Geological Society of America Bulletinyin/05-Publications/papers/076-Murphy...24 Geological Society...

Geological Society of America Bulletin

doi: 10.1130/0016-7606(2003)115<0021:SEASOT>2.0.CO;2 2003;115, no. 1;21-34Geological Society of America Bulletin

M.A. Murphy and An Yin Indus-Yalu suture zone, southwest TibetStructural evolution and sequence of thrusting in the Tethyan fold-thrust belt and

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

GSA Bulletin; January 2003; v. 115; no. 1; p. 21–34; 6 figures.

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

M.A. Murphy†

An YinDepartment of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, USA

ABSTRACT

Regional mapping of a north-south tra-verse from the India-Nepal-China borderjunction to Mount Kailas in southwest Ti-bet—combined with previously publishedgeochronologic and stratigraphic data—isthe basis for an incremental restoration ofthe Tethyan fold-thrust belt and deforma-tion along the Indus-Yalu suture zone.From north to south, the major structuralfeatures are (1) the Indus-Yalu suture zone,composed of five south-dipping thrustfaults involving rocks interpreted to repre-sent parts of the former Indian passivemargin and Asian active margin, (2) theTethyan fold-thrust belt, composed of adominantly north-dipping system of imbri-cate thrust faults involving Precambrianthrough Upper Cretaceous strata, and (3)the Kiogar-Jungbwa thrust sheet. A line-length cross-section reconstruction indi-cates a minimum of 176 km of north-southhorizontal shortening partitioned by theTethyan fold-thrust belt (112 km) and Indus-Yalu suture zone (64 km). Sequential res-toration of the cross section shows that thelocus of shortening prior to the late Oligo-cene occurred significantly south (possibly.60 km) of the Indus-Yalu suture zonewithin the Tethyan fold-thrust belt and thata significant amount of unsubducted oce-anic lithosphere was present south of thesuture in southwest Tibet until that time.An implication of this result is that postcol-lision (Oligocene/Miocene) high-K, calc-alkaline magmatism may be explained bymelting due to active subduction of oceaniccrust beneath the Kailas magmatic complexuntil the late Oligocene. A regional profileacross the Tibetan-Himalayan orogen from

†Present Address: Department of Geosciences,University of Houston, Houston, Texas 77204-5007;e-mail: [email protected].

the Subhimalaya to the Gangdese Shan(Transhimalaya), along with previously re-ported shortening estimates in the centralHimalaya, yields a minimum shortening es-timate across the orogen of ;750 km.

Keywords: Tethys, Himalayan orogeny, Ti-betan plateau, suture zones, thrust faults.

INTRODUCTION

The deformation histories of suture zonesare in general tremendously complex becausethey are commonly modified by multiple gen-erations of syncollisional faulting (e.g., Dew-ey, 1977; Yin et al., 1994; Puchkov, 1997).The major tectonic elements of a ‘‘model’’ su-ture zone are well preserved, however, alonga segment of the Indus-Yalu suture (alsoknown as Indus-Yarlung suture) in southwestTibet. This suture zone separates rocks withan Asian affinity to the north from those withan Indian affinity to the south. Plate-tectonicreconstructions of the Tibetan-Himalayan col-lision zone suggest that ;1000 km of short-ening has been accommodated between south-ern Tibet and the Indian Shield (Patriat andAchache, 1984; Dewey et al., 1989; Chen etal., 1993; Patzelt et al., 1993) since the initialcollision between India and Asia at ca. 55–50Ma (Patriat and Achache, 1984; Rowley,1996; de Sigoyer et al., 2000) or earlier (Yinand Harrison, 2000). Attempts to corroboratethis total estimate with field-based structuralstudies have viewed the Tibetan-Himalayancollision zone as consisting of three belts thatcould have absorbed a significant proportionof the convergence: (1) the Himalayan fold-thrust belt (Gansser, 1964; Sinha, 1986; Schel-ling, 1992; DeCelles et al., 1998, 2001), (2)the Tethyan fold-thrust belt (Burg and Chen,1984; Searle, 1986, 1996a; Steck et al., 1993;Ratschbacher et al., 1994), and (3) the Indus-Yalu suture zone (Heim and Gansser, 1939;Gansser, 1964; Burg and Chen, 1984; Yin et

al., 1994, 1999; Harrison et al., 2000) (Figs.1A and 1B).

Although constraints have been placed onthe magnitude of shortening of the TethyanSedimentary Sequence in the northwesternTethyan Himalaya (Zanskar) and eastern Teth-yan Himalaya in southern Tibet (Searle, 1986;Burg et al., 1987; Ratschbacher et al., 1994),there have been no estimates of Cenozoicshortening for the central Tethyan Himalayain southwest Tibet. This paper presents a firstattempt to better understand the sequence ofCenozoic thrusting in southwest Tibet and toquantify the magnitude of contraction withinthe Tethyan fold-thrust belt and Indus-Yalu su-ture zone between the Mount Kailas region inthe north and the Nepal-India-China borderjunction to the south in southwest Tibet.

REGIONAL GEOLOGY

The Himalayan fold-thrust belt lies betweenthe Indian shield to the south and the Indus-Yalu suture zone to the north (Figs. 1A and1B). It consists of four lithotectonic unitsbounded by four north-dipping Cenozoic faultsystems: the Main Frontal Thrust, the MainBoundary Thrust, the Main Central Thrust,and the South Tibetan Detachment System,from south to north. Immediately south of thestudy area, in the Garhwal Himalaya, the MainCentral Thrust and South Tibetan DetachmentSystem correlate with the Vaikrita thrust andMalari fault, respectively (Valdiya, 1981,1989) (Figs. 1A and 1B). The Lesser Hima-laya is the structurally lowest thrust slice. It isbounded at the base by the Main BoundaryThrust and at the top by the Main CentralThrust and consists of Middle Proterozoic me-tasedimentary rocks, Paleozoic to Eocene sed-imentary and volcanic rocks, and Cambrian–Ordovician granitic rocks (Brookfield, 1993;Parrish and Hodges, 1996; DeCelles et al.,2000). The Greater Himalaya Crystalline Se-quence (also known as the High Himalayan



22 Geological Society of America Bulletin, January 2003

MURPHY and YIN

Figure 1.



