Abstract

The COCORP deep seismic reflection profiles in eastern Nevada, part of the 40°N traverse of the North American Cordillera, reveal crustal features probably developed or strongly modified during Mesozoic shortening and Cenozoic extension. A west-dipping crustal reflection fabric is interpreted to represent structures of the Mesozoic hinterland which are largely intact, although uplifted and locally cut by Tertiary normal faults. The discordance of the northern Snake Range décollement and the west-dipping zones of reflections below it are consistent with a simple-shear-zone model for the evolution of the décollement. The dipping crustal fabric and a deeply penetrating fault of eastern Nevada contrast with the subhorizontally layered lower crust of central and western Nevada and suggest that a pure-shear model of pervasive, horizontal ductile stretching of the lower crust is not the favored model of Basin-Range extension here.

Subhorizontal reflections and diffractions beneath the Diamond Mountains, which lie in the transition from the generally west-dipping reflection fabric of eastern Nevada to the subhorizontal reflection pattern of the lower crust of central and western Nevada, may represent structures like those exposed in the metamorphic-plutonic complex of the Ruby Mountains to the north, which are largely Mesozoic but were modified during Cenozoic extension.

A zone of subhorizontal Moho reflections which separate the reflective crust from the seismically transparent upper mantle is nowhere demonstrably offset and varies only regionally from 10 to 11 sec two-way traveltime. The present smooth Moho in eastern Nevada locally appears to postdate the Schell Creek fault, a unique crustal-penetrating Basin-Range fault which may be traced into the lower crust along a prominent east-dipping boundary defined by the termination of subhorizontal reflections. Offset at depth along this fault has probably been accommodated mainly by intrusions into and at the base of the footwall; the presently smooth reflection Moho may evolve in part by a process of magma underplating and may be quite young.

The eastward extent of well-developed Moho reflections corresponds generally to a location where important Cenozoic detachments and Basin-Range normal faults of opposite vergence focus extensional displacements deeply into the crust. This relationship may indicate that the prominent Moho represents a basal-crust zone of decoupling for extension and westward translation of much of the Basin-Range crust relative to the Colorado Plateau. Such decoupling may be enhanced by a lubricating zone of underplated magma.

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