Heat Flow Density Values Research Articles

Geotermalny przepływ ciepła w Portugalii kontynentalnej

Heat flow density measurements in Portugal were made in boreholes used for mining prospecting. Heat flow density values in the sedimentary basins of the western and southern margins of the country were obtained using data from oil or gas prospecting works. Due to the scarcity of data in the central and northern regions of the country, numerical models based on some characteristics of the region were used to obtain more heat flow values. The total of heat flow values used in this work is 69, with 17 values obtained using numerical models, 36 obtained in boreholes and 16 obtained from oil/gas prospecting wells data. The analysis and location of these data allows to define different regions in the territory and characterize them using average values of heat flow density, temperature gradient and thermal conductivity. Statistical values with all the data in the territory are also presented.

Thermal conductivity of sediments from well-logs and its application to determine heat flow density in the Pannonian Basin, Hungary

The thermal conductivity of rocks can be deduced from available data of exploration wells such as core samples, cuttings, lithological descriptions and geophysical well logs. We present a methodology for determining the thermal conductivity of the Neogene and Quaternary clastic sediments in the Pannonian Basin using geophysical well logs and thermal conductivity data measured in laboratory. The lithological composition consisting of shale, sand and water was identified and the volumetric fractions of these components were derived from wireline logging data that included natural gamma ray, resistivity, bulk density and neutron porosity logs. The bulk thermal conductivity was computed by applying an appropriate mixing law using the thermal conductivity values of the lithological components. The thermal conductivities of the lithological components were determined by the best fit of the calculated bulk thermal conductivity values to the laboratory data. Different mixing models are recommended to calculate thermal conductivity of shales and sandstones. The thermal conductivity of shales was calculated with the weighted harmonic mean of the constituents, where the volumetric fractions and the porosity were the weights. The thermal conductivity of sandstones can be equally well estimated by the weighted geometric mean and the three component lower Hashin-Shtrikman bound. The geometric mean was used in calculations due its simplicity. Shale to sand ratio was utilized to separate shaly and sandstone samples. Heat flow density determination was carried out using the Bullard-plot technique with thermal conductivities calculated by our method. Heat flow density calculated by our method is in the range of heat flow density values previously conducted but with substantially lower uncertainties. The significance of the method is that it allows the harmonization of the heat flow density in the countries lying in the Pannonian Basin, because the Neogene and Quaternary sedimentary cycle resulting in the filling of the basin was uniform throughout the basin.

Using Heat Flow Density Values Obtained in the Gulf of Cadiz and Gorringe Bank, Atlantic Ocean

The geothermal heat flow measured at the surface of the Earth is originated by different heat sources located at different depths of the planet. The main sources of heat flow in the crust are associated with radioactive decay of Uranium, Thorium and Potassium, in rocks. In some regions, additional heat sources must be considered such as exothermic chemical reactions. The value of the heat flow coming from deep regions, designated by “heat from the mantle”, must be obtained using indirect methods. In this work, the geoid height was used as indicator of alterations “in heat from the mantle” values, considering that the density decrease in regions with geoid height increase is related to high temperature values in the upper part of the mantle. The region on study is located in the Atlantic Ocean, SW of Cape St. Vincent and Cadiz Gulf. Temperature-depth values were obtained in twelve points of the region considering heat flow by conduction in the vertical direction, using published heat flow and thermal conductivity data. Layered models were made using data obtained in published seismic profiles. Moho depth values were used as lower boundary of the crust and mantle heat flow variations were made according geoid height increases. Ocean depth values between 2.5 and 4.3 km were used. A value of 5°C was used for temperature at the upper boundary (ocean bottom) of the models. Temperature calculus stops when a value of 1350 °C was attained. Lithosphere thickness is obtained considering this temperature value as temperature at the bottom of the lithosphere. Heat flow density values from 36 to 65.8 mW m−2 were used in the work with “heat from the mantle” values from 33 to 35 mw m−2. Curie Point Temperature (600°C) depths from 33 to 36 km were obtained. Lithosphere thickness values about 97 km were obtained in all the models.

