Regional Thermal Structure Research Articles

Reconstruction of the Curie depth distribution on the Arabian Plate: Implications for its regional thermal structure

Heat flow measurements in the Arabian Plate are inconsistent with anticipated heat flow levels and are not uniformly distributed. They originate mostly from shallow temperature measurements for geothermal surveys and are not specifically targeted for determining the thermal field at the lithosphere scale. We employed spectral analysis of aeromagnetic data based on the fractal distribution of sources to estimate the depth to Curie isotherm (580 °C) in 22 overlapping windows along a southwest-northeast oriented transect of 1000 km across the Arabian Plate. We used the estimate of depth to Curie isotherm and present depth to the lithosphere-asthenosphere boundary to constrain a set of geotherms and to predict surface heat flow and bulk radiogenic heat production across the Arabian Plate. The results indicate that the depth to Curie isotherm is relatively deep (37 km) at the center of the plate and becomes shallower westwards (22 km) and eastwards (27 km), in the Arabian Shield and the Arabian Platform, respectively. The estimated bulk radiogenic heat production in the Arabian Shield is relatively low (0.4 μW/m3) and increases gradually to 1.12 μW/m3 towards the eastern part of the plate. The estimated surface heat flow is low at the center of the plate (45.6 m/Wm2) and rises to about 65 m/Wm2 in the west of the Shield and also in the east of the plate. The average surface heat flow on the entire Arabian Shield is around 50 mW/m2, in accordance with the average heat flow observed on Late Precambrian crust worldwide, but contrasts with the heat flow measurements in the Saudi Arabian part (36 and 45 mW/m2). The elevation of the upper mantle as well as the relatively high radiogenic heat production in the crust can be considered as the causes of the higher level of heat flow at the eastern side of the platform. The results of our study provide a better understanding of the thermal structure of the Arabian Plate and show how potential field data can be employed as a constraint when calibrating basin models.

Variation in thermal structure with crustal thickness for the crust beneath the Peninsular Malaysia

We present a regional thermal structure as well as a new crustal thickness model beneath the Peninsular Malaysia and the surrounding regions. Curie point depth estimates for the entire area range between ∼ 17 – 46 km with a mean of 29 km. The estimated crustal thickness for the study area varies from ∼ 28 – 35 km with an average of ∼ 31 km. Uplifted Curie depths (20 – 25 km) and deeper Moho depths (30 – 34 km) are observed over most parts of Peninsular Malaysia continent. In contrast, the NW Peninsular Malaysia continent and the southern Thailand are characterized by deeper Curie depths between 30 and 40 km. These regions with deeper Curie depths are coincident with the oldest dated rocks on the Sibumasu terrane. The observation of larger Curie depths in NW Peninsular Malaysia implies that the region is presently thermally stable than the remaining parts of the Peninsular. Consistent with deeper crustal thickness (30 – 35 km), the west Sumatra block yields the deepest Curie point depth that ranges between 30 and 46 km. The west Sumatra block and the NW Peninsular Malaysia have low Bouguer anomalies and comparable crustal thickness indicating similarity in regional features. This suggests that these regions are linked by a common ancient continental core. A comparison between the Curie depths and crustal thickness shows that the upper mantle beneath the Island of Sumatra, Singapore, Malay basin, NW Peninsular Malaysia continent, and southern Thailand are significantly magnetized.

Geothermal Mechanism and Reservoir Characterization with Multiscale Analysis of the Potential Field and MT Profile in the Yanqing-Huailai Basin, North China

