Heat flow and thermal modeling of the Yinggehai Basin, South China Sea

Geothermal gradients are estimated to vary from 31 to 43 °C/km in the Yinggehai Basin based on 99 temperature data sets compiled from oil well data. Thirty-seven thermal conductivity measurements on core samples were made and the effects of porosity and water saturation were corrected. Thermal conductivities of mudstone and sandstone range from 1.2 to 2.7 W/m K, with a mean of 2.0±0.5 W/m K after approximate correction. Heat flow at six sites in the Yinggehai Basin range from 69 to 86 mW/m², with a mean value of 79±7 mW/m². Thick sediments and high sedimentation rates resulted in a considerable radiogenic contribution, but also depressed the heat flow. Measurements indicate the radiogenic heat production in the sediment is 1.28 μW/m³, which contributes 20% to the surface heat flow. After subtracting radiogenic heat contribution of the sediment, and sedimentation correction, the average basal heat flow from basement is about 86 mW/m².Three stages of extension are recognized in the subsidence history, and a kinematic model is used to study the thermal evolution of the basin since the Cenozoic era. Model results show that the peak value of basal heat flow was getting higher and higher through the Cenozoic. The maximum basal heat flow increased from 65 mW/m² in the first stage to 75 mW/m² in the second stage, and then 90 mW/m² in the third stage. The present temperature field of the lithosphere of the Yinggehai Basin, which is still transient, is the result of the multistage extension, but was primarily associated with the Pliocene extension.     

Heat flow, heat production and fission track data from the Hercynian basement around the Provençal Basin (Western Mediterranean)

In the complex structural framework of the Western Mediterranean. Hercynian areas are expected to be thermally preserved from the recent tectonic evolution. The thermal regime of these areas is studied using heat flow, heat production and fission track data. The surface heat flow is significantly higher in Corsica (76 ± 10 mW m⁻²) than in the Maures and Estérel (58 ± 2 mW m⁻²). Neither heat production nor erosion subsequent to the Alpine orogeny in Corsica can explain such a difference. It is suggested that a deep thermal source related to the asymmetric evolution of the Provençal basin could explain the higher heat flow in Corsica. A model of thermal structure based on the present day thermal regime of the Maures and Estérei is proposed for the stable Hercynian crust in this area. The mantle heat flow is 20–25 mW m⁻² and the temperature at Moho level is 375–500°C, depending on the thermal parameter distribution with depth.     

Burial and thermal history of the West Bashkirian sedimentary basins

The GALO system is applied to the numerical reconstruction of burial and thermal histories of the West Bashkirian lithosphere from the Riphean to the present. An analysis of the variation in tectonic subsidence of the basin during its development is utilized to estimate approximately the mantle heat flow variations. Our variant of basin evolution suggests that after cooling in the Early Riphean, the rather weak thermal reactivations have not led to considerable heating of the lithosphere in the study region. Surface heat flow decreased from relatively high values in the Early Riphean (60–70 mW/m² in the eastern area and 40–50 mW/m² in the western part) to present-day values of 32–40 mW/m². In spite of the relatively low temperature regime of the basin as a whole, a syn-rifting deposition of more than 10 km of limestone, shale and sandstone in the Riphean resulted in rather high temperatures (180–190 °C) at the base of present-day sedimentary blanket in the eastern area. In agreement with the observed data, computed present-day heat flow through the sediment surface increases slightly from 32 to 34 mW/m² near the west boundary of the region to 42 mW/m² near the boundary of the Ural Foldbelt, whereas the heat flow through the basement surface decreases slightly from 28–32 to 24–26 mW/m² in the same direction. The mantle heat flow is only 11.3–12.7 mW/m², which is considerable lower than mean heat flow of the Russian Platform (16–18 mW/m²) and comparable with the low heat flow of Precambrian shields.     

