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Subglacial melt has important implications for ice-sheet dynamics. Locating and identifying subglacial lakes are expensive and time-consuming, requiring radar surveys or satellite methods. We explore three methods to identify source regions for lakes using seven continent-wide environmental characteristics that are sensitive to or influenced by ice-sheet temperature. A simple comparison of environmental properties at lake locations with their continent-wide distributions suggests a statistical relationship (high Kolmogorov-Smirnov statistic) between stable lake locations and ice thickness and surface temperatures, indicating melting under passive conditions. Active lakes, in contrast, show little correlation with direct thermally influenced parameters, instead exhibiting large statistical differences with horizontal velocity and bedrock elevation. More sophisticated techniques, including principal component analysis (PCA) and machine learning (ML) classification, provide better spatial identification of lake types. Positive PCA scores derived from the environmental characteristics correlate with stable lakes, whereas negative values correspond to active lakes. ML methods can also identify regions where subglacial lake melt sources are probable. While ML provides the most accurate classification maps, the combination of approaches adds deeper knowledge of the primary controls on lake formation and the environmental settings in which they are likely to be found.

Refereed/Peer-reviewed Multi-mineral petrochronology can effectively track changes in the thermochemical environment experienced by rocks during metamorphism. We demonstrate this concept using garnet–chlorite schists from the Walter-Outalpa Shear Zone of the southern Curnamona Province, South Australia, which reveal a cryptic and protracted (c. 39 Myr) record of high thermal gradient metamorphism. Petrochronological data including in situ monazite U–Pb and garnet Lu–Hf and Sm–Nd dating suggest elevated geotherms were persistent between at least c. 519–480 Ma, throughout the duration of garnet growth. Additional in situ xenotime U–Pb dating implies that partial garnet breakdown occurred between c. 480–440 Ma, likely induced by fluid-rock interaction or exhumation. Although metamorphism temporally overlaps with the timing of the regional Delamerian Orogeny (c. 520–480 Ma), the thermal mechanism to sustain elevated temperatures has remained enigmatic. One-dimensional thermal models are used to appraise the role of radiogenic heat production in driving the observed high thermal gradient metamorphism. The models reveal that with only modest crustal thickening during orogenesis, the endogenous radiogenic heat production hosted within the basement rocks could plausibly provide the thermal impetus for metamorphism.

Igneous and metamorphic rocks contain the mineralogical and geochemical record of thermally driven processes on Earth. The generally accepted thermal budget of the mantle indicates a steady cooling trend since the Archean. The geological record, however, indicates this simple cooling model may not hold true. Subduction-related eclogites substantially emerge in the rock record from 2.1 Ga to 1.8 Ga, indicating that average mantle thermal conditions cooled below a critical threshold for widespread eclogite preservation. Following this period, eclogite disappeared again until ca. 1.1 Ga. Coincident with the transient emergence of eclogite, global granite chemistry recorded a decrease in Sr and Eu and increases in yttrium and heavy rare earth element (HREE) concentrations. These changes are most simply explained by warming of the thermal regime associated with granite genesis. We suggest that warming was caused by increased continental insulation of the mantle at this time. Ultimately, secular cooling of the mantle overcame insulation, allowing the second emergence and preservation of eclogite from ca. 1.1 Ga until present.

Accurate spatial models of tectonic plates and geological terranes are important for analyzing and interpreting a wide variety of geoscientific data and developing compositional and physical models of the lithosphere. We present a global compilation of active plate boundaries and geological provinces in a shapefile format with interpretive attributes (e.g., crust type, plate type, province type, last orogeny). The initial plate and province boundaries are constructed from a combination of published global and regional models that we refine using a variety of geoscientific constraints including, but not limited to, relative GPS motions, earthquakes, mapped faults, potential field characteristics, and geochronology. These new plate model show improved correlation to observed earthquake and volcano occurrences within deformation zones and microplates, compared to existing models, capturing 73 and 80% of these criteria, respectively. Deformation zones and microplates only account for 16% of Earth's surface area. We estimate 57.5% of the Earth’s surface is covered by oceanic crust, which is a slight increase relative to the most recent seafloor age model. The model of last orogenies agrees well with peaks in the globally summed geochronology data. There is room for improvement in future editions of our global plate and geologic provinces model where basins, ice, or lack of geological data fidelity obscure bedrock geology, particularly in the eastern Central Asian Orogenic Belt, much of Africa, East Antarctica, and eastern Australia. Additionally, some province types—orogens, shields, and cratons that are homogenized within our global scheme—can likely be partitioned into smaller terranes with more precise geodynamic attributes. Despite some of these shortcomings, the digital maps presented here form a self-consistent data standard for adding spatial metadata to geoscientific databases. The database is available on GitHub where the geoscience community can provide updates to improve the models and their contemporaneity as new knowledge is acquired. The files are also released in formats suitable for use in Generic Mapping Tools and GoogleEarth.

