Regional Heat Flow Research Articles

The composition of back‐arc basin lower crust and upper mantle in the Mariana Trough: A first report

Abstract The Mariana Trough is an active back‐arc basin, with the rift propagating northward ahead of spreading. The northern part of the Trough is now rifting, with extension accommodated by combined stretching and igneous intrusion. Deep structural graben are found in a region of low heat flow, and we interpret these to manifest a low‐angle normal fault system that defines the extension axis between 19°45′ and 21°10′N. A single dredge haul from the deepest (∼5.5 km deep) of these graben recovered a heterogeneous suite of volcanic and plutonic crustal rocks and upper mantle peridotites, providing the first report of the deeper levels of back‐arc basin lithosphere. Several lines of evidence indicate that these rocks are similar to typical back‐arc basin lithosphere and are not fragments of rifted older arc lithosphere. Hornblende yielded an 40Ar/39Ar age of 1.8 ± 0.6 Ma, which is interpreted to approximate the time of crust formation. Harzburgite spinels have moderate Cr# (<40) and coexisting compositions of clinopyroxene (CPX) and plagioclase (PLAB) fall in the field of mid‐ocean ridge basalt (MORB) gabbros. Crustal rocks include felsic rocks (70‐80% SiO2) and plutonic rocks that are rich in amphibole. Chemical compositions of crustal rocks show little evidence for a ‘subduction component’, and radiogenic isotopic compositions correspond to that expected for back‐arc basin crust of the Mariana Trough. These data indicate that mechanical extension in this part of the Mariana Trough involves lithosphere that originally formed magmatically. These unique exposures of back‐arc basin lithosphere call for careful study using ROVs and manned submersibles, and consideration as an ocean drilling program (ODP) drilling site.

Skin to skin care:heat balance.

Skin to skin care has been practised in primitive and high technology cultures for body temperature preservation in neonates. Regional skin temperature and heat flow was measured in moderately hypothermic term neonates to quantitate the heat transfer occurring during one hour of skin to skin care. Nine healthy newborns with a mean rectal temperature of 36.3 degrees C were placed skin to skin on their mothers' chests. The mean (SD) rectal temperature increased by 0.7 (0.4) degrees C to 37.0 degrees C. The heat loss was high (70 Wm-2) from the unprotected skin of the head to the surrounding air. Minute heat losses occurred from covered areas; and heat was initially gained from areas in contact with the mother's skin. The total dry heat loss during skin to skin care corresponded to heat loss during incubator care at 32-32.5 degrees C. The reduced heat loss, and to a minor extent, the initial heat flux from the mothers allowed heat to be conserved, leading to rewarming.

Isostatic response of the Central Indian Ridge (Western Indian Ocean) based on transfer function analysis of gravity and bathymetry data

The Central Indian Ridge (CIR) is a part of the major active mid-ocean ridge system trending north-south between 2°N and the triple junction at 25°S. Previous investigations suggest that the half-spreading rate varies from 1.6 to 2.5 cm/yr between the northern and southern ends of the ridge, and the diffuse plate boundary dividing the Indian and Australian plates intersects the Central Indian Ridge near the equator. In the present study, I have attempted to investigate the nature of isostatic response of young oceanic topography created by spreading along the Central Indian Ridge. Gravity and bathymetric profiles, four in northern part of the ridge within or close to the diffuse boundary zone, and six in southern part outside the zone have been utilized separately to obtain their isostatic response based on transfer function analysis. The admittance estimates for the long wavelengths suggest that the elastic plate model fits the data for the compensation of sea floor topography near the ridge crest. An effective elastic thickness ranging from 5 to 10 km with an average T e of 7 ± 2 km fits best to the data for the northern part of the Central Indian Ridge, and a value from 10 to 17 km with an average T e of 13 ± 3 km fits the data for the southern part of the ridge. Isostatic response function estimates confirm that the difference in elastic plate thicknesses between the northern and southern segments is real and significant. The observed low effective elastic thickness for the data in the northern part of the Central Indian Ridge close to the diffuse boundary zone is interpreted as due to higher than average temperatures beneath the ridge. These higher temperatures have been inferred due to the perturbations of the regional heat flow anomaly associated with the diffuse plate boundary.

