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In this work, we calculated the Curie Point Depth (CPD) for the Brazilian Equatorial Margin using the Fourier and continuous wavelet (CWT) transforms considering a linear inversion for random and fractal magnetization models with four different magnetic datasets. After comparing our results with Curie maps reported by previous studies, we concluded that the more consistent Curie isotherm was obtained by applying the random model based on CWT to the Brazilian Equatorial Margin Magnetic Map dataset, a compilation of airborne and marine magnetic surveys complemented with magnetic anomaly data extracted from the World Digital Magnetic Anomaly Map in the deep-water oceanic domain. In addition, we estimated heat flow using CPD results and compare with a global heat flow model based on geothermal measurements. In the continental domain, Curie values range from 18 to 35 km, shallowing westwards until reaching anomalously 10 km in the Tocantins Province. In turn, the heat flow decreases eastwards from 88 to 58 mW/m2 as the Curie depth increases within the continental crust. Meanwhile, in the deep-water oceanic domain, the heat flow is overall higher (between 72 and 87 mW/m2), indicating a younger and warmer oceanic crust, whereas the Curie isotherm is shallower, varying between 10 and 30 km. Even so, it is deeper than Moho, suggesting a magnetized lithospheric mantle.

Surface heat flow provides essential information on the thermal state and thickness of the lithosphere. Southern Africa is a mosaic of the best‐preserved and exposed crustal blocks, assembled in the early late Archean and then modified by a series of major tectono‐thermal events, both of Precambrian and Phanerozoic. Understanding the thermal and compositional structure of the southern African lithosphere provides crucial information for the actual causes, processes of lithospheric stability, and modification. Curie depth, interpreted as the depth to 580°C, provides a valuable constraint on the thermal structure of the lithosphere. Due to the sparse distribution of surface heat flow data, we examine the degree to which the thermal structure of the crust can be constrained from Curie depth estimates in southern Africa. We provide a Curie depth map for southern Africa (obtained from the inversion of magnetic anomaly data via power spectral methods and wavelet analysis) alongside with a heat flow map (based on the previous Curie depth estimates), both equipped with uncertainties via a Bayesian approach. Opposed to other cratonic regions, the observation of a shallow Curie depths and low heat flow over the Kaapvaal Craton suggests a thermochemical reworking of the cratonic lithosphere in this region. Furthermore, a comparison with a model for the Moho depth reveals significant variations of the Curie depth, which may be located below or above the Moho in large regions. Both observations indicate that in certain regions magnetic anomaly based Curie depth estimates may reflect a compositional rather than a temperature controlled constraint., Plain Language Summary: The thermal state and thickness of the lithosphere are reflected, among other quantities, in the surface heat flow. While heat flow data are rather sparse, magnetic anomaly maps are widely available and allow, under certain conditions, the estimation of the bottom of the magnetized layer within the lithosphere. Latter can be associated with the 580°C‐isotherm (frequently called the Curie depth), therefore allowing inferences on the thermal state of the lithosphere from magnetic data. Here, we use classical power spectral methods in combination with wavelet analysis and Bayesian methods to estimate the Curie depth and its uncertainty from magnetic anomaly maps, and subsequently, use it to estimate the surface heat flow over southern Africa. Comparison with the sparsely available measured heat flow allows us to assess the quality of the estimation and to interpret it with respect to the lithospheric structure. Southern Africa is particularly suitable for such a study due to its well‐preserved and well‐studied crustal blocks. The shallow Curie depths and low heat flow observed over the Kaapvaal Craton suggest a thermal reworking of the old lithospheric structures in this region. Comparison with a Moho depth model further suggests that in some regions the obtained Curie depth estimates do not necessarily reflect a temperature controlled but rather a compositional boundary., Key Points: Curie depth is estimated over southern Africa via power spectral methods and wavelet transforms within a Bayesian framework. Heat flow and its uncertainty is estimated based on Curie depth. Shallow Curie depth and low heat flow suggest thermochemical reworking of the Kaapvaal Craton., Bundesministerium für Wirtschaft und Energie (BMWi) http://dx.doi.org/10.13039/501100006360, Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) http://dx.doi.org/10.13039/501100003246

The Gulf of Cadiz has been affected by a long and complex geodynamic evolution. The lithospheric structure is poorly understood in this region, and it also shows a diffuse seismicity, spanning over a broad area. The Canary Archipelago has been extensively studied. Nevertheless, there are fundamental topics that are still under debate. No studies have addressed or suggested the possibility of a plausible geodynamic connection between both remarkable locations. In this study we integrate total tectonic subsidence (TTS), Curie point depth (CPD), Bouguer gravity anomaly, and seismic information. TTS shows the existence of a basement bulge in the area of the Canary Archipelago that extends to the north, and in the area of Madeira Island, which extends in a more subtle way to the north, too. Likewise, the CPD reaches the shallowest values in the same location at the Canary Archipelago. These two aspects suggest a cause-effect relationship between TTS and CPD at this specific area. Gravity data and CPD show a linear feature, which links the NW of the Canary Archipelago and the Gulf of Cadiz. The data we manage in this work show remarkable clues as: (a) the absence of a similar signal in the TTS, (b) the fact that CPD it is rather constant along this track, (c) CPD amplitude also almost doubles the values obtained on Canary Archipelago, and (d) the existence of a clear correlation between seismogenic depths and CPD, which points to the existence of a correlation between seismicity and the thermal architecture of the lithosphere. All of these evidences support the presence of a lithospheric thinning between Canary Islands and Gulf of Cadiz area, and in turn, the presence of an asthenospheric channel which feeds and alter locally the Eurasia–Africa Plate Boundary.

