THE THERMAL STRUCTURE OF COLLISIONAL OROGENS AS A RESPONSE TO ACCRETION, EROSION, AND RADIOGENIC HEATING

Thermal models of collisional orogens generally predict temperature st ructures that are much cooler than those recovered by thermobarometric studies. Here we demonstrate that high-temperature, low-pressure meta morphism and the development of inverted geotherms within collisional belts may be the result of accretion and erosion acting on crust enric hed with heat-producing elements. A new two-dimensional finite differe nce model, described here, incorporates the subduction of lithosphere with heat-producing material in the upper crust, accretion of crustal material from the subducting plate to the upper plate, and surface ero sion of the upper plate. These processes result in the development of a wedge of heat-producing material within the upper plate. The rate of heat production within the wedge and maximum depth of the wedge are t he most important parameters controlling the magnitude of upper plate temperatures. Our model yields inverted upper plate geotherms when hea t production rates exceed 0.75 mu W/m(3) and the heat-producing wedge extends to a depth greater than 35 km. Temperatures in excess of 500 d egrees C at depths of 20-30 km are computed when heat production rates are greater than similar to 1.75 mu W/m(3) and the wedge extends to a depth > 50 km. Other processes, such as shear heating, fluid flow, or mantle delamination, need not be invoked to explain geologic evidence of high temperatures or inverted thermal gradients in collisional sys tems.