Heat flow in the Altai-Sayan Area: new data

Eleven new estimates of heat flow (q) from the southern Altai-Sayan Folded Area (ASFA) have provided update to the heat flow map of Gorny Altai. Measured heat flow in the area varies from 33 to 90 mW/m², with abnormal values of >70 mW/m^q at four sites. The anomalies may have a deep source only at the Aryskan site in the East Sayan (q = 77 mW/m²) while high heat flows of 75–90 mW/m² obtained for the Mesozoic Belokurikha and Kalguty plutons appear rather to result from high radiogenic heat production in granite, which adds a 25–30 W/m² radiogenic component to a deep component of 50–60 mW/m². The latter value is consistent with heat flow estimates derived from helium isotope ratios (54 mW/m² in both plutons). Heat flow variations at other sites are in the range from 33 to 60 mW/m². The new data support the earlier inferences of a generally low heat flow over most of ASFA (average of 45–50 mW/m²) and of a “cold” Cenozoic orogeny in the area (except for southeastern ASFA), possibly driven by shear stresses associated with India indentation into Eurasia.     

Contribution to the thermal regime of the Provençal Basin based on Flumed heat flow surveys and previous investigations

We present original heat flow determinations carried out during the Flumed surveys by the CEPM along three transects of the Provençal Basin (Gulf of Lions-West Sardinia; Toulon-Ajaccio; Nice-Calvi). A total of 121 thermal gradients and 37 conductivities are examined together with previous heat flow determinations along depth sections based on previous geophysical investigations. The mean observed heat flows are clearly shown to increase from NW to SE along the profiles (expect for the Toulon-Calvi transect, where results are ambiguous). The observed heat flow increases from 55–65 mW m⁻² (Gulf of Lions) to 85 ± 14 mW m⁻² (West Sardinia) and from 55–65 mW m⁻² (Var Basin) to 103–108 mW m⁻² (lower Corsican margin), suggesting an asymmetrical distribution of the observed heat flow. We examine whether this asymmetry could be caused by thermal refraction above salt structures or by any other superficial cause (sedimentation, topography, etc.) and conclude that an asymmetrical distribution of the subcrustal heat flow is probably the cause of this thermal regime. The elevated heat flows observed to the east in the abyssal plain, corrected for sedimentation, cannot be accounted for by the standard age/heat flow relations established for oceanic or attenuated continental lithosphere. The geodynamic significance of this speculative subcrustal origin remains poorly constrained, but could be related to post-rifting magmatic activity. Further investigations are necessary to elucidate the apparent high local variability of the heat flow on the upper margin of the Gulf of Lions and on the Provençal margin of the Ligurian Sea.     

The variability of heat flow both regional and with depth in southern Alberta, Canada: Effect of groundwater flow?

Detailed studies of terrestrial heat flow in southern and central Alberta estimated on the basis of an order of magnitude larger data base than ever used before (33653 bottom-hole temperature data from 18711 wells) and thermal conductivity values based on detailed rock studies and measured rock conductivities show significant regional and local variations and variations with depth. Heat flow values were estimated for each 3 × 3 township/range area (28.8 × 28.8 km). A difference in heat flow exists between Paleozoic and Mesozoic strata. Generally lower heat flow values are observed in the strata above the Paleozoic erosional surface (20–75 mW m⁻²). Much higher values are estimated for the Younger Paleozoic formations, with large local and regional variations between 40 and 100 mW m⁻².Average heat flow values based on heat flow determinations below and above the Paleozoic surface that agree within 20% show an increase from values less than 40 mW m⁻² in southern and southwestern Alberta to values as high as 70 mW m⁻² in central Alberta. The predominance of regional downward groundwater flows in Mesozoic strata seem to be responsible for the generally observed heat flow increase with depth.The results show that the basin heat flow pattern is influenced by water movement and even careful detailed heat flow measurements will not give correct values of background steady-state heat flow within the sedimentary strata.     