SUMMARY The variation of temperature in the crust is difficult to quantify due to the sparsity of surface heat flow observations and lack of measurements on the thermal properties of rocks at depth. We examine the degree to which the thermal structure of the crust can be constrained from the Curie depth and surface heat flow data in Southeastern Australia. We cast the inverse problem of heat conduction within a Bayesian framework and derive its adjoint so that we can efficiently find the optimal model that best reproduces the data and prior information on the thermal properties of the crust. Efficiency gains obtained from the adjoint method facilitate a detailed exploration of thermal structure in SE Australia, where we predict high temperatures within Precambrian rocks of 650 °C due to relatively high rates of heat production (0.9–1.4 μW m−3). In contrast, temperatures within dominantly Phanerozoic crust reach only 520 °C at the Moho due to the low rates of heat production in Cambrian mafic volcanics. A combination of the Curie depth and heat flow data is required to constrain the uncertainty of lower crustal temperatures to ±73 °C. We also show that parts of the crust are unconstrained if either data set is omitted from the inversion.

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 µW 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.

Spectral analysis of NAMAG magnetic data enabled us to estimate depths to the Curie isotherm along the Pacific-North America plate boundary. The estimated depths for the Gulf of California, and southern Baja California peninsula, correlate quite well with the depths to the Moho available from previous studies based on receiver function, and seismic refraction. In northernmost Baja California, and southwestern USA no correlation is observed between depths to the Curie isotherm and depths to the mantle obtained from receiver function analysis. Depths to the Curie isotherm are shallower. Along the coastal plains of Sonora and Sinaloa, differences between receiver function based Mantle depths and Curie depths are equally large. However, there is more consistency with depths established by seismic refraction studies. Four zones with shallow Curie isotherm are identified in southern Gulf of California (GS1, GS2, GS3, and GS4). GS1 is located at the Gulf of California mouth. The Alarcon rise is located between GS1 and GS2. Pescaderos Basin is located northwest of GS2. GS3 stretches from the Carmen Basin to the Farallon Basin. GS4 extends from Tiburon Island up to northern Guaymas Basin. Northernmost shallow Curie depth zone (GN1) comprises Upper and Lower Delfin Basins, as well as the Consag and Wagner Basins. A northern prolongation also covers Pinacate Volcanic center. The southern shallow Curie isotherm zones are associated with lithospheric-asthenospheric material upwelling zones. GN1 also includes a convective component related to the younger magmatism of the Pinacate Volcanic Field. Deeper Curie depths are found at southern Baja California peninsula, interpreted as associated with thicker continental or stretched crust. It is possible that the crust is underlain or underplated by magnetic oceanic crust as suggested by satellite magnetic anomalies centered at the Magdalena shelf. A contrasting pattern features northern Baja California: deep Curie zones to the west contrasting with relative shallower Curie isotherm areas to the east. The prolongation towards the peninsula of the extinct Shirley transform fracture zone roughly separates the two domains observed along the peninsula, and suggests that the Curie isotherm along the peninsula is controlled by differences subduction of the Guadalupe and Magdalena microplates. Major differences pointed out between northern and southern Gulf of California might indicate that subduction also played a key role at the beginning of the opening of the Gulf of California, however, currently, the Curie pattern is controlled by the rifting processes (shallow Curie zones correlating quite well with the rift basins and the lithosphere-asthenosphere upwelling material).

The energy of Egypt is generally derived from petroleum products. However, advancements in the development of the generation capacity using sustainable and renewable resources, for example, solar, wind and geothermal resources and investment in nuclear power plants are planned. The geology and structure setting of Egypt affirms that reasonable geothermal resources are available, particularly along the Gulf of Suez, the Red Sea, the Western Desert and parts of the Nile Valley. The main objective of this research is to develop a geothermal favourability map of Egypt, which may be considered as a screening tool for the assessment of optimal areas for geothermal development. Digital data layers and maps were employed in a GIS model to select the most promising areas for geothermal potentiality. The ArcGIS model includes six thematic layers: distance to faults, Bouguer anomaly, distance to seismic activity, Curie depth, heat flow, and temperatures at different depths. The validation of the generated geothermal favourability map was performed by a comparison with thermal water wells and hot springs. Generally, the most encouraging zones for geothermal probability in Egypt are located near the shorelines of the Gulf of Suez and the Red Sea.

The Eastern and Southeastern Asian regions witness the strongest land–ocean and lithosphere–asthenosphere interactions. The extreme diversity of geological features warrants a unified study for a better understanding of their geodynamic uniqueness and/or ubiquity from a regional perspective. In this paper we have explored a large coverage of potential field data and have detected high resolution Moho and Curie depths in the aforementioned regions. The oldest continental and oceanic domains, i.e. the North China craton and the Pacific and Indian Ocean have been found thermally perturbed by events probably linked to small-scale convection or serpentinization in the mantle and to numerous volcanic seamounts and ridges. The thermal perturbation has also been observed in proximity of the fossil ridge of the western Philippine Sea Basin, which shows anomalously small Curie depths. The western Pacific marginal seas have the lowest Moho temperature, with Curie depths generally larger than Moho depths. The contrary is true in most parts of easternmost Eurasian continent. Magmatic processes feeding the Permian Emeishan large igneous province could have also been genetically linked to deep mantle/crustal processes beneath the Sichuan Basin. The regionally elongated magnetic features and small Curie depths along the Triassic Yangtze-Indochina plate boundary suggest that the igneous province could be caused by tectonic processes along plate margins, rather than by a deep mantle plume. At the same time, we interpret the Caroline Ridge, the boundary between the Pacific and the Caroline Sea, as a structure having a continental origin, rather than as hotspot or arc volcanism. The surface heat flow is primarily modulated by a deep isotherm through thermal conduction. This concordance is emphasized along many subduction trenches, where zones of large Curie depths often correspond with low heat flow. Local or regional surface heat flow variations cannot be faithfully used in inferring deep thermal structures, which can be better constrained overall through Curie depths detected from surface magnetic anomalies.