Abstract Geothermal energy has attracted increasing attention worldwide due to its abundant reserves, stability, and sustainability. Based on the survey of geothermal resources in the Yanqing-Huailai Basin, this paper conducts comprehensive data study, demonstrates geothermal distribution and multiscale characteristics, and constructs the geothermal mechanism and conceptual model to promote regional geothermal energy development and utilization. Our study proposes to characterize geothermal reservoirs using geophysical data ranging from the representative geothermal field scale to the basin scale. Hydrochemical samples were also collected. According to the results of multiscale analysis and interpretation, combined with field geological and geothermal investigations, it is concluded that the Dahaituo pluton is composed of acid granite and monzonite granite, with low gravity and high magnetic anomaly. Its intrusion and development are responsible for the local geothermal anomaly. Multiscale analysis and source depth estimation of potential field data and MT profile interpretation were carried out. The regional thermal structure and reservoir characteristics were demonstrated from the deep to the shallow. In addition, according to the regional geological background and geophysical interpretation, a three-dimensional geological model is constructed to characterize the distribution law and genetic mechanism of geothermal anomalies. The study shows that strong Cenozoic tectonic movement and the intrusion and development of rock mass are the main factors causing local geothermal anomaly. The development of regional deep and major faults and extensional structures, as well as the channels and space formed by the breaking of rocks at the intersection of faults, provides advantageous conditions for the upwelling, migration, and convergence of geothermal fluids. The recharge of surface water in the intermountain basin and thermal convection drive the circulation of the geothermal system.

Characteristics of Precambrian basement intruded by Cretaceous geological intrusions in Monteregian Igneous Province and their impacts on regional thermal structure

With the progress of geothermal exploration in deep buried geological bodies, high radiogenic geological intrusions have become the hot spot in recent years. However, the assessment of the complex structure, lithology of geological intrusions by the geophysical methods has uncertainty, making it a challenging to accurately predict the thermal structure around the geological intrusions. In southern Québec, Canada, recent studies show that a relative high surface heat flux has been detected in the region enclosed by Montréal, Salaberry-de-Valleyfield and Saint-Jean-sur-Richelieu, around the southwest of the Monteregian Hills, which belong to the Early Cretaceous alkaline and carbonatite intrusions. It is not clear whether these Monteregian intrusions have impacts on the thermal anomaly of the Montréal, Salaberry-de-Valleyfield and Saint-Jean-sur-Richelieu region. The objective of this paper is to numerically investigate the thermal structure in the thermal anomaly region, considering the impact of different Monteregian intrusions. The simplified Monteregian intrusions are embedded into a three-dimensional geological model consisting of the sedimentary formations in the St. Lawrence Lowlands and the simulator Underworld2 is used for the thermal modelling. Simulation results show that the geological intrusions in this region have large impacts on the thermal structure at the local-scale, depending on the radiogenic heat production, thermal conductivity, emplacement depth and size. Temperature in the sedimentary formations may be lower or higher than that of the adjacent geological intrusions, highly depending on the thermal physical characteristics of these intrusions. Furthermore, the complex fault systems also strongly control the thermal distribution in different fault blocks, making the Potsdam Group sandstone located between the Grand-St-Esprit and Notre-Dame-du-Bon-Conseil faults as the potential geothermal reservoir. Cited as: Liu, H., Ban, S., Bédard, K., Giroux, B. Characteristics of Precambrian basement intruded by Cretaceous geological intrusions in Monteregian Igneous Province and their impacts on regional thermal structure. Advances in Geo-Energy Research, 2022, 6(3): 206-220. https://doi.org/10.46690/ager.2022.03.04

Thermal Structure of the Northern Outer Albanides and Adjacent Adriatic Crustal Sector, and Implications for Geothermal Energy Systems

Using an analytical methodology taking into account heat flow density data, frictional heating, temperature variations due to the re-equilibrated conductive state after thrusting and geological constrains, we calculated surface heat flow, geotherms and isotherms along a balanced and restored regional geological cross-section. Our results highlight the impact of frictional heating produced by thrusts on the thermal structure of the study area, leading to a raising of the isotherms both in the inner Albanides to the E and in the Adriatic sector offshore. Minimum values of Qs in the surroundings of Tirana and the reconstructed 2D thermal structure suggest less favorable conditions for exploitation of geothermal energy, besides the direct use (Borehole Heat Exchanger-Geothermal Heat Pump systems). Nevertheless, the occurrence of the “Kruja geothermal zone”, partially overlapping this area and including hot spring manifestations, emphasize the structural control in driving hot fluids to the surface with respect to the regional thermal structure.