Thermal regime at the Upper Stillwater dam site, Uinta mountains, Utah: Implications for terrain, microclimate and structural corrections in heat flow studies

A detailed study of the subsurface thermal regime at the Upper Stillwater dam site, Uinta Mountains, northeast Utah, has been made. Temperature measurements were made in 36 drillholes located within a 1 km² area and ranging in depth from 20 to 97 m. Holes less than about 40 m deep were used only to obtain information about spatial variations in mean annual surface temperature. Several holes in or near talus slopes at the sides of the canyons have temperature minima approaching 0°C between 10 and 20 m indicating the presence of year-round ice at the base of the talus. Another set of holes show transient thermal effects of surface warming resulting from clearing of a construction site 3.5 years prior to our measurements. Most of the remaining holes show conductive behavior and have gradients ranging from 13° to 17°C km⁻¹. Measurements made on 44 core samples yield a thermal conductivity of 5.6 (std. dev. 0.35) W m⁻¹ K⁻¹ for the Precambrian quartzite present. Surface heat flow estimates for these holes range from 70 to 100 mW m⁻². However, the local disturbance of the thermal field by topography and microclimate is considerable. A finite difference method used to model these effects yielded a locally corrected Upper Stillwater heat flow of about 75 mW m⁻². A final correction to account for the effects of refraction of heat from the low conductivity sedimentary rocks in the Uinta Basin into the high conductivity quartzite at the dam site, produced a regionally corrected Upper Stillwater heat flow between 60 and 65 mW m⁻². This value is consistent with the observed heat flow of 60 mW m⁻² in the Green River Basin to the north and the Uinta Basin to the south.     

Geothermal regime and Jurassic source rock maturity of the Junggar basin, northwest China

We analyze the thermal gradient distribution of the Junggar basin based on oil-test and well-logging temperature data. The basin-wide average thermal gradient in the depth interval of 0–4000 m is 22.6 °C/km, which is lower than other sedimentary basins in China. We report 21 measured terrestrial heat flow values based on detailed thermal conductivity data and systematical steady-state temperature data. These values vary from 27.0 to 54.1 mW/m² with a mean of 41.8 ± 7.8 mW/m². The Junggar basin appears to be a cool basin in terms of its thermal regime. The heat flow distribution within the basin shows the following characteristics. (1) The heat flow decreases from the Luliang Uplift to the Southern Depression; (2) relatively high heat flow values over 50 mW/m² are confined to the northern part of the Eastern Uplift and the adjacent parts of the Eastern Luliang Uplift and Central Depression; (3) The lowest heat flow of smaller than 35 mW/m² occurs in the southern parts of the basin. This low thermal regime of the Junggar basin is consistent with the geodynamic setting, the extrusion of plates around the basin, the considerably thick crust, the dense lithospheric mantle, the relatively stable continental basement of the basin, low heat generation and underground water flow of the basin. The heat flow of this basin is of great significance to oil exploration and hydrocarbon resource assessment, because it bears directly on issues of petroleum source-rock maturation. Almost all oil fields are limited to the areas of higher heat flows. The relatively low heat flow values in the Junggar basin will deepen the maturity threshold, making the deep-seated widespread Permian and Jurassic source rocks in the Junggar basin favorable for oil and gas generation. In addition, the maturity evolution of the Lower Jurassic Badaowan Group (J_1b) and Middle Jurassic Xishanyao Group (J_2x) were calculated based on the thermal data and burial depth. The maturity of the Jurassic source rocks of the Central Depression and Southern Depression increases with depth. The source rocks only reached an early maturity with a R₀ of 0.5–0.7% in the Wulungu Depression, the Luliang Uplift and the Western Uplift, whereas they did not enter the maturity window (R₀ < 0.5%) in the Eastern Uplift of the basin. This maturity evolution will provide information of source kitchen for the Jurassic exploration.     

