Interplay Between Crustal-Scale Thrusting, High Metamorphic Heating Rates, and the Development of Inverted Thermal-Metamorphic Gradients: Numerical Models and Examples From the Caledonides of Northern Scotland

Understanding the processes that produce high prograde metamorphic heating rates and the development of inverted metamorphic sequences in collisional thrust belts remains a fundamental challenge for tectonics and metamorphic petrology. New 2D finite element models of crustal-scale thrusts with variable slip rates (10, 20, 35, 50 km Myr⁻¹) are used to examine how thrust sheet emplacement contributes to these processes. In the models, average prograde heating rates of 31–118 °C Myr⁻¹ are observed in the footwall, with maximum transient heating rates of ∼167 °C Myr⁻¹ occurring at the highest slip rate. Also, thrust sheet emplacement produces an inverted thermal gradient in the model footwall. At slip rates of 10 km Myr⁻¹, heating magnitudes >200 °C are observed >7 km structurally beneath the thrust plane. At slip rates of 20–50 km Myr⁻¹, models produce thermal penetration depths of 4–5 km for similar heating magnitudes. Petrologically determined heating rates for thrust footwalls in the Scandian orogenic wedge of northern Scotland mostly yield rates consistent with model results (10–230 °C Myr⁻¹). The model-derived magnitude of inverted thermal gradients is also similar to those indicated in texturally determined deformation temperature transects across the Scandian orogenic wedge (100 °C–180 °C). Combined, these results indicate that high heating rates can be produced by fault slip at typical plate velocities. Additionally, this implies that crustal-scale thrusts are likely to produce inverted metamorphic sequences in most systems, provided that P-T conditions during slip allow for metamorphic or recrystallization processes that would preserve evidence for such features.