Estimating Curie Point Depth and Heat Flow Map for Northern Red Sea Rift of Egypt and Its Surroundings, from Aeromagnetic Data

In this study, we aim to map the Curie point depth surface for the northern Red Sea rift region and its surroundings based on the spectral analysis of aeromagnetic data. Spectral analysis technique was used to estimate the boundaries (top and bottom) of the magnetized crust. The Curie point depth (CPD) estimates of the Red Sea rift from 112 overlapping blocks vary from 5 to 20 km. The depths obtained for the bottom of the magnetized crust are assumed to correspond to Curie point depths where the magnetic layer loses its magnetization. Intermediate to deep Curie point depth anomalies (10–16 km) were observed in southern and central Sinai and the Gulf of Suez (intermediate heat flow) due to the uplifted basement rocks. The shallowest CPD of 5 km (associated with very high heat flow, ~235 mW m^?2) is located at/around the axial trough of the Red Sea rift region especially at Brothers Island and Conrad Deep due to its association with both the concentration of rifting to the axial depression and the magmatic activity, whereas, beneath the Gulf of Aqaba, three Curie point depth anomalies belonging to three major basins vary from 10 km in the north to about 14 km in the south (with a mean heat flow of about 85 mW m^?2). Moreover, low CPD anomalies (high heat flow) were also observed beneath some localities in the northern part of the Gulf of Suez at Hammam Fraun, at Esna city along River Nile, at west Ras Gharib in the eastern desert and at Safaga along the western shore line of the Red Sea rift. These resulted from deviatoric tensional stresses developing in the lithosphere which contribute to its further extension and may be due to the opening of the Gulf of Suez and/or the Red Sea rift. Furthermore, low CPD (with high heat flow anomaly) was observed in the eastern border of the study area, beneath northern Arabia, due to the quasi-vertical low-velocity anomaly which extends into the lower mantle and may be related to volcanism in northern Arabia. Dense microearthquakes seem to occur in areas where the lateral gradients of the CPD are steep (e.g. entrance of the Gulf of Suez and Brothers Island in the Red Sea). These areas may correspond to the boundaries between high and low thermal regions of the crust. Thus, the variations in the microseismic activity may be closely related to thermal structures of the crust. Indeed, shallow cutoff depths of seismicity can also be found in some geothermal areas (e.g. western area of Safaga city along the Red Sea coastal region and at Esna city along the River Nile). These facts indicate that the changes in the thickness of the seismogenic layer strongly depend on temperature. Generally, the shallow Curie point depth indicates that some regions in our study area are promising regions for further geothermal exploration particularly in some localities along the River Nile, Red Sea and Gulf of Suez coastal regions.     

南北地震带北段地温场的初步探讨

本文利用39个地温梯度值及估算的热流值讨论了本区地温场的分布特征。并求取了居里等温面。结果表明:(1)全区平均地温梯度为3.2℃/100m,平均热流值为1.5HFU;(2)居里面的深度为14—34公里,强震主要发生在居里等温线的梯度带上。最后就地温场与地震的关系做了初步分析。     

Curie point depth from spectral analysis of aeromagnetic data from Cerro Prieto geothermal area,Baja California,México

Using aeromagnetic data acquired in the area from the Cerro Prieto geothermal field, we estimated the depth to the Curie point isotherm, interpreted as the base of the magnetic sources, following statistical spectral-based techniques. According to our results the Curie point isotherm is located at a depths ranging from 14 to 17 km. Our result is somewhat deeper than that obtained previously based only in 2-D and 3-D forward modeling of previous low-quality data. However, our results are supported by independent information comprising geothermal gradients, seismicity distribution in the crust, and gravity determined crustal thickness. Our results imply a high thermal gradient (ranging between 33 and 38 °C/km) and high heat flow (of about 100 mW/m²) for the study area. The thermal regime for the area is inferred to be similar to that from the Salton trough.     

GEOTHERMAL PROSPECTING BY GEOELECTRIC SOUNDINGS IN NE GREECE*

In total 77 direct current resistivity soundings were carried out during a geothermal exploration survey of the Genisea, NE Greece, geothermal field. The data revealed a high electrical conductivity zone at the center of the investigated area and suggested that an anomalous heat source lay beneath the study area. This was confirmed by subsequent drilling data. Temperature measurements, from 11 boreholes, were used for the construction of isotherms that correlated very closely with the geoelectric data.     