The sinking of the ocean sea bottom is produced by thermal cooling of the lithosphere. This evolution is determined by the underlying asthenospheric mantle. Estimation of the Curie Depth variations in the Scotia Sea by using a spectral approach and applied on magnetic anomaly data led us to determine a thermal model and derive a heat flux map. Using multichannel seismic and bathymetry data, we show that the West Scotia Sea reaches thermal equilibrium more quickly than other oceans do and thermally behaves like old oceanic crust in large oceans, following a different empirical age (t, in Ma)-depth (d, in m) relationship, d(t) = 4,480 ¿ 19,380 exp(¿t/4). For oceanic crusts of the same age, underlain by different shallow mantle controlling the heat supply, low heat flux values imply older ages than those predicted for large oceans based on the empirical relationships of the standard plate model. These circumstances, together with the new heat flux map, shed light on the anomalous evolution of the Scotia Sea, a consequence of the present Pacific mantle outflow through the Drake Passage. Two branches of elevated heat flux surround the Shackleton Fracture Zone and extend to the northern and southern boundaries of the Scotia Plate. Most of the heat sources are located in the flanks, whereas the colder parts are centrally located. This signature supports the Drake Passage's role as a main mantle gateway for Pacific outflow toward the Atlantic reservoir favoring the oceanic spreading activity of this ocean. ©2019. American Geophysical Union. All Rights Reserved., We thank the CTM2008‐06386‐C01/ ANT, CTM2011‐30241‐CO2‐01, CGL2016‐80687‐R AEI/FEDER, and RTI2018‐099615‐B‐I00 projects funded by the Spanish Government. Foundations We thank the CTM2008‐06386‐C01/ ANT, CTM2011‐30241‐CO2‐01, CGL2016‐80687‐R AEI/FEDER, and RTI2018‐099615‐B‐I00 projects funded by the Spanish Government. Foundations of this study were initially carried out as a part of the PhD of Y.M. M. who wishes to thank Dr. Andres Maldonado as a PhD advisor. We wish to thank the two anonymous reviewers who helped us to improve our manuscript. The data to evaluate this manuscript are available at Martos, Catalán, et al., 2014 and at the http:// wdmam.org/. The resulting Curie Depth and geothermal heat flux models canbefoundatPANGAEA(https://doi. org/10.1594/PANGAEA.905428). of this study were initially carried out as a part of the PhD of Y.M. M. who wishes to thank Dr. Andres Maldonado as a PhD advisor. We wish to thank the two anonymous reviewers who helped us to improve our manuscript. The data to evaluate this manuscript are available at Martos, Catalán, et al., 2014 and at the http:// wdmam.org/. The resulting Curie Depth and geothermal heat flux models canbefoundatPANGAEA(https://doi. org/10.1594/PANGAEA.905428).

Ground and aeromagnetic data are combined to characterize the onshore and offshore magnetic properties of the central Philippines, whose tectonic setting is complicated by opposing subduction zones, large-scale strike-slip faulting and arc–continent collision. The striking difference between the magnetic signatures of the islands with established continental affinity and those of the islands belonging to the island arc terrane is observed. Negative magnetic anomalies are registered over the continental terrane, while positive magnetic anomalies are observed over the Philippine Mobile Belt. Several linear features in the magnetic anomaly map coincide with the trace of the Philippine Fault and its splays. Power spectral analysis of the magnetic data reveals that the Curie depth across the central Philippines varies. The deepest point of the magnetic crust is beneath Mindoro Island at 32 km. The Curie surface shallows toward the east: the Curie surface is 21 km deep between the islands of Sibuyan and Masbate, and 18 km deep at the junction of Buruanga Peninsula and Panay Island. The shallowest Curie surface (18 km) coincides with the boundary of the arc–continent collision, signifying the obduction of mantle rocks over the continental basement. Comparison of the calculated Curie depth with recent crustal thickness models reveals the same eastwards thinning trend and range of depths. The coincidence of the magnetic boundary and the density boundary may support the existence of a compositional boundary that reflects the crust–mantle interface.

Aeromagnetic data were analyzed to determine the bottom of magnetic bodies that might be related to the Curie point depth (CPD) by 2D spectral and 3D inversion methods within the Salton Trough and the surrounding region in southern California. The bottom of the magnetic bodies for 55 × 55 km windows varied in depth between 11 and 23 km in depth using 2D spectral methods. Since the 55 × 55 km square window may include both shallow and deep source, a 3D inversion method was used to provide better resolution of the bottom of the magnetic bodies. The 3D models indicate the depth to the bottom of the magnetic bodies varied between 5 and 23 km. Even though both methods produced similar results, the 3D inversion method produced higher resolution of the CPD depths. The shallowest depths (5–8 km) occur along and west of the Brawley Seismic Zone and the southwestern portion of the Imperial Valley. The source of these shallow CPD values may be related to geothermal systems including hydrothermal circulation and/or partially molten material. Additionally, shallow CPD depths (7–12 km) were found in a northwest-trending zone in the center of the Salton Trough. These depths coincide with previous seismic analyses that indicated a lower crustal low velocity region which is believed to be caused by partially molten material. Lower velocity zones in several regions may be related to fracturing and/or hydrothermal fluids. If the majority of these shallow depths are related to temperature, they are likely associated with the CPD, and the partially molten material extends over a wider zone than previously known. Greater depths within the Salton Trough coincide with the base of basaltic material and/or regions of intense metamorphism intruded by mafic material in the middle/lower crust.

