Curie Isotherm Research Articles

Structural modelling of subsurface geologic structures in Anambra and adjoining Bida Basins using aeromagnetic data: Implications for mineral explorations.

Aeromagnetic data over parts of the Northern Anambra and Southern Bida basins have been evaluated with emphasis on the subsurface geologic structures controlling mineralization and delineating the geomorphology of the basin. Four aeromagnetic data sheets (246 (Kabba), 247 (Lokoja), 266 (Auchi), and 267 (Idah)) were acquired through purchase, assemblage, analysis, and interpretation using integrated processing techniques of Euler deconvolution, analytical signal (AS), source parameter imaging (SPI), and 3D magnetic inversion model. The data are collected laterally in a series of NW-SE trajectories spaced 2 km apart, with an average flight altitude of approximately 150 m, and tielines occurring at approximately 20 km intervals. Qualitative observation of TMI and RAM shows that the research region is greatly fractured, with structures orienting in the E-W direction. The spectral analysis result shows that the sedimentary infillings range from 0.60 to 4.03 km. 3D model and curie isotherm depth subsurface maps reveal deeper curie depths at Kabba, Adavi, and Itakpe zones (28.0–40.0 km) and shallow curie depths at Auchi, Osara, Idah, and Anyigba areas (18.0–27.0 km). The result from the heat flow model shows an inverse relationship with the Curie isotherm depth. The contact depth sources calculated from Euler depth assessments show that the majority of these contact sources are trending in the E-W directions. The overall structural geomorphology of the research region conforms to the general pattern of structural trends within the research region. Most of the structures are trending NNE-SSW, NNW-SSE, and NE-SW, while the minor structures trend in the E-W direction. 3D inversion results show a discrete subsurface geologic structure that may house minerals within the region.

The upper crustal thermal structure of the Cameli Basin and its surroundings (SW Anatolia, Türkiye) by using the fractal-based centroid method of aeromagnetic data and its relationship with earthquakes occurring in the region

The Cameli Basin and its surroundings are located in southwestern Anatolia, Türkiye, and are one of the tectonically active regions in western Türkiye. The Curie point depth of continental crust can help us to determine the regional tectonic and geothermal structures. The aeromagnetic magnetic data of the study region were used to determine the Curie Point Depth estimates. The fractal-based centroid method is used for this purpose. The fractal approach removes the effect of fractal magnetization in the power spectrum. The depth to the bottom of the deepest magnetic sources in the study region ranges between 6.9 and 14.05. The estimated thermal gradient varies from 41.28°C/km to 89.23°C/km and the average value is 58.59°C/km (580°C for magnetite). The distributions of earthquakes are compatible with NW-SE trending estimated Curie depth anomalies. Interpretation indicates that the depth to the Curie isotherm is considerably shallower than the Moho depth.

Lithospheric structural dynamics and geothermal modeling of the Western Arabian Shield

Understanding the dynamics of suturing and cratonisation and their implications are vital in estimating the link between the lithospheric mantle architecture and geothermal resources. We propose new interpretations of the Western Arabian Shield’s geodynamic styles and geothermal anomalies. In this work, features of the crust and mantle were interpreted from geophysical modeling to unravel the structural dynamics between the Arabian Shield and the Red Sea rift, as well as the influence of these mechanisms on the uplift of the Cenozoic basalts. Estimates of the lower crust thermal properties were also achieved. Spectral properties of the potential field were used to define the Curie isotherm, heat fluxes, geothermal gradients, radiogenic heat production, Moho configuration, and lithosphere-asthenosphere boundary. Results show new structural styles, micro-sutures, and significant thermal anomalies. The defined geothermal patterns were inferred to be due to localized initiation of tectonic and asthenospheric disequilibrium during the rifting episodes within the Red Sea. Also, magma mixing is initiated by the northward migration of magma from the Afar plume towards the Western Arabian Shield which drives local mantle melts beneath the western Arabia, thereby providing the pressure field required for magma ascent. The ascendant magma flow provides the heating source of geothermal reservoirs within the Western Arabian Shield. However, there are indications that during the episodes of rifting within the Red Sea and/or ancient Pan-African activities, the mixing process may have been altered resulting in crustal thinning and creating pathways of ascendant magma flow along the MMN volcanic line. Integrating geophysical and geothermal models indicated new zones of suturing and extensional tectonics between the amalgamated terranes. The geodynamic interpretation shows a new redistribution of terranes and continuous compressional and transtentional movements within the Arabian Shield.

