Butte detachment: how pre‐rift geological structure and drainage integration drive escarpment evolution at rifted continental margins

Abstract

AbstractThe erosional pattern of passive margins often follows the fabric of ancient, compressional geological structures exposed by the topographic energy of rifting. As erosion cuts into these belted outcrop systems they impose initial and boundary conditions that steer drainage recession into the plateau edge and control escarpment‐forming conditions. Pattern therefore controls process. Although generic surface process models predict scarp patterns and retreat in settings devoid of geological heterogeneity, they tend to do so only at isolated locations and for periods shorter than the lifespan of the escarpments. Thus, to focus on relatively narrow strike‐perpendicular swaths of passive margin topography misses important aspects of drainage integration, which involves mobile drainage basin boundaries shifting across but also along the strike of inherited geological structures and through continental‐scale bioclimatic zones. Space‐for‐time substitution along three passive margin escarpments (Blue Ridge, Western Ghats, Eastern Ghats) reveals the significance of escarpment jumps and the detachment of topographic outliers, here generically termed ‘buttes’, as key processes of escarpment evolution. The examples show that these continental escarpments are strongly patterned after pre‐rift structural and lithological heterogeneities. As seaward sloping drainages cut into the rift margin, they extend their drainage heads in a non‐uniform and unsteady fashion. As a result escarpments can form, be destroyed, reform, and leave topographic vestiges (buttes) of the retreating escarpment. Given the pre‐rift geological heterogeneities, there are no a priori reasons why escarpment landscape change should be uniform, steady or self‐similar. Copyright © 2010 John Wiley & Sons, Ltd.