Tectonic stress field in the Indian subcontinent

Abstract

A map of maximum horizontal compressive stress orientation in the Indian subcontinent has been prepared using orientations derived from three different stress indicators: borehole elongation breakouts, in situ hydraulic fracturing measurements, and earthquake focal mechanisms. Most part of the subcontinent appears to be characterized by a compressional stress regime (thrust and strike-slip faulting) imposed by plate boundary forces although S_Hmax orientations do not, in general, show clear correlation with the direction of motion of the Indian plate. Four provinces are recognized on the basis of regionally consistent orientations. These are the midcontinent stress province, the southern shield, the Bengal basin, and the Assam wedge. Their boundaries have been determined taking into consideration regional tectonics and seismicity. Central and northern India, including the Shillong Plateau stretching up to the great Himalaya, Pakistan, and Nepal are included in a broad midcontinent stress province characterized by NNE-ENE oriented S_Hmax.The mean orientation of S_Hmax in this province is N23°E, subparallel to the direction of compression expected to arise from he net resistive forces at the Himalayan collision zone, suggesting that it is largely determined by the tectonic collision processes. Much of southern India (Mysore plateau and the high-grade metamorphic terrain south of the plateau) appears to be part of a second stress province characterized by NW oriented S_Hmax. These appear close to those of the intraplate stress field prevailing in the central Indian Ocean. A third stress province was recognized in the Bengal basin including parts of West Bengal, Tripura, Manipur, and Mizoram in northeastern India and most of Bangladesh. This province extends eastward from the marginal fault in the western margin of the Bengal basin to the Indo-Burma subduction zone and is bounded on the north by the E-W striking Dauki fault.S_Hmax within the sedimentary pile of the Bengal basin is oriented in E-W direction, while P axes of earthquakes within the basement and the crust beneath the basin and within the subducted slab of the Indian plate beneath the Indo-Burman ranges generally trend north-N30°E. S_Hmax orientations within the sedimentary pile of the basin are parallel to the local (approximately E-W) direction of the convergence of Indian and Burmese plates, suggesting a casual relation to the resistive forces at the subduction zone in he Indo-Burma region. Interestingly, the stress field in the basement and the crust beneath the Bengal basin and in the subducted slab is similar to the one prevailing in the midcontinent stress province. Assam wedge stress region occupies the northeastern corner of the Indian plate, including Upper Assam, Arunachal Pradesh, and much of Nagaland. This region subducts beneath a sharply bent continental collision boundary consisting of the northeastern limb of the Himalayan and northern limb of the Indo-Burman fold belts. As a result, the stress field in this province is depth-differentiated and most likely responsible for the absence of consistent S_Hmax directions.