中国陆域居里等温面深度特征

基于最新编制的1/100万全国航陆域磁异常图数据,采用功率谱法对中国陆域的居里点深度进行了估算,获得了8004个居里点深度,完成了中国陆域居里面深度图的编制,首次完整的展现了中国陆域的居里面起伏特征.研究表明,居里面在稳定地块表现为坳陷,埋深为28~45km,如塔里木盆地,准噶尔盆地,柴达木盆地,可可西里—巴颜喀拉坳陷区,扬子盆地区,华北盆地区,松辽盆地,二连盆地,巴彦浩特—武威—潮水盆地,珠江口—琼东南盆地等.华北盆地区的居里面深度与塔里木陆块和扬子陆块有较大的差异,相对偏浅,这可能与华北陆块遭受了复杂的后期改造,导致软流圈上隆和岩石圈减薄有关.可可西里—巴颜喀拉地块是青藏高原北部发育的呈NWW向展布的巨型居里面坳陷带,其原因是该地区发育大面积的三叠系沉积地层和较少的岩浆活动,这些稳定的地块都具有莫霍面隆起和居里面坳陷的特征.在活动频繁的造山带居里面以隆起为特征,埋深为18~26km,如东北部山岭区、西北部山岭区、秦岭—大别山地区、西昆仑—西藏—三江—康滇地区、东南沿海地区等.这反映了构造运动及岩浆活动所引起的地壳地温梯度的差异.根据我国816个大地热流数据,对比研究居里面深度与地温梯度和大地热流的关系,结果显示居里面深度与热流值并非线性关系,居里面深度大于30km时,热流值较低,均小于100mW·m~(-2);在居里面深度小于30km的地区,热流值变化范围较大.并且,随着热流值的升高,热流值有向中国东部沿海、藏南—三江地区、秦岭—大别地区、辽东等集中的趋势,这些地区都呈现出居里面隆起的特征,是地热资源勘探开发的重要远景区.     

Deep crustal structures of eastern China and adjacent seas revealed by magnetic data

Through reduction to the North Pole and upward continuation of the total field magnetic anomalies, we analyze magnetic patterns and spatial distributions of different tectonic blocks and crustal faults in eastern China and adjacent seas. Depths to the Curie isotherms are further estimated from radially averaged amplitude spectra of magnetic data reduced to the pole. Data reductions effectively enhance boundaries of regional tectonic belts, such as the Dabie ultra-high metamorphic belt, the Tanlu Fault, and the Diaoyudao Uplift. Curie depths are estimated at between 19.6 and 48.9 km, with a mean of 31.7 km. The Subei Basin and the south Yellow Sea Basin in the lower Yangtze block show relatively deep Curie isotherms, up to about 35 km in depth, whereas in the surrounding areas Curie depths are averaged at about 25 km. This implies that the lower Yangtze Block has experienced a unique tectonic evolution and/or has unique basement lithology and structures. From a regional perspective, sedimentary basins, such as the Subei Basin, the south Yellow Sea Basin, and the East China Sea Basin, normally show deeper Curie isotherms than surrounding uplifts such as the Diaoyudao Uplift and the Zhemin Uplifts. Curie isotherms also upwell significantly in volcanically active areas such as the Ryukyu Arc and the Cheju Island, confirming strong magmatic and geothermal activities at depth. Supported by National Natural Science Foundation of China (Grant Nos. 40776026 and 40876022) and National Basic Research Program of China (Grant No. 2007CB411702)     

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.     

Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation

The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by extensive volcanism and by high rates of magma production. Associated with this volcanism are numerous high-temperature (> 250 °C) geothermal systems through which the natural heat output of 4200 ± 500 MW is channelled. Outside the geothermal fields the heat flow is negligible. The average heat flux from the central 6000 km² of the TVZ, which contains most of the geothermal fields, is 700 mW/m³. This heat flux appears to be more concentrated along the eastern margin of the TVZ.Schlumberger resistivity measurements (AB/2 of 500 m and 1000 m) have identified 17 distinct geothermal fields with natural heat outputs greater than 20 MW. An additional six, low-heat-output geothermal fields also occur, and may represent formerly more active systems now in decline. Two extinct fields have also been identified. The average spacing between fields is 10–15 km. The distribution of geothermal fields does not appear to be directly associated with individual volcanic features except for the geothermal system that occurs within Lake Taupo and which occupies the vent of the 1800 yr.B.P. Taupo eruption. The positions of the geothermal fields do not appear to have varied for at least the last 200,000 years. These data are consistent with a model of large-scale convection occurring throughout the TVZ, in which the geothermal fields represent the upper portion of the rising, high-temperature, convective plumes. The majority of the recharge to the convection system is provided by the downward movement of cold meteoric water between the fields which suppresses the heat flow in these regions.Gravity measurements indicate that to a depth of about 2.5 km the upper layers of the TVZ consist of low-density pyroclastic infill. A seismic refraction interface with velocity change from 3.2 km/s to 5.5 km/s occurs at a similar depth. The cross-sectional area of the convection plumes (identified electrically) appears to increase at depths of 1–2 km, consistent with a decrease in permeability at the depth at which the velocity and density increase.The seismicity is dominated by swarm activity which accounts for about half of all earthquakes and is highly variable in both space and time. The small number of seismic events (and swarms) that have well determined depths show a cut off of seismicity at depths of 7–9 km. The depth of the transition from brittle to ductile behaviour of the rocks is identified with the transition from a regime where heat is transported by (hydrothermal) convection and pore pressures are near-hydrostatic to a regime where heat transport is dominantly conductive and pore pressures are lithostatic. Within the convective region, temperatures are moderated by the circulation of water so that the depth of the transition from convective to conductive heat transfer can be linked to the bottom of the seismogenic zone. Rocks must become ductile within about 1 km of the bottom of the overlying convective zone.Seismic refraction studies suggest that the crust beneath the TVZ is highly thinned with a seismic velocity of about 7.5 km/ s, typical of the upper mantle, occurring at depth of 15 km. Seismological studies indicate the upper mantle is highly attenuating beneath the TVZ. Conductive heat transfer between the bottom of the convective system, at about 8 km, and the base of the material with crustal velocities, at 15 km, is not able to provide all the heat that is discharged at the surface. Repeated intrusion from the mantle may provide the additional heat transport required.     

Curie Point Depth Investigation of Central Anatolia,Turkey

The residual aeromagnetic total field intensity anomalies in central Anatolia were calculated from the regional aeromagnetic anomalies surveyed by the Mineral Research and Exploration (MTA) of Turkey. The residual aeromagnetic data were analyzed to produce Curie point estimates by the method of OKUBO et al. (1985). The Curie point depth of central Anatolia varies from 7.9 km and 22.6 km. The shallowest Curie point depths were observed around the Cappadocia and Erciyes Volcanic complexes in central Anatolia. A good correlation was deduced between the Curie point depths and the heat-flow data measured previously, which is most certainly important for the geothermal resources of the region. The shallow Curie point depths also correlate well with the hot spring locations in central Anatolia.     

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?C30?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?C16?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?C32?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?C20?km) to east (10?C12?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.     

北京平原区西北部大地热流与深部地温分布特征

北京平原区蕴藏着丰富的中-低温水热型地热资源,其西北部分布着小汤山地热田和京西北地热田,两大地热田以南口—孙河断裂带为界.地热田及其外围地区基础的地热地质研究工作较少.为给地热学研究和地热资源精细勘探提供科学依据,本文基于前人23眼钻孔的温度测量数据以及近期完成的548件热导率和100件放射性生热率实测数据,研究了区域大地热流和0~4 km深部地温特征.结果表明：（1）研究区现今地温梯度为11.31~94.89℃·km^-1,平均值为31.79℃·km^-1;岩石热导率为0.895~5.111 W·（m·K）^-1,放射性生热率为0.257~2.305 μW·m^-3,大地热流为48.1~99.1 mW·m^-2,平均值为68.3 mW·m^-2,热流的分布受基底形态和断裂构造控制.研究区东部南口—孙河断裂带两侧小汤山和郑各庄地区为高热流异常区,中部马池口地区也存在局部高热流异常区.（2）在南口—孙河断裂带的不同位置,不同深度地层温度差异明显,体现出区域现今地温场不只受控于该活动断裂,更是多期次构造事件复合叠加的结果.（3）南口—孙河断裂带南侧存在两处有意义的较高地温异常区,分别为郑各庄异常区和马池口异常区,其中马池口异常区是未来地热开发利用有一定潜力的地区.     

