Heat Flow Density Research Articles

Thermal state of the west carpathian lithosphere

In the recent time the notions about the temperature field of the West Carpathians, obtained by direct measuring methods were supplemented by various simple 1-D and/or 2-D analytical and numerical models. We constructed our model using geological density models of the crust along six profiles crossing the Carpathian arc and a suitable construction method of the distribution of thermal parameters within the cross-section (determination of thermal conductivities and heat productions adequate to the petrophysical composition and stratigraphical assignated to geologically well investigated subsurface structures, calculation of heat productions from densities in deeper parts of the lithosphere, etc.). The two-dimensional solution of the equation of heat conduction in an inhomogeneous medium, obtained by means of the finite difference method, was used. The constructed 3-D model is represented by the heat flow density and temperature distribution at the Moho-discontinuity, at four horizontal levels (5, 10, 15 and 50 km) including the description of geological structures, and by the map of thicknesses of the West Carpathian lithosphere. Seismic maps of the lithosphere thickness and other geological and geophysical data enabled us to analyse our model from the point of view of tectonic evolution and of its regional influence on the thermal state of the West Carpathian lithosphere.

Heat flow in the Baltic Shield—results of the lithospheric geothermal modelling

The results of heat flow measurements performed within the Baltic Shield and in its closest neighbourhood are reviewed. The regional distribution of the heat flow density is presented as a heat flow map. The characteristic heat flow density amounts to 35–40 mW m−2 and is notably smaller than heat flow in the surrounding areas. The radiogenic heat production within the crust contributes to the observed surface heat flow by about 15–20 mW m−2. This permits to estimate the Moho heat flow in the most stable parts of the shield to about 20 mW m−2, and over 25–30 mW m−2 in the surrounding platform to the south and east. Relatively low Moho temperatures of 350–500°C characterize the crust/mantle boundary of the shield, increasing to 600–700°C beneath the shield margins. The thermal thickness of the lithosphere of the shield can attain more than 200 km.

Heat flow map of northern and central parts of the Fennoscandian Shield based on geochemical surveys of heat producing elements

The heat flow-heat production ( Q- A) relationship is a useful tool in geothermal research, and it has been widely used for delineating geothermal provinces and determining characteristic parameters of heat production in the continental crust. In this study, a simple but rarely used technique of utilizing the heat flow-heat production relationship is discussed. In central and northern parts of the Fennoscandian Shield extensive geochemical surveys have produced 1483 samples taken from glacial till with a sampling density of 1 sample/300 km 2. Heat production values determined from U, Th and K concentrations in these samples were used to calculate a map of heat flow density. A previously determined Q- A relationship, Q = 15.8 + 10.8 · A, was applied. The compiled map covers all Finland, northern Sweden and northern Norway, about 35% of the exposed shield area in Fennoscandia. Heat flow density and heat production increase with decreasing geological age and correlate with granitoid types. The calculated heat flow density values on the map were controlled with 12 drill hole measurements not used in calculating the applied Q- A regression line. Nine of them are from previously unpublished data from the Finnish part of the Shield. The agreement with drill hole measurements and the geochemical estimate is reasonable, although not perfect in all cases. The differences can be attributed to anomalous vertical variation in heat production, reliability of the applied Q- A plot, reliability of till geochemistry in bedrock studies, convective groundwater disturbances or local structural effects. The calculated heat flow map can be used as a data set supplementing drill hole measurements to determine representative values of heat flow density in areas with low numbers of drill hole measurements.

Paleo heat flow of eastern Canada's passive margins

Tectonic subsidence rates, derived from several wells in the Labrador Sea and offshore Nova Scotia and Newfoundland, were used to estimate the paleo heat flow along the margins of eastern Canada. If tectonic subsidence is caused by the cooling of the lithosphere only, the transient component of the heat flow (i.e., the heat flow in excess of background heat flow) can be estimated directly from subsidence data. For the Nova Scotia and the Labrador Sea margins, the analysis shows that the transient component of the heat flow decreased markedly from 28–56 mW · m −2, immediately after rifting, to a present value between 7 and 14 mW · m −2 depending on boundary conditions. The analysis shows a distinctive difference between the evolution of the Labrador Sea and that of the northeastern Newfoundland margin where it is estimated that the transient component of the heat flow exceeded 100 mW · m −2 after rifting and dropped rapidly to very low values. The study suggests that the subsidence of the Newfoundland margin was not caused by cooling only but required an additional mechanism such as lower-crustal phase transformations or continuing extension after continental breakup with ductile deformation in the lower crust and upper mantle. The estimated transient heat flow was used to calculate the average present surface heat flow in eastern Canada's margins, which is the sum of heat flow from the mantle, crustal heat production, and the transient cooling of the lithosphere. The calculated heat flow compares well with heat flow density data obtained from bottom hole temperature measurements.