Seafloor structural observations, Costa Rica Accretionary Prism

By studying seafloor morphology we can make associations between near surface deformation, fluid flow and the overall structural framework of accretionary prisms. In February, 1994 a DS/RV ALVIN program to the Costa Rica accretionary prism investigated the relationship of fluid seepage and sediment deformation by using the distribution of chemosynthetic communities and heat flow anomalies as indicators of fluid flow. The active normal faults that cut the hemipelagic section on the Cocos plate may provide conduits for fluids that cause the regional heat flow to be extremely low. These normal faults intersect the toe of the prism at an oblique angle, creating localized regions of increased deformation. Positive heat flow anomalies observed at the deformation front indicate diffuse fluid flow, however, we discovered no seep communities indicative of focused flow. The seaward‐most seep communities discovered are in a region of active out‐of‐sequence thrusts that cut a sediment apron which covers the complex to within 3 km of the prism toe. Vents occur consistently at the base of the fault scarps. Dives on a mud diapir show extensive seep communities, pock marks, and authigenic carbonates. Evidence of fluid release is on the crest which implies a low viscosity fluid migrating upward in the center of the structure. Normal faults on the upper slope can be seen in cross‐section in the walls of a submarine canyon. The faults cut the slope apron and displace the seafloor, actively maintaining the critical taper of the prism.

Anomalous heat flow regime in the Western margin of the North American Craton, Canada

A large scale northern Prairies heat flow anomaly (NPHFA) extends from north-eastern British Columbia and northern Alberta to the southern Northwestern Territories between the Precambrian shield (Slave Province) and the Cordillera (Mackenzie Fold Belt). The large aerial scale of the anomaly (order of magnitude of 10 2 km) is of the order of magnitude of lithospheric thickness. In parts of the basin close to areas of high topographic elevation, including the Rocky Mountains and the Mackenzie Fold Belt, the magnitude of the heat flow anomaly is significantly reduced by downward cross-formational fluid flow. The position of the heat flow anomaly (NPHFA) in the central part of the basin is offset with respect to the position of the major carbonate reef, carrying fluids north eastward. It is hypothesised that the large regional scale heat flow anomaly is related to increased mantle upflow of heat under this region. The NPHFA is aligned with the other heat flow anomalies along the longitude 120 ° west ±5 °. Anomalous heat flow associated with an upflow in the mantle under the western part of the westwardly drifting continent, is the most probable explanation of the observed thermal regime in the western margin of the North American Craton in Canada.

A Two-Dimensional Regional Basin Model of Williston Basin Hydrocarbon Systems

Institut Francais du Petrole's two-dimensional model, temispack, is used to discuss the functioning of petroleum systems in the Williston basin along a 330-km-long section, focusing on four regional source intervals: Ordovician Yeoman formation, Lower Devonian Winnipegosis Formation, Upper DevonianuLower Mississippian Bakken Formation, and Mississippian Lodgepole formation. Thermal history calibration against present temperature and source rock maturity profiles suggests that the Williston basin can be divided into a region of constant heat flow of about 55 mW/m2 away from the Nesson anticline, and a region of higher heat flow and enhanced thermal maturity in the vicinity of the Nesson anticline. Original kinetic parameters used in the calibration were derived or each of the four source rocks from Rock-Eval yield curves. Bakken overpressures are entirely due to oil generation, not compaction disequilibrium. Very low Bakken vertical permeabilities range from 0.01 to 0.001 nd are matched against observed overpressures, whereas Bakken porosities based on the model and confirmed by measurements are inferred to be also unusually low, around 3%. Mature Bakken shales do not seem to have reached hydraulic fractionation thresholds, except perhaps locally in regions of extensional tectonic stress. Hydraulic fracturing cannot be viewed as a pervasive mechanism driving Bakken oil expulsion. Our expulsion model confirms the high residual Bakken oil saturations and explains the low residual S1/TOC by the low Bakken shale porosities. Approximately 85% of the oil generated in the Bakken shales is predicted to have been expelled out of Bakken strata, which agrees with observed S1/TOC trends. Madison Group accumulations at the subcrop edge are found to be fed by Lodgepole-sourced oil only, in particular by the rich middle Lodgepole interval. These accumulations probably result from a three-dimensional migration pattern. Madison accumulations in the Nesson anticline are found to be fed mostly by Lodgepole-sourced oil mixed with minor amounts of Bakken-sourced oil. The vast majority of the expelled Bakken oils are lost in the Madison Group at very low saturations. This explains the low conventional oil resource associated with Bakken sources by recent geochemical studies. Expulsion and migration occurred no earlier than the atest Cretaceous-Paleocene in the Yeoman and no earlier than the Eocene in other source rocks, consistent with temporal controls on oil migration. Flow models show no restriction to expulsion and migration of Yeoman oil. This suggests a potential undiscovered oil resource in Ordovician and Silurian strata of Williston basin, northeast of the Nesson anticline.