Numerical modelling of the thermal evolution of the northwestern Dniepr–Donets Basin (Ukraine)

The Dniepr–Donets Basin (DDB) is a Late Devonian rift structure located within the East-European Craton. Numerical heat flow models for 13 wells calibrated with new maturity data were used to evaluate temporal and lateral heat flow variations in the northwestern part of the basin.The numerical models suggest that heat flow was relatively high during Late Carboniferous and/or Permian times. The relatively high heat flow is probably related to an Early Permian re-activation of tectonic activity. Reconstructed Early Permian heat flow values along the axial zone of the rift are about 60 mW/m² and increase to 90 mW/m² along the northern basin margin. These values are higher than those expected from tectonic models considering a single Late Devonian rifting phase. The calibration data are not sensitive to variations in the Devonian/Carboniferous heat flow. Therefore, the models do not allow deciding whether heat flows remained high after the Devonian rifting, or whether the reconstructed Permian heat flows represent a separate heating event.Analysis of the vitrinite reflectance data suggest that the northeastern Dniepr–Donets Basin is characterised by a low Mesozoic heat flow (30–35 mW/m²), whereas the present-day heat flow is about 45 mW/m².     

Water movement within lac du bonnet batholith as revealed by detailed thermal studies of three closely-spaced boreholes

Successive temperature logs have been obtained over a period of two years in three closely-spaced boreholes in the Lac du Bonnet batholith of the Superior Province of the Canadian Shield. Two of the boreholes, of depth 450 m and 830 m, intersect a dipping fracture zone at 435–450 m. In both holes water is flowing from near the surface to the fracture zone at approximately 1.5–1.9·10⁻⁵ m³ s⁻¹, the flow being inferred from analysis of the temperature logs. Below 25 m, temperatures in these two holes are 0.22–0.28 K lower than those in the third, 145 m, hole.The temperature data have been combined with over 200 thermal conductivity measurements on core samples to produce heat flow values. In the deepest hole heat flow above the fracture zone is 16% higher than that below the zone. This indicates that water is flowing up the fracture zone. The flow rate is approximately 0.3 g s⁻¹ m⁻¹, and the flow has existed for thousands of years.Observation of thermal effects of water flow in massive, relatively unfractured plutons in a region having little topographic relief causes one to be concerned about the reliability of heat flow values measured in similar environments.The regional heat flow is taken to be 50 mW m⁻² after correction for glaciation effects. The average value of 24 determinations of radioactive heat generation in granitic core samples is 5.23 ± 1.11 μW m⁻³, which is more than three times higher than expected for such a heat flow in the Superior Province. This implies that the layer of high radioactive heat generation is thin, being not more than 4 km and probably about 1.3 km thick.     

Heat flow and heat generation in the Churchill Province of the Canadian Shield, and their palaeotectonic significance

Six new heat flow determinations are presented for Proterozoic mobile belts of the Churchill Province of the Canadian Shield, an area that was affected by several stages of the Hudsonian orogenic sequence (1.9-1.6 Ga ago). With other, previously published, values the mean of eight determinations considered reliable and representative and corrected for the effects of Pleistocene glaciation is 44 ± 7 mW m⁻². Heat generation measurements have also been made; values range from 0.1–1.04 μW m⁻³.A linear relation between heat flow and heat production is apparent. The heat flow axis intercept is 37 mW m⁻², and the scale depth is 11 km, compared with 28 mW m⁻² and 13.6 km for the Archaean Superior Province. Approximately 20% of the Churchill heat flow appears to be derived from radioactive decay in the upper crust, compared with 30% for the Superior Province and shields as a whole. The observations imply that the heat flow-heat production relation for the Churchill Province should be written as Q = Q_c + Q_e + A₀b where Q_c is equivalent to the reduced heat flow for the Archaean terrain, b is similar for the two, and Q_e is an additional component of heat flow in the Proterozoic mobile belts of the Churchill Province.A speculative tectonic model is presented. It is suggested that rifting along two axes of an original craton, which had lateral variations in near surface radiogenic element concentration, followed by erosion of the radiogenic layer and subsequent reconvergence of the cratonic segments, led to widespread redistribution of radioactive elements into the reactivated inter-rift crustal block. One result would be that crustal temperatures are higher in that part of the Churchill Province than in the Superior.     

