STRUCTURE AND SHORTENING OF THE KANGRA AND DEHRA DUN REENTRANTS, SUB-HIMALAYA, INDIA

Surface geology, oil-well, seismic-reflection, and magnetostratigraphi c data are integrated to evaluate the structural style and the shorten ing rate at the Himalayan front (Sub-Himalaya) of northwest India. The Sub-Himalaya, between the Main Boundary thrust and the Himalayan Fron tal fault, is the primary surface expression of shortening between the Himalaya and the Indian plate. At certain locations, the Himalayan Fr ontal fault is a blind thrust beneath anticlines of Siwalik (Tertiary) molasse, parallel to the Himalayan orogen. The Main Boundary thrust i s sinuous, so the width of the Sub-Himalaya ranges from 30 to 80 km, W here the Sub-Himalaya is narrow (Nahan salient), Tertiary rocks are ex posed in imbricate thrust sheets; where the Sub-Himalaya is broad (Kan gra 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 t he base of the Tertiary section. Reflection profiles and well data als o indicate that the basement lithology changes northward from Precambr ian crystalline rocks beneath the Indo-Gangetic plains to Precambrian and Cambrian metasedimentary rocks beneath the Sub-Himalaya. The Sub-H imalayan decollement dips 2.5 degrees northward beneath the Kangra ree ntrant, but it is steeper, 6 degrees, beneath the Dehra Dun reentrant. The Kangra and Dehra Dun reentrants display fault-propagation folds h aving steep limbs in the north, and fault-propagation and fault-bend f olds that have gently north-dipping limbs in the south,A balanced cros s section of the Kangra reentrant shows that a minimum of 23 km shorte ning 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 shorte ning rates and indicate that approximately 25% of the total India-Eura sia convergence at this longitude is accommodated within the Sub-Himal aya Given continued convergence and the presence of overpressured well s in the Kangra reentrant, the region is likely at risk from moderate and/or great earthquakes in the future.