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Abstract.The issue of whether subduction is still active in the India-Burma plate boundary zone has been rather controversial. While the presence of an eastward dipping Indian lithospheric slab is undisputed, different opinions have been voiced regarding the continuance of subduction at present. Analysis of the Harvard CMT data in comparison with major subduction zones of the world demonstrates that the Burmese arc is a unique region where there is a subducted slab but the direction of plate motion is nearly perpendicular to the down-dip direction. We propose a major right-lateral shearing of the Indian plate along with its subducted slab past the Burmese plate in the NNE direction.

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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.

We incorporate the effects of anisotropy to refine the continental‐scale 3‐D isotropic velocity model previously produced for India and Tibet by inverting 52,050 teleseismic P wave residuals. We have exploited a total of 1648 individual SKS splitting parameters to calculate the P wave travel time corrections due to azimuthal anisotropy. Our results suggest that anisotropy affects the P wave delays significantly (−0.3 to +0.5 s). Integration of these corrections into the 3‐D modeling is achieved in two ways: (a) a priori adjustment to the delay time vector and (b) inverting only for anisotropic delays by introducing strong damping above 80 km and below 360 km depths and then subtracting the obtained anisotropic artifact image from the isotropic image, to get the corrected image. Under the assumption of azimuthal anisotropy resulting from lattice preferred orientation (LPO) alignment due to horizontal flow, the bias in isotropic P wave tomographic images is clear. The anisotropy corrected velocity perturbations are in the range of ±1.2% at depths of around 150 km and reduced further at deeper levels. Although the bias due to anisotropy does not affect the gross features, it does introduce certain artifacts at deeper levels.