High heat flow anomalies on an old oceanic plate observed seaward of the Japan Trench

Thirty-three new measurements on the seaward slope and outer rise of the Japan Trench along a parallel of 38°45′N revealed the existence of high heat flow anomalies on the subducting Pacific plate, where the seafloor age is about 135 m.y.. The most prominent anomaly with the highest value of 114 mW/m² is associated with a small mound on the outer rise, which was reported to be a kind of mud volcano. On the seaward slope of the trench, heat flow is variable: high (70–90 mW/m²) at some locations and normal for the seafloor age (about 50 mW/m²) at others. The spatial variation of heat flow may be related to development of normal faults and horst/graben structures due to bending of the Pacific plate before subduction, with fluid flow along the fault zones enhancing the vertical heat transfer. Possible heat sources of the high heat flow anomalies are intra-plate volcanism in the last several million years like that discovered recently on the Pacific plate east of the Japan Trench.     

Heat flow and lithospheric thermal regime in the Northeast German Basin

New values of surface heat flow are reported for 13 deep borehole locations in the Northeast German Basin (NEGB) ranging from 68 to 91 mW m^− 2 with a mean of 77 ± 3 mW m^− 2. The values are derived from continuous temperature logs, measured thermal conductivity, and log-derived radiogenic heat production. The heat-flow values are supposed free of effects from surface palaeoclimatic temperature variations, from regional as well as local fluid flow and from thermal refraction in the vicinity of salt structures and thus represent unperturbed crustal heat flow. Two-D numerical lithospheric thermal models are developed for a 500 km section along the DEKORP-BASIN 9601 deep seismic line across the basin with a north-eastward extension across the Tornquist Zone. A detailed conceptual model of crustal structure and composition, thermal conductivity, and heat production distribution is developed. Different boundary conditions for the thickness of thermal lithosphere were used to fit surface heat flow. The best fit is achieved with a thickness of thermal lithosphere of about 75 km beneath the NEGB. This estimate is corroborated by seismological studies and somewhat less than typical for stabilized Phanerozoic lithosphere. Modelled Moho temperatures in the basin are about 800 °C; heat flow from the mantle is about 35 to 40 mW m^− 2. In the southernmost part of the section, beneath the Harz Mountains, higher Moho temperatures up to 900 to 1000 °C are shown. While the relatively high level of surface heat flow in the NEGB obviously is of longer wave length and related to lithosphere thickness, changes in crustal structure and composition are responsible for short-wave-length anomalies.     

孤岛油田馆陶组热储地热资源开发利用分析

孤岛油田蕴藏丰富的中、低温地热资源,对其进行合理的开发利用,对推进该区新、旧热能转换,促进地方经济发展具有重要意义。在总结以往勘探成果的基础上,查明了孤岛油田为大地热流高值异常区,平均值为72.62 mW/m²。重点研究馆陶组热储地热地质条件,查明了馆陶组下段热储厚度为106~145 m,平均孔隙度约为30%,热储温度为75.5~82 ℃,单位降深涌水量为3.71~10.55 m³/(h·m),是地热资源开发的有利目标热储。采用热储法估算区内馆陶组下段热储中蕴藏的地热资源量为3.745×10¹⁸ J,折合标准煤量1.28亿t,地热水储存量约为60.87×10⁸ m³; 采用开采强度法估算的该区地热水允许开采量约为253万m³/a,可支持供暖面积约100万m²。     

Absence of a regional surface thermal high in the Baikal rift; new insights from detailed contouring of heat flow anomalies

