High Heat Flow Values Research Articles

P–T–t modelling of Proterozoic terranes in Lithuania: geodynamic implications for accretion of southwestern Fennoscandia

The East European Craton (EEC) consists of Archaean nucleii and accreted Proterozoic terranes. Pressure–Temperature–time (P–T–t) paths for the West Lithuanian Granulite Domain (WLG), the East Lithuanian Domain (ELD) and the Mid-Lithuanian Suture Zone (MLSZ) in the southwestern part of the EEC constrain the sequence of tectonic events and their deformational and metamorphic conditions. A kinematic and thermal model has been developed for the evolution of these terranes, given the accretion of the ELD to the Sarmatian and the WLG to the Fennoscandian continents, respectively, based on their structural and compositional differences. For initial surface heat flow values ranging from 80–90 mWm−2, the numerical model predicts a thermal relaxation phase of at least 30–50 m.y., following very slow accretion (0.54 cm/yr) of the WLG in a subduction-type environment, prior to granulite facies metamorphism at 1.80 Ga. Only the use of very high initial surface heat flow values (c. 120 mWm−2) allows the direct link of accretion to the observed granulite facies metamorphism. Exhumation of rocks to shallower crustal levels occurs still within granulite facies conditions, being accompanied by partial melting. An observed cooling-reheating event can be explained by underthrusting of the ELD beneath the WLG at c. 1.70–1.65 Ga in the MLSZ, which is a boundary between accretionary belts related to Fennoscandia and another set of such belts formed at the margin of Sarmatia. Subsequent amphibolite facies metamorphism within the MLSZ is followed by what is interpreted as a collapse related exhumation phase. The predicted temperature field indicates that the collapse might have led to a substantial elevation of isotherms, possibly triggering Rapakivi magmatism at 1.60–1.50 Ga.

Three‐dimensional velocity structure of crust and upper mantle in southwestern China and its tectonic implications

Using P and S arrival times from 4625 local and regional earthquakes recorded at 174 seismic stations and associated geophysical investigations, this paper presents a three‐dimensional crustal and upper mantle velocity structure of southwestern China (21°–34°N, 97°–105°E). Southwestern China lies in the transition zone between the uplifted Tibetan plateau to the west and the Yangtze continental platform to the east. In the upper crust a positive velocity anomaly exists in the Sichuan Basin, whereas a large‐scale negative velocity anomaly exists in the western Sichuan Plateau, consistent with the upper crustal structure under the southern Tibetan plateau. The boundary between these two anomaly zones is the Longmen Shan Fault. The negative velocity anomalies at 50‐km depth in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with temperature and composition variations in the upper mantle. The Red River Fault is the boundary between the positive and negative velocity anomalies at 50‐km depth. The overall features of the crustal and the upper mantle structures in southwestern China are a low average velocity, large crustal thickness variations, the existence of a high‐conductivity layer in the crust or/and upper mantle, and a high heat flow value. All these features are closely related to the collision between the Indian and the Asian plates.

Characteristics of surface heat flow in the South China Sea

A total of 592 heat flow measurements, ranging from 8 to 192 mW/m 2, 78 percent of which are between 50 and 100 mW/m 2, have been collected in the South China Sea (SCS). To overcome shortcomings such as the uneven distribution of the measurements and the occurrence of abnormal heat flow values, the tectonic evolution of different areas and their crustal thickness have been combined in analyzing geothermal characters. The results show that the oceanic basins, where they are floored by oceanic crust, the western part of the southern margin and the western fault system have high heat flow values. Heat flow along the northern margin of the SCS increases from 61 mW/m 2 on the shelf to 73–80 mW/m 2 in the slope area, and in the Xisha–Zhongsha area increases from about 70 mW/m 2 to about 85 mW/m 2 from the NW to the SE. Heat flow in the Nansha (Spratley) Islands is about 60 mW/m 2 or even higher, and decreases from the NW to SE; The average heat flow of the Mekong Basin is about 60 mW/m 2, which is similar to that of the Beibuwan Basin on the northern margin. Heat flow on the eastern margin and on the eastern part of the southern margin is lower, especially in the Luzon Trough, where the average heat flow is lower than 40 mW/m 2. Observed heat flow values in the SW subbasin are generally lower than predicted theoretically from the age of the ocean floor, unlike values in the eastern subbasin. A high heat flow zone in the lower slope area of the northern margin is recognized for the first time.

