ANISOTROPY AT THE BASE OF THE SIERRA NEVADA BATHOLITH AS INFERRED FROM PASSIVE SEISMIC OBSERVATIONS: TRIGGER OR RESULT OF ROOT REMOVAL

Anisotropy within the crust can result from the presence of dikes or the alignment of either minerals or fractures. Examination of azimuthal patterns in the timing and amplitude of receiver function arrivals has proven to be an effective method for characterizing crustal structure and fabrics. Methods based upon this approach have been used to distinguish between dipping layers, anisotropic layers, anisotropic layers with plunging axes of symmetry, as well as combinations of these. Seismograms from both the 1993 Southern Sierra and 1997 Sierran Paradox Continental Dynamics Projects reflect anisotropy in the lower crust. Amplitudes of receiver function arrivals from a mid-crustal seismic converter vary with azimuth by a factor of three; this variation was interpreted by Jones and Phinney (1998) as caused by anisotropic fabrics generated by WNW directed shear on a low-angle normal fault extending down under the eastern Sierra from exposures in the Death Valley area. Somewhat similar anisotropy, albeit with a noticeable plunge, was inferred by Zandt et al. (2004) from variations in both radial and transverse receiver functions from stations across much of the eastern and central portion of the Southern Sierra. Interestingly, however, evidence for anisotropy was not found for stations in the western Sierra. Combining this observation with other evidence that a seismically fast dense blob is present below the western Sierra lead to their preferred interpretation; that the anisotropy reflects melt-filled cracks developed in the early stage of bottom-to-the-west flow of the batholithic root, but an alternative is that S-C fabrics from the extensional top-to-the-west system extend to the base of the crust under the Sierra. Although the presence of anisotropy can be identified with this technique, the sense of shear remains ambiguous. The recognition of this major deformational event under a region virtually undeformed in the Cenozoic illustrates the power of seismological identification of anisotropy. However, the conflicting interpretations of the same seismological inference suggest the present limitations and provide motivation for developing geologic and seismological techniques to resolve between different mechanisms for generating anisotropy.