How hot are the Cairngorms?

Heat flow measured over the East Grampians batholith in the 1980s was found to be unexpectedly low and at odds with high radiogenic heat production within the outcropping granites and a very large volume of granite predicted from an interpretation of gravity data. Past climate variations perturb temperature gradients in the shallow subsurface leading to erroneous estimates of heat flow. A reconstruction of the surface temperature history during the last glacial cycle has enabled a rigorous palaeoclimate correction to be applied to the heat flow that shows an increase of 25% over previously reported values; revised to 86 +/- 7 mW m(-2). An interpretation of recent mapping reveals that the surface exposures of the East Grampians granites are the roof zones of a highly evolved magma system. Rock composition, therefore, is likely to become more mafic with depth and the heat production will decrease with depth. This petrological model can be reconciled with the gravity data if the shape of the batholith is tabular with deep-seated feeder conduits. The increased heat flow value leads to revised predictions of subsurface temperatures of 129 degrees C at 5 km depth and 176 degrees C at 7 km depth, increases of 40% and 49%, respectively, compared to previous estimates. These temperatures are at the lower end of those currently required for power generation with Engineered Geothermal Systems, but could potentially be exploited as a direct heat use resource in the Cairngorm region by targeting permeable fractures with deep boreholes.