Barrovian regional metamorphism: where's the heat?

Coupled thermal-mechanical models of convergent orogens offer a novel way to investigate the interactions between heat and tectonics that lead to regional metamorphism. In this study, the effects of different distributions of heat-producing material in the crust and upper mantle on crustal thermal histories and deformation fields are investigated. The models involve subduction-driven collision with moderate convergence and erosion rates. For models involving standard continental crust, where heat production is initially concentrated in the upper crust, P-T-t paths do not intersect the held of typical Barrovian P-T conditions. However, heat-producing material can be tectonically redistributed, for example, by subduction of crustal rocks to upper mantle depths, or by formation of thick accretionary wedges or continental margin sequences during convergence. Models that include a wedge of heat-producing material in the upper mantle generate high temperatures in the lower crust and upper mantle that lead to a change in orogenic style; radioactive heating of partially subducted crustal material on time scales of 10-30 Ma yields temperatures high enough for partial melting. However, crustal P-T-t paths are unlikely to intersect the Barrovian field unless erosion or convergence rates change. Models that include a crustal-scale region with moderate, uniform heat production, simulating a large accretionary wedge or tectonically thickened continental margin sequence, generate P-T-t paths that intersect the Barrovian field. However, as convergence proceeds, the heat-producing region is deformed, eroded, and reduced in volume, so that the model orogen begins to cool down after about 20 Ma. The model results provide an explanation for many first-order tectonic and metamorphic features of small orogens, including metamorphic styles ranging from blueschists to the Barrovian series to granulites, late-orogenic granitoid magmatism, and the crustal-scale tectonic features associated with regional metamorphic belts. We conclude that the thermal state of an orogen is controlled by the evolving competition between cooling by subduction and radioactive heating within the deforming orogen.