The study area includes the most important carbonatite and kimberlite complexes in Brazil, located in the Brazilian states of Goias and Minas Gerais. The central portion of this area involves the Azimuth 125° lineament (Az 125°) that consists of an extensive set of faults (oriented in the NW–SE direction) that served as a conduit for magma ascent. This lineament is the main structural feature associated with these complexes. The Goias (GAP) and Alto Paranaiba (APAP) Alkaline Provinces occur along the Az 125° and include highly economically valuable mineralizations. In this study, we aim to map the depth to the curie isotherm (or Curie Point Depths: CPD) of the study area (mainly the Gap and APAP regions) based on spectral analysis of aeromagnetic data. The CPD estimations were achieved from a spectral approach known as the centroid method, providing the relationship between the spectra of magnetic anomalies and the depths of the magnetic source of a 2-D magnetic data. The CPD estimates from approximately 500 overlapping blocks vary from 7 km to 40 km deep. The shallower depths are related to the GAP and APAP regions, and the deeper ones are related to the Sao Franciscana Plate. The Curie depths related to the Az 125° are between 30 km and 15.7 km deep. According to the results, the GAP and APAP intrusive bodies have shallower roots the major faults of the Az 125°.

International audience; We present an innovative study to generalize Curie Point Depth (CPD) determinations at the scale of oceanic volcanic islands, an approach which has previously focused largely on continental areas. In order to determine the validity of this technique in oceanic environments, we first tested the approach on sets of sea-floor-spreading anomalies. Assuming that magnetic anomalies are concentrated within the oceanic crust and uppermost mantle, the Curie depth should deepen as oceanic lithosphere increases in age and thickness away from spreading centers. The calculated depths to the magnetic bottom are in agreement with this general pattern. On the basis of this test, we then applied the method to La Réunion Island and surrounding oceanic lithosphere. The calculated extent of magnetic sources lies at depths between 10 and 30 km and exhibits a complex topography, presumably caused by a combination of various magmatic and tectonic lithospheric structures. These calculations indicate that magnetic sources extend well below the crust-mantle interface at this location. To the first order, the bottom of the magnetic surface shallows beneath Réunion and Mauritius Islands due to the thermal effect of the hot spot, and deepens away from La Réunion edifice. On the scale of the Mascarene Basin, several discontinuities in the CPD correlate well with major fracture zones.

Curie-point depth and heat flow values of the Erciyes region are determined to identify the thermal regime of the Central Anatolia by applying the spectral analysis method to the magnetic anomaly data. To compute the spectrum of the data, the magnetic anomaly of the region is transformed into 2-D Fourier domain to attain the average Curie depth. This method is useful in determining the top boundary of magnetic anomaly sources and reveals the Curie depth as 13.7 km in the study area. The obtained results imply a high thermal gradient (42.3°C km−1) and corresponding heat flow values (88.8 mWm−2) in the research area. Using the temperature value measured at borehole drilled by the General Directorate of Mineral Research and Exploration of Turkey (MTA), the values for the thermal gradient and heat flow value were computed as 50.7°C km−1, 106.5 mWm−2. From the heat flow value, the Curie-point depth was determined as 11.4 km in this region. It is concluded from the obtained values that the region has very high geothermal potential caused by partial melting of the lower crust.

Estimating depth to the bottom of the magnetic crust (synonymously called the Curie isotherm) on a regional scale from long wavelength magnetic anomalies requires that large areas of survey data be used for the calculations. There is still no consensus on the minimum survey area required to arrive at a reliable estimate of the Curie isotherm depth. From the available aeromagnetic data over India, depth to the Curie isotherm is estimated using spectral techniques. We found that the significant spectral maxima required to calculate the depth to the Curie isotherm existed only for 4° × 4° blocks in southern peninsular India up to 18 °N latitude and for 5° × 5° blocks for the rest of the region in Central India. As aeromagnetic data coverage over India is incomplete, we also calculate the magnetic crustal thickness from the lithospheric model of the CHAMP satellite data for the whole country, using an iterative forward modeling approach. The calculated Curie isotherm is shallow in the mobile belts and deeper in the cratons in both the derivations. As the Curie isotherms calculated from the two data sets collected at very different altitudes using totally different techniques, match reasonably well, it lends credence to the methodology adopted. The derived Curie isotherm depth map is in accordance with the basic structural trend of the major tectonic units within the Indian subcontinent. A comparison is made of the calculated Curie isotherm depth with Moho depths along available DSS profiles over India and it is found that the Curie depth is generally shallower than the Moho depth implying that it possibly represents a thermal boundary rather than a compositional change. Further, we find that high magnitude earthquakes are associated with high gradients in Curie depth.

