Journal of Structural Geology, Vol. 10, No. 1, pp. 129 to 132, 1988 0191-8141/88 $03.00 + 0.00 Printed in Great Britain Pergamon Press pie
Structural evolution and sequence of thrusting in the High Himalayan, Tibetan-Tethys and Indus Suture zones of Zanskar and Ladakh, Western Himalaya: Discussion
P. B. KELEMEN
Department of Geological Sciences A J-20, University of Washington, Seattle, WA 98195, U.S.A.
Laboratoire de G~ologie Stratigraphique et Structurale, Universit~ de Poitiers, 40, Avenue du Recteur Pineau, 86022 Poitiers C6dex, France
Geologische Bundesanstalt, Rasumofskygasse 23, A-1031 Wien, Austria
(Received 19 May 1987; accepted 29 July 1987)
M. P. Searle's recent paper in the Journal of Structural Reuber 1986). In addition, Eocene strata have been Geology (Searle 1986) included a major departure from identified in the melange at the base of the klippe published structural interpretations of the Ladakh (Colchen et al. in press). Thus the final emplacement of Himalaya. The geologic history of Ladakh is a vital key the klippe must post-date Lower Eocene sedimentation to understanding the timing and sequence of events (at least as young as 55 Ma). during the Himalayan orogeny. Ophiolitic rocks and Thrusting of the klippe may have begun substantially island arc volcanics along the Indus Suture zone (Frank earlier than its final emplacement, especially if the possi- et al. 1977, and many others) constitute remnants of a bility of intra-oceanic faulting (Reuber 1986) is con- broad oceanic basin, formerly north of the Indian craton. sidered as part of the emplacement 'event'. There is little The closure of this basin, and the 'collision' of the Indian data constraining the time at which the klippe was first continental margin with a subduction-related magmatic thrust over the northern Indian passive margin. Searle arc, marked an important event early in the orogeny. It presents the Maastrichtian Kangi La Formation as a is important to reconstruct the sequence of other major "syn-emplacement 'flysch' deposit" which accompanied thrust events, for instance the emplacement of the thrusting of the Spongtang Klippe. However, the Kangi ophiolitic Spongtang Klippe southward over platform La Formation is quartz-rich and apparently contains sediments of the northern Indian continental margin, in sediments derived only from the passive margin of north- relation to the time of closure. In Ladakh, closure along ern India. The first known sedimentologic evidence for the Indus Suture zone is thought to have occurred about the presence of oceanic basement and arc volcanics in 55 Ma (Klootwijk et al. 1979). the Indian succession is found in the post-Paleocene Searle's more general postulates regarding collisional Chulung La slates (Garzanti & Gaetani in press). tectonics are difficult to test, and thus are beyond the The relations shown on Searle's map contradict other scope of our criticism. The most important regional published maps of the area. It is not clear whether Searle hypothesis advanced in his paper is that the ophiolitic was unaware of this contradiction or whether he dis- Spongtang Klippe, about 30 km south of the Indus agrees with earlier observations. In the latter case, of Suture zone, was emplaced between 75 and 60 Ma, prior course, the reasons for such disagreement should have to deposition of the Paleocene to Lower Eocene been stated. The interpretation of Searle's phrase: "orig- Lingshet Limestone. He specifically states that "the inal direct contact" [emphasis added] is open to ques- Upper Paleocene Lingshet limestones . . . are not in tion. If it means that the klippe was thrust onto the original direct contact with the Spongtang Ophiolite", northern Indian platform earlier than the post-Lower and shows the two units as spatially separate (about 5 km Eocene and later moved along a reactivated thrust fault apart) on his geologic map (Searle 1986, fig. 2). How- to its present position, then this should have been ever, observations made by three independent teams explicitly stated and justified. show that the basal thrust of the Spongtang Klippe The relative timing of emplacement of the klippe and directly overlies, and truncates antiformal hinges in, the deposition of the limestone is of great tectonic signifi- Paleocene-Lower Eocene limestone (Fuchs 1982, Kele- cance. Paleocene limestone, equivalent to that beneath men & Sonnenfeld 1983, Colchen & Reuber 1986, the klippe, is reported to overlie the Cretaceous and 129 130 Discussion/Reply earliest Tertiary arc volcanics and volcanoclastic rocks tural history including long-lasting episodes of thrusting of the Indus Suture zone (Wadia 1937, van Haver 1984). (Fuchs 1982, Reuber 1986), but it is beyond the scope of It seems likely that the presence of this regionally exten- this comment to propose a detailed sequence of events. sive limestone indicates formation of a shallow, quies- Any such hypothesis must be rigorously justified by cent basin after cessation of plate-scale convergence and detailed mapping and accurate stratigraphic data. closure along the suture. If this is true then final move- To summarize, it is beyond doubt that final emplace- ment of the klippe post-dated closure, whereas Searle ment of the Spongtang Klippe was post-Lower Eocene, (1986) suggests that emplacement of the klippe pre-dated not between 75 and 60 Ma (late Cretaceous to Lower closure, and many authors tentatively infer that Paleocene) as stated by Searle. Final movement was at emplacement was synchronous with closure (e.g. Frank least coeval with, and probably post-dated, closure along et al. 1977). The klippe probably had a polyphase struc- the Indus Suture zone approximately 55 Ma.