Regional skin temperature, heat flow and conductance in preterm neonates nursed in low and in neutral environmental temperature.

The changes of regional dry heat loss and skin temperature in 15 healthy preterm babies, 8 with a gestational age (GA) of 33-36 weeks and 7 with a GA of 28-31 weeks, were studied under controlled conditions at environmental temperatures ranging from 29.5 to 34.0 degrees C. In both groups of babies the skin temperature for all body regions followed the changes in operative temperatures. Regional dry heat losses also closely followed the external temperature gradient. In the 33-36 weeks GA neonates the regional changes in thermal conductance (index of cutaneous blood flow) indicated that only the foot responded to low environmental temperature with vasoconstriction while vasodilatation was indicated for the trunk. In the 28-31 weeks GA neonates similar but not significant changes of thermal conductance were calculated. The limited ability to reduce heat loss by reducing the skin conductance over a major part of the body surface area contributes to the vulnerability to low environmental temperature in preterm neonates.

On the correlation of electrical conductivity and heat flow in Middle Valley, Juan de Fuca Ridge

The near‐surface electrical conductivity has been mapped within an area of Middle Valley, a sediment‐filled axial valley at the northern end of the Juan de Fuca ridge. The conductivity in the uppermost 20 m of sediment was determined by analyzing the magnetic field signal transmitted between a source coil and a receiver that were towed along the seafloor. The instrument is a version of a time domain electromagnetic (TDEM) sounding system. The heat flow pattern within Middle Valley, with a large central anomaly above a basement high, is reproduced by the conductivity measurements, the result of enhanced pore fluid electrical conductivity due to higher near‐surface temperatures in the high heat flow regions. The high correlation between heat flow and conductivity requires that porosity as inferred from Archie's law must be nearly uniform in the central part of the study area. Porosities derived from the conductivity measurements are in close agreement with measurements from the Ocean Drilling Project (ODP) boreholes. Higher near‐surface porosities are required in the eastern part of the valley to match the observed heat flow, consistent with the higher porosity seen at ODP site 855. A small region of apparently lower porosity was detected to the west of the center of the valley.

Strength and rheology of the western U.S. Cordillera

Effective elastic thickness Te depends primarily on temperature, composition, and state of stress of the lithosphere. In this paper, we examine high‐resolution spectral estimates of Te and their relationships to regional heat flow, age of the lithosphere, seismic properties, stress orientations, and earthquake focal depths of the western U.S. Cordillera. The relationship of Te to heat flow indicates that ductile flow accommodates long‐term (∼106 to 108 years) isostatic response at different levels of the crust and upper mantle, depending principally on age (and, by implication, bulk composition) of the lithosphere. Isostatic response is primarily controlled by the upper mantle in Archean lithosphere of the middle Rocky Mountains, whereas Te depends on lower crustal flow in Early Proterozoic lithosphere of the Colorado Plateau. The Yellowstone‐Snake River Plain volcanic field and significantly extended regions in the Basin‐Range and northern Rocky Mountains are associated with latest Proterozoic aged lithosphere and indicate middle to upper crustal control of long‐term Te. We also show that azimuthal variations of Te reflect deviatoric stress in the lithosphere. Te is found empirically to approximate the 95th percentile focal depth of background seismicity. The latter relationship is inconsistent with brittle‐ductile control of focal depth, indicating that another rheological transition (e.g., from stick‐slip to stable sliding frictional behavior) is responsible. Tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere. Moreover, we find a spatial correlation of the Intermountain Seismic Belt to a marked transition in Te, implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current‐day plate boundary interactions.

