HR: 10:40h
AN: U52A-02    [Abstracts]
TI: A Two-Layer Model of Deformation Along the San Andreas Fault System: Evidence from Shear-Wave Splitting, GPS, and Geologic Data
AU: * Flesch, L M
EM: lmflesch@purdue.edu
AF: Dept. of Earth & Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015 United States
AB: Several SKS shear-wave splitting studies have observed two distinct anisotropic layers along the northern section of the San Andreas Fault System (SAFS), a feature only observed at station LAC along the southern portion of the SAFS, and not observed within the remainder of the western United States where only a single anisotropic layer is measured. We investigate the two-layer phenomena to understand the sources of deformation along the SAFS region with depth, and determine the factors that control the development of a second anisotropic layer. Because the direction of the SKS shear-wave splitting fast polarization direction, Φ, can be inferred to reflect the direction of maximum shear, we first compare existing two-layer estimates of the top and bottom layers, φ_t and φ_b, to the predicted directions of maximum shear for an upper lithospheric and lower athenospheric source. For the top anisotropic layer, it is assumed that the lithosphere deforms like a thin viscous sheet, in which case the maximum shear orientation determined from GPS and Quaternary fault slip rates should be parallel to φ_t. For an asthenospheric source, we assume that φ_b is parallel to the difference vector between an unknown uniform sub-asthenospheric velocity field and the base-of-lithosphere velocity field, taken to be equal to the observed surface velocity field. Using the data set of two-layer solutions where φ_t was assumed to be parallel to SAFS-related shear, we then inverted for the sub-asthenospheric velocity that provides the best fit to φ_b. The resultant sub-athenospheric velocity field is similar to that determined by Silver and Holt [2002] for a much larger data set that was dominated by single-layer observations, thus compatible with a model containing these two sources of anisotropy. There is, however, a misfit in φ_b of 11° much of which is systematic and increases closer to the fault, and may represent interaction in the mantle between vertical shear from the SAFS and horizontal shear produced from the motion of the sub-athenospheric mantle. Finally, we observe a direct correlation between magnitude of observed surface strain rates and regions where two-layer anisotropy is observed. Regions with strain rate magnitudes equal to or in excess of 3x10^-7/yr (northern SAF, a section of the Eastern California Shear Zone, and Baja California) all produce two-layer anisotropy. Thus strain rate magnitudes less than 3x10^-7/yr may not be large enough to generate a distinct fabric within the lithospheric mantle.
DE: 8106 Continental margins: transform
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Union [U]
MN: Fall Meeting 2005