SUMMARY Based on a critical evaluation of several different spectral magnetic depth determination techniques on areally large synthetic layered and random magnetization models, we recommend the following considerations in the usage of the methods as necessary prerequisites to successful bottom depth determinations: (1) using windows with sufficient width to ascertain that the response of the deepest magnetic layer is captured and by verifying the spectra and computing the depth estimates with the largest possible windows (>300‐500 km); (2) avoiding filtering to remove arbitrary regional fields, accomplished by compiling magnetic anomalies derived from modern spherical harmonic degree 13 Earth’s main field models [e.g. recent International Geomagnetic Reference Field models (IGRF) or Comprehensive models (CM)]; (3) ascertaining the near-circularity of the autocorrelation function to avoid analysing biased spectra containing strong anomaly trends; and (4) avoid determining the slopes from the exponential, low wavenumber part of the spectra in the cases of layered magnetization. We also describe the details of the new spectral peak forward modelling method and discuss the conditions under which the method can lead to useful results. We found that, despite all these precautions, in some cases, the results can still be erroneous and, therefore, we recommend a critical evaluation of the results by modelling heat flow and taking into account seismic information on the crustal and lithospheric thicknesses and seismic velocities wherever possible. In the southcentral US, east of the Rockies, where the surface heat flow ranges between 40 and 65 mW m −2 ,w e obtained the magnetic bottom depth of 40 ± 10 km using the approach of the forward modelling of the spectral peak. This range is similar to the seismically derived crustal thickness of 45‐ 50 km, suggesting, therefore, that the entire crust may be magnetic in this region. Because of the uncertainties in the various heat flow contributing parameters, such as the variations in thermal conductivity, radiogenic heat and hydraulic regime, we could not constrain the lithospheric thickness beyond an estimate ranging approximately from 100 to 200 km.

Using California as a test region, we have examined the feasibility of using Curie-isotherm depths, estimated from magnetic anomalies, as a proxy for lithospheric thermal structure. Our method follows previous studies by dividing a regional aeromagnetic database into overlapping subregions and analyzing the power-density spectrum of each subregion, but we have improved on previous studies in two important ways: We increase subregion dimensions in a stepwise manner until long-wavelength anomalies are appropriately sampled, and each subregion spectrum determined from the magnetic anomalies is manually fit with a theoretical expression that directly yields the depth to the bottom of the magnetic layer. Using this method, we have obtained Curie-isotherm depths for California that show a general inverse correlation with measured heat flow, as expected. The Coast Ranges of California are characterized by high heat flow [Formula: see text] and shallow Curie depths [Formula: see text], whereas the Great Valley has low heat flow (less than [Formula: see text] and deeper Curie depths [Formula: see text].

SUMMARY A Curie-point isotherm map of the whole of Turkey has been prepared from aeromagnetic data by means of a spectral analysis technique. The most characteristic signature of this map is that the shallow depths generally correlated with the young volcanic rocks and the thinned crustal thicknesses in the west reveal geothermal fields and several hot springs. The orogenic Pontide belt of the Black Sea region in the north, the western Taurus belt in the south, and some high plateaus in eastern Turkey show that the deepest Curie isotherm depths range between 20 and 29 km. Depth contour lines elongating in an almost E–W direction are due to the overlapping percentage of adjacent data blocks of 50 per cent in the E–W direction and to magnetic anomalies trending in the E–W direction. Curie isotherm depths are in the range 6 to 10 km in the Aegean region and are characterized by E–W- and NE–SW-trending graben structures, as expected, since the region has crustal thinning and many hot springs and geothermal fields. These imply the existence of shallow heat sources that could be utilized for electricity, heating of towns and cities, greenhouse and health purposes.

Ground total magnetic field data of Albania were used to produce estimates of the Curie point isotherm. The strategy followed was to estimate the depth to the bottom of the deepest magnetic sources. Firstly, the average depth to the top of the deepest crustal block, z t, was computed by linear fitting to the second lowest-frequency segment of the azimuthally averaged power spectrum of the total magnetic field data. Then, the depth to the centroid of the deepest crustal block, z 0, was computed by linear fitting to the lowest-frequency segment of the azimuthally averaged power spectrum of a distribution of magnetic dipoles. Finally, the depth to the bottom, the inferred Curie point depth, z b, was calculated from z b=2z 0−z t. Curie depth estimates for Albania vary from about 17 to 25.5 km (below sea level). These results are consistent with the depths inferred by extrapolating geothermal gradient and heat-flow values, suggesting that the Curie point depth analysis is useful to estimate the regional thermal structure. It also suggests that the approach was valid and that ground total magnetic field data can be used for this purpose.

— The aeromagnetic data of Macedonia and Thrace were used to produce Curie point estimates. The data were high pass filtered to remove components arising from topography and magnetic core fields which were not adequately modeled by a DGRF. The depth to the centroid, z 0, of the deepest distribution of the magnetic dipoles was obtained by computing a least-squares fit to the lowest-frequency segment of the azimuthally averaged log power spectrum. The average depth to the top of the deepest crustal block was computed as the depth to the top, z t , of the second lowest-frequency segment of the spectrum. The depth to the bottom of the deepest magnetic dipoles, the inferred Curie point depth, was then calculated from z b =2z 0 − z t . The Curie depth estimates for Macedonia and Thrace range between 11.2 and 17.3 km. These results are consistent with the depths inferred by extrapolating known geothermal gradient and heat-flow values.

—The aeromagnetic data of the island of Crete were inverted to produce Curie point estimates. The data were high-pass filtered to remove components arising from topography and magnetic core fields which were not adequately modeled by IGRF. The depth to the centroid, z 0 , of the deepest distribution of the magnetic dipoles, was obtained by computing a least-squares fit to the lowest-fre quency segment of the azimuthally averaged log power spectrum. The depth to the top of the deepest crustal block was computed as the depth, z t , to the centroid of the second deepest distribution, using the second lowest-frequency segment of the spectrum. The depth to the bottom of the deepest dipoles, the inferred Curie point depth, is then z b = 2z 0−z t . The Curie depth estimates range between 24 and 28 km. This is in accordance with the depths inferred by extrapolating heat-flow values measured in boreholes.