A model for thermal gradient and heat flow in central Chile: The role of thermal properties

The aim of this work is to quantify variations in heat flow and thermal gradient patterns at the latitude of central Chile and to evaluate the role of thermal properties of macro-geological units within the lithosphere. We developed a numerical thermal model for a continental-scale cross section at 33∘S latitude, integrating available and new data on geometry and dynamics of subduction, as well as thermal and mechanical properties for the continental and oceanic lithosphere, and asthenosphere. The model compares heat flow and thermal gradient curves against homogeneous inputs for radiogenic heat production (RHP) and thermal conductivity. The results of this model were calibrated with results of thermal gradient measurements at different morpho-tectonic domains. The results show that both, cold slab subduction and mantle wedge advection, play major roles in the regional thermal structure. Variations with respect to regional tendency are due to changes in thermal properties. The fore-arc has an average thermal gradient from 12 to 16∘C/km, whereas in the High Andes Cordillera it reaches up to values in the range between 20 and 28∘C/km. Model results indicate that thermal gradient increases eastward up to ∼25∘C/km in the easternmost foreland, which is a stable domain. The consistency of the model with respect to seismic record is discussed. Variation in composition and thermal properties is common at subduction zones due to dynamic petrologic processes. The RHP of the exposed upper crust units was obtained from more than 1000 in-situ measurements, whereas thermal conductivity, specific heat capacity and density of samples were obtained in the laboratory. The calculated RHP of upper crust is ∼2.0 μW/m3, which is responsible for ∼15–40% of the heat flow that reaches the surface.

Compositional zoning and inclusions of garnet in Sanbagawa metapelites from the Asemi-gawa route, central Shikoku, Japan

Compositional and textural characteristics of the zonal structure and inclusions of garnet in metapelites from areas of high-grade greenschist facies (garnet zone) to areas of epidote-amphibolite facies (albite-biotite and oligoclase biotite zones) located along the Asemi-gawa route were examined using EPMA and Raman spectroscopy to investigate evolution of the P-T history of Sanbagawa metamorphism. Garnet grains exhibit two types of compositional zoning patterns: bell-shaped and composite types with spessartine variation. Bell-shape zoning is characterized by a monotonous decrease in the spessartine component from the crystal core towards the margin. Almandine and pyrope components exhibit the opposite trend. The grossular content reaches a maximum at an intermediate position between the core and the rim, and then decreases towards the outermost rim. The composite-zoned garnet is divided into core and mantle parts and it exhibits discontinuous compositional variations according to the associated boundary, evidencing resorption of the core and overgrowth of the mantle during crystallization; implying a two-stage growth of garnet during the Sanbagawa metamorphism. Composite-zoned garnet is characteristically observed in metapelites from a part of the albite-biotite zone, and quartz grains included in its core retain a higher residual pressure than those in garnet from other mineral zones. Isolated paragonite crystal occurs particularly as an inclusion in the core part of this composite-zoned garnet. These data suggest that part of the Sanbagawa metamorphic rocks in the Asemi-gawa region recrystallized under higher-pressure conditions up to the eclogite facies, prior to regional metamorphism from the greenschist, to the epidote-amphibolite facies that formed the regional thermal structure of the Sanbagawa belt of central Shikoku.

Investigation into regional thermal structure of the Thrace Region, NW Turkey, from aeromagnetic and borehole data

The aeromagnetic values over the study region are relatively uniform except for a few anomalies in the northeastern and southwestern areas. Analyses of aeromagnetic data were performed in NW Turkey, in order to have a look into the subsurface regional thermal structure of the region. For this purpose, power spectra, reduced to pole (RTP), and band-pass filtered anomalies were produced using geophysical techniques. Band-pass filtered data were produced from the RTP aeromagnetic anomalies to isolate near surface and undesired deep effects. Based on the aeromagnetic data interpretation, the thickness of the magnetized crust, named the Curie Point Depth (CPD), in the study area lies between 9.7 and 20.3 km. The CPD estimates in the Thrace region of Turkey indicate two shallow CPD (SCPD1 and SCPD2) zones (the Istranca Massif and the Saros Graben area). The deep CPD are located within the Thrace Basin with sediment thickness of about 9 km. The corresponding heat flow map prepared from the averaged thermal conductivities and thermal gradients from the CPD reveals the existence of one low heat flow zone (75 mW/m2) over the center of Thrace Basin, and two high heat flow zones over the Istranca Masif (100–125 mW/m2) in the northern side and Saros Graben (125–135 mW/m2) areas in the southern side of the Thrace Basin.