Geological Society of America Bulletin, January 2003 23

STRUCTURE IN TETHYAN FOLD-THRUST BELT AND INDUS-YALU SUTURE ZONE, SOUTHWEST TIBET

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MURPHY and YIN

Crystalline Sequence) is bounded by the MainCentral Thrust below and the South TibetanDetachment System above (Burg and Chen,1984; Burchfiel et al., 1992) and is composedof Neoproterozoic to lower Paleozoic meta-sedimentary, granitic, and volcanic rocks, andTertiary granitic rocks (Le Fort, 1986; Parrishand Hodges, 1996; DeCelles et al., 2000). TheTethyan (or North) Himalaya lies between theSouth Tibetan Detachment System and theGreat Counter Thrust, a major north-directedthrust system located along the Indus-Yalusuture zone (Heim and Gansser, 1939;Ratschbacher et al., 1994; Yin et al., 1999).It consists of late Precambrian to lower Pa-leozoic sedimentary and metasedimentaryrocks (Gansser, 1964; Yin et al., 1988;Burchfiel et al., 1992; Brookfield, 1993; Gar-zanti, 1999) and thick Permian to Upper Cre-taceous continental-margin sequences(Cheng and Xu, 1987; Brookfield, 1993; Gar-zanti, 1999). The entire sequence is com-monly referred to as the Tethyan Sedimen-tary (or metasedimentary) Sequence.

Sedimentologic characteristics, trace-elementisotope data, and U-Pb detrital zircon agesplace the Greater Himalayan Crystalline Se-quence north of, and partially at a higherstratigraphic level than, the Lesser HimalayanSequence (Brookfield, 1993; Parrish andHodges, 1996; DeCelles et al., 2000). A prov-enance study by DeCelles et al. (2000) sug-gested on the basis of U-Pb detrital zirconages and Nd model ages that the Greater Hi-malayan Crystalline Sequence was accretedonto the Indian plate during Late Cambrian–Early Ordovician time along a paleosuturemarked by the present Main Central Thrust.Sedimentologic characteristics, trace-elementisotope data, and U-Pb detrital zircon agessuggest that the Greater Himalayan Crystal-line Sequence may have served as the base-ment to the Paleozoic–Mesozoic succession ofthe Tethyan Sedimentary Sequence (Brook-field, 1993; Parrish and Hodges, 1996; De-Celles et al., 2000). Cenozoic convergence be-tween India and Asia has resulted inshortening of the Tethyan Sedimentary Se-quence across the entire Himalayan thrustfront, forming the Tethyan fold-thrust belt(Fig. 1A inset map). Although schists andgneisses do exist in the Tethyan Himalaya,most are interpreted to have been exhumed bymotion on syncollisional extensional struc-tures (e.g., Gurla Mandhata—Murphy et al.[2002]; Kangmar dome—Chen et al. [1990];Lee et al. [2000]) that postdate the Tethyanfold-thrust belt, therefore indicating that thethrust belt may be thin-skinned. If so, the bas-al detachment may be located along the con-tact between the Tethyan Sedimentary Se-

quence and the Greater Himalayan CrystallineSequence.

The magnitude of shortening within the Hi-malayan fold-thrust belt is undoubtedly un-derestimated because of the lack of hanging-wall cutoffs and pervasive penetrativedeformation within the hanging wall of theMain Central Thrust (e.g., Gansser, 1964; Bru-nel and Kienast, 1986; MacFarlane et al.,1992; Schelling, 1992; Hodges et al., 1996;DeCelles et al., 2001). Nonetheless, minimumshortening estimates north of the MainBoundary Thrust, but excluding the Tethyanfold-thrust belt, range from 193 to 260 km inthe northwestern Himalaya of northern India(Srivastava and Mitra, 1994), 316 km in thewestern Nepal Himalaya (DeCelles et al.,2001), and 175–210 km in the eastern NepalHimalaya (Schelling, 1992). Field studies sug-gest that the Tethyan Sedimentary Sequencehas been shortened 100–126 km in the Zan-skar region (Searle, 1986, 1996a; Steck et al.,1993) and 60 km (Hauck et al., 1998), 139km (Ratschbacher et al., 1994), and 325 km(Burg et al., 1987) in south-central Tibet.

Over the past decade, field investigations andthermochronologic studies along the Indus-Yalu suture zone have recognized two phasesof north-south shortening (Harrison et al.,1992, 2000; Ratschbacher et al., 1994; Yin etal., 1994, 1999; Quidelleur et al., 1997). Theearlier phase (30–23 Ma) is associated withthe south-directed Gangdese thrust system, re-sulting in juxtaposition of Gangdese grani-toids in its hanging wall over the Tethyan Sed-imentary Sequence in its footwall (Yin et al.,1994; Harrison et al., 2000). Modification ofthis earlier configuration occurred between 19and 13 Ma in southwest Tibet and is associ-ated with thrusting toward the north along theGreat Counter Thrust (Yin et al., 1999), alsoreferred to as the South Kailas thrust in south-west Tibet (Cheng and Xu, 1987; Murphy etal., 2000), the backthrust system (Ratschbach-er et al., 1994), and the Renbu-Zedong thrustin southeast Tibet (Yin et al., 1994; Quidelleuret al., 1997; Harrison et al., 2000). Where ob-served, the thrust places the Tethyan Sedi-mentary Sequence in its hanging wall abovethe Gangdese batholith in its footwall. TheGangdese thrust system does not crop out insouthwest Tibet (Yin et al., 1999). However,thermal modeling of 40Ar/39Ar step-heating re-sults from K-feldspar in a granite from theKailas magmatic complex indicates a rapidcooling event between 30 and 25 Ma (Yin etal., 1999). In the absence of extensional struc-tures of such age cutting the Gangdese bath-olith, the cooling ages may best be explainedby slip on a structure equivalent to the Gang-dese thrust system in southwest Tibet (Yin et

al., 1999). Thermal modeling of the Gangdesehanging wall in the Zedong area of southeastTibet yields a slip rate of 7 mm/yr between30 and 23 Ma, placing a minimum constrainton its displacement of ;50 km (Harrison etal., 2000). Thermal modeling of the hangingwall of the younger Great Counter Thrust inthe Lian Xian area, east of Zedong in south-east Tibet, by Quidelleur et al. (1997) placesa minimum constraint on the total displace-ment of 12 km. The total displacement on theGreat Counter thrust in southwest Tibet hasnot been investigated prior to this study.

GEOLOGY OF SOUTHWEST TIBET

In the following section, we summarize thegeologic relationships between the MountKailas area in the north and the China-Nepal-India border junction in the south (Fig. 21),based on observations by Heim and Gansser(1939), Cheng and Xu (1987), Yin et al.(1999), and our recent field work in the re-gion. Several components of the former Indianpassive continental margin and Asian activemargin are exposed along this profile. Theyare, from south to north, (1) the Tethyan Sed-imentary Sequence (Indian passive-margin se-quence), (2) the Kiogar-Jungbwa ophiolite(Neotethyan oceanic lithosphere), (3) the Indus-Yalu suture zone (Triassic–Cretaceous me-lange and accretionary-wedge materials), (4)the Xigaze Group (forearc basin deposits), and(5) the Gangdese batholith (magmatic arc),which is also called the Kailas magmatic com-plex) (Figs. 1–3). Figure 3 shows a schematicprofile displaying the inferred tectonic frame-work across the India-Asia convergent marginprior to collision.

