Regional Heat Flow Research Articles

Geochemistry of Bazman thermal springs, southeast Iran

Thermal springs of the Bazman geothermal field, 27km south of the Holocene Bazman volcano caldera in Makran continental margin, SE Iran, were studied for the first time. New data on major components, selected trace elements (Li, Rb, Cs, Ba, Sr, Fe, Al), water isotopic composition (H, O and S), chemical and isotopic (δ13C of CO2 and CH4, 3He/4He and 40Ar/36Ar) composition of dissolved and bubbling gases from the hottest vents are presented. Four groups of springs with temperature range of 27–44°C discharge Na-Cl waters with total dissolved solids (TDS) of ∼800 to ∼7000mg/L from different aquifers composed of diverse type of rocks including granites and limestone. Only the hottest and most saline springs of the Bazman field with low bicarbonate are close to equilibrium with surrounding rocks whereas the others discharge immature waters. Geothermometry based on SiO2 concentrations, Na-K and Na-K-Ca-Mg systems shows low equilibrium temperatures of up to 130°C, consist with the temperatures estimated by alumino-silicate minerals saturation indices. The water isotopic composition (δ18O and δD) indicates meteoric origin with a small oxygen isotopic shift measured in the saline waters. The δ34S values of SO4 indicate influence of gypsum and anhydrite dissolution from the host rock. Dissolved and free gases are N2-rich (>95vol.%) with a high He content (0.5vol.%). A biogenic origin may be suggested for CH4 and CO2 based on their carbon isotopic characteristics (−62‰ and −13‰ vs. V-PDB, respectively). The R/Ra value of 0.5 indicates a contribution of about 6 % of He from the mantle. It is suggested that the Bazman thermal waters are heated at considerable depth (4−5km) in a deep fault system most probably by the regional heat flow according to the local geothermal gradient.

Extreme fractionation and magmatic–hydrothermal transition in the formation of the Abu Dabbab rare-metal granite, Eastern Desert, Egypt

The Abu Dabbab rare-metal granite in the Central Eastern Desert of Egypt is a peraluminous alkali-feldspar leucogranite stock with minimummelt composition of albite-rich haplogranite and is characterized by economic resources of columbite group minerals (CGM) and cassiterite. The bulk-rock trace element composition is characterized by very low Sr, Ba, REE+Y, Zr, Ti, Ni, Cr concentrations, elevated Li and F, and very high Sn, Ta and Ga. The non-chondritic, low Y/Ho ratios and extremely low Nb/Ta and Zr/Hf ratios of the Abu Dabbab granite are consistent with highly evolved peraluminous granites. The chondrite-normalized REE patterns with significant tetrad effects (TE1,3 ~ 1.6–2.6) and strongly negative Eu anomalies are indicative of the magmatic to hydrothermal transition. Heterogeneous 147Sm/144Nd (0.20411–0.51923) and 143Nd/144Nd (0.512968–0.514056) values of the bulk-rock reflect a disturbed Sm-Nd isotope system. Nevertheless, the initial εNd values calculated for a wide range of Neoproterozoic ages (500–700 Ma) are consistently positive, suggesting a juvenile source.Laser ablation ICP-MS analyses of the CGM, cassiterite, wolframite and Mn-ilmenite show elevated rare metal concentrations and complex REE tetrad patterns, reflecting fluid-melt interactions. The metasomatic fluids associated with the Abu Dabbab rare-metal granite were primarily low-salinity (<6 wt.% NaCl eq.), high-density (~0.8–0.9 g/cm3), CO2-rich aqueous fluids with up to ~50 mol% CO2 (+CH4). The minimum entrapment conditions of carbonic-aqueous inclusions are calculated as 2–3 kbar and 350–360°C, whereas aqueous inclusions in the granite and crosscutting quartz veins most likely correspond to lower-pressure conditions and fluid phase separation by decompression. The extreme level of fractionation and complex tetrad effects of the bulk-rock and ore minerals coupled with criteria indicating the magmatic-hydrothermal transition are in accord with regional seismic tomography and heat flow data. A much larger, unexposed parental granite system, from which the exposed Abu Dabbab rare-metal tip has fractionated, must be present at depth. The interaction between extreme fractionation and fluid-melt interaction at shallow crustal levels gave rise to economic-grade Ta-Nb-Sn±W mineralization in the granite body and associated quartz veins.

