MORPHOLOGICAL DATING OF CUMULATIVE REVERSE FAULT SCARPS: EXAMPLES FROM THE GURVAN BOGD FAULT SYSTEM, MONGOLIA

Abstract

We relate reverse fault scarp morphology formed by several earthquake dislocations to the average deformation rate, using a morphological dating model based on a diffusion analogue of erosion. Our scarp degradation model includes diffusive erosion during the interseismic period, the gravitational collapse of the coseismic fault scarp just after formation, and the variation of the surface rupture location. Interactions between thrusting and geomorphic processes acting on scarp morphology are analysed along the Gurvan Bogd Range in Mongolia. Four main processes acting on scarp morphology were distinguished: 1) gravitational collapse of the frontal scarp, resetting the diffusive scarp if fault offsets are big and faulting is localized; 2) progressive erosion of the fault scarp during the interseismic period; 3) folding associated with the frontal thrust and backthrusts; 4) competing alluvial deposition on mountain piedmont slopes and abrasion of the fault scarp by wash processes. The growth of cumulative reverse fault scarps is suppressed when they are located in the outwash of major drainage basins. They can grow higher in distance from major catchment discharges. The modelling suggests that the morphology of the scarp and its apparent degradation stage, depend on the parameters controlling the amount of frontal collapse; the magnitude of coseismic offsets, the dip of the fault near the surface and the step distance between faults. Folding associated with thrusting creates a convexity on the upper part of the scarp and increases its height. The comparison of different scarp profiles suggests that folding leads to an overestimate of the morphological age. We estimate a diffusion coefficient at 3.3 ± 1.7 m2 ka−1. Morphological ages calculated with our model confirm that slip rate along reverse faults of the Gurvan Bogd range has not been constant over the last 100 ka.