Heat flow in active tectonic zones as the Baikal rift is a crucial parameter for evaluating deep anomalous structures and lithosphere evolution. Based on the interpretation of the existing datasets, the Baikal rift has been characterized in the past by either high heat flow, or moderately elevated heat flow, or even lacking a surface heat flow anomaly. We made an attempt to better constrain the geothermal picture by a detailed offshore contouring survey of known anomalies, and to estimate the importance of observed heat flow anomalies within the regional surface heat output. A total of about 200 new and close-spaced heat flow measurements were obtained in several selected study areas in the North Baikal Basin. With an outrigged and a violin-bow designed thermoprobe of 2–3-m length, both the sediment temperature and thermal conductivity were measured. The new data show at all investigated sites that the large heat flow highs are limited to local heat flow anomalies. The maximum measured heat flow reaches values of 300–35000 mW/m², but the extent of the anomalies is not larger than 2 to 4 km in diameter. Aside of these local anomalies, heat flow variations are restricted to near background values of 50–70 mW/m², except in the uplifted Academician zone. The extent of the local anomalies excludes a conductive source, and therefore heat transport by fluids must be considered. In a conceptual model where all bottom floor heat flow anomalies are the result of upflowing fluids along a conduit, an extra heat output of 20 MW (including advection) is estimated for all known anomalies in the North Baikal Basin. Relative to a basal heat flow of 55–65 mW/m², these estimations suggest an extra heat output in the northern Lake Baikal of only 5%, corresponding to a regional heat flow increase of 3 mW/m². The source of this heat can be fully attributed to a regional heat redistribution by topographically driven ground water flow. Thus, the surface heat flow is not expected to bear a signal of deeper lithospheric thermal anomalies that can be separated from heat flow typical for orogenically altered crust (40–70 mW/m²). The new insights on the geothermal signature in the Baikal rift once more show that continental rifting is not by default characterized by high heat flow.     

Research on the characteristics and influencing factors of terrestrial heat flow in Guizhou Province

Terrestrial heat flow is an important physical parameter in the study of heat transfer and thermal structure of the earth and it has great significance in the genesis and development and utilization potential of regional geothermal resources. Although several breakthroughs in geothermal exploration have been made in Guizhou Province. The terrestrial heat flow in this area has not been properly measured, restricting the development of geothermal resources in the province. For this reason, the terrestrial heat flow in Guizhou was measured in this study, during which the characteristics of heat flow were determined using borehole thermometry, geothermal monitoring and thermal property testing. Moreover, the influencing factors of the terrestrial heat flow were analyzed. The results show that the thermal conductivity of rocks ranges from 2.0 W/(m·K) to 5.0 W/(m·K), with an average of 3.399 W/(m·K); the heat flow varies from 30.27 mW/m² to 157.55 mW/m², with an average of 65.26 ± 20.93 mW/m², which is slightly higher than that of the average heat flow in entire land area in China. The heat flow in Guizhou generally follows a dumbbell-shaped distribution, with high values present in the east and west and low values occurring in the north and south. The terrestrial heat flow is related to the burial depths of the Moho and Curie surface. The basaltic eruptions in the Emeishan led to a thinner lithosphere, thicker crust and lateral emplacement, which dominated the basic pattern of heat flow distribution in Guizhou. In addition, the dichotomous structure of regional active faults and concealed deep faults jointly control the heat transfer channels and thus influence the terrestrial heat flow.     

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/m². 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/m². 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.     

