Large modern fluvial fans are partial cones of sediment, each fan generally formed by a single dominant river. These features display gentle slopes (compared with small alluvial fans) and their apices are tied to mountain fronts or low scarps. Large fans are mesoscale continental sedimentary systems (radii arbitrarily defined as >100 km) barely recognized in geological literature, mainly because they are not readily visible on the ground or from low-altitude photographs because of their great size and gentle slopes. Space Shuttle and International Space Station photographs, supported by 1:1,000,000 maps, reveal basinal geological settings with relatively young sediments and distributary drainage in many parts of the world, leading to the identification of 96 large fans. Large fans appear on all continents between 55°N and 55°S, which constitutes our survey area. Fan areas range between ~7000 and 210,000 km². Most radii (90%) fall between 100 and 300 km. Fifty-eight percent lie in foreland and peripheral basins; 39% lie in cratonic settings. The remaining 3% occur in rifts and intra-orogenic settings. Asia displays the largest number of modern large fans and North America the smallest number. Large areas of nested fans occur in Australia and Africa. The largest group of contiguous fans occurs in South America, from northern Bolivia to central Argentina where the basin between the Andes Mts. and the Brazilian craton is entirely occupied by seventeen fans, covering ~750,000 km². Large fluvial fans are thus a significant feature of the global landscape, and probably have been so throughout much of Earth's history.

Understanding the distribution of large modern fans may help direct exploration for such features in past landscapes. Large modern fans may also provide models for detailing the architecture of paleo-fluvial reservoir systems. For example, the early Paleozoic oil and gas reservoirs of North Africa accumulated in settings which may have been analogous to the modern foreland basins of Central Asia and India. The large fan analog may also prove appropriate for the Witwatersrand basin of South Africa.

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Unnumbered Figure. The densely forested distributaries of the Okavango Swamps, also known as the "jewel of the Kalahari Desert," appear dark green in this Space Shuttle view of semiarid northwest Bostwana. The swamps constitute a large fan within the Okavango Graben, an extension of the East Africa rift system. The 150 km radius of the fan is determined by the distance between faults at the apex and the toe (arrows). Major mountain fronts are not the only environments in which large fans form. Low fault scarps such as that at the Okavango Swamp apex (only -5m) are also sufficient to allow rivers to avulse through a wide angle to form large fans.

Unnumbered Figure. The Okavango River, reflecting the sun, enters the megafan apex from the high-rainfall Angolan plateau. In world terms, the fan is unusual for its size, its swampy nature and existence within a rift. Numerous fires in the thicker Angolan savannas appear top left in this oblique, northwest-looking view.

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