Lithosphere Delamination and Small-Scale Convection Beneath California Imaged with High Resolution Rayleigh Wave Tomography Donald W. Forsyth and Yingjie Yang Brown University Forsyth, D.W., and A. Li, Array-analysis of Two-dimensional Variations in Surface Wave Velocity and Azimuthal Anisotropy in the Presence of Multipathing Interference, in Seismic Earth: Array Analysis of Broadband Seismograms, (A. Levander and G. Nolet, ed.), AGU Geophysical Monograph 157, 81-97, 2005. Yang, Y. and D.W. Forsyth, Regional tomographic inversion of amplitude and phase of Rayleigh waves with 2-D sensitivity kernels, Geophys. J. Int., 166, 1148-1160, 2006. Yang, Y. and D.W. Forsyth, Rayleigh wave phase velocities, small-scale convection and azimuthal anisotropy beneath southern California, J. Geophys. Res., 111, B07306, doi:10.1029/2005JB004180, 2006.

Rayleigh Wave Dispersion Seismograms for an earthquake on the Juan de Fuca plate boundary, Ms = 5.5 14 s 15 s 16 s 18 s 20 s 22 s 25 s 29s 33 s 40 s 50 s Seismograms filtered at different periods provide sensitivity to different depths.

Rayleigh Wave Sensitivity Kernels Depth (km) 16 s 40 s 100 s Depth of peak sensitivity increases with period. Range of sensitivity is larger for longer periods.

We use finite frequency response kernels to represent sensitivity of incoming Rayleigh waves to perturbations in structure. Triangle indicates station location. Use of amplitudes gives better control on phase velocity through focusing and defocusing effects. These kernels represent response to a local velocity increase.

Propagate Rayleigh waves from all azimuths through random velocity field with imbedded checkerboard. Seismometers only within checkerboard region. (Yang and Forsyth, GJI, 2006) km

Relative amplitudes within checkerboard region for initially plane Rayleigh wave propagating from north. Period is 50 s. Example of amplitude variations in MELT array for event propagating from Kuriles. Represent incoming wavefield as sum of two plane waves. Focusing effects from heterogeneities within and near array represented by response kernels.

Top, inversion for phase velocity with no external heterogeneities Top, inversion for phase velocity with no external heterogeneities. Triangles indicate station locations. Bottom, inversion with random external heterogeneities.

Station amplitude correction factors for Rayleigh waves at 25 s.

We use well-distributed sources to perform surface wave tomography within S. California. Average phase velocities as a function of period are inverted to give a reference model of shear velocity versus depth. There is a high velocity lid or lithosphere overlying a pronounced low velocity zone.

At each period from 25 s to ~145 s we determine the lateral variations in phase velocities using finite frequency sensitivity kernels for both amplitude and phase. A sampling of some of the periods shows gradual evolution in pattern from one period to the next. Longer periods penetrate deeper into the earth. Note very low velocities beneath eastern edge of the Sierra Nevada at short periods. Resolution degrades somewhat towards edges of maps, but 1% variation is significant at ~ 95% confidence level.

A slice near the base of the lithosphere shows pattern of both upwellings and downwellings. The low velocity anomaly beneath the Sierra Nevada and Walker Lane (SNWLA) indicates delamination of the mantle part of the lithosphere. GVA is Great Valley anomaly, sometimes called Central Valley anomaly or Isabella anomaly, which has been described as a lithospheric drip. ETRA and WTRA are Eastern and Western Transverse Range anomalies. STA is Salton Trough anomaly.

At 70 to 90 km, low velocity region coincides with region of Quaternary volcanism (black dots). Dashed line indicates extent of high-K volcanism at ~ 3.5 Ma. Solid line indicates total extent of volcanism at that time (after Manley et al., Geology, 2000). Note more circular shape of the Great Valley anomaly.

Great Valley drip merges with high velocity anomaly beneath eastern Sierra Nevada at 110-130 km depth and is absent deeper than 130 km. Delaminated lithosphere beneath the Sierras seems to have sunk vertically. Transverse Range anomaly clearly separates into two separate drips. Western one dips to the north. Although perhaps initiated by shortening across San Andreas bend, the pattern of anomalies does not appear to be kinematically driven by surface plate motions. The Peninsular Range may have undergone delamination similar to that in the Sierra Nevada. At 70-90 km, low velocities beneath eastern PR merges with Salton Trough anomaly. High velocity anomaly begins at ~130 km and strengthens downward.

Great Valley drip may be connected to delaminated lithosphere beneath Sierra Nevada and Owens Valley. Very low velocities at shallow depths indicate partial melt. Transverse Range anomalies separate into two separate drips that extend no deeper than 150 km. Peninsular Range delamination does not extend to as shallow depths as beneath Sierra Nevada and top of delaminated lithosphere is deeper.

Conclusions Small-scale convection is active beneath Southern California in a complex, 3-D pattern with scale on the order of 100 km Delamination may have occurred beneath the Peninsular Ranges in addition to the eastern Sierra Nevada Downwelling may be largely vertical and tends to take the form of drips, but dripping lithosphere may still be connected to lithosphere at the surface (Transverse Ranges and Great Valley anomaly) Shear velocities below the lithosphere and within delaminated regions are as low as 4.0 km/s, suggesting the presence of partial melt