RAPID RATES OF OROGENIC COLLAPSE IN THE SCOTTISH CALEDONIDES

Recently obtained ⁴⁰Ar/³⁹Ar cooling ages from the Scandian (c. 435-415 Ma) orogenic retrowedge indicate cooling through the hornblende and muscovite windows from c. 420-411 Ma, suggesting that extensional collapse of the Scottish Caledonides initiated during peak orogenesis. Muscovites from Moine Supergroup pelites and semipelites located throughout Sutherland yield ages of 449, 421-414, 411, and 413 Ma in the Moine, Ben Hope, Naver, and Skinsdale thrust nappes, respectively. The older age obtained in the Moine nappe is currently interpreted to be the result of a prominent thermal break structurally below the Ben Hope thrust. Hornblendes from amphibolite gneiss of the Cambrian Strathy Complex in the northern Naver nappe and Lewisian orthogneiss in the southeastern Ben Hope nappe yield ages of 416 Ma and 420 Ma, respectively. These ages are interpreted to represent cooling following peak orogenic conditions at c. 425 Ma. The spatial distribution of all ages may support relatively consistent unroofing rates across the wedge structurally above the Ben Hope thrust. The ages also indicate cooling rates may have increased after c. 416 Ma from ~20-25° C Myr^-1 to ~45-80° C Myr^-1based on ⁴⁰Ar/³⁹Ar closure temperatures of ~550° C and ~400° C for hornblende and muscovite, and early Emsian (407-403 Ma) depositional ages for lower Old Red Sandstone conglomerate that lies directly above the exhumed orogenic wedge. Prior geobarometric analyses indicate maximum pressures of ~8-10 kbar in the core of the retrowedge, suggesting exhumation of currently exposed rocks in the Naver and Skinsdale nappes from depths of 27-35 km. When coupled with the ~4-9 Myr thermochronological lag times calculated in this study, these estimates suggest unroofing of the core of the Scandian retrowedge from maximum depth to surface conditions proceeded at a sustained and rapid rate of ~3-9 km Myr^-1. Erosional denudation rates following peak metamorphic conditions in similar orogens are generally much lower (<0.01 km Myr^-1in the Appalachians) than these Scandian rates, establishing a need to identify the mechanism(s) that can help explain the rates encountered in Scotland. Ongoing thermal-kinematic and thermomechanical finite element modeling seeks to analyze possible scenarios for the rapid orogenic collapse of the Scandian retrowedge.