>Mesozoic and Cenozoic extension recorded by metamorphic rocks in the Funeral Mountains, California

The Funeral Mountains metamorphic core complex, like so many of these complexes in the North American Cordillera, contains early ductile structures related to Mesozoic compressional tectonics that have been overprinted by structures related to later extension. The core-bounding detachment system formed in late Miocene time, and ductile extensional fabrics in its footwall have been viewed as evidence of a higher-temperature stage in the same event. Integrated structural analysis and geochronology, however, indicate that ductile fabrics with a transport direction similar to the northwest direction of movement on the detachment are part of an older extensional event of Late Cretaceous age. This event produced mylonitic fabrics, a dominant foliation, and well-developed mineral and stretching lineations. Mesoscopic folds and a widespread crenulation lineation that developed during a late stage in this event also are consistent with northwest-directed shear. Late Cretaceous mylonitic fabrics are related to extensional shear zones within the core complex that have small stratigraphic offsets but may represent significant structural and metamorphic discontinuities. Rocks in the footwall of the shear zones experienced mid-crustal depths that cannot be explained by the overburden available if the Funeral Mountains core rocks represent an intact block of Mesozoic crust. Instead, it appears that a significant section of mid-crustal material may have been excised in Late Cretaceous time by the combined Monarch Canyon and Chloride Cliff shear zones as the result of gravitational instability of an overthickened crust. In our interpretation, this early extension was made possible by a waning of compression within the Mesozoic thrust belt combined with a high regional heat flow due to the Sierran arc. This early extension took place 50 m.y. before development of the core-bounding detachment system, movement on which took place at low enough temperatures that deformation was predominantly brittle with some calc-mylonite fabric development. The youngest structures in the core are normal faults that cut the detachment and indicate a southwest transport direction orthogonal to those of both the earlier extensional events.

Evaluation of methods for measurement of regional skin temperature and heat flow in neonates

A reliable method for measurement of regional heat loss is a prerequisite for studies of heat loss in the neonate with different clothing and under varying environmental conditions. In the present study we evaluated methods for measuring regional skin temperature and regional dry heat loss in neonates, and specifically the influence of contact sensors applied to the skin surface. A modified incubator with controlled climate conditions was used as the climate chamber. The operative temperature was measured continuously with a black globe thermometer. Skin temperature was recorded with small-mass contact sensors and their influence on skin temperature was verified by simultaneous measurement with an infrared camera. The correlation was 0.99 and 1 SD within 0.3 degree C from the line of identity. The influence of the regional heat flow meter on skin temperature was also checked with an infrared camera and was found to be neglible.

Mapping the fluid flow of the Mariana Mounds ridge flank hydrothermal system: Pore water chemical tracers

We present a conceptual model of fluid circulation in a ridge flank hydrothermal system, the Mariana Mounds. The model is based on chemical data from pore waters extracted from piston cores and from push cores collected by deep‐sea research vessel Alvin in small, meter‐sized mounds situated on a local topographic high. These mounds are located within a region of heat flow exceeding that calculated from a conductive model and are zones of strong pore water upflow. We have interpreted the chemical data with time‐dependent transport‐reaction models to estimate pore water velocities. In the mounds themselves pore water velocities reach several meters per year to kilometers per year. Within about 100 m from these zones of focused upflow velocities decrease to several centimeters per year up to tens of centimeters per year. A larger area of low heat flow surrounds these heat flow and topographic highs, with upwelling pore water velocities less than 2 cm/yr. In some nearby cores, downwelling of bottom seawater is evident but at speeds less than 2 cm/yr. Downwelling through the sediments appears to be a minor source of seawater recharge to the basaltic basement. We conclude that the principal source of seawater recharge to basement is where basement outcrops exist, most likely a scarp about 2–4 km to the east and southeast of the study area.

Heat flow and regional uplift at the north-eastern border of the Ebro basin,NE Spain

