Structural evolution and sequence of thrusting in the Tethyan -thrust belt and Indus-Yalu zone, southwest

M.A. Murphy² An Yin Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, USA

ABSTRACT the Subhimalaya to the Gangdese Shan al., 1994, 1999; Harrison et al., 2000) (Figs. (Transhimalaya), along with previously re- 1A and 1B). Regional mapping of a north-south tra- ported shortening estimates in the central Although constraints have been placed on verse from the -Nepal- border Himalaya, yields a minimum shortening es- the magnitude of shortening of the Tethyan junction to Mount Kailas in southwest Ti- timate across the orogen of ϳ750 km. Sedimentary Sequence in the northwestern betÐcombined with previously published Tethyan Himalaya (Zanskar) and eastern Teth- geochronologic and stratigraphic dataÐis Keywords: Tethys, Himalayan , Ti- yan Himalaya in southern Tibet (Searle, 1986; the basis for an incremental restoration of betan plateau, suture zones, thrust faults. Burg et al., 1987; Ratschbacher et al., 1994), the Tethyan fold-thrust belt and deforma- there have been no estimates of Cenozoic tion along the Indus-Yalu suture zone. INTRODUCTION shortening for the central Tethyan Himalaya From north to south, the major structural in southwest Tibet. This paper presents a ®rst features are (1) the Indus-Yalu suture zone, The deformation histories of suture zones attempt to better understand the sequence of composed of ®ve south-dipping thrust are in general tremendously complex because Cenozoic thrusting in southwest Tibet and to faults involving rocks interpreted to repre- they are commonly modi®ed by multiple gen- quantify the magnitude of contraction within sent parts of the former Indian passive erations of syncollisional faulting (e.g., Dew- the Tethyan fold-thrust belt and Indus-Yalu su- margin and Asian active margin, (2) the ey, 1977; Yin et al., 1994; Puchkov, 1997). ture zone between the Mount Kailas region in Tethyan fold-thrust belt, composed of a The major tectonic elements of a ``model'' su- the north and the Nepal-India-China border dominantly north-dipping system of imbri- ture zone are well preserved, however, along junction to the south in southwest Tibet. cate thrust faults involving Precambrian a segment of the Indus-Yalu suture (also through Upper Cretaceous strata, and (3) known as Indus-Yarlung suture) in southwest REGIONAL GEOLOGY the Kiogar-Jungbwa thrust sheet. A line- Tibet. This suture zone separates rocks with length cross-section reconstruction indi- an Asian af®nity to the north from those with The Himalayan fold-thrust belt lies between cates a minimum of 176 km of north-south an Indian af®nity to the south. Plate-tectonic the Indian shield to the south and the Indus- horizontal shortening partitioned by the reconstructions of the Tibetan-Himalayan col- Yalu suture zone to the north (Figs. 1A and Tethyan fold-thrust belt (112 km) and Indus- lision zone suggest that ϳ1000 km of short- 1B). It consists of four lithotectonic units Yalu suture zone (64 km). Sequential res- ening has been accommodated between south- bounded by four north-dipping Cenozoic toration of the cross section shows that the ern Tibet and the Indian Shield (Patriat and systems: the , the Main locus of shortening prior to the late Oligo- Achache, 1984; Dewey et al., 1989; Chen et Boundary Thrust, the Main Central Thrust, cene occurred signi®cantly south (possibly al., 1993; Patzelt et al., 1993) since the initial and the South Tibetan Detachment System, Ͼ60 km) of the Indus-Yalu suture zone collision between India and Asia at ca. 55±50 from south to north. Immediately south of the within the Tethyan fold-thrust belt and that Ma (Patriat and Achache, 1984; Rowley, study area, in the Garhwal Himalaya, the Main a signi®cant amount of unsubducted oce- 1996; de Sigoyer et al., 2000) or earlier (Yin Central Thrust and South Tibetan Detachment anic lithosphere was present south of the and Harrison, 2000). Attempts to corroborate System correlate with the Vaikrita thrust and suture in southwest Tibet until that time. this total estimate with ®eld-based structural Malari fault, respectively (Valdiya, 1981, An implication of this result is that postcol- studies have viewed the Tibetan-Himalayan 1989) (Figs. 1A and 1B). The Lesser Hima- lision (Oligocene/Miocene) high-K, calc- collision zone as consisting of three belts that laya is the structurally lowest thrust slice. It is alkaline magmatism may be explained by could have absorbed a signi®cant proportion bounded at the base by the Main Boundary melting due to active subduction of oceanic of the convergence: (1) the Himalayan fold- Thrust and at the top by the Main Central crust beneath the Kailas magmatic complex thrust belt (Gansser, 1964; Sinha, 1986; Schel- Thrust and consists of Middle me- until the late Oligocene. A regional pro®le ling, 1992; DeCelles et al., 1998, 2001), (2) tasedimentary rocks, Paleozoic to sed- across the Tibetan-Himalayan orogen from the Tethyan fold-thrust belt (Burg and Chen, imentary and volcanic rocks, and Cambrian± 1984; Searle, 1986, 1996a; Steck et al., 1993; Ordovician granitic rocks (Brook®eld, 1993; Parrish and Hodges, 1996; DeCelles et al., ²Present Address: Department of Geosciences, Ratschbacher et al., 1994), and (3) the Indus- University of Houston, Houston, Texas 77204-5007; Yalu suture zone (Heim and Gansser, 1939; 2000). The Greater Himalaya Crystalline Se- e-mail: [email protected]. Gansser, 1964; Burg and Chen, 1984; Yin et quence (also known as the High Himalayan

GSA Bulletin; January 2003; v. 115; no. 1; p. 21±34; 6 ®gures.

For permission to copy, contact [email protected] ᭧ 2003 Geological Society of America 21 MURPHY and YIN

Figure 1.

22 Geological Society of America Bulletin, January 2003 STRUCTURE IN TETHYAN FOLD-THRUST BELT AND INDUS-YALU SUTURE ZONE, SOUTHWEST TIBET n oned tening different SÐGang- DeCelles et have fed into rakoram fault set shows location me. nding from the Subhimalaya (A) Regional geologic map of southwest Tibet and northwestern Nepal compiled from Heim and Gansser (1939), Valdiya (1981), Cheng and Xu (Continued.) Figure 1. (1987), Shrestha et al. (1987),to DeCelles et the al. Gangdese (1998), Shan Yin (Transhimalaya)of et (Fig. al. study (1999), 1B) area and is and Murphy shown.system; et cross The al. BNSÐBangong-Nujiang sections A±B (2000). suture from Location segment of Searle zone; ofFrontal regional the (1986) IYSZÐIndus-Yalu cross-section Thrust; regional line in suture MCTÐMain (A±F) cross Zanskar zone; exte Central section andal. STDSÐSouth is Thrust. Ratschbacher (1998), taken Tibetan (B) et and directly Detachment Regional al. from this System; cross (1994)dese DeCelles study. MBTÐMain section in et thrust Additional al. Boundary across south system. abbreviations: (1998). Tibet. Thrust; the Formations RTÐRamgarh In orientations Abbreviations: MFTÐMai Himalaya in thrust; JSÐJinsha on are the DTÐDadeldhura suture; the shown thrust; Tethys KFSÐKa basis KFÐKarakorambetween Himalaya for of fault; the and the pro®les GCTÐGreat Subhimalaya Gangdese published Counter Main and Shan Thrust; inthe the Central (between GT Heim Indus-Yalu Main Thrust, C and suture Himalayan and Gansser on zone.beneath Thrust (1939), F If the the on and the basis Kailas the displacement underthrusting magmatic of cross-section) on is arc correlate all observations complex assumed thrusts to in shortly south to those the thereafter, of shown have thereby Garhwal and in begun implying including and Figure that the at 2. in oceanic Main 20 Two lithosphere the Central Ma, had Thrust Karnali. the is been We assumed subducted leading to estimate beneath edge Asia 763 of until km the that of ti Indian horizontal plate's shor lower crust would have been positi