Tethyan Sedimentary Sequence

On the basis of stratigraphic data collectedby the Tibetan Bureau of Geology and Min-eral Resources (Cheng and Xu, 1987) and ourown geologic mapping (Fig. 2), we have com-piled stratigraphic thicknesses of the Precam-brian–Cretaceous units (Fig. 4). Although thetotal thickness of the units in the study area isnearly constant (9.6–9.3 km from south tonorth), the thickness of individual units doesvary (Cheng and Xu, 1987).

Precambrian–Devonian strata are well ex-posed in the southern part of the study area,near Pulan (Figs. 1A and 2). These rocks con-sist of quartzites, calc-silicates, and minorlimestone and dolostone, ;4.9 km thick (Fig.4). Carboniferous–Permian strata are variably

1Figure 2 is on a separate sheet accompanyingthis issue.





Figure 3. Schematic north-south profile showing the major tectonic elements of the northern (passive) margin of the Indian plate andthe southern (active) margin of the Asian plate prior to the onset of collision. Position of Lesser Himalayan Sequence, Greater HimalayanCrystalline Sequence, Indian plate’s lower crust (Indian Shield–type rocks), and Tethyan Sedimentary Sequence after DeCelles et al.(2000).

exposed in southwest Tibet. Carboniferous strataare missing south of the Kiogar-Jungbwa ophiol-ite sheet, and only 333 m of Permian lime-stone is present in the thrust belt immediatelywest of Pulan (Cheng and Xu, 1987). Northof the Kiogar-Jungbwa ophiolite thrust sheet,both Carboniferous and Permian strata arepresent and are as thick as 2100 m (Figs. 1Aand 2). Elsewhere in the Tethyan Himalaya,two regional unconformities have been ob-served in the Upper Carboniferous (Pennsyl-vanian)–Lower Permian strata that have beenattributed to rifting of the Lhasa block off themargin of India or other part of Gondwana-land (Leeder et al., 1988; Brookfield, 1993; LeFort, 1996), which may explain missing Low-er Carboniferous (Mississippian) strata alongthe southern part of the traverse. No Paleozoicrift-related structures have been observed insouthwest Tibet, although they are probablypresent below the younger Mesozoic strata.The Triassic strata (1000 to 1122 m thick)were deposited in a shallow-marine environ-ment and consist of interbedded silty lime-stone and fine-grained sandstone, containingabundant ammonites particularly to the westof Pulan (Fig. 4). The Jurassic through Cre-taceous strata (3400–2250 m thick) consists ofshale and minor siltstone, sandstone, andlimestone. The Jurassic strata, south of theKiogar-Jungbwa ophiolite thrust sheet, containbivalves that indicate that a shallow-marineenvironment prevailed on the Indian passivemargin at least until this time. The upper partof the Jurassic strata are dominated by fine-grained sandstone and shale. Tertiary stratawithin the Tethyan Himalaya are only presentin the Pulan basin, which postdates thrustingand is interpreted to be related to late Mioceneslip on the Gurla Mandhata detachment sys-tem (Fig. 1A).

Kiogar-Jungbwa OphioliteBetween Mapam Yum Co and La’nga Co

(‘‘Co’’ is ‘‘Lake’’ in Tibetan) on their southside (Figs. 1 and 2), a north-dipping thrust

places nearly unaltered norite and mantle-typerocks (dunite and harzburgite) in its hangingwall over fossiliferous sandstone, siltstone,and minor limestone of Jurassic to Cretaceousage in its footwall (Fig. 2). At the base of thethrust sheet is a foliated cataclasite containingnumerous veins of calcite. Field evidencefrom slickensides, foliation patterns, chattermarks, and offset lithologic markers definethree sets of faults: east-trending thrust faults,north-trending normal faults, and northwest-trending right-slip faults (Fig. 2). Faults dis-playing north-trending striations are cut bythose displaying east-trending lineations. Weinterpret strike-slip and extensional structureswithin the thrust sheet to be related to recentslip on the Karakoram fault system and GurlaMandhata detachment system (Murphy et al.,2002). The thrust sheet is poorly exposed nearMapam Yum Co (Fig. 2), and we thereforewere unable to recognize any complete rocksequence. A more complete section of thethrust sheet was observed by Augusto Gansser(Heim and Gansser, 1939) near the southwestcorner of La’nga Co (Fig. 1A). Gansser doc-umented a north-dipping thrust sheet flooredby an ;200-m-thick flysch sequence consist-ing of a Cretaceous limestone, Jurassic sand-stone matrix, and blocks of Triassic carbonatesand andesite of unknown age. Gabbroic rockslie in tectonic contact above the flysch se-quence (Gansser, 1979). Gansser (1979)linked the Cretaceous limestone with theflysch sequence, therefore placing a maximumage constraint on the emplacement of theophiolitic rocks. Farther north, we observedthat peridotite crops out for nearly 10 km. Un-like other ophiolite exposures in the TethyanHimalaya, such as the Spontang ophiolite inZanskar (Searle, 1986; Searle et al., 1997) orthe Xigaze ophiolite in southern Tibet (Nico-las et al., 1981), a metamorphic sole at thebase of the thrust sheet was not found. Thefact that the flysch sequence documented byGansser near La’nga Co is missing near Ma-pam Yum Co farther to the east suggests that

the thrust may have been reactivated, resultingin excision of the flysch sequence along strike(Fig. 1A). Moreover, the Kiogar-Jungbwathrust sheet is bounded both to the south andnorth by Tethyan Sedimentary Sequencerocks, suggesting out-of-sequence-thrusting,similar to the Spontang thrust sheet (Searle,1986; Searle et al., 1997). As mentioned al-ready, the upper age constraint on slip alongthe Kiogar-Jungbwa thrust sheet is Creta-ceous. In southeast Tibet, constraints on thecrystallization age of the Indus-Yalu sutureophiolites are similar (late Albian–early Cen-omanian according to Marcoux et al. [1981];119 6 25 Ma according to Allegre et al.[1984]).