Eurasia spreading basin to Laptev Shelf transition: structural pattern and heat flow

New geophysical data have become available from shipborne and satellite measurements allowing a re-evaluation of the largely unknown junction of the Arctic spreading centre and the northeastern Siberian continental margin where the transpolar mid-ocean Gakkel Ridge abuts against the continental slope of the Laptev Sea. Based on multichannel seismic reflection and gravity data, this sediment-covered spreading axis can be traced to the continental rise where it is cut-off by a transcurrent fault. Further continuation of the extensional axis into the continental slope can be attributed to two asymmetric grabens, which terminate against the prominent Khatanga–Lomonosov Fracture. Remnants of hydrothermal fauna and high heat-flow values of approximately 100 mW m−2 documented around these grabens in the up-slope area are typical for an oceanic spreading axis. Thus we consider these grabens to be morphotectonic termination of the global Atlantic–Arctic spreading system with plate motions shifting to the Khatanga–Lomonosov Fracture. The high heat flow and the distribution of earthquake epicentres allow us to assume that the present-day divergent plate tectonic boundary passes from the Gakkel Ridge to the eastern part of Laptev Sea with an offset of initial rifting along the Bel'kov–Svyatoi Nos Rift to the projected prolongation of the buried spreading axis by 140–150 km.

Seismological study on the crustal structure of Tengchong volcanic-geothermal area

Based upon the deep seismic sounding profile conducted in the Tengchong volcanic-geothermal area, a two-dimensional crustal P velocity structure is obtained by use of the finite-difference inversion and the forward travel-time fitting method. The crustal model shows that there is a low velocity zone in upper crust in the Tengchong area, which may be related to the volcanic-geothermal activities, and two intracrustal faults (the Longling-Ruili fault and Tenchong fault) exist on the profile, where the Tengchong fault may extend to the Moho discontinuity. Meanwhile, based on teleseismic data recorded by a temporary seismic network, we obtained the S-wave velocity structures beneath the Rehai-Retian region in the Tengchong area, which show the low S-wave velocity anomaly in upper crust. The authors discuss the causes of Tengchong volcanic eruption based on the deep crustal structure. The crustal structure in the Tengchong volcanic-geothermal area is characterized by low P-wave and S-wave velocity, low resistivity, high heat-flow value and low Q value. The P-wave velocity in the upper mantle is also low. For this information, it can be induced that the magma in the crust is derived from the upper mantle, and the low velocity anomaly in upper crust in the Tengchong area may be related to the differentiation of magma. The Tengchong volcanoes are close to an active plate boundary and belong to ‘plate boundary’ volcanoes.

Global heat flow simulation based on a kinematic model of mantle flow

SUMMARY The global heat flow is the surface representation of thermal processes within the earth's mantle. The long-wavelength pattern of observed heat flow closely resembles the plate tectonics and its most prominent feature is higher values along ocean ridge systems. Theoretically, to determine the thermal state of the Earth's mantle, the heat transfer problem and the mantle convection problem have to be solved simultaneously since they are coupled with each other. However, the development of global seismic tomography provides us with a possibility that, at least under certain assumptions, these problems can be decoupled from each other and solved separately. This allows us to calculate mantle flow velocities first based on the internal loading theory and then use the velocity field as the input to solve the thermal problem. In addition to the internal density anomalies, surface plate movements also excite mantle circulations and, under certain circumstance, they may dominate the structure of the mantle flow. In this study, using a kinematic model of mantle convection and heat transfer, we investigated the underlying processes that generated the observed global heat flow. Buoyancy from the density anomalies and the coupling from the overlying plates are treated as the mantle flow driving force. Both advection and conduction heat transfers are included in the energy equation. Results show that calculated depth derivatives of the near surface temperature are closely correlated to the observed surface heat flow pattern. Higher heat flow values around midocean ridge systems can be reproduced very well. The predicted average temperature as a function of depth reveals that there are two thermal boundary layers, one is close to the surface and another is close to the core-mantle boundary. The rest of the mantle is nearly isothermal. Although, in most of the mantle, advection dominates the heat transfer, the conductive heat transfer is still locally important in the boundary layers and plays an important role for the surface heat flow pattern. The existence of surface plates is responsible for the long wavelength surface heat flow pattern.