Summary The north-eastern border of the Ebro foreland basin is characterized by relatively high elevation. the presence of Neogene to Present volcanism, and a high thermal gradient. These facts suggest that the area is affected not only by the topographic load of the Pyrenees but also by the opening of the Valencia trough. To examine this hypothesis, we first present the results of a surface heat-flow survey carried out in the Osona depression. After selecting those wells which are essentially affected by conductive heat transport and applying topographic and palaeoclimatic corrections, we obtain a corrected surface heat-flow density of 88 ± 16m Wm−2. Assuming this value as representative of the regional conductive heat flow, we relate the resulting lithospheric thermal structure to the topographic elevation assuming local isostatic equilibrium and considering a plausible range of crustal thickness and crustal density. The results obtained together with geothermometric and geobarometric data from xenoliths sampled in the nearby Olot volcanic zone indicate a regionally thinned lithosphere about 60–70 km thick. This thinning is related to the Neogene opening of the Valencia trough whose effects extend beyond the rift shoulder (Catalan Coastal Ranges) affecting a broad area of the Ebro basin. Two different uplift mechanisms are proposed as having taken place in the study area: a mechanical uplift related to Eocene compression, followed by a thermal uplift phase associated with asynchronous differential stretching produced during Neogene extension. The combination of these two mechanisms accounts for the observed vertical movements and the migration of the Middle Miocene and Plio-Quaternary volcanic activity.

Convective hydrothermal C0 2 emission from high heat flow regions

In addition to volatiles released from volcanoes, the flux of CO 2 to the atmosphere from other sources (e.g., metamorphism and subsurface magmatism) represents an important aspect of the global carbon cycle. We have obtained a direct estimate of the present-day atmospheric CO 2 flux from convective hydrothermal systems within subaerial, seismically-active, high heat flow regions. Geothermal systems of the Salton Trough (California, U.S.A.) and the Taupo Volcanic Zone (New Zealand) provide benchmarks for quantifying convective hydrothermal CO 2 fluxes from such regions. CO 2 fluxes from the Salton Trough ( ∼ 10 9 mol yr −1) and the Taupo Volcanic Zone (∼ 8·109 mol yr −1) were computed using data on convective heat flow and the temperatures and CO 2 concentrations of reservoir fluids. The similarity in specific CO 2 flux ( ∼ 10 6 mol km −2 yr −1) from these two disparate geologic/tectonic settings implies that this flux may be used as a baseline to compute convective hydrothermal CO 2 emission from other areas of high heat flow. If this specific flux is integrated over high heat flow areas of the circum-Pacific and Tethyan belts, the total global CO 2 flux could equal or exceed 10 12 mol yr −1 Adding this flux to a present-day volcanic CO 2 flux of ∼ 4·10 12 mol yr −1 the total present-day Earth degassing flux could balance the amount of CO 2 consumed by chemical weathering ( ∼ 7·10 12 mol yr −1).

95/01153 Liptinite in Indonesian tertiary coals

Secondary macropores and mosaic structure, common characteristics in metallurgical and natural cokes, are considered to be the solid residue after the release of liquid and gas during the heating process. In north-central Henan province, People's Republic of China, secondary macropores and “mosaic” structure are found in semi-anthracite and anthracite of lower Permian telemagmatic (not in contact with intrusion) metamorphic coal. Coal rank exhibits a zoned distribution cored by a northwest-southeast trending zone of anthracite. Coal rank decreases from this central zone toward both the northeast and southwest margins from low-volatile bituminous through medium-volatile bituminous to high-volatile bituminous. The zoned rank distribution of coal in this area is interpreted as being the result of regional metamorphism overprinted by telemagmatic metamorphism. Coal metamorphism in this area occurred in three stages: (1) pre-orogenic (late Carboniferous-late Triassic). Regional metamorphism produced coals of subbituminous to high volatile bituminous rank. (2) Orogenic (early Jurassic-late Cretaceous). Telemagmatic metamorphism resulted in zones of higher rank coal (medium-volatile bituminous through anthracite rank). (3) Post-orogenic (Tertiary-Quaternary). Shallow burial depth due to the tectonic uplift followed by erosion had a negligible effect on coal rank. Secondary macropores and “mosaic” structure found in semi-anthracite and anthracite in the research area are similar in form, by microscopic and SEM observation, to those of metallurgical cokes and other natural cokes, indicating extraordinary heating effect on the coal. High temperature, high-pressure experiments on high volatile bituminous coal from the marginal area show that the pores develop in the 250–300°C range and “mosaic” structure develops in the 350–500°C range. It is therefore postulated that the high rank coal in this area may have been subjected to temperatures similar to carbonization temperatures during the orogenic period. Because the heat sources for telemagmatic metamorphism are deep-seated plutons, the rate of heating on this coal might be relatively slow, the time involved in the metamorphism was considerably longer, and the affected area might be extensive. During the heating process, bituminous coal near the center of high heat flow region was heated to a thermoplastic temperature range and coal underwent a soft stage (plastic). Gases evolved in this phase and left the pore system in their wake. Mesophase growth units may also develop in this phase, which could get compressed into each other to form mosaic structure. The existence of secondary macropores and mosaic structure in coal can be used as an indicator of heating effect and gas generation. Ubiquitous macropores developed in the coal in a considerable area indicate that a regional gas-generation process occurred. The gases generated by thermal cracking of the coal during the heating process could be captured later by the macropores, migrate through the network of pores, and form the underground gas reservoirs where the storage conditions are favorable for gas retention. Therefore secondary macropores and mosaic structure in telemagmatic metamorphic coal have potential significance in natural gas exploration.

