2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 5
Presentation Time: 9:00 AM


FORSYTH, Donald W. and YANG, Yingjie, Geological Sciences, Brown University, Box 1846, Providence, RI 02912, Donald_Forsyth@brown.edu

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 finer features in the shear wave structure than has been possible previously. 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. 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 the dripping lithosphere beneath the 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. Previously unknown features include: a high velocity anomaly dipping to the north, centered beneath the Channel Islands at depths of 60 to 120 km; a high velocity anomaly at depths greater than 120 km beneath the Peninsular Range at the California-Mexico border; 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. There is dynamic convection in the asthenosphere on a scale as fine as can be resolved by the surface wave data - a few hundred km or less.