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Curie depth offers a valuable constraint on the thermal structure of the lithosphere, based on its interpretation as the depth to 580 ∘ C, but current methods underestimate the range of uncertainty. We formulate the estimation of Curie depth within a Bayesian framework to quantify its uncertainty across the British Isles. Uncertainty increases exponentially with Curie depth but this can be moderated by increasing the size of the spatial window taken from the magnetic anomaly. The choice of window size needed to resolve the magnetic thickness is often ambiguous but, based on our chosen spectral method, we determine that significant gains in precision can be obtained with window sizes 15–30 times larger than the deepest magnetic source. Our Curie depth map of the British Isles includes a combination of window sizes: smaller windows are used where the magnetic base is shallow to resolve small-scale features, and larger window sizes are used where the magnetic base is deep in order to improve precision. On average, the Curie depth increases from Laurentian crust (22.2±5.3 km) to Avalonian crust (31.2±9.2 km). The temperature distribution in the crust, and associated uncertainty, was simulated from the ensemble of Curie depth realizations assigned to a lower thermal boundary condition of a crustal model (sedimentary thickness, Moho depth, heat production, thermal conductivity), constructed from various geophysical and geochemical datasets. The uncertainty in the simulated heat flow field substantially increases from ±10 mW m -2 for shallow Curie depths at ∼15 km to ±80 mW m -2 for Curie depths >40 km. Surface heat flow observations are concordant with the simulated heat flow field except in regions that contain igneous bodies. Heat flow data within large batholiths in the British Isles exceed the simulated heat flow by ∼25 mW m -2 as a result of their high rates of heat production (4–6 µ W m -3). Conversely, heat refraction around thermally resistive mafic volcanics and thick sedimentary layers induce a negative heat flow misfit of a similar magnitude. A northward thinning of the lithosphere is supported by shallower Curie depths on the northern side of the Iapetus Suture, which separates Laurentian and Avalonian terranes. Cenozoic volcanism in Northern Britain and Ireland has previously been attributed to a lateral branch of the proto-Icelandic mantle plume. Our results show that high surface heat flow (>90 mW m -2) and shallow Curie depth (∼15 km) occur within the same region, which supports the hypothesis that lithospheric thinning occurred due to the influence of a mantle plume. The fact that the uncertainty is only ±3 –8 km in this region demonstrates that Curie depths are more reliable in hotter regions of the crust where the magnetic base is shallow. [ABSTRACT FROM AUTHOR]

Regions where surface temperature has increased since past glaciation events, such as Ireland, underestimate the heat output of the Earth unless paleoclimate corrections are applied. We apply probabilistic techniques to quantify the uncertainty of 22 paleoclimate‐corrected heat flow estimates in Ireland, which assimilate multiple surface temperature histories associated with 130 ka of glacial oscillation in the British Isles. Heat flow values increase by ∼15 mW/m2 after a paleoclimate correction and provide new insights into the thermochemical structure of the lithosphere. The heat flow regime is broadly delineated by the Iapetus Suture Zone that separates Laurentian to the north and Avalonian terranes to the south (mean surface heat flow of 73 ± 14 and 65 ± 14 mW/m2, respectively). The degree to which heat‐producing elements are partitioned into the uppermost crust is described by the differentiation index of a heat flow province. From Bayesian inversion, we determine that radiogenic elements are substantially more differentiated in the uppermost crust of Laurentia (DI=2.8 ± 1.4) than Avalonia (DI=1.5 ± 1.3), despite a moderately enriched lower crust (0.8 ± 0.3 μW/m3). This is facilitated by a thin yet highly radiogenic layer in the uppermost crust of Laurentia (3.9 ± 1.8 μW/m3). Extrapolating these results across the British Isles and Newfoundland suggests that heat‐producing elements have been more successfully reworked into the upper crust to the north of the Iapetus Suture Zone during continental accretion between Laurentia and Avalonia. Key Points: Average surface heat flow has increased by 20% after probabilistic paleoclimate corrections are appliedLaurentian and Avalonian heat flow provinces are identified, delineated by the Iapetus Suture ZoneA higher degree of radiogenic differentiation is recognized in the upper crust of Laurentia, compared to that of Avalonia [ABSTRACT FROM AUTHOR]

High quality maps of the geothermal gradient and temperature are essential when assessing geothermal potential for a region. However, determining geothermal potential is a challenge as direct measurements of in situ temperature are sparse and individual geophysical methods are sensitive to a range of parameters, not solely temperature. Here, we develop a novel approach to determine the geothermal gradient using a new joint geophysical-petrological inversion where seismic velocities and density in the mantle are related to temperature and bulk composition within a thermodynamic framework. Large datasets of phase velocities of seismic surface-waves are now incorporated into the inversion, and provide essential constraints on the lithospheric thickness and temperature, which shape the crustal geotherms to a significant extent. We also include all available measurements of the surface heat flow, radiogenic heat production (RHP) and thermal conductivity within the crust, to further constrain the temperature and geothermal gradient, in particular in the top few kilometres of the crust. We use Ireland as a case study and show how our new methodology can reproduce the results of previous work but also improve on them, thanks to the complementary sensitivities of the full range of data. Lithospheric and crustal thicknesses have primary controls on the temperature gradient, with areas of thinner lithosphere showing higher geothermal gradients. In some locations, anomalously warm geotherms result from high RHP within crustal granitic rocks, mudstone and shales. RHP is above continental averages across all Ireland, likely due to a crust with mostly felsic lithology. The new methodology provides a robust workflow for determining the geothermal potential in areas with limited direct temperature measurements, facilitating knowledge creation for the transition to sustainable energy sources and energy self-sufficiency. • We have developed a new method to determine subsurface temperature. • The method works in regions with sparse borehole measurements, such as Ireland. • Lithospheric and crustal thickness have primary control on the temperature gradient. • Thermal properties influence temperature with warmer geotherms in granites and muds. • Model results are within ±2 °C at 1 km depth for sedimentary borehole geotherms. [ABSTRACT FROM AUTHOR]