Structure of the Sierra Nevada from receiver functions and implications for lithospheric foundering

Abstract

Receiver functions sampling the Sierra Nevada batholith and adjacent regions exhibit signifi cant 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 Nevada, portions of the Basin and Range, and near young volcanic fi elds in the eastern Sierra Nevada 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 Nevada to low amplitude and deep (<10%, 45-55 km) beneath the western batholith. We propose that dense mafi c-ultramafi c residue has foundered in the east-central and southern Sierra Nevada but still resides beneath its western portion. The central and northern Sierra Nevada shows inherited, prebatholithic structure at the Moho that was not completely overprinted by emplacement of the massive end-stage batholith. Evidence for the development and/ or loss of substantial residue in the northern Sierra Nevada is equivocal. The asymmetric structure of the lithosphere beneath the central Sierra, which we model using constraints from petrophysical analyses, suggests that foundering progresses from southeast to northwest. This process sharpens the seismic response of the Moho by removing its underlying lithospheric mantle and allows upwelling asthenosphere to replace the detached material. Deep crustal seismicity and recent volcanism observed to 38° N appear linked to this process and correlate 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.

Evidence for Moho‐lower crustal transition depth diking and rifting of the Sierra Nevada microplate

“…Resulting SV hypocentral depths are consistent with Moho depths determined from receiver functions at this latitude [Frassetto et al, 2011]. Cross sections D′ and P′ of Frassetto et al [2011] show Moho depths in the vicinity of the deep swarms within a range of 30-35 km.…”

Fossil slabs attached to unsubducted fragments of the Farallon plate

“…The absence of a Ps signal could be caused by a steeply dipping Moho pulled down by a dripping lower lithosphere (18). However, later work showed that this weak Ps conversion is not confined to the vicinity of the Isabella anomaly; it extends along the entire western Sierra Nevada and is now thought to be due to the presence of a dense mafic-ultramafic residue in the lower crust that reduces the contrast between crust and mantle (20), an interpretation that is consistent with our tomography model that has unusually high S-wave velocities in the lower crust beneath the western Sierra Nevada (Fig. 3C).…”

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“…The crust of the Colorado Plateau between parts of the Rockies and Basin and Range exhibits a broad transition over a wide depth interval between the crust and mantle, with crustal thicknesses between 40 and 45 km (Wilson et al, 2005). Along the western margin of the Basin and Range, the crust thickens into the Sierra Nevada to as much as 40 km or more (e.g., Ruppert et al, 1998;Fliedner et al, 2000;Frassetto et al, 2011). Crustal thicknesses close to 40 km continue farther to the north of the Sierra Nevada into the active Cascade volcanic arc, to the east of which the crust thins in the backarc to close to 30 km within the High Lava Plains (Cox and Keller, 2010;Eagar et al, 2011).…”