The Salina, Lipari, and Vulcano volcanic ridge and the surrounding sea sectors (Aeolian Archipelago, Southern Tyrrhenian Sea, Italy) are characterized by vents responsible for a recent (

Heat flow measurements are high on the back arc side and low on the forearc side of the Tohoku arc, but convective heat transfer in volcanic areas affects the heat flow measurements, complicating the determination of the thermal structure from heat flow measurements alone. Curie point depths deduced from previous magnetic analyses show a sudden change of the Curie isotherm between the volcanic front and the Japan Trench. A magnetic belt extends from west Hokkaido to the eastern limit of Tohoku. The location is west of and parallel to the Cretaceous subduction zone. To determine the geometry of the source of the magnetic belt, detailed magnetic analyses were carried out using forward modeling and recently developed spectral analysis techniques. The magnetic analyses indicate that sources of the magnetic belt extend to significant depths. The result reveals that the Curie isotherm varies from 10 km in the back arc to 20 km or deeper at the eastern limit of Tohoku. The boundary between seismic and aseismic zones in the overriding plate correlates with the inferred Curie isotherm, indicating that seismicity in the overriding plate is related to temperature.

Evidence from borehole susceptibility logs and the spectral analysis of aeromagnetic data suggests that the three-dimensional distribution of magnetization within the crust can be described as fractal. This property can be exploited in magnetic interpretation methods which explicitly require statistical information on the spatial variation of magnetization. Specifically, we address the problem of magnetic source depth estimation through downward continuation and gridding aeromagnetic survey data using the method of kriging. When magnetic data are continued downwards the depth at which the power spectrum flattens out (the ‘white’ depth) can be taken to be an estimate of the top of the source distribution. This procedure assumes that individual sources are uncorrelated with each other. Taking into account the correlation of the magnetization using a fractal description leads to a reduction in this depth estimate. Gridding of randomly distributed magnetic measurements using kriging requires an estimate of the covariance of the data. Compared with the assumption of a white (uncorrelated) magnetization distribution, using fractal covariances for kriging produces gridded estimates which more closely reflect the statistics of the underlying magnetization process and produce maps with a justifiable degree of smoothness.

We propose a modified centroid method to compute the depth to the bottom of magnetic sources (DBMS) based on a fractal source distribution. This approach provides better estimates than the assumption of an uncorrelated source distribution. We apply our approach to a recently compiled homogeneous set of aeromagnetic data from Germany. The deepest DBMS values are found for some large basin areas, i.e., the Molasse basin and parts of the North German basin. Smaller DBMS were estimated for the Moldanubian region in southern Germany and the northern part of the North German basin. A comparison of DBMS with heat-flow data, crustal temperatures at 3-km depths, and Moho depth indicates that DBMS is controlled by the geothermal condition of the earth’s crust in Germany and lithologic changes. Although the Upper Rhine graben and the Moldanubian region are characterized by small DBMS, a change in DBMS values in northern Germany seems to be related to the Elbe lineament.

Ground and airborne magnetic data are severely disturbed due to random susceptibility variations in Deccan flood basalts. However, Magnetic Satellite (MAGSAT) data over the Deccan flood basaltic region of the Indian subcontinent exhibit filtering of surficial noise. Three passes over Deccan traps show a “low” at about 20°N latitude and a “high” at about 23°N latitude. Spectral analysis of these passes and an available 2-D MAGSAT vertical intensity map indicate a deep (40±4 km) magnetic interface. It is interesting to note that the determination of Curie-depth from MAGSAT matches and confirms the geothermal data model. The estimates correspond to the Moho depth derived from gravity and deep seismic sounding studies. The study suggests a continental shield-like geothermal gradient of about 14°C/km below the area.

Surface heat flows are calculated from elastic lithosphere thicknesses for the heavy cratered highlands of Mars, in terms of the fraction of the surface heat flow derived from crustal heat sources. Previous heat flow estimations for Mars used linear thermal gradients, which is equivalent to ignoring the existence of heat sources within the crust. We compute surface heat flows following a methodology that relates effective thickness and curvature of an elastic plate with the strength envelope of the lithosphere, and assuming crustal heat sources homogeneously distributed in a radioactive element-rich layer 20 or 60 km thick. The obtained results show that the surface heat flow increases with the proportion of heat sources within the crust, and with the decrease of both radioactive element-rich layer thickness and surface temperature. Also, the results permit us to calculate representative temperatures for the crust base, rock strength for the upper mantle, and lower and upper limits to the crustal magnetization depth and intensity, respectively. For Terra Cimmeria, an effective elastic thickness of 12 km implies between 30% and 80% of heat sources located within the crust. In this case the uppermost mantle would be weak at the time of loading, and temperatures in the lower crust cold enough to favor unrelaxed crustal thickness variations and to permit deep Curie depths in the highlands, as suggested by the observational evidence. D 2005 Elsevier B.V. All rights reserved.

