Pre India-Asia convergence history (>110 Ma)

Several major fault zones north of the Himalayan Orogen bound tectonic blocks already sutured before the India-Asia convergence (Pozzi et al., 1972; Burg, 1983; Allègre et al., 1984; Sengör, 1984; Dewey et al., 1988; Matte et al., 1986). Most of these blocks are of small size, probably originating from the break-up of Gondwanaland in the Early Palaeozoic (Bond et al., 1984). They were progressively accreted from then on, contributing to the growth of the Asian (Laurasian) margin southwards (Allègre et al., 1984; Matte et al., 1996). In Early Palaeozoic, Altaids were accreted to the Siberian Platform (Burret, 1974; Burret et al., 1990), the Kunlun-Qinling blocks were accreted in the Silurian. Following the formation of the Siberian Craton, three successive cycles of accretion can be distinguished (Sengör, 1984; 1987; Sengör et al., 1993; Van der Voo, 1993, Klootwijk, 1996): (1) Variscan, (2) Cimmerian and (3) Alpine. The Variscan cycle is mainly concerned with the Tarim block in the Late Carboniferous-Permian (Sengör, 1987). The Cimmerian cycle corresponds to the break-up of the Cimmerian continent (CC) from the Gondwana margin, which began in the Late Permian and was complete during the Triassic (Sengör et al., 1988). At this stage, Panjal trap volcanism occurred in the northern part of the Indian margin (Honnegger et al., 1982), as well as in the Karakoram block (Rolland et al., accepted), possibly associated with the opening of the Neo-Tethys Ocean andthe separation of the Karakoram / Lhasa blocks from India. Following Sengör et al. (1988), the CC is complex. It extends in an E-W direction, comprising two parallel strips of continent separated by a small ocean, the Tangulla-Waser Ocean. The northern continental strip of the CC comprised the north Tibetan (or Qiantang) block in the west and the north China block in the east. The southern continental strip of the CC comprised the south Tibetan (or Lhasa) block in the west and the south China block in the east. The northward progression of the CC lead to the subduction of the Palaeo-Tethys Ocean along the southern margin of the Tarim block and to the formation of the Neo-Tethys Ocean between the CC and the Gondwana margin (Sengör, 1984). The closure of the Palaeo-Tethys Ocean and the suturation of the CC occurred between Middle Triassic and Early-Middle Cretaceous (Sengör, 1984; Van der Voo et al., 1999; Zanchi et al., 2000). The Qiantang Block was accordingly accreted along the Kilik suture during the Late Triassic (e.g., Matte et al., 1996; Fig. 1), while the Lhasa block was fully sutured along the Bangong suture in Lower-Middle Cretaceous after closure of the Tangulla-Waser Ocean (Besse et al., 1984; Sengör et al., 1988; Zanchi et al., 2000).

Figure 1. Schematic map of the Pamir-Karakoram-NW Himalaya syntaxis, with location of principal tectonic blocks and sutures

Schematic map of the Pamir-Karakoram-NW Himalaya syntaxis, with location of principal tectonic blocks and sutures

The main geological units of the Karakoram are shown. The location of the study area is indicated. MKT: Main Karakoram Thrust.

Our study in the SE Karakoram region of Skardu has shown that the south Karakoram Series are part of Cambro-Ordovician sequences overlying a Precambrian basement (Rolland et al., 2002b). Such successions have also been described by Le Fort et al. (1994) in SW Karakoram, and confirm continuity of the Karakoram block, with similar geological formations along its southern rim and on its northern part (Gaetani, 1996; Gaetani et al., 1996; Zanchi et al., 1997). Similar successions, comprising Cambro-Ordovician series overlying Cambrian gneisses, have also been described in the Lhasa block (Xu et al., 1985; Yin et al., 1988). These similarities suggest that the Karakoram may be an original part of Lhasa block later offset by the Karakoram Fault (Rolland et al., 2002b). The Early Cretaceous accretion of the Karakoram-Lhasa block was followed by that of the Kohistan-Ladakh Arc terranes in Late Cretaceous (Petterson & Windley, 1985). The India-Asia collision and Himalayan orogeny are therefore part of a long accretion history, originating in the Palaeozoic.