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Early Himalayan Exhumation: Isotopic Constraints from the Indian Foreland

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Early Himalayan Exhumation: Isotopic Constraints from the Indian Foreland Paper 268 Disc Early Himalayan exhumation: Isotopic constraints from the Indian foreland basin Yani Najman{,{, Mike Bickle*,{ and Hazel Chapman{ {Department of Earth Sciences, Cambridge University, Downing St., Cambridge CB2 3EQ, UK, {Department of Geology & Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, UK ABSTRACT Nd- and Sr-isotopic compositions of Palaeogene foreland basin at least this time until the present day. The transition is sediments are used to provide insights into early Himalayan interpreted to reflect exhumation of `basement rocks' of the evolution,particularly the timing of exposure of high 87Sr/86Sr Indian plate,when the High Himalaya became a sufficient units,erosion of which may have caused the late Tertiary increase topographic barrier to separate suture zone rocks from the in oceanic Sr-isotopic ratios. During the late Palaeocene±early foreland basin. The marked rise in seawater 87Sr/86Sr from 40 Ma Eocene,erosion was from mixed sources including suture zone is consistent with the erosion of a Himalayan source with a high rocks. Exhumation of the High Himalaya was occurring by the 87Sr/86Sr ratio. time of deposition of alluvial sediments after mid-Oligocene times and this source has dominated Himalayan sediments from Terra Nova, 12, 28±34, 2000 chain from six main tectonic units (Fig. cambrian to early Permian and mainly Introduction 1) (listed from north to south below). shelf carbonate in the late Permian to The Himalayas are the prime example 1 The Trans-Himalayan zone,the An- early Eocene,unconformably overlain of an active collisional orogen. Their dean-type northern margin of Tethys by the Chulung La collisional deposits kinematics form the basis of our under- formed by Cretaceous to Eocene calc- (Critelli and Garzanti,1994; Searle et standing of crustal deformation pro- alkaline plutons intruding the Eurasian al.,1997a). The Tibetan Sedimentary cesses in collisional zones (e.g. England continental crust (Honegger et al., Series is now juxtaposed against the and McKenzie,1982; Tapponnier et al., 1982; Le Fort,1996); High Himalayan Crystalline Series to 1982; Houseman and England,1993). 2 The Indus±Tsangpo Suture Zone, the south by normal faulting on the The associated uplift and erosion may comprising deep-water Indian conti- South Tibetan Detachment Zone. moderate long-term changes in global nental rise sediments,ophiolite-bearing 4 The High Himalayan Crystalline Ser- climate (e.g. Raymo,1991). A critical accretionary complexes,island arc vol- ies is derived from late Proterozoic and limit to our understanding of the evolu- canics and fore-arc basin sediments,as younger Indian continental crust which tion of mountain belts is that observa- well as post-early Eocene molasse-type underwent high-grade regional meta- tions on their kinematics are only pos- sediments (e.g. Garzanti and Van Haver, morphism and intrusion by leucogra- sible at the present day. Past kinematics, 1988; Robertson and Degnan,1993); nites during Himalayan orogenesis (e.g. responsible for today's crustal expres- 3 The Tibetan Sedimentary Series,de- Le Fort et al.,1987; Parrish and Hodges, sion,must be inferred from the geologi- posited on the continental margin of 1996). This unit is emplaced over the cal record. In this paper we use the Sr the Indian plate consisting of mixed Lesser Himalayan Series to the south by and Nd isotopic composition of Hima- clastics and carbonates in the late Pre- the Main Central Thrust (MCT). layan-derived sediments,deposited in the foreland basin,to map the history of exhumation and erosion of various isotopically distinct crustal units. Our results put further constraints on the timing of erosion of the 87Sr/86Sr en- riched rocks,which may have had a significant influence on the rise in sea- water 87Sr/86Sr since *40 Ma (Hodell et al.,1990; Richter et al.,1992). Geological setting Himalayan tectonics Southwards directed and propagating thrusting has constructed the Himalayan *Correspondence: Tel.: +44/ 1223 333400; Fig.1 HimalayanorogenicbeltshowingpotentialHimalayansourcerocksforthesediments Fax: +44/ 1223 333450; E-mail: mb72@esc. of the foredeep. Study area boxed. MCT,Main Central Thrust; STDZ,South Tibetan cam.ac.uk Detachment Zone; MBT,Main Boundary Thrust; HFT=MFT,Main Frontal Thrust. 28 *C 2000 Blackwell Science Ltd Ahed Fig marker Bhed Table marker Ched Ref end Dhed Ref start Ref marker Paper 268 Disc Terra Nova, Vol 12, No. 1, 28±34 Early Himalayan exhumation . Y. Najman et al. ............................................................................................................................................................................................................................................. 