In this study, we explore small-scale (~1 to 20 km) thermal-refractive effects on basal geothermal heat flux (BGHF) at subglacial boundaries resulting from lateral thermal conductivity contrasts associated with subglacial topography and geologic contacts. We construct a series of two-dimensional, conductive, steady-state models that exclude many of the complexities of ice sheets in order to demonstrate the effect of thermal refraction. We show that heat can preferentially flow into or around a subglacial valley depending on the thermal conductivity contrast with underlying bedrock, with anomalies of local BGHF at the ice–bedrock interface between 80 and 120% of regional BGHF and temperature anomalies on the order of ±15% for the typical range of bedrock conductivities. In the absence of bed topography, subglacial contacts can produce significant heat flux and temperature anomalies that are locally extensive (>10 km). Thermal refraction can result in either an increase or decrease in the likelihood of melting and ice-sheet stability depending on the conductivity contrast and bed topography. While our models exclude many of the physical complexities of ice behavior, they illustrate the need to include refractive effects created by realistic geology into future glacial models to improve the prediction of subglacial melting and ice viscosity.

The Yorke Peninsula region in the southeastern Gawler Craton forms the southern portion of the Olympic Cu-Au Province that hosts the giant Olympic Dam, Carrapateena and Prominent Hill deposits. The Yorke Peninsula hosts substantial Cu-Au mineralisation, with deposits at Hillside and in the Moonta–Wallaroo district that are broadly considered to have formed during the same event as Olympic Dam, the c. 1595–1575 Ma Hiltaba thermal event. Despite the region's economic significance, there is very little information on the nature and history of metamorphism in the Yorke Peninsula region, leaving a considerable gap in the knowledge of the tectonic controls on mineralisation in this area. New U–Pb LA-ICP-MS geochronology on metamorphic titanite, monazite and apatite from Wallaroo Group metasediments indicate the Yorke Peninsula region has a protracted thermal history, with three groupings of metamorphic ages at c. 1585 Ma, 1560–1550 Ma and 1530–1500 Ma, and apatite cooling ages that cluster around c. 1450 Ma. Phase equilibria modelling of andalusite-sillimanite bearing schists and cordierite-sillimanite bearing migmatitic gneisses indicate that metamorphism occurred between 1560 and 1500 Ma occurred at P–T conditions between 3.6 and 4.5 kbar and 660–750 °C. These high thermal gradient conditions (45–50 °C km−1) are recorded at least 20 million years after Hiltaba Suite magmatism; however, they occur within the thermo-chemically anomalous crust of the South Australian Heat Flow Anomaly, which predominantly consists of rock packages with heat production rates 60–100% greater than the global median for rocks of their age. One-dimensional thermal models constrained by the crustal architecture and thermal regime of the SE Gawler Craton suggest the metamorphic conditions recorded between 1560 and 1500 Ma can be achieved by burial of this heat producing element -enriched crust, without the influence of an advective heat source. Within this thermally-anomalous crustal regime in which high thermal gradients can be sustained, small changes in burial depth are considered sufficient to stimulate metal remobilisation in the metal-rich crust of the Yorke Peninsula region. This potential for burial-triggered, thermally-induced fluid liberation and subsequent metal remobilisation may be applicable to the entire eastern Gawler Craton, including the Olympic Cu-Au Province. Refereed/Peer-reviewed

Numerical models of glaciers suggest variations in geothermal heat flux influences basal melting. In this study, we demonstrate the effect of thermal refraction on heat flux variations at the glaci...

Viscosity and melt generation at the base of ice sheets are critically dependent upon heat flow. Yet subglacial heat flow is poorly constrained due to the logistical challenges of obtaining boreholes that intersect the bedrock beneath thick ice cover. Currently, continental estimates of Antarctic heat flow are derived from geophysical methods that provide ambiguous constraints of crustal heat sources, despite their demonstrated importance for accurate predictions of future ice sheet behaviour. This study pursues an alternative approach by using heat flow measurements from rock units in the Coompana Province of southern Australia, which represent the geological counterparts of those beneath Wilkes Land in East Antarctica. We present nine new surface heat flow estimates from this previously uncharacterised region, ranging from 40–70 mW m‐2 with an average of 57 ± 3 mW m‐2. These values compare favourably to recent geophysically‐derived estimates of 50–75 mW m‐2 for the Totten Glacier catchment of East Antarctica, and to the single in situ measurement of 75 mW m‐2 from the Law Dome deep ice borehole. However, they are appreciably lower than the range of 56–120 mW m‐2 (83 ± 13 mW m‐2 average) for the abnormally enriched Proterozoic terranes of the Central Australian Heat Flow Province. This study provides the first regional heat flow map of geological provinces formerly contiguous with East Antarctica through the application of continent‐scale heat flow datasets tied to a Jurassic plate tectonic reconstruction for Gondwana. Our approach reveals several discrepancies with current heat flow models derived from geophysical methods and provides a more robust analysis of subglacial heat flow using this plate tectonic synthesis as a proxy for East Antarctica. Refereed/Peer-reviewed

