Seismic anisotropy under extended crust: evidence from upper mantle xenoliths, Cima Volcanic Field, California

A model of P-wave velocity anisotropy and shear-wave splitting for upper ma ntle under extended crust is derived from comparisons of seismic field meas urements and observed xenolith anisotropies. Seismic velocity measurements and anisotropies calculated from petrofabric measurements of 15 upper mantl e xenoliths from the Cima Volcanic Field, Eastern Mojave Desert, California show average maximum anisotropies of 6.9% for V-p and 4.5% for V-s. Xenoli th compressional wave velocities corrected for upper mantle temperature and pressure agree well with field observations in the vicinity of the Cima Vo lcanic Field. The velocities calculated from the olivine fabrics suggest th at upper mantle foliations beneath the region are horizontal, and lineation s trend ENE. The fabrics are believed to originate from early Cenozoic exte nsion in the areal which had the same trend as the lineations. The unusuall y high upper mantle temperatures in this region apparently have not signifi cantly affected upper mantle fabrics. (C) 2000 Elsevier Science B.V. All ri ghts reserved.