Coupled Rapid Erosion and Foreland Sedimentation Control Orogenic Wedge Kinematics in the Himalayan Thrust Belt of Central Nepal

摘要

Spatial and temporal coincidence among rapid Pliocene-Holocene bedrock exhumation, development of a topographic bight, abundant monsoonal precipitation, accumulation of anomalously thick proximal foreland basin deposits, and development of an opposite-polarity salient-reentrant couple on the two most frontal major thrust faults in the Himalayan orogenic wedge of central Nepal provide a basis for a model that links these diverse phenomena and could be operating in other parts of the frontal Himalaya. Rapid bedrock erosion is documented by a concentration of young (<5 Ma) low-temperature thermochronologic ages in the Narayani River catchment basin. Where the river exits the Lesser Himalayan Zone, the Main Boundary thrust has a 15-km-amplitude reentrant. Directly south of the reentrant lies the similar to 50 km wide Chitwan wedge-top basin, which is confined by a large salient on the Main Frontal thrust. Rapid erosion and sediment flux out of the Narayani catchment basin, possibly due to anomalously intense monsoonal precipitation in this topographically depressed region of central Nepal, causes greater flexural subsidence and surface aggradation in the foreland, both of which increase initial wedge taper and render this region more susceptible to anomalous forward propagation of the thrust front. Analysis of the modern and post-early Miocene taper history of the thrust belt suggests that rapid erosion hindered forward propagation of the contemporaneous Main Boundary thrust, but simultaneously produced conditions in the foreland that eventually elevated initial taper to a critical/supercritical value promoting forelandward propagation of the Main Frontal thrust. This analysis has implications for large damaging earthquakes in the Himalaya.