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. Teleseismi c receiver functions for Global Seismographic Network station RAYN on the e astern edge of the Arabian Shield contain a strong converted-phase P(H)s th at 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 boun dary of a zone of depth-dependent anisotropy within the upper mantle. The R AYN receiver functions indicate a similar zone at the crust-mantle transiti on at 40 km. Forward modeling in simple one-dimensional anisotropic models indicates that the receiver functions require, assuming hexagonal anisotrop y, 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 propertie s extend beneath the discontinuities. Within both zones the anisotropic sym metry 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 zo nes developed during late Proterozoic continent-continent collision, when t he Ar-Rayn tectonic block underwent northward 'escape' along the northern f lank of the Najd fault system. (C) 2000 Elsevier Science B.V. All rights re served.