Gravity and magnetic investigations of the Sierra Nevada batholith, California

Missing roots and mantle “drips”: Regional Pn and teleseismic arrival times in the southern Sierra Nevada and vicinity, California

“…The principal observations that constrained the inference of a thick crustal root were seismic refraction profiles. The large Bouguer gravity anomaly, while consistent with a thick root farther to the north [Oliver, 1977;Oliver and Robbins, 1982], does not by itself require a thick crust and will be discussed later.…”

“…Because of the large-scale vertical transport of material in the Mesozoic magmatic arc [Saleeby, 1990], it seems reasonable to assume that the gross variations in density observed at the surface extend down many kilometers into the crust. Gravity gradients associated with individual plutons in the upper crust indicate that these plutons extend down to about 10 to 13 km depth [Oliver, 1977;Oliver et al, 1987Oliver et al, , 1993. A more diffuse density contrast below 10 km between the San Joaquin crust and the Sierran crust could exist, for the deep levels of the Sierran batholith exposed in the southernmost part of the range remain fairly silicic to paleodepths approaching 30 km, and densities rarely exceed 2.8 Mg m· 3 even in these deep levels of exposure [Ross, 1989;Saleeby, 1990].…”

“…East-west profiles across the Sierra have often been used to demonstrate the presence of an Airy root beneath the range [e.g., Oliver, 1977;Kennelly and Chase, 1989). This is curious because gradients on the west side of the Sierra are so steep that they require substantial density contrasts in the upper crust [Kennelly and Chase, 1989;Oliver and Robbins, 1982]; thus even overcompensation at the base of the crust will not fit the observed anomaly without upper crustal anomalies.…”

“…Proponents of a crustal root point to two longitudinal seismic refraction profiles along the range [Eaton, 1966;Pakiser and Brune, 1980], the likely existence of thick crust beneath the Mesozoic arc, and gravity profiles constructed using constraints from the seismic profiles [Bateman and Eaton, 1967;Oliver, 1977]. Those proposing a low-velocity upper mantle note observations of low seismic velocities from a teleseismic study extending north from Mono Lake [Mavko and Thompson, 1983], surface wave phase velocities [Crough and Thompson, 1977], and low Pn velocities observed on refraction profiles across the range LCarder, 1973].…”

Seismic structure of the lithosphere from teleseismic converted arrivals observed at small arrays in the southern Sierra Nevada and vicinity, California

“…Gravity observations require the presence of a large low-density body, an isostatic root, in the crust or upper mantle to support the modern elevation of the range [e.g., Oliver and Robbins, 1982;Oliver, 1977]. Several previous seismic observations indicated that the Sierra might have a thick crustal root that supports the >2800 m average elevation of the range [Byerly, 1938[Byerly, , 1939Eaton, 1966;Pakiser and Brune, 1980].…”