Abstract
Surface geology, oil-well, seismic-reflection, and magnetostratigraphic data are integrated to evaluate the structural style and the shortening rate at the Himalayan front (Sub-Himalaya) of northwest India. The Sub-Himalaya, between the Main Boundary thrust and the Himalayan Frontal fault, is the primary surface expression of shortening between the Himalaya and the Indian plate. At certain locations, the Himalayan Frontal fault is a blind thrust beneath anticlines of Siwalik (Tertiary) molasse, parallel to the Himalayan orogen. The Main Boundary thrust is sinuous, so the width of the Sub-Himalaya ranges from 30 to 80 km. Where the Sub-Himalaya is narrow (Nahan salient), Tertiary rocks are exposed in imbricate thrust sheets; where the Sub-Himalaya is broad (Kangra and Dehra Dun reentrants), alluvium fills wide synclinal valleys (duns).
Seismic-reflection data reveal that surface anticlines form in association with south-vergent thrusts that root in a decollement at the base of the Tertiary section. Reflection profiles and well data also indicate that the basement lithology changes northward from Precambrian crystalline rocks beneath the Indo-Gangetic plains to Precambrian and Cambrian metasedimentary rocks beneath the Sub-Himalaya. The Sub-Himalayan decollement dips 2.5° northward beneath the Kangra reentrant, but it is steeper, 6°, beneath the Dehra Dun reentrant.
The Kangra and Dehra Dun reentrants display fault-propagation folds having steep limbs in the north, and fault-propagation and fault-bend folds that have gently north-dipping limbs in the south. A balanced cross section of the Kangra reentrant shows that a minimum of 23 km shortening has occurred since 1.9–1.5 Ma, yielding a shortening rate of 14 ± 2 mm/yr. Shortening has occurred at a rate of 6–16 mm/yr across the Dehra Dun reentrant. These data are similar to other published shortening rates and indicate that approximately 25% of the total India-Eurasia convergence at this longitude is accommodated within the Sub-Himalaya. Given continued convergence and the presence of overpressured wells in the Kangra reentrant, the region is likely at risk from moderate and/or great earthquakes in the future.
