Basin and Range Topography

Basin and range topography is a type of landscape where ridges and mountains alternate with depressions in a somewhat regular pattern. Such topography occurs wherever diastrophic processes have produced faults that have uplifted blocks and dropped down intervening basins. These features are not tied genetically in any way to arid areas but many of the best-known examples do occur in such regions, which allows for greater recognition of them partly because of good exposures of the causative structural processes. Many of the basins have entirely interior or endorheic drainage so that no water drains out to the sea. Such landscapes occur in the well-known Great Basin geomorphologic province of the southwestern United States and northwestern Mexico, as well as the dry littoral regions of Chile and Peru, parts of Iran, Western Afghanistan, and Southern Pakistan, in Central Asia, and in parts of the Sahara Desert in Northern Africa. Additional examples are known from northeastern Japan and the Gulf of Corinth, Greece.

The primary genetic attributes characteristic of basin and range topography are down-dropped graben rock structures and uplifted horst block rock structures that are bounded with tensional normal faults. In addition are numerous cases of tilted fault blocks that are lifted up on one side and dropped down on the other into what are termed half-grabens. In either case the graben and half-graben basins are generally filled in with sediment derived from the erosion of the nearby uplifted horst blocks or mountains. In arid regions, most of these graben or basin-filling sediments are comprised of alluvial fan sands and gravels, with the basin centers being largely alluvial plains down which ephemeral stream channels flow, or lacustrine (lake) infillings left as former water bodies dried up and became dry or playa lakes, the most well-known of these playa lakes is Lake Bonneville that existed during the Pleistocene Ice Age. Lake Bonneville was a large lake centered on what is now the State of Utah, which dried up as the climate warmed and dried after about 10,000 years ago to leave the shrunken remnant of what is now the Great Salt Lake, as well as the Bonneville Salt Flats that are used for attempts at land speed records.

The cause of faulting of basin and range structure and topography in the American Southwest is crustal extension or stretching of the rocks of the crust, perhaps several tens of millions of years ago. In the course of the disruption of the crust, huge outpourings of lava occurred, which left extensive lava-capped tilted fault blocks at the edges of the half-grabens. The cause for this crustal extension is not known with certainty but apparently when the great slab of the North American tectonic plate was pushed up on top of the San Andreas transform fault zone, a compressional gap developed. This seems to have reduced the compressional forces that the Great Basin area was under and allowed extension of the crustal rocks toward the slab gap and out more rapidly to the west. The result was the extensive north-south normal fault system that allowed the crust to fragment so extensively.

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