Seismic anisotropy of mantle xenoliths and constraints on upper mantle structure beneath the southern Canadian Cordillera

Seismic properties of upper mantle xenoliths from the southern Canadian Cor dillera were determined by numerical calculations from petrofabric analyses in order to provide constraints on the structural interpretation of field seismic data over the region. The samples studied include harzburgite, spin el lherzolite, wehrlite and websterite. The average anisotropy calculated f rom petrofabrics is 7.5% for V-p and 4.8% for V-s. Comparison of xenolith d ata with SKS and refraction/wide-angle reflection data yields possible stru ctural orientations in the upper mantle beneath the southern Canadian Cordi llera. Our results suggest that foliation in the asthenosphere is likely ho rizontal, and the stretching lineation (X-direction) is oriented NEE, which coincides with the absolute plate motion (APM) of the North American conti nent. The lack of Pn anisotropy in the region may imply lithologic and stru ctural heterogeneity of the lithospheric upper mantle, which results in an apparent isotropy at regional scales. Our data also suggest that the upper mantle reflection at a depth of similar to 50 km revealed on wide-angle dat a may be caused by the anisotropy of underlying asthenosphere. (C) 2001 Els evier Science B.V. All rights reserved.