Radiogenic heat production variations in the Gonghe basin, northeastern Tibetan Plateau: Implications for the origin of high-temperature geothermal resources

Hot dry rock is an almost inexhaustible source of geothermal energy. Since the first attempt to extract thermal energy from hot dry rock in America in the 1970s, successive studies have been conducted in many countries. Recently, high-temperature rock (exceeding 180 °C) was drilled in the Gonghe basin, northeastern Tibetan Plateau, marking a significant breakthrough in the exploration of hot dry rock resources in China. A proper understanding of the origin of the hot dry rock in the Gonghe basin is essential to the evaluation of the geothermal resource potential and to the establishment of an enhanced geothermal system in the basin.In the present research, we attempt to seek clues from the radiogenic heat production of the rocks. In all, 52 core samples collected from the main boreholes in the Gonghe basin were measured for the concentrations of radioisotopes. The heat production values were determined to be 1.21–2.02 μW m−3 for the sedimentary rocks, yielding an arithmetic mean of 1.67 ± 0.29 μW m−3, and 1.17–5.81 μW m−3 for the basal granitic rocks, yielding a mean of 3.20 ± 1.07 μW m−3. The results show that the radiogenic heat production of the granitic rocks is approximately that of global Mesozoic-Cenozoic granites, which is further corroborated by an extensive compilation of heat production data for granitic rocks in the immediate vicinity of the study area (average of 2.07 ± 0.97 μW m−3, n = 136). Based on the heat production data sets combined with available geological and geophysical information, the crustal heat production contribution is determined. The results show that the heat contribution from the crust is about 48.3 mW m−2, accounting for ∼47.3% of terrestrial heat flow. Compared with the regional background heat flow, the high heat flow in the Gonghe basin may be attributed to an additional heat flow contribution (∼27 mW m−2) from a heat anomaly (probably related to partial melting) at shallow depth, which is evidenced by the low-resistivity anomalies in the magnetotelluric survey results. Thus, this study emphasizes that the anomalous heat flow and the hot dry rock geothermal resources in the Gonghe basin may be the combined effects of the radiogenic heat contribution from the thickened crust and the heat contribution from a local heat anomaly beneath the Gonghe basin.

Una anomalía térmica local persistente en el gneis de Ahorn recargada por el agua de deshielo de los glaciares (Austria)

In the unlined Tuxbach water transfer tunnel, running between Hintertux (1,500 m asl) and the Schlegeis Reservoir (Austria), a local geothermal anomaly with temperatures up to 14.6 °C exists. These temperatures are around 3 °C higher than expected, considering the tunnel’s shallow depth, together with its surrounding alpine environment and regional heat flow. This is especially noticeable because the temperatures have remained stable since the tunnel’s construction in 1969, although the tunnel is generally cooling the surrounding rock massive. The objective of this investigation is to explain the origin of the anomaly with hydrogeological methods and to evaluate the hydrogeological properties of the gneisses exposed in the tunnel. The anomaly is caused by the high hydraulic conductivity (~2.5∙10−5 m s−1) within a narrow shear-zone core, part of the Tux Shear Zones in the Ahorn Gneiss Core. The zone triggers fast groundwater transport over 1.5 km from both sides towards the tunnel. One reason is that the morphology provides thicker overburden with growing distance from the tunnel and therefore higher temperatures on the same horizontal level in the directions of the fault plane. The second explanation is that the narrowness of the shear zone permits effective heat transfer similar to a heat exchanger. No hydrothermal water share is recognizable; instead, mainly cold glacial melt water and snow contribute to the section of the anomaly and all other runouts of the tunnel. Factually based results show the disproportionately high contribution of snow and glaciers to the groundwater recharge in this alpine hard-rock aquifer.

