Automated alluvial fan discrimination, Quaternary fault identification, and the distribution of tectonically reactivated crust in the Gobi Altai region, southern Mongolia

Abstract

In this study, we demonstrate the utility of SRTM topographic data as a tool for mapping alluvial fans and documenting the distribution of tectonically reactivated crust in the Gobi Altai region of southern Mongolia. The topographic data clearly reveal three diagnostic indicators of crustal reactivation: visible fault scarps, sharply defined faulted mountain fronts, and alluvial fan complexes adjacent to steep mountain fronts. These three variables are combined to produce a new surface tectonic activity map of the region. By selecting terrain with slope thresholds between 1 and 8° and then applying a topographic roughness filter, alluvial fan complexes are automatically discriminated and mapped throughout the region for the first time. This allows clear identification of Quaternary uplifted regions (and associated thrust faults) and the distribution of clastic basins which have experienced a two‐stage history: Mesozoic rifting and Late Cenozoic alluvial sedimentation. By analysing mountain front sinuosity and alluvial fan topographic roughness, the tectonic activity of individual mountain fronts is quantitatively compared and ranked. Most ranges are upthrust fault blocks with outward directed thrusts and bajadas bounding their N‐NE and S‐SW fronts. The spatial distribution of youthful mountain ranges, flanking fan complexes, and Quaternary thrust and strike‐slip faults is consistent with ongoing sinistral transpressional crustal reactivation throughout the region.