Graduate Thesis Or Dissertation
 

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

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  • Surface-geology, oil-well, seismic-reflection, and magnetostratigraphic data are 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 and the Himalayan Frontal fault, is the actively deforming front of the Himalaya. At certain locations, the Himalayan Frontal fault is a blind thrust beneath anticlines of Siwalik (Tertiary) molasse, parallel to the Himalayan arc. The Main Boundary thrust, in contrast, is sinuous, so that 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 at 6° beneath the Dehra Dun reentrant. The Kangra and Dehra Dun reentrants are characterized by fault-propagation folds with steep limbs in the north and by broad anticlines with 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 7-15 mm/yr across the Dehra Dun 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.
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