Crustal thermal regime of Ikogosi warm spring, Nigeria inferred from aeromagnetic data

Spectral analysis method was applied to aeromagnetic data obtained for Ikogosi warm spring (IWS) area of southwestern Nigeria. This was done with the objective of determining the bottom of the magnetized crust called Curie point depth (CDP) and understand the nature and extent of the local geothermal system at depth beneath IWS. The depth to the centroid, Zo, of the deepest distribution of the magnetic dipoles was obtained by computing 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, Zt, 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 Zb = 2Zo − Zt. The Curie depth estimates for IWS range between 4.68 and 11.38 km (below sea level). We also estimate the heat flow and Curie temperature using a one-dimensional conductive heat transport model. The average heat flow, 42 mW m−2, and geothermal gradient, 32°C/km, obtained suggest a low enthalpy thermal regime. The Curie temperature for the region varies between 153°C and 350°C. Also, an inverse linear relationship between heat flow and Curie depths was determined. Good agreement between the Curie point depths derived from heat flow data and magnetic data suggests that the Curie point depth analysis is useful to estimate the regional thermal structure and the tectonic settings.

Curie Point Depth Estimates and Correlation with Subduction in Mexico

We investigate the regional thermal structure of the crust in Mexico using Curie Point Depth (CPD) estimates. The top and bottom of the magnetized crust were calculated using the power-density spectra of the total magnetic field from the freely available “Magnetic Anomaly Map of North America”. We applied this method to estimate the regional crustal thermal structure in overlapping square windows of 2° × 2°. The CPD estimates range between 10 and 40 km and show several regions of relatively shallow and deep magnetic sources, with a general inverse correlation with measured heat flow. A deep CPD region (20–30 km) is located in the fore-arc area where the subducting Cocos plate has a flat-slab geometry. This deep region is bound to the NW and SE by shallow CPD areas beneath the states of Michoacan (CPD = 12–16 km) and Oaxaca (CPD = ~16 km), respectively. There is a good spatial correlation between this deep CPD area and two main fracture zones located on the incoming Cocos plate (Orozco and O’Gorman fracture zones), suggesting that subduction plays an important role in setting apart different CPD provinces along the Mexican coast. Another deep CPD (16–32 km) area corresponds to the region where the Rivera plate subducts beneath Jalisco block. The Trans-Mexican Volcanic Belt is characterized by a decrease in Curie depths from west (16–20 km) to east (10–12 km). Finally, several deep CPD areas are situated in the back-arc region where old Mesozoic terrains are present. Our results suggest that the main control on the crust’s regional thermal structure in the fore-arc and volcanic arc regions is due to the subduction of the Cocos and Rivera plates beneath Mexico.

Modeling the Magnitudes and Directions of Regional Metamorphic Fluid Flow in Collisional Orogens

We present two-dimensional numerical models that simulate fluid flow in an orogenic overthrust setting in the absence of magmatism. The modeling is intended to test hypotheses regarding the direction of regional fluid flow in the middle and deep crust, including upward, convective, sub-horizontal, up-temperature, and downtemperature flow regimes. Several geological factors that have received relatively little attention in previous numerical models of regional metamorphic fluid flow are considered: (1) retrograde hydration metamorphic reactions; (2) anisotropy and spatial heterogeneity in crustal permeability structure; (3) temporal evolution of permeability as a result of porosity production and consumption during metamorphic reactions; (4) heat of metamorphic reactions. Our results suggest that the typical fluid flow pattern in overthrust settings is towards the surface, in the direction of decreasing temperature. The flow is mainly driven by metamorphic dehydration reactions; the integrated fluxes in the deep crust below 20 km are of the order of 10 3 m 3 /m 2 over 30 Myr for a background permeability of 10 ^19 m 2 . At higher crustal permeability, the buoyancy flux generally exceeds the metamorphic flux and large-scale fluid circulation becomes possible.Transient upward flow in the up-temperature direction is observed during the first 1^3 Myr of the model evolution within the thrust core that undergoes retrograde hydration; this flow is the result of the initially inverted geotherm in the thickened crustal section. Downward fluid infiltration caused by metamorphic hydration may occur in the flanks of the model orogen, but the integrated fluxes in these areas are only � 10 m 3 /m 2 . Hydration-driven downward flow may also develop in the core of the orogen at late stages of exhumation. Low-permeability layers lead to sub-horizontal (but down-temperature) flow over distances corresponding to the lateral dimensions of such layers. Heterogeneities in permeability structure, as well as porosity^permeability variations produced by metamorphic reactions, result in focusing of fluid flow and locally elevated fluid fluxes (up to 10 4 m 3 /m 2 ). These fluxes are large enough to produce significant chemical and isotopic metasomatism, but are not sufficient to significantly affect the temperature distribution in the model orogen. However, metamorphic hydration and dehydration reactions are found to exert considerable control on the regional thermal structure. We conclude that convective, downward, or uptemperature fluid flow is probably very limited during amagmatic prograde metamorphism of the middle and lower crust, even in the presence of permeability anisotropy, high-permeability channels, or low-permeability barriers.

