Annales Societatis Geologorum Poloniae (1986), vol. 56: 3-31 PL ISSN 0208-9068 INDUS-TSANGPO SUTURE IN THE HIMALAYA: CRUSTAL EXPRESSION OF A PALAEO-SUBDUCTION ZONE N. S. Virdi Wadia Institute o f Himalayan Geology, Dehra Dun-248001, India Virdi, N. S., 1986. Indus-Tsangpo suture in the Himalaya: crustal expression of a palaeo- -subduction zone. Ann. Soc. Geol. Polon., 56: 3-31. Kraków. Abstract: The Indus-Tsangpo suture zone extending from Kohistan through Ladakh to southern Tibet represents the line along which the Indian plate embraced the Tibetan plate subse­ quent to its northward drift during the Mesozoic and collision during the Eocene. The Indus-Tsang­ po suture zone is constituted by more or less parallel belts composed of rock types attributable from south to north to: 1) platform at the northern margin of Indian shield, 2) trench zone, 3) fore- -arc basin, 4) magmatic arc, 5) back-arc basin and 6) platform at the southern margin of Tibet. These belts now occur with more or less vertical or steep tectonic junctions. The trench sequence of ophiolites and ophiolitic melange exhibits metamorphism of high pressure blueschist fades whereas the magmatic arc and back-arc sequences together exhibit a low pressure-high tempera­ ture metamorphism. The two jointly constitute paired metamorphic belts. The glaucophane-law- sonite bearing rocks are widespread in Kohistan, Ladakh and Southern Tibet. These features in­ dicate that the Indus-Tsangpo suture zone represents the site of an old subduction zone which lay at the southern margin of Tibet. Key words: Indus-Tsangpo suture, tectonic evolution, palaeo-subduction, magmatic arc, paired metamorphic belts, Himalaya, southern Tibet, review. Manuscript received July 1985, accepted August 1985 INTRODUCTION The Indus-Tsangpo suture zone is one of the most fascinating regions in the Himalaya due to its unique position and broad spectrum of constituent petro- -tectonic assemblages widely regarded as formed at and indicative of zones of crustal convergence or subduction. The Himalaya are regarded the type example of colli­ sion type orogenic belts (Dewey & Bird, 1970), the suture thus represents the line along which the Indian plate embraced the Tibetan plate subsequent to its northward drift during the Mesozoic and collision during the Eocene. N . S. VIRDI о О Д ев u CD а Nо i О и.О к I 3 3 *± С) и О 4> а О. HS cd во о с J3 СПcd н £ сл ТЭ3 с cdс v cd ^ -е N «в i ^ Й ed ад «ч С cd Uli « Z cd <u в •о I а> Ло •s i S «-*о < cd 2; 'S лс/i С 2 & g .a й i еcd Д cd o J3 oo Ś **5 J 2 H jap 1 uT И u)0> 1 o ыw j Сc/5 СЛ . « cd a> <ЛСЛ * g o 2 «> C2 4>*- O • M rtС £5u +-*СЛ 11 S ■a*o i J3 je« *O £ '3 cd ^-T s fi ■<-' л§ I •.8ih u h i •a •a с O oocd cd _ o ęd 3 i 4 ,2 cd <di & I a i * o § _l« o. - 00 Ng Sу Йe P 5, £ •a o < o £ t>o 'S 3С cd Ph Ut Q INDUS-TSANGPO SUTURE 5 The Indus-Tsangpo suture zone and its constituent lithotectonic belts can be traced more or less continuously from Kohistan region of Pakistan in the west through Ladakh in India and Kailash and Lhasa regions of southern Tibet in the east (Fig. 1). Stoliczka (1866a, b), Lydekker (1883), McMahon (1901), Kraft (1902) and Hayden (1904, 1907, 1916) were the earliest workers who presented preliminary accounts of the geology of the Indus-Tsangpo suture zone and the surrounding regions. Subsequent and notable contributions came from Dainelli (1933—1934), De Terra (1935), Wadia (1932, 1937), Heim & Gansser (1939), Norin (1946), Ber- thelsen (1953), Tewari (1964), Gansser (1964, 1966) and Gupta et al. (1970). Amongst the recent contributors to our knowledge of this important zone we have Gupta and Kumar (1975), Shanker et al. (1976), Shah et al. (1976), Frank et al. (1977), Gansser (1977, 1980), Virdi et al. (1977, 1978), Thakur & Virdi (1979), Thakur (1981), Farah & De Jong (1979), Thakur & Sharma (1983), Shackleton (1981), Jixiang (1981), Science Press (1981), Fuchs (1977, 1979, 1982), Coward et al. (1981a, 1982b), Wan (1982), Rai (1982, 1983), Virdi (1981a, b, 1983, 1984a, b), Thakur & Gupta (1984), Xuchang et al. (1983), Tapponnier et al. (1981), Tahirkheli (1982), Honegger et al. (1982), Searle (1983), Allegre et al. (1984), Scharer et al. (1984a, b) and Klootwijk (1984). As a result of these studies a more or less unified picture has emerged and it has been clearly established that the Indus-Tsangpo suture zone marked the northern boundary of the Indian plate and during its evolution, like other zones of subduction, it was characterized by not only a well developed system of volcano-magmatic arc, fore-arc and rear-arc basins and a trench, but also paired metamorphic belts. The present paper discusses the geotectonic evolution of the Indus-Tsangpo suture zone and the role it played in the development of the Himalayan mountains, particularly within the Indian territory of Ladakh. An attempt is also made to com­ pare the sequence of events in Ladakh with those in the western and eastern exten­ sion of the suture zone in Kohistan and southern Tibet, respectively. LITHO-TECTON1C SETTING OF THE LADAKH REGION The Indus-Tsangpo suture zone in Ladakh region extends as a longitudinal zone bounded by the Karakorum mountains in the north and Zanskar mountains in the south. The two mountain ranges are constituted by Phanerozoic shelf se­ diments deposited at the northern shelf of the Indian shield and southern shelf o f the Tibetan landmass, respectively. The suture zone is characterized by a number of lithotectonic belts composed of rock assemblages developed in different parts of a subduction zone and related volcano-magmatic arc. Table 1 presents comparative litho-tectonic setting of the suture zone in Kohistan, Ladakh and southern Tibet, while Figure 2 shows the areal distribution of these units in Ladakh region. A brief description of characters of various uni ts is given in the sequel. 6 N. s. 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