Shear zones in the Proterozoic lithosphere of the Arabian Shield and the nature of the Hales discontinuity

Seismic velocity discontinuities are commonly found within the upper 100 km of the mantle lithosphere, with great variability in their depth, lateral extent, and the polarity of velocity jump. Among the more commonly observed is the '80 km' or Hales discontinuity, identified in a variety of tectonic environments, and sometimes associated with seismic anisotropy. Teleseismic receiver functions for Global Seismographic Network station RAYN on the eastern edge of the Arabian Shield contain a strong converted-phase P(H)s that we believe arises from the Hales discontinuity at the depth of 70 km. On the basis of the polarization, azimuthal behavior and frequency dependence of P(H)s, we characterize the Hales discontinuity as marking an upper boundary of a zone of depth-dependent anisotropy within the upper mantle. The RAYN receiver functions indicate a similar zone at the crust-mantle transition at 40 km. Forward modeling in simple one-dimensional anisotropic models indicates that the receiver functions require, assuming hexagonal anisotropy, an inclined symmetry-axis with N-S strike. In our preferred model there are two anisotropic zones bounded, respectively, by the sharp Moho and the Hales discontinuities on the top. Smooth gradients of anisotropic properties extend beneath the discontinuities. Within both zones the anisotropic symmetry axes, indicative of systematic fabric in the olivine-dominated upper mantle material, are oriented along the N-S direction (+/- 10 degrees), and are tilted from the vertical 50 degrees toward the south. We propose that these regions of coherent fabric within the upper mantle represent shear zones developed during late Proterozoic continent-continent collision, when the Ar-Rayn tectonic block underwent northward 'escape' along the northern flank of the Najd fault system. (C) 2000 Elsevier Science B.V. All rights reserved.