Curie point depths of Bulgarian territory inferred from geomagnetic observations and its correlation with regional thermal structure and seismicity

The basal depth of the magnetic layer (Curie point depths) of Bulgarian territory was calculated using geomagnetic field observation and the variations of this border were mapped for the first time. The adopted computational method transforms the spatial data into the frequency domain and provides a relationship between radially averaged power spectrum of the magnetic anomalies and the depths to the magnetic sources. The magnetic layer bottom thus obtained has “normal” (28–32 km) values in the Moesian platform and central part of Bulgaria which are the relatively most stable regions of the investigated territory. In the Moesian platform CPD and Moho are very close but the magnetic layer's bottom is still marked as a thermal boundary. In the southern part, calculated depths gradually reach the minimal values of the derived Curie isotherm (up to 14–16 km) with relatively high gradient. According to the results from geothermal modeling and paleomagnetic investigations, magnetic layer's bottom of the Bulgarian territory corresponds to different Curie temperatures. They vary from 400–425 °C in the regions of Rila-Rhodope unit, up to 550–600 °C in the Moesian platform, West- and East-Balkan unit. The obtained results comprise useful information concerning regional estimates of the correlation between different geophysical and geological characteristics of the earth's crust of the Bulgarian territory.

Thermal maturity of a fold–thrust belt based on vitrinite reflectance analysis in the Western Foothills complex, western Taiwan

Burial depth, cumulative displacement, and peak temperature of frictional heat of a fault system are estimated by thermal analysis in the fold–thrust belt of the Western Foothills complex, western Taiwan based on the vitrinite reflectance technique. The regional thermal structure across the complex reveals that the rocks were exposed to maximum temperatures ranging from 100 °C to 180 °C, which corresponds to a burial depth of 3.7–6.7 km. A large thermal difference of 90 °C were observed at the Shuilikeng fault which make the eastern boundary of the fold–thrust belt where it is in contact with metamorphic rock of Hsuehshan Range. The large thermal difference corresponds to cumulative displacements on the Shuilikeng fault estimated to be in the range of 5.2–6.9 km. However, thermal differences in across the Shuangtung and Chelungpu faults cannot be determined apparently due to small vertical offsets. The large displacement observed across the Shuilikeng fault is absent at the other faults which are interpreted to be younger faults within the piggyback thrust system. Localized high temperatures adjacent to fault zones were observed in core samples penetrating the Chelungpu fault. Three major fracture zones were observed at core lengths of 225 m, 330 m, and 405 m and the two lower zones which comprise dark gray narrow shear zones. A value of vitrinite reflectance of 1.8%, higher than the background value of 0.8%, is limited at a narrow shear zone of 1 cm thickness at the fracture zone at 330 m. The estimated peak temperature in the range of 550–680 °C in the shear zone is far higher than the background temperature of 130 °C, and it is interpreted as due to frictional heating during seismic faulting.