High resolution magnetic anomalies, volcanism and tectonics of the active “La Fossa” vulcanic system (Vulcano island) and Lipari island (South Italy)

In this paper we present the new high-resolution magnetic anomaly map of the La Fossa Caldera system and Lipari island (Southern Italy), obtained by merging two low-altitude aeromagnetic surveys. In these islands a variegated ensemble of magnetic anomalies develops in the north-south direction. The La Fossa Caldera is characterized by very high-intensity and short-wavelength magnetic anomalies, related to mafic intrusive/effusive sources, mainly aligned along the NNW-SSE and NS faults. Instead, the Lipari island is characterized by lower intensity anomalies related to sources with more evolved chemistry, elongated in the NE-SW, NNE-SSW and EW of the subordinate faults. Both the two sets of structures belong to the Tindari-Letojanni strike slip fault-system, a regional lineament along which the southern side of the Aeolian Archipelago is emplaced. The study identifies three distinctive magnetic zones on the Lipari island: the southern, the central, and the north-western ones. The southern zone is characterized by negative magnetic monopoles in correspondence with the younger rhyolitic domes and a Curie Isotherm upwelling. The central part of the island has an average lower intensity of positive anomalies, corresponding to the less evolved products of the intermediate Lipari volcanic epochs. The north-western side is characterized by higher intensity anomalies related to older volcanic epochs. The interplay among strike-slip tectonics, chemistry of the uprising magmas, and the thermal setting has contributed to the overall anomaly pattern in this sector of the Aeolian Archipelago. This interpretation is supported by the application of digital enhancement to the total intensity magnetic anomaly field and by the spatial correlation analysis of the magnetic and volcano-tectonic features. The obtained insights are useful to better understand the relationship between arc volcanism and tectonics. Moreover, they can also outline new inferences to forecast future eruptions of the active La Fossa volcanic system. In fact, recent unrest signals were registered at La Fossa Cone, which is one of the present-day active volcanoes of the Aeolian Archipelago, along with Stromboli island. Key pointsLa Fossa volcanic system of Vulcano and Lipari islands high resolution aeromagnetic surveys; Strike slip tectonics magnetic anomaly signature; Spatial correlation analysis of volcano-tectonic and magnetic features.

The Curie depths of the United Arab Emirates: Implications for regional thermal structures and tectonic terranes

We have determined the depth of the Curie isotherm in the United Arab Emirates (UAE) using a modified centroid method based on the fractal distribution of sources. The derived spatial coverage of Curie depths and geothermal gradients are consistent with the geothermal gradients derived from the bottom hole temperatures measured in exploration wells and reveal three NNE–SSW to NE–SW-trending regions of shallow Curie depths. These are: (1) the north-western offshore region of the UAE with Curie depths of 11–25 km; (2) in central UAE ranging from the Shah field in south-central regions of the UAE to the northern offshore region of Dubai, with Curie depths of 10–25 km, high geothermal gradients of 22.1–55.3 °C/km, and high heat flows of 55.3–138.3 mW/m2; and (3) in the eastern region of the UAE from Al Ain to Dibba and the Gulf of Oman, with medium Curie depths of 25–30 km. These regions of shallow Curie depths are flanked by two NS to NE–SW-trending regions of deeper Curie depths between 35 and 50 km with low geothermal gradients and heat flows of 11.1–15.8 °C/km and 27.8–39.5 mW/m2 respectively. We interpreted these regional NE–SW trends to be related to tectonic terranes composed of magmatic arcs and microcontinental fragments that have amalgamated to form the crust beneath the UAE. In addition, many large hydrocarbon fields are located immediately above or on the margins of these shallow Curie depth zones, suggesting that the shallow Curie depth could have played a critical role in the maturation, migration, and entrapment of hydrocarbons in the UAE. Furthermore, since these zones delineate areas of high heat flow (>100 mW/m2) and high geothermal gradients (∼40 °C/km) they represent areas in which future geothermal exploration could be focused.

