Rayleigh Wave Phase Velocities, Small-Scale Convection and Azimuthal Anisotropy beneath Southern California

Rayleigh Wave Phase Velocities, Small-Scale Convection and Azimuthal Anisotropy beneath Southern California Distribution of volcanism (black dots) in southern Sierra Nevada during the Quaternary (1.5-0 Ma) period. Bold solid line outlines area with which Pliocene (chiefly 4-3 Ma) volcanism was prevalent; note that Quaternary volcanic fields (LV-Long Valley, BP-Big Pine, GT-Golden Trout, C-Coso) are all within area of Pliocene event. Dashed line outlines the area of Pliocene potassic volcanism 4-3 Ma. Colors show shear-wave velocity anomalies at depths of 70-90 km. Note that the Quaternary volcanism coincides with the region of lowest velocities.
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We use Rayleigh waves to invert for shear velocities in the upper mantle beneath southern California [Yang and Forsyth, 2006]. A one-dimensional shear velocity model reveals a pronounced low- velocity zone (LVZ) from 90 to 210 km. The pat- tern of velocity anomalies indicates that there is active small-scale convection in the asthenosphere and that the dominant form of convection is three-dimensional (3-D) lithospheric drips and asthenospheric upwellings, rather than 2-D sheets or slabs. Several of the features that we observe have been previously detected by body wave tomography: these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern Sierra Nevada and Walker Lane region; sinking lithosphere beneath the southern Central Valley; upwelling beneath the Salton Trough; and downwelling beneath the Transverse Ranges. Our new observations provide better constraints on the lateral and vertical extent of these anomalies. In addition, we detect two previously undetected features: a high-velocity anomaly beneath the northern Peninsular Range and a low-velocity anomaly beneath the northeastern Mojave block. We also estimate the azimuthal anisotropy from Rayleigh wave data. The strength is 1.7% at periods shorter than 100 s and decreases to below 1% at longer periods. The fast direction is nearly E-W. The anisotropic layer is more than 300 km thick. The E-W fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms: pure shear in the lithosphere due to N-S tectonic shortening and simple shear in sublithosphere mantle due to mantle flow.
</p><p>References
</p><p>Yang, Y. and D.W. Forsyth, Rayleigh wave phase velocities, small-scale convection and azimuthal anisotropy beneath southern California, J. Geophys. Res., 111, B07306,2006
</p><p>Acknowledgements: This work was supported by National Science Foundation grants OCE-9911729 and EAR-0510621.</p>

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