Statistical analysis for thermal data in the Japanese Islands

We calculated statistical average of thermal data to speculate regional thermal structure of the forearc area of the Japanese Islands. The three thermal statistical averages show a difference of a high thermal regime in the western part of forearc inner zone and a low in the Kanto forearc outer zone. The Kanto zone marks 18 K km −1 for mean geothermal gradient, 44 mW m −2 for mean heat flow, while the western inner zone shows 27 K km −1 for mean geothermal gradient, 63 mW m −2 for mean heat flow. The geothermal gradients of the Nobi Plain and the Osaka Plain in the western inner zone are 29 and 36 K km −1, respectively, while the value of the Kanto Plain in the Kanto zone is 21 K km −1. Taking account of the effect of accumulation of sediments, we see the difference in the thermal regime between the plains and conclude that the difference is significant. Heat flux in the crust depends on the volume of granite rich in radioactive elements. There are few granitic rocks in the Kanto zone, while granitic rocks are dominant in the western inner zone. The heat flow of 20 mW m −2 is attributed to the granitic rocks of about 8 km in thickness. There are two oceanic plate subductions of the Pacific plate and the Philippine Sea plate under the Kanto zone, while only the Philippine Sea plate has been subducting under the western inner zone. The model simulation based on thermal and subduction model shows a heat flow ranging 50–60 mW m −2 in the southwest Japan forarc area and a low value of about 20 mW m −2 in the northeast Japan forearc area. The heat flux from the cooling oceanic lithosphere depends on the age of plate. The Shikoku Basin, a part of the Philippine Sea plate, off the western inner zone is 15–30 Ma, while the Pacific plate off the Kanto zone is 122–132 Ma. Theoretically, heat flux values of 15 and 50 Ma oceanic plates range 60–120 mW m −2 and those of 122–132 Ma could be about 10 mW m −2. If the heat flux contribution from the Philippine Sea plate under the Kanto zone is smaller than the plate under the western inner zone, there could be a thermal regime difference in order of several tens of mW m −2. Conclusively, the cause of the difference of heat flux could be the uneven granitic rocks distribution and/or the difference of heat flux between the two subducting plate.

Crustal thermal regime inferred from magnetic anomaly data and its relationship to seismogenic layer thickness: The Japanese islands case study

The seismogenic layer thickness correlates with surface heat flow beneath the Japanese islands. However, this correlation is shown at restricted area, where seismic activity is high. In order to overcome this spatial limitation, we used another approach to estimate the regional thermal structure in the crust beneath the Japanese islands with more uniform coverage. The bottom depths of the magnetized crust determined from the spectral analysis of residual magnetic anomalies is generally interpreted as the level of the Curie point isotherm. We applied this method to estimate the crustal thermal structure in square windows of 2.125° × 2.125°. The obtained depths ranging from 11 to 30 km with average value of 18 km, correlate with the seismogenic layer thickness. It suggests that the Curie point depth is a useful indicator of the crustal thermal structure in these regions.

Curie Point Depths of Albania Inferred from Ground Total Field Magnetic Data

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, zt, 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, z0, 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, zb, was calculated from zb=2z0−zt. 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.

Curie Point Depth variations to infer thermal structure of the crust at the African-Eurasian convergence zone, SW Turkey

Abstract We examined the thermal structure of the crust across complex deformation zones in SW Turkey using the Curie Point Depth (CPD) estimates and made comparisons of the thermal state of the crust with the seismic activity to provide insights for spatial limits of brittle failure in this region. The CPD estimates of SW Turkey from 80 overlapping blocks vary from 9 to 20 km. SW Turkey has two regions of shallow CPD. The shallow CPD region in the Uşak-Afyon zone in western part of the study area is caused by upper crustal thinning and shallowing of high conductivity lower crust. The other shallow CPD region is in the Central Anatolian Volcanic Province in the eastern part of the study area and is thought to be related to the presence of silicate melts in the shallow-level crust. A NNW-SSE trending belt of deep CPD region separates these two zones and is located along the boundary of high (west) and low (east) seismic activities. It is interpreted that the regional thermal structure in SW Turkey is mainly controlled by the processes associated with the African-Eurasian plate convergence zone. The N-S lithospheric extension above the subducting slab created a thermal dome in Western Anatolia in response to upwelling of asthenosphere. Post-collisional magmatism of Neogene-Quaternary age generated another thermal dome in the eastern area. Comparison of the CPD variations with the seismic activity has shown that large earthquakes occur near the margins of the inferred regional thermal domes. Low seismic activity within the regionally active seismic areas seems to be associated with shallow CPD and high heat flow.