The fundamental origins of metamorphic rocks as sedimentary or igneous are integral to the proper interpretation of a terrane’s tectonic and geodynamic evolution. In some cases, the protolith class cannot be determined from field relationships, texture, and/or compositional layering. In this study, we utilize machine learning to predict a metamorphic protolith from its major element chemistry so that accurate interpretation of the geology may proceed when the origin is uncertain or to improve confidence in field predictions. We survey the efficacy of several machine learning techniques to predict the protolith class (igneous or sedimentary) for whole rock geochemical analyses using 9 major oxides. The data are drawn from a global geochemical database with > 533 000 geochemical analyses. In addition to metamorphic samples, igneous and sedimentary analyses are used to supplement the dataset based on their similar chemical distributions to their metamorphic counterparts. We train the classifiers on most of the data, retaining ∼ 10% for post-training validation. We find that the RUSBoost algorithm performs best overall, achieving a true-positive rate of > 95% and > 85% for igneous- and sedimentary-derived samples, respectively. Even the traditionally-difficult-to-differentiate metasedimentary and metaigneous rocks of granitic–granodioritic composition were consistently identified with a > 75% success rate (92% for granite; 85% for granodiorite; 88% for wacke; 76% for arkose). The least correctly identified rock types were iron-rich shale (58%) and quartzolitic rocks (6%). These trained classifiers are able to classify metamorphic protoliths better than common discrimination methods, allowing for the appropriate interpretation of the chemical, physical, and tectonic contextual history of a rock. The preferred classifier is available as a MATLAB function that can be applied to a spreadsheet of geochemical analyses, returning a predicted class and estimated confidence score. We anticipate this classifier’s use as a cheap tool to aid geoscientists in accurate protolith prediction and to increase the size of global geochemical datasets where protolith information is ambiguous or not retained.

Crustal heat production accounts for 30 to 40% of continental heat loss and enhances thermally controlled processes such as melting and metamorphism, yet is in general poorly constrained. We present a new model for continental igneous heat production from ∼4 Ga to the present using a global geochemical database of 75,800 whole-rock analyses providing the highest resolution record to date. Hypotheses advanced to explain past heat production–age variations include erosion of the enriched felsic upper crust, decay of the radioactive elements, geodynamic processes such as the supercontinent cycle, secular cooling, lithological controls, and/or a major shift in the bulk composition in the crust during the late Archean. However, previous heat production–age models are often coarsely resolved, poorly sampled, and/or spatially biased. To test these hypotheses and refine secular trends in crustal heat production, we construct a model by correcting for radioactive decay and normalizing by SiO2 content to remove the gross influence of lithology. The variations through time are highly correlated among both plutonic and volcanic samples, as well as mafic and felsic distributions. Unsurprisingly, compositional normalization indicates lithological control is the dominant factor on heat production after the influence of decay is removed. Following these adjustments, we find that heat production has been relatively constant through time. We suggest the heat production–age pattern does not significantly reflect the influences of erosion, secular cooling, depletion, or the supercontinent cycle as suggested by some previous studies. Heat producing element distributions might be expected to be similar regardless of the age of melt generation, once compositionally normalized and adjusted for the decay of the various isotopes. Compared to this reference model, we observe a heat production and heat producing element enrichment deficit, particularly for the Archean and Paleoproterozoic. This deficit is accounted for by a rapid increase in heat producing element concentrations associated with a shift in the bulk composition of the crust from the early Archean to ∼2.7 Ga. Additionally, we suggest that the heat production record may also represent a biased sample set, perhaps as a result of selective preservation due to thermal stability. This new model will lead to better crustal heat production and global heat loss estimates both at present and within Earth's past.

The composition of Earth's early crust is challenging to assess as only a fragmented record remains. As a consequence, deciphering the composition of early crust as a means to understand early processes on our planet often relies on the isotopic composition of resistive minerals. Here we present a new tool for investigating igneous petrogenesis and crustal evolution by combining 87Sr/86Sr measurements of apatite inclusions with U–Pb and Hf isotope analysis of their host zircon crystals. This approach takes advantage of the complementary inverse fractionation behaviour of Rb/Sr and Lu/Hf, and the cogenetic host/inclusion relationship, to link the three isotopic systems and construct an evolution model in triple isotope space. By applying this triple isotope system modelling to Mesoarchean igneous rocks from the Akia terrane in SW Greenland, we reconstruct a bulk SiO2 composition of 63-68 wt% for the precursor source crust and infer an average crustal residence time of 300–350 Ma. Our modelling implicates involvement of intermediate to felsic Paleo-Eoarchean crust in the genesis of voluminous 3.0 Ga magmatic rocks of the Akia terrane, one of the largest components of the Archean North Atlantic Craton. A further outcome of this approach is the ability to place more robust empirical constraints on the Lu/Hf ratio of the precursor crust by comparison of the modelling output to a global whole-rock dataset. The approach in this work provides a template for further application to ancient rocks in order to better understand the evolution of early Earth.

