Shallow Mantle Discontinuities Beneath the North Anatolian Fault

Abstract:

The goal of this study is to better understand deformation in the mantle lithosphere beneath continental strike-slip plate boundaries through Sp receiver function analysis of mantle discontinuities beneath the North Anatolian Fault (NAF). This fault zone marks the boundary between the Anatolian and Eurasian plates. Sp receiver functions were calculated using waveforms filtered with a variety of bandpasses, and 33 s to 4 s was found to produce coherent images while reducing interference between phases from the Moho and shallow mantle discontinuities. Using common conversion point stacking, we calculated 3D images of scattering beneath the region for two different cases: migration with the S-wave and P-wave velocities from the waveform tomography model of Fichtner et al. (2013) and with the AK135 velocity model. In both cases, a clear Moho is observed and Moho depth does not systematically vary between sides of the NAF. In contrast, shallow mantle discontinuities show pronounced changes in depth that correlate with strong lateral variations in mantle velocity observed in the Fichtner et al. tomography model, and some of these discontinuity depth variations occur within 50 km horizontally of the surface expression of the NAF. Where the tomography contains a clear high velocity lithospheric mantle layer, the Sp stack contains a negative velocity gradient at depths comparable to the base of the high velocity layer that can be interpreted as the seismological LAB. Where the tomography shows low mantle velocities that persist to the crust, a feature that may represent either anomalously low velocity lithosphere or upwelling asthenosphere beneath thin lithosphere, mantle discontinuities differ significantly from surrounding regions. Mantle discontinuity depth variations beneath the NAF also occur when the data are migrated with the AK135 model. Hence, the presence of the discontinuity depth variations is not an artifact of the assumed velocity model, although depths differ between models. These initial results suggest that blocks of lithosphere with contrasting mantle discontinuity structures are juxtaposed across the NAF system.