Structural evolution and sequence of thrusting in the Tethyan fold-thrust belt and Indus-Yalu suture zone, southwest Tibet 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 India-Nepal-China 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 ;750 km. Sedimentary Sequence in the northwestern betÐcombined with previously published Tethyan Himalaya (Zanskar) and eastern Teth- geochronologic and stratigraphic dataÐis Keywords: Tethys, Himalayan orogeny, 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 ;1000 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 fault toration of the cross section shows that the ern Tibet and the Indian Shield (Patriat and systems: the Main Frontal Thrust, 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, .60 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 Proterozoic me- until the late Oligocene. A regional pro®le ling, 1992; DeCelles et al., 1998, 2001), (2) tasedimentary rocks, Paleozoic to Eocene 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] q 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 Geological Society of America Bulletin, January 2003 23 Figure 1. (Continued.) (A) Regional geologic map of southwest Tibet and northwestern Nepal compiled from Heim and Gansser (1939), Valdiya (1981), Cheng and Xu (1987), Shrestha et al. (1987), DeCelles et al. (1998), Yin et al. (1999), and Murphy et al. (2000). Location of regional cross-section line (A±F) extending from the Subhimalaya to the Gangdese Shan (Transhimalaya) (Fig. 1B) is shown. The A±B segment of the regional cross section is taken directly from DeCelles et al. (1998). Inset shows location of study area and cross sections from Searle (1986) in Zanskar and Ratschbacher et al. (1994) in south Tibet. Abbreviations: JSÐJinsha suture; KFSÐKarakoram fault system; BNSÐBangong-Nujiang suture zone; IYSZÐIndus-Yalu suture zone; STDSÐSouth Tibetan Detachment System; MBTÐMain Boundary Thrust; MFTÐMain Frontal Thrust; MCTÐMain Central Thrust. (B) Regional cross section across the Himalaya on the basis of pro®les published in Heim and Gansser (1939), DeCelles et al. (1998), and this study. Additional abbreviations: RTÐRamgarh thrust; DTÐDadeldhura thrust; KFÐKarakoram fault; GCTÐGreat Counter Thrust; GTSÐGang- dese thrust system. Formations in the Tethys Himalaya and Gangdese Shan (between C and F on the cross-section) correlate to those shown in Figure 2. Two different orientations are shown for the Main Central Thrust, on the basis of observations in the Garhwal and in the Karnali. We estimate 763 km of horizontal shortening between the Subhimalaya and the Indus-Yalu suture zone. If the displacement on all thrusts south of and including the Main Central Thrust is assumed to have fed into the Main Himalayan Thrust and underthrusting is assumed to have begun at 20 Ma, the leading edge of the Indian plate's lower crust would have been positioned beneath the Kailas magmatic arc complex shortly thereafter, thereby implying that oceanic lithosphere had been subducted beneath Asia until that time. 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 ;50 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).