Geological Society of America Bulletin, January 2003 23 MURPHY and YIN

Crystalline Sequence) is bounded by the Main quence and the Greater Himalayan Crystalline al., 1999). Thermal modeling of the Gangdese Central Thrust below and the South Tibetan Sequence. hanging wall in the Zedong area of southeast Detachment System above (Burg and Chen, The magnitude of shortening within the Hi- Tibet yields a slip rate of 7 mm/yr between 1984; Burch®el et al., 1992) and is composed malayan fold-thrust belt is undoubtedly un- 30 and 23 Ma, placing a minimum constraint of Neoproterozoic to lower Paleozoic meta- derestimated because of the lack of hanging- on its displacement of ϳ50 km (Harrison et sedimentary, granitic, and volcanic rocks, and wall cutoffs and pervasive penetrative al., 2000). Thermal modeling of the hanging Tertiary granitic rocks (Le Fort, 1986; Parrish deformation within the hanging wall of the wall of the younger Great Counter Thrust in and Hodges, 1996; DeCelles et al., 2000). The Main Central Thrust (e.g., Gansser, 1964; Bru- the Lian Xian area, east of Zedong in south- Tethyan (or North) Himalaya lies between the nel and Kienast, 1986; MacFarlane et al., east Tibet, by Quidelleur et al. (1997) places South Tibetan Detachment System and the 1992; Schelling, 1992; Hodges et al., 1996; a minimum constraint on the total displace- Great Counter Thrust, a major north-directed DeCelles et al., 2001). Nonetheless, minimum ment of 12 km. The total displacement on the thrust system located along the Indus-Yalu shortening estimates north of the Main Great Counter thrust in southwest Tibet has suture zone (Heim and Gansser, 1939; Boundary Thrust, but excluding the Tethyan not been investigated prior to this study. Ratschbacher et al., 1994; Yin et al., 1999). fold-thrust belt, range from 193 to 260 km in It consists of late Precambrian to lower Pa- the northwestern Himalaya of northern India GEOLOGY OF SOUTHWEST TIBET leozoic sedimentary and metasedimentary (Srivastava and Mitra, 1994), 316 km in the rocks (Gansser, 1964; Yin et al., 1988; western Nepal Himalaya (DeCelles et al., In the following section, we summarize the Burch®el et al., 1992; Brook®eld, 1993; Gar- 2001), and 175±210 km in the eastern Nepal geologic relationships between the Mount zanti, 1999) and thick Permian to Upper Cre- Himalaya (Schelling, 1992). Field studies sug- Kailas area in the north and the China-Nepal- taceous continental-margin sequences gest that the Tethyan Sedimentary Sequence India border junction in the south (Fig. 21), (Cheng and Xu, 1987; Brook®eld, 1993; Gar- has been shortened 100±126 km in the Zan- based on observations by Heim and Gansser zanti, 1999). The entire sequence is com- skar region (Searle, 1986, 1996a; Steck et al., (1939), Cheng and Xu (1987), Yin et al. monly referred to as the Tethyan Sedimen- 1993) and 60 km (Hauck et al., 1998), 139 (1999), and our recent ®eld work in the re- tary (or metasedimentary) Sequence. km (Ratschbacher et al., 1994), and 325 km gion. Several components of the former Indian Sedimentologic characteristics, trace-element (Burg et al., 1987) in south-central Tibet. passive continental margin and Asian active isotope data, and U-Pb detrital zircon ages Over the past decade, ®eld investigations and margin are exposed along this pro®le. They place the Greater Himalayan Crystalline Se- thermochronologic studies along the Indus- are, from south to north, (1) the Tethyan Sed- quence north of, and partially at a higher Yalu suture zone have recognized two phases imentary Sequence (Indian passive-margin se- stratigraphic level than, the Lesser Himalayan of north-south shortening (Harrison et al., quence), (2) the Kiogar-Jungbwa ophiolite Sequence (Brook®eld, 1993; Parrish and 1992, 2000; Ratschbacher et al., 1994; Yin et (Neotethyan oceanic lithosphere), (3) the Indus- Hodges, 1996; DeCelles et al., 2000). A prov- al., 1994, 1999; Quidelleur et al., 1997). The Yalu suture zone (Triassic±Cretaceous meÂ- enance study by DeCelles et al. (2000) sug- earlier phase (30±23 Ma) is associated with lange and accretionary-wedge materials), (4) gested on the basis of U-Pb detrital zircon the south-directed Gangdese thrust system, re- the Xigaze Group (forearc basin deposits), and ages and Nd model ages that the Greater Hi- sulting in juxtaposition of Gangdese grani- (5) the Gangdese batholith (magmatic arc), malayan Crystalline Sequence was accreted toids in its hanging wall over the Tethyan Sed- which is also called the Kailas magmatic com- onto the during Late Cambrian± imentary Sequence in its footwall (Yin et al., plex) (Figs. 1±3). Figure 3 shows a schematic Early Ordovician time along a paleosuture 1994; Harrison et al., 2000). Modi®cation of pro®le displaying the inferred tectonic frame- marked by the present Main Central Thrust. this earlier con®guration occurred between 19 work across the India-Asia convergent margin Sedimentologic characteristics, trace-element and 13 Ma in southwest Tibet and is associ- prior to collision. isotope data, and U-Pb detrital zircon ages ated with thrusting toward the north along the suggest that the Greater Himalayan Crystal- Great Counter Thrust (Yin et al., 1999), also Tethyan Sedimentary Sequence line Sequence may have served as the base- referred to as the South Kailas thrust in south- ment to the Paleozoic±Mesozoic succession of west Tibet (Cheng and Xu, 1987; Murphy et On the basis of stratigraphic data collected the Tethyan Sedimentary Sequence (Brook- al., 2000), the backthrust system (Ratschbach- by the Tibetan Bureau of Geology and Min- ®eld, 1993; Parrish and Hodges, 1996; De- er et al., 1994), and the Renbu-Zedong thrust eral Resources (Cheng and Xu, 1987) and our Celles et al., 2000). Cenozoic convergence be- in southeast Tibet (Yin et al., 1994; Quidelleur own geologic mapping (Fig. 2), we have com- tween India and Asia has resulted in et al., 1997; Harrison et al., 2000). Where ob- piled stratigraphic thicknesses of the Precam- shortening of the Tethyan Sedimentary Se- served, the thrust places the Tethyan Sedi- brian±Cretaceous units (Fig. 4). Although the quence across the entire Himalayan thrust mentary Sequence in its hanging wall above total thickness of the units in the study area is front, forming the Tethyan fold-thrust belt the Gangdese batholith in its footwall. The nearly constant (9.6±9.3 km from south to (Fig. 1A inset map). Although schists and Gangdese thrust system does not crop out in north), the thickness of individual units does gneisses do exist in the Tethyan Himalaya, southwest Tibet (Yin et al., 1999). However, vary (Cheng and Xu, 1987). most are interpreted to have been exhumed by thermal modeling of 40Ar/39Ar step-heating re- Precambrian±Devonian strata are well ex- motion on syncollisional extensional struc- sults from K-feldspar in a granite from the posed in the southern part of the study area, tures (e.g., Gurla MandhataÐMurphy et al. Kailas magmatic complex indicates a rapid near Pulan (Figs. 1A and 2). These rocks con- [2002]; Kangmar domeÐChen et al. [1990]; cooling event between 30 and 25 Ma (Yin et sist of quartzites, calc-silicates, and minor Lee et al. [2000]) that postdate the Tethyan al., 1999). In the absence of extensional struc- limestone and dolostone, ϳ4.9 km thick (Fig. fold-thrust belt, therefore indicating that the tures of such age cutting the Gangdese bath- 4). Carboniferous±Permian strata are variably thrust belt may be thin-skinned. If so, the bas- olith, the cooling ages may best be explained al detachment may be located along the con- by slip on a structure equivalent to the Gang- 1Figure 2 is on a separate sheet accompanying tact between the Tethyan Sedimentary Se- dese thrust system in southwest Tibet (Yin et this issue.

