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Propagation of the thrust system and erosion in the Lesser Himalaya: Geochemical and sedimentological evidence

¹ Maison des Géosciences, Université Joseph Fourier, BP 53, 38041 Grenoble, France
² Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
³ Maison des Géosciences, Centre National de Recherche Scientifique, BP 53, 38041 Grenoble, France
⁴ Centre de Recherches Pétrographiques et Géochimiques, BP 20, 54501 Vandoeuvre-les-Nancy, France

Sedimentological and Nd isotope data of two sections of the sub-Himalaya of western Nepal are used as new constraints for understanding the erosion history of the Himalaya. Throughout the deposition of the middle and upper members of the Siwalik Group, the Lesser Himalaya contribution to the total detrital input progressively increased from less than 20% to 40%. The increasing proportion of Lesser Himalaya sediments started at ca. 10–8 Ma and is associated with a coarsening of the maximum grain size at both microscopic and macroscopic scales. Thin-skinned tectonics of the Lesser Himalaya thrust system would have controlled the exhumation of the Lesser Himalaya rocks and would have begun at 12–10 Ma, taking into account the delay for denudation. Together with other studies, these data restrict the onset of movement on the Lesser Himalaya thrust system to less than 3 m.y. along more than 1750 km. This short time frame implies a ratio of lateral propagation rate to shortening rate far too high for the propagation of a single crustal thrust; thus we suggest instead the simultaneous initiation of several thrusts ahead of the Main Central thrust at ca. 12 Ma. We suggest that a rapid rise of the Tibetan Plateau at this time has transformed the Himalaya to an overcritical thrust wedge that has propagated forward to return to a stable state. This regional rising could be the prime cause of the increase of sediment influx at ca. 11 Ma around the Himalaya.

Key Words: Siwalik formations • neodymium • grain size • erosion • thrust sheets • Himalaya

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