Curie-point Depth from Spectral Analysis of Magnetic Data in Erciyes Stratovolcano (Central TURKEY)

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.     

Three-dimensional crustal and upper mantle structure beneath Chalkidiki (northern Greece)

We invert teleseismic relative residuals recorded in the Thessaloniki seismological network, for the crustal and upper mantle structure beneath Chalkidiki (Northern Greece) where extension is observed to have occurred since Eocene time. After conducting several tests to insure the reliability of the results, a low-velocity anomaly (5–8%) is observed which is located beneath Chalkidiki for the top two layers (0–35 km and 35–85 km); this anomaly is probably related to the fact that the crust is thicker here than beneath the neighboring basins. For the two other layers, with depths between 85 and 145 km and between 145 and 205 km, relatively low velocities (3–5%) are observed beneath the Thermaico Gulf and the Kavala Basin. These results are compared with Lyberis's (1985) [8] calculated shallowing of the isotherms due to extension and thinning of the lithosphere. We conclude that the velocity anomalies that we observe are likely to be due to the intrusion of hot material from the asthenosphere into the gap in the extended lithosphere.     

沉积速率对渤中坳陷大地热流的影响

盆地内快速剧烈的构造作用可导致热异常,在利用盆地热历史揭示深部动力学过程时,需消除热异常的影响. 本文根据瞬时热传导原理,校正了渤海湾盆地渤中坳陷低热异常,准确地约束了盆地深部动力学状态. 对渤中坳陷内3口典型井进行热流校正结果表明,渤中坳陷古近纪以来快速沉积导致其现今（~60.9 mW·m^-2）未达到热平衡（低热异常）. 校正后的热流值平均约为67.4 mW·m^-2,比现今高5~10 mW·m^-2. 利用校正后的热流值计算得到的渤中坳陷的“热”岩石圈的平均厚度约为70 km,比修正前的厚度（82~100 km）减少了13~28 km.     

Interpretation of Magnetic Anomalies and Estimation of Depth of Magnetic Crust in Slovakia

The magnetic map of Slovakia used in the paper was compiled as part of a project titled Atlas of Geophysical maps and profiles in 2001. The residual magnetic data were analyzed to produce Curie point estimates. To remove distortion of magnetic anomalies caused by the Earth’s magnetic field, reduction to pole transformation was applied to the magnetic anomalies using the magnetization angle of the induced magnetization. Anomalies reduced to the pole tend to be better correlated with tectonic structures. We applied a 3-km upward continuation to the residually compiled magnetic anomalies in order to remove effects of topography. The depth of magnetic dipoles was calculated by an azimuthally averaged power spectrum method for the entire area. Such estimates can be indicative of temperatures in the crust, since magnetic minerals lose their spontaneous magnetization according to Curie temperature of the dominant magnetic minerals in the rocks. The computed Curie point depths in the Slovakia region vary between 15.2 km and 20.9 km. Heat flow higher than 100 mWm⁻² occurs at the central volcanics and eastern part of Slovakia, where the Curie point depths values are shallow. The correlation between Curie point depths, heat flow and crust depth was investigated for two E-W cross sections. Heat flow and Curie point depth values are correlated with each other however, these values could not be correlated with crust depth. The Curie point isotherm, which separates magnetic and non-magnetic parts of the crust, is represented in two cross sections.     

四川盆地钻孔温度测量及现今地热特征

基于四川盆地9口钻孔的稳态测温资料和297块岩石样品的热导率数据,报道了9个高质量的大地热流数据,提出了沉积地层岩石热导率系列柱.结合前人的数据资料,绘制了地温梯度和大地热流等值线图.四川盆地沉积地层的岩石热导率变化主要由岩性控制,与现今埋藏深度没有明显的相关性.盆地的地温梯度为17.7～33.3℃/km,平均值为22...     