24 Geological Society of America Bulletin, January 2003 STRUCTURE IN TETHYAN FOLD-THRUST BELT AND INDUS-YALU SUTURE ZONE, SOUTHWEST TIBET

Figure 3. Schematic north-south pro®le showing the major tectonic elements of the northern (passive) margin of the Indian plate and the southern (active) margin of the Asian plate prior to the onset of collision. Position of Lesser Himalayan Sequence, Greater Himalayan Crystalline Sequence, Indian plate's lower crust (Indian Shield±type rocks), and Tethyan Sedimentary Sequence after DeCelles et al. (2000). exposed in southwest Tibet. Carboniferous strata places nearly unaltered norite and mantle-type the thrust may have been reactivated, resulting are missing south of the Kiogar-Jungbwa ophiol- rocks (dunite and harzburgite) in its hanging in excision of the ¯ysch sequence along strike ite sheet, and only 333 m of Permian lime- wall over fossiliferous sandstone, siltstone, (Fig. 1A). Moreover, the Kiogar-Jungbwa stone is present in the thrust belt immediately and minor limestone of Jurassic to Cretaceous thrust sheet is bounded both to the south and west of Pulan (Cheng and Xu, 1987). North age in its footwall (Fig. 2). At the base of the north by Tethyan Sedimentary Sequence of the Kiogar-Jungbwa ophiolite thrust sheet, thrust sheet is a foliated containing rocks, suggesting out-of-sequence-thrusting, both Carboniferous and Permian strata are numerous veins of calcite. Field evidence similar to the Spontang thrust sheet (Searle, present and are as thick as 2100 m (Figs. 1A from , patterns, chatter 1986; Searle et al., 1997). As mentioned al- and 2). Elsewhere in the Tethyan Himalaya, marks, and offset lithologic markers de®ne ready, the upper age constraint on slip along two regional unconformities have been ob- three sets of faults: east-trending thrust faults, the Kiogar-Jungbwa thrust sheet is Creta- served in the Upper Carboniferous (Pennsyl- north-trending normal faults, and northwest- ceous. In southeast Tibet, constraints on the vanian)±Lower Permian strata that have been trending right-slip faults (Fig. 2). Faults dis- crystallization age of the Indus-Yalu suture attributed to rifting of the Lhasa block off the playing north-trending striations are cut by ophiolites are similar (late Albian±early Cen- margin of India or other part of Gondwana- those displaying east-trending lineations. We omanian according to Marcoux et al. [1981]; land (Leeder et al., 1988; Brook®eld, 1993; Le interpret strike-slip and extensional structures 119 Ϯ 25 Ma according to AlleÁgre et al. Fort, 1996), which may explain missing Low- within the thrust sheet to be related to recent [1984]). er Carboniferous (Mississippian) strata along slip on the fault system and Gurla the southern part of the traverse. No Paleozoic Mandhata detachment system (Murphy et al., Indus-Yalu Suture Zone -related structures have been observed in 2002). The thrust sheet is poorly exposed near The boundary between rocks with an Asian southwest Tibet, although they are probably Mapam Yum Co (Fig. 2), and we therefore af®nity and those with an Indian af®nity is present below the younger Mesozoic strata. were unable to recognize any complete rock currently de®ned by a south-dipping thrust The Triassic strata (1000 to 1122 m thick) sequence. A more complete section of the system referred to as the Great Counter Thrust were deposited in a shallow-marine environ- thrust sheet was observed by Augusto Gansser (Heim and Gansser, 1939; Yin et al., 1999) ment and consist of interbedded silty lime- (Heim and Gansser, 1939) near the southwest (Figs. 1A and 2); it has been referred to lo- stone and ®ne-grained sandstone, containing corner of La'nga Co (Fig. 1A). Gansser doc- cally as the South Kailas thrust (Cheng and abundant ammonites particularly to the west umented a north-dipping thrust sheet ¯oored Xu, 1987). The Great Counter Thrust is a of Pulan (Fig. 4). The Jurassic through Cre- by an ϳ200-m-thick ¯ysch sequence consist- thrust system composed of at least ®ve south- taceous strata (3400±2250 m thick) consists of ing of a Cretaceous limestone, Jurassic sand- dipping thrust faults in southwest Tibet. From shale and minor siltstone, sandstone, and stone matrix, and blocks of Triassic carbonates south to north, they place phyllitic schist over limestone. The Jurassic strata, south of the and andesite of unknown age. Gabbroic rocks a sequence of limestone and sandstone that Kiogar-Jungbwa ophiolite thrust sheet, contain lie in tectonic contact above the ¯ysch se- may be part of the Tethys Sedimentary Se- bivalves that indicate that a shallow-marine quence (Gansser, 1979). Gansser (1979) quence. In the hanging wall of this thrust, ser- environment prevailed on the Indian passive linked the Cretaceous limestone with the pentinized ophiolitic rocks are thrust over the margin at least until this time. The upper part ¯ysch sequence, therefore placing a maximum phyllitic schist unit. We speculate, on the basis of the Jurassic strata are dominated by ®ne- age constraint on the emplacement of the of the rock types juxtaposed, that the thrust is grained sandstone and shale. Tertiary strata ophiolitic rocks. Farther north, we observed an older, folded, south-directed thrust that sep- within the Tethyan Himalaya are only present that peridotite crops out for nearly 10 km. Un- arates an accretionary-wedge complex above in the Pulan basin, which postdates thrusting like other ophiolite exposures in the Tethyan from metamorphosed Triassic turbidites be- and is interpreted to be related to late Miocene Himalaya, such as the Spontang ophiolite in low. Fault-slip analysis by Ratschbacher et al. slip on the Gurla Mandhata detachment sys- Zanskar (Searle, 1986; Searle et al., 1997) or (1994) determined that the transport direction tem (Fig. 1A). the Xigaze ophiolite in southern Tibet (Nico- on this fault is toward the southwest. Approx- las et al., 1981), a metamorphic sole at the imately 2 km to the north, a north-directed Kiogar-Jungbwa Ophiolite base of the thrust sheet was not found. The thrust places a folded limestone unit over pur- Between Mapam Yum Co and La'nga Co fact that the ¯ysch sequence documented by ple sandstone. The limestone is in turn thrust (``Co'' is ``Lake'' in Tibetan) on their south Gansser near La'nga Co is missing near Ma- northward over a Ͼ1-km-thick sequence of side (Figs. 1 and 2), a north-dipping thrust pam Yum Co farther to the east suggests that Cretaceous limestone and sandstone referred

Geological Society of America Bulletin, January 2003 25 MURPHY and YIN suture zone. Figure 4. Representative stratigraphicData sources: column Heim from and each Gansser of (1939), the Cheng main and Xu tectonic (1987), units Yin from et the al. southern (1999), Tethyan and our Sedimentary own Sequence observations. to the Indus-Yalu

