INFLUENCE OF THRUST-RELATED ADVECTION AND MASS TRANSFER ON METAMORPHIC HEATING RATES IN OROGENS (Invited Presentation)

Traditionally, numerical models of orogenic metamorphism have yielded rather modest heating rates (5-20 °C), however metamorphic and geochronologic studies from a number of collisional systems indicate that these rates may be much faster than those often considered by modeling studies (>100 °C). For example, in the immediate footwall of the crustal-scale Ben Hope thrust in northwest Scotland, diffusion modeling of quartz inclusions in garnet yields heating rates of ~150–250 °C Myr^-1. In most numerical modeling studies that represent the wedge as a continuum, bulk thermal architecture is governed by the inherent kinematic asymmetry of the pro- and retro- sides of the wedge. In natural systems, major orogenic components such as the high-grade hinterland, low-grade hinterland, and exterior foreland fold-thrust belt are generally separated by discrete faults and/or shear zones. These boundaries commonly accommodate substantial thrust-, and in some cases, normal-sense displacement of 10’s to 100’s of kms. Displacement of this magnitude can lead to juxtaposition of rocks that do not share a common protolith and, perhaps most importantly, finite deformational and thermal history. The substantial apparent thermal break across these boundaries reflects their fundamental influence on the thermal architecture of the system and suggests that thrust advection (and fault-aided exhumation in the case of normal faults) can substantially influence the orogenic metamorphism. In this study, we use 2D finite element models to examine if thrust advection, particularly at normal (10-20 km Myr^-1) to high (>50 km Myr^-1) lateral slip rates, can substantially impact metamorphic heating rates. Simple models that involve a single thrust with a dip of ~30° and geothermal gradients that are initially equal in the hanging wall and footwall yield maximum footwall heating rates of 15, 32, 75, and 150 °C Myr^-1, for imposed thrust rates of 5, 20, 50, and 100 km Myr^-1, respectively. Although the heating rates derived from models with relatively high slip rates are the same order of magnitude as those observed in mineral diffusion studies, they still appear to suggest that other processes may need to contribute to the thermal budget. Future studies will further examine how anatectic magmatism, erosion, and isostasy impact these results.