Heat flow and deep temperature in South Portugal

Heat-flow density determinations were made in South Portugal. The region of study covers the Algarve Basin, the South Portuguese Zone, part of the Ossa Morena Zone and part of the Lusitanian Basin.

Rheological consequences of the lithospheric thermal structure in the Fennoscandian Shield

The surface heat-flow density of the Fennoscandian Shield, after removing the disturbances due to palaeoclimatic changes, shows a remarkable contrast from the Archaean terrains to the Late Proterozoic provinces which derive from tectonic reactivation and reheating of older materials. This involves lateral variations in the rheological behaviour of the lithosphere. On the basis of seismic structural data and assumptions about the petrological composition and flow parameters of steady-state dislocation creep, strength profiles and lateral viscosity variations have been deduced for several sites. In the northern and northeastern parts of the shield, where the Moho temperature and mantle heat-flow density are typically cratonic, the rheological thickness of the lithosphere, given by the depth at which the strength is reduced to 1 MPa, ranges from 120 to 140 km. In the southwestern shield, were enhanced Moho temperatures and mantle heat-flow density occur, the rheological thickness is reduced to 60–80 km. The depth of the brittle-ductile transition in the upper crust, above which most earthquakes occur, varies on average from 30 km in the northeast to 18 km in the southwest. The limiting temperature of the brittle uppermost layer on average is 365 ± 70°C. The essentially aseismic behaviour of the mantle agrees with the subcrustal predominating ductile deformations predicted by the models. The average lithospheric strength falls within the range 70–200 MPa, typical of the older stable areas. The viscosity of the upper mantle, at a reference depth level of 60 km, ranges from 10 21 to 10 24 Pa s and increases with the geological age, being maximum beneath the Archaean nucleus.

Vertical variation of heat flow density in the continental crust

Terrestrial heat flow density is a key parameter in understanding the past, present and future development of our planet. Most phenomena studied in deep crustal geophysics are temperature dependent and therefore reliable assesments of deep temperatures are necessary. Most heat flow measurements have been made in drill holes which are shallow (< 1 km) in comparison to the thicknesses of the crust and lithosphere. The recent findings in deep drilling projects (e.g. the Kola deep hole in Russia and the KTB hole in Germany) have yielded results which suggest that there is a distinct contrast between heat flow densities measured in the uppermost 1 km and values measured at deeper levels. The factors contributing to the vertical variation in the uppermost few kilometres are discussed with special emphasis on palaeoclimatic ground surface temperature changes and groundwater circulation in the bedrock.

Heat flow in the Bastar Craton, central Indian Shield: implications for thermal characteristics of Proterozoic cratons

Measurements of surface heat flow density in various lithologies from the central parts of the Proterozoic Bastar Craton of the central Indian Shield have yielded heat flow values ranging from 51 to 64 mW m −2, with a mean value of 56 mW m −2 (standard deviation = ±6.1 mW m −2). These values are considerably higher than in the Archaean and Archaean to Early Proterozoic cratons of many Precambrian shields, but are compatible with the heat flow data from the Proterozoic Gawler Craton, central Australian Shield. It is inferred that (1) the contrast in surface heat flow between the Archaean and Proterozoic cratons is a consequence of the difference in the radiogenic heat production in their upper (10–20 km thick) crustal layers, and (2) the Proterozoic cratons, as these could also have enclaves of basic Archaean rocks, show a wide range of surface heat flow values controlled mainly by the varying amounts of upper crustal radiogenic heat source.

Geothermics of the Essaouira basin in Morocco

The mean values of geothermal gradient and heat flow density have been estimated from bottom-hole temperatures taken from 75 petroleum exploration wells in the Essaouira basin in Morocco and are found to be 21.0 ± 0.3 mK/m and 68 ± 8 mW/m2, respectively. The regional thermal gradient varies with depth, from 30–35 mK/m near to the surface to about 18 mK/m between 2000 and 3000 m depth and then increasing again to over 24 mK/m at depths greater than 3000 m. The decrease at intermediate depths results from the presence of Triassic evaporites (with high thermal conductivities). Comparison of the temperature pattern and petroleum occurences indicates that the hydrocarbon matured in the deep Paleozoic formations and ascended from the source rocks by both upward migration along active faults and uplift during compressional tectonics.