Structural evolution and sequence of thrusting in the High Himalayan, Tibetan-Tethys and Indus Suture zones of Zanskar and Ladakh, Western Himalaya: Reply
M. P. SEARLE
Department of Geology, University of Leicester, Leicester LE1 7RH, U.K.
(Received 27 July 1987; accepted in revised form 12 October 1987)
The crux of the discussion by Kelemen, Reuber and (notably Oman and Western Newfoundland) show that Fuchs relates to the age of obduction of the Spongtang a complex history of thrusting spanning ca 20-25 Ma Ophiolite thrust sheet in the Zanskar Range of the NW involves hundreds of km of translation (see Searle & Indian Himalaya, a topic that has been hotly debated in Stevens 1984 for review and references). Obduction the recent Himalayan Workshop meetings at Leicester processes (Dewey 1970, Coleman 1977) begin as intra- in 1985 and Nancy in 1986. Firstly much confusion in the oceanic mantle-tapping ductile shear-zones (e.g. Reuber literature has arisen due to the different interpretations 1986) or thrust faults which are subduction-related (e.g. of terminology, notably the terms ophiolite, obduction Searle & Malpas 1980), and generally evolve into thin- and emplacement. The Spongtang Ophiolite refers solely ner-skinned brittle thrust-related structures. Deep level to the ophiolite sensu stricto sequence (i.e. ultramafic ductile detachment zones with high-temperature plastic mantle sequence, gabbros, cumulates, dykes or sills, flow fabrics become shallower level brittle thrust faults pillow lavas) following the Penrose ophiolite definition with decreasing depth and increasing time. (Coleman 1977). It does not include the various sedimen- The stratigraphy of the Zanskar Shelf sequence has tary (Lamayuru Complex, Goma Formation, etc.) or been extensively studied (e.g. Fuchs 1979, 1982, Kele- andesitic volcanic (Dras formation) rocks immediately men & Sonenfeld 1983, Gaetani et al. 1983, 1985, etc.) beneath the ultramafic rocks. Ophiolite obduction, a and it is possible to constrain the timing of closure of term originally proposed by Coleman (1971), refers to Tethys along the Indus Suture zone in Ladakh. Palaeo- the process of displacing oceanic crust and mantle onto a magnetic studies indicate closure at around 55 Ma continental margin. Emplacement is generally used (Klootwijk et al. 1979). Stratigraphic studies indicate synonymously with obduction. If we agree that the closure at around 50 Ma, which is the age of the youngest ophiolite was oceanic crust and mantle then obduction marine sediments on the Zanskar Shelf-Spanboth For- or emplacement must have been prior to ocean closure mation shallow marine carbonates of Palaeocene age in (T1). Subsequent (post-collision) deformation involved Western Zanskar, or Palaeocene-Lower Eocene several phases of complex thrust stacking (T2, T3), and Lingshet limestones in central Zanskar. Overlying the is related to continental collison tectonics, not to earlier Spanboth Formation are a sequence of purple and green subduction-obduction-emplacement tectonics. continental ferruginous slates of early Eocene age The Spongtang Ophiolite is a slab of Tethyan oceanic (Chulung La Formation). There are no marine sedi- crust and mantle sequence rocks in which the volcanic ments younger than early Eocene on the Zanskar Shelf component has a dominant MORB-chemistry and the or along the Indus Suture zone (van Haver 1984). Conti- harzburgite--dunite-lherzolite mantle component is over nental molasse deposition (Indus Group) dominated the 2 km thick. It was thrust southwards onto the north suture zone after this time and closure of Tethys and Indian continental margin from an oceanic site north of collision of India with the northern plate can be con- the Zanskar Shelf margin. Similar obducted ophiolite strained at ca 50 Ma. There are few certainties in slabs which are better constrained and better studied Ladakhi geology but one can say with certainty that