New models of thermal evolution and fluid flow in collisional orogens

This work explores the thermal evolution and fluid flow patterns that arise during regional metamorphism of a collisional orogenic terrane. A set of two-dimensional forward numerical models is constructed to evaluate existing hypotheses bearing on the directions and magnitudes of regional fluid flow in the middle and deep crust. The models specifically test for large-scale upward, lateral, and convective flow regimes, as well as for flow in a direction of increasing temperature (up-T). Several geologic factors that have rarely been introduced in modeling of regional metamorphism are considered: (1) prograde and retrograde metamorphic reactions; (2) anisotropy and spatial heterogeneity in crustal permeability; (3) temporal evolution of permeability as a result of metamorphic reactions and hydrofracturing; (4) variable crustal lithology; and (5) synmetamorphic magmatic intrusions. The simulation results suggest that deep and mid-crustal flow of fluid in overthrust settings is mainly driven by dehydration reactions; the typical fluid flow pattern is towards the surface, in the direction of decreasing temperature. Convective, downward, or up-T fluid flow is very limited during amagmatic metamorphism of the middle and lower crust, even in the presence of permeability anisotropy and heterogeneities. However, in the models with synmetamorphic magmatic intrusions, a long-lived prograde up-T flow of fluid may appear as a consequence of the inverted thermal profile within the depth range of magmatism. Thermal calculations suggest that heat advection by fluid flow is relatively minor in the overall metamorphic heat budget, at least for the parameter space investigated herein. Dehydration and decarbonation reactions, on the other hand, may consume significant amounts of heat and noticeably affect the pressure-temperature paths of metamorphic rocks. The strongest effect on the thermal evolution of collisional terranes is produced by synmetamorphic magmatism. Repetitive sills of basaltic or intermediate---felsic composition emplaced at different depths within overthickened crust may produce Barrovian and Buchan-type metamorphic sequences and rocks in the granulite facies of metamorphism. Several phenomena predicted by the numerical models of shallow and mid-crustal plutonism are observed in the type locality of Barrovian and Buchan metamorphism in the Scottish Dalradian, suggesting that magmatism was important in driving the metamorphic reactions.