The Basin and Range Province

The Basin and Range province is a broad, highly extended terrane embedded within the North Ameri-can western Cordillera that extends from Canada, through the western United States, and across much of Mexico. The province roughly occupies the space between the Cascade Ranges and Rocky Mountains in the north and the Sierra Nevada and the Colorado Plateau in the middle, and it engulfs the Sierra Madre Occidental Range in northern Mexico (Fig. 7-1). Seismicity, high heat flow, and recent basaltic vol-canism indicate that the Basin and Range province is actively extending. Its descriptive name is derived from a particular (and most recent) mode of exten-sional block-faulting that left the characteristic pattern of alternating basins and ranges across the province (e.g., Gilbert, 1928). This pattern is especially prominent in the Great Basin, a region of internal drainage that occupies most of the northern Basin and Range province. Estimates of the total crustal extension across the Basin and Range province converge to between 50 and 100% (e. though it is recognized that this extension is not uniformly distributed across the province, but instead occurs as extreme extension (100-300%) in some areas, and as minor extension (<10%) in others. The Basin and Range is unlike most continental rifts because of the breadth of extended lithosphere there. At its widest points, the Basin and Range province is more than 900 km across, much wider than continental riffs such as the 100-300 km wide Rio Grande (Chapter 6) and East African (Chapter 5) rifts. Other broadly extended regions of crust are observed, but are usually found beneath sea level, as in the case of the extended crust beneath the North Sea, the Bering Strait, and the South China Sea, or beneath thick basinal sedimentary rocks. In contrast, the northern Basin and Range stands at an average 1.5 km above sea level. Much of the Basin and Range province lies within a rain shadow behind the Sierra Nevada and Cascade Ranges, and the resulting dearth of erosion and sedimentation has preserved excellent exposures of extended crust. Good exposures, combined with the enigmatic nature of actively extending continental crust residing at high elevations, has invited a great deal of study; as of 1992 more than 7200 papers and books have been written under a subject heading of "Basin and Range". An unfortunate sampling bias in many types of geologic and geophysical data towards the Basin and Range province …