SUMMARY The Earth's crust is magnetized down to the Curie-temperature depth at about 10 to 50 km. This limited depth extent of the crustal magnetization is discernible in the power spectra of magnetic maps of South Africa and Central Asia. At short wavelengths, the power increases as rapidly towards longer wavelengths as expected for a self-similar magnetized crust with unlimited depth extent. Above wavelengths of about 100 km the power starts increasing less rapidly, indicating the absence of deep-seated sources. To quantify this effect we derive the theoretical power spectrum due to a slab carved out of a self-similar magnetization distribution. This model power spectrum matches the power spectra of South Africa and Central Asia for a self-similarity parameter of β= 4 and Curie temperature depths of 15 to 20 km.

SUMMARY Depth estimation from potential field power spectra requires a realistic assumption of the statistical properties of the source distributions. Density and susceptibility distributions in the Earth’s crust exhibit a long-range dependence, which is adequately described by scaling random fields with a spectral density proportional to some power of the wavenumber. The theoretical power spectrum for a half-space model of scaling sources explains the shape of observed power spectra of real potential field data very well. Minimizing the misfit between the model and the observed power spectrum yields an estimate for the depth to the top of the sources. After demonstrating this approach on synthetic magnetic data, we reinterpret power spectra of gravity and aeromagnetic data from Utah, Hawaii and Saskatchewan, finding depth values that differ significantly from earlier interpretations. All three power spectra are best explained by source distributions starting at surface level, even the power spectrum from an aeromagnetic survey of a sedimentary basin with virtually non-magnetic basin fill. In the latter case, a priori information on the intensity and the scaling exponent of the field caused by the basement had to be included to obtain an approximate estimate of the basin depth. In general, potential field power spectra are dominated by scaling properties of their source distributions and contain only limited depth information.

Summary. We study aeromagnetic maps from four areas in the Western United States: Utah High Plateaus, Yellowstone National Park, Southern Great Basin, and Uinta Basin. In the first area we try to infer depth-to-bottom of magnetic source by comparison of individual anomalies with theoretical fields of vertical prisms. We conclude that this task is impossible, at least for the few anomalies considered. Assuming a bottomless prism a fit can be obtained which is not significantly worse than the best fit obtainable with any prism model. This is documented by both nonlinear and linear inverse theory. We then turn to a spectral theory like that of Spector & Grant. The advantage of spectral analysis is that the broad, low-amplitude negative flanks of positive anomalies are included even though they may be covered by adjacent positive anomalies. The disadvantage is sensitivity to finite data length: the spectra are interpreted with models derived by Fourier transformation in an infinite domain. Anomalies which are only partly included in the map can degrade the estimated spectrum. We try several techniques to deal with this problem, but none seem universally applicable. These techniques include elimination of part of the map and subtraction of a low-order polynomial. For computation of the spectral estimate our favourite procedure is: (1) average autocovariance over azimuth, (2) extrapolate to large lags as r-5, (3) Hankel transform the radial autocovariance profile to a radial power profile. Our usual model for interpretation of the low-frequency spectra is a truncated right circular cone defined by the four parameters ZT (depth to top), ZB (depth to bottom), RT (top radius) and RB (bottom radius). For the Utah High Plateaus and Uinta Basin we find a distinct preference for sloping sides, i.e. RB > RT. In analysing individual anomalies as well as spectra we find anticorrelation of ZT and ZB. We attribute this to the centroid of the average source body being better determined than its thickness.

As part of a comprehensive, nationwide evaluation of geothermal resources for Japan, the first of the Curie point depth maps, covering the island of Kyushu, has been prepared. The map was created by inverting gridded, regional aeromagnetic data. Two satisfactory algorithms were developed to invert the gridded data based upon a distribution of point dipoles. The first algorithm estimates [Formula: see text],[Formula: see text], and [Formula: see text], the coordinates of the centroid of the distribution, by computing a least‐squares fit to the radial frequency of the Fourier transform; the second algorithm estimates centroid depth only by computing a least‐squares fit to the squared amplitude of the frequency estimates. The average depth to the top, [Formula: see text] of the collection of point dipoles, was estimated by a variation of the second algorithm. The depth to the bottom of the dipoles, inferred Curie point depth, is [Formula: see text]. The depth estimates are hand contoured to produce the final map. The Curie point depth map is then compared to regional geology and heat flow data, and to a limited set of gravity data. Good correlations are found between the Curie point depths and the heat flow and regional geology. A spatial correlation observed between gravity and Curie point depths is considered a secondary, structural effect. Locations of the currently operating geothermal power plants correspond to the shallowest Curie point depths. Based on these comparisons, we conclude that the methods provide geologically reasonable results which are usable in a nationwide geothermal assessment program.

Magnetic total intensity anomalies in northern and central Arizona have been analyzed to locate the bases of the polarized source bodies. The base of the magnetic crust is interpreted as the position of the Curie point isotherm. Results indicate a zone of shallow Curie depth (∼10 km) in a belt, about 60 km wide, running through the center of the state. This zone, near the northern border of the Basin‐Range province, is flanked on the north and south by areas of greater Curie depth (∼20 km). The results are in agreement with regional variations in [Formula: see text] velocity in Arizona.

Shallow Curie‐point isotherm depths, indicated by the analysis of magnetic anomalies, young silicic volcanism, hot springs, and high heat flow mark the Cascade Range of central Oregon as a potentially important geothermal resource area. Aeromagnetic measurements in the central Cascades between 43°00′ and 44°15′N latitude and 121°00′ and 122°30′W longitude exhibit predominant northwest‐southeast lineations and less prominent north‐northeast by south‐southeast lineations. Longer wavelength components of the magnetic anomalies (greater than 25 km), possibly related to deeper sources, trend approximately north‐south parallel to the Cascade Range. Magnetic anomalies show a right‐lateral offset of the High Cascades Volcanism at 43°30′N latitude, which is consistent with the tectonic structures of the Basin and Range province southeast of the area. Magnetic source depth calculations show that the High Cascades occupy a structural depression or graben on the eastern side of the Western Cascades. Spectral analysis of the magnetic anomalies suggests that the High Cascades are underlain by Curie‐point isotherm depths as shallow as 9 km. These shallow Curie‐point isotherm depths imply thermal gradients greater than 50°C/km and surface heat flow greater than [Formula: see text].

