Structural evolution and sequence of thrusting in the High Himalayan, Tibetan-Tethys and Indus Suture zones of Zanskar and Ladakh, Western Himalaya: Discussion

M. P. Searle's recent paper in the Journal of Structural Geology (Searle 1986) included a major departure from published structural interpretations of the Ladakh Himalaya. The geologic history of Ladakh is a vital key to understanding the timing and sequence of events during the Himalayan orogeny. Ophiolitic rocks and island arc volcanics along the Indus Suture zone (Frank et al. 1977, and many others) constitute remnants of a broad oceanic basin, formerly north of the Indian craton. The closure of this basin, and the 'collision' of the Indian continental margin with a subduction-related magmatic arc, marked an important event early in the orogeny. It is important to reconstruct the sequence of other major thrust events, for instance the emplacement of the ophiolitic Spongtang Klippe southward over platform sediments of the northern Indian continental margin, in relation to the time of closure. In Ladakh, closure along the Indus Suture zone is thought to have occurred about 55 Ma (Klootwijk et al. 1979). Searle's more general postulates regarding collisional tectonics are difficult to test, and thus are beyond the scope of our criticism. The most important regional hypothesis advanced in his paper is that the ophiolitic Spongtang Klippe, about 30 km south of the Indus Suture zone, was emplaced between 75 and 60 Ma, prior to deposition of the Paleocene to Lower Eocene Lingshet Limestone. He specifically states that "the Upper Paleocene Lingshet limestones . . . are not in original direct contact with the Spongtang Ophiolite", and shows the two units as spatially separate (about 5 km apart) on his geologic map (Searle 1986, fig. 2). However, observations made by three independent teams show that the basal thrust of the Spongtang Klippe directly overlies, and truncates antiformal hinges in, the Paleocene-Lower Eocene limestone (Fuchs 1982, Kelemen & Sonnenfeld 1983, Colchen & Reuber 1986, Reuber 1986). In addition, Eocene strata have been identified in the melange at the base of the klippe (Colchen et al. in press). Thus the final emplacement of the klippe must post-date Lower Eocene sedimentation (at least as young as 55 Ma). Thrusting of the klippe may have begun substantially earlier than its final emplacement, especially if the possibility of intra-oceanic faulting (Reuber 1986) is considered as part of the emplacement 'event'. There is little data constraining the time at which the klippe was first thrust over the northern Indian passive margin. Searle presents the Maastrichtian Kangi La Formation as a "syn-emplacement 'flysch' deposit" which accompanied thrusting of the Spongtang Klippe. However, the Kangi La Formation is quartz-rich and apparently contains sediments derived only from the passive margin of northern India. The first known sedimentologic evidence for the presence of oceanic basement and arc volcanics in the Indian succession is found in the post-Paleocene Chulung La slates (Garzanti & Gaetani in press). The relations shown on Searle's map contradict other published maps of the area. It is not clear whether Searle was unaware of this contradiction or whether he disagrees with earlier observations. In the latter case, of course, the reasons for such disagreement should have been stated. The interpretation of Searle's phrase: "original direct contact" [emphasis added] is open to question. If it means that the klippe was thrust onto the northern Indian platform earlier than the post-Lower Eocene and later moved along a reactivated thrust fault to its present position, then this should have been explicitly stated and justified. The relative timing of emplacement of the klippe and deposition of the limestone is of great tectonic significance. Paleocene limestone, equivalent to that beneath the klippe, is reported to overlie the Cretaceous and