THRUST STACKS, SEISMOTECTONICS AND GRAVITY MODELLING IN THE HIMALAYA

The Himalaya typify a continent-continent collision mountains system built as thrust stacks of various ages. They are the locale of earthquakes of various magnitudes including ۵ of higher than ۸ magnitude since ۱۸۹۱. From north to south, the major thrusts are the Indus Tsangpo Suture, Main Central Thrust, Main Boundary Thrust, and Main Frontal Thrust. Since ۱۹۸۲, a number of teams have carried out multidisciplinary geophysical investigations in the Himalaya that have "delineated major seismotectonic features. These studies provide constraints for gravity modelling and help reduce ambiguity associated with potential field data interpretation. The Bouguer anomalies, as expected, continually decrease with the increasing elevation to -۵۰۰ mGals over the Himalaya from an average of -۴۰ to -۵۰ mGals ove,r,. the northern Indian Shield. Removal of the isostatic effect or crustal thickening leaves +۱۰۰ mGals isostatic anomalies over the Himalaya which can be explained by thickening of the lower denser crust. When the regional effect of the isostatic roots is removed, the residual anomalies can be further modelled by assuming imbricated thrusts and undualations of the Precambrian basement. Thus by gravity data two structural elements are modelled. One at the deeper level where lower crust and or oceanic crust is presumed to be obducted to give positive isostatic anomaly. The other envisages juxtaposition of supracrustal rocks of differing densities in a thrust regime. The gravity modelling also leads to the inference of an under-thrust Tertiary Sedimentry Basin beneath the Main Boundary Thrust. These inferences may help seismotectonic studies in the Himalaya and its Indus-Ganga-Brahmputra Foredeep (IGBF).