Evidence for the multi-stage formation of the south-western Valencia Trough

A basin evolution model for the south-western Valencia Trough is established using thermal and palaeo-thermal data. Bottomhole temperature and drillstem test data from oil wells have been used to determine the geothermal gradient in the study area. A combination of well log data and X-ray diffractometry analyses carried out on rock samples from the Ibiza Marino well has allowed an estimation of the bulk thermal conductivity and heat flow density. Integrating these data with previous seafloor heat flow measurements yields a regional surface heat flow density of about 90–100 mW m −2. Vitrinite reflectance data available from the Ibiza Marino well have been used as a palaeo-thermal constraint for the proposed tectonic models. Two different modelling approaches are tested: a single rift phase model and a multi-phase formation model. The single rifting phase, which considers different eroded thicknesses, is ruled out as it does not match the observed data. A multi-stage evolution coherent with the geological and geophysical data is proposed and consists of three Mesozoic rifting events, Palaeogene compression (to which erosion of about 5 km of Late Jurassic and Cretaceous sediments is related) and a Neogene rifting event that gave rise to the present day Valencia Trough. Surface heat flow, vitrinite data and eroded thickness are successfully integrated in the proposed model.

Utility of Drill-Stem Tests in Determination of the Geothermal Regime of Railroad Valley, Nye County, Nevada: ABSTRACT

Accurate representation of geothermal conditions is necessary to determine generation potential of source rocks buried in Railroad Valley. Boreholes, provide the best source of geothermal information, but formation temperature data must be screened for variations caused by drilling. Bottomhole temperatures from wireline logs are affected by initial formation conditions, drilling fluid that moves into the formation while drilling, and lag time between cessation of drilling fluid circulation and acquisition of logs. More accurate indicators of formation conditions are temperatures recorded during drill-stem tests, especially for tests that recovered large amounts of fluid. Over 130 drill-stem tests were examined to establish the viability of this source of data and to determine the geothermal conditions of the Railroad Valley basin. Results indicate that 500 feet or more of fluid recovery on a test is necessary to get a temperature recorded that is not influenced by drilling perturbations. The formation temperature data collected for Railroad Valley indicate the possibility of 2 thermal regimes. A low-temperature gradient regime is probably influenced by meteoric water. The high-temperature gradient regime probably reflects the regional heat flow associated with the thin crust of the Great Basin.

Heat flow in the Kenya rift zone

An understanding of the processes of continental rifting is fundamental to understanding the evolution of the continents. Considerable evidence exists to suggest that continental rift zones are associated with high heat flow and elevated lithospheric geotherms, but direct heat-flow measurements from young rifts do not clearly define surface heat-flow anomalies associated with deep-seated thermal processes in these rifts. The first detailed compilation of heat-flow data from the Neogene Kenya rift is presented here. Heat-flow data are presented from traditional heat-flow determinations in water drill-holes, from bottom-hole-temperature measurements in oil wells, and from heat-flow estimates from groundwater silica data. These data define generally low heat flow on the flanks of the Kenya rift, with high, but variable heat flow on the rift floor. There is a spatial association among high heat-flow values, Quaternary volcanism and faulting, and hydrothermal manifestations on the rift floor. We interpret these results to suggest that any deep-seated thermal anomaly associated with the Kenya rift has not yet been conducted to the surface. The high heat-flow values are interpreted to result from heat advected into the axial rift zone with local redistribution of this heat by hydrothermal convection. Normal to moderately high heat flow was measured in eastern Kenya between the rift zone and the coast. The regional heat flow in eastern Kenya is interpreted to be normal, with local shallow modification by groundwater flow eastward from the Kenya dome. These interpretations support a model of relatively young evolution of the asthenospheric anomaly beneath the Kenya rift zone, with the age of heating of the mantle at the Mono no older than about 10 Ma.

