2.10 Basin and Range Province of Nevada, Fault Block Mountains: Plate Tectonics

Most mountain belts, including the Alps, Himalayas, and Appalachians, form in compressional environments, as evidenced by the predominance of large thrust faults and folded strata. However, other tectonic processes, such as continental rifting, also produce uplift and the formation of topographic mountains. The mountains that form in these settings, termed fault-block mountains, are bounded by high-angle normal faults that gradually flatten with depth. Most fault block mountains form in response to broad uplifting, which causes elongation and faulting. Such a situation is exemplified by the fault blocks that rise high above the rift valleys of East Africa.

Playlist of Geology and Earth Science Videos from Snow Mountain
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Mountains in the United States in which faulting and gradual uplift has contributed to their lofty stature include the Sierra Nevada of California and the Grand Tetons of Wyoming. Both are faulted along their eastern flanks, which were uplifted as the blocks tilted downward to the west. Looking west from Owens Valley, California, and Jackson Hole, Wyoming, the eastern front of these ranges, the Sierra Nevada and the Grand Tetons respectively, rise more than 2 km, making them two of the most imposing mountain fronts in the United States.

The Basin and Range Province: Located between the Sierra Nevada and Rocky Mountains is one of Earth’s largest regions of fault block mountains, the Basin and Range Province. This region extends in a roughly north to south direction for nearly 3,000 kilometers, 2,000 miles, and encompasses all of Nevada and portions of the surrounding states, as well as parts of southern Canada and western Mexico. Here, the brittle upper crust has literally been broken into hundreds of fault blocks. Tilting of these faulted structures, half grabens, gave rise to nearly parallel mountain ranges, averaging about 80 km in length, which rise above adjacent sediment-laden basins.

Extension in the Basin and Range Province began about 20 million years ago and appears to have stretched the crust as much as twice its original width. High heat flow in the region, three times average, and several episodes of volcanism provide strong evidence that mantle upwelling caused doming of the crust, which in turn contributed to extension in the region.

It has also been suggested that the change in the nature of the plate boundary along the western margin of California may have contributed to the formation of the Basin and Range. About 40 million years ago the dominant forces acting on the western margin of North America were compressional, caused by the buoyant subduction of a segment of the Pacific basin. Subduction gradually ceased along the coast of California as the convergent boundary separating the Pacific and North American plates turned into the transform boundary we call the San Andreas Fault. Approximately 20 million years ago, a warm rising mantle plume began to uplift and fault the crust between the Sierra Nevada and Rocky Mountains. According to one model, these elevated crustal blocks began to gravitationally slide off their lofty perches to generate the fault-block topography of the Basin and Range Province.

Playlist of Geology and Earth Science Videos from Snow Mountain
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