Indus-Yalu Suture ZoneThe boundary between rocks with an Asian

affinity and those with an Indian affinity iscurrently defined by a south-dipping thrustsystem referred to as the Great Counter Thrust(Heim and Gansser, 1939; Yin et al., 1999)(Figs. 1A and 2); it has been referred to lo-cally as the South Kailas thrust (Cheng andXu, 1987). The Great Counter Thrust is athrust system composed of at least five south-dipping thrust faults in southwest Tibet. Fromsouth to north, they place phyllitic schist overa sequence of limestone and sandstone thatmay be part of the Tethys Sedimentary Se-quence. In the hanging wall of this thrust, ser-pentinized ophiolitic rocks are thrust over thephyllitic schist unit. We speculate, on the basisof the rock types juxtaposed, that the thrust isan older, folded, south-directed thrust that sep-arates an accretionary-wedge complex abovefrom metamorphosed Triassic turbidites be-low. Fault-slip analysis by Ratschbacher et al.(1994) determined that the transport directionon this fault is toward the southwest. Approx-imately 2 km to the north, a north-directedthrust places a folded limestone unit over pur-ple sandstone. The limestone is in turn thrustnorthward over a .1-km-thick sequence ofCretaceous limestone and sandstone referred




MURPHY and YIN

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ions

.





to as the Yiema Formation by Cheng and Xu(1987) and correlated to the Xigaze forearcbasin deposits by Liu (1988). The Yiema For-mation is thrust northward over the Kailasconglomerate. Ratschbacher et al. (1994) de-termined a compression direction trend of0338 for the northernmost and southernmostbackthrust. Yin et al. (1999) investigated thegeneral stratigraphic framework of the Kailasconglomerate. They advocated a two-phasehistory, with the deposition of its lower sec-tion coeval with slip on a south-directed thrustequated to the Gangdese thrust system (Yin etal., 1994), whereas its upper section recordsnorth-directed motion on the Great CounterThrust. Fault-slip data collected by Ratsch-bacher et al. (1994) (stations 8–9F and KAF)from the northernmost backthrust can be in-terpreted to record two deformation events, anearlier south-directed thrusting event followedby a later northwest-directed thrusting event.The earlier south-directed thrusting event mayreflect motion along a fault equivalent to theGangdese thrust. A late Oligocene–late Mio-cene age of the Kailas conglomerate is definedby 40Ar/39Ar cooling ages of both clasts andtheir apparent source in the Gangdese batho-lith (Harrison et al., 1993; Yin et al., 1999).The Kailas conglomerate lies unconformablyabove a granite that is a part of the Kailasmagmatic complex (Honegger et al., 1982)correlated with the Gangdese batholith (Chengand Xu, 1987; Liu, 1988).

South Tibetan Detachment System

The main trace of the South Tibetan De-tachment System crops out ;20 km south ofthe study area. It is a down-to-the-north, low-angle normal-fault system that is traceablealong the length of the Himalaya (Burg andChen, 1984; Burchfiel et al., 1992). This fea-ture places generally low-grade Tethyan me-tasedimentary rocks against the Greater Him-alaya crystalline basement with variablydeformed leucogranites commonly exposed inthe footwall of the detachment system (Burgand Chen, 1984; Herren, 1987; Burchfiel etal., 1992; Edwards et al., 1996; Hodges et al.,1996). Augusto Gansser mapped such a rela-tionship along the Kali river in Nepal, wherethe fault dips ;408 to the north (Heim andGansser, 1939). Within our study area, duewest of the town of Pulan, we speculate that,on the basis of similar orientations, the top-to-the-north normal fault may be part of theSouth Tibetan Detachment System (Fig. 2).The timing of slip on the South Tibetan De-tachment System in southwest Tibet is un-known. Elsewhere, however, U-Th-Pb datingof accessory minerals from synkinematic

dikes yields crystallization ages between 21and 12 Ma (e.g., Zanskar—Noble and Searle[1995]; Shisha Pangma—Searle et al. [1997];Nyalam—Scha ¨rer et al. [1986] and Xu [1990];Rongbuk valley—Hodges et al. [1998] andMurphy and Harrison [1999]; Dinggye—Xu[1990]; Wagye La—Wu et al. [1998]; GontaLa—Edwards and Harrison [1997]).

Karakoram-Gurla Mandhata FaultSystem

The dextral strike-slip Karakoram fault sys-tem and Gurla Mandhata detachment systemmerge within the study area (Figs. 1A and 2).On the basis of offset of recent geomorphicfeatures, both faults are considered active (Ka-rakoram fault—Molnar and Tapponnier[1978], Armijo et al. [1989], Liu [1993],Ratschbacher et al. [1994], Searle [1996b],and Murphy et al. [2000]; Gurla Mandhata de-tachment system—Ratschbacher et al. [1994],Yin et al. [1999], and Murphy et al. [2002]).Movement on them likely began in the lateMiocene to early Pliocene in southwest Tibet(Ratschbacher et al., 1994; Searle, 1996b; Yinet al., 1999; Murphy et al., 2000). The Kara-koram fault system extends into our study areaas an ;36-km-wide zone of northwest-strikingdextral strike-slip faults (Fig. 2). On the basisof timing, kinematics, and net-slip estimates,Murphy et al. (2002) suggested that the Ka-rakoram fault system is linked to the GurlaMandhata detachment system, thus making alarge right step in the fault system (Fig. 1).The slip direction on the two fault systems isnearly coincident. The mean slip direction ofthe Karakoram fault system at its southernend, near Mount Kailas, is N708W, 308 (a95

confidence cone, 308). The mean slip directionof the Gurla Mandhata detachment is N808W,208 (a95 confidence cone, 108). The magnitudeof slip on the Karakoram fault system at itssouthern end is estimated to be 66 km on thebasis of offset of the Great Counter Thrust.The footwall of the Gurla Mandhata detach-ment systems contains rocks that can be cor-related with both the Greater Himalayan Se-quence and Lesser Himalayan Sequence onthe basis of their Sr-Nd isotopic ratios, imply-ing these rocks originated below the TethyanSedimentary Sequence. Consideration of GASP(garnet-aluminosilicate-quartz-plagioclase) andGARB (garnet-biotite) thermobarometric re-sults yields equilibration depths for the foot-wall rocks between 13.3 and 26.7 km, if alithostatic gradient of 0.275 kbar/km (Murphyet al., 2002) is assumed. The shallowest equil-ibration depth imposes a limit to the maxi-mum depth of the Tethyan Sedimentary Se-quence. Moreover, considerations for the

original dip angle of the Gurla Mandhata de-tachment during exhumation of the footwallyield total-slip estimates of between 66 and 35km across the Gurla Mandhata detachmentsystem (Murphy et al., 2002).

