Basin and Range Topography

Basin and range topography is characterized by tilted fault blocks forming sub-parallel mountain ranges and intervening sediment-filled basins. These elements are typical of the basin and range physiographic topography in the western United States. This province is bounded on the east by the Colorado Plateau, the Columbia and Snake River Plateaus to the north, the Sierra Nevada to the west, and extends southward through eastern California and southern Arizona into northern Mexico. Nearly the entire state of Nevada and western Utah exhibit features distinctive to basin and range topography.

Within the basin and range province, steep mountain ranges are bounded by normal faults, with ground motion along the faults resulting in the relative uplift of the mountains and dropping of the valleys. The longitudinal mountain ranges lie generally parallel to each other and trend northward, leading one early geologist to compare a map view of the ranges within the province to a group of caterpillars crawling slowly north. The bare mountains are cut by numerous drainages that carry the products of weathering into the basins below. Sediment in the resultant alluvial fans eventually fills the intermontane basin. In some cases, as much as 10,000 feet of sand, gravel, and clay has accumulated. Ultimately, these relatively featureless alluvial slopes conceal the majority of the fault-block mountains and the faults from which they were formed.

The fundamental structural element of the region is the ever-present north-south trending normal fault. The presence of the normal faults is indicative of tensional stress over the region oriented in an east-west direction. This stress has produced dramatic crustal extension in this same direction, thus allowing the valley blocks to drop between the bounding normal faults as the ranges were stretched apart. Directly related to the tensional stress is the general tectonic uplift of the entire region than began approximately 15 million years ago.

Although geologists have an understanding of the mechanisms by which the basin and range formed, the thinner crust and higher heat flow of the area present strong evidence that conditions within the upper mantle are responsible. The most common explanation for these conditions is related to the subduction of a crustal plate producing chemical and physical changes within the mantle. Upwelling of heated, lower-density material caused the crust to bow upward. This, in turn, produced the high heat flow, uplift, crustal thinning,

and regional extension observed in basin and range topography.

Alluvial System; Faults and Fractures; Plate Tectonics

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