Seismic Anisotropy Beneath the Pamir and the Hindu Kush: Evidence for Contributions From Crust, Mantle Lithosphere, and Asthenosphere

Abstract

We use local and teleseismic earthquakes to analyze shear wave splitting within the Pamir-Hindu Kush region, north of the western syntaxis of the India-Asia collision zone. These two data sets allowed us to map the distribution of azimuthal anisotropy, to put constraints on the depth range where it is accumulated, and to deduce characteristics of ongoing deformation. From 1,073 SKS (core-mantle refracted phases) measurements at 107 stations, we derived time delays of 0.7–2.25 s and dominantly ENE-WSW oriented fast polarization directions. Fast polarization directions only deviate adjacent to the subducting slabs and major strike-slip faults, aligning parallel to these structures. From 461 direct S measurements along a transect perpendicular to the Pamir seismic zone, we obtain fast directions parallel to those from SKS measurements but smaller delay times (average 0.4 s), which vary depending on depth. Time delays exhibit 0.1–0.3 s crustal contribution and increase to 0.8 s in a narrow domain coinciding with the inferred subcrustal contact of the two colliding plates. We find measurements from the same event-station paths at different filter frequencies to be frequency-independent, allowing a comparison with SKS results along the studied profile. The smaller average time delays of local events imply that the crust and uppermost mantle only make a minor contribution to the SKS splitting. Thus, the coherent fast direction pattern suggests a strain field dominated by the indentation of India and the escape of sublithospheric material north of the indenter. Crustal anisotropy is likely also controlled by this regional deformation pattern with locally highest strain rates closest to the continental subduction front.