Seismic character of the crust and upper mantle beneath the Sierra Nevada

The Sierra Nevada Mountains form a prominent segment of the Mesozoic-age, Cordilleran volcanic arc of western North America. The Sierra Nevada EarthScope Project has focused on relating seismic study of the central and northern Sierra Nevada to the evolution of the continental lithosphere following the emplacement of a massive batholith ca. 85 Ma. Here receiver functions exhibit significant variations in the structure of the crust and upper mantle. Crustal Vp/Vs values are lower in the core of the batholith and higher in the northern Sierra, portions of the Basin and Range, and near young volcanic fields in the eastern Sierra and Owens Valley. P- to S-wave conversions from the Moho vary from high amplitude and shallow (>25% of the direct P-arrival amplitude, 25-35 km depth) along the eastern Sierra to low amplitude and deep (<10%, 45-55 km) beneath the western batholith. The region of reduced Moho amplitudes correlates closely with results from P-wave and surface-wave tomographic studies, which resolve regionally fast material in the lower crust and upper mantle beneath the west-central Sierra. Unusual crustal earthquakes have been located to nearly 40 km depth within this anomalous zone of high wavespeeds. We propose that dense, seismically fast mafic-ultramafic residue has foundered beneath the east-central and southern Sierra but still resides under its western portion. Evidence for the development and/or loss of substantial residue in the northern Sierra is equivocal. The asymmetric structure of the lithosphere beneath the central Sierra, which we model using constraints from petrophysical analyses and the tomography suggests that foundering progresses from southeast to northwest. This process sharpens the seismic response of the Moho by removing its underlying lithospheric mantle, while upwelling asthenosphere replaces the detached material. Unusually fast crust and upper mantle, deep crustal seismicity and recent volcanism observed to 38 degree N appears linked to this process and correlates spatially with the change in the character of the Moho, measurements of high crustal Vp/Vs, and presence of prominent negative conversions in the crust and uppermost mantle. These findings advance our understanding of how felsic continental crust originates within Cordilleran batholiths.