Conductivity Structure along the Gediz Graben, West Anatolia, Turkey: Tectonic Implications

Groups of grabens in west Anatolia have contrasting E-W and NE-SW orientations and are the subject of debate as to their relative ages and relationships. We investigated the E-W-trending Gediz graben and its neighboring NE-SW-trending Gördes, Demirci, and Selendi grabens, which form an important graben system representative of the region. We studied gravity data from one profile and magnetotelluric (MT) data from two profiles, 73 km and 93 km long. The data supports the hypothesis that the Gediz graben was superimposed onto the (older) NE-SW grabens. 2D gravity and MT modelling revealed an undulating graben floor, varying in depth between 500 and 3000-4000 m (gravity-MT); within the graben two apparent basins 3–4 and 1.5-2.5 km deep (gravity-MT) are separated by a subsurface horst. The residual gravity map appears to indicate the continuation of NE-SW grabens from north of Gediz graben to beyond its southern border. The MT model revealed three main zones of varying thickness within the crust. The britde upper crust comprises two zones: sedimentary fill (apparent resistivity 15-50 ohm.m) and Menderes massif basement (200 ohm.m). The third zone is highly conductive lower crust (10 ohm.m), identified by our MT modeling at an average depth of 10 km. This conductive layer was considered in conjunction with two other regional features, high heat flow values and shallow earthquake focal depths. A heat flow map shows a very high average value of 108 mWm−2 for west Anatolia and 120-300 mWm−2 for the Gediz graben area specifically, compared with the world average of 80 mWm−2. Seismological records showing shallow earthquake focal depths together with the high conductivity zone were taken to indicate a partially melted, viscoelastic lower crust.

Regional‐scale permeability by heat flow calibration in the Powder River Basin, Wyoming

Forward modeling of coupled fluid and heat flow in the Powder River basin, Wyoming, is used to explain anomalously high heat flow values observed in the southern portion of the basin. Effective basin‐scale permeabilities of selected Powder River basin aquifers and aquitards were calibrated by matching surface heat flow measurements to simulation results. Fractures associated with a large anticline in the southwestern part of the basin were found to play a major role in the basin's thermal regime. While the model results are non‐unique, they demonstrate that regional structural features play an important role in a basin's overall energy budget and fluid flow regime. With the results of the basin‐scale model it is possible to evaluate regional‐scale flow and transport processes.

Lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone: an example of decreasing plate coupling

Abstract The Okinawa Trough is a backarc basin located behind the Ryukyu arc–trench system. The southernmost part of the Okinawa Trough (SPOT) displays different tectonic features from the rest of the Okinawa Trough. The SPOT area includes abundant seamounts with active hydrothermal venting and high heat-flow values. To understand better the rifting and magmatism context of the SPOT area, we examine the lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone. The results show that beneath the SPOT area the continental crust and mantle lithosphere thickness of ∼25–30 and 120 km, respectively, are thick with little continental thinning. The analysis of mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone shows strong plate coupling between the overriding and subducting plates. However, the two plates are actually decoupled as indicated by present-day interface earthquakes. This situation indicates that the southernmost Ryukyu subduction zone displays a transitory case of a changing plate coupling, from a strong to a weak plate coupling. Such a coupling/decoupling transition is probably associated with the collision of the Luzon arc with the Asian continental margin. Additionally, the curve of the mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone indicates that the submarine magmatism in the SPOT area is located within the volcanic arc area, suggesting early arc magmatism in the SPOT area.

The effects of variable thermal conductivity on mantle heat‐transfer

We have examined the styles of steady‐state convective heat transfer for two kinds of mantle thermal conductivity models in an aspect‐ratio one box. We have employed the phonon portion of the thermal conductivity model of Hofmeister (1999) and varied the radiative component in this model. We have also considered a second model which emphasizes radiative heat transfer and has a high contribution in the lower mantle. We have conducted constant viscosity calculations out to a surface Ra of 1010. Variable conductivity enhances in most cases the surface heat flow. The smallest deviation from the constant conductivity model comes from the phonon model with the parameters associated with oxides. Larger heat flow is obtained with silicates and increasing radiative contributions associated with the values of Shankland et al. (1979). The highest heat flow values are obtained for the radiatively dominated model, where the thermal conductivity in the deep mantle can be a factor of 50 greater than the surface conductivity value.

