Variations across and along a major continental rift: An interdisciplinary study of the Basin and Range Province, western USA

Abstract

Geological, geochemical, and geophysical data gathered within the central part of the Basin and Range and adjacent areas of the western USA suggests that considerable heterogeneity characterizes Cenozoic extension in this region. Good exposure and an abundance of pre-rifting markers indicate 250 km of extension of the upper crust over the past 16 m.y. Extension of several hundred percent has occurred in two distinct deformational domains, Death Valley and Lake Mead, separated by a relatively unextended block, the Spring Mountains. The limited topographic differences between extended and unextended regions imply that material with a crustal density has been added to the extended regions. Although igneous activity can provide some of this added material, kinematics of extension within the Death Valley region suggest that lateral flow of the middle and lower crust into the extended areas accounts for much of the needed material. Such flow is consistent with geochemical analysis of intermediate to silicic volcanic rocks in the Death Valley area. These volcanic rocks contain isotopic and geochemical trends similar to Mesozoic plutonic rocks from the western part of the Sierra Nevada, about 150–200 km to the west, thus suggesting that the upper crust has moved by that amount relative to deeper crustal levels. Geochemical analyses of basaltic magmas in the region indicate that two mantle reservoirs are present: an OIB-type asthenosphere, and an old, Precambrian continental lithosphere. The ancient lithospheric mantle is preserved beneath the Central Basin and Range, but to the west and north the basaltic rocks have a signature compatible with an asthenospheric origin. These differences indicate that the degree of thinning and removal of the mantle lithosphere varies considerably across the Central Basin and Range. These differences are compatible with the inference from geological and geophysical arguments that thinning of the mantle lithosphere at the latitude of the Central Basin and Range is localized beneath the Sierra Nevada. Geophysical measurements have shown that the thickness of the crust varies little from a mean of about 30 km over the entire Basin and Range; the crust under the high Sierra Nevada to the west might have about the same thickness. Estimates of the buoyancy of the crust and mantle based on P-wave crustal structures suggest that the most buoyant, and thus probably the warmest, mantle lies under the Sierra Nevada and not under areas of strongly thinned upper crust of the Death Valley and Lake Mead regions to the east. Similar analyses indicate that the extended upper crust of the Northern Basin and Range overlies an upper mantle more buoyant than that of the Southern and Central Basin and Range; this is in accord with geochemical and seismological inferences. Thus, the style of lithospheric extension varies considerably both along and across the strike of the Basin and Range.