Heat Flow in SW of Galicia And NW of Portugal. The Interpretation of a Seismic Anomaly

Four heat flow density values and five temperature-depth profiles were obtained in a heterogeneous region considering heat flow by conduction in the vertical direction, and heat sources in the crust due to radioactivity decay. The heat produced in the upper layers of the crust was obtained based on gamma-ray charts and published data of laboratory radioactivity measurements on samples taken from the region. Seismic data were used to obtain the thickness of the different layers of the upper and middle crust and the depth of the Moho discontinuity considered coincident with the crust/mantle boundary. The heat flow at the surface is obtained by adding the heat flow from the mantle with the heat generated by the radioactive sources in the crust. The method was applied to a region with one measured heat flow density value (Ourense) and four points without any heat flow density measurement. A heat flow value from the mantle was obtained with Ourense data. Special attention was given to a region (A) with a seismic anomaly near the surface. The anomaly was explained by mass deficit near the surface that gives rise to an abnormal density value and an increase in seismic wave velocity values. The decrease in density is due to the presence of water in the region forming an aquifer. Geothermometer values obtained from samples of water in the thermal springs of Tuy (Spain) and Monção (Portugal) were used as the water temperature at the bottom of the aquifer. Isostatic balance in the region was considered to obtain density values and the amount of water in the region. Thermal conductivity and radioactivity heat source values in the region were obtained considering the amount of water in the region. Values from 86 to 97 mW/m2 were obtained using the same value of heat flow from the mantle. This is due to different values of heat produced in the crust due to different thickness layer values or/and to different heat production values.

New heat flow data from the Veiholmen and Raudsand boreholes, middle Norway

New thermal data for Fennoscandia have been recently acquired in two boreholes, Veiholmen and Raudsand 5, which are located in the northern part of Møre and Romsdal county of middle Norway. The calculated values of the uncorrected average heat flow density are 55 and 56 mW/m2 within the Veiholmen and Raudsand 5 boreholes, respectively. But, the values of average heat flow density, corrected for topography and palaeoclimate, are the following: 71 mW/m2 within the Veiholmen borehole and 79 mW/m2 within the Raudsand 5 borehole. The estimated palaeoclimatic corrections are relatively high and reach almost 20 mW/m2, showing the significance of taking into account the influence of the last glacial periods on the present-day thermal regime within the uppermost crust of Fennoscandia for geothermal calculations. The heat flow density is slightly higher in the Raudsand 5 borehole than in the Veiholmen borehole, most likely, due to the presence of the massive gneisses within the Raudsand area, which are characterized by higher radiogenic heat production compared to the diorites on the Innveien Island, where the Veiholmen borehole is located. The latter demonstrates the importance of internal heat production within the crust in addition to the heat coming from the Earth’s interior.

Numerical Simulations of Terrestrial Heat Flow in the Beiras Region, Mainland Portugal

Numerical simulations of heat flow density have been made for ten localities in the Beiras region of central Portugal where observational data are absent. The procedure adopted is based on results of deep crustal geophysical surveys and consider that the heat flow measured at the surface of the Earth results from the addition of heat generated in the crust by radioactive sources to that coming from the mantle. Radioactive heat sources in the region are heterogeneous and heat flow values at the surface depends on the thickness of upper crustal layers. Geotherms were obtained considering heat flow by conduction in the vertical direction. The models employed make use of data derived from geophysical surveys of Moho depths and detailed results related with seismic velocity distribution in the crust. In addition, results of radiometric surveys were employed in deriving heat production values for upper layers of the crust. A value around 35 mW m-2 was assumed for heat flow from the mantle. The resulting heat flow density values are similar to those found for areas with similar tectonic characteristics in NW Africa and in Southern Portugal.