A boundary condition on temperature at depth in the continental crust can in principle be obtained by mapping of the Curie isotherm where it forms the base of the magnetic crust. An approach to mapping magnetic crustal thickness using satellite magnetic anomaly data is described. Because magnetic crustal thickness is small with respect to satellite altitude, inversion of the anomaly data yields magnetization models expressing regional variations in the vertical integral of magnetization. This parameter is inversely correlated with regional heat flow in the western United States, suggesting that areas of low apparent magnetization are areas of thin magnetic crust resulting from a shallow Curie isotherm. For a simple model of a uniformly magnetized magnetic crust (or intra-crustal layer),Curie depth is linearly related to apparent magnetization. If the relation can be calibrated by two or more Curie depth estimates, regional apparent Curie isotherm configuration can be mapped. In an alternative approach, a family of Curie depth models can be computed using assumed magnetization values. In each case, the Curie depth configuration is used as a boundary condition in a thermal model. That model is selected which gives a heat flow distribution best fitting observations. Conductive heat flow models for a traverse across the Rio Grande Rift using a finite element technique and for a map grid using a one-dimensional thermal model approximation are given as examples.

The spectra of gravity and magnetic anomalies due to a prismatic body can be expressed as sums of exponentials. The complex exponents of these exponentials are functions of frequency and locations of the corners of the body. An exponential approximation method is used for the analysis of the radial spectra of an anomaly and its first order moments for obtaining accurate estimates of the depths to the top and bottom of the body. A method has also been developed for determining approximately the location of the centroid of the body. When the location of the centroid and the depths to the top and bottom are known for the causative body, it is possible to calculate the horizontal dimensions with the help of the spectrum of the anomaly.

The bottom of the magnetized crust determined from the spectral analysis of residual magnetic anomalies is generally interpreted as the level of the Curie point isotherm. This paper studies the spatial variation of the Curie point isotherm level in Yellowstone National Park with the help of aeromagnetic data. A very shallow isothermal surface at a depth of only 5–6 km below sea level is associated with the central part of the Yellowstone caldera. It seems to extend along a narrow corridor toward the southwestern and eastern edges of the map. Except in a few localized spots, the isotherm deepens considerably in the areas outside the caldera. Because the caldera encloses most of the areas of hydrothermal alteration, fumaroles, and boiling springs in the park, this study indicates a strong correlation between the spatial variation of the Curie isotherm level and the concentration of subsurface geothermal energy.

Summary A first order estimate of Curie depth and lithospheric thickness variation beneath the Indian region has been obtained using the available geothermic data. The Curie depth is found to vary from 18 to 68 km and the lithospheric thickness from 38 to 186 km. These estimates conform well with the MAGS AT and seismological findings. Both the parameters exhibit covariancy and suggest the possibility of approximating heat flow values and lithospheric thickness in geothermally uncharted areas using Curie point geotherm depths derived from air/satellite-borne magnetic anomalies. Interestingly, the areas with shallow Curie depth and lithospheric thickness are found to be associated with a thin crust, higher heat flow, higher geothermal gradients and positive gravity anomalies, and vice versa. Large thickness estimate of magnetic crustal layer, extending below the Moho, demands a new conceptual understanding. The study has brought out a good case for wider use of magneto-thermometry and provides understanding of the geodynamic evolution of the Indian subcontinent.

A mathematical basis for the application of power spectrum analysis to aeromagnetic map interpretation is developed. An ensemble of blocks of varying depth, width, thickness, and magnetization is considered as a statistical model. With the use of the fundamental postulate of statistical mechanics, a formula which can be used to analyze the power spectrum of an aeromagnetic map is developed. The influences of horizontal size, depth, thickness, and depth extent of the blocks on the shape of the power spectrum are assessed. Examples which include power spectra of maps from Canada and Central America demonstrate the application of the approach. In the cases studied a double ensemble of blocks appears to best explain the observed power spectrum characteristics.

Estimates of the depth to the Curie temperature isotherm in Nevada are in accordance with other regional geologic and geophysical information and together can be explained in the context of present-day tectonism. A method to estimate the depth extent of magnetic sources from the statistical properties of magnetic anomalies was applied to a statewide compilation of aeromagnetic data from Nevada. Basal depths of magnetic sources show no apparent correlation with the so-called magnetic quiet zone, which trends northerly through the eastern part of the state, or with basin-and-range topography. However, certain correlations with published heat flow measurements are apparent and suggest that undulations in basal depth of magnetic sources are related in part to undulations in the Curie temperature isotherm. For example, an area of shallow basal depth (

Two-dimensional Curie depth models along two sections across the western United States are derived using an equivalent layer magnetization model derived from Magsat data, and the results are used to constrain finite element thermal models developed by Mayhew and Majer (1980). Regional heat flow variations predicted by the models compare favorably with those inferred from direct measurements. The methodology is applied to upward-continued aeromagnetic data, and the results are found to be in good agreement with previous Curie depth estimates and average measured heat flow.