[S-IT16] 地球深部科学

[SIT16-P05] Decoupled deformation patterns between crust and mantle beneath Indo-Burmese Wedge inferred from shear wave splitting analysis: a comprehensive study on complex tectonics

*Debasis D Mohanty^1,2,3、Satyapriya Biswal^2,3、Kazunori Yoshizawa¹ (1.Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, Japan、2.Council of Scientific and Industrial Research (CSIR-NEIST), Assam, India 、3.Academy of Scientific and Innovative Research, New Delhi, India)

キーワード：Shear Wave Splitting, Indo-Burmese Region, Seismic Anisotropy, Mantle Deformation, Oblique Subduction, Crustal Deformation

We investigated seismic anisotropy of the crust and upper mantle beneath the Indo-Burmese Wedge (IBW) from the shear wave splitting (SWS) analysis of core-refracted phases and local direct S-waves. The IBW, a complex and major tectonic unit in the northeastern part of the Indian subcontinent, possesses complex geodynamics due to the major tectonic influences from the Indo-Eurasian collision. Pop-up dynamics of Shillong plateau and oblique subduction of Indian slab beneath the Burmese sliver plate, in its close vicinity. Though major debates remain unsolved regarding the present subduction patterns and tectonics of this region, our present study entrusts a decoupled deformation between crust and mantle, with 565 and 230 shear wave splitting (SWS) measurements from mantle and crust, respectively. While the SWS of core refracted phases point a trench parallel deformation (N-S) in the southern part of IBR due to the subduction mechanism, a major contribution from asthenosphere drag controlling the absolute plate motion (APM) of the Indian plate (NE-SW) is responsible for deformation in the northern IBR region. Contrary to the mantle anisotropy patterns, the deformation in the crust differed completely in IBW. The crustal anisotropy measurements are nearly orthogonal to the mantle deformation patterns, suggestive of a decoupled motion between the crust and mantle. The gravitational pull of huge crustal mass flow in an N-S trend from the Eastern Himalayan Syntaxis due to massive overloading and unique bending seems to be responsible for crustal deformation in the northern part of IBW, whereas the E-W trend crustal-scale wedge shortening due to oblique subduction mechanism supplements the deformation patterns in the southern mass.