Heat flow density estimates in the Upper Rhine Graben using laboratory measurements of thermal conductivity on sedimentary rocks

The Upper Rhine Graben (URG) has been extensively studied for geothermal exploitation over the past decades. Yet, the thermal conductivity of the sedimentary cover is still poorly constrained, limiting our ability to provide robust heat flow density estimates. To improve our understanding of heat flow density in the URG, we present a new large thermal conductivity database for sedimentary rocks collected at outcrops in the area including measurements on (1) dry rocks at ambient temperature (dry); (2) dry rocks at high temperature (hot) and (3) water-saturated rocks at ambient temperature (wet). These measurements, covering the various lithologies composing the sedimentary sequence, are associated with equilibrium-temperature profiles measured in the Soultz-sous-Forêts wells and in the GRT-1 borehole (Rittershoffen) (all in France). Heat flow density values considering the various experimental thermal conductivity conditions were obtained for different depth intervals in the wells along with average values for the whole boreholes. The results agree with the previous heat flow density estimates based on dry rocks but more importantly highlight that accounting for the effect of temperature and water saturation of the formations is crucial to providing accurate heat flow density estimates in a sedimentary basin. For Soultz-sous-Forêts, we calculate average conductive heat flow density to be 127 mW/m2 when considering hot rocks and 184 mW/m2 for wet rocks. Heat flow density in the GRT-1 well is estimated at 109 and 164 mW/m2 for hot and wet rocks, respectively. Results from the Rittershoffen well suggest that heat flow density is nearly constant with depth, contrary to the observations for the Soultz-sous-Forêts site. Our results show a positive heat flow density anomaly in the Jurassic formations, which could be explained by a combined effect of a higher radiogenic heat production in the Jurassic sediments and thermal disturbance caused by the presence of the major faults close to the Soultz-sous-Forêts geothermal site. Although additional data are required to improve these estimates and our understanding of the thermal processes, we consider the heat flow densities estimated herein as the most reliable currently available for the URG.

Heat Flow, Geotherms, Density and Lithosphere Thickness in SW of Iberia (South of Portugal)

Thermal structure, density distribution and lithosphere thickness in the SW part of the Iberian Peninsula are studied using data obtained in the South Portuguese Zone (SPZ) and SW border of the Ossa Morena Zone (OMZ) in the South of Portugal. Five different regions were defined, and models were built for each region. Geotherms were obtained using average density values from data published. The high values of heat flow density in these regions are attributed to occurrence of anomalous heat sources due to radioactivity content and exothermic chemical reactions associated to ore deposits in the zone. The results obtained with models based on isostasy in the region led to lithosphere thickness values between 95 and 96 km in the SPZ and a lower value of 94.5 km in the SW border of the OMZ. Analysis of geotherms shows lateral variations of temperature at the same depth. These lateral variations are compared with information obtained with seismic data.

Hydrocarbons from near-surface sediments of the Barents Sea north of Svalbard – Indication of subsurface hydrocarbon generation?

