HR: 1340h
AN: T33A-1148    [Abstracts]
TI: Mysterious Moho Beneath the Southern Sierra Nevada, Analyzed with Beam-Formed Receiver Functions
AU: * Levandowski, W
EM: wlevando@gmail.com
AF: Princeton University Department of Geosciences, Guyot Hall, Princeton, NJ 08540, United States
AU: Jones, C
AF: University of Colorado Geological Sciences, Benson Hall, Boulder, CO 80302, United States
AU: Nolet, G
AF: Princeton University Department of Geosciences, Guyot Hall, Princeton, NJ 08540, United States
AU: Phinney, B
AF: Princeton University Department of Geosciences, Guyot Hall, Princeton, NJ 08540, United States
AB: Single station receiver functions from the southern Sierra Nevada, California (36-38 N latitudes) show considerable variation in amplitude of Moho P-S conversions. Specifically, in the range's western foothills, a region termed the "Moho hole" (Zandt et al., 2004), the Moho is absent on single station receiver functions, proposed to be the result of a dramatic cusp on the Moho that destroys Moho P-S coherence. I explore the possibility that signal generated reverberations have sufficiently obscured the seismograms to disguise the Moho P-S conversion. Using data from the 1997 Sierra Paradox Experiment, seismograms from groups of 3 stations spaced ~20 km apart are slant-stacked to make the vertical and horizontal components of the "beam" used in deconvolution. Exploiting improved signal-to-noise ratio, radial receiver functions calculated for the beams better recover from the Moho and intracrustal discontinuities while reflections off of structure near individual stations interfere destructively before deconvolution and are thus not imaged. In some cases, the linear (post- deconvolution) stack is incoherent while the receiver function calculated by first beam forming is clear and interpretable. Of paramount interest is the use of this technique in the region interpreted as the "Moho hole"; here even small amplitude yet coherent receiver functions can be used to constrain crustal geometry. Calculations agree with previous models based on active source seismology and do not invoke dramatic topography at the base of the crust. To investigate the cause of the severely diminished Moho signals, the relative Moho P-S-to- direct P ratio, I have calculated predictions for these relative ratios. Models match very closely with observations, with the low amplitudes under the western foothills primarily the result of coupled geometric considerations and low impedance contrasts across the Moho. Therefore, no dramatic cusp on the Moho is necessary to explain receiver function observations, and overall crustal architecture is consistent with the removal of a large mafic root from the base of the Sierra, strengthening the case that modern topography of the southern Sierra Nevada has resulted from a ~3.5 Ma delamination event.
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: 2007 Fall Meeting