FLUVIAL MEGAFANS ARE THE CONSEQUENCE OF PALEOCHANNEL-ENHANCED AVULSION DYNAMICS

Fluvial megafans are large, low-gradient depositional environments that appear in foreland basin settings worldwide. Where they occur, megafans form the bulk of the geologic record of foreland basins, provide useful records of paleoclimate and tectonic signals, and create substantial hazards (e.g., the Kosi river avulsion in India and Nepal, 2008). Despite their importance, however, it is unclear why some rivers leaving the topographic confinement of mountain-fronts generate megafans and others instead create simple channelbelts. In addition, the controls on fluvial megafan size and shape are unknown. We demonstrate through modeling that megafan generation and scaling can be explained by the competition between two parameterized processes: river avulsions and paleochannel healing. We created a simplified cellular model of landscape evolution in a subsiding foreland basin (O:10¹-10⁷ years; 10⁰-10² km) and implemented simple rules for subsidence, sediment transport, and overbank processes. In this setting, we show that fluvial megafans are only created when avusions occur on a much shorter timescale than paleochannels are removed from the floodplain. Additionally, we demonstrate that avulsions can occur much more rapidly than expected by previous analytical methods involving superelevation timescales. This is possible because we propose that paleochannels effectively shortcut the amount of aggradation necessary to achieve superelevation. Our findings provide a mechanistic understanding to apparently contradictory results from empirical studies that debated the role of climatic setting on megafan generation. By placing the focus on avulsion dynamics and paleochannel processes, we help explain megafan generation and bring clarity to their use as records of paleoclimate and tectonic signals.