26 Geological Society of America Bulletin, January 2003 STRUCTURE IN TETHYAN FOLD-THRUST BELT AND INDUS-YALU SUTURE ZONE, SOUTHWEST TIBET to as the Yiema Formation by Cheng and Xu dikes yields crystallization ages between 21 original dip angle of the Gurla Mandhata de- (1987) and correlated to the Xigaze forearc and 12 Ma (e.g., ZanskarÐNoble and Searle tachment during exhumation of the footwall basin deposits by Liu (1988). The Yiema For- [1995]; Shisha PangmaÐSearle et al. [1997]; yield total-slip estimates of between 66 and 35 mation is thrust northward over the Kailas NyalamÐSchaÈrer et al. [1986] and Xu [1990]; km across the Gurla Mandhata detachment conglomerate. Ratschbacher et al. (1994) de- Rongbuk valleyÐHodges et al. [1998] and system (Murphy et al., 2002). termined a compression direction trend of Murphy and Harrison [1999]; DinggyeÐXu 033Њ for the northernmost and southernmost [1990]; Wagye LaÐWu et al. [1998]; Gonta CROSS-SECTION RECONSTRUCTION backthrust. Yin et al. (1999) investigated the LaÐEdwards and Harrison [1997]). general stratigraphic framework of the Kailas conglomerate. They advocated a two-phase Karakoram-Gurla Mandhata Fault In order to better understand the structural history, with the deposition of its lower sec- System relationship between these lithotectonic zones tion coeval with slip on a south-directed thrust in southwest Tibet in the vicinity of the Indus- equated to the Gangdese thrust system (Yin et The dextral strike-slip Karakoram fault sys- Yalu suture zone and their original paleogeo- al., 1994), whereas its upper section records tem and Gurla Mandhata detachment system graphic position, we have restored a regional north-directed motion on the Great Counter merge within the study area (Figs. 1A and 2). cross section (Fig. 5). Our geologic map is Thrust. Fault-slip data collected by Ratsch- On the basis of offset of recent geomorphic compiled from mapping by Augusto Gansser bacher et al. (1994) (stations 8±9F and KAF) features, both faults are considered active (Ka- (Heim and Gansser, 1939), the Tibetan Bureau from the northernmost backthrust can be in- rakoram faultÐMolnar and Tapponnier of Geology and Mineral Resources (Cheng terpreted to record two deformation events, an [1978], Armijo et al. [1989], Liu [1993], and Xu, 1987), A. Yin and M.A. Murphy in earlier south-directed thrusting event followed Ratschbacher et al. [1994], Searle [1996b], 1995 (Yin et al., 1999), and ®eld mapping by by a later northwest-directed thrusting event. and Murphy et al. [2000]; Gurla Mandhata de- M.A. Murphy during 1997 and 1998 ®eld sea- The earlier south-directed thrusting event may tachment systemÐRatschbacher et al. [1994], sons (Fig. 2). We make four general assump- re¯ect motion along a fault equivalent to the Yin et al. [1999], and Murphy et al. [2002]). tions in our cross section: (1) The transport Gangdese thrust. A late Oligocene±late Mio- Movement on them likely began in the late direction of the Tethyan fold-thrust belt was cene age of the Kailas conglomerate is de®ned Miocene to early Pliocene in southwest Tibet perpendicular to the trend of axial hinges of by 40Ar/39Ar cooling ages of both clasts and (Ratschbacher et al., 1994; Searle, 1996b; Yin thrust-related asymmetric folds, i.e., 203Њ (Fig. their apparent source in the Gangdese batho- et al., 1999; Murphy et al., 2000). The Kara- 2). The transport direction of the Great Coun- lith (Harrison et al., 1993; Yin et al., 1999). koram fault system extends into our study area ter Thrust system (Fig. 6), which was deter- The Kailas conglomerate lies unconformably as an ϳ36-km-wide zone of northwest-striking mined from the orientations of compression- above a granite that is a part of the Kailas dextral strike-slip faults (Fig. 2). On the basis related structures (Fig. 2) and fault-slip magmatic complex (Honegger et al., 1982) of timing, kinematics, and net-slip estimates, analysis (Ratschbacher et al., 1994), was toward correlated with the Gangdese batholith (Cheng Murphy et al. (2002) suggested that the Ka- 033Њ. The transport direction for the Kiogar- and Xu, 1987; Liu, 1988). rakoram fault system is linked to the Gurla Jungbwa ophiolite thrust sheet is assumed to Mandhata detachment system, thus making a be parallel to the Tethyan fold-thrust belt, al- South Tibetan Detachment System large right step in the fault system (Fig. 1). though measured thrust-related striations in- The slip direction on the two fault systems is dicate a north-south motion. (2) The fold- The main trace of the South Tibetan De- nearly coincident. The mean slip direction of thrust belt is thin-skinned, and the basal thrust tachment System crops out ϳ20 km south of the Karakoram fault system at its southern (deÂcollement) lies between the Tethyan Sedi- the study area. It is a down-to-the-north, low- end, near Mount Kailas, is N70ЊW, 3 0 Њ (␣ 95 mentary Sequence and the Greater Himalayan angle normal-fault system that is traceable con®dence cone, 30Њ). The mean slip direction Crystalline Sequence. (3) Thrusts propagated along the length of the Himalaya (Burg and of the Gurla Mandhata detachment is N80ЊW, toward the foreland within the Tethyan fold- Chen, 1984; Burch®el et al., 1992). This fea- 20Њ (␣ con®dence cone, 10Њ). The magnitude 95 thrust belt, with the exception of the thrust ture places generally low-grade Tethyan me- of slip on the Karakoram fault system at its directly north of the Kiogar-Jungbwa ophiolite tasedimentary rocks against the Greater Him- southern end is estimated to be 66 km on the thrust sheet. (4) Out-of-plane motion is not alaya crystalline basement with variably basis of offset of the Great Counter Thrust. considered (Fig. 2). The cross section is line- deformed leucogranites commonly exposed in The footwall of the Gurla Mandhata detach- the footwall of the detachment system (Burg ment systems contains rocks that can be cor- length balanced. Figures 5A through 5G sum- and Chen, 1984; Herren, 1987; Burch®el et related with both the Greater Himalayan Se- marize our proposed sequential development al., 1992; Edwards et al., 1996; Hodges et al., quence and Lesser Himalayan Sequence on of the Tethyan fold-thrust belt and Indus-Yalu 1996). Augusto Gansser mapped such a rela- the basis of their Sr-Nd isotopic ratios, imply- suture zone beginning with the precollisional tionship along the Kali river in Nepal, where ing these rocks originated below the Tethyan setting. Figure 5G indicates 112 km of hori- the fault dips ϳ40Њ to the north (Heim and Sedimentary Sequence. Consideration of GASP zontal shortening in the Tethyan Sedimentary Gansser, 1939). Within our study area, due (garnet-aluminosilicate-quartz-plagioclase) and Sequence and 64 km of horizontal shortening west of the town of Pulan, we speculate that, GARB (garnet-biotite) thermobarometric re- in the Indus±Yalu suture zone. As no subsur- on the basis of similar orientations, the top- sults yields equilibration depths for the foot- face data exist, stratigraphic and structural re- to-the-north normal fault may be part of the wall rocks between 13.3 and 26.7 km, if a lationships shown in the reconstruction are a South Tibetan Detachment System (Fig. 2). lithostatic gradient of 0.275 kbar/km (Murphy ®rst-order approximation at best. Moreover, The timing of slip on the South Tibetan De- et al., 2002) is assumed. The shallowest equil- because we do not take into account internal tachment System in southwest Tibet is un- ibration depth imposes a limit to the maxi- deformation (, , and isoclinal known. Elsewhere, however, U-Th-Pb dating mum depth of the Tethyan Sedimentary Se- folding) within thrust sheets, our shortening of accessory minerals from synkinematic quence. Moreover, considerations for the estimate should be viewed as a minimum.