CROSS-SECTION RECONSTRUCTION

In order to better understand the structuralrelationship between these lithotectonic zonesin southwest Tibet in the vicinity of the Indus-Yalu suture zone and their original paleogeo-graphic position, we have restored a regionalcross section (Fig. 5). Our geologic map iscompiled from mapping by Augusto Gansser(Heim and Gansser, 1939), the Tibetan Bureauof Geology and Mineral Resources (Chengand Xu, 1987), A. Yin and M.A. Murphy in1995 (Yin et al., 1999), and field mapping byM.A. Murphy during 1997 and 1998 field sea-sons (Fig. 2). We make four general assump-tions in our cross section: (1) The transportdirection of the Tethyan fold-thrust belt wasperpendicular to the trend of axial hinges ofthrust-related asymmetric folds, i.e., 2038 (Fig.2). The transport direction of the Great Coun-ter Thrust system (Fig. 6), which was deter-mined from the orientations of compression-related structures (Fig. 2) and fault-slipanalysis (Ratschbacher et al., 1994), was toward0338. The transport direction for the Kiogar-Jungbwa ophiolite thrust sheet is assumed tobe parallel to the Tethyan fold-thrust belt, al-though measured thrust-related striations in-dicate a north-south motion. (2) The fold-thrust belt is thin-skinned, and the basal thrust(decollement) lies between the Tethyan Sedi-mentary Sequence and the Greater HimalayanCrystalline Sequence. (3) Thrusts propagatedtoward the foreland within the Tethyan fold-thrust belt, with the exception of the thrustdirectly north of the Kiogar-Jungbwa ophiolitethrust sheet. (4) Out-of-plane motion is notconsidered (Fig. 2). The cross section is line-length balanced. Figures 5A through 5G sum-marize our proposed sequential developmentof the Tethyan fold-thrust belt and Indus-Yalusuture zone beginning with the precollisionalsetting. Figure 5G indicates 112 km of hori-zontal shortening in the Tethyan SedimentarySequence and 64 km of horizontal shorteningin the Indus–Yalu suture zone. As no subsur-face data exist, stratigraphic and structural re-lationships shown in the reconstruction are afirst-order approximation at best. Moreover,because we do not take into account internaldeformation (stylolites, cleavage, and isoclinalfolding) within thrust sheets, our shorteningestimate should be viewed as a minimum.




MURPHY and YIN

Figure 5. Sequential cross-section restoration across the Tethyan Himalaya and Indus-Yalu suture zone. Cross sections are orientedN238E. Reconstruction shows 60% horizontal shortening (176 km), corresponding to 112 km within Tethyan fold-thrust belt and 64 kmwithin the Indus-Yalu suture zone faults (Great Counter Thrust and Gangdese thrust system) (see text for details). (A) Initial configu-ration prior to significant underthrusting of the Indian subcontinent (initial length 290 km). (B) Emplacement of the Kiogar-Jungbwaophiolite thrust sheet over the Tethyan Sedimentary Sequence. C, D, and E: Propagation of the Tethyan fold-thrust belt toward theforeland. The largest of the thrusts juxtaposes Ordovician strata over Jurassic strata. (F) South-directed thrusting on the Gangdesethrust system (GTS) resulting in burial of the Xigaze forearc and accretionary-wedge complex and deposition of the lower section ofthe Kailas conglomerate as wedge-top deposits on the Kailas magmatic complex. (G) North-directed thrusting on the Great CounterThrust system resulting in significant 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

Stage A (Early Cretaceous to LateCretaceous)

The Neotethys oceanic lithosphere subductsbeneath the Asian active margin (Fig. 5A).The configuration of the Asian active marginprior to collision is defined by those tectonicunits exposed in the Great Counter Thrust sys-tem near Mount Kailas and in south-centralTibet where the Xigaze forearc basin deposits(Xigaze Group) are better exposed. U-Pb zir-con data from the Kailas magmatic complexindicate that arc-related magmatism was oc-curring by the Early Cretaceous (120 Ma—Miller et al., 2000) in southwest Tibet. Fartherto the east, geochronologic data indicate anolder age for arc-related magmatism (130Ma—Lhasa area, Harris et al., 1988; 147Ma—Coqin area, 318 N, 858 E, Murphy et al.,1997). We assume the existence of a forearcbasin in southwest Tibet prior to the India-Asia collision, although its present exposureis small compared to south-central Tibet (Liu,1988). The discontinuous occurrence of theXigaze Group along strike of the Indus-Yalusuture zone has been interpreted in two dif-ferent ways. Yin et al. (1994) suggested thatthe Xigaze Group is preserved in south-centralTibet because the Gangdese thrust system out-crops to the south. In the Mount Kailas area,Yin et al. (1999) inferred the existence of anequivalent structure on the basis of thermalmodeling results from the Kailas magmaticcomplex. Alternatively, Einsele et al. (1994)suggested that primary pinching out oftrapped oceanic or transitional crust in theforearc could have controlled the occurrenceof forearc basin deposits. On the basis of thelimited occurrence of forearc deposits insouthwestern Tibet, Einsele et al. (1994) in-terpreted very little, if any, oceanic or transi-tional crust to be trapped in the forearc. Al-though their model does offer an alternativeexplanation for the limited occurrence of fore-arc deposits in southwestern Tibet, the strongcorrelation between missing Xigaze forearcdeposits and significant tectonic denudation ofthe Gangdese arc led us to favor the interpre-

tation that the Xigaze forearc strata have beenunderthrust below the Gangdese batholith.The dimensions of the forearc shown are cal-culated from stratigraphic studies and struc-tural reconstructions of the Xigaze Group insouth-central Tibet (Ratschbacher et al., 1992;Durr, 1993, 1996; Einsele et al., 1994; Durr etal., 1995). Between Xigaze (828529 E) and di-rectly north of Saga (858109 E), the overallstructure of the Xigaze Group is a large syn-clinorium (Burg et al., 1987; Ratschbacher etal., 1992; Einsele et al., 1994). In the Ji-angqinzhe area (888109 E), the Xigaze Grouprestores to an original north-south length of;65 km, implying a similar width of the fore-arc basin (Einsele et al., 1994). Burg et al.(1987) estimated a higher predeformationalwidth of 80–100 km for the forearc basin insouth-central Tibet. Measured stratigraphicsections in the Xigaze area by Durr (1996)indicate that the original thickness of the Xig-aze Group was ;12 km. It should be notedthat these forearc basin dimensions are mini-mum values, as the margin of the forearc basinmay have been removed by tectonic and ero-sional processes. As discussed in the next sec-tion, the Kailas magmatic complex is differ-entially denuded from south to north, whichis consistent with a tilted thrust hanging wallover a ramp. In constructing the early crosssections, we have added a volume of graniteto the Kailas magmatic complex (light redrocks in Fig. 5A) equal to that which, on thebasis of thermochronology data presented inYin et al. (1999), we estimate to have beenremoved by erosion. The accretionary-wedgecomplex is shown to be rather small (Fig. 5A).Its minimum thickness is defined by a separatereconstruction of the Great Counter Thrust(Fig. 6).