Geomagnetic depth sounding over the Singhbhum and the surrounding regions of eastern India

Magnetovariational studies have been carried out in Singhbhum and surrounding regions during 1987 and 1989. Three deep-seated linear conductors have been identified. One of them is located to the north of Ranchi, Bokaro and Purulia extending in E-W direction coinciding with high heat flow region and Gondwana sediments. The trend of anomaly at Ranchi and Purulia at longer periods suggests a conductivity anomaly due to the mafic and ultramafic intrusions, considered to be responsible for the uplift of Chhotanagpur plateau. The second conductor is associated with the basin margin fault that separates the Singhbhum craton and Chhotanagpur plateau from the West Bengal basin. This conductive zone appears to extend further south and join the high heat flow region of Attri-Tarabalo. This conductor could be isolated only after eliminating the coast effect from the observed induction vectors. The third conductive zone follows the trend of Mahanadi valley located south of the Sukinda thrust. Conductive anomaly associated with the Sukinda and Singhbhum thrust zones could not be resolved due to the interference from neighbouring conductive structures. These two thrusts may not be very deep-seated structures. The Singhbhum granite batholith is found to be highly resistive and seems to extend to greater depths.

Heat flow and geothermal processes in Iceland

Heat flow values, derived from temperature measurements in shallow boreholes in Iceland, vary substantially across the country. The near-surface temperature gradients range from almost 0 to 500°C/km. The thermal conductivity of water-saturated rocks varies from 1.6 to 2.0 W/m°C. The temperature gradient in Iceland is mainly dependent on four factors: 1. (1) the regional heat flow through the crust, 2. (2) hydrothermal activity, 3. (3) the permeability of the rock, and (4) residual heat in extinct volcanic centers. As Iceland is mainly made of basaltic material the radiogenic heat production is almost negligible. The thermal conductivity is, on the other hand, mainly influenced by the porosity of the rock; it increases as the porosity decreases. Iceland is made of sequences of flood basalts that formed within the volcanic rift zone—a continuation of the axis of the Mid-Atlantic ridge—and subsequently drifted sideways. Fresh basaltic lava is usually highly porous (30%) and fractured, and heat is mainly transported by convection. Therefore, a very low or even no temperature gradient is observed at shallow levels within the volcanic rift zone. As the basalt becomes buried the pores close due to lithostatic pressure and formation of secondary minerals. Below 500–1000 m depth in an uneroded lava pile, the heat is mainly transported by conduction. In the lowlands and valleys of Iceland outside the volcanic rift zone, 1000–1500 m of the original lava pile has been eroded, leaving thermal conduction as the most important heat transport mechanism. The regional temperature gradient has been measured in drillholes in dense and poorly permeable rocks away from the geothermal fields. The results show that the temperature gradient varies from 50 to 150°C/km. The highest values are found close to the volcanic rift zone and the gradient decreases with distance from the spreading axis. This result is mainly based on numerous shallow boreholes (60–500 m) but in some cases the results have been confirmed by 1000–2000 m deep boreholes. By extrapolating the temperature gradient down and assuming a slight increase in the thermal conductivity with depth, partially molten material can be expected at 10–30 km depth. Geothermal reservoirs are quite common in Iceland. They are primarily convective systems associated with young tectonic fractures, carrying heat from several kilometers depth towards the surface. Within the volcanic rift zone the heat sources seem to be hot intrusions; away from it, the heat is mined from the underlying crust. The highest values of the near-surface temperature gradient are found above the geothermal systems. Drilling of 30–60 m deep boreholes is therefore a powerful tool for geothermal prospecting outside the volcanic rift zone. In the deeper parts of the geothermal systems, a gentle temperature gradient is observed and the temperature is lower than would be expected from the regional temperature gradient. This is due to geothermal convection which has removed heat from the deeper parts of the geothermal systems. Convective geothermal systems must have a downflow part, where cold water is flowing down into the deeper parts of the geothermal systems along fractures. Such downflow areas have been observed and appear as areas with an anomalously low temperature gradient.