Evidence for asthenospheric partial melt beneath the Anatolian region: Constraints from Sp receiver functions

Seismic velocity gradients beneath the Anatolian region have been imaged by Sp receiver function common-conversion point stacking, helping to reveal the properties of the lithosphere and asthenosphere. P and SV waveform components were calculated with a free-surface transform, and the free-surface velocities were determined with a new method that takes advantage of P and S wave free-surface behavior. We obtained 18,992 individual receiver functions with time-domain deconvolution after careful quality control, and mapped them to three-dimensional space with common-conversion point stacking using a weighting scheme based on Sp sensitivity kernels. Strong negative Sp phases are observed at depths of 100 - 150 km depth over much of the region, consistent with the lower boundary of a low velocity asthenospheric layer constrained by full-waveform inversion (Fichtner et al., 2013). Sp phases also clearly image the Moho and the 410-discontinuity. However, unambiguous positive Sp phases indicative of a localized velocity decrease at the lithosphere-asthenosphere are not widely observed, suggesting that the mantle lithosphere is too warm to produce a large velocity contrast with the underlying asthenosphere. To investigate the origins of the low-velocity asthenospheric layer, we jointly inverted the Sp receiver function CCP stack and surface wave phase velocities across the region, using the full-waveform tomography as a starting model. When the resulting shear wave velocity model is scaled to temperature, it implies an anomalously high mantle potential temperature. The intersection of the mantle adiabat and a nearly anhydrous solidus lies just below the depth of the strong negative Sp arrival, implying that the anomalously low asthenospheric velocities are in part due to partial melt. Melt fractions required to match asthenospheric velocities are less than 1%.