Pascale Huyghe¹, Albert Galy², Jean-Louis Mugnier³, Christian France-Lanord⁴

(1) Grenoble University, Grenoble, France
(2) Cambridge University
(3) Laboratoire de Geodynamique des Chaines Alpines, Grenoble, France
(4) CNRS and CRPG Nancy, Nancy

ABSTRACT: Sedimentary and Geochimical Signature of the Himalayan Thrust System Propagation

Two new data sets, based on sedimentological and Nd isotopic composition of two sections of the subHimalayas of Western Nepal, suggest that erosion of the Lesser Himalayan (LH) zones began only at 8.5-10 Ma. The Bengal Fan Nd isotopic data set also shows an increase of the (LH) erosion at 8-9 Ma. Taking into account the delay for the denudation of the (LH) located either beneath a paleo-foreland basin or beneath Higher Himalayan (HH) nappes, the thin-skinned tectonics of the Lesser Himalayas thrust system (LHTS) must have begun prior to 9-11 Ma. It appears from 8 independent data sets (three presented in this paper) that the onset of the LHTS ranges from 9 to 12 Ma along a more than 1500-km long Himalayan segment. This implies a lateral propagation of the thrust system at a rate greater than 500 mm/yr. As the convergence rate through the Himalayas is permanently in the order of 20 mm/ yr., the ratio lateral propagation /shortening rate is greater than 25, i.e. greater than the one expected for the propagation of a single crustal thrust. We suggest that the propagation of the (LHTS) corresponds to several thrusts that initiated simultaneously ahead of the Main Central Thrust, and synchronously with the initiation of basaltic volcanism and of east-west extension of the Tibetan plateau, the change in the deformation both north and south of Tibet, including the Indian lithosphere south of the Bengal fan. A rapid rise of the Tibetan plateau and of the Himalayan at this time is inferred to explain all these geodynamic events, including the LHTS initiation. This regional rising could be the primary cause of the increase of the sediment influx between 11 and 8 Ma around the Himalayan belt and to the sedimentological transition between lower and middle Siwalik members.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado