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In the later Cainozoic, approx 18 m.y. ago, the first basin and range faulting developed in the central part of the Great Basin, this extensional tectonic system resulting from drag on the North American plate as the Pacific plate moved obliquely to the NW along the San Andreas fault. The northern boundary of the Great Basin at the Snake River plain and W across SE Oregon is the tectonic zone along which the E-W extending Basin and Range province has been moving for the past 18 m.y. In the Great Basin axis a narrow N-trending zone of basalt intruded the crust at the same time that basin and range faulting developed; this belt widens northwards as it approaches the N edge of the Great Basin and becomes diffuse and widespread in SE Oregon and SW Idaho, reaching enormous dimensions in the Columbia Plateau farther N. The basalt, which replaced andesitic igneous activity in the mid-Cainozoic, was produced by widespread partial melting in the upper mantle when the tectonic regime changed from a convergent- and subduction-related system to the extensional basin and range system. The locus of magma generated migration to the E and W margins of the Great Basin simultaneously and, as it migrated, it produced a series of eruptive centres along the N boundary of the Great Basin.-R.A.H.
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Tectonic and magmatic development of the Great Basin of western United States during the late Cenozoic time.