High geothermal gradient metamorphism during thermal subsidence

Abstract

The burial of a basement sequence enriched in heat producing elements during thermal subsidence following rifting produces two concomitant changes in the thermal structure of the crust. Firstly, the burial of the 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 lithospheric thickening that drives thermal subsidence reduces the heat flowing into the deeper crust from the mantle. Because the process of thermal subsidence promotes burial, it naturally increases the depth extent of the high geothermal gradients in the upper crust, potentially inducing significant temperature increases in the mid-upper crust during burial. The lowering of the thermal gradients in the deep crust accompanying burial severely limits the temperature changes affecting the Moho; potentially allowing Moho cooling while the mid-upper crust heats. These effects can promote high geothermal gradient (>40°C/km) metamorphism in the mid-upper crust without inducing significant melting in the lower crust, providing the basement heat production contributes > ~70 mW m-2 to the surface heat flow and that the horizontal length scale for the basement heat production anomaly is > ~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 enigmatic. One of these terrains, the Mt. Painter province in the northern Flinders Ranges, South Australia, is used to illustrate some of the attributes of the model.