Heat Flow of the Southwestern End of the Okinawa Trough

Detailed heat flow measurements were carried out around the ODP contingency drilling site KS-l (24°48'N, 122°30'E) located on the southern slope of the southwestern end of the Okinawa Trough. A total of 26 heat flow values were obtained. Within about 10 km of the site, the heat flow is lower than 50 m W/m2. Heat flow increases southward to about 65 m W/m2 on the south edge of the trough. Due to the extraordinarily high sedimentation rates, the sedimentation-corrected heat flow values have increased by as much as 76%, so only the most recent surface sedimentation effects are considered. Distribution of heat flow patterns shows that high heat flow values are associated with igneous activities and are observed in both the northern and southern flank and the western end of the Okinawa Trough. Heat flow values in the E- W trending central rift zone, on the other hand, appear to be lower. This is probably due to hydrothermal activities in the rift zone. The correlation between the heat flow and bathymetric contours is discernable, but with a slight southward shift in the heat flow pattern. Considering the fact that abnormally high and low heat flows are observed near volcanic intrusions, extrusions and faults, and that much higher heat flows would be expected if the full sedimentation effects were eliminated, we suggest that the magma associated with the extension of the southwestern Okinawa Trough has at least extended into the southwestern end of the Okinawa Trough, in northeast Taiwan.

Heat flow variations from bottom simulating reflectors on the Cascadia margin

Abstract A bottom simulating reflector (BSR), representing the base of the gas hydrate stability field, was observed over a closely spaced grid of seismic lines on the continental slope of the northern Cascadia margin. A simple conductive model was used to calculate heat flow from the depth of the BSR. A regional trend was observed, in which heat flow decreased landward across the margin from an average of ∼80 mW/m2 at a distance of 15 km from the deformation front to ∼65 mW/m2 at a distance of 40 km. This trend reflects the processes of tectonic thickening of accretionary wedge sediments and subduction of the Juan de Fuca plate, and contrasts with the near-constant heat flow across the Oregon margin where the dip of the subducting plate and thus the rate of sediment thickening are slightly smaller. Consistent local variations were also observed, notably low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs. At some localities over a horizontal distance of 1–2 km, heat flow increased by as much as 50%, from typical values of 65 to 100 mW/m2. Much of this variation may be due to focusing and defocusing effects of the topography alone, and indicates the importance of carrying out topographic corrections to heat flow in regions of significant relief such as the continental slope. However, a component of the local heat flow patterns may be due to dynamic effects, including the displacement of isotherms by thrust faulting and the upward migration of warm fluids along permeability channels such as faults.

Crustal and upper mantle velocity structure of the Hoggar swell (Central Sahara, Algeria)

The Hoggar region is known as one of the most important swells in the African continent. Its altitude culminates at 2908 m in the Tahat hill (Atakor). The Hoggar and other massifs of central Africa (Aı̈r, Eghei, Tibesti, Darfur, Cameroon mount, …) form a system of domal uplifts with similar scale, morphology and volcanic activity. The knowledge of the structure beneath the Hoggar swell will help us to understand the origin of continental swells. In order to get an image of the lithosphere in this region, we have performed a teleseismic field experiment. The 33 short-period seismic stations have been maintained for 2 1/2 month along a 700-km long NNW–SSW profile. This experiment crossed the Central Hoggar and extended northward into the In-Salah Sahara basin which is characterized by high heat flow values of deep origin. The high quality of the data recorded during this experiment allows us to perform a velocity inversion. The Hoggar appears to be characterized by lower mantle velocities. The anomalous zone extends from the upper lithosphere to the mantle. The weak velocity contrast is interpreted in agreement with gravity, geothermal and petrological data as due to extensive mantle modifications inherited from Cenozoic volcanic activity. It confirms that the Hoggar swell is not due to a large-scale uplift of hot asthenospheric materials but corresponds to a now cooled-off modified mantle. On the contrary, local low-velocity zones associated with the Atakor and Tahalra volcanic districts show that hot materials still exist at depths in relation with recent basaltic volcanism.

Heat flow in the Trans‐Hudson Orogen of the Canadian Shield: Implications for Proterozoic continental growth

