SOURCE-SIDE SHEAR WAVE SPLITTING AND UPPER MANTLE FLOW BENEATH THE ARAKAN SLAB, INDIA-ASIA-SUNDALAND TRIPLE JUNCTION REGION

Shear wave splitting of S waves from earthquakes in the Arakan slab are consistent with strong asthenospheric anisotropy beneath and adjacent to the slab. Indian Ocean lithosphere comprising the Arakan slab is dismembered into three segments as a result of its collision with Asian lithosphere at the East Himalayan syntaxis. GPS site velocities in the India-Asia-Sundaland triple junction region show that deformation along the Arakan subduction zone is partitioned into a strong dextral strike-slip motion, as India moves northwards with respect to Asia, and contraction across the Arakan trench and Chittagong-Tripura fold Belt. Offsets of intermediate depth earthquake hypocenters at two locations delineate slab segments that form a left-stepping en echelon structure. Arakan slab focal mechanisms include dextral strike-slip along the slab and also regions of N-S contraction within the slab, particularly at the northern slab segment offset. Teleseismically-recorded S waves from earthquakes within the three slab segments, and surroundings, are split systematically: once corrected for receiver-side splitting, fast shear trends are predominantly trench-parallel beneath the east-dipping slab segments; are more nearly trench-normal on the Sundaland (east) side of the Arakan lithosphere; parallel the southern ~E-W gap between Arakan slab segments; and turn sharply around the extreme northern and southern edges of subducted Arakan lithosphere. Source-side shear wave splitting beneath India is consistent with limited ~E-W trending fast shear polarizations of SK(K)S splitting in northeastern India. The general pattern of both surface site velocities from GPS and upper mantle flow is consistent with material flow around the eastern Himalayan syntaxis and into the mantle wedge above the Arakan slab, and around the northern terminus of the Arakan slab. Further to the south, upper mantle may flow through the gap between the central and southern Arakan slab segments.