Interpretation of gravity and magnetic data in the Central Indian Ocean

The crustal deformation in the Central Indian Ocean is due to major undulations of the oceanic crust, longitudinal fracture zones, and sea-floor topography. The gravity and magnetic data along with six long profiles across the Central Indian Ocean Basin on W–E tracks between 6°S – 1°S latitudes and 77°E – 90°E longitudes are used to study this deformation. It has been observed that the crustal depths obtained from spectral analysis of gravity and magnetic data are in good agreement with 2D forward gravity modelling results which supports seismic results. The computed seismic velocities for the sediments are 2.0 – 5.7 km/s and 6.1 – 7.7 km/s for the oceanic igneous layer and 8.3 – 8.5 km/s for the oceanic upper mantle are used to determine the densities of oceanic crust with the velocity-density relationship. The average basement depths for all the gravity and magnetic profiles are obtained as ~5 km with deviations of about 1 – 2 km from the mean and for the deeper marker, the crustal depths vary from 9 km to 12 km. In the case of curie isotherm, the crustal depths vary from 9 km to 12 km for all magnetic profiles which may indicate deformation. The crustal top depths vary in the range of 3.5 – 8 km (3.2 – 6 km) and the bottom depth varies in the range of 8.2 – 13.5 km (8.5 – 13 km) for magnetic field anomaly data using the spectral method (the Werner method). The crustal top depths vary in the range of 3.6 – 6.5 km and the bottom depth varies in the range of 7.5 – 11.5 km for free-air anomaly data using the spectral method. The above depths are almost correlated with interpreted 2D gravity modelling and available Seismic results.

Lithospheric Thermal Structure in Jinggangshan City: Implications for High Geothermal Background

Jinggangshan City, which is located in the southwestern part of Jiangxi Province, is rich in hot springs. However, lack of geothermal studies has limited the exploration and utilization of the geothermal resources in this city. This study estimated the terrestrial heat flow in Jinggangshan City through well logging and analyses of rock thermal conductivity and heat production. Based on this and the surrounding geoscience transect and the Crust 1.0 model, this study constructed a one-dimensional lithospheric thermal structure by solving the steady-state heat conduction equation. Furthermore, the deep temperature distribution in Jinggangshan City was obtained to better understand the geodynamic condition of the geothermal resources in this city. The results show that the heat flow in this city is around 83.52 mW/m2, indicating the high heat background for the formation of geothermal resources. According to the lithospheric thermal structure of Jinggangshan City, the mantle contributes more to the terrestrial heat flow (qm/qc > 1) than the crust. The temperature of the Mohorovicic discontinuity (the Moho) is 671.7°C, which is consistent with that below the Tanlu deep fault (620–690°C). Moreover, the calculated depth of the Curie surface (585°C) is 27 km, which is consistent with the Curie isotherm depth estimated from aeromagnetic data. This consistency verifies the validity of the lithospheric thermal structure of Jinggangshan City constructed in this study. In summary, the high heat background plays an important role in the formation of geothermal resources in Jinggangshan City.

