Source-side shear-wave splitting and upper-mantle fl ow beneath the Arakan slab, India-Asia-Sundalind triple junction

摘要

Shear-wave splitting of S waves from nearthquakes in the Arakan slab is consistent nwith strong asthenospheric anisotropy that ndeveloped as a consequence of the India-Asia ncollision. Global positioning system (GPS) nsite velocities in the India-Asia-Sundaland ntriple junction region show that deforma-ntion along the Arakan subduction zone is npartitioned into dextral strike-slip motion, as nIndia moves northwards with respect to Asia, nand contraction across the Arakan trench nand Chittagong-Tripura fold belt. Indian nOcean lithosphere comprising the Arakan nslab is dismembered into three segments as na result of its collision with Asian lithosphere nat the East Himalayan syntaxis. Offsets of nintermediate-depth earthquake hypocenters nat two locations delineate slab segments that nform a left-stepping en echelon structure. nArakan slab focal mechanisms are consistent nwith slab sinking and along-strike compres-nsion and bending, and, south of 25°N, dex-ntral strike-slip along the slab. Two regions of nN-S contraction within the slab appear to be nlocalized at the slab segment offsets. Teleseis-nmically recorded S waves from earthquakes nwithin the three slab segments, and sur-nroundings, are split systematically: once cor-nrected for receiver-side splitting, fast shear ntrends are predominantly trench-parallel nbeneath the east-dipping slab segments; are nmore nearly trench-normal on the Sundaland n(east) side of the Arakan lithosphere; paral-nlel the southern ~E-W gap between Arakan nslab segments; and turn sharply around the nextreme northern and southern edges of nsubducted Arakan lithosphere. Source-side nshear-wave splitting beneath India is con-nsistent with observed ~E-W–trending fast nshear polarizations of SK(K)S splitting in nnortheastern India. The general pattern of nboth surface site velocities from GPS and shear-wave splitting studies is consistent with nmaterial fl ow around the eastern Himalayan nsyntaxis and into the mantle wedge above the nArakan slab, and around the northern ter-nminus of the Arakan slab. The upper mantle nmay also fl ow through the gap between the ncentral and southern Arakan slab segments.