The Basin and Range Province: Origin and Tectonic Significance

Abstract

The Basin and Range province is a vast arid tract of regionally corrugated, angular topography of high relief in the western Cordillera. It is characterized by evenly spaced parallel mountain ranges and intervening desert basins (Fig­ ure la). The range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward, interrupted here and there by low fault scarps that paral­ lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges. Thermal springs located at, or near, range-bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting. In the southern part of the province, especially in southeastern California and southwestern Arizona, range fronts have been worn back by erosion, leaving a thin veneer of gravel on an erosion-cut, bedrock surface that slopes gently outward. The range-bounding faults of these mountain blocks are buried at the outer edge of such pediments, often at considerble distances from the erosional remnants of the ranges themselves. The American physiographer N. M. Fenneman (1928, 1931) named the Basin and Range province and defined its general boundaries. As thus circum­ scribed, the province includes some 800,000 km2 of area in eight western states. Later students (Pardee 1950, Lawrence 1976, Reynolds 1979, Eaton 1979b) have observed that many of the fundamental geological and geo­ physical characteristics of the province are found well beyond the boundaries drawn by Fenneman, which were based on physiography alone. As a tec­ tonophysical entity, its areal extent is greater than 1 million km2, more than 10% of the area of the United States (Figure 1) . Fenneman (1931) subdivided the province into five physiographic sections, the largest of which is the Great Basin (see Figure 1b) . It is not, as its name implies, a single regional depression with a common topographic center, but is characterized instead by isolated networks of interior drainage, divisible into