Digging deeper: The influence of historical mining on Glasgow's subsurface thermal state to inform geothermal research

Studies of the former NE England coalfield in Tyneside demonstrated that heat flow perturbations in boreholes were due to the entrainment and lateral dispersion of heat from deeper in the subsurface through flooded mine workings. This work assesses the influence of historical mining on geothermal observations across Greater Glasgow. The regional heat flow for Glasgow is 60 mW m −2 and, after correction for palaeoclimate, is estimated as c . 80 mW m −2 . An example of reduced heat flow above mine workings is observed at Hallside ( c . 10 km SE of Glasgow), where the heat flow through a 352 m deep borehole is c . 14 mW m −2 . Similarly, the heat flow across the 199 m deep GGC01 borehole in the Glasgow Geothermal Energy Research Field Site is c . 44 mW m −2 . The differences between these values and the expected regional heat flow suggest a significant component of horizontal heat flow into surrounding flooded mine workings. This deduction also influences the quantification of deeper geothermal resources, as extrapolation of the temperature gradient above mine workings would underestimate the temperature at depth. Future projects should consider the influence of historical mining on heat flow when temperature datasets such as these are used in the design of geothermal developments. Supplementary material : Background information on the chronology of historical mining at each borehole location and a summary of groundwater flow in mine workings beneath Glasgow are available at https://doi.org/10.6084/m9.figshare.c.4681100 Thematic collection: This article is part of the ‘Early Career Research’ available at: https://www.lyellcollection.org/cc/SJG-early-career-research

Geophysical Investigation of the Geothermal Potential Under the Largest Volcanic Cover in Anatolia: Kars Plateau, NE Turkey

In this study, Curie-point depth (CPD), geothermal gradient, radiogenic heat production, and heat flow maps were constructed based on different thermal conductivity coefficients using magnetic anomaly data for the Kars Plateau, which has the largest volcanic cover in Turkey. The bottom depths of the magnetic crust in the research area were revealed by the CPD map for the first time in this investigation. There are two apparent magnetic anomaly trends in the study area: the first is the Horasan–Senkaya–Sarikamis–Selim–Arpacay trend in the NE–SW direction, and the other is the Hanak–Ardahan–Arpacay trend in the NW–SE direction. Two other prominent elongations extend into the Ardahan–Gole–Senkaya and Kars–Digor axes. All these trends represent mountain chains and/or stratovolcanoes in the region, and no anomalies are observed around the non-volcanic outcrops. Curie depths are shallow, up to 14 km between Horasan and Kagizman towns, and 12 km in the northwestern part of the study area. Gradient values can reach 50 °C km−1 in the northwestern sector, together with the high heat flows represented by the 150 Wm−1 K−1 contours. The deepest CPD region lies between Gole and Susuz towns, where the geothermal gradient decreases to 27 °C km−1. Heat flows decrease 60 Wm−1 K−1 in the same area. An apparent gap around the Kars Plateau was observed in previous regional heat flow maps of Turkey by other authors (who used the bottom hole temperatures of boreholes and hot springs temperatures). This gap has been accurately filled from the results of this study, and geothermal exploration areas and the geothermal potential of the Kars Plateau have thus been determined for future exploration activity on the basis of the tectonic elements and earthquake data.

Thermal evolution of the Northern Cordillera Volcanic Province: implication for heat flow in remnant back-arc regions

ABSTRACT Active and remnant back-arc regions do not follow a typical conductive lithosphere cooling model, but instead have an apparent two-stage cooling, defined by a high heat flow back-arc region during subduction and a second post-subduction heating event that extends elevated heat flow for several 10s million years. Numerical one-stage cooling models have not reproduced observed heat flow anomalies in active subduction zones using physically realistic parameters and require a secondary post-subduction heating mechanism. Here, an extension driven-volcanism model is developed to examine extension driven heating and volcanism as a mechanism to produce a prolonged thermal anomaly within back-arc lithosphere. This model is tested using the recorded thermal evolution of the Northern Cordillera Volcanic Province (NCVP), a Neogene-Quaternary alkaline volcanic province located in the remnant back-arc region of the Pacific-North American subduction zone in British Columbia, Canada. A single steady-state lithosphere geotherm does not intersect all previously published temperature estimates, suggesting previous data record the thermal evolution of the NCVP. Calculated geotherms at equilibrium with the minimum and maximum MELTS temperatures predict an increase in reduced mantle heat flow (Qm ) from 43 to 72 mW/m2 and lithosphere thinning from a depth of 87 to 48 km. The newly developed extension-volcanism model reproduced the calculated pre- and post-volcanism thermal regimes for the NCVP and supports that extension within the remnant back-arc could produce the present heat flow anomaly and volcanism. The model most readily produces volcanism when Qm is ~45–65 mW/m2, a typical range for back-arcs. Back-arc regions are prime locations for limited volcanism because their warmer thermal regime minimizes tectonic stress requirements to produce volcanism. Additionally, two-stage cooling of back-arcs can be explained with a time-dependent extension-volcanism thermal feedback mechanism that is possible because of the subduction driven pre-heating of back-arc regions.

