HIGH GEOTHERMAL GRADIENT METAMORPHISM DURING THERMAL SUBSIDENCE

The burial of a basement sequence enriched in heat producing elements during thermal subsidence following rifting produces two concomitant c hanges in the thermal structure of the crust. Firstly, the burial of t he enriched layer produces high geothermal gradients in the overlying sedimentary succession, with the high gradients propagating down into, but not through, the enriched basement sequence. Secondly, the lithos pheric thickening that drives thermal subsidence reduces the heat flow ing into the deeper crust from the mantle. Because the process of ther mal subsidence promotes burial, it naturally increases the depth exten t of the high geothermal gradients in the upper crust, potentially ind ucing significant temperature increases in the mid-upper crust during burial. The lowering of the thermal gradients in the deep crust accomp anying burial severely Limits the temperature changes affecting the Mo ho; potentially allowing Moho cooling while the mid-upper crust heats. These effects can promote high geothermal gradient (>40 degrees C/km) metamorphism in the mid-upper crust without inducing significant melt ing in the lower crust, providing the basement heat production contrib utes, similar to 70 mW m(-2) to the surface heat flow and that the hor izontal length scale for the basement heat production anomaly is > sim ilar to 50 km. These conditions appear to be met in several Australian intermediate- to high-temperature, low-pressure metamorphic terranes where the thermal causes of metamorphism have hitherto remained enigma tic. One of these terrains, the Mt. Painter province in the northern F linders Ranges, South Australia, is used to illustrate some of the att ributes of the model. (C) 1998 Elsevier Science B.V. All rights reserv ed.