Stage B (Late Cretaceous to Paleocene)

We interpret the earliest thrusting event toinvolve emplacement of the Kiogar-Jungbwaophiolite thrust sheet onto the Tethyan Sedi-mentary Sequence (Fig. 5B). We estimate that53 km of displacement (51 km horizontal

shortening) has occurred on the Kiogar-Jungbwa thrust. The magnitude of displace-ment is defined by the present surface expo-sure of the ophiolitic complex (;20 km)(Figs. 1A and 2) and the observation thatTethyan Sedimentary Sequence rocks that pre-date Permian rifting (Gaetani and Garzanti,1991) of the Lhasa block from the northernmargin of India are present north of the ultra-mafic rocks. As we show in Figure 5B, an out-of-sequence thrust may explain the presenceof Tethyan Sedimentary Sequence rocks northof the Kiogar-Jungbwa ophiolitic thrust sheet.In stage B of our proposed sequence of events,the Kiogar-Jungbwa thrust sheet moved southover the passive continental margin; this sce-nario provides the necessary precondition forthe inferred out-of-sequence thrust in the Teth-yan Sedimentary Sequence. We view the 53km as a minimum displacement because wechose the thrust sheet to have originated closeto the continental borderland. Had the thrustoriginated farther north, within the oceaniclithosphere, there could be substantially moreslip on the fault. The thrust that now juxta-poses the Kiogar-Jungbwa ophiolitic sequenceagainst the Jurassic strata may well be a re-activated thrust, as seen by the lack of anmetamorphic sole beneath the ophiolitic se-quence. However, the initial stripping of theophiolitic rocks from either transitional oroceanic lithosphere must have occurred priorto the collision, and prior to underthrusting ofthe Indian subcontinent. The mechanism forophiolite obduction is uncertain. Studies byAitchison et al. (2000) in southeast Tibet nearZedong (298159 N, 918459 E) suggest the pres-ence of remnants of a south-facing (north-dipping) intraoceanic subduction system,south of the Indus-Yalu suture zone. There-fore, the ophiolite may have been obductedduring the development of the subductionzone. The timing of its emplacement is likelyslightly older than estimates for the timing ofthe initial collision between India and Asia.However, estimates for the onset of collisionremain uncertain, varying between 70 Ma and45 Ma, and probably reflecting the different




STRUCTURE IN TETHYAN FOLD-THRUST BELT AND INDUS-YALU SUTURE ZONE, SOUTHWEST TIBET on August 31, 2012gsabulletin.gsapubs.orgDownloaded from



MURPHY and YIN

Figure 6. Development of the Great Counter Thrust system in southwest Tibet. Cross section is oriented N338 E and shows a total of30.5 km (;40%) of horizontal shortening accommodated by three north-directed thrusts, which were initiated in the footwall of thepreceding thrust. Configuration of the Indus-Yalu suture zone prior to slip on the Great Counter Thrust approximates that shown instage F of Figure 5, in which the Kailas magmatic complex, in the hanging wall of the Gangdese thrust system (GTS), has been juxtaposedover the Yiema Formation (forearc deposits), in the thrust’s footwall.

types of data used (Yin and Harrison, 2000).Examination of magnetic seafloor anomaliesand fracture zones in the Atlantic and IndiaOceans indicates that India’s northward mo-tion slowed dramatically at ca. 45 Ma (Deweyet al., 1989; Le Pichon et al., 1992). Strati-graphic (Rowley, 1996) and metamorphicconstraints (de Sigoyer et al., 2000) suggestinitiation of collision in the northwest Hima-laya at 55–50 Ma. These estimates are an up-per bound for the timing of collision. Colli-sion may have been earlier as sediments mayhave been subducted, and metamorphism dueto burial by thrusting must have occurred ear-

lier than growth of peak metamorphic assem-blages, depending on the regional thermal re-gime (e.g., England and Thompson, 1984). Asmentioned earlier, the maximum age limit onslip of the Kiogar-Jungbwa ophiolite thrustsheet is Cretaceous, on the basis of the age offlysch-type rocks within the thrust zone (Heimand Gansser, 1939). The minimum age limitis poorly determined in southwest Tibet, butmust predate the Gurla Mandhata detachmentsystem because related structures cut thethrust sheet (Fig. 2). Elsewhere in the TethyanHimalaya, the timing of ophiolite obductionwas between Late Cretaceous and Paleocene

(see Burg et al. [1987] and Makovsky et al.[1999] for south-central Tibet and Searle[1996a] and Searle et al. [1997] for the north-west Himalaya), suggesting that the emplace-ment age for ophiolites along the Indian mar-gin may be similar along strike.

Stage C (Eocene to Early Oligocene)

Development of two south-directed thrustsand one north-directed normal fault resulted in;31 km of horizontal shortening. As just dis-cussed, we interpret the existence of an out-of-sequence thrust to explain the presence of





Tethyan Sedimentary Sequence rocks to thenorth of the Kiogar-Jungbwa thrust sheet. Thisthrusting event may also explain the exposureof Precambrian–Cambrian rocks east of Ma-pam Yum Co immediately north of the Kiogar-Jungbwa ophiolitic complex (Fig. 1A). A sim-ilar kinematic interpretation was conceived bySearle (1986) to explain the position of theSpontang ophiolite. We infer the existence ofa top-to-the-north normal fault in order to ex-plain Permian strata juxtaposed against Or-dovician strata. An unconformity may also ex-plain this relationship. However, because anunconformity at these stratigraphic levels wasnot recognized in other parts of the study area,its aerial extent would necessarily need to berestricted to the northern parts of the TethyanSedimentary Sequence. Although we interpretthe normal fault to be active at this stage, itis also possible that it slipped later and maybe related to slip on the South Tibetan De-tachment System (Figs. 1A and 1B). Alter-natively, it may have formed during the initialdevelopment of the passive margin during thePermian.

Stage D (Eocene to Early Oligocene)

The thrust active during stage D shows thelargest magnitude of slip, ;40 km of south-ward movement, resulting in ;30 km of hor-izontal shortening. Moreover, this thrust hasthe greatest observed stratigraphic throwalong the profile and, as interpreted at depth,doubles the thickness of the Tethyan Sedi-mentary Sequence in the immediate area.

Stage E (Eocene to Early Oligocene)

The development of a south-directed thrustand two north-directed normal faults result in,1 km of total horizontal shortening acrossthe profile compared to that shown in stage D(Fig. 5E). The south-directed thrust was notobserved in the field, but it is inferred to ac-count for the large antiform in the hangingwall of the inferred thrust where we show itto have developed as a fault-bend fold. Therelationship between the normal faults, includ-ing the one shown in stage C, and thrust faultsis uncertain. Either the normal faults are re-lated to development of the thrust belt in asimple shear zone (Yin and Kelty, 1991) orare part of the South Tibetan Detachment Sys-tem (Burchfiel et al., 1992). If the latter is true,then they probably were active later than whatthe reconstruction shows.

