CALIBRATION OF SHEAR-WAVE SPLITTING IN THE SUBCONTINENTAL UPPER-MANTLE BENEATH ACTIVE OROGENIC BELTS USING ULTRAMAFIC XENOLITHS FROM THE CANADIAN CORDILLERA AND ALASKA

Despite the abundance of measurements of shear-wave splitting around t he world, some fundamental questions about its geological interpretati on have not yet been answered. In order to constrain (i) the orientati on and magnitude of S-wave anisotropy, (ii) the thickness of the aniso tropic layer and (iii) the possible variation of the anisotropy with d epth in the subcontinental upper mantle beneath active orogenic belts, we have carried out a systematic investigation on shear-wave properti es of mantle xenoliths from recent alkaline basalts of the Canadian Co rdillera and Alaska. It is found that the polarization direction (Phi) of the fast S-wave as well as the, time delay (delta t) between the t wo arrivals are strongly dependent on the propagation direction with r espect to the structural frame. At a single station, the variations of measured Phi and delta t values for SKS phases from different events can result from this dependence. Because the average grain size of oli vine in upper mantle samples is commonly larger than 0.5 mm, dislocati on creep prevails over a thickness of at least 250 km and results in o livine LPO and hence anisotropy and shear-wave splitting. A single thi ck anisotropic layer in mountain-parallel, subvertical shear zones may cause high values of delta t, while multiple, subhorizontal layers wi ll produce multiple splitting, making the measurement and interpretati on of S-wave splitting difficult. Our results also suggest that the S- wave splitting observed in cold, old, stable cratons such as the Canad ian Shield is likely dominated by fossil anisotropy, whereas that obse rved in hot, active orogenic regions, such as the Northwest American C ordillera and the Tibetan Plateau, may be dominated by modern tectonic deformation in the lithosphere and flow in the asthenosphere.