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The Snake River Plain is a major late Cenozoic tectonic feature in the North American crust. The plain consists of distinctive western and eastern parts, which differ in structural trend and geology: a northwest-trending sedimentary basin in the west, and a northeast-trending volcanic plain in the east (Fig. 1). Structurally, the western Snake River Plain separates the Cretaceous Idaho batholith of west-central Idaho from outliers of the batholith in southwestern Idaho, and the eastern Snake River Plain interrupts the continuity of mountain ranges and valleys of the Basin and Range Province. The Snake River Plain is entirely covered by Cenozoic volcanic and sedimentary deposits, of which the exposed material is largely of Quaternary age.

The Snake River Plain is an area of subdued, locally almost featureless, relief surrounded by mountains and highlands. The plain is as much as 70 km wide in the western part, 90 to 100 km wide in the eastern part, and about 600 km long. Its surface rises with a gradually increasing gradient from altitudes of less than 700 m at the western end to about 2,000 m at the eastern end.

The geology of substantial parts of the Snake River Plain has been studied in moderate detail, a chronologic framework based on radiometric ages of the volcanic rocks is fairly well established, and considerable geophysical data pertinent to the crustal structure has been gathered?including seismic profiles, gravity and aeromagnetic surveys, magnetotelluric and resistivity soundings, and measurements of thermal gradients and heat flow. Furthermore, the subsurface.

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