Stage F (Late Oligocene to Early Miocene)

South-directed thrusting on the Gangdesethrust system resulted in overthrusting of the

Yiema Formation (correlated to the Xigazeforearc basin deposits) and deposition of theGangdese foreland basin system (Fig. 5F). Yinet al. (1999) interpreted the lower part of theKailas conglomerate as wedge-top deposits re-lated to slip on a thrust equivalent to theGangdese thrust recognized in southeasternTibet near Zedong (Yin et al., 1994). If cor-rect, their interpretation would predict accu-mulation of the rest of the foreland basin system(foredeep, forebulge, back-bulge) (DeCelles andGiles, 1996) south of Mount Kailas. Theramp-flat geometry of the Gangdese thrust issuggested by the 40Ar/39Ar-derived thermalhistory from a sample of granite (95-6-11-3)presented in Yin et al. (1999) and the erosion-al pattern of the Gangdese batholith in thisregion. The volcanic cover is eroded away inthe south, but preserved to the north of MountKailas (Fig. 1A). Yin et al. (1999) showed thatthe granite was emplaced at an ambient tem-perature of ;400 8C and rapidly cooled from30 to 25 Ma. A assumed geothermal gradientof 358C/km puts this rock’s intrusion level ata depth of 11.5 km. Harrison et al. (2000) es-timated 50 km of displacement on the Gang-dese thrust in southeast Tibet (Zedong win-dow). To explain the increased denudation ofthe Kailas magmatic complex southward andoverlying volcanic rocks to the south (K-Tvin Fig. 1A), we have drawn the Gangdesethrust to have a 30-km-long ramp dipping 308to the north and a nearly horizontal, 44-km-long flat. Although we expect that the forearcbasin is significantly shortened as its beddingis isoclinally folded and locally transposed inthe study area, no attempt was made to defineits magnitude owing to limited exposure in thestudy area.

Stage G (Early Miocene to MiddleMiocene)

North-directed thrusting on the Great Coun-ter Thrust system resulted in (1) stacking ofthe Cretaceous Xigaze forearc basin deposits,accretionary-wedge materials, and Triassicstrata, (2) burial of the Gangdese thrust sys-tem, and (3) deposition of the upper Kailasconglomerate (Fig. 5G). Figure 6 shows a sep-arate reconstruction of the Great CounterThrust in which its development is modeledas a imbricate thrust system propagating to-ward the foreland. Approximately 20 km ofshortening is shown to have been accommo-dated along a basal thrust lying along thePermian/Carboniferous contact. This positionwas chosen to explain the widespread occur-rence of imbricated Triassic strata along theIndus-Yalu suture zone in southern Tibet (Liu,1988). The rest of the displacement is accom-

modated along a later north-dipping thrust thatextends to deeper structural levels. We havechosen not to root this thrust into the basaldecollement between the Greater HimalayanCrystalline Sequence and the Tethyan Sedi-mentary Sequence, but instead, project it intothe Greater Himalayan Crystalline Sequence.This choice is motivated by the potential linkbetween the Great Counter Thrust and theSouth Tibetan Detachment System that isbased on their compatible slip direction andcoeval fault movement (Yin et al., 1999; Leeet al., 2000). The dip-slip component of faultsrelated to the Gurla Mandhata detachment sys-tem and the Karakoram fault system have alsobeen restored (Murphy et al., 2002).

Because the Karakoram fault system inter-sects the cross section, the assumption in ourrestoration that no out-of-the-plane motion hasoccurred is not valid. However, because dex-tral strike-slip faults that are part of the Ka-rakoram fault system intersect the cross sec-tion at angles between 858 and 908, ,5 km ofapparent shortening along the cross section isexpected to have resulted from slip on thesefaults.

DISCUSSION

Our reconstruction shows that the locus ofCenozoic crustal shortening (112 km) oc-curred significantly south of the Indus-Yalusuture zone during the early stages of the In-dia-Asia collision. Although highly dependentupon the assumptions we made in our recon-struction, the geometry shown in our recon-struction results in preserving a .60-m-widepiece of oceanic lithosphere between the Teth-yan fold-thrust belt and the Kailas magmaticcomplex until activation of the Gangdesethrust system in the Oligocene. Major andtrace element contents and geochronologicdata of the Kailas magmatic complex present-ed in Miller et al. (2000) indicate that high-K,calc-alkaline magmatism did not end until 40Ma. Further consideration of the age andchemistry of younger volcanic rocks in theMount Kailas area extends the arc-type igne-ous activity to the Miocene (Miller et al.,1999). A similar time-span for arc-type mag-matism has been reported in Ladakh (Honeg-ger et al., 1982) and southeastern Tibet (Har-rison et al., 2000). These data indicate thatfluid and thermal conditions remained appro-priate for arc-type magmatism well after theonset of collision between India and Asia, re-gardless of which data set is used (Yin andHarrison, 2000). Rather than calling upon acomplex mechanism to explain fluid influxand elevated temperatures, our reconstructionis consistent with melting due to active sub-




MURPHY and YIN

duction of oceanic lithosphere and the pres-ence of an asthenospheric wedge beneath theKailas magmatic complex until the late Oli-gocene or early Miocene.

In light of this conclusion, it is necessaryto account for the amount of underthrusting ofthe Indian plate’s lower crust beneath Asia insouthwest Tibet. In Figure 1B we have com-bined our cross section with the balancedcross section by DeCelles et al. (1998) and theprofile by Heim and Gansser (1939). We haveattempted to line up all three cross sections byprojecting the Jahla normal fault and Vaikritathrust, which are equated with the South Ti-betan Detachment System (and Zanskar nor-mal fault [Herren, 1987; Burchfiel et al.,1992]) and the Main Central Thrust, respec-tively. DeCelles et al. (1998) calculated ;228km of horizontal shortening of the Subhima-laya and Lesser Himalaya and later refinedthis estimate with more detailed mappingalong the Seti River corridor to 460 km(DeCelles et al., 2001) (Fig. 1B). A minimum-shortening estimate for the Dadeldhura thrustand Main Central Thrust is 117 km (DeCelleset al., 2001). Along the Seti River and KarnaliRiver corridors, considerably more shorteningis required if both thrusts reached as far southas the Dadeldhura synform (DeCelles et al.,2001; Upreti and Le Fort, 1999). DeCelles etal. (2001) emphasized that the shortening es-timate for the Main Central Thrust is a mini-mum estimate because internal deformationwithin its hanging wall is not accounted for.Srivastava and Mitra (1994) estimated be-tween ;193 and 260 km on the Main CentralThrust and Almora thrust (equivalent to theDadeldhura thrust) in northern India. Com-bining the shortening estimates for (1) theSubhimalaya and Lesser Himalaya along theDadeldhura-Baitadadi road transect (DeCelleset al., 1998) (Fig. 1A), (2) the Main CentralThrust and Almora thrust (Srivastava and Mi-tra, 1994), and (3) the Tethyan fold-thrust beltand Indus-Yalu suture zone (this study) yieldsa total horizontal shortening estimate acrossthe central Himalaya of 597–664 km. Alter-natively, combining the shortening estimatesalong the Seti River corridor (DeCelles et al.,2001) with those in the Tethyan fold-thrustbelt and Indus-Yalu suture zone yields a totalshortening estimate of 763 km, which is clear-ly a minimum estimate because internal de-formation within the hanging wall of the MainCentral Thrust is not accounted for. If we as-sume that (1) all the southward displacementwas accommodated along the Main CentralThrust, (2) thrusts south of the Main CentralThrust root into a thrust equivalent to theMain Himalayan Thrust (Zhao et al., 1993;Brown et al., 1996; Nelson et al., 1996), as

