Uplift and tectonics of the Shillong Plateau, Northeast India

Abstract

Different views prevail regarding the uplift and tectonics of the Shillong Plateau in northeastern India. In light of these, we discuss the cause of the current uplift, the time of initiation of the uplift, and the current tectonic scenario. Various geophysical results indicate that neither a thermal anomaly nor isostatic compensation could have caused the uplift as suggested by many workers in the past. Several coinciding factors point towards a combination of tectonic forces in this unique thrust environment, comprising the Himalayan thrust and the Burmese thrust, to be responsible instead. Stratigraphic evidence suggests that the initiation of the current uplift was during Mio-Pliocene. While most tectonic models attribute the Shillong Plateau uplift to a N-S compressive stress regime, it appears that an E-W compressive stress owing to the Indo-Burman subduction active during that period also aided the uplift by providing the necessary and timely impetus. Even at present, a small component of stress in the E-W direction seems to be operational as inferred from a computation of strain rates and velocity vectors. The predominance of thrust type focal mechanisms and their P axis orientations possibly indicate that the uplift is sustained by compressive forces acting on the plateau from all sides.