Processes and Consequences of Deep Subduction

Oceanic lithosphere subduction is the phenomenon responsible for some of the most spectacular and powerful expressions of the ceaseless motion of the Earth's tectonic plates. Subduction is manifested at the Earth's surface by a global system of oceanic trenches, the deep abyssal scars in the ocean floor that extend over tens of thousands of kilometers and cut up to 4 km deep into the planet's surface. Subduction zones are also associated with regional gravity anomalies, island arc formation, and a large portion of the world's earthquakes and volcanoes, including the deepest and most energetic earthquakes, which have been observed at depths of 660—700 km.In addition to these dramatic surficial features, the process of subduction affects mineral physics, geochemistry, petrology structural geology, and rock mechanics. The conditions associated with subduction provide a unique natural laboratory in which the entrainment of water and sediment with anomalously cold slabs of subducted oceanic lithosphere creates a dynamic and volatile environment as plates sink and encounter hot ambient mantle, mineralogical phase boundaries, and rheological transitions. Part of the difficulty in unravelling the complexity of these systems lies in understanding the feedback that occurs between phenomena operating at macroscopic and microscopic scales. For example, phase transformation rates affect buoyancy, and therefore subduction rates. This affects the regional thermal structure and thereby the phase transformations. Slab rheology and morphology may similarly be affected by mineral transformations and their kinetics.

Curie point depth based on spectrum analysis of the magnetic anomaly data in East and Southeast Asia

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. A method to estimate the depth extent of magnetic sources (Curie point depth analysis) was applied to the magnetic anomalies of East and Southeast Asia. Although the geologic and physiographic complexities of this area constrain the method, certain correlations between the Curie point depths and heat flow data are apparent and the Curie point depths are consistent with the tectonic settings. Shallow basal depths of magnetic sources that extend in back-arc regions correspond to high heat flow values. The correspondence of deep basal depths with low heat flow values along the trench axis suggests that they are both related to the subducting plate. We also estimate the Curie point depths from heat flow data using a one-dimensional heat conductive transport model. Good agreement between the Curie point depths derived from heat flow data and magnetic data suggests that the Curie point depth analysis is useful to estimate the regional thermal structure.

Pumpellyite–actinolite facies of the Sanbagawa metamorphism

The pumpellyite–actinolite facies proposed by Hashimoto is defined by the common occurrence of the pumpellyite–actinolite assemblage in basic schists. It can help characterize the paragenesis of basic and intermediate bulk compositions, which are common constituents of various low‐grade metamorphic areas. The dataset of mutually consistent thermodynamic properties of minerals gives a positive slope for the boundary between the pumpellyite–actinolite and prehnite–pumpellyite facies in P–T space. In the Sanbagawa belt in Japan, the mineral parageneses of hematite‐bearing and ‐free basic schists, as well as pelitic schists have been well documented. The higher temperature limit of this facies is defined by the disappearance of the pumpellyite+epidote+actinolite+chlorite assemblage in hematite‐free basic schists with XFe3+ of epidote around 0.20–0.25 and the appearance of epidote+actinolite+chlorite assemblage with XEpFe3+≤0.20. In hematite‐bearing basic schists, there is a continuous change of paragenesis to higher grade, epidote–glaucophane or epidote–blueschist facies. In pelitic schists, the albite+lawsonite+chlorite assemblage does occur but only rarely, and its assemblage cannot be used to determine the regional thermal structure. The lower temperature equivalence of the pumpellyite–actinolite assemblage is not observed in the field. The Mikabu Greenstone complex and the northern margin of the Chichibu complex, which are located to the south of the Sanbagawa belt, are characterized by clinopyroxene+chlorite or lawsonite+actinolite assemblages, which are lower temperature assemblages than the pumpellyite+actinolite assemblage. These three metamorphic complexes belong to the same subduction‐metamorphic complex. The pumpellyite–actinolite facies or subfacies can be useful to help reveal the field thermal structure of metamorphic complexes