The widespread occurrence of high to ultra-high temperature (HT-UHT) metamorphism in continental crust has been widely documented worldwide. However, there has been ongoing debate on the heat sources responsible for generating these HT-UHT conditions. Generating HT-UHT temperatures is thought to require either singularly, or in combination, long-lived crustal thickening (e.g. orogenic systems) with high radioactive heat production and low erosion rates, or large supplies of heat from the mantle either through conduction within thinned lithosphere (e.g. back-arc) or by advective heating linked to large-scale mantle-derived magma’s. Distinction between these two major thermal sources can made on the crustal heat generation rates and timescales of the HT-UHT metamorphism and the volumes of externally derived high-temperatures magmas. Therefore, a detailed understanding of the terrain-scale heat generation rates, and the metamorphic P-T-­t path inferred from the integration of petrochronology and phase equilibria modelling can provide important information. The Paleoproterozoic Khondalite (metasedimentary) rock system in the North China Craton is thought to represent a typical Paleoproterozoic HT metamorphic belt with local areas reaching UHT conditions and it has been extensively studied. In terms of the thermal drivers, most workers suggest advective heating from the emplacement of mantle related mafic magma, although the apparent volume of clearly-mantle derived magma appears generally insufficient to account for the regional extent of HT-UHT conditions. To better understand the mechanisms leading the HT-UHT conditions, we need (1) regional-scale measurements of in-situ heat producing elements and (2) a better understanding of the duration of HT-UHT conditions on a regional scale. To better characterise in-situ thermal sources we have determined heat generation rates using quantitative in-field gamma ray spectrometer (GRS) analysis. Volume averaging of rock types indicates terrain-scale U-Th concentrations would have generated around 3mWm-3 at the time of metamorphism. Given that U-Th would have been lost from the metamorphic system during extraction of high-temperature crustal melts, simple modelling shows the crustal U-Th concentrations would have contributed substantially to the generation of the high-temperature thermal regime. Furthermore, a preliminary compilation of concordant zircon and monazite metamorphic ages from published literature shows a range of ca. 1950-1850 Ma in both western and eastern Khondalite Belt, suggesting possible long-lived metamorphism. Therefore, we argue that the role of the mantle derived advective heat in generating the UHT regime in the North China Khondalite Belt may have been overestimated. Key words: heat generation, HT-UHT metamorphism, Khondalite Belt, North China Craton

Sedimentary rocks cover ∼73% of the Earth's surface and metamorphic rocks account for approximately 91% of the crust by volume. Understanding the average behavior and variability of heat production for these rock types are vitally important for developing accurate models of lithospheric temperature. We analyze the heat production of ∼204,000 whole rock geochemical data to quantify how heat production of these rocks varies with respect to chemistry and their evolution during metamorphism. The heat production of metaigneous and metasedimentary rocks are similar to their respective protoliths. Igneous and metaigneous samples increase in heat production with increasing SiO2 and K2O, but decrease with increasing FeO, MgO and CaO. Sedimentary and metasedimentary rocks increase in heat production with increasing Al2O3, FeO, TiO2, and K2O but decrease with increasing CaO. For both igneous and sedimentary rocks, the heat production variations are largely correlated with processes that affect K2O concentration and covary with other major oxides as a consequence. Among sedimentary rocks, aluminous shales are the highest heat producing (2.9 μW m−3) whereas more common iron shales are lower heat producing (1.7 μW m−3). Pure quartzites and carbonates are the lowest heat producing sedimentary rocks. Globally, there is little definitive evidence for a decrease in heat production with increasing metamorphic grade. However, there remains the need for high resolution studies of heat production variations within individual protoliths that vary in metamorphic grade. These results improve estimates of heat production and natural variability of rocks that will allow for more accurate temperature models of the lithosphere. Keywords: Heat generation, Density, Metamorphic rocks, Sedimentary rocks, Igneous rocks, Continental lithosphere

The concentration of atmospheric oxygen (pO2) is thought to have increased throughout Earth history, punctuated by rapid increases ca. 2.4 and 0.8 billion years ago near the beginning and end of the Proterozoic Eon. As photosynthesis is the largest source of free O2, the reigning paradigm of rising O2 levels centres around biologic metabolism. Here we show that the phosphorus content of igneous rocks correlates, in a first-order sense, with secular increases in O2 through time, suggesting that rising O2 levels are affected by long-term mantle cooling and its effect on the continental phosphorus inventory. Because phosphorus is the limiting nutrient for primary productivity, its availability has fundamental control over the efficiency of oxygenic photosynthesis, pointing to a previously unrecognized role of the solid Earth in biologic and atmospheric evolution. Furthermore, as many bio-essential elements are effectively incompatible in the mantle, this relationship has implications for any terrestrial planet. All planets will cool, and those with efficient plate tectonic convection will cool more rapidly. We are left concluding that the speed of such cooling may affect pattern of biological evolution on any habitable planet.