suggested by microseismic activity in far-western Nepal (Pandey et al., 1999) (Fig. 1B),(3) a shallow subduction angle for the Indianlithosphere, and (4) the configuration of theGreater Himalayan Crystalline Sequence,Lesser Himalayan Sequence, and the Indianplate’s lower crust and lithospheric mantle thatis shown in Figure 2, our reconstruction in-dicates that the northern edge of the Indianplate’s lithosphere has been underthrust be-tween 421 and 587 km north of the presentposition of the Indus-Yalu suture zone. If theshortening in the Himalaya was completelyaccommodated by flat subduction, then itwould imply that the northern edge of the In-dian continent has already reached the Jinshasuture separating the Qiangtang terrane fromthe Songpan Ganzi terrane (Yin and Harrison,2000) (Fig. 1A). This possibility may be test-ed by geophysical observations of the litho-spheric structure beneath the western part ofthe Tibetan plateau. In fact, preliminary re-sults from seismic tomography, using theP1200 global data set (Zhou, 1996), appear tosupport this interpretation (H.-W. Zhou, 2001,personal commun.). However, our assumptionthat Indian lithosphere is inserted horizontallybeneath Tibet appears to be inconsistent withthe depth estimates for Indian coesite-bearingrocks in the Greater Himalayan CrystallineSequence (O’Brien et al., 2001) and heliumisotopic studies in southern Tibet by Hoke etal. (2000), who defined the southern limit ofmantle-derived geothermal helium in south-west Tibet to coincide with the surface traceof the Karakoram fault (Fig. 1A). Becausemetamorphism of coesite-bearing rocks in thewestern Himalaya is estimated to have oc-curred in the Eocene (O’Brien et al., 2001), itis possible that the flat subduction of Indiawas initiated later in the Oligocene with thearrival of less dense lithosphere (Indian con-tinental lithosphere) as implied by our recon-struction. Regarding the observations made byHoke et al. (2000), we note that their samplescame from exclusively the active rifts or pull-apart basins in Tibet. As pointed out by Yin(2000), these rifts most likely involve thinningof both the Tibetan and Indian mantle litho-sphere in southern Tibet, which may have alsoinduced upwelling of the asthenospheric flow.Such upwelling would permit flat subductionof the Indian lithosphere beneath Tibet, asshown in our reconstruction.

The timing of underthrusting can be deter-mined from the temporal estimates of dis-placements on individual thrusts shown inFigure 1B. The Main Central Thrust (Vaikritathrust) in the Garhwal Himalaya was active atca. 20 Ma (Metcalfe, 1993). Farther east, incentral Nepal, the Main Central Thrust is also

known to have been active in the early Mio-cene (Hubbard and Harrison, 1989; Hodges etal., 1996; Harrison et al., 1998), but was alsoreactivated in the Pliocene (Harrison et al.,1995, 1998; Catlos, 2000). DeCelles et al.(1998) interpreted offset on the Dadeldhurathrust to have begun at ca. 15–14 Ma, the old-est age of the Siwalik Group. Sediment-accumulation rates and magnetostratigraphicdata from northern India suggest that offset onthe the Main Boundary Thrust was initiated atca. 11 Ma (Meigs et al., 1995; Burbank et al.,1996). DeCelles et al. (1998) interpreted thethrust to have formed somewhat later in thecentral Himalaya. The Main Frontal Thrust isactive today (Nakata, 1989; Lave and Avouac,1999) and is interpreted to have been initiatedcoevally with the Pliocene upper SiwalikGroup (DeCelles et al., 1998). These timingconstraints indicate that underthrusting of theIndian Shield in southwest Tibet did not beginuntil the early Miocene and underthrustingrates have been on the order of ;21.3 mm/yr.By using this slip-rate estimate, the leadingedge of the subducted part of the Indian Shieldwould have reached the Indus-Yalu suturezone around the early Miocene, thereby allow-ing arc-type magmatism to continue until thattime.

CONCLUSIONS

Cross-section restoration of a 111-km-longtraverse from the India-Nepal-China border toMount Kailas in southwest Tibet is used toestimate a minimum of 176 km of north-southCenozoic horizontal shortening, which waspartitioned between the Tethyan fold-thrustbelt, including the Kiogar-Jungbwa ophiolitethrust sheet (112 km) and contractional struc-tures along the Indus-Yalu suture zone (64km). Sequential reconstruction of the crosssection shows that the locus of Cenozoicshortening due to the India-Asia collisionsince the late Oligocene occurred significantlysouth (.60 km) of the Indus-Yalu suture zonewithin the Tethyan fold-thrust belt. Moreover,oceanic lithosphere was present south of theIndus-Yalu suture zone to the north and theTethyan fold-thrust belt to the south until thelate Oligocene when movement on the Gang-dese thrust system was initiated in southwestTibet. An implication of this reconstruction isthat postcollision high-K, calc-alkaline mag-matism in southwestern Tibet may be ex-plained by melting due to active subductionof a remnant oceanic lithosphere beneath theKailas magmatic complex until the Miocene.A regional profile across the Tibet-Himalayaorogen from the Subhimalaya to the GangdeseShan (Transhimalaya), together with previous-





ly reported shortening estimates in the centralHimalaya, yields a total shortening estimateacross the orogen of 593–763 km. Timingconstraints for thrusts south of and includingthe Main Central Thrust indicate that the un-derthrusting northern edge of the IndianShield along the Main Himalayan Thrust didnot reach the Indus-Yalu suture zone until theearly Miocene, thereby allowing arc-typemagmatism to persist north of the suture untilthat time.

ACKNOWLEDGMENTS

We gratefully acknowledge support in the fieldfrom Paul Kapp (University of Arizona), Ding Lin,and Guo Jinghui (Chinese Academy of Sciences–Institute of Geology and Geophysics) and fundingfrom U.S. National Science Foundation grant EAR-9628178 (to An Yin and T.M. Harrison). Jean-PierreBurg, Richard Law, and Lothar Ratschbacher pro-vided careful reviews and many useful suggestions.

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MANUSCRIPT RECEIVED BY THE SOCIETY 23 APRIL 2001REVISED MANUSCRIPT RECEIVED 7 NOVEMBER 2001MANUSCRIPT ACCEPTED 2 MARCH 2002

Printed in the USA



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