The Barents Sea is considered as an important target for oil and gas exploration, but the petroleum potential of its shelf and slope regions is unknown. Here we present results of a research cruise to the Northern Hinlopen Margin at the transition to the Southern Nansen Basin and the Eastern Yermak Plateau. Multichannel reflection seismic data acquisition, heat flow measurements, and geochemical analyses of near-surface sediments obtained by gravity coring were conducted to study the northern Barents Sea shelf and the early opening of the Nansen Basin and decipher their petroleum potential. Seismic data indicate high thicknesses of up to ∼2000 m of Cenozoic sediments. Heat flow density values in the study area range between 67 and 108 mW/m2. The sediment samples were analysed for bulk geochemistry and sorbed hydrocarbon gases and for two sites for extractable hydrocarbons. Data from extractable (n-alkanes > n-C25) and bulk (HI and OI from Rock Eval) organic matter demonstrate predominantly terrigenous organic material, most likely derived from ice-transported allochthonous sediments. None of the sediments revealed substantial amounts of methane in pore waters, arguing against active hydrocarbon seepage in the studied areas. However, thermogenic gases sorbed to the sediment matrix (clay minerals, organic matter and/or carbonates) were found in concentrations of up to 600 ppb (on sediment wet wt. basis). For the samples from the Northern Hinlopen Margin and particularly from the adjacent Nansen Basin, a paleo fluid flow of thermogenic gas is indicated and accompanied by higher n-alkanes with a modal, petroleum-like distribution. δ13C values of methane, ethane and propane and gas compositions point at a mainly marine source rock origin of all studied gases with early oil window maturities of the associated rocks (0.6–0.9%VR). From this data an admixture of Type III derived thermogenic gases is indicated for some of the Yermak Plateau sediments for which also the lowest abundances of sorbed gases (50–100 ppb) were observed. Gas geochemical characteristics in the samples with low gas abundances can partially be explained by an input of gases through ice-transport of allochthonous hydrocarbons, which were bound to mature organic matter. For a site on the Northern Hinlopen Margin NE of Svalbard, right at the southern termination of the Nansen Basin a different situation is indicated. In this area the highest concentrations of sorbed gases most likely derived from sediments with an early-oil window maturity and a marine kerogen Type II-typical isotopic distribution. At this location a pseudo well was constructed from 2D seismic data for reconstruction of thermal and maturity evolution. The simulation results indicate that an Early to Middle Eocene source rock would be in the early oil window since the Early Miocene. A possible source rock here and in the circum-Arctic region could have been formed by Azolla algae and other flourishing primary producers.

Modeling of heat and mass transfer processes in phase transformation cycle of sprayed water into gas: 3. Energy and thermal states analysis of slipping droplet in a humid air flow

In this work an unsteady phase transformation mode for droplets that are slipping in humid air was numerically simulated. Initial slipping for droplet of 150 micrometre diameter is defined by Reynolds criterion values of 0, 5, 10, 20, 40 and 80. Warming droplet energy state is evaluated by balance method according to average heat flow density values in distinguished Fourier criterion time intervals at condensing and unsteady evaporation modes. At numerical experiment a balance control of heat flow on droplet surface was carried out, when allowable imbalance is defined by less than hundredth percentage point. A bright water slipping in humid gas impact for their heat transfer in condensation phase transformation mode was based on. This impact reveals due to reactivation of leaded heat to central layers as well as an equal droplet warming and duration grow of condensing mode. DOI: https://dx.doi.org/10.5755/j01.mech.21.5.12167

New heat flow data from three boreholes near Bergen, Stavanger and Moss, southern Norway

New heat flow data have recently been obtained for three boreholes, Fyllingsdalen, Ullrigg and Årvollskogen, which are located in southern Norway near Bergen, Stavanger and Moss, respectively. The obtained topographically and palaeoclimatically corrected values of average heat flow density are 51mW/m2 within the Ullrigg borehole, 72mW/m2 within the Fyllingsdalen borehole and 80mW/m2 within the Årvollskogen borehole, in the depth interval of 120–400m. According to the preferred palaeoclimatic scenario, the highest tentative palaeoclimatic corrections vary from 21 to 26mW/m2 within the shallow parts of the investigated boreholes. Therefore, a significant decrease of the Earth's surface temperatures as a result of the continuous cooling during the two last glaciations in Weichselian and Saalian still affects the subsurface thermal field of the study areas in terms of the reduced heat flow density within the uppermost crystalline crust. Topographic corrections are characterised by rather minor values compared to the palaeoclimatic ones. Moreover, the groundwater flow can be a significant factor for the reduction of heat flow density in the Fyllingsdalen and Ullrigg boreholes, whereas hypothesised subsurface radioactive sources may have contributed to a higher heat flow density at Årvollskogen. The variation in heat production related to different lithologies appears to be one of the main reasons for the higher heat flow density in the Fyllingsdalen and Årvollskogen boreholes in comparison with the Ullrigg borehole.