The composition of Earth's early crust is challenging to assess as only a fragmented record remains. As a consequence, deciphering the composition of early crust as a means to understand early processes on our planet often relies on the isotopic composition of resistive minerals. Here we present a new tool for investigating igneous petrogenesis and crustal evolution by combining 87Sr/86Sr measurements of apatite inclusions with U–Pb and Hf isotope analysis of their host zircon crystals. This approach takes advantage of the complementary inverse fractionation behaviour of Rb/Sr and Lu/Hf, and the cogenetic host/inclusion relationship, to link the three isotopic systems and construct an evolution model in triple isotope space. By applying this triple isotope system modelling to Mesoarchean igneous rocks from the Akia terrane in SW Greenland, we reconstruct a bulk SiO2 composition of 63-68 wt% for the precursor source crust and infer an average crustal residence time of 300–350 Ma. Our modelling implicates involvement of intermediate to felsic Paleo-Eoarchean crust in the genesis of voluminous 3.0 Ga magmatic rocks of the Akia terrane, one of the largest components of the Archean North Atlantic Craton. A further outcome of this approach is the ability to place more robust empirical constraints on the Lu/Hf ratio of the precursor crust by comparison of the modelling output to a global whole-rock dataset. The approach in this work provides a template for further application to ancient rocks in order to better understand the evolution of early Earth.

Radiogenic heat production is a physical parameter crucial to properly estimating lithospheric temperatures and properly understanding processes related to the thermal evolution of the Earth. Yet heat production is, in general, poorly constrained by direct observation because the key radiogenic elements exist in trace amounts making them difficulty image geophysically. In this study, we advance our knowledge of heat production throughout the lithosphere by analyzing chemical analyses of 108,103 igneous rocks provided by a number of geochemical databases. We produce global estimates of the average and natural range for igneous rocks using common chemical classification systems. Heat production increases as a function of increasing felsic and alkali content with similar values for analogous plutonic and volcanic rocks. The logarithm of median heat production is negatively correlated ( r 2 = 0.98) to compositionally-based estimates of seismic velocities between 6.0 and 7.4 km s −1 , consistent with the vast majority of igneous rock compositions. Compositional variations for continent-wide models are also well-described by a log-linear correlation between heat production and seismic velocity. However, there are differences between the log-linear models for North America and Australia, that are consistent with interpretations from previous studies that suggest above average heat production across much of Australia. Similar log-linear models also perform well within individual geological provinces with ∼1000 samples. This correlation raises the prospect that this empirical method can be used to estimate average heat production and natural variance both laterally and vertically throughout the lithosphere. This correlative relationship occurs despite a direct causal relationship between these two parameters but probably arises from the process of differentiation through melting and crystallization.

SUMMARY Thermal conductivity is a physical parameter crucial to accurately estimating temperature and modelling thermally related processes within the lithosphere. Direct measurements are often impractical due to the high cost of comprehensive sampling or inaccessibility and thereby require indirect estimates. In this study, we report 340 new thermal conductivity measurements on igneous rocks spanning a wide range of compositions using an optical thermal conductivity scanning device. These are supplemented by a further 122 measurements from the literature. Using major element geochemistry and modal mineralogy, we produce broadly applicable empirical relationships between composition and thermal conductivity. Predictive models for thermal conductivity are developed using (in order of decreasing accuracy) major oxide composition, CIPW normative mineralogy and estimated modal mineralogy. Four common mixing relationships (arithmetic, geometric, square-root and harmonic) are tested and, while results are similar, the geometric model consistently produces the best fit. For our preferred model, $k_{\text{eff}} = \exp ( 1.72 \, C_{\text{SiO}_2} + 1.018 \, C_{\text{MgO}} - 3.652 \, C_{\text{Na}_2\text{O}} - 1.791 \, C_{\text{K}_2\text{O}})$, we find that SiO2 is the primary control on thermal conductivity with an RMS of 0.28 W m−1 K−1or ∼10 per cent. Estimates from normative mineralogy work to a similar degree but require a greater number of parameters, while forward and inverse modelling using estimated modal mineralogy produces less than satisfactory results owing to a number of complications. Using our model, we relate thermal conductivity to both P-wave velocity and density, revealing systematic trends across the compositional range. We determine that thermal conductivity can be calculated from P-wave velocity in the range 6–8 km s−1 to within 0.31 W m−1 K−1 using $k({V_p}) = 0.5822 \, V_p^2 - 8.263 \, V_p + 31.62$. This empirical model can be used to estimate thermal conductivity within the crust where direct sampling is impractical or simply not possible (e.g. at great depths). Our model represents an improved method for estimating lithospheric conductivity than present formulas which exist only for a limited range of compositions or are limited by infrequently measured parameters.

The depth to the Curie isotherm provides a snapshot into the deep thermal conditions of the crust, which helps constrain models of thermally controlled physical properties and processes. In this study, we present an updated global Curie depth model by employing the equivalent source dipole method to fit the lithospheric magnetic field model LCS-1 from spherical harmonic degree 16 to 100. In addition to the new field mode, we utilise all three vector components and include a laterally variable magnetic susceptibility model. We also employ an improved thermal model, TC1, to supplement the degree 1 to 15 components that are otherwise contaminated by the core field. Our new Curie depth model differs by as much as ±20 km relative to previous models, with the largest differences arising from the low order thermal model and variable susceptibility. Key differences are found in central Africa due to application of a variable susceptibility model, and shield regions, but continents with poor constraints such as Antarctica require additional improvement. This new Curie depth model shows good agreement with continental heat flow observations, and provides further evidence that Curie depth estimates may be used to constrain evaluations of the thermal state of the continental lithosphere, especially in regions with sparse or surface contaminated heat flow observations.

