NUMERICAL INVESTIGATIONS OF RAPID HEATING IN THRUST BELTS AND IMPLICATIONS FOR METAMORPHISM IN THE SCANDIAN OROGENIC WEDGE, NW SCOTLAND

Understanding the rates and driving mechanisms of metamorphism in collisional orogens remains a fundamental challenge for tectonics and metamorphic petrology. Classic studies of orogen thickening predicted prograde heating at timescales generally >20 Myr, and this view has guided thinking about the timescales required to produce regional metamorphic events. However, multiple studies focused on constraining heating and cooling rates in orogens recognize that many preserved evidence for very fast (0.1 - 10 Myr) prograde heating and subsequent retrograde cooling. These fast heating rates in thrust belts may result from foreland propagation of relatively thin (3 - 5 km) thrust slices at very high slip rates (>100 km/Myr). In this scenario, an incipient thrust sheet at the front of the orogenic wedge is buried by the overriding wedge, driving prograde metamorphism. As the thrust system breaks forward, the incipient thrust sheet is accreted into the wedge and is rapidly exhumed as the thrust structurally below the thrust sheet becomes active.

To test this hypothesis, 2D finite-element models of a crustal-scale thrust with variable slip rates (10, 20, 35 and 50 km/Myr) and durations (1.6 - 7.5 Myr) were constructed to examine the rate, magnitude, and distribution of footwall heating and hanging wall cooling. All models include an erosion approximated by the stream power law. The highest modeled heating rates are confined to a fault parallel zone directly beneath the thrust ramp. At thrust rates of 10 km/Myr, the models yield maximum footwall heating rates >30°C/Myr, with progressively higher maximum heating rates of ~40 - 130°C/Myr in models with slip rates of 20 - 50 km/Myr, respectively. These rates cover the range of footwall heating rate estimates in the Moine and Ben Hope thrust sheets in the Scandian orogenic wedge. Because these rates represent an average over the entire history of thrusting, higher rates may occur in very fast, transient slip events. The models also yield hanging wall cooling rates that range from ~45 - >300°C/Myr over a range of slip rates from 10 - 50 km/Myr, with the slower slip rates more closely aligning with cooling (30 - 55°C/Myr) and exhumation rates (1.0 - 2.5 km/Myr) calculated for the Scandian thrust sheets. These results indicate that high heating rates can occur at slip rates significantly <100 km/Myr.