NEOGENE FORELAND BASIN DEPOSITS, EROSIONAL UNROOFING, AND THE KINEMATIC HISTORY OF THE HIMALAYAN FOLD-THRUST BELT, WESTERN NEPAL

Sedimentological and provenance data from the lon cr Miocene-Pliocene Dumri Formation and Siwalik Group in western Nepal provide new informa tion about the timing of thrust faulting and the links between erosion al unroofing of the Himalaya and the Cenozoic Sr-87/Sr-86 record of th e ocean. In western Nepal, the Dumri Formation is an similar to 750-13 00-m-thick fluvial sandstone and overbank mudstone unit. The Siwalik G roup is >4200 m thick and consists of a lower member (>850 m) of 2-12- m-thick fluvial channel sandstones and oxidized calcareous paleosols, a middle member (>2400 m of very thick (>20 m) channel sandstones and mainly organic-rich Histosols, and an upper member (>1000 m) composed of gravelly braided river deposits. Paleocurrent data indicate that mi ddle Miocene-Pliocene rivers in western Nepal flow ed southward, trans verse to the thrust belt, throughout deposition of the Siwalik Group. No evidence was found for an axial fluvial trunk system (i.e., the pal eo-Ganges River) in Siwalik Group sandstones. A major increase in fluv ial channel size is recorded ba the transition from lower to middle Si walik members at similar to 10.8 Ma, probably in response to an increa se in seasonal discharge. Modal petrographic data from sandstones in t he Dumri Formation and the Siwalik Group manifest an upsection enrichm ent in potassium feldspar, carbonate Lithic fragments, and high-grade metamorphic minerals. Modal petrographic analyses of modern ri, er san ds provide some control on potential source terranes for the Miocene-P liocene sandstones, The Dumri Formation was most likely derived from e rosion of sedimentary and low-grade metasedimentary rocks in the Tibet an (Tethyan) Himalayan zone during early Miocene emplacement of the Ma in Central thrust, The presence in Dumri sandstones of plagioclase gra ins suggests exposure of crystalline rocks of the Greater Himalayan zo ne, perhaps in response to tectonic unroofing by extensional detachmen t faults of the South Tibetan detachment system, During deposition of the lower Siwalik Group (similar to 15-11 hla), emplacement of the Dad eldhura thrust sheet (one of the synformal crystalline thrust sheets o f the southern Himalaya on top of the Dumri Formation supplied abundan t metasedimentary lithic fragments to the foreland basin. A steady sup ply of plagioclase grains and high-grade minerals was maintained by de eper erosion into the Main Central thrust sheet, From similar to 11 Ma to the present, K-feldspar sand increased steadily, suggesting that g ranitic source rocks became widely exposed during deposition of the up per part of the loner Siwalik Group. This provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the Dadeldhura thrust sheet above a large duplex in the Lesser Hima layan rocks. Since the onset of deposition of the conglomeratic upper Siwalik Group (similar to 4-5 Ma), fault slip in this duplex has been fed updip and southward into the Main Boundary and Main Frontal thrust systems. We obtained 113 U-Pb ages on detrital zircons from modern ri vers and Siwalik Group sandstones that cluster at 460-530;Ma, similar to 850-1200 Ma, similar to 1.8-2.0 Ga, and similar to 2.5 Ga, An abund ance of Cambrian-Ordovician grains in the Siwalik Group suggests sourc es of Siwalik detritus in the granites of the Dadeldhura thrust sheet and possibly the Greater Himalayan orthogneisses. The older ages are c onsistent with sources in the Greater and Lesser Himalayan zones, An o verall upsection increase in zircons older than 1.7 Ga suggests increa sing aerial exposure of Lesser Himalayan rocks, None of the detrital z ircons (even in the modern river samples) yielded a Cenozoic age that might suggest derivation from the Cenozoic Greater Himalayan leucogran ites, but this may be attributable to the inheritance problems that ch aracterize the U-Pb geochronology of the leucogranites. when compared with recent studies of the Sr-87/Sr-86 composition of paleosol carbona te nodules and detrital carbonate in paleosols from the Siwalik Group, the provenance data suggest that erosion and weathering of meta-morph osed carbonate rocks in the Lesser Himalayan zone and Cambrian-Ordovic ian granitic rocks of the crystalline thrust sheets in central and eas tern Nepal map hare played a significant role in elevating the Sr-87/S r-86 ratio of middle Miocene synorogenic sediments in the Indo-Gangeti c foreland basin and the Bengal fan, as well as global seawater.