First heat flow density determinations from Southeastern Zaïre (Central Africa)

First heat flow density determinations from southeastern Zaïre are presented. Sites are located in the late Proterozoic metasedimentary cover of the Pan-African belt (600 Ma.). For each individual boreholes, heat flow ranges between 48 and 72 mWm −2. The average value of 62 mW m −2 for the sites is similar to that of 66 mW m −2 observed in Zambia. Both values are higher than what is expected for Pan-African terranes. These heat flow determinations in Shaba province of southeastern Zaïre, together with gravity and seismological observations, support the hypothesis of lithospheric thinning for this area. As already suggested for Zambia, this lithospheric thinning may be associated with a southwestern extension of the East African Rift System from Tanganyika across the central African plateau.

Dependence of the seismotectonic regime on the thermal state in the northern italian apennines

Abstract In order to study the relationships between recent seismotectonic processes and thermal conditions of the lithosphere, we analyzed the lateral variations both in surface heat-flow density and in seismic activity of the northern Italian Apennines. The focal depths have been related to the mechanical properties of the lithosphere. The hypocentre distribution indicates that the thickness of the seismogenic crustal layer increases towards the outer part of the range. Rheological models confirm this result, showing that in the western areas, marked by heat-flow density greater than 60 mW/m 2 and by positive gravity anomalies, the depth of the crustal brittle-ductile transition is rather shallow, being about 10 km. In the cold external parts of the chain, this depth increases, ranging from 15 to 25 km; brittle seismogenic levels can be also found in the upper mantle, below an essentially aseismic, ductile crustal layer. The estimated limiting temperatures of the brittle strength zones are about 320°–380°C and 550°–600°C for crustal and upper mantle materials, respectively.

Relationship of geological and geothermal field properties: Midcontinent area, USA, an example

Quantitative approaches to data analysis in the last decade have become important in basin modeling and mineral-resource estimation. The interrelation of geological, geophysical, geochemical, and geohydrological variables is important in adjusting a model to a real-world situation. Revealing the interdependences of variables can contribute in understanding the processes interacting in sedimentary basins. It is reasonably simple to compare spatial data of the same type but more difficult if different properties are involved. Statistical techniques, such as cluster analysis or principal components analysis, or some algebraic approaches can be used to ascertain the relations of standardized spatial data. In this example, structural configuration on five different stratigraphic horizons, one total sediment thickness map, and four maps of geothermal data were copared. As expected, the structural maps are highly related because all had undergone about the same deformation with differing degrees of intensity. The temperature gradients derived (1) from shallow borehole logging measurements under equilibrium conditions with the surrounding rock, and (2) from non-equilibrium bottom-hole temperatures (BHT) from deeper depths are mainly independent of each other. This was expected and confirmed also for the two temperature maps at 1000 ft which were constructed using both types of gradient values. Thus, it is evident that the use of a 2-point (BHT and surface temperature) straightline calculation of a mean temperature gradient gives different information about the geothermal regime than using gradients from temperatures logged under equilibrium conditions. Nevertheless, it is useful to determine to what a degree the larger dataset of nonequilibrium temperatures could reflect quantitative relationships to geologic conditions. Comparing all maps of geothermal information vs. the structural and the sediment thickness maps, it was determined that all correlations are moderately negative or slightly positive. These results are clearly shown by the cluster analysis and the principal components. Considering a close relationship between temperature and thermal conductivity of the sediments as observed for most of the Midcontinent area and relatively homogeneous heat-flow density conditions for the study area these results support the following assumptions: (1) undifferentiated geothermal gradients, computed from temperatures of different depth intervals and differing sediment properties, cannot contribute to an improved understanding of the temperature structure and its controls within the sedimentary cover, and (2) the quantitative approach of revealing such relations needs refined datasets of temperature information valid for the different depth levels or stratigraphic units.

Paleoclimate change and heat flow density inferred from temperature data in the Superior Province of the Canadian sheild

Four boreholes, located near Cochrane, Hearst, Minchin Lake, and Otoskwin River in the Superior Province of the Canadian Shield, were logged in 1969/1970, and again in the summer of 1985. We have used the obtained temperature-depth ( T- z) data and the thermal property measurements to estimate the ground surface temperature (GST) histories and the heat flow densities at these sites, using a least squares inverse method. The method employs the theory of heat conduction in a laterally homogeneous Earth and the concept of incorporating a priori information to stabilize and uniquely determine the solution. We formulate the least squares optimization problem in the functional spaces and solve it with a quasi-Newton iterative gradient method. The functional space formulation allows the gradient operator to be obtained by solving only forward problems and the quasi-Newton method, which has a second order convergence, allows the a posteriori covariance operator to be readily computed. Except for the Cochrane hole, where systematic errors appear to exist in the temperature measurements, the two T- z logs for each hole, whether inverted individually or simultaneously, yield highly consistent, though not identical, results. All of the estimated GST histories, except that for Otoskwin River, exhibit a broad low centered around 1800 A.D. (the Little Ice Age), preceeded by a warm period in the Mid-Holocene (the Little Climatic Optimum) and followed by a warming trend in the last 100 years. However, the GST histories differ significantly from hole to hole in terms of the amplitude and the timing of their temporal variations. This suggests that the T- z data may have been significantly perturbed by local topography, lateral heterogeneities in the subsurface thermal properties, groundwater circulation, etc., which are not taken into account in the theory of one-dimensional heat conduction.