Fourteen new heat flow and radiogenic heat production measurements have been obtained in the Paleo‐Proterozoic Trans‐Hudson Orogen of the Canadian Shield. This orogen, which consists of several distinctive belts, corresponds to a pulse of crustal growth through island arc magmatism between 1.9 and 1.8 Ga. The data now available include 17 previously published measurements. Heat flow variations that are related to the history of magmatism and internal differentiation of the belts provide constraints on the crustal assemblages in the different belts of the orogen. The average and standard deviation of heat flow values for the entire orogen, 42±11 mW m−2, are identical to those of the older Superior Province and of the younger Grenville Province. For the orogen as a whole, heat flow is weakly correlated to the heat production of surface rocks. High heat flow values are found in the Thompson belt, consisting of metasedimentary rocks deposited on the ancient continental margin of the Superior craton. There the accumulation of sediments derived from older and differentiated continental upper crust has resulted in significant concentrations of radioelements in large volumes of rocks. The heat flow is low in the belts that expose juvenile Proterozoic crust consisting mostly of arc‐related volcanic rocks. In the Flin Flon‐Snow Lake Belt, the average heat flow is the same as the average of the orogen. The low heat production and the lack of correlation between heat flow and heat production suggest that the supracrustal volcanics exposed at the surface are thin and rest on a basement richer in radioelements. In the Lynn Lake belt, the heat flow is significantly lower than the average for the orogen although the surface heat production is not low. The heat flow data require a thin (<10 km) surface layer overlying the mid and lower crust depleted in radioelements. Around the town of Lynn Lake, heat flow is consistently low over a distance of ≈40 km. The coincidence between this “cold spot” and anomalously thick crust suggests that deep crustal roots may be preserved because of the stronger rheology implied by the low temperatures. The evolution of the Trans‐Hudson Orogen exemplifies the interplay between the processes generating rocks of evolved composition, which require crustal thickening, and those forming “normal” continental crust with average thickness, which require crustal flow and soft crustal rheology.

Mantle heat flow and geotherms for the main geologic domains in Morocco

Thermal and deep seismic soundings data are used to study the dependence between the compressional Pn velocity and the surface heat flow or the temperature at the Moho discontinuity in Morocco. This correlation indicates a significant decrease in Pn velocity where high heat flow and Moho temperature are observed. This result is consistent with respect to other regions of the world. Crustal heat generation models and geotherms are constructed for the major Moroccan geological domains extending from the Precambrian units in the south to the Alpine units in the north. The crustal contribution in surface heat flow is on average 35 mWm–2, with high values of 41–42 mWm–2 in the western and eastern Meseta where Hercynian granite intrusions could enrich the crust in radioactive heat sources. High mantle heat flow values are obtained beneath the Alboran neogene basin (62 mWm–2), the Rif (47 mWm–2), the Middle Atlas (41 mWm–2), and the south Atlantic margin (40 mWm–2) where the crust is thinned by an extensional tectonic regime. Despite their similar formation context, the intra-continental belts of the Middle and the High Atlas show different geothermal field components. A lithospheric heating process in the Middle Atlas could be the result of a Plio-Quaternary basaltic volcanism. Finally, the Precambrian basement of the Anti-Atlas like all the West African shield is a stable domain showing the lowest subsurface temperatures.

Curie point depth based on spectrum analysis of the magnetic anomaly data in East and Southeast Asia

The bottom of the magnetized crust determined from the spectral analysis of residual magnetic anomalies is generally interpreted as the level of the Curie point isotherm. A method to estimate the depth extent of magnetic sources (Curie point depth analysis) was applied to the magnetic anomalies of East and Southeast Asia. Although the geologic and physiographic complexities of this area constrain the method, certain correlations between the Curie point depths and heat flow data are apparent and the Curie point depths are consistent with the tectonic settings. Shallow basal depths of magnetic sources that extend in back-arc regions correspond to high heat flow values. The correspondence of deep basal depths with low heat flow values along the trench axis suggests that they are both related to the subducting plate. We also estimate the Curie point depths from heat flow data using a one-dimensional heat conductive transport model. Good agreement between the Curie point depths derived from heat flow data and magnetic data suggests that the Curie point depth analysis is useful to estimate the regional thermal structure.

Geophysical and geological constraints on the evolution of the Guadalquivir foreland basin, Spain

Abstract This paper presents a compilation and reinterpretation of available geophysical and geological data recently acquired for the ENE-WSW Guadalquivir foreland basin, located on the northern margin of the Betic orogen in southern Iberia. The data include seismic reflection and refraction profiles, well logs, gravity, geoid, surface heat-flow data and field observations. The deep structure of the southern Iberian margin is characterized by large variations in crustal thickness and high heat-flow values, which result in a very low lithospheric rigidity for the whole area. Geoid and gravity data show that deformation affected the crust and the lithospheric mantle differently, producing anomalous mass distributions that could act as subsurface loads. Seismic sequence analysis of the basin infill has permitted the re-assessment of the depositional sequential arrangement of the sediments deposited from Late Langhian-Early Serravallian to Messinian. They are arranged in six sequences and do not show any E-W progradational pattern indicating that during this period the acting loads moved essentially in a NNW direction. A careful analysis of the southern border of the basin shows that the ‘so-called olistostromes’ correspond to lateral diapirs of squeezed Triassic evaporites and internally imbricated Miocene wedges. We discuss the results obtained in terms of palaeo-geographic environments, time distribution and nature of acting loads, and constraints for future basin modelling approaches.

