N-Q 20 8 10 Kvc 25 J-K 1MK 34 10 18 J-K 20 High-angle normal fault; faults in red show 4MK neotectonic activity; 16 29 Kvc Lithologic Thrust fault; arrow shows bar-ball on hanging wall J-K N-Q 118 ± 3Ma dip; diamond shows trend mapped largely from of striations Strike-slip fault satellite imagery Q1 Folds J-K Solid where well located, dashed where approximately located or Q2 N-Q inferred, dotted where buried; arrow shows direction of plunge

87°30'E Trend and plunge of 89 Anticline Q2 Syncline small-scale fold axis Q1 N-Q N-Q J-K 84 Strike and dip J-K Transposed 4600 Horizontal Inclined Vertical Overturned bedding Cleavage Other J-K 6-2-98-2 3MC Start of J-K Marker-bed Sample locality measured section J-K 4600 32°00'N J-K 32°00'N Q1 82 Q1 4600 Q1 Q1 J-K J-K J-K

4500 J-K

Q2 J-K

shorelines J-K Q2 4500

J-K Q2

Puzuo 4500 Q1 Lake A'' A'' Jr PazuoPuzuo thrust 4600 Puzuo 33 7-14-98-3 35 thrust 64 17 Puzuo Kv granite J-K 7-14-98-2b Kv

7-14-98-2 Dagze Lake shorelines gr 46 46 20 124 ± 4 Ma 32 59 46 Q1 J-K 7-14-98-1 ca. 110 Ma Kv 48 Mochang R. South Puzuo 35 thrust Q2 35 Q2 27 65 Q1 Kr 22

22 River

Q2 Q2 Q1 1DC Mochang 26-25 Ma 62 shorelines 40 Tr 23 Q2 18 21 41 33 12 21 20 42 41 20 Nima Q1 Tr 26 56 N-Q synclin e 5DC Tr N-Q Q1 Nima syncline N-Q 74 Nima 23.5 Ma N-Q 20 42 A' 52 Tr Tr 67 N-Q A' Tr 72 80 Tr Nima thrust 75 Tr 85 45 Tr 2DC 75 Kcl QueriQuer 78 Tr 4DC i rRange 76 Tr 82 Tr 45 68 45 ange Kr 66 85 Tr Tr 58 ? 1NM 69 83 67 68 85 70 42 7-22-98-4 7-22-98-5 79 69 72 80 3DC 30 75 4700 Tr 60 Tr 52 7-13-98-3 N-Q Nf Q1 J-K 83 2NM (26-25 Ma) Q1 Nima thrust 27 Tr 42 Kv 33 58 40 8 36 7-11-05-2 80 52 Tr Quer Tr Tr 40 78 35 52 <125 Ma 30 23 20 i - M 32 16 18 Kv alai 33 30 3 37 Kv 50 6-11-04-4 10 thrust 86 7-22-98-3 Q1 30 50 5 42 ~106 Ma 6-10-04-1 30 Kcl 50 Kr 55 76 7-22-98-1 55 70 40 7-13-98-2 64 Kv Kv Nf 70 55 Kv 2 29 Kcl 70 65 60 7-13-98-1 Kl Q1 Tr 72 42 43 Malai Peakpeak Kr Kr 85 Tr 46 43 Queri - Malai thrust 42 35 58 38 0 3MC 18 J-K Q1 Kv 480 ~99 Ma 36 7-22-98-2 67 78 Kv Kcl Gaize - Siling 36 J-K J-K 60 45 Kr Co thrust Tr J-K Q2 40 4 40 4900 6-11-04-1 54 Q1 700 Kr Kl Nf 6-11-04-2 69 Kcv ~97 Ma Q1 57 2MC 6214 Kl Kcv Kcv 70 22 Kcl Gaize - Siling Kcv 34 27 1MC 30 Co thrust 65 Kcv Kl Kcl 24 54 60 5000 Kl Kcl 65Q2 Kl area unmapped 4900 Kl Q1 area unmapped area unmapped area unmapped Gaize -Siling Co thrust A 31°40'N 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 1. Geometry of Tertiary Nima basin constrained from bedding attitudes; thickness of >4 South Gaize - Queri - Malai Nima North km for southern Nima basin confirmed from measured sections. Puzuo A Siling Co thrust thrust A' A'' 0 2 46 8 10 km A''' 2. Nima thrust is assumed to be active during Cretaceous deposition.Space between thrust Muggar thrust Southern no vertical exaggeration Zanggenong Tertiary Nima basin and underlying Cretaceous volcanic rocks is filled by wedges of Puzuo thrust km Kcl thrusts km syncontractional deposits derived from the hanging wall of the Nima thrust. thrust Kml 5 Kl 5 3. The elevation of the Cretaceous unconformity beneath the Tertiary Nima basin is Shortening Estimates 4 Tmu 1 Jr 4 Kcv Kv Tr taken to mark the initial regional elevation of the unconformity. 3 4 Kr 3 Kcv Kv Kv 4. Gaize -Siling Co thrust soles out at base ofKl , as no older strata are observed along Present-day length = 64.6 km J-K 1 vc 2 2 K Xiabie 2 its entire strike length. Kl is shown with a structural thickness of ~2 km, likely obtained J-K granite 1 J-K Kr 7 1 by folding/imbrication of a limestone unit <500 m in stratigraphic thickness. Restored length = 123.0 km 0 5 J-K 0 3 5. Thickening ofJ-K unit is required to explain the regional structural elevation of rocks -1 Cretaceous unconformity -1 J-K 6 in the hanging wall of the Nima thrust; accomplished here by emplacing a horse of J-K. Minimum Tertiary shortening = 25 km -2 J-K -2 6. The location of this footwall ramp is constrained from the regional geometry of the (28%) -3 -3 -4 Cretaceous unconformity -4 contact betweenJ-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. Minimum total shortening (post-mid- -5 -5 7. Duplexing in the J-K unit is inferred to elevate theK vc unit above its initial regional Cretaceous) = 58 km (47%) -6 -6 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. ca. 99 Ma tuff 108-104 Ma Kcl B Cretaceous shortening and basin development rapid cooling Kl Kv future location of Kcv Kcv Tertiary Nima basin future location of J-K Kr Kr Tertiary Northern Nima basin Kvc Jr J-K Kv J-K Xiabie J-K granite

original Aptian-Albian folding between separation ca. 106 Ma ca. 110 Ma stratigraphic pinch-out required marine 106 and 99 Ma 118 Ma tuffs unknown volcanics ca. 123 Ma volcanics but location uncertain due to erosion C Restored cross section carbonate Kv granitoid Kl Kvc Kv

Assumption: No initial relief Xiabie granite (ca. 118 Ma) Position of undeformed on unconformity beneath Cretaceous; >10 km below surface footwall is pinned restored to horizontal

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