Structure and shortening of the Kangra and Dehra Dun reentrants, Sub-Himalaya, India

Graduation date: 1997Surface-geology, oil-well, seismic-reflection, and magnetostratigraphic data are\ud integrated to evaluate structural style and shortening rate at the Himalayan front (Sub-Himalaya) of northwest India. The Sub-Himalaya, between the Main Boundary thrust\ud and the Himalayan Frontal fault, is the actively deforming front of the Himalaya. At\ud certain locations, the Himalayan Frontal fault is a blind thrust beneath anticlines of\ud Siwalik (Tertiary) molasse, parallel to the Himalayan arc. The Main Boundary thrust, in\ud contrast, is sinuous, so that the width of the Sub-Himalaya ranges from 30 to 80 km.\ud Where the Sub-Himalaya is narrow (Nahan salient), Tertiary rocks are exposed in\ud imbricate thrust sheets; where the Sub-Himalaya is broad (Kangra and Dehra Dun\ud reentrants), alluvium fills wide synclinal valleys (duns).\ud Seismic-reflection data reveal that surface anticlines form in association with\ud south-vergent thrusts that root in a decollement at the base of the Tertiary section.\ud Reflection profiles and well data also indicate that the basement lithology changes\ud northward from Precambrian crystalline rocks beneath the Indo-Gangetic plains to\ud Precambrian and Cambrian metasedimentary rocks beneath the Sub-Himalaya. The\ud Sub-Himalayan decollement dips 2.5° northward beneath the Kangra reentrant, but it is\ud steeper at 6° beneath the Dehra Dun reentrant.\ud The Kangra and Dehra Dun reentrants are characterized by fault-propagation\ud folds with steep limbs in the north and by broad anticlines with gently north-dipping\ud limbs in the south. A balanced cross section of the Kangra reentrant shows that a\ud minimum of 23 km shortening has occurred since 1.9-1.5 Ma, yielding a shortening rate\ud of 14±2 mm/yr. Shortening has occurred at a rate of 7-15 mm/yr across the Dehra Dun\ud reentrant. These data compare with other published shortening rates and indicate that [approximatelyt] 25% of the total India-Eurasia convergence is accommodated within the Sub-Himalaya of northwest India