While surface heat flow relates to the heat loss through the lithosphere, it can be difficult to quantify and separate the heat produced internally through radiogenic decay from the heat transferred across the base of the lithosphere by mantle convection. In this study, we apply a thermo-isostatic analysis to Australia and estimate the sub-lithospheric and radiogenic heat flow components by employing a simple 1-D conservation of energy model. We estimate an anomalous radiogenic heat production across much of eastern Australia generally accounting for >50 mW m −2 , while western Australia appears to have high crustal compositionally corrected elevation, possibly related to chemical buoyancy of the mantle lithosphere. A moderately high sub-lithospheric heat flow (∼40 mW m −2 ) along the eastern and southeastern coast, including Tasmania, is coincident with locations of Cenozoic volcanism and supports an edge-driven convection hypothesis. However, the pattern of sub-lithospheric heat flow along the margin does not support the existence of hotspot tracks. Thermo-isostatic models such as these improve our ability to identify and quantify crustal from mantle sources of heat loss and add valuable constraints on tectonic and geodynamic models of the continental lithosphere's physical state and evolution.

Thermal structure of the lithosphere exerts a primary control on its strength and density and thereby its dynamic evolution as the outer thermal and mechanic boundary layer of the convecting mantle. This contribution focuses on continental lithosphere. We review constraints on thermal conductivity and heat production, geophysical and geochemical/petrological constraints on thermal structure of the continental lithosphere, as well as steady-state and non-steady state 1D thermal models and their applicability. Commonly used geotherm families that assume that crustal heat production contributes an approximately constant fraction of 25–40% to surface heat flow reproduce the global spread of temperatures and thermal thicknesses of the lithosphere below continents. However, we find that global variations in seismic thickness of continental lithosphere and seismically estimated variations in Moho temperature below the US are more compatible with models where upper crustal heat production is 2–3 times higher than lower crustal heat production (consistent with rock estimates) and the contribution of effective crustal heat production to thermal structure (i.e. estimated by describing thermal structure with steady-state geotherms) varies systematically from 40 to 60% in tectonically stable low surface heat flow regions to 20% or lower in higher heat flow tectonically active regions. The low effective heat production in tectonically active regions is likely partly the expression of a non-steady thermal state and advective heat transport.

The Reynolds Range in the Paleo-Mesoproterozoic Aileron Province of central Australia exposes a differentially exhumed metamorphic rock system varying from greenschist-to granulite-facies. Calculated P-T pseudosections indicate that peak metamorphic conditions in greenschist-facies areas reached temperatures of ∼480 °C and pressures of 3.2-5 kbar. Peak conditions at amphibolite-facies are constrained to 500-620 °C and 1.3-3.8 kbar. P-T modelling of two petrologically different granulite-facies samples indicate that peak conditions in the exhumed granulite-facies rocks reached 780-920 °C and 3.5-5.4 kbar, corresponding to high apparent thermal gradients of 152-218 °C/kbar. In the amphibolite-facies rocks, in situ LA-ICP-MS U-Pb monazite geochronology records ages that range between 1680 and 1550 Ma. In contrast, in the granulite-facies rocks, monazite records ages of ca. 1560 Ma. The terrane contains significantly elevated rates of crustal heat production compared to global norms, and when incorporated into lithospheric thermal models with reasonable boundary conditions and average thermal properties, can plausibly account for the long-lasting metamorphic conditions. The thermal modelling shows that the long lasting (>100 Myr) record of monazite U-Pb ages, supported by previous studies, reflects a continuous duration of elevated crustal temperatures, an interpretation consistent with texturally simple mineral assemblages that lack any evidence for polymetamorphism. In high-crustal heat production terranes, high thermal gradients and high temperatures will persist until exhumation removes the upper crust. This physical reality provides a basis for the existence of arbitrarily long-lived metamorphic systems, even under high thermal gradients that are generally considered to be thermally anomalous. However, such thermal anomalism is not remarkable, as there is abundant evidence for elevated rates of crustal heat production in the Reynolds Range and worldwide. In terranes where accessory minerals record large continuous ranges of age, robust volumetric measurements of crustal heat production rates are a necessary step in evaluating the potential thermal significance of such geochronological records. Failure to document rates of crustal heat generation invalidates many interpretations for drivers of metamorphism. Refereed/Peer-reviewed

The radiogenic heat produced by granites has a significant influence on the thermal state of the crust due to both their relatively high heat production with respect to most rock types and high abundance. However, the variations in present day heat production with age are generally based on relatively few measurements that are poorly distributed geographically. In this study, we construct a global model for the heat production of granitic rocks for the past 4 Ga using 13,400 geochemical analyses. We observe a nearly monotonic increase in radiogenic heat production from 4.0 to 2.0 Ga, which mirrors a shift from more TTG-like calcic to more alkalic compositions. This shift towards high-heat-producing granites post-2.0 Ga is often attributed to enrichment related to reworking and/or erosion. However, there is a strong correlation between granitic heat production with that of similarly-aged basalts and gabbros, which suggests a dominant mantle-level component to granite generation rather than crustal reworking. Secular cooling and mantle depletion may affect heat production, but the signal is complex and cannot easily explain the heat production with age profile. The most likely mechanism to describe the observed heat production–age pattern is one of selective preservation as a consequence of thermal stability. High heat producing terranes that were not stable during the Archean become increasingly stable towards the present. This selective preservation model has significant implications for the growth and composition of the continental crust. Ferroan, alkalic and felsic compositions were less thermally stable in the Archean due to their generally higher heat production and thus may have been more common in the early Earth than assumed by most compositional models. The temporal heat production model determined in this study can be used to improve geotherm models, particularly within ancient terranes.