Double seismic zone beneath the middle of Hokkaido, Japan, in the southwestern side of the Kurile Arc

The vertical section of microearthquakes, determined accurately by using the Hokkaido University network, shows two dipping zones (the double seismic zone) 25–30 km apart in the depth range of 80–150 km beneath the middle of Hokkaido in the southwestern side of the Kurile arc. Hypocentral distribution of large earthquakes (m_b > 4) based on the ISC (International Seismological Centre) bulletin also shows the double seismic zone beneath the same region. The hypocentral distribution indicates that the frequency of events occurring in the lower zone is four times greater than that in the upper zone. The difference in seismic activity between the two zones beneath Hokkaido is in contrast with the region beneath northeastern Honshu in the northeastern Japan arc.Composite focal mechanisms of microearthquakes and individual mechanisms of large events mainly characterize the down-dip extension for the lower zone as is observed beneath northeastern Honshu. For the upper zone, however, the stress field is rather complex and not necessarily similar to that beneath northeastern Honshu. This may be considered to indicate the influence of slab contortion or transformation in the Hokkaido corner between the Kurile and the northeastern Japan arcs.     

Thermal and mechanical structure of the central Iberian Peninsula lithosphere

The central Iberian Peninsula (Spain) is made up of three main tectonic units: a mountain range, the Spanish Central System and two Tertiary basins (those of the rivers Duero and Tajo). These units are the result of widespread foreland deformation of the Iberian plate interior in response to Alpine convergence of European and African plates. The present study was designed to investigate thermal structure and rheological stratification in this region of central Spain. Surface heat flow has been described to range from 80 to 60 mW m⁻². Highest surface heat flow values correspond to the Central System and northern part of the Tajo Basin. The relationship between elevation and thermal state was used to construct a one-dimensional thermal model. Mantle heat flow drops from 34 mW m⁻² (Duero Basin) to 27 mW m⁻² (Tajo Basin), and increases with diminishing surface heat flow. Strength predictions made by extrapolating experimental data indicate varying rheological stratification throughout the area. In general, in compression, ductile fields predominate in the middle and lower crusts and lithospheric mantle. Brittle behaviour is restricted to the first 8 km of the upper crust and to a thin layer at the top of the middle crust. In tension, brittle layers are slightly more extended, while the lower crust and lithospheric mantle remain ductile in the case of a wet peridotite composition. Discontinuities in brittle and ductile layer thickness determine lateral rheological anisotropy. Tectonic units roughly correspond to rheological domains. Brittle layers reach their maximum thickness beneath the Duero Basin and are of least thickness under the Tajo Basin, especially its northern area. Estimated total lithospheric strength shows a range from 2.5×10¹² to 8×10¹² N m⁻¹ in compression, and from 1.3×10¹² to 1.6×10¹² N m⁻¹ in tension. Highest values were estimated for the Duero Basin.Depth versus frequency of earthquakes correlates well with strength predictions. Earthquake foci concentrate mainly in the upper crust, showing a peak close to maximum strength depth. Most earthquakes occur in the southern margin of the Central System and southeast Tajo Basin. Seismicity is related to major faults, some bounding rheological domains. The Duero Basin is a relative quiescence zone characterised by higher total lithospheric strength than the remaining units.     

Spatial characteristics of groundwater temperature in the Ishikari Lowland, Hokkaido, northern Japan: analytical and numerical applications

In porous sediments of the Ishikari Lowland, there is a gradual increase in the background geothermal gradient from the Ishikari River (3–4 °C 100 m^–1) to the southwest highland area (10 °C 100 m^–1). However, the geothermal gradient at shallow depths differs in detail from the background distribution. In spite of convective heat-flow loss generally associated with groundwater flow, heat flow remains high (100 mW m^–2) in the recharge area in the southwestern part of the Ishikari basin, which is part of an active geothermal field. In the northeastern part of the lowland, heat flow locally reaches 140 mW m^–2, probably due to upward water flow from the deep geothermal field. Between the two areas the heat flow is much lower. To examine the role of hydraulic flow in the distortion of the isotherms in this area, thermal gradient vs. temperature analyses were made, and they helped to define the major components of the groundwater-flow system of the region. Two-dimensional simulation modeling aided in understanding not only the cause of horizontal heat-flow variations in this field but also the contrast between thermal properties of shallow and deep groundwater reservoirs. Electronic Publication