Crustal deformation of the Central Indian Ocean, south of Sri Lanka as inferred from gravity and magnetic data

Bathymetry, gravity, and magnetic data across the Central Indian Ocean Basin (CIOB) along a WE track between 5°N to 1°N latitudes and 77°E to 90°E longitudes are used to identify crustal deformation due to tectonic features such as the Comorin Ridge, 85°E ridge, Ninety East Ridge, and major fracture zones. The tectonic features were interpreted along the North Central Indian Ocean using 2D gravity modelling to understand the origin and tectonic activity of the subsurface features. The Comorin Ridge is coupled with gravity anomalies with small amplitude varying 25–30 mGal in comparison with the ridge relief which suggests that the ridge is compensated at deeper depths. The focus of the present study is to prepare a reasonable crustal model of the Central Indian Ocean using gravity and magnetic data. The crustal depths of the Central Indian Ocean Basin (CIOB) determined from gravity data using the spectral method are compared with the 2D gravity modelling results. It has been observed that the crustal depths obtained from the Spectral method are in good correlation with results obtained from 2D gravity modelling. The average basement depths for the profiles were obtained as ~5 km and perhaps deviated approximately 1–2 km from the mean. In the case of curie isotherm, the crustal depths vary 9–12 km for all magnetic profiles which may indicate deformation.

Volcanism and Curie Isotherm Distribution in the Jalisco Block, Mexico

A suitable combination of magnetic field determinations is assembled in the whole area of the Jalisco block in central-western Mexico; it serves as the basis for calculations of the Curie Point Isotherm by means of Spectral Analysis. The Jalisco block contains numerous volcanic manifestations; an attempt is made at correlating the Curie Isotherm (CI) with volcanic manifestations and its implicit risk to nearby populated regions. A preliminary analysis of this isotherm is carried out using areas of 60 × 60 km2, where the volcanic regions at the NW and SE portions of the Tepic-Zacoalco (T-Z) rift coincide with shallow layers of the CI. Results show an unexpected region in the middle of the T-Z rift, where the isotherm deepens to 12 km depth and volcanism appears to be missing. The authors argue that this phenomenon may be associated with a flare up episode occurring at 5-3 Ma along the rift. Varying the area used for the calculation of the CI, from 30 × 30 km2 to 120 × 120 km2, illustrates how the concomitant changes in volume affect the depth of penetration. The authors find exceptional regions in which the CI shows shallow depths at all area sizes used in the calculations; this consistency is interpreted as a magnetic alteration of the crust originating at mid-crustal depths. These regions also correspond to high values of the Bouguer anomaly reported elsewhere. Six magneto-stratigraphic profiles are presented for the results of the 60 × 60 km2 calculations, where geologic, topographic, and magnetic properties are displayed along their length.

Linearized Bayesian estimation of magnetization and depth to magnetic bottom from satellite data

SUMMARY Estimating the depth to magnetic bottom (DTB) from magnetic data is one of the most important and difficult potential field inversion problems. Since DTB can often be linked to the Curie isotherm depth of magnetite (∼580 °C), it could provide crucial constraints on heat flow, even in remote or inaccessible areas. Spectral methods are the most popular approach to estimate DTB, but their reliability has been challenged on many grounds. In contrast, space-domain methods have received relatively little attention, even though they might avoid some of the limitations of spectral methods. Furthermore, many DTB estimation methods are to some extent ad hoc, which makes uncertainty estimation and effective communication of the results difficult. In this work, we develop a Bayesian approach to estimate susceptibility and DTB from magnetic data. We describe the subsurface in terms of tesseroids and use a two-step inversion procedure that consists of a Monte Carlo Markov Chain hyperparameter optimization and a linearized inversion. This way, the uncertainties due to unknown hyperparameter are rigorously propagated to the final maps of susceptibility and DTB. Additionally, pointwise constraints based on heat flow measurements can be easily included into the inversion. Synthetic tests are used to determine the accuracy and reliability of the new algorithm. We find that heat flow constraints are necessary to achieve reliable results, although already a small number of points is sufficient. Finally, we apply the algorithm to the Australian continent and demonstrate applicability to real data.