Geological Society of America Bulletin, January 2003 27 MURPHY and YIN

Stage A (Early Cretaceous to Late tation that the Xigaze forearc strata have been shortening) has occurred on the Kiogar- Cretaceous) underthrust below the Gangdese batholith. Jungbwa thrust. The magnitude of displace- The dimensions of the forearc shown are cal- ment is de®ned by the present surface expo- The Neotethys oceanic lithosphere subducts culated from stratigraphic studies and struc- sure of the ophiolitic complex (ϳ20 km) beneath the Asian active margin (Fig. 5A). tural reconstructions of the Xigaze Group in (Figs. 1A and 2) and the observation that The con®guration of the Asian active margin south-central Tibet (Ratschbacher et al., 1992; Tethyan Sedimentary Sequence rocks that pre- prior to collision is de®ned by those tectonic DuÈrr, 1993, 1996; Einsele et al., 1994; DuÈrr et date Permian rifting (Gaetani and Garzanti, units exposed in the Great Counter Thrust sys- al., 1995). Between Xigaze (82Њ52Ј E) and di- 1991) of the Lhasa block from the northern tem near Mount Kailas and in south-central rectly north of Saga (85Њ10Ј E), the overall margin of India are present north of the ultra- Tibet where the Xigaze forearc basin deposits structure of the Xigaze Group is a large syn- ma®c rocks. As we show in Figure 5B, an out- (Xigaze Group) are better exposed. U-Pb zir- clinorium (Burg et al., 1987; Ratschbacher et of-sequence thrust may explain the presence con data from the Kailas magmatic complex al., 1992; Einsele et al., 1994). In the Ji- of Tethyan Sedimentary Sequence rocks north indicate that arc-related magmatism was oc- angqinzhe area (88Њ10Ј E), the Xigaze Group of the Kiogar-Jungbwa ophiolitic thrust sheet. curring by the Early Cretaceous (120 MaÐ restores to an original north-south length of In stage B of our proposed sequence of events, Miller et al., 2000) in southwest Tibet. Farther ϳ65 km, implying a similar width of the fore- the Kiogar-Jungbwa thrust sheet moved south to the east, geochronologic data indicate an arc basin (Einsele et al., 1994). Burg et al. over the passive continental margin; this sce- older age for arc-related magmatism (130 (1987) estimated a higher predeformational nario provides the necessary precondition for MaÐLhasa area, Harris et al., 1988; 147 width of 80±100 km for the forearc basin in the inferred out-of-sequence thrust in the Teth- MaÐCoqin area, 31Њ N, 85Њ E, Murphy et al., south-central Tibet. Measured stratigraphic yan Sedimentary Sequence. We view the 53 1997). We assume the existence of a forearc sections in the Xigaze area by DuÈrr (1996) km as a minimum displacement because we basin in southwest Tibet prior to the India- indicate that the original thickness of the Xig- chose the thrust sheet to have originated close Asia collision, although its present exposure aze Group was ϳ12 km. It should be noted to the continental borderland. Had the thrust is small compared to south-central Tibet (Liu, that these forearc basin dimensions are mini- originated farther north, within the oceanic 1988). The discontinuous occurrence of the mum values, as the margin of the forearc basin lithosphere, there could be substantially more Xigaze Group along strike of the Indus-Yalu may have been removed by tectonic and ero- slip on the fault. The thrust that now juxta- suture zone has been interpreted in two dif- sional processes. As discussed in the next sec- poses the Kiogar-Jungbwa ophiolitic sequence ferent ways. Yin et al. (1994) suggested that tion, the Kailas magmatic complex is differ- against the Jurassic strata may well be a re- the Xigaze Group is preserved in south-central entially denuded from south to north, which activated thrust, as seen by the lack of an Tibet because the Gangdese thrust system out- is consistent with a tilted thrust hanging wall metamorphic sole beneath the ophiolitic se- crops to the south. In the Mount Kailas area, over a ramp. In constructing the early cross quence. However, the initial stripping of the Yin et al. (1999) inferred the existence of an sections, we have added a volume of granite ophiolitic rocks from either transitional or equivalent structure on the basis of thermal to the Kailas magmatic complex (light red oceanic lithosphere must have occurred prior modeling results from the Kailas magmatic rocks in Fig. 5A) equal to that which, on the to the collision, and prior to underthrusting of complex. Alternatively, Einsele et al. (1994) basis of thermochronology data presented in the Indian subcontinent. The mechanism for suggested that primary pinching out of Yin et al. (1999), we estimate to have been ophiolite is uncertain. Studies by trapped oceanic or transitional crust in the removed by erosion. The accretionary-wedge Aitchison et al. (2000) in southeast Tibet near forearc could have controlled the occurrence complex is shown to be rather small (Fig. 5A). Zedong (29Њ15Ј N, 91Њ45Ј E) suggest the pres- of forearc basin deposits. On the basis of the Its minimum thickness is de®ned by a separate ence of remnants of a south-facing (north- limited occurrence of forearc deposits in reconstruction of the Great Counter Thrust dipping) intraoceanic subduction system, southwestern Tibet, Einsele et al. (1994) in- (Fig. 6). south of the Indus-Yalu suture zone. There- terpreted very little, if any, oceanic or transi- fore, the ophiolite may have been obducted tional crust to be trapped in the forearc. Al- Stage B (Late Cretaceous to Paleocene) during the development of the subduction though their model does offer an alternative zone. The timing of its emplacement is likely explanation for the limited occurrence of fore- We interpret the earliest thrusting event to slightly older than estimates for the timing of arc deposits in southwestern Tibet, the strong involve emplacement of the Kiogar-Jungbwa the initial collision between India and Asia. correlation between missing Xigaze forearc ophiolite thrust sheet onto the Tethyan Sedi- However, estimates for the onset of collision deposits and signi®cant tectonic denudation of mentary Sequence (Fig. 5B). We estimate that remain uncertain, varying between 70 Ma and the Gangdese arc led us to favor the interpre- 53 km of displacement (51 km horizontal 45 Ma, and probably re¯ecting the different

Figure 5. Sequential cross-section restoration across the Tethyan Himalaya and Indus-Yalu suture zone. Cross sections are oriented N23؇E. Reconstruction shows 60% horizontal shortening (176 km), corresponding to 112 km within Tethyan fold-thrust belt and 64 km within the Indus-Yalu suture zone faults (Great Counter Thrust and Gangdese thrust system) (see text for details). (A) Initial con®gu- ration prior to signi®cant underthrusting of the Indian subcontinent (initial length 290 km). (B) Emplacement of the Kiogar-Jungbwa ophiolite thrust sheet over the Tethyan Sedimentary Sequence. C, D, and E: Propagation of the Tethyan fold-thrust belt toward the foreland. The largest of the thrusts juxtaposes Ordovician strata over Jurassic strata. (F) South-directed thrusting on the Gangdese thrust system (GTS) resulting in burial of the Xigaze forearc and accretionary-wedge complex and deposition of the lower section of the Kailas conglomerate as wedge-top deposits on the Kailas magmatic complex. (G) North-directed thrusting on the Great Counter Thrust system resulting in signi®cant shortening of the Permian±Triassic strata, the accretionary-wedge complex, and the Yiema For- mation (Xigaze forearc deposits) (see Fig. 6 for a detailed reconstruction of the Great Counter Thrust system). M