Studies on the relationships of the Curie surface with heat flow and crustal structures in Yunnan Province, China, and its adjacent areas

A Curie surface indicates the distribution of the thermal fields underground, providing a clear marker for the thermodynamic effect in the crust and mantle. In this paper, based on a geomagnetic field model (NGDC-720) and aeromagnetic data, we use power spectrum analysis of magnetic anomalies to estimate the Curie surface in Yunnan Province, China, and its adjacent areas. By combining the distribution of the Curie surface with regional heat flow, the geothermal gradient, crustal wave velocity ratio anomalies, high-conductivity layer anomalies, and the Moho surface, we reveal the connection between the undulation of the magnetic basement and the crustal structures. The results indicate that the uplift and depression of the Curie surface in the research area are distinct. The Curie surface is approximately inversely correlated to the surface heat flow. The Lijiang-Jianchuan-Baoshan-Tengchong and Jianchuan- Chuxiong- Kunming-Yuxi zones are two Curie surface uplift zones, and their crust-mantle heat flows are relatively high. The Curie surface uplift zone along the Lijiang-Xiaojinhe fault and Red River fault is consistent with the heading direction of the fault zone and is partially in agreement with the eastward mass flow of the Tibetan Plateau. The Curie surface uplift zone is consistent with the high wave velocity ratio and high-conductivity layer anomaly region of the crust. The depth of the Curie surface is less than the depth of the Moho surface.

Regional heat flow and subsurface temperature patterns at Elysium Planitia and Oxia Planum areas, Mars

Elysium Planitia and Oxia Planum are plains located near the Martian dichotomy. Lately, both regions have been extensively analyzed due to the major role that they play in the InSight and ExoMars missions. InSight landed in Elysium Planitia and will obtain the first direct measurement of surface heat flow on Mars. Similarly, the Rosalind Franklin rover on ExoMars 2020 will also provide useful information to understand the thermal state of the planet from data acquired in Oxia Planum, which is the preferred landing site. The proximity of the Martian dichotomy to the area surrounding both landing locations is an important source of spatial variability. In this work, we have modeled the heat flow and the subsurface temperature in the regions adjacent to both landing sites considering the regional context. In order to do so, we have solved the heat conduction equation by means of a finite element analysis and by taking into account topography, crustal composition, and crustal and megaregolith thicknesses. Our results indicate that the spatial variation in these parameters for the region surrounding the InSight landing site involves maximum differences in subsurface temperatures and surface heat flows between highlands and lowlands of about 67% and 16%, respectively. In regard to the area surrounding ExoMars landing site, these differences can reach 28% for subsurface temperatures, and 3% for surface heat flows. Crustal and megaregolith thicknesses together with the thermal properties of the megaregolith layer are the most influential factors affecting heat flows and temperature patterns. We also find that regional variations related to the dichotomy boundary are unlikely to have a large effect on the geothermal heat flux at the InSight and ExoMars landing sites.

Lithosphere and shallow asthenosphere rheology from observations of post-earthquake relaxation

In tectonically active regions, post-earthquake motions are generally shaped by a combination of continued fault slippage (afterslip) on a timescale of days to months and viscoelastic relaxation of the lower crust and upper mantle on a timescale of days to years. Transient crustal motions have been observed following numerous magnitude >~7 earthquakes in various tectonic settings: continental rift zones (Basin and Range), continental plate boundary zones (San Andreas fault corridor; Alaska; Turkey), subduction zones (Japan, Chile, Sumatra), ongoing continental collision zones (Arabia; Tibet), and mid-ocean rifting zones (Iceland). When afterslip can be discriminated from viscoelastic relaxation and when temporal coverage of the postseismic measurements is broad (i.e., geodetic surveys of at least several years duration are available), a wide spectrum of relaxation timescales are usually identified. Current temporal resolution and modeling approaches (e.g., Burgers body analog) allow identification of transient (Kelvin) and steady-state (Maxwell) viscosities that are operable in the short-term and long-term, respectively. I compile results from 40 studies of post-earthquake motions, augmented by ten studies of contemporary surface loading or unloading, that illuminate current estimates of transient and steady-state viscosity of the lower crust and/or uppermost mantle. Lower crust viscosity estimates range from ~1018 to 1021 Pa s, with most estimates near the upper end except in areas of overthickened crust. Mantle lithosphere and asthenosphere viscosity estimates are particularly abundant and yield a picture of transient viscosity ranging from ~1016 to 1019 Pa s and steady-state viscosity ranging from ~1018 to 1021 Pa s. To first order, both transient and steady-state viscosities are well correlated with regional heat flow, and steady-state viscosities are comparable with temperature and strain-rate dependent rock viscosities from laboratory-based flow laws.

