Deep Drilling and Current Models of Cenozoic Crustal Deformation in Western United States: ABSTRACT

Abstract

Seismic reflection, heat-flow, earthquake, gravity, and magnetic data have been interpreted in recent years to suggest that the crust of the actively deforming Basin and Range province may consist of two layers that have contrasting physical properties and mechanical behavior. The boundary between them, a subhorizontal zone of decoupling to weak coupling, is pictured as a thin zone of decollement. Above this zone the crust fails in extension through the formation of listric faults tangential to it. Below it, the crust dilates horizontally through the injection of dikes of basalt from the mantle. The boundary is pictured as inherited from master thrusts that formed during Mesozoic and Paleogene compressional regimes. The upper crustal layer is essentially brittle; the lowe layer is relatively ductile. This model, if correct, has implications for a wide variety of matters of practical interest. Prominent among them are: the design of strategies for oil and gas, geothermal energy, and mineral resources exploration; the underground storage of hazardous wastes; and the occurrence and surface effects of regional earthquakes. The model is amenable to testing by drilling to depths of 16,000 to 40,000 ft (5 to 12 km). Outcrops of subhorizontally foliated metamorphic rocks as young as Miocene have been interpreted as locally uplifted with exposed parts of the zone and may provide information on its lithologic characteristics; however, deep drilling (and attendant in-situ testing of rock and fluid properties, measurements of stress and their variation with depth, and surface-to-borehole geophysical communication) offers the only viable means for determining the state of the crust and for testing the postulated dynamic processes associated with it. End_of_Article - Last_Page 703------------