Thrust geometries and deep structure of the outer and lesser Himalaya, Kumaon and Garhwal (India): Implications for evolution of the Himalayan fold‐and‐thrust belt

Abstract

The Kumaon‐Garhwal region of the Himalaya lies near the center of the Himalayan fold‐and‐thrust belt. We have drawn two balanced cross sections, 100 km apart, through the Outer and Lesser Himalaya. The cross sections incorporate all the surface, well log, and earthquake seismic data currently available from the region. Two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn. The three dimensional deep structure of the Outer and Lesser Himalaya is interpreted based on the balanced cross sections and the branch line maps. Deep structure of the Higher and Tethyan Himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas. A sequential evolutionary model for the Kumaon Himalaya along the eastern (Pindari) section is proposed. According to this model, the Kumaon Himalaya evolved by an overall forelandward progression of thrusting, with some reactivation along the Munsiari thrust (MT), the Main Boundary thrust (MBT), and the Main Central thrust (MCT). We use structural, stratigraphic, and radiometric criteria to place time constraints on the motion of these thrusts. Earliest motion along the MBT may have occurred in Early‐Middle Paleocene, but the main episode probably started in Late Eocene and may still be continuing. Emplacement of the MT had occurred by Middle‐Late Eocene, whereas the MCT shows activity around 20 Ma, thus exhibiting break back thrusting with respect to the MT. Shortening estimates are obtained from the Pindari section. Minimum shortening in the sedimentary thrust sheets of the Outer and Lesser Himalaya is 161 km or 65%. As a first approximation, we have also restored the crystalline sheets in order to obtain shortening estimates for the entire Himalaya. Minimum shortening for the Himalaya after restoring the MCT sheet varies from 354 (76%) to 421 km (79%). These estimates were further combined with published data from the area between the MCT sheet and the Indus‐Tsangpo Suture Zone (ITSZ). Minimum shortening between the Indo‐Ganga foreland and the ITSZ thus obtained lies in the range 687–754 km or 69–72%. We compare our shortening estimates with those available from the Pakistan and Nepal Himalaya.