Timing of Main Boundary Thrust Initiation, Deduced from the Petrography of Late Miocene Foreland Basin Fill Sandstones, from the Northwestern Himalayas

Initiation and cessation of tectonic activity in an active orogenic belt results in varied responses of the foreland basin in terms of subsidence, sedimentary character, geochemical properties etc. The initiation of the Main Boundary Thrust (MBT) in the North Western Himalayas has already been estimated using the modeling of gravel progradation rates using an assumption of syntectonic gravel progradation. On the other hand, episodes of subsidence of the foreland basin and a shallower foreslope during active thrusting phases make it difficult to explain gravel progradation as a syntectonic phenomenon.The exhumation of the foreland basin during the exhumation phase, resulting in a steep foreslope renders gravel progradation to be arguably a post-tectonic phenomenon, especially for an overfilled basin. We have constrained the ages of MBT initiation for the same part of the Himalayas using a syntectonic proxy, namely sandstone modal percentages. We report a minor phase of thrust initiation at 11.3 Ma or possibly earlier followed by a major phase at 10.5 Ma, when the source rocks were mostly igneous and metamorphic, possibly derived from the Greater Himalayan Crystallines (GHC). Another active phase of thrusting at 9 Ma corresponds to sedimentary tock fragments indicates a possible source change from the GHC to the Lesser Himalayas (LHS). A possible drainage organization event is also hypothesized based upon available published literature.