Mapping Curie Depth Across Western Canada From a Wavelet Analysis of Magnetic Anomaly Data

AbstractIn western Canada, geophysical studies infer an abrupt change in crustal temperatures between the Canadian Cordillera and the adjacent North American craton, with important implications for the tectonics and geodynamics of the area. We use a wavelet analysis of magnetic anomaly data in western Canada to map the depth to the bottom of the magnetic source, or Curie depth. This depth corresponds to the point at which crustal rocks reach their Curie temperature, thus providing an estimate of geothermal gradient. Our model is defined by a fractal distribution of magnetization characterized by the parameter β, as well as the depths to the top (zt) and bottom (zb) of the magnetized layer. Synthetic tests reveal the increased accuracy of the estimated zb when values for zt and β are fixed prior to the inversion. We set zt to the thickness of sedimentary rocks overlying the magnetic bedrock and use various values of β to estimate zb. We determine β a posteriori by comparing Monte Carlo simulations of predicted heat flow values (assuming a Curie temperature of 580 °C) with observed heat flow in various regions. Our results suggest a β value of 2.5 for the Canadian Cordillera and Slave craton, and 2 for the remaining North American craton. The Curie depths resolve geological domains and important structural features, with estimates for zb averaging 15 ± 1 km in the Cordillera, 32 ± 3 km in the Slave craton, and 34 ± 3 km in the North American craton to the south.

3D-Basin Modeling of the Changling Depression, NE China: Exploring Petroleum Evolution in Deep Tight Sandstone Reservoirs

The Changling Depression is the largest and most resource-abundant reservoir in the South Songliao Basin, NE China. The petroleum evolution rules in the Lower Cretaceous deep tight sandstone reservoir are unclear. In this study, 3D basin modeling is performed to analyze the large-scale petroleum stereoscopic migration and accumulation history. The Changling Depression has a complex fault system and multiple tectonic movements. The model is calibrated by the present formation temperatures and observed maturity (vitrinite reflectance). We consider (1) three main erosion episodes during the burial history, one during the Early Cretaceous and two during the Late Cretaceous; (2) the regional heat flow distribution throughout geological history, which was calibrated by abundant measurement data; and (3) a tight sandstone porosity model, which is calibrated by experimental petrophysical parameters. The maturity levels of the Lower Cretaceous source rocks are reconstructed and showed good gas-generation potential. The highest maturity regions are in the southwestern sag and northern sag. The peak hydrocarbon generation period contributed little to the reservoir because of a lack of seal rocks. Homogenization temperature analysis of inclusions indicated two sets of critical moments of gas accumulation. The hydrocarbon filling in the Haerjin and Shuangtuozi structures occurred between 80 Ma and 66 Ma, while the Dalaoyefu and Fulongquan structures experienced long-term hydrocarbon accumulation from 100 Ma to 67 Ma. The homogenization temperatures of the fluid inclusions may indicate a certain stage of reservoir formation and, in combination with the hydrocarbon-accumulation simulation, can distinguish leakage and recharging events.