5 The Lesser Himalayan Series,a low- grade,mid-Proterozoic clastic and car- bonate sequence,probably originally deposited on Indian continental crust (e.g. Valdiya,1980; Parrish and Hodges, 1996). The Lesser Himalayan Series is emplaced on the sub-Himalayan units by the Main Boundary Thrust. 6 The Sub-Himalaya comprising sedi- ments eroded from the Himalayan oro- gen and deposited in the peripheral fore- deep south of the mountain belt (e.g. Najman et al.,1993; Critelli and Garzan- ti,1994; Najman and Garzanti,2000). The structural evolution of the Hi- malayan orogen initiated with conti- nental collision in the latest Palaeocene at * 50 Ma (e.g. Le Fort,1996; Row- ley,1996). Crustal thickening resulting from thrusting of suture zone rocks,the Tibetan Sedimentary Series and the precursors of the High Himalayan Crystalline Series,resulted in Barro- vian metamorphism that peaked at 40±30 Ma (Hodges and Silverberg, 1988; Inger and Harris,1992; Hodges et al.,1994; Vance and Harris,1999). The High Himalaya were then thrust over the Lesser Himalaya along the MCT,active at * 22 Ma (Hubbard and Harrison,1989; Hodges et al., 1996) with the youngest movement da- ted at * 6 Ma (Harrison et al.,1997). Extensional faulting along the South Tibetan Detachment Zone took place between * 23 Ma and *15 Ma (e.g. Searle et al.,1999). Southerly propaga- tion of the thrust stack continued with Sample locations and distribution of major tectonic units (compiled by Najman, the emplacement of the Lesser Hima- Fig. 2 1995). Mapping of distribution of Subathu,Dagshai and Kasauli Formations is laya over the Sub Himalaya along the restricted mainly to road cuts and river sections. Main Boundary Thrust,active in the middle±late Miocene (e.g. Meigs et al., 1995),and the presently active Main Bhatia,1982; Fig. 2) are often poorly tion stratigraphically. The Dagshai Frontal Thrust to the south which em- exposed,largely lacking in fossils,and Formation,consisting of red-coloured places older units of the Sub-Himalaya complexly intercalated. Correlation mudstones,sandstones and caliche, on the youngest sediments in the Indo- and identification of these formations was dated using palaeomagnetic incli- Gangetic Plain. is primarily by their lithostratigraphic nations at 36 + 7 Myr age (2 s)by attributes (Najman et al.,1993). Najman et al. (1994); however,it was The Subathu Formation is well dated dated subsequently to be younger than The foreland basin by a marine fauna between late Palaeo- mid-Oligocene by Najman et al. (1997), Sediments eroded from the Himalayan cene and lower mid-Eocene (Lower with ages in the range 25 to 32 Myr chain have been deposited in a foreland Lutetian) established by Mathur using Himalayan detrital muscovite basin that extends along the entire (1980),and consists of limestones, 40Ar±39Ar (single grain,total fusion), southern flank of the orogen. We mudstones and very fine-grained sand- and by Najman et al. (1999),with fis- sampled older units of the foreland stones. Detrital material is derived pre- sion-track zircon ages of 30 Myr at the basin in Himachal Pradesh,where the dominantly from pre-existing sedimen- base of the succession. Derivation was Sub-Himalaya consists of a series of tary rocks with subordinate mafic predominantly from a very low-grade Tertiary sediment thrust slices beneath igneous input identified by lithic frag- metamorphic source (Najman and the Main Boundary Thrust (Fig. 2). ments and Cr-spinel (Najman and Gar- Garzanti,2000). The conformably Within these thrust sheets,the sedimen- zanti,2000). Najman et al. (1993,1997) overlying alluvial Kasauli Formation tary Subathu,Dagshai,Kasauli and showed that the alluvial Dagshai For- comprises mainly grey sandstones and Siwalik Formations (Gansser,1964; mation overlies the Subathu Forma- subordinate fine-grained material de- *C 2000 Blackwell Science Ltd 29 Paper 268 Disc Early Himalayan exhumation . Y. Najman et al. Terra Nova, Vol 12, No. 1, 28±34 ............................................................................................................................................................................................................................................. rived from a garnet-grade meta- morphic source (Najman and Garzanti, 2000). It is dated by its early to mid- Miocene fossil flora (Fiestmantel, 1882),with Himalayan detrital musco- vite Ar±Ar ages mostly between 22 and 32 Myr (Najman et al.,1997),and it is older in age than the overlying alluvial Siwalik Group,magnetostratigraphi- cally dated at 14 Myr old (Johnson et al.,1985; Meigs et al.,1995; Burbank et al.,1996). Methodology Foreland basin deposits preserve a well-mixed average of sediment pro- duced by collisional orogenic belts be- cause transport distances are long prior to deposition by the major river in
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