Article history: Using an updated global heat flow dataset with >14000 oceanic measurements, we revise the estimated global power deficit due to ventilated hydrothermal circulation. This study differs from previous estimates by taking into account (1) non-Gaussian statistics, (2) an improved seafloor age model, (3) a new plate cooling model calibrated directly to heat flow, and (4) the effect of sediment cover on the heat flow deficit and ventilated cutoff age. We obtain the maximum heat flow deficit (difference between predicted and observed) when the data are separated by seafloor areas with

Plate tectonic theory predicts subsidence and decreasing heat flow as young, hot buoyant lithosphere ages and cools. Oceanic cooling models are re-calibrated to a new ‘hydrothermal-free’ heat flow dataset to provide a better fit to heat flow on old ( > 80 Ma ) seafloor while simultaneously allowing for a more reasonable mantle potential temperature (1350 °C). The plate model (constant basal temperature) and chablis (constant basal temperature and constant basal heat flow) are tested as viable solutions to the observed data. Both models fit to reasonable values of misfit, but only the plate cooling model is acceptable after considering additional statistical constraints. The best-fitting plate model results in a thinner lithospheric thickness (90 km) than previous estimates. These results improve estimates of global heat loss rate through oceanic crust (29.4 TW, ∼ 44 TW globally) and improves the estimated temporal evolution of the thermal structure of oceanic lithosphere suggesting a shallower lithosphere–asthenosphere boundary. This model can serve as a reference for estimating the global redistribution of heat by ventilated hydrothermal circulation through young seafloor.

An accurate viscosity structure is critical to truthfully modeling lithosphere dynamics. Here, we report an attempt to infer the effective lithospheric viscosity from a high-resolution magnetotelluric (MT) survey across the western United States. The high sensitivity of MT fields to the presence of electrically conductive fluids makes it a promising proxy for determining mechanical strength variations throughout the lithosphere. We demonstrate how a viscosity structure, approximated from electrical resistivity, results in a geodynamic model that successfully predicts short-wavelength surface topography, lithospheric deformation, and mantle upwelling beneath recent volcanism. We further show that this viscosity is physically consistent with and better constrained than that derived from laboratory-based rheology. We conclude that MT imaging provides a practical observational constraint for quantifying the dynamic evolution of the continental lithosphere.

As you receive this issue of The Leading Edge the Summer of Applied Geophysical Experience (SAGE) will be starting our 30th field season. SAGE is a unique educational program that combines teaching and research as a partnership between universities, industry, government agencies and professional societies. SAGE includes a four-week period based in Santa Fe, New Mexico and one-week workshop in the following January for undergraduates, at San Diego State University, which allows us to enhance their research experience. We teach the principles and applications of refraction and reflection seismics, magnetics, gravity, GPS, several electromagnetic (EM) methods and ground-penetrating radar (GPR) in a field-based, hands-on setting. The central research activity of SAGE is the acquisition and interpretation of geophysical field data. Students learn geophysics by doing geophysics—a discovery oriented approach.

A new dataset of nearly 15,000 oceanic heat flow measurements is analyzed to determine the conductive heat loss through the seafloor. Many heat flow values in seafloor younger than 60 Ma are lower than predicted by models of conductively cooled lithosphere. This heat flow deficit is caused by ventilated hydrothermal circulation recharge and discharge at crustal outcrops or areas of thin sedimentary cover. Filtering heat flow data, retaining only sites with > 400 m of sediment cover and located > 60 km from the nearest seamount, minimizes the effect of hydrothermal ventilation. Filtered heat flow exhibits a much higher correlation coefficient with seafloor age (up to 0.95 for filtered data in contrast to 0.5 for unfiltered data) and lower variability (reduction by 30%) within an age bin. A small heat flow deficit still persists at ages

We propose a continental lithosphere heat production model based on the petrology of crust and mantle, heat production measurements of surface and xenolith samples, and tectono-thermal constraints. Continental elevation considered within a thermal isostasy rubric is used to partition crustal heat production into upper crustal and lower crustal contributions. The best-fitting partition model using elevation data from 33 North American tectonic provinces suggests upper crustal heat production on average accounts for ~ 6% of observed surface heat flow. An average heat production for the lower crust of 0.4 μW/m 3 is based on measurements from exposed granulite terranes while a lithospheric mantle heat production of 0.02 μW/m 3 is based on chemical analyses of mantle xenoliths. Results are relatively insensitive to mantle composition and thickness of the upper crustal heat producing layer. Continental geotherms are computed using the generalized heat production model and incorporating thermal conductivity results from a number of recent laboratory studies. P–T conditions of xenoliths provide further constraints to ensure that our geotherms and hence the heat production model are reasonable. P–T conditions of 10 Precambrian regions are consistent with surface heat flow of 40 mW/m 2 and a lithospheric thickness of 200 km. Our generalized model for heat production can serve as a reference model from which anomalies are identified.