28 Geological Society of America Bulletin, January 2003 STRUCTURE IN TETHYAN FOLD-THRUST BELT AND INDUS-YALU SUTURE ZONE, SOUTHWEST TIBET

Geological Society of America Bulletin, January 2003 29 MURPHY and YIN

Figure 6. Development of the Great Counter Thrust system in southwest Tibet. Cross section is oriented N33؇ E and shows a total of 30.5 km (ϳ40%) of horizontal shortening accommodated by three north-directed thrusts, which were initiated in the footwall of the preceding thrust. Con®guration of the Indus-Yalu suture zone prior to slip on the Great Counter Thrust approximates that shown in stage F of Figure 5, in which the Kailas magmatic complex, in the hanging wall of the Gangdese thrust system (GTS), has been juxtaposed over the Yiema Formation (forearc deposits), in the thrust's footwall. types of data used (Yin and Harrison, 2000). lier than growth of peak metamorphic assem- (see Burg et al. [1987] and Makovsky et al. Examination of magnetic sea¯oor anomalies blages, depending on the regional thermal re- [1999] for south-central Tibet and Searle and zones in the Atlantic and India gime (e.g., England and Thompson, 1984). As [1996a] and Searle et al. [1997] for the north- Oceans indicates that India's northward mo- mentioned earlier, the maximum age limit on west Himalaya), suggesting that the emplace- tion slowed dramatically at ca. 45 Ma (Dewey slip of the Kiogar-Jungbwa ophiolite thrust ment age for ophiolites along the Indian mar- et al., 1989; Le Pichon et al., 1992). Strati- sheet is Cretaceous, on the basis of the age of gin may be similar along strike. graphic (Rowley, 1996) and metamorphic ¯ysch-type rocks within the thrust zone (Heim constraints (de Sigoyer et al., 2000) suggest and Gansser, 1939). The minimum age limit Stage C (Eocene to Early Oligocene) initiation of collision in the northwest Hima- is poorly determined in southwest Tibet, but laya at 55±50 Ma. These estimates are an up- must predate the Gurla Mandhata detachment Development of two south-directed thrusts per bound for the timing of collision. Colli- system because related structures cut the and one north-directed normal fault resulted in sion may have been earlier as sediments may thrust sheet (Fig. 2). Elsewhere in the Tethyan ϳ31 km of horizontal shortening. As just dis- have been subducted, and metamorphism due Himalaya, the timing of ophiolite obduction cussed, we interpret the existence of an out- to burial by thrusting must have occurred ear- was between Late Cretaceous and Paleocene of-sequence thrust to explain the presence of

30 Geological Society of America Bulletin, January 2003 STRUCTURE IN TETHYAN FOLD-THRUST BELT AND INDUS-YALU SUTURE ZONE, SOUTHWEST TIBET

Tethyan Sedimentary Sequence rocks to the Yiema Formation (correlated to the Xigaze modated along a later north-dipping thrust that north of the Kiogar-Jungbwa thrust sheet. This forearc basin deposits) and deposition of the extends to deeper structural levels. We have thrusting event may also explain the exposure Gangdese system (Fig. 5F). Yin chosen not to root this thrust into the basal of Precambrian±Cambrian rocks east of Ma- et al. (1999) interpreted the lower part of the deÂcollement between the Greater Himalayan pam Yum Co immediately north of the Kiogar- Kailas conglomerate as wedge-top deposits re- Crystalline Sequence and the Tethyan Sedi- Jungbwa ophiolitic complex (Fig. 1A). A sim- lated to slip on a thrust equivalent to the mentary Sequence, but instead, project it into ilar kinematic interpretation was conceived by Gangdese thrust recognized in southeastern the Greater Himalayan Crystalline Sequence. Searle (1986) to explain the position of the Tibet near Zedong (Yin et al., 1994). If cor- This choice is motivated by the potential link Spontang ophiolite. We infer the existence of rect, their interpretation would predict accu- between the Great Counter Thrust and the a top-to-the-north normal fault in order to ex- mulation of the rest of the foreland basin system South Tibetan Detachment System that is plain Permian strata juxtaposed against Or- (foredeep, forebulge, back-bulge) (DeCelles and based on their compatible slip direction and dovician strata. An unconformity may also ex- Giles, 1996) south of Mount Kailas. The coeval fault movement (Yin et al., 1999; Lee plain this relationship. However, because an ramp-¯at geometry of the Gangdese thrust is et al., 2000). The dip-slip component of faults unconformity at these stratigraphic levels was suggested by the 40Ar/39Ar-derived thermal related to the Gurla Mandhata detachment sys- not recognized in other parts of the study area, history from a sample of granite (95-6-11-3) tem and the Karakoram fault system have also its aerial extent would necessarily need to be presented in Yin et al. (1999) and the erosion- been restored (Murphy et al., 2002). restricted to the northern parts of the Tethyan al pattern of the Gangdese batholith in this Because the Karakoram fault system inter- Sedimentary Sequence. Although we interpret region. The volcanic cover is eroded away in sects the cross section, the assumption in our the normal fault to be active at this stage, it the south, but preserved to the north of Mount restoration that no out-of-the-plane motion has is also possible that it slipped later and may Kailas (Fig. 1A). Yin et al. (1999) showed that occurred is not valid. However, because dex- be related to slip on the South Tibetan De- the granite was emplaced at an ambient tem- tral strike-slip faults that are part of the Ka- tachment System (Figs. 1A and 1B). Alter- perature of ϳ400 ЊC and rapidly cooled from rakoram fault system intersect the cross sec- natively, it may have formed during the initial 30 to 25 Ma. A assumed geothermal gradient tion at angles between 85Њ and 90Њ, Ͻ5kmof development of the during the of 35ЊC/km puts this rock's intrusion level at apparent shortening along the cross section is Permian. a depth of 11.5 km. Harrison et al. (2000) es- expected to have resulted from slip on these timated 50 km of displacement on the Gang- faults. Stage D (Eocene to Early Oligocene) dese thrust in southeast Tibet (Zedong win- dow). To explain the increased denudation of DISCUSSION The thrust active during stage D shows the the Kailas magmatic complex southward and largest magnitude of slip, ϳ40 km of south- overlying volcanic rocks to the south (K-Tv Our reconstruction shows that the locus of ward movement, resulting in ϳ30 km of hor- in Fig. 1A), we have drawn the Gangdese Cenozoic crustal shortening (112 km) oc- izontal shortening. Moreover, this thrust has thrust to have a 30-km-long ramp dipping 30Њ curred signi®cantly south of the Indus-Yalu the greatest observed stratigraphic throw to the north and a nearly horizontal, 44-km- suture zone during the early stages of the In- along the pro®le and, as interpreted at depth, long ¯at. Although we expect that the forearc dia-Asia collision. Although highly dependent doubles the thickness of the Tethyan Sedi- basin is signi®cantly shortened as its bedding upon the assumptions we made in our recon- mentary Sequence in the immediate area. is isoclinally folded and locally transposed in struction, the geometry shown in our recon- the study area, no attempt was made to de®ne struction results in preserving a Ͼ60-m-wide Stage E (Eocene to Early Oligocene) its magnitude owing to limited exposure in the piece of oceanic lithosphere between the Teth- The development of a south-directed thrust study area. yan fold-thrust belt and the Kailas magmatic and two north-directed normal faults result in complex until activation of the Gangdese Ͻ1 km of total horizontal shortening across Stage G (Early Miocene to Middle thrust system in the Oligocene. Major and the pro®le compared to that shown in stage D Miocene) trace element contents and geochronologic (Fig. 5E). The south-directed thrust was not data of the Kailas magmatic complex present- observed in the ®eld, but it is inferred to ac- North-directed thrusting on the Great Coun- ed in Miller et al. (2000) indicate that high-K, count for the large antiform in the hanging ter Thrust system resulted in (1) stacking of calc-alkaline magmatism did not end until 40 wall of the inferred thrust where we show it the Cretaceous Xigaze forearc basin deposits, Ma. Further consideration of the age and to have developed as a fault-bend fold. The accretionary-wedge materials, and Triassic chemistry of younger volcanic rocks in the relationship between the normal faults, includ- strata, (2) burial of the Gangdese thrust sys- Mount Kailas area extends the arc-type igne- ing the one shown in stage C, and thrust faults tem, and (3) deposition of the upper Kailas ous activity to the Miocene (Miller et al., is uncertain. Either the normal faults are re- conglomerate (Fig. 5G). Figure 6 shows a sep- 1999). A similar time-span for arc-type mag- lated to development of the thrust belt in a arate reconstruction of the Great Counter matism has been reported in Ladakh (Honeg- simple zone (Yin and Kelty, 1991) or Thrust in which its development is modeled ger et al., 1982) and southeastern Tibet (Har- are part of the South Tibetan Detachment Sys- as a imbricate thrust system propagating to- rison et al., 2000). These data indicate that tem (Burch®el et al., 1992). If the latter is true, ward the foreland. Approximately 20 km of ¯uid and thermal conditions remained appro- then they probably were active later than what shortening is shown to have been accommo- priate for arc-type magmatism well after the the reconstruction shows. dated along a basal thrust lying along the onset of collision between India and Asia, re- Permian/Carboniferous contact. This position gardless of which data set is used (Yin and Stage F (Late Oligocene to Early Miocene) was chosen to explain the widespread occur- Harrison, 2000). Rather than calling upon a rence of imbricated Triassic strata along the complex mechanism to explain ¯uid in¯ux South-directed thrusting on the Gangdese Indus-Yalu suture zone in southern Tibet (Liu, and elevated temperatures, our reconstruction thrust system resulted in overthrusting of the 1988). The rest of the displacement is accom- is consistent with melting due to active sub-

