Texture of mantle lithosphere along the Dead Sea Rift: Recently imposed or inherited?

Seismic anisotropy, a property linked to the texture of the mantle rock, should be distributed with depth along the trace of the Dead Sea Rift (DSR), owing to a combination of present day and ancient tectonics. Using data from four permanent and one temporary seismic observatories we evaluate birefringence (splitting) of 91 teleseismic core-refracted shear waves, primarily SKS phases. We find significant levels of birefringence in the bulk of observed phases. We also find that birefringence parameters (fast directions and delays) vary as a function of source-receiver geometry. Notably, the pattern of this directional variation in birefringence is quite similar at all sites we have examined. We interpret observed birefringence in SKS phases in terms of one- and two-layer models. Single-layer models for all stations exhibit a fast polarization oriented 12-19° east of north, with anisotropy sufficient to generate 1.3-s time delay. We find strong evidence for at least two distinct anisotropic layers. For the two layer models, the upper layers resemble the single-layer models, showing near-north fast polarizations and delays on the order of 1 s. Three out of four sites show fast polarizations in the lower layer that strike 50-80° CW from north with time delays 0.3-0.6 s. One site, at the northern end of the DSR, displays a higher degree of anisotropy in the lower layer, and a more northerly fast polarization. Overall, the lower layers at all sites appear to be consistent with the deformation caused by plate motion relative to the asthenosphere. The fabric in the upper layer is sub-parallel to the present-day transcurrent motion on the DSR, but also matches the typical orientation of lithospheric seismic anisotropy in the Arabian shield. Our overall conclusion is that the impact of the DSR on the rock fabric of the mantle lithosphere is probably quite weak.