Teleseismic Shear wave splitting and lithospheric anisotropy beneath and across the Altyn Tagh Fault

Sixteen 3‐component, 5‐sec stations along a profile crossing the Altyn Tagh fault (ATF) at ∼90°E were used to measure shear wave splitting beneath Tibet's northern edge. Splitting delay times (δt) of 0.3 to 1.3 sec show a marked increase in a narrow zone centered on the fault, implying anisotropy in the lithospheric mantle. Fast Polarization Directions (FPD) (ϕ) swing counterclockwise, from a mean azimuth of ∼110°E in the Qaidam basin south of the fault, to a mean azimuth of ∼60°E in the Altyn mountains. This swing occurs in less than 40 km. Both the δt increase and FPD swing are consistent with ductile sinistral shear along the 40 km‐wide, low P‐wave velocity anomaly (−6–8%) imaged to a depth of ∼140 km beneath the fault by teleseismic tomography. This confirms that the ATF guides the northeastwards extrusion of the Tibet‐Qaidam's lithosphere relative to the Tarim's.