THERMAL EVOLUTION OF THE SCANDIAN OROGENIC RETROWEDGE, NORTHERN SCOTLAND: TECTONIC IMPLICATIONS OF RAPID COLLAPSE

Recent work in the Scandian (ca. 435-405 Ma) orogenic retrowedge in northern mainland Scotland has indicated increasingly rapid cooling and subsequently rapid unroofing rates following the constructional phase of orogenesis. Cooling from >700 °C to surface conditions in the high-grade orogenic core increased from rates of ~25 °C Myr^-1 during peak orogenesis to as high as 90 °C Myr^-1 during late-stage orogenic collapse, suggesting unroofing rates as high as ~3.8 mm yr^-1 during a time when erosional efficiency is assumed to decrease as topography is diminished. Due to this seemingly paradoxical behavior, it is proposed that the collapse of the Scandian retrowedge in this region was at least partially assisted by extension driven by a similar lower crustal flow mechanism to that previously proposed for the East Greenland Caledonides (EGC). The hot, high-grade core of the Scottish retrowedge may have mobilized material away from the orogen core and toward the flanks, acting as a smaller-scale version of the “infrastructure” recognized in the EGC. Using the thermal evolution of the wedge presented here as a time-temperature constraint for the system, current work is focused on determining the efficacy of both erosional (fluvial incision and hillslope diffusion) and coupled erosional-geodynamic models for collapse of the retrowedge. Preliminary surface process modeling using the Python-based Landlab code indicates the persistence of significant, spatially-consistent topographic relief after 10 Myr of post-collisional collapse. This is incompatible with the paleo-depositional surface and elevation of the early Emsian (ca. 407-403 Ma) lower Old Red Sandstone (ORS), which can locally exceed 300-400 meters thickness and is interpreted to mark the end of the Scandian orogeny. This apparent inability of erosional processes to solely and completely denude the retrowedge in the short time following construction and prior to ORS deposition suggests another mechanism may have been influential in the collapse, similar to the lower crustal flow mechanism proposed for the EGC. Further modeling efforts will explore this possibility and seek to provide a comprehensive thermal-kinematic reconstruction of the collapse of the retrowedge as well as a range of threshold rates for erosion in collapsing collisional systems.