Gridded maps of geological methane emissions and their isotopic signature

Abstract. Methane (CH4) is a powerful greenhouse gas, whose natural and anthropogenic emissions contribute ∼20 % to global radiative forcing. Its atmospheric budget (sources and sinks), however, has large uncertainties. Inverse modelling, using atmospheric CH4 trends, spatial gradients and isotopic source signatures, has recently improved the major source estimates and their spatial–temporal variation. Nevertheless, isotopic data lack CH4 source representativeness for many sources, and their isotopic signatures are affected by incomplete knowledge of the spatial distribution of some sources, especially those related to fossil (radiocarbon-free) and microbial gas. This gap is particularly wide for geological CH4 (geo-CH4) seepage, i.e. the natural degassing of hydrocarbons from the Earth's crust. While geological seepage is widely considered a major source of atmospheric CH4, it has been largely neglected in 3-D inverse CH4 budget studies given the lack of detailed a priori gridded emission maps. Here, we report for the first time global gridded maps of geological CH4 sources, including emission and isotopic data. The 1∘×1∘ maps include the four main categories of natural geo-CH4 emission: (a) onshore hydrocarbon macro-seeps, including mud volcanoes, (b) submarine (offshore) seeps, (c) diffuse microseepage and (d) geothermal manifestations. An inventory of point sources and area sources was developed for each category, defining areal distribution (activity), CH4 fluxes (emission factors) and its stable C isotope composition (δ13C-CH4). These parameters were determined considering geological factors that control methane origin and seepage (e.g. petroleum fields, sedimentary basins, high heat flow regions, faults, seismicity). The global geo-source map reveals that the regions with the highest CH4 emissions are all located in the Northern Hemisphere, in North America, in the Caspian region, in Europe and in the East Siberian Arctic Shelf. The globally gridded CH4 emission estimate (37 Tg yr−1 exclusively based on data and modelling specifically targeted for gridding, and 43–50 Tg yr−1 when extrapolated to also account for onshore and submarine seeps with no location specific measurements available) is compatible with published ranges derived using top-down and bottom-up procedures. Improved activity and emission factor data allowed previously published mud volcanoes and microseepage emission estimates to be refined. The emission-weighted global mean δ13C-CH4 source signature of all geo-CH4 source categories is about −49 ‰. This value is significantly lower than those attributed so far in inverse studies to fossil fuel sources (−44 ‰) and geological seepage (−38 ‰). It is expected that using this updated, more 13C-depleted, isotopic signature in atmospheric modelling will increase the top-down estimate of the geological CH4 source. The geo-CH4 emission grid maps can now be used to improve atmospheric CH4 modelling, thereby improving the accuracy of the fossil fuel and microbial components. Grid csv (comma-separated values) files are available at https://doi.org/10.25925/4j3f-he27.

New Regional Data and Advances in Understanding of the Stratigraphy, Tectonics, Structure and Prospectivity of the Gulf of Papua (Papua New Guinea)

Between 2015 and 2017, Searcher Seismic acquired 32,478 km of long offset PSDM 2D seismic data and reprocessed an additional 12,972 km of previously acquired 2D data in the Gulf of Papua, Papua New Guinea (PNG). The new data has resulted in a significant improvement in subsurface imaging and areal coverage, providing the foundation for a new integrated analysis of the region. In addition, a regional drop core geochemistry and heat flow survey provides important clues regarding the existence of working petroleum systems in the Gulf of Papua. The evaluation of these new datasets has improved the current understanding of the stratigraphy, plate tectonics, local structure petroleum prospectivity of the Gulf of Papua.New seismic allowed identification of several depositional packages that are often bounded by regional unconformities related to the tectonic development of the area. Seismic and shipborne gravity/magnetics analyses allowed a confident identification of the following events/packages: Moho event allowing estimation of the crustal thickness and differentiation between oceanic and crust and calibration of the heat flow measurements;Paleozoic severely folded succession analogous to eastern Australia accretionary terrains;Permian analogous to the Bowen Basin in Queensland, Australia;Triassic to Jurassic succession supported by the existence of the Jurassic seep identified by the Davaria geochemical survey;Presence of previously unidentified block faulted highs with Miocene reefs and carbonate platform build-ups;Pliocene and younger basin sandstone floor fans; andExtension of the compressional front into deep water Gulf of Papua.These observations have been integrated into an updated plate tectonic model that predicts widespread deposition of the Permian and Triassic to Tertiary source rocks estimated to be often within the hydrocarbon generative window.

