C. M. Green

Antarctic geothermal heat flow (GHF) is one of the least constrained basal boundary conditions affecting subglacial hydrology and ice sheet dynamics. Furthermore, the paucity of knowledge about GHF hampers our understanding of the linkages between geodynamic evolution and tectono-thermal conditions in Antarctica.Here we present the results of enhanced spectral analysis of a new Antarctic aeromagnetic anomaly compilation, conformed at long wavelengths with SWARM satellite magnetic data. We apply manual picking of defractal magnetic power spectra on several different major subglacial lake districts in both West and East Antarctica and compare our results with those obtained using automated workflows implemented in PyCurious. Furthermore, we compare our results with independent GHF estimates from seismology, multivariate-similarity approaches and previous magnetic studies.We show that in the Amundsen Sea Embayment in West Antarctica manual spectral picking resolves the spatial heterogeneity in GHF anomalies better than automated approaches. We newly define a wide coastal region of relatively lower values corresponding to recently inferred mafic intrusions within this sector of the West Antarctic Rift System and higher GHF in the Byrd Subglacial Basin. This is highly significant as it suggests that elevated GHF may contribute to the onset of enhanced ice flow in the interior of the Thwaites Glacier catchment. Additionally, we find localised GHF anomalies in the area of the Thwaites active lakes that may affect subglacial water availability and promote reduced basal shear stress despite the widespread hard bed conditions related to the occurrence of predominantly crystalline rocks.In East Antarctica, the manual approach confirms the existence of elevated GHF beneath the Dome C subglacial lake district. However, the anomaly is more linear than previously recognised and better aligned with the trend of major aeromagnetic anomalies interpreted as reflecting extensive Paleo to Mesoproterozoic basement in the sector of East Antarctica. Notably, remarkably similar magnetic anomalies are imaged in formerly contiguous Australia where highly radiogenic igneous provinces significantly enhance GHF.Overall, we find that the choice of appropriate window sizes and spectral ranges coupled with careful inspection of individual power spectra (including the recognition of outliers) and the choice of defractal parameters is important to better define regional scale heterogeneity in Curie Depth estimates. We also find that incorporating the results of independent seismic, multivariate approaches, and expert knowledge in the geological settings of the different study regions is beneficial to better define the realistic ranges of average Curie Depth and for the conversion from Curie Depth to GHF.The results of our magnetic studies need to be integrated into thermal modelling frameworks together with the evolving knowledge of crustal and lithospheric properties in Antarctica, including intracrustal heat production and sedimentary basin distribution. This approach will yield improved spatial resolution and accuracy of Antarctic GHF and better understanding of the geological origin and significance of major GHF anomalies.

Direct geological information in Antarctica is limited to ice free regions along the coast, high mountain ranges, or isolated nunataks. Therefore, indirect methods are required to reveal subglacial geology and heterogeneities in crustal properties, which are critical steps toward interpreting geological history. We present a 3D crustal model of density and susceptibility distribution in the Wilkes Subglacial Basin (WSB) and the Transantarctic Mountains (TAM) based on joint inversion of airborne gravity and magnetic data. The applied “variation of information” technique enforces a coupling between inferred susceptibility and density, relating these quantities to the same gravity and magnetic sources to give an enhanced inversion result. Our model reveals a large body located in the interior of the WSB interpreted as a batholithic intrusive structure, as well as a linear dense body at the margin of the Terre Adélie Craton. Density and susceptibility relationships are used to inform the interpretation of petrophysical properties and the reconstruction of the origin of those crustal bodies. The petrophysical relationship indicates that the postulated batholitic intrusion is granitic, but independent from the Granite Harbor Igneous Complex described previously in the TAM area. Emplacement of a large volume of intrusive granites can potentially elevate local geothermal heat flow significantly. Finally, we present a new conceptual tectonic model based on the inversion results, which includes development of a passive continental margin with seaward dipping basalt horizons and magmatic underplating followed by two distinct intrusive events associated with the protracted Ross Orogen.

SUMMARY: Bedrock geology from Antarctica remains largely unknown since it is hidden beneath thick ice sheets. Geophysical methods such as gravity and magnetic inverse modelling provide a framework to infer crustal rock properties indirectly in Antarctica. However, due to limited availability of rock samples, validation against direct geological information is challenging. We present a new rock property catalogue containing density and susceptibility measurements on 320 rock samples from northern Victoria Land. This catalogue is used to assess the reliability of local and regional scale inverse results, including a new local high resolution magnetic inversion in the Mesa Range region and a previously published regional scale joint inversion of gravity and magnetic data in northern Victoria Land and the Wilkes Subglacial Basin. We compare our density and susceptibility measurements to global and local measurements from the literature to access the correlation to rock types and geological units. Furthermore, the measured values are compared against inverted values. The close correspondence between inverted and measured rock properties allows us to predict locations of rock types where currently such information is missing. The utility of measured susceptibility and density relationships for interpreting inversion output provides a strong incentive to incorporate local rock samples into geophysical studies of subglacial geology across Antarctica.