[1] To illuminate rifting processes across the Transition Zone between the extensional Great Basin and stable Colorado Plateau interior, we collected an east-west profile of 117 wideband and 30 long-period magnetotelluric (MT) soundings along latitude 38.5°N from southeastern Nevada across Utah to the Colorado border. Regularized two-dimensional inversion shows a strong lower crustal conductor below the Great Basin and its Transition Zone in the 15–35 km depth range interpreted as reflecting modern basaltic underplating, hybridization, and hydrothermal fluid release. This structure explains most of the geomagnetic variation anomaly in the region first measured in the late 1960s. Hence, the Transition Zone, while historically included with the Colorado Plateau physiographically, possesses a deep thermal regime and tectonic activity like that of the Great Basin. The deep crustal conductor is consistent with a rheological profile of a brittle upper crust over a weak lower crust, in turn on a stronger upper mantle (jelly sandwich model). Under the incipiently faulted Transition Zone, the conductor implies a vertically nonuniform mode of extension resembling early stages of continental margin formation. Colorado Plateau lithosphere begins sharply below the western boundary of Capitol Reef National Park as a resistive keel in the deep crust and upper mantle, with only a thin and weak Moho-level crustal conductor near 45 km depth. Several narrow, steep conductors connect conductive lower crust with major surface faulting, some including modern geothermal systems, and in the context of other Great Basin MT surveying suggest connections between deep magma-sourced fluids and the upper crustal meteoric regime. The MT data also suggest anisotropically interconnected melt over a broad zone in the upper mantle of the eastern Great Basin which has supplied magma to the lower crust, consistent with extensional mantle melting models and local shear wave splitting observations. We support a hypothesis that the Transition Zone location and geometry ultimately reflect the middle Proterozoic suturing between the stronger Yavapai lithosphere to the east and the somewhat weaker Mojave terrane to the west. We conclude that strength heterogeneity is the primary control on locus of deformation across the Transition Zone, with modulating force components.

[1] Continental elevations can be partitioned into contributions from thermal and compositional buoyancy and geodynamic forces. In order to isolate the thermal component, elevations of 36 North American tectonic provinces are adjusted for the effects of compositional thickness and density variations by computing an isostatic adjustment relative to a standard crustal section (2850 kg m−3 density and 39 km thickness). Crustal densities are estimated using an empirical velocity-density relationship. Compositional elevation adjustments applied to North American provinces range from ∼−1100 m in the southern Rocky Mountains to 2300 m in the Gulf of California, with uncertainties ranging from ∼200 m to >600 m. Theoretical thermal buoyancy is estimated by integrating the difference between a geotherm derived from observed values of heat flow and a reference geotherm. The best fitting continental heat flow–elevation model has a reference heat flow of 46.6 mW m−2 at 0 km and a 60:40 partitioning of surface heat flow between reduced and upper crustal radiogenic heat flow. Raw elevations of continental provinces show little correlation with heat flow, while compositionally adjusted elevations show a clear trend with ∼3 km difference between hot and cold provinces. A continental heat flow–elevation plot is used to identify outliers in adjusted province elevations. Anomalous elevations may reflect a nonsteady state thermal regime, dynamically supported elevation, anomalous mantle, or some combination of these states. Discriminating between these elevation sources provides insight into the geodynamics of North America, demonstrating the usefulness of this approach in continental geodynamic studies.

[1] Continental elevations result from a combination of compositional and thermal buoyancy and geodynamic forces. Thermal effects are often masked by compositional variations to crustal density and thickness that produce equal or greater relief. We have developed a method by which compositional variations in the crust may be removed, thereby isolating the thermal contribution to elevation. This isostatic correction normalizes the composition of a region to a standard crustal column 39 km thick having an average density of 2850 kg m−3. The crustal thickness and density are computed using one-dimensional seismic VP models and an empirical velocity to density conversion. Continental regions adjusted for compositional effects show that thermal isostasy can produce nearly 3 km of relief between cold shield platforms and hot rift zones, comparable to observed relief between young (hot) and old (cold) regions of oceanic lithosphere. A Monte Carlo analysis is used to estimate the uncertainties in the isostatic correction. Uncertainties in the seismic parameters, surface heat flow, and regression coefficients of the velocity-density relationship are all incorporated into the analysis. The Wyoming Craton is used as a case study to demonstrate the effectiveness of the elevation adjustment. Analyses of seismic VP models yield a crustal thickness of 49.5 ± 4.9 km and density of 2945 ± 13 kg m−3. The computed compositional correction to elevation for the Wyoming Craton is −131 ± 180 m, shifting the raw elevation of 1069 m to an adjusted elevation of 938 m.