Spatial variation of radiogenic heat production related to the crystalline rock types in the western Himalaya-Karakoram region of Pakistan

The western Himalaya-Karakoram region in northern Pakistan has such hydrothermal features as hot springs and alteration zones. The heat source for these features remains unclear, with suggested mechanisms including radiogenic heat production from minerals, frictional heating caused by shearing along faults, residual heat from Miocene plutonic intrusions, metamorphic heat caused by tectonic collision, and heat advection related to rapid exhumation. In this study, we provide a quantitative estimation of the radiogenic heat production of 158 locations from different crystalline lithologies exposed in the three distinct tectonic domains of the western Himalayan-Karakoram region, i.e., Nanga Parbat-Haramosh Massif, Kohistan-Ladakh Batholith, and Karakoram Batholith. The radiogenic heat production values are calculated from the concentrations of the uranium (ppm), thorium (ppm), and potassium (wt%), which are determined directly in the field using a portable gamma spectrometer on exposures of Proterozoic to Tertiary crystalline rocks. The radiogenic heat production in the Nanga Parbat-Haramosh Massif ranges between 0.72 and 18.46 µWm^-3, with mean and median values of 7.12 and 6.74 µWm^-3, respectively. Furthermore, Proterozoic gneisses, Tertiary granites, and pegmatites within the Nanga Parbat-Haramosh Massif have mean radiogenic heat production values of 7.86, 10.67, and 6.47 µWm^-3, respectively. The radiogenic heat production in the Kohistan-Ladakh Batholith ranges between 0.42 and 5.16 µWm-3, averaging at 2.49 µWm^-3 with the highest mean of 3.68 µWm^-3 in granites and lowest 0.74 µWm^-3 in tonalites. The radiogenic heat production of the Karakoram Batholith ranges between 1.04 and 23.54 µWm^-3 with a mean of 5.84 µWm^-3 and a median of 4.45 µWm^-3. Within the Karakoram Batholith, the Tertiary granites have the highest mean radiogenic heat production of 11.17 µWm^-3,  while the lowest mean radiogenic heat production of 2.86 µWm^-3 is found in the Cretaceous diorites. Our results suggest that the Nanga Parbat-Haramosh Massif, which is composed of Proterozoic Indian plate basement rocks, has high concentrations of uranium, thorium, and potassium, and consequently a higher radiogenic heat production. This also correlates with similar high radiogenic heat-producing basement rocks exposed in southern India. The presence of high radiogenic heat production in Tertiary granites and pegmatites indicates mobilization and enrichment of incompatible uranium and thorium due to crustal evolution processes related to the Himalayan Orogeny. We suggest that high radiogenic heat production in Proterozoic rocks may have contributed significantly to the enhanced heat flux in the active Himalayan Orogen.