Direct geological information in Antarctica is limited to ice free regions along the coast, high mountain ranges or isolated nunataks. Therefore, indirect methods are required to reveal subglacial geology and heterogeneities in crustal properties, which are critical steps towards interpreting geological history. We present a 3D crustal model of density and susceptibility distribution in the Wilkes Subglacial Basin and the Transantarctic Mountains (TAM) based on joint inversion of airborne gravity and magnetic data. The applied “variation of information” technique enforces a coupling between gravity and magnetic sources to give an enhanced inversion result. Our model reveals a large-scale body located in the interior of the Wilkes Subglacial Basin interpreted as a batholithic intrusive structure, as well as a linear dense body at the margin of the Terre Adélie Craton. Density and susceptibility relationships are used to inform the interpretation of petrophysical properties and the reconstruction of the origin of those crustal blocks. The petrophysical relationship indicates that the postulated batholitic intrusion is granitic, but independent from the Granite Harbour Igneous Complex previous described in the TAM area. Emplacement of a large volume of intrusive granites can potentially elevate local geothermal heat flow significantly. Finally, we present a tectonic evolution sketch based on the inversion results, which includes development of a passive continental margin with seaward dipping basalt horizons and magmatic underplating followed by two distinct intrusion events in the Wilkes Subglacial Basin with Pan-African ages (700 - 551 Ma) and Ross ages (550 - 450 Ma).

Geothermal heat flux (GHF) is a critical basal boundary condition that exerts important influences on the initiation of flow of the Antarctic ice sheet and is related to crustal and lithospheric structure and composition. Despite its importance, our knowledge of Antarctic GHF heterogeneity remains limited and this hinders interdisciplinary efforts to better constrain Solid Earth influences on subglacial hydrology and ice sheet behaviour.Within the framework of the 4D Antarctica ESA project we produced a new continent-wide aeromagnetic anomaly compilation for Antarctica, conformed at longer wavelengths with SWARM satellite magnetic data. It includes recent data collected after the ADMAP 2.0 compilation, over the Ross Ice Shelf, the Amundsen Sea Embayment and the Recovery and South Pole regions, as well as enhanced maps for the Gamburtsev Subglacial Mountains and Wilkes and Dome C regions, based on relevelling. We applied Curie Depth Point (CDP) estimation using the centroid, modified centroid and fractal and defractal approaches. We tested different window sizes at continental scale and for detailed analysis (200x200 km; 300x300 km; 400x400 km) and centroid distances, and both automated ranges and hand-picked intervals over selected features. The estimates reveal regions of enhanced GHF along the coast of the Amundsen Sea Embayment, in general agreement with independent seismological estimates, and are interpreted as reflecting dynamic interactions between the West Antarctic Rift System and anomalously warm Pacific upper mantle at depth. A higher degree of continuity of potential thermal anomalies related to the Byrd Subglacial Basin is evident between the Thwaites and Pine Island catchments compared to a recent magnetic estimate (Dziadek et al., 2021). A large area of enhanced GHF under the Siple Coast ice streams and active subglacial lake districts is confirmed, but has lower values and greater complexity than previously imaged (Martos et al., 2017). This can be correlated with regions of thinner and thicker crust and different magnetic patterns as revealed from inversion of satellite and airborne gravity and aeromagnetic data respectively. In East Antarctica, the new CDP estimates suggest that any Mesozoic to Cenozoic extension is restricted to upper crustal levels and is focussed in narrow regions. Intriguing, relatively shallow CDP anomalies (given their location within the composite East Antarctic craton) are revealed in the Dome C lake district and also Gamburtsev Subglacial Mountains lake district regions. These may speculatively stem either from intraplate Mesozoic to Cenozoic fault reactivation and/or enhanced intracrustal heat production. We conclude that our new Curie depth estimates yield geologically reasonable thermal boundary conditions, which can be used together with independent estimates derived e.g. from seismology, machine leaning and multi-variate analysis to initialise new thermal models that incorporate crust and lithosphere thickness variations and intracrustal composition (as a proxy for ranges of radiogenic heat production and thermal conductivity).