Geological Society of America Bulletin, January 2003 31 MURPHY and YIN duction of oceanic lithosphere and the pres- suggested by microseismic activity in far- known to have been active in the early Mio- ence of an asthenospheric wedge beneath the western Nepal (Pandey et al., 1999) (Fig. 1B), cene (Hubbard and Harrison, 1989; Hodges et Kailas magmatic complex until the late Oli- (3) a shallow subduction angle for the Indian al., 1996; Harrison et al., 1998), but was also gocene or early Miocene. lithosphere, and (4) the con®guration of the reactivated in the Pliocene (Harrison et al., In light of this conclusion, it is necessary Greater Himalayan Crystalline Sequence, 1995, 1998; Catlos, 2000). DeCelles et al. to account for the amount of underthrusting of Lesser Himalayan Sequence, and the Indian (1998) interpreted offset on the Dadeldhura the Indian plate's lower crust beneath Asia in plate's lower crust and lithospheric mantle that thrust to have begun at ca. 15±14 Ma, the old- southwest Tibet. In Figure 1B we have com- is shown in Figure 2, our reconstruction in- est age of the Siwalik Group. Sediment- bined our cross section with the balanced dicates that the northern edge of the Indian accumulation rates and magnetostratigraphic cross section by DeCelles et al. (1998) and the plate's lithosphere has been underthrust be- data from northern India suggest that offset on pro®le by Heim and Gansser (1939). We have tween 421 and 587 km north of the present the the Main Boundary Thrust was initiated at attempted to line up all three cross sections by position of the Indus-Yalu suture zone. If the ca. 11 Ma (Meigs et al., 1995; Burbank et al., projecting the Jahla normal fault and Vaikrita shortening in the Himalaya was completely 1996). DeCelles et al. (1998) interpreted the thrust, which are equated with the South Ti- accommodated by ¯at subduction, then it thrust to have formed somewhat later in the betan Detachment System (and Zanskar nor- would imply that the northern edge of the In- central Himalaya. The Main Frontal Thrust is mal fault [Herren, 1987; Burch®el et al., dian continent has already reached the Jinsha active today (Nakata, 1989; Lave and Avouac, 1992]) and the Main Central Thrust, respec- suture separating the Qiangtang from 1999) and is interpreted to have been initiated tively. DeCelles et al. (1998) calculated ϳ228 the Songpan Ganzi terrane (Yin and Harrison, coevally with the Pliocene upper Siwalik km of horizontal shortening of the Subhima- 2000) (Fig. 1A). This possibility may be test- Group (DeCelles et al., 1998). These timing laya and Lesser Himalaya and later re®ned ed by geophysical observations of the litho- constraints indicate that underthrusting of the this estimate with more detailed mapping spheric structure beneath the western part of Indian Shield in southwest Tibet did not begin along the Seti River corridor to 460 km the . In fact, preliminary re- until the early Miocene and underthrusting (DeCelles et al., 2001) (Fig. 1B). A minimum- sults from seismic tomography, using the rates have been on the order of ϳ21.3 mm/yr. shortening estimate for the Dadeldhura thrust P1200 global data set (Zhou, 1996), appear to By using this slip-rate estimate, the leading and Main Central Thrust is 117 km (DeCelles support this interpretation (H.-W. Zhou, 2001, edge of the subducted part of the Indian Shield et al., 2001). Along the Seti River and Karnali personal commun.). However, our assumption would have reached the Indus-Yalu suture River corridors, considerably more shortening that Indian lithosphere is inserted horizontally zone around the early Miocene, thereby allow- is required if both thrusts reached as far south beneath Tibet appears to be inconsistent with ing arc-type magmatism to continue until that as the Dadeldhura synform (DeCelles et al., the depth estimates for Indian coesite-bearing time. 2001; Upreti and Le Fort, 1999). DeCelles et rocks in the Greater Himalayan Crystalline al. (2001) emphasized that the shortening es- Sequence (O'Brien et al., 2001) and helium CONCLUSIONS timate for the Main Central Thrust is a mini- isotopic studies in southern Tibet by Hoke et mum estimate because internal deformation al. (2000), who de®ned the southern limit of Cross-section restoration of a 111-km-long within its hanging wall is not accounted for. mantle-derived geothermal helium in south- traverse from the India-Nepal-China border to Srivastava and Mitra (1994) estimated be- west Tibet to coincide with the surface trace Mount Kailas in southwest Tibet is used to tween ϳ193 and 260 km on the Main Central of the Karakoram fault (Fig. 1A). Because estimate a minimum of 176 km of north-south Thrust and Almora thrust (equivalent to the metamorphism of coesite-bearing rocks in the Cenozoic horizontal shortening, which was Dadeldhura thrust) in northern India. Com- western Himalaya is estimated to have oc- partitioned between the Tethyan fold-thrust bining the shortening estimates for (1) the curred in the Eocene (O'Brien et al., 2001), it belt, including the Kiogar-Jungbwa ophiolite Subhimalaya and Lesser Himalaya along the is possible that the ¯at subduction of India thrust sheet (112 km) and contractional struc- Dadeldhura-Baitadadi road transect (DeCelles was initiated later in the Oligocene with the tures along the Indus-Yalu suture zone (64 et al., 1998) (Fig. 1A), (2) the Main Central arrival of less dense lithosphere (Indian con- km). Sequential reconstruction of the cross Thrust and Almora thrust (Srivastava and Mi- tinental lithosphere) as implied by our recon- section shows that the locus of Cenozoic tra, 1994), and (3) the Tethyan fold-thrust belt struction. Regarding the observations made by shortening due to the India-Asia collision and Indus-Yalu suture zone (this study) yields Hoke et al. (2000), we note that their samples since the late Oligocene occurred signi®cantly a total horizontal shortening estimate across came from exclusively the active or pull- south (Ͼ60 km) of the Indus-Yalu suture zone the central Himalaya of 597±664 km. Alter- apart basins in Tibet. As pointed out by Yin within the Tethyan fold-thrust belt. Moreover, natively, combining the shortening estimates (2000), these rifts most likely involve thinning oceanic lithosphere was present south of the along the Seti River corridor (DeCelles et al., of both the Tibetan and Indian mantle litho- Indus-Yalu suture zone to the north and the 2001) with those in the Tethyan fold-thrust sphere in southern Tibet, which may have also Tethyan fold-thrust belt to the south until the belt and Indus-Yalu suture zone yields a total induced upwelling of the asthenospheric ¯ow. late Oligocene when movement on the Gang- shortening estimate of 763 km, which is clear- Such upwelling would permit ¯at subduction dese thrust system was initiated in southwest ly a minimum estimate because internal de- of the Indian lithosphere beneath Tibet, as Tibet. An implication of this reconstruction is formation within the hanging wall of the Main shown in our reconstruction. that postcollision high-K, calc-alkaline mag- Central Thrust is not accounted for. If we as- The timing of underthrusting can be deter- matism in southwestern Tibet may be ex- sume that (1) all the southward displacement mined from the temporal estimates of dis- plained by melting due to active subduction was accommodated along the Main Central placements on individual thrusts shown in of a remnant oceanic lithosphere beneath the Thrust, (2) thrusts south of the Main Central Figure 1B. The Main Central Thrust (Vaikrita Kailas magmatic complex until the Miocene. Thrust root into a thrust equivalent to the thrust) in the Garhwal Himalaya was active at A regional pro®le across the Tibet-Himalaya Main Himalayan Thrust (Zhao et al., 1993; ca. 20 Ma (Metcalfe, 1993). Farther east, in orogen from the Subhimalaya to the Gangdese Brown et al., 1996; Nelson et al., 1996), as central Nepal, the Main Central Thrust is also Shan (Transhimalaya), together with previous-