Thermal conditions of the central Sinai Microplate inferred from new surface heat-flow values and continuous borehole temperature logging in central and southern Israel

This paper reports ten new surface heat-flow density (qs) values for central and southern Israel (central Sinai Microplate), whose crystalline crust and lithosphere formed as part of the Neoproterozoic Arabian-Nubian Shield. Heat flow was calculated in Mesozoic sediments using the classical approach of heat-flow determination by implementing in the analysis high-precision continuous temperature logs obtained in air- and/or water-filled boreholes. Thermal conductivity (TC) measured for a large suite of rock samples of lithotypes making up the sequence was assigned to temperature gradients in intervals for which the lithology was known. The heat-flow values obtained for different depth intervals in a borehole as well as the average values for the individual borehole locations cover a narrow range, attesting heat-conduction conditions. A steady-state thermal model along an E–W crustal cross section through the area shows that the observed systematic spatial distribution of the qs values, which range between 50 and 62mWm−2, can primarily be explained by variations in the thickness of the upper crust and in the ratio between sedimentary and crystalline rocks therein. Given the time lapse of thermal heat transfer through the lithosphere, the qs data monitor the crustal thermal conditions prior to rift- and plume-related lithospheric thermal perturbations that have started in the larger area ca. 30Ma ago. Observed and modeled qs display the best fit for a pre-Oligocene lithosphere–asthenosphere boundary (LAB) at ∼150km, which would be at the upper end of LAB depths determined from stable areas of the Arabian Shield (150–120km) not affected by the young, deep-seated thermal processes that have caused a further uprise of the LAB. Our data imply or predict that the surface heat flow of the Sinai Microplate generally tends to increase along N–S and W–E traverses, from ∼45–50mWm−2 to ∼55–60mWm−2. Surface heat flows on the order of 55–60mWm−2 may be common in the northern Arabian Shield, where it exhibits typical lithosphere structure and composition and is unaffected by young heating processes, compared to values of ≤45mWm−2 recently determined in the southern Arabian Plate for the Arabian Platform.

One-Dimensional Thermal Modeling of the Eastern Pontides Orogenic Belt (NE Turkey)

The eastern Pontides orogenic belt is one of the most complex geodynamic settings in the Alpine–Himalayan belt due to the lack of systematical geological and geophysical data. In this study, 1D crustal structure and P-wave velocity distribution obtained from gravity modeling and seismological data in the area has been used for the development of the thermal model of the eastern Pontides orogenic belt. The computed temperature-depth profiles suggest a temperature of 590 ± 60°C at a Moho depth of 35 km indicates the presence of a brittle-ductile transition zone. This temperature value might be related to water in the subducted crust of the Tethys oceanic lithosphere. The Curie temperature depth value of 29 km, which may correspond to the crustal magma chambers, is found 5–7 km below the Moho depth. Surface heat flow density values vary from 66.5 and 104.7 mW m−2. High mantle heat flow density value of 48 mW m−2 is obtained for the area should be related to melting of the lithospheric mantle caused by upwelling of asthenosphere.

A note on the lithosphere thickness and heat flow density of the Indian Craton from MAGSAT data

The Curie depth map of India compiled from MAGSAT data has been used for preparing the lithosphere thickness and the surface heat flow density maps of the Indian Craton, utilizing the concept of magnetothermometry. The lithosphere thicknesses of the major Indian geological units/provinces, as obtained from the prepared map, are found to be in reasonably good agreement with the previously published values for these regions. Also, the surface heat flow density values obtained from the prepared maps closely follow the previously published results. The maps are useful in providing first order estimates of lithosphere thickness and surface heat flow density of the important geological units/provinces of India.