Paleoclimate change and heat flow density inferred from temperature data in the Superior Province of the Canadian sheild

Four boreholes, located near Cochrane, Hearst, Minchin Lake, and Otoskwin River in the Superior Province of the Canadian Shield, were logged in 1969/1970, and again in the summer of 1985. We have used the obtained temperature-depth ( T- z) data and the thermal property measurements to estimate the ground surface temperature (GST) histories and the heat flow densities at these sites, using a least squares inverse method. The method employs the theory of heat conduction in a laterally homogeneous Earth and the concept of incorporating a priori information to stabilize and uniquely determine the solution. We formulate the least squares optimization problem in the functional spaces and solve it with a quasi-Newton iterative gradient method. The functional space formulation allows the gradient operator to be obtained by solving only forward problems and the quasi-Newton method, which has a second order convergence, allows the a posteriori covariance operator to be readily computed. Except for the Cochrane hole, where systematic errors appear to exist in the temperature measurements, the two T- z logs for each hole, whether inverted individually or simultaneously, yield highly consistent, though not identical, results. All of the estimated GST histories, except that for Otoskwin River, exhibit a broad low centered around 1800 A.D. (the Little Ice Age), preceeded by a warm period in the Mid-Holocene (the Little Climatic Optimum) and followed by a warming trend in the last 100 years. However, the GST histories differ significantly from hole to hole in terms of the amplitude and the timing of their temporal variations. This suggests that the T- z data may have been significantly perturbed by local topography, lateral heterogeneities in the subsurface thermal properties, groundwater circulation, etc., which are not taken into account in the theory of one-dimensional heat conduction.

An attempt to interpret the temperature profile of the KTB pilot drillhole (Germany) by paleoclimatic considerations

The temperature profile of the KTB pilot drillhole, T( z) KTB-PH,is distinctly nonlinear: a temperature deficit ΔT (relative to a linear temperature-depth profile) is especially pronounced in the depth range 500–3500 m. The depth dependence of the deficit, Δ( z) is compared to be anticipated effect of surface paleoclimatic variations, ΔT pc( z), at the drillsite on the temperature profile. The latter can be calculated from available paleo-climatic models. If ΔT pc( z) is added to T( z) KTB-PH, a nearly linear temperature-depth curve results with an average geothermal gradient of 27.9°C/km. This, together with an average vertical thermal conductivity of 3.0 W/mK, estimated from KTB drillcore data, implies a heat flow density at the KTB site of 84 mW/m 2. This modelled value is in good agreement with heat flow determinations in the adjacent Eger graben structure (Western Bohemian massif).

An attempt to interpret the temperature profile of the KTB pilot drillhole (Germany) by paleoclimatic considerations

The temperature profile of the KTB pilot drillhole, T( z) KTB-PH,is distinctly nonlinear: a temperature deficit ΔT (relative to a linear temperature-depth profile) is especially pronounced in the depth range 500–3500 m. The depth dependence of the deficit, Δ( z) is compared to be anticipated effect of surface paleoclimatic variations, ΔT pc( z), at the drillsite on the temperature profile. The latter can be calculated from available paleo-climatic models. If ΔT pc( z) is added to T( z) KTB-PH, a nearly linear temperature-depth curve results with an average geothermal gradient of 27.9°C/km. This, together with an average vertical thermal conductivity of 3.0 W/mK, estimated from KTB drillcore data, implies a heat flow density at the KTB site of 84 mW/m 2. This modelled value is in good agreement with heat flow determinations in the adjacent Eger graben structure (Western Bohemian massif).

