An Evaluation of Erosional‐Geodynamic Thresholds for Rapid Orogenic Denudation. Issue 12 (13th December 2021)

• Record Type: Journal Article
• Authors: Spencer, Brandon M.; Thigpen, J. Ryan; Gallen, Sean F.; Dortch, Jason M.; Hodges, Kip V.; Law, Richard D.; Mako, Calvin A.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 12(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-12-13
• Subjects: numerical models -- erosion -- orogenic decay -- Scotland -- orogenic collapse; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB022353
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Tectonic and surface processes (e.g., erosion) act in concert to denude and flatten elevated topography during and following collisional uplift. In orogens that collapsed rapidly (<10 Myr), the effectiveness and relative contribution of erosion remains poorly understood. Here, we use 1‐D and 2‐D finite‐difference models to test plausible magnitudes of erosion during denudation of the Scandian orogenic wedge, in northern Scotland, which previous work suggests may have been very rapid. Thermochronologic, stratigraphic, and thermobarometric data constrain initial post‐collision uplift rates as well as the cumulative post‐collisional exhumation magnitude. Using these constraints, we consider an idealized orogen subjected to fluvial incision, hillslope diffusion, and subsequent isostatic compensation for a duration of 10 Myr. Using a range of geomorphic parameters, we evaluate whether erosion alone is sufficient to achieve the observed cumulative exhumation. Additionally, we apply a thermal model during the same period to determine the viability of the previous erosion estimates in the presence of a transient geothermal gradient. For most cases with initial uplift rates of 0.5–2.5 mm yr −1, total erosion is insufficient to account for the observed Scandian denudation history, regardless of whether a fixed or transient geothermal gradient is assumed. Average elevations near the crest of the modeled orogen after 10 Myr are >1.4 km, which is inconsistent with the post‐Scandian landscape relief interpreted from the reconstructed depositional surface of the Emsian (407–403 Ma) Old Red Sandstone, Collectively these findings imply a combination of rapid erosion and tectonic denudation likely accomplished the rapid collapse of the Scandian wedge. Plain Language Summary: The destruction of mountain ranges is accomplished by numerous physical processes, which include erosion of the landscape and subsurface deformation of the Earth's crust. In mountains where this destruction occurred very rapidly, such as in northern Scotland, there is poor understanding of the relative effectiveness of each process in wearing mountains down. In this paper, we use computer models to calculate how much erosion might have occurred in the geologically short time frame after the mountains in Scotland stopped growing. These models track the effects of river erosion, hillslope movement, and crustal buoyancy on elevation change after mountain‐building has essentially ceased. We compare the model results to pressure‐temperature‐time data recorded in rocks exposed at the surface today that give us information about how fast the rocks were brought up to the Earth's surface from depth. Our inference based on these models is that erosion alone cannot remove material quickly enough to explain available geologic data, implying the importance of crustal thinning by deformation in the Scottish Highlands. Key Points: Published thermochronologic data for metamorphic rocks of the Scottish Highlands indicate rapid denudation following orogenesis The models in this work suggest that denudation was not accomplished by surface processes alone, but required additional tectonic processes This may indicate a role for channelized crustal flow similar to that proposed for the Himalaya and the East Greenland Caledonides