Rapid Orogenic Collapse—Efficient Erosion, or Something More?

Abstract:

Recent work has proposed that the Scandian (ca. 435-405 Ma) orogenic retrowedge in Scotland may have completely collapsed in less than 10 Myr following peak orogenesis. This interpretation lies in stark contrast to other orogenic systems wherein topography persists for tens to hundreds of millions of years following collision. In Scotland, integration of thermochronological data with palynological data from the early Emsian (407-403 Ma) lower Old Red Sandstone (LORS) yields estimated unroofing rates of ~3.8 mm/yr in the orogenic core during the orogenic collapse phase, a marked increase from estimated rates of ~1.2 mm/yr during construction. It remains unclear whether enhanced exhumation was primarily a result of very efficient surficial processes or driven by a geodynamic mechanism such as lower crustal flow. To better understand the processes and/or mechanisms that may have contributed to this rapid orogenic destruction, it is necessary to understand the threshold rates at which these processes may occur. This contribution presents preliminary landscape evolution model results inspired by the Scandian wedge to assess the potential role of surface processes in facilitating rapid unroofing from peak orogenesis through the presumed end of orogenesis at ca. 405 Ma, which is marked by the deposition of the LORS. Using the Python-based Landlab modeling package, 35 km of total crustal section (based on the estimated maximum depth determined from thermochronological and thermobarometric analyses just prior to exhumation) is denuded by fluvial incision and hillslope diffusion processes during the constructional and collapse phases, with compensation by isostatic rebound during the collapse phase. Our preliminary results indicate persistence of topographic relief that is incompatible with the reconstructed paleo-depositional surface derived from the distribution and elevation of LORS sediments. If surficial processes alone cannot produce the completely denuded topographic surface interpreted for LORS deposition immediately following Scandian orogenesis, then other exhumation and/or denudation mechanisms may need to be considered. Further modeling efforts will investigate the viability of geodynamic mechanisms that might have contributed to the destruction of the Scandian wedge in Scotland.