Preliminary results of a magneto-telluric survey over a geothermal anomaly in Portugal

During summer 1990 a magneto-telluric (MT) survey was conducted over a geothermal anomaly discovered a few years ago in south central Portugal. This anomaly, known as the Alentejo Geothermal Anomaly (AGA), exhibits heat flow density values (greater than 160 mW m 2) at its centre. The area covered by the MT survey was about 2500 km 2, and 34 MT sites were occupied. The preliminary results indicate that the central area of the AGA coincides with a region of high electrical resistivity which continues to depths greater than 10 000 m. As no intense hydrothermal manifestations are known in the study area, it is suggested that the heat flow density anomaly is the result of high heat production in some of the geological formations that outcrop in the region. Two major faults cross the area, intersecting at approximately a right angle. Low-resistivity zones appear to be associated with the fault zones, and a possible interpretation is that these conductive zones may be related to fluids contained in fractures surrounding the faults.

Heat loss and tectonic style of Venus

The tectonic style of a terrestrial planet depends strongly on the mechanisms of heat release from the mantle through the lithosphere to the surface. Three types of lithospheric heat transfer have been proposed. (1) Lithospheric conduction, (2) (hot spot) volcanism, (3) plate recycling (mainly at spreading plate margins). In the case of the Earth the total heat flow is determined by plate recycling 65%, heat conduction through the lithosphere 20%, decay of radioactive elements in the crust 15%, hot spot volcanism <1%. Scaling the mean surface heat flow density of the Earth to venusian conditions leads to 66 mW/m2. In the case of Venus plate tectonics play only a minor role. Thus, two processes remain for heat release: (hot spot) volcanism and conduction. The term “hot spot” is written in brackets because volcanism on Venus occurs globally, not necessarily associated with hot spots. The volcanic lava production has been estimated from Venera 15/16 scenes. Arecibo and Magellan images revealed that the surface character south of 30° N is very similar to the area covered by Venera. The main results of the estimation are: (i) The maximum thickness of the plain lavas is 3 km. (ii) With plain lava thicknesses larger than 200 m the lava production from central volcanoes is negligible, (iii) Two age models have been used for the mean age of the area obseved: Δt 1 = 109 a, Δt 2 = 400 x 106 a. Δt 1 leads to the maximum lava production rate of 3 km3/a compared to 20 to 25 km3/a of the Earth; this gives a maximum contribution of 0.75mW/m2 to the heat flow density of Venus, i.e. about 1%. This implies that either heat conduction is the only dominating process for heat release or there is a hidden reservoir of the “missing basalt” somewhere or there is another unknown tectonic process. Assuming pure conduction and correcting the surface heat flow density for radioactive elements in the crust leads to a thickness of the thermal lithosphere of 45km. A reservoir for the “missing basalt” could be basaltic underplating to a depth of 100 km. This gives a contribution of about 20 mW/m2 with the age model δt 2 to the heat flow density from first order calculations. While the tectonic style of the Earth can be described to be linear formed at the plate margins, the surface of Venus is characterized by global spotty volcanism. The surface is more dominated by volcanic landforms than in the case of the Earth despite the relatively low lava production rate with a maximum of 3 km3/a. As plate tectonics is a minor process on Venus, conduction through a rather thin lithosphere should play an important role for heat release.

Heat‐flow constraints on the West African lithosphere structure

Heat production measurements from the West‐African shield have been compiled using various samples of crustal rocks. There is no clear indication for a systematic variation of heat production with age which could explain the regional heat flow density (HFD) variations by corresponding variations of the crustal component. HFD values are positively correlated with the P‐wave travel time residuals deduced from teleseismic observations and negatively correlated with gravity isostatic anomalies. We propose that regional HFD variations are significative of change in the upper mantle structure.

Heat-flow data and shallow thermal regime on Mallorca and Menorca (western Mediterranean)

The first heat-flow density values acquired on the Balearic Islands are presented. A total of 39 thermal gradient determinations were carried out, 23 on Mallorca and 16 on Menorca. The sampling points consist of water exploration wells whose depths range between 100 m and 250 m. Most of the thermometric logs are strongly affected by water circulation through the formation and/or through the borehole. Heat-flow density measurements have been performed from 18 boreholes where conductive heat transport is inferred. Thermal conductivity values were obtained from 52 available well core samples covering the main lithologies present in the area. After topographic and paleoclimatic corrections, surface heat-flow values ranging from 24 ± 13 mW/m 2 to 88 ± 23 mW/m 2 for Mallorca and from 46 ±6 mW/m 2 to 92 ± 20 mW/m 2 for Menorca are obtained. A well-defined positive thermal anomaly associated to upward ground-water flow is described on Mallorca. Water flowing through the Mesozoic basement would be responsible for an overall cooling effect affecting the northeastern half of the island where the lower heat-flow values are measured. Water perturbations on Menorca are mainly due to hydraulic connection through the borehole. Taking into account the effects of the regional ground-water flow, background heat-flow densities of about 70–80 mW/m 2 for Mallorca and 75–90 mW/m 2 for Menorca are proposed.