The cratonic nature of the interior of East Antarctica means elevated geothermal heat flux is an unexpected result8. A number of geological factors could cause the observed elevated geothermal flux and basal melting. The adjacent West Antarctic region is predicted by many studies to be a region of elevated geothermal flux5,7,36,37, linked to the existence of a major Cretaceous to Cenozoic rift system38. Notably, recent passive seismic studies have revealed warm West Antarctic mantle impinging beneath the Transantarctic Mountains towards the South Pole region39 (Fig. 3b). However, hot mantle as the direct source for the observed geothermal anomaly is unlikely given the localised area of identified ice sheet melting and low background geothermal flux values we found. Cenozoic magmatism associated with the warm mantle could generate a localised geothermal anomaly via intrusions or volcanism. Outcrops40 and erratics41 show Cenozoic (17–20.6 Ma) volcanics are present in neighbouring sectors of the Transantarctic Mountains (Fig. 3b). Aeromagnetic data suggests the sub-ice outcrops, which are the likely source of the volcanic erratics, are within the region of the warm mantle intrusion, but do not extend further towards South Pole41,42. This observation, in conjunction with the magnetotelluric43 and seismic imaging of thick cold lithosphere39 towards the area of elevated geothermal heat flux we identify means we do not favour a West Antarctic, or associated Cenozoic volcanic source for the observed geothermal feature.

Although cratons generally show low geothermal flux8 the Central Australian Heat Flow Province44 provides an example of a cratonic region with anomalously high geothermal flux. High heat flux values within a craton are due to radiogenic granitoids in the upper crust, which give rise to local geothermal anomalies. In the Central Australian Heat Flow Province, Meso and Paleoproterozoic (ca 1800-1600 Ma) granitoids cause locally high geothermal heat flux and likely also underlie parts of the East Antarctic conjugate margin11. In other areas of East Antarctica Cambrian granites11 have been recognised which could create local geothermal anomalies of up to 120 mW m−2, and radiogenic Jurassic granites in the Ellsworth-Whitmore Mountains microcontinent45 are modelled to give geothermal anomalies of up to 95 mW m−2. Geothermal anomalies in the East Antarctic interior generated by radiogenic source rocks depend on such rocks being present. Provenance studies from the catchments of the Byrd and Nimrod glaciers (Fig. 3b) indicate the basement is composed largely of Proterozoic (1.2–2.0 Ga) granitoids46. Low heat production is recorded in 16 out of the 17 basement samples47, but one 1850 Ma sample stands out as being highly radiogenic, giving a predicted heat flux of 83.6 mW m−2. The presence of this sample indicates that although sparse, it is reasonable to infer that high heat producing granitoids exist in the South Pole region of East Antarctica. Additionally, it is important to consider that radiogenic intrusions could be buried and hence be more widespread than erratics alone appear to indicate.

The peak geothermal flux predicted from erratics in the Transantarctic Mountains47 is 83.6 mW m−2, and requires a heat production of ~7.5 μW m−3. This heat flux is ~35 mW m−2 above background, but significantly less than our modelled value of 120 mW m−2. Studies in other parts of Antarctica45 have shown that heat flux 30 mW m−2 above the regional background requires a highly radiogenic granite 8 km thick with a mean heat production of 5.35 μW m−3. Models11 indicate that a thermal anomaly of ~120 mW m−2 requires heat production of ~50 μW m−3, almost an order of magnitude above anything measured in the Transantarctic Mountains erratics47. An additional mechanism is therefore likely required to explain the observed geothermal anomaly. Hydrothermal circulation, in particular where faults provide a rapid conduit to the surface for water heated at depth, is a mechanism that can act to enhance local geothermal flux. Studies in the US Basin and Range province48, along the Têt fault system in the French Pyrenees49, and in the Rhine Graben50 demonstrate that thermal anomalies resulting from fluid circulation within the brittle upper crust are significant and can more than double local geothermal flux. The geothermal anomaly we identify lies at the foot of a ~450 m escarpment at the boundary between a broad flat plain and a topographic highland (Fig. 2a). Although radar data alone does not uniquely define faults, this bedrock configuration is consistent with the presence of a partly eroded fault scarp adjacent to a half graben, as seen for example in mainland Greece51. We therefore propose that hydrothermal circulation, influenced by a fault system, is a potential additional explanation for the high amplitude of the geothermal anomaly we model. Notably, the fault scarp we infer lies along strike from the Recovery Highlands (Fig. 3b). This uplifted fault-bounded block is located at an elevation of ~1000 m above a branch of the Permian to Cretaceous East Antarctic Rift System that dissects East Antarctica4. Recent earthquake focal solutions support the interpretation that this is a fault bounded mountain range and indicates that some of the faults systems flanking the range are still active52. Such active faulting is likely to help maintain high permeability along the fault system, facilitating continued geothermal circulation, thereby contributing to enhanced geothermal flux49.

Our recognition of a major geothermal anomaly at the ice divide close to South Pole and within East Antarctica has several significant implications. Firstly, it implies that the preservation of ancient climatic records around the South Pole region is likely not as promising as proposed by several previous continental scale ice sheet models14. Inclusion of both our maximum and minimum values of heat flux in glaciological models will allow for a better estimation of the potential spatial distribution of >1 Ma old ice. Secondly, our discovery of an additional water source at the ice divide helps explain the numerous lakes further downstream, including the dynamic systems in the lower parts of the glacial catchments. Such an active hydrological system may in turn influence the onset and maintenance of enhanced ice flow beneath some of Antarctica’s largest glaciers, as suggested for the adjacent Recovery Glacier system34. Thirdly, given our interpretation of the origin of the geothermal anomaly we suggest that highly radiogenic rocks and intraplate faulting may exert important influences on basal melting patterns around South Pole. Similar processes may affect other largely unexplored areas of interior East Antarctica. We conclude that higher resolution future geophysical studies and drilling are required to better constrain how geology and geothermal heat flux influences the variability in subglacial and englacial conditions in both East and West Antarctica.