Partial melting below the magmatic arc in the central Andes deduced from geoelectromagnetic field experiments and laboratory data

Magnetotelluric and geomagnetic deep soudings in northern Chile revealed a pronounced high conductivity zone (HCZ). Below the Western Cordillera, which constitutes the present magmatic arc with active volcanism of the South American continental margin, conductivities in the range of 1 S/m are observed. The anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km, are interpreted in terms of partial melting. Other geophysical observations, such as a zone of low seismic velocities (LVZ) at similar depths, high heat flow values (> 100 mW/m 2) and a pronounced negative anomaly in the residual gravity field, are also considered. Impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities. At sub-solidus temperatures, electrical behavior is described by defect electrons with an activation energy of 1.34 eV and a conductivity of 2.5 mS/m at 900°C. Model calculations using a modified-brick-layer model (MBL) were compared with experimental observations. A good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 S/m at 1250°C assuming an activation energy of 1 eV. The same MBL model is used to calculate melt proportions beneath the Western Cordillera. Between 14 and 27 vol.% of interconnected melt are necessary to explain the observed HCZ. The stability of the melt rich crust is explained by a dynamic melting-crystallisation behavior during crustal anatexis and by magma filled dikes.

Seismic, gravity and petrological evidence for partial melt beneath the thickened Central Andean crust (21–23°S)

On the basis of seismic refraction investigations and gravimetric data we have modelled the crustal structure of the southern Central Andes (21–23°S). A pronounced variation in crustal parameters is seen in N-S- and W-E-crossing seismic profiles over the entire Andean orogene, characterized by a crustal thickness of up to 70 km under the magmatic arc and backarc, strongly reduced seismic velocities and a Bouguer minimum of −450 mGal. Anomalously low velocities of 5.9–6.0 km/s in the deeper crust of the Western Cordillera and Altiplano regions lead to an over-compensation of the Bouguer minima resulting in values of crustal densities higher than estimates based purely on seismic velocity measurements. In an attempt to reconcile these differences, the behavior of crystalline rocks based on published laboratory data was studied under varying pressure and temperature conditions up to the range of partial melting. If the temperature is increased above the melting point, a rapid decrease in seismic velocity is accompanied by a slow decrease in density. For the Central Andes, a good fit of the observed and calculated Bouguer anomalies is obtained if the densities of the rocks from the low-velocity zone (LVZ) beneath the Western Cordillera and the Altiplano are varied. Model calculations lead to a velocity-density relation for partial molten rocks that allows the melt proportions of rocks to be estimated. Model calculations indicate that 15–20 vol.% of basaltic to andesitic melt at depth is necessary to explain the LVZ and Bouguer anomaly beneath the arc and parts of the backarc. High heat flow values (100 mW/m 2) support the idea that large areas of the deeper Andean crust are strongly weakened by the presence of partially molten rocks, resulting in reduced seismic velocities, with the Western Cordillera, the active volcanic arc of the Andean mountain range, acting as a ductile buffer between the two more rigid crustal blocks of the forearc and backarc regions.

Thermal analysis of the southern Powder River Basin, Wyoming

Temperature and geologic data from over 3000 oil and gas wells within a 180 km × 30 km area that transect across the southern Powder River Basin in Wyoming, U.S.A., were used to determine the present thermal regime of the basin. Three‐dimensional temperature fields within the transect, based on corrected bottomhole temperatures (BHTs) and other geologic information, were assessed using: (1) A laterally constant temperature gradient model in conjunction with an [Formula: see text] norm inversion method, and (2) a laterally variable temperature gradient model in conjunction with a stochastic inversion technique. The mean geothermal gradient in the transect is 29°C/km, but important lateral variations in the geothermal gradient exist. The average heat flow for the southern Powder River Basin is [Formula: see text] with systematic variations between [Formula: see text] and [Formula: see text] along the transect. Extremely high local heat flow (values up to [Formula: see text]) in the vicinity of the Teapot Dome and the Salt Creek Anticline and low heat flow of [Formula: see text] occurring locally near the northeast end of the transect are likely caused by groundwater movement.