Multiproxy Isotopic and Geochemical Analysis of the Siwalik Sediments in NW India: Implication for the Late Cenozoic Tectonic Evolution of the Himalaya

摘要

Provenance analysis of the Sub-Himalayan Late Miocene-Pleistocene foreland basin deposits (Siwaliks) from the Dehradun reentrant area provides a 10-Myr long record of the denudation history and tectonic evolution of the northwestern Indian Himalaya. We studied Siwalik sediments exposed along the Mohand-Rao and Haripur-Khol sections, using detrital zircon U-Pb geochronology, major and trace elements, and Sr-Nd isotope geochemistry. Results suggest that the erosion pattern has been relatively stable since the Late Miocene with sediments derived from the Tethyan Himalayan (THS), Greater Himalayan (GHS), and outer- (oLHS) and inner-Lesser Himalayan (iLHS) sequences. Provenance data indicate that erosional unroofing of the Lesser Himalayan Crystalline sequences (LHCS) initiated around 6Ma, possibly related to out-of-sequence movement of the Ramgarh-Munsiari Thrust. Our data also suggest erosional recycling of older foreland basin deposits into younger Siwaliks since 5.5Ma, which may indicate the time of thrust propagation from the Lesser Himalaya into the foreland basin. While the iLHS has been exposed to erosion since at least 10Ma, the Siwaliks were dominated by materials derived from the GHS and THS sources. We interpret these results as an indication that tectonic uplift and erosion of the orogenic wedge occurred in response to duplexing of the iLHS and concomitant high topography and rock uplift rate in the Greater and Tethyan Himalaya. Comparing the provenance of the Siwalik sediments with that of the modern Ganga and Yamuna river sediments further indicates that deposition during the Late Cenozoic was most likely accomplished by southward flowing transverse Himalayan rivers, analogous to the modern ones.