Shear-wave splitting of S waves from earthquakes in the Arakan slab is consistent with strong asthenospheric anisotropy that developed as a consequence of the India-Asia collision. Global positioning system (GPS) site velocities in the India-Asia-Sundaland triple junction region show that deformation along the Arakan subduction zone is partitioned into dextral strike-slip motion, as India moves northwards with respect to Asia, and contraction across the Arakan trench and Chittagong-Tripura fold belt. 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. Offsets of intermediate-depth earthquake hypocenters at two locations delineate slab segments that form a left-stepping en echelon structure. Arakan slab focal mechanisms are consistent with slab sinking and along-strike compression and bending, and, south of 25°N, dextral strike-slip along the slab. Two regions of N-S contraction within the slab appear to be localized at the slab segment offsets. 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 observed ∼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 shear-wave splitting studies 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. The upper mantle may also flow through the gap between the central and southern Arakan slab segments.
- Received 15 July 2009.
- Revision received 26 September 2011.
- Accepted 16 October 2011.
- © Geological Society of America