Low Geothermal Gradient Research Articles

Thermal state and structure of lithospheric mantle beneath the Xing'an Massif, northeast China: Constraints from mantle xenoliths entrained by Cenozoic basalts

The lithospheric mantle beneath the Xing'an Massif (XM), northeast China, was formed at the Paleoproterozoic or even earlier, but its thermal state and structure remain poorly understood. To address this issue, we calculate pressure–temperature (P–T) arrays for mantle xenoliths entrained by Cenozoic basalts from the XM, using major‐element‐based and rare earth element (REE)‐based two‐pyroxene thermometry as well as single clinopyroxene barometry. Samples (include spinel or garnet lherzolites and harzburgites, dunite and pyroxenite) come from the Keluo, Nuomin, and Aershan regions, which are located on either side of the North–South Gravity Lineament (NSGL). To the west of the NSGL, in the Nuomin and Aershan regions, mantle xenoliths yield consistent REE‐based and major‐element‐based temperatures, suggesting that these samples were derived from stable thermal conditions. Garnet peridotites from these regions yield pressures of 2.37–3.01 GPa and temperatures of 1066–1303°C, revealing a high geothermal gradient (corresponding to a surface heat flow of 70–80 mW/m2) and a crustal thicknesses of ~37 km. The lithosphere–asthenosphere boundary determined from the intersection of this geotherm with the mantle adiabat occurs at a depth of ~100 km, which is much shallower than the lithospheric thickness