Geometry of the Magma Chamber and Curie Point Depth Beneath Hawaii Island: Inferences From Magnetic and Gravity Data

This study used land gravity and airborne magnetic data to investigate the depth to the magmatic chamber and map the heat flow distribution beneath the active volcanoes of Hawaii Island using the Curie point depth (CPD) and gravity modeling. Obtaining some of the ground-based geophysical measurements was problematic due to accessibility limitations; therefore, this study used available data. The CPD and magnetic data were used to map the depth to the bottom of the magnetic layer by calculating the depth to the Curie isotherm (540°C) beneath Hawaii Island. The spectral peak method was used to calculate the depths to the shallow and deep magnetic sources for the entire island, and the CPD was calculated using the centroid method. A two-dimensional density model for two Earth layers was constructed using forward modeling of the gravity data. A large plume of dense intrusive material was observed beneath the three adjacent volcanoes of Mauna Loa, Mauna Kea, and Kilauea, and two small chambers were found to be located beneath the Kohala and Hualalai volcanoes. Based on the gravity modeling results, the depth to the magma layer varied from 0.5 to 10 km, and the heat flow was higher close to the volcanic eruption zones. The current study is informative and cost effective for the world’s most active volcanic areas.

Geothermal structure revealed by curie isothermal surface under Guangdong Province, China

Guangdong Province in Southeast China is noted for its numerous geothermal resources due to tectonic episodes, mainly occurred during the Cretaceous. The surface heat flow and geothermal gradient are the most direct ways to understand the temperature of the Earth. However, geothermal resources are poorly utilized in Guangdong Province due to limited numbers of boreholes and surficial hydrothermal fluids. To improve the understanding of underground temperature distribution in Guangdong Province, we have applied power-density spectral analysis to aeromagnetic anomaly data to calculate the depth of the Curie isothermal surface. Upward continuation is applied and tested to the magnetic data. The calculated Curie isotherm is between 18.5 km and 25 km below surface. The fluctuation in the depth range reflects lateral thermal perturbations in the Guangdong crust. In particular, the eastern, northern, western and coastline areas of the province have a relatively shallow Curie isotherm. By comparing the surface heat flow, geothermal gradient, distribution of Mesozoic granite-volcanic rocks, and natural hot springs, we conclude that during Mesozoic, magmatism exerted great influence on the deep thermal state of Guangdong Province. A shallow Curie isotherm surface, as well as numerous natural hot springs and high heat flow, show clear signatures of shallow heat sources.

A crustal magnetic model of Britain obtained by 3D inversion

The national baseline aeromagnetic survey of Britain allows a uniform assessment of the shallow and deep magnetic properties of the British tectonic terranes. The most significant is that associated with destruction of early Palaeozoic oceanic lithosphere across the Iapetus Suture separating Baltica and Avalonia from the Laurentian terranes. Here a formal 3D inversion of a continuous swathe of the data is considered. The study provides a uniform volumetric whole crust assessment extending for over 1000 km. Normally a 3D inversion of magnetic data is controlled using a variety of constraints however this is not appropriate at the crustal scale due to our increasingly imprecise knowledge of lithology at increasingly greater depths. The main crustal interface encountered occurs at the Curie isotherm depth. We demonstrate the behaviour of introducing different magnetic crustal depths and suggest the crustal ‘magnetic depth’ of our models can be independently constrained using global or regional studies of the deep geotherm. Static magnetic data have no inherent depth resolution. Here an empirical ‘1D depth’ weighting and a more formal ‘3D distance’ weighting are assessed. The inversion procedure is regularised to provide stable models appropriate to the data and their errors. To gain confidence when using such a ‘geologically-unconstrained’ inversion, we compare our 3D inversion results with an existing geologically-constrained 2.5D profile inversion across northern Britain. A surprising agreement in the 3D susceptibility magnitudes is observed. The chosen study area traverses 10 British terranes and images their tectonic fabric by way of non-magnetic zones (i.e. susceptibilities <0.0001 to 0.001 SI) and magnetic zones displaying geological relevance and tectonic significance at deeper crustal levels. Here we discuss the more significant 3D model features which, by virtue of a continuous crustal-scale assessment and fitting the data with a high degree of fidelity, provide additional structural insights.