A tentative 2D thermal model of central India across the Narmada-Son Lineament (NSL)

This work deals with 2D thermal modeling in order to delineate the crustal thermal structure of central India along two Deep Seismic Sounding (DSS) profiles, namely Khajuriakalan–Pulgaon and Ujjan–Mahan, traversing the Narmada-Son-Lineament (NSL) in an almost north–south direction. Knowledge of the crustal structure and P-wave velocity distribution up to the Moho, obtained from DSS studies, has been used for the development of the thermal model. Numerical results reveal that the Moho temperature in this region of central India varies between 500 and 580 °C. The estimated heat flow density value is found to vary between 46 and 49 mW/m 2. The Curie depth varies between 40 and 42 km and is in close agreement with the Curie depth (40±4 km) estimated from the analysis of MAGSAT data. Based on the present work and previous work, it is suggested that the major part of peninsular India consisting of the Wardha–Pranhita Godavari graben/basin, Bastar craton and the adjoining region of the Narmada Son Lineament between profiles I and III towards the north and northwest of the Bastar craton are characterized with a similar mantle heat flow density value equal to ∼23 mW/m 2. Variation in surface heat flow density values in these regions are caused by variation in the radioactive heat production and fluid circulation in the upper crustal layer.

A 3-D density–velocity model between the Cretan Sea and Libya

A 3-D density model for the Cretan and Libyan Seas and Crete was developed by gravity modelling constrained by five 2-D seismic lines. Velocity values of these cross-sections were used to obtain the initial densities using the Nafe–Drake and Birch empirical functions for the sediments, the crust and the upper mantle. The crust outside the Cretan Arc is 18 to 24 km thick, including 10 to 14 km thick sediments. The crust below central Crete at its thickest section, has values between 32 and 34 km, consisting of continental crust of the Aegean microplate, which is thickened by the subducted oceanic plate below the Cretan Arc. The oceanic lithosphere is decoupled from the continental along a NW–SE striking front between eastern Crete and the Island of Kythera south of Peloponnese. It plunges steeply below the southern Aegean Sea and is probably associated with the present volcanic activity of the southern Aegean Sea in agreement with published seismological observations of intermediate seismicity. Low density and velocity upper mantle below the Cretan Sea with ρ ≅ 3.25 × 10 3 kg/m 3 and V p velocity of compressional waves around 7.7 km/s, which are also in agreement with observed high heat flow density values, point out at the mobilization of the upper mantle material here. Outside the Hellenic Arc the upper mantle density and velocity are ρ ≥ 3.32 × 10 3 kg/m 3 and V p = 8.0 km/s, respectively. The crust below the Cretan Sea is thin continental of 15 to 20 km thickness, including 3 to 4 km of sediments. Thick accumulations of sediments, located to the SSW and SSE of Crete, are separated by a block of continental crust extended for more than 100 km south of Central Crete. These deep sedimentary basins are located on the oceanic crust backstopped by the continental crust of the Aegean microplate. The stretched continental margin of Africa, north of Cyrenaica, and the abruptly terminated continental Aegean microplate south of Crete are separated by oceanic lithosphere of only 60 to 80 km width at their closest proximity. To the east and west, the areas are floored by oceanic lithosphere, which rapidly widens towards the Herodotus Abyssal plain and the deep Ionian Basin of the central Mediterranean Sea. Crustal shortening between the continental margins of the Aegean microplate and Cyrenaica of North Africa influence the deformation of the sediments of the Mediterranean Ridge that has been divided in an internal and external zone. The continental margin of Cyrenaica extends for more than 80 km to the north of the African coast in form of a huge ramp, while that of the Aegean microplate is abruptly truncated by very steep fractures towards the Mediterranean Ridge. Changes in the deformation style of the sediments express differences of the tectonic processes that control them. That is, subduction to the northeast and crustal subsidence to the south of Crete. Strike-slip movement between Crete and Libya is required by seismological observations.