Structural modeling of lobate scarps in the NW margin of Argyre impact basin, Mars

Martian lobate scarps are prominent tectonic structures interpreted to be the topographic expression of large thrust faults generated under compression. Here, we present a structural modeling performed on three large lobate scarps (Ogygis Rupes, Bosporos Rupes and Phrixi Rupes) located in the heavily cratered highlands of Mars, specifically in Aonia Terra, between Thaumasia Montes and Argyre impact basin. These lobate scarps, formed in the Late Noachian/Early Hesperian, strike parallel to the edge of the Thaumasia Montes, and were generated by ESE-vergent thrust faults. Structural analysis of craters deformed by these lobate scarps gives minimum estimates for the faults slip of ∼1700–3100 m. We applied two structural methods in order to constrain the geometry of these thrust faults at depth, area balanced cross sections and forward mechanical dislocation modeling, obtaining a depth of faulting in this area between ∼18 and ∼45 km, and dip angles between 23° and 35°. These results are consistent with previous studies of lobate scarps on Mars. The depth of faulting gives an estimation of the depth of the brittle-ductile transition at the time of its formation giving a range of depth in which the state of the lithosphere change from brittle to ductile-dominated deformation. The heat flow values calculated from the obtained depths of the brittle–ductile transition range from 25 to 51 mW m−2. We show that the brittle-ductile transition depth in Aonia Terra is set in 18–27 km at a larger distance from the basin center, while it is deeper closer to the Argyre rim (∼33–45 km). This difference seems to indicate a thickening of the brittle domain under Argyre main rim with respect to the external area but, attending to regional geology and heat flow values calculated, this high value (∼33–45 km) might be an overestimation of the depth of faulting caused by the presence of the crater rim elevation before the formation of the lobate scarps.

Disturbances of Temperature-Depth Profiles by Surface Warming and Groundwater Flow Convection in Kumamoto Plain, Japan

Subsurface temperatures depend on climate and groundwater flow. A lack of observations of subsurface temperature collected over decades limits interpretation of the combined influences of surface warming and groundwater flow on subsurface thermal regimes. Subsurface temperature-depth profile data acquired for Kumamoto Plain, Japan, between 1987 and 2012 were collected and analyzed to elucidate regional groundwater and heat flows. The observed and simulated temperature-depth profiles showed the following: subsurface water flows from northeast to southwest in the study area; the combined influence of surface warming and water flow perturbation produces different temporal changes in thermal profiles in recharge, intermediate, and discharge areas; and aquifer thermal properties contribute more than hydraulic parameters to the perturbation of temperature-depth profiles. Spatial and temporal evolution features of subsurface thermal regimes may be utilized to investigate the influence of surface warming events on subsurface water and heat flows at the basin scale.

Quantitative Relationships Between Basalt Geochemistry, Shear Wave Velocity, and Asthenospheric Temperature Beneath Western North America

AbstractWestern North America has an average elevation that is ∼2 km higher than cratonic North America. This difference coincides with a westward decrease in average lithospheric thickness from ∼240 to &lt;100 km. Tomographic models show that slow shear wave velocity anomalies lie beneath this region, coinciding with the pattern of basaltic magmatism. To investigate relationships between magmatism, shear wave velocity, and temperature, we analyzed a suite of &gt;260 basaltic samples. Forward and inverse modeling of carefully selected major, trace, and rare earth elements were used to determine melt fraction as a function of depth. Basaltic melt appears to have been generated by adiabatic decompression of dry peridotite with asthenospheric potential temperatures of 1340 ± 20 °C. Potential temperatures as high as 1365 °C were obtained for the Snake River Plain. For the youngest (i.e., &lt;5 Ma) basalts with a subplate geochemical signature, there is a positive correlation between shear wave velocities and trace element ratios such as La/Yb. The significance of this correlation is explored by converting shear wave velocity into temperature using a global empirical parameterization. Calculated temperatures agree with those determined by inverse modeling of rare earth elements. We propose that regional epeirogenic uplift of western North America is principally maintained by widespread asthenospheric temperature anomalies lying beneath a lithospheric plate, which is considerably thinner than it was in Late Cretaceous times. Our proposal accounts for the distribution and composition of basaltic magmatism and is consistent with regional heat flow anomalies.

