Tectonic deformation zones across the Himalaya of northwest India

Abstract:

Although the large-scale tectonic features of the Himalayan wedge are now thought to be understood, e.g. spatio-temporal distribution of deformation across the orogen is still unknown. For instance, it has been favored that crustal shortening dominantly accommodates along the toe of the wedge and that the Main Frontal Thrust (MFT) forms the direct surface expression of the Main Himalayan Thrust (MHT). The oblique convergence of NW-Himalaya, provides a unique opportunity to detect important differences in deformation patter compared to the arc-perpendicular convergence in Nepal and further east.

The following observation provide a key for understanding the location of the main decoupling horizon and where strain is accommodated between the under thrusting India and the Himalayan wedge.

(a) In the NW-Himalaya segments of the MFT grow arc-parallel in contrast to the strongly undulating trend of the MBT resulting in a strongly curved topographic front.

(b) Majority of Holocene shorting is accommodated along the out-of-sequence thrust, e.g. Jwalamukhi-Thrust, that is located in the center of the Sub-Himalaya and probably only minor parts of the total shortening is accommodated along the MFT.

(c) We observed top-to-west thrust ramps that are uplifting early tertiary Subathu and Dhramsala formation within the footwall of the here north-south trending segment of the MBT. This pattern of deformation strongly suggest that the Sub-Himalaya is more strongly connected and effected by the under-thrusting of India rather than triggered by deformation of the Himalayan Wedge, behaving here as regit indentor.

(d) Low temperature thermochronology transects across the Dhauladar Rangeindicate continuous uplift and fault displacement with rates in the range of 1-2 mm/yr along the MBT-fault zone hanging wall since the late Miocene. In summary these deformation pattern demonstrates that MBT and MFT developed independently from each other and that deformation within Sub-Himalaya is decoupled from growth of Himalayan wedge. Therefore we would suggest that rather the MBT is directly linked with the MHT forming the main decoupling horizon rather than the MFT, as it is usually proposed.