MOUNTAINS, MONSOONS, AND FLUVIAL MEGAFANS

Fluvial megafans are volumetrically significant sedimentary depositional systems that form as a result of a complex combination of tectonic and climatic processes in basins adjacent to orogenic systems. The factors responsible for fluvial megafan formation and the controls that govern where megafans will, or will not, form are poorly understood. Compiling and comparing numerous characteristics of more than 150 rivers that exit mountain belts from around the world we found climate, specifically highly seasonal precipitation, to be the dominant driver behind fluvial megafan formation. All megafan-forming rivers experience large seasonal fluctuations in discharge that result from highly seasonal to monsoonal precipitation. Under such conditions, fluvial channel stability decreases, leading to higher rates of avulsion and channel migration, thereby enabling megafans to form. No correlation exists between megafan occurrence and drainage basin characteristics, such as area, maximum elevation, or relief. The modern global distribution of fluvial megafans is restricted to 15¨C35 degrees latitude in the northern and southern hemispheres, where atmospheric conditions alternate seasonally between baroclinic and barotropic. In the stratigraphic record, fluvial megafan deposits in North and South America and Asia coincide with the onset of paleomonsoonal conditions. Because highly seasonal to monsoonal precipitation is necessary for megafan formation, the recognition of fluvial megafan deposits in the stratigraphic record can provide crucial information for paleoclimate reconstructions.