Origin of a foreland-dipping seismogenic zone and its basal decollement in southwestern Taiwan

An unusual foreland-dipping seismicity band has been identified at a depth between 7 and 25 km beneath the Coastal Plain, a foreland basin, in southwestern Taiwan. This seismogenic zone is closely associated with abrupt changes in Vp/Vs based on our new high-resolution seismic tomography. Rocks with high Vp/Vs ratios of up to 1.84 were found directly above the inclined seismicity band. Below it, the Vp/Vs ratio dramatically drops to about 1.72–1.78. We found that this change may result from illite-bearing slates transforming to muscovite-bearing phyllite/mica-schist. Previous studies suggested that the frictional instability regime for illite–quartz gouge and muscovite–quartz gouge falls in the range of 250–500 °C. We calculated the isotherms based on regional heat flow data and found that the foreland-dipping seismicity band can well be bounded between the 250 °C and 500 °C isotherms. In addition, in the same temperature range wet illite/hydromuscovite would transform to muscovite and release water. This dehydration may elevate pore pressure and facilitate the formation of an incipient decollement at the base of the seismicity band in front of the mountain belt. The hypothesis that such a decollement exists is supported by the existence of a corresponding zone of low resistivity and also by the geometry of the earthquake of October 22, 1999. The latter clearly shows a fault branched off the basal decollement with an angle about 45°. This finding implies that the effective friction coefficient of the basal decollement is at least 23% less than that of the overlying rocks.

Regional thermo-rheological field related to granite emplacement in the upper crust: implications for the Larderello area (Tuscany, Italy)

ABSTRACTWe modelled thermo-rheological perturbations, related to the emplacement of a magmatic body in the upper crust. This approach was considered relevant for the areas characterized by elevated surface heat flow and chiefly for the geothermal fields. The numerical conductive thermal model applied to the Larderello geothermal area in Tuscany, allowed to constrain size, depth and timing of emplacement of the pluton. We inferred that the emplacement of a magmatic body, at a minimum depth of 3 km, having a horizontal extension of 14 km and a maximum thickness of 8 km, can reasonably reproduce the observed regional surface heat flow anomaly of the Larderello area, when 300 (± 100) kyr are elapsed from the magma emplacement. Even assuming an incremental growth, the first magma injection should not be older than 1 ± 0.3 Ma.Results of the thermal model were used to set up a rheological model and to simulate the drifting of the brittle-ductile transition during the cooling of the pluton. A comparison with the K-horizon profile, a prominent seismic reflector in the Larderello area, was then performed. It was found that the K-horizon approximately corresponds with the pluton roof and with the current location of the brittle-ductile transition.

Terrestrial heat flow and crustal thermal structure of the Gonghe-Guide area, northeastern Qinghai-Tibetan plateau

The Gonghe-Guide area, which is located at the northeastern edge of the Qinghai-Tibetan Plateau, has the greatest hot dry rock (HDR) geothermal resources exploration and development potential in China. Separated by the Waligong tectonomagmatic belt, the study area includes the Gonghe and Guide basins, which are characterized by distinctly different thermal state and surface thermal manifestations. Although a number of geophysical explorations have been undertaken, studies on fundamental geothermal theory remain scarce, including a lack of high-quality heat flow determinations and thermal structure studies. In this study, we obtained continuous steady-state temperature logs from three deep boreholes (GR2 and DR3 in the Gonghe basin and ZR1 in the Guide basin). The thermal conductivities and radiogenic heat production rates of the outcrop samples gathered from the two basins were measured. The heat-flow values were determined to be 116.3–122.3mWm−2 for the Gonghe basin, yielding a mean of 119.3mWm−2. Two existed heat flow sites (R2 and R3 boreholes) gave an average heat flow of 75mWm−2 for the Guide basin, which is approximately that of the regional background heat flow level of the northeastern Tibetan Plateau. The 1-D theoretical crustal thermal structures of the Gonghe and Guide basins were constructed by solving steady-state heat conduction equation. The results show that the Moho temperature and mantle heat flow in the Guide basin were 1016°C and 27mWm−2, respectively. However, the temperature field in the Gonghe basin indicated partial melting in the crust. The high temperatures and thus heat flow in the Gonghe basin may suggest the existence of anomalous heat source body in crust. Successful inversions of vertical and horizontal temperature variations of boreholes in the Gonghe basin were obtained by numerically simulating a cooling rectangular magma chamber with a buried depth of 5–6km and horizontal dimensions of 12–14km. The study emphasizes that the anomalous heat flow in the Gonghe basin may be the joint thermal effect of: (1) the high regional background heat flow level of the northeastern Tibetan Plateau, which is mainly due to the radiogenic heat of thickened crust; (2) the cooling of a shallow magma chamber, which may be caused by the uplift and denudation of the plateau.