The Transantarctic Mountains (TAMs) separate the warmer lithosphere of the Cretaceous‐Tertiary West Antarctic rift system and the colder and older provinces of East Antarctica. Low velocity zones beneath the TAM imaged in recent seismological studies have been interpreted as warm low‐density mantle material, suggesting a strong contribution of thermal support to the uplift of the TAM. We present new Curie Point Depth (CPD) and geothermal heat flow (GHF) maps of the northern TAM and adjacent Wilkes Subglacial Basin (WSB) based exclusively on high resolution magnetic airborne measurements. We find shallow CPD and high GHF beneath the northern TAM, reinforcing the hypothesis of thermal support of the topography of the mountain range. Additionally, this study demonstrates, that limiting spectral analysis to areas with a high density of aeromagnetic measurements increases the resolution of CPD estimates revealing localized shallow CPD and associated high heat flow in the Central Basin of the WSB and the Rennick Graben (RG). Across the study area the CPD ranges from 15 to 35 km and the GHF values range from 30 to 110 mW/m². The recovered CPD range is compatible with recent Moho depth estimates, as the CPD predominantly lies within the crust, rather than in the magnetite‐poor mantle. GHF estimates, based on the CPD estimates, show a good agreement to sparse in situ GHF measurements and the location of active volcanoes. Comparison to existing continent‐wide GHF estimates shows strong differences from magnetically‐derived heat flow estimates, while seismologically‐derived heat flow estimates show the best agreement to our results.

The Transantarctic Mountains (TAM) separate the warmer lithosphere of the West Antarctic rift system and the colder East Antarctic craton. Low velocity zones beneath the TAM imaged in recent seismological studies have been interpreted as warm low-density mantle material, suggesting a strong contribution of thermal support to the uplift of the TAM. We present new Curie Depth Point (CDP) and geothermal heat flow (GHF) maps of the northern TAM and adjacent Wilkes Subglacial Basin (WSB) based on high resolution magnetic airborne measurements. We find shallow CDP and high GHF, beneath the northern TAM reinforcing the idea of thermal support of the topography of the mountain range. Additionally, locally high GHF is observed in the Central Basin of the WSB and the Rennick Graben, which has not been resolved previously, while the broader WSB show lower GHF. Across the study area the GHF values range from 30 to 110 mW/m ² . Lastly, we compare our CDP estimates to recent Moho depth estimates and our GHF estimates to sparse in situ GHF measurements as well as to existing continent-wide GHF estimates, which shows closed agreement to previous seismic estimates.

An Andean-style convergent margin was active between ca 580 and 460 Ma along the margin of Gondwana. It led to the emplacement of a major magmatic arc, which is in parts exposed along the much younger Transantarctic Mountains (TAM). Arc magmatism, thrusting, deformation and metamorphism are hallmarks of the long-lived subduction-related Ross Orogen (RO).Despite the wealth of knowledge on the RO, the location, structure and evolution of the unexposed boundary between the Precambrian Mawson Craton and the RO remains very poorly known, particularly in the South Pole (SP) region- one of the largest poles of ignorance in the whole of East Antarctica.  Here we combine new aeromagnetic data collected during the ESA PolarGAP campaign with vintage ADMAP 2.0 (Golynsky et al., 2018- GRL) aeromagnetic datasets in the SP region and level these using the satellite magnetic LCS-1 model to investigate the craton margin and RO. The final levelled data were draped at 2800 m above the bedrock topography (Morlighem et al., 2020, Nature Geo.) and reduced to the pole.  To enhance magnetic signatures and reveal subglacial basement terranes we applied pseudo-gravity transforms, derivatives and upward continuation. We also computed new airborne gravity residual maps and compared these with enhanced magnetic anomaly images. We applied a variety of depth to source of the magnetic and gravity residual anomalies, including tilt depth, Werner and Euler Deconvolution and constructed simple 2D models of the crustal architecture of the RO and the adjacent Precambrian craton margin.  Using the information from enhanced aeromagnetic imaging and combined magnetic and gravity modelling we propose a new tectonic model for the region. In our model, a former late Neoproterozoic rifted margin that developed along an irregular cratonic margin of the Mawson continent evolved during the Ross Orogen in a wide back-arc basin tectonic setting, linked to a predominantly retreating accretionary subduction-related setting from ca 530 Ma to 500 Ma. This led to the emplacement of magnetite-rich ribbons of arc crust, which are magnetically imaged for the first time in this sector of the active margin. Complex deformation of these ribbons is also imaged from aeromagnetic signatures and appears to resemble some of the deformation patterns observed in the Tasmanides in Australia in evolving retreating accretionary arc and back arc systems (e.g Moresi et al., 2014, Nature).

The shapes and directionality of bathymetry at slow-spreading ridges are key to understanding the magmatic or tectonic emplacement of the crust. Magmatic terrain is marked by linearly fault-bounded abyssal hills, while tectonic terrain is marked by long-lived detachment faults, forming Oceanic Core Complexes (OCCs). However, the quantitative description of these crustal regimes is still limited. We develop a novel automated terrain classification technique and test it at the 13-15° N section of the Mid-Atlantic Ridge. The algorithm uses the Slope-Weighted Eccentricity (SWE) of the horizontal eigenvalues to represent surface directionality and reveal crustal tectonic fabric. The application of this new technique yields results consistent with qualitative interpretation. Thus, it provides both new insights into the mid-oceanic ridge spreading and the potential to automate such mapping with different sets of grids, such as gravity and magnetic data in regions further away from the ridge where sediments mask sea-bed features.