渤海湾盆地冀中坳陷现今地热特征

渤海湾盆地冀中坳陷是我国最典型的潜山油气藏富集区.本文借助117口钻井地层测温资料和45块实测岩石热导率数据系统研究了冀中坳陷现今地温梯度、大地热流、热岩石圈厚度、岩石圈热结构等地热特征参数.研究表明,冀中坳陷0~3000m统一深度现今地温梯度为20.8~41.0℃·km-1,平均值为31.6℃·km-1,比未校正值减小1~3℃·km-1;现今大地热流介于48.7~79.7mW·m-2,平均值为59.2mW·m-2.平面上,冀中坳陷现今地温梯度和热流由西向东(从盆地边缘向内部)逐渐增大,并且凸起区地温梯度和热流相对较高,而凹陷区则偏低,与基底地形起伏具有很好的对应关系.同时,冀中坳陷腹部高热流凸起区广泛分布地热田.冀中坳陷现今热岩石圈厚度为98~109km,其岩石圈热结构为一典型的"冷壳热幔"型.本研究不仅对冀中坳陷油气勘探与地热能开发具有重要的指导意义,而且为深部岩石圈研究(华北克拉通破坏科学问题)提供了新依据.     

沁水盆地大地热流与地温场特征

分析了20口井的温度数据和39个岩石样品的热导率数据，计算了20个大地热流值.结果表明：沁水盆地具有偏高的大地热流背景，现今热流变化于448～1018 mW/m2之间,平均627±152 mW/m2，现今地温梯度介于209～476 ℃/km之间,平均地温梯度282±103 ℃/km. 热流的分布与本区上、下主煤层含气量的分布特征相似，在一定程度上反映了现代热流继承了煤层气生气期的古地热背景.     

Geothermal data analysis at the high-temperature hydrothermal area in Western Sichuan

The western Sichuan hydrothermal area is located at the northeastern margin of the eastern syntaxis of the Qinghai-Tibet Plateau, which is also the eastern end of the Mediterranean-Himalayan geothermal activity zone. There are 248 warm or hot springs in this area, and 11 have temperatures beyond the local boiling temperature. Most of these hot springs are distributed along the Jinshajiang, Dege-Xiangcheng, Ganzi-Litang, and Xianshuihe faults, forming a NW-SE hydrothermal belt. A geothermal analysis of this high-temperature hydrothermal area is an important basis for understanding the deep geodynamic process of the eastern syntaxis of the Qinghai-Tibet Plateau. In addition, this study offers an a priori view to utilize geothermal resources, which is important in both scientific research and application. We use gravity, magnetic, seismic, and helium isotope data to analyze the crust-mantle heat flow ratio and deep geothermal structure. The results show that the background terrestrial heat flow descends from southwest to northeast. The crustal heat ratio is not more than 60%. The high temperature hydrothermal active is related to crustal dynamics processes. Along the Batang-Litang-Kangding line, the Moho depth increases eastward, which is consistent with the changing Qc/Qm(crustal/mantle heat flow) ratio trend. The geoid in the hydrothermal zone is 4–6 km higher than the surroundings, forming a local "platform". The NW-SE striking local tensile stress zone and uplift structure in the upper and middle crust corresponds with the surface hydrothermal active zone. There is an average Curie Point Depth(CPD) of 19.5–22.5 km in Batang, Litang, and Kangding. The local shear-wave(S-wave) velocity is relatively low in the middle and lower crust. The S-wave shows a low velocity trap(Vs3.2 km s.1) at 15–30 km, which is considered a high-temperature partial melting magma, the crustal source of the hydrothermal active zone. We conclude that the hydrothermal system in this area can be divided into Batang-type and Kangding-type, both of which rely on a crustal heating cycle of atmospheric precipitation and surface water along the fracture zone. The heat is derived from the middle and lower crust: groundwater penetrates the deep faults bringing geothermal energy back to the surface and forming high-temperature springs.