A global Curie depth model utilising the equivalent source magnetic dipole method

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.

Crustal structure of Sicily from modelling of gravity and magnetic anomalies

We aim at modeling the main crustal and thermal interfaces of Sicily (Italy), a key area for understanding the geological complexity at the collisional boundary between the African and European plates. To this end, we analyze the gravity and magnetic fields, integrated with information from well logs, geology, heat flow, and seismic data. In order to make the most accurate description of the crustal structure of the area, we modeled with different methodologies the carbonate and crystalline top surfaces, as well as the Moho and the Curie isotherm surface. The reconstruction of the carbonate platform is achieved using a nonlinear 3D method constrained by the available seismic and borehole data. The crystalline top, the Curie, and the Moho are instead estimated by spectral analysis of both gravity and magnetic data. The results show a complex carbonate basement and a deep crystalline crust in central Sicily, with a prominent uplift beneath the Hyblean Plateau. Maps of the Moho and the Curie isotherm surface define a variable thermal and structural setting of Sicily, with very thin crust in the southern and eastern sectors, where high heat flow is found, and deep and cold crust below the Caltanissetta Basin.

Geothermal distribution characteristics and sedimentary basin geothermal system in the severe cold region of Northeast China

In this study, Shengli fault depression, Tangyuan fault basin, and northern Songliao Basin in Yitong-Yilan fault zone of Heilongjiang province are considered the research areas for geothermal anomaly. Based on the temperature of the deep thermal reservoir, the hydrothermal fluid channel, caprock thickness, and the mode of heat transfer, which are the main factors controlling the geothermal reservoir formation, we examined geothermal resource system of the underground HDR in this area. First, we inversed the aeromagnetic data, calculated the Curie isotherm depth, analyzed the geothermal distribution characteristics, and estimated the temperature of the deep heat source. Second, we applied the controlled source audio frequency magnetotelluric (CSAMT) and magnetotelluric (MT) methods to obtain the deep electrical structure of the study area. We determined the thickness of the caprock and the hydrothermal fluid channel. Finally, we obtained the borehole geothermal steady-state temperature measurement data and water sample chemical analysis data from the logging temperature curves of 24 wells to infer the mode of heat transfer. Based on the results, we built a model of the geothermal system of the sedimentary basin in this area. The results show that the depth of Curie isotherm in the study area is 17–39 km. The resistivity of sedimentary caprock in the north of Songliao basin is low, and there exists a deep heat source, which is mainly thermal convection. In contrast, in Shengli and Tangyuan fault basins, heat conduction is dominant. Based on the geothermal system model, we conclude that the area from Daqing to Lindian in Songliao basin has a thermal-convection-dominated sedimentary basin geothermal system. Heat exchange is realized by the upwelling of mantle-derived thermal materials through fracture channels. The thick sedimentary caprock reduces the heat loss. It can be a target for sustainable development and utilization of HDR.

Geological and Thermal Control of the Hydrothermal System in Northern Yellowstone Lake: Inferences From High‐Resolution Magnetic Surveys

AbstractA multiscale magnetic survey of the northern basin of Yellowstone Lake was undertaken in 2016 as part of the Hydrothermal Dynamics of Yellowstone Lake Project (HD‐YLAKE)—a broad research effort to characterize the cause‐and‐effect relationships between geologic and environmental processes and hydrothermal activity on the lake floor. The magnetic survey includes lake surface, regional aeromagnetic, and near‐bottom autonomous underwater vehicle (AUV) data. The study reveals a strong contrast between the northeastern lake basin, characterized by a regional magnetic low punctuated by stronger local magnetic lows, many of which host hydrothermal vent activity, and the northwestern lake basin with higher‐amplitude magnetic anomalies and no obvious hydrothermal activity or punctuated magnetic lows. The boundary between these two regions is marked by a steep gradient in heat flow and magnetic values, likely reflecting a significant structure within the currently active ~20‐km‐long Eagle Bay‐Lake Hotel fault zone that may be related to the ~2.08‐Ma Huckleberry Ridge caldera rim. Modeling suggests that the broad northeastern magnetic low reflects both a shallower Curie isotherm and widespread hydrothermal activity that has demagnetized the rock. Along the western lake shoreline are sinuous‐shaped, high‐amplitude magnetic anomaly highs, interpreted as lava flow fronts of upper units of the West Thumb rhyolite. The AUV magnetic survey shows decreased magnetization at the periphery of the active Deep Hole hydrothermal vent. We postulate that lower magnetization in the outer zone results from enhanced hydrothermal alteration of rhyolite by hydrothermal condensates while the vapor‐dominated center of the vent is less altered.