32 Geological Society of America Bulletin, January 2003 STRUCTURE IN TETHYAN FOLD-THRUST BELT AND INDUS-YALU SUTURE ZONE, SOUTHWEST TIBET

ly reported shortening estimates in the central Thrust zone, Himalayan orogen [Ph.D. thesis]: Los and Zhang, Y., 1988, Isotope geochemistry of the Angeles, University of California, 300 p. 1985 Tibet Geotraverse, Lhasa to Golmund: Royal So- Himalaya, yields a total shortening estimate Chen, Z., Liu, Y., Hodges, K.V., Burch®el, B.C., Royden, ciety of London Philosophical Transactions, Ser. A, across the orogen of 593±763 km. Timing L.H., and Deng, C., 1990, The Kangmar : A v. 327, p. 263±285. constraints for thrusts south of and including metamorphic core complex in southern Xizang (Ti- Harrison, T.M., Copeland, P., Kidd, W.S.F., and Yin, A., bet): Science, v. 250, p. 1552±1556. 1992, Raising Tibet: Science, v. 255, p. 1663±1670. the Main Central Thrust indicate that the un- Chen, Y., CogneÂ, J.-P., Courtillot, V., Tapponnier, P., and Harrison, T.M., Copeland, P., Hall, S.A., Quade, J., Burner, derthrusting northern edge of the Indian Zhu, X.Y., 1993, Cretaceous paleomagnetic results S., Ojha, T.P., and Kidd, W.S.F., 1993, Isotopic pres- from western Tibet and tectonic implications: Journal ervation of Himalayan/Tibetan uplift, denudation, and Shield along the Main Himalayan Thrust did of Geophysical Research, v. 98, p. 17,981±17,999. climatic histories of two molasses deposits: Journal of not reach the Indus-Yalu suture zone until the Cheng, J., and Xu, G., 1987, Geologic map of the Gerdake Geology, v. 100, p. 157±175. early Miocene, thereby allowing arc-type region at a scale of 1:1,000,000 and geologic report: Harrison, T.M., McKeegan, K.D., and LeFort, P., 1995, De- Xizang Bureau of Geology and Mineral Resources, tection of inherited monazite in the Manaslu leuco- magmatism to persist north of the suture until 363 p. (in Chinese). granite by 208Pb/232Th ion microprobe dating: Crystal- that time. DeCelles, P.G., and Giles, K.A., 1996, Foreland basin sys- lization age and tectonic implications: Earth and tems: Basin Research, v. 8, p. 1±19. Planetary Science Letters, v. 133, p. 271±282. DeCelles, P.G., Gehrels, G.E., Quade, J., Ojha, T.P., Kapp, Harrison, T.M., Grove, M., Lovera, O.M., and Catlos, E.J., ACKNOWLEDGMENTS P., and Upreti, B.N., 1998, Neogene foreland basin 1998, A model for the origin of Himalayan anatexis deposits, erosional unroo®ng, and kinematic history of and inverted metamorphism: Journal of Geophysical We gratefully acknowledge support in the ®eld the Himalayan fold-thrust belt, western Nepal: Geo- Research, v. 103, p. 27,017±27,032. from Paul Kapp (University of Arizona), Ding Lin, logical Society of America Bulletin, v. 110, p. 2±21. Harrison, T.M., Yin, A., Grove, M., Lovera, O.M., Ryerson, and Guo Jinghui (Chinese Academy of Sciences± DeCelles, P.G., Gehrels, G.E., Quade, J., LaReau, B., and F.J., and Zhou, X., 2000, The Zedong : A re- Institute of Geology and Geophysics) and funding Spurlin, M., 2000, Tectonic implications of U-Pb zir- cord of superposed Tertiary convergence in south- con ages of the Himalayan in Nepal: eastern Tibet: Journal of Geophysical Research, from U.S. National Science Foundation grant EAR- Science, v. 288, p. 497±499. v. 105, p. 19,211±19,230. 9628178 (to An Yin and T.M. Harrison). Jean-Pierre DeCelles, P.G., Robinson, D.M., Quade, J., Copeland, P., Hauck, M.L., Nelson, K.D., Brown, L.D., Zhao, W., and Burg, Richard Law, and Lothar Ratschbacher pro- and Upreti, B.N., 2001, , structure, and Ross, A.R., 1998, Crustal structure of the Himalayan vided careful reviews and many useful suggestions. tectonic evolution of the Himalayan fold-thrust belt in orogen at ϳ90 degrees east longitude from Project western Nepal: , v. 20, p. 487±509. INDEPTH deep re¯ection pro®les: Tectonics, v. 17, de Sigoyer, J., Chavagnac, V., Blichert-Toft, J., Villa, I.M., p. 481±500. 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34 Geological Society of America Bulletin, January 2003 Figure 2. Geologic map of the Mount Kailas-Gurla Mandhata area, southwest Tibet. The equal-area Structural evolution and sequence of thrusting in the Tethyan fold-thrust stereonet plots show (1) distributions of fold axes within the Tethyan fold-thrust belt, and (2) fault-slip belt and Indus-Yalu suture zone, southwest Tibet data collected at the base of the Kiogar-Jungbwa thrust sheet. M.A. Murphy and An Yin Figure 2 Supplement to: Geological Society of America Bulletin, v. 114, no. 12.