Detailed spatially distributed geothermal heat-flow data for modeling of basal temperatures and meltwater production beneath the Fennoscandian ice sheet

AbstractAccurate modeling of ice sheets requires proper information on boundary conditions, including the geothermal heat flow (or heat-flow density (HFD)). Traditionally, one uniform HFD value is adopted for the entire modeled domain. We have calculated a distributed, high-resolution HFD dataset for an approximate core area (Sweden and Finland) of the Scandinavian ice sheet, and imbedded this within lower-resolution data published for surrounding regions. Within the Last Glacial Maximum ice margin, HFD varies with a factor of as much as 2.8 (HFD values ranging between 30 and 83 mWm–2), with an average of 49 mWm–2. This average value is 17% higher than 42 mWm–2, a common uniform value used in ice-sheet modeling studies of Fennoscandia. Using this new distributed dataset on HFD, instead of a traditional uniform value of 42 mWm–2, yields a 1.4 times larger total basal meltwater production for the last glacial cycle. Furthermore, using the new dataset in high-resolution modeling results in increased spatial thermal gradients at the bed. This enhances and introduces new local and regional effects on basal ice temperatures and melt rates. We observed significant strengthening of local ‘ice streaming’, which in one case correlates to an ice-flow event previously interpreted from geomorphology. Regional to local variations in geothermal heat flow need to be considered for proper identification and treatment of thermal and hydraulic bed conditions, most likely also when studying Laurentide, Greenland and Antarctic ice sheets.

Magnetic spectral analysis in Portugal and its adjacent seas

Magnetic spectral analysis, which has often been applied to estimate Curie point depths, was used to delineate thermal and crustal structures of Portugal and its adjacent seas. The magnetic data set was the grid file of 5 km interval. Assuming random samplings from magnetic prisms, we obtained power spectra of two-dimensional square areas by the double Fourier transform and estimated statistical average depths. The cell-size for the area we used is 320 km. The average centroid depth, z 0, was obtained automatically from the gradient of natural log of [(power spectrum)/(radial frequency)] in the range between 0.005 and 0.02 km −1 in frequency (50–200 km in wavelength). The average depth to the top of the magnetic layer, z t, was estimated from the gradient of the power spectrum between 0.02 and 0.04 km −1 in frequency (25–50 km in wavelength). The average depth to the bottom of the magnetic layer is calculated from 2 z 0− z t. The results show that the bottom depths of land areas range between 20 and 55 km below sea level. The statistical errors of centroid depth are within 8 km. Marine linear magnetic anomalies widely cover the offshore areas, where random samples of prisms is inconsistent. We take one-dimensional analysis for the areas. The south of Portugal marks shallow depths of 20–30 km, while deep depth areas greater than 30 km spread widely in the north. One-dimensional analysis of the offshore area indicates the depth of marine magnetic layer of oceanic crust. The wide variation of depth is interpreted to be a reflection of the crustal and the thermal structures. Since the Moho must correspond to the base of the magnetic layer, the results could be the depths to the Moho instead of the Curie point isotherm in the northern Portugal. South Portuguese Zone is in the area ranging from 20 to 30 km in depth. These depths are shallower than the Moho. Two-dimensional forward modeling confirms that there is a throw of the bottom of the magnetic layer across the Ficalho over thrust from 35 to 40 km in the Ossa Morena Zone to 20–30 km in the South Portuguese Zone. There is a geothermal anomaly of high heat flow density values more than three times the normal average value. The Curie point isotherm estimated by the anomaly is shallower than the depth to the bottom of the magnetic layer. One interpretation is that the bottom of the magnetic layer corresponds to a Moho depth that could be 28–30 km in the Ossa Morena Zone and to a Curie point isotherm that could be 20–30 km in the South Portuguese Zone. The high heat flow density values of this area could be produced by hydrothermal convection along tectonic lines such as the Ferreira–Ficalho over thrust.

Gas hydrates acting as cap rock to fluid discharge in the Makran accretionary prism?