Geothermal Heat Flux in Antarctica: Assessing Models and Observations by Bayesian Inversion

Geothermal heat flux under the Antarctic ice is one of the least known parameters. Different methods (based on e.g. magnetic or seismic data) have been applied in recent years to quantify the thermal structure and the geothermal heat flux, resulting in vastly different estimates. In this study, we use a Bayesian Monte-Carlo-Markov-Chain approach to explore the consistency of such models and to which degree lateral variations of the thermal parameters are required. Hereby, we evaluate the input from different lithospheric models and how they influence surface heat flux. We demonstrate that both Curie isotherm and heat production are dominating parameters for the thermal calculation and that use of incorrect models or sparsely available data lead to unreliable results. As an alternative approach, geological information should be coupled with geophysical data analysis, as we demonstrate for the Antarctic Peninsula.

A three-dimensional temperature model of the Acoculco caldera complex, Puebla, Mexico, from the Curie isotherm as a boundary condition

Three-dimensional temperature simulations of the Acoculco caldera complex are undertaken to improve our understanding of its thermal regime. This volcanic caldera has been considered a Conduction-Dominated Crystalline-Rock geothermal play based on evidences of low permeability of the formations hosting the heat source. An estimated Curie isotherm at depth is obtained from the de-fractal method to establish the background heat flow in the area. Additionally, local thermal anomalies and radiogenic heat generation are considered. The description of the structures controlling the heat flow is based on limited borehole data along with field observations, it includes stratigraphic columns and fracture zones. The heat transfer equation is solved in steady state with a Finite Volume numerical method and a Conjugate Gradient algorithm. The resulting computational model constitutes a workable instrument to test assumptions on the thermal regime in the Acoculco caldera that can be gradually constrained, as more data become available. Numerical results show that shallow and localized thermal anomalies (750 °C) lead to geothermal gradients as high as those measured in the field. The thermal anomalies turn out between 3300 and 3800 m below ground surface as the mean depth, shallower than those assumed in previous 1D thermal models in this caldera complex.

Thermomagnetic features of Pirapora region, central Brazil

We report results of a detailed analysis of aeromagnetic survey data of Pirapora region, situated in the São Francisco Craton region, central Brazil. The main residual anomaly is wide, spanning over an area of about 9000 square kilometers, and has a maximum intensity of ± 300 nT. The analytic signal of the anomaly is located between 44.5°and 45.5° W and between 16.5° and 18.5 S and has a maximum value of about 0.028 nTm-1. Results of spectral analysis, based on matched bandpass filtering method, indicate that the anomaly is composed of signals arising from three different layers. The top layer is at depth shallower than 5 km while the intermediate one extends from 5 to 15 km depth. The bottom of deepest magnetic layer extends over depths varying from 20 to 55 km. Along an east-west belt, south of Pirapora anomaly, the depth to bottom of magnetized crust is less than 35 km and comparable to the thickness of the local crust obtained in seismic surveys. However, the thickness of the magnetized crust can exceed 35 km to the north and to the south of the Pirapora anomaly. This implies that the top layer of the mantle itself is magnetized. Results of geothermal model studies indicate Curie isotherm of 580oC may lie at depths greater than 35 km, along wide sections of the Pirapora region, within the São Francisco Craton.