HR: 09:15h
AN: T21D-06    [Abstracts]
TI: Small-scale Convection and Anisotropy in the Mantle Beneath Southern California From High-Resolution Rayleigh Wave Tomography
AU: * Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Dept. of Geological Sciences, Brown University, Box 1846, Providence, RI 02912 United States
AU: Yang, Y
EM: YingjiE_yang@brown.edu
AF: Dept. of Geological Sciences, Brown University, Box 1846, Providence, RI 02912 United States
AB: High-resolution imaging of the upper mantle beneath central and southern California reveals many features associated with delaminated lithosphere, lithospheric drips, and associated small-scale convection in the underlying asthenosphere. Using amplitude and phase of Rayleigh waves from teleseismic events recorded at stations of the USArray/Trinet network and finite-frequency sensitivity kernels, we resolve features in the shear wave structure on a horizontal scale of ~100 km down to a depth of about 200 km. The continous coverage provided by surface waves yields better resolution of the shape of known anomalies reported by others, as well as the detection of previously unknown features of apparent convective origin. Despite the evidence of a 3-D convective pattern, shear wave-splitting directions are relatively uniform, perhaps due to the strong anisotropy in the lithosphere revealed by Rayleigh wave azimuthal anisotropy. The images reveal very low velocities requiring the presence of melt beneath the volcanic fields of the southern Sierra Nevada and Walker Lane, centered at a depth of 50-70 km. The lithosphere beneath this region has been removed, with a corresponding high-seismic-velocity lithospheric drip existing now beneath the adjacent southern Great Valley, which we image at depths of 70 to 120 km. This Great Valley drip may still be connected to foundering lithosphere at depths of 110 to 160 km beneath the southern Sierra Nevada. We also image dripping lithosphere beneath the eastern and western Transverse Ranges at depths of roughly 50 to 130 km and anomalously low velocities near the Salton trough from near the surface to 100 km or more. There is a high velocity anomaly at depths greater than 120 km beneath the Peninsular Range at the California-Mexico border that may be a detached drip, and a low velocity anomaly beneath the eastern edge of the Mojave block, trending north into the Basin and Range Province at depths greater than 120 km.
DE: 7255 Surface waves and free oscillations
DE: 8110 Continental tectonics: general (0905)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Tectonophysics [T]
MN: Fall Meeting 2005