Abstract We present a numerical model of the geothermal field of the Makran accretionary prism and of the slab being subducted below it. Calculated heat flow density values for the sea floor of the abyssal plain and the shelf slope are compared with in situ measured and bottom simulating reflector (BSR)-derived heat flow density values. The result suggests a predominance of conductive heat transport within the accretionary complex. Little evidence is found to suggest that fluid flow or frictional heat modifies the observed geothermal field to any great extent. We also studied the geothermal field associated with the decay of the potential gas hydrate layers (indicated by the presence of BSRs), as gas hydrate layers are being tectonically uplifted out of the gas hydrate stability field into shallower and warmer sea water. Theoretical considerations suggest a complete disappearance of gas hydrates at a water depth of about 750 m. The observed presence of numerous gas seeps almost exclusively at water depths of less than 800 m suggests that gas hydrate layers in the Makran accretionary prism act as a very effective cap rock to upward-directed flow of fluids containing notable amounts of dissolved gas from within the prism to the sea floor.

Variation of radiogenic heat production rate in 2.8–1.8 Ga old rocks in the central Fennoscandian shield

We present results of a geochemical survey based on 1150 outcrop samples for heat production studies in a E–W oriented band ( 120 km×500 km) located approximately between 62 and 63°N in Finland, the central Fennoscandian shield. The results provide representative averages of heat production rate in the studied tectonic units, but also demonstrate the heterogeneity and spatial variation in radiogenic heat production rate of the Precambrian lithosphere. The study area covers formations from the Archaean granite-greenstone terrain in the east via a Palaeoproterozoic mobile belt to a major granitoid complex surrounded by schist belts in central and western Finland. In this study we review the heat production rates as calculated from the U, Th and K total analyses and densities of the samples, and the relations of heat production rate values with major tectonic setting, lithological types, major composition and petrophysical properties of rocks. The data is further correlated with heat flow values measured in boreholes and applied in a lithospheric heat production model in the central part of the shield. Generally, heat production rate increases from Archaean to Proterozoic rocks in E–W direction but this trend is relatively weak and often overrun by lithological variations. The arithmetic means are 1.1±1.3 μW m −3 in the Archaean domain, 1.3±0.6 μW m −3 in the Höytiäinen autochthonous Proterozoic domain, 1.2±0.5 μW m −3 in the Suvasvesi domain characterized by allochthonous Proterozoic cover, 1.0±0.6 μW m −3 in the Proterozoic Raahe–Ladoga Mobile belt, 1.3±1.1 μW m −3 in the Proterozoic Rantasalmi–Haukivuori domain, 1.6±0.8 μW m −3 in the Central Finland Granitoid Complex and 1.4±0.8 μW m −3 in the Bothnian Schist Belt. The standard deviations of the mean values are considerable and reflect the heterogeneity of heat production. All distributions overlap. Heat production rate shows spatial variations which range in scale from kilometres to tens of kilometres. Well-defined systematic variations of heat production rate values with either SiO 2 content, density or P-wave velocity could not be found, although the correlations are statistically significant at 1% risk level. The correlations are weak and scattered, and sometimes opposite in sign in the major rock groups (plutonic, metavolcanic and metasedimentary). Heat flow density measured in boreholes increases with increasing heat production rate, but the present data do not support a strict linear dependence between the variables, and the regression line parameters vary depending on the applied heat production values (drill core analyses versus hole site averages from the present study). This indicates that the heat production rate is spatially heterogeneously distributed in the scale of the boreholes and their formations. A lithospheric heat production model was constructed using data from the present study for the upper crust, literature data for lower crustal xenoliths in the study area and a xenolith-derived heat flow density value for the mantle. The total crustal contribution to the surface heat flow density (37 mW m −2) from radiogenic heat sources is calculated as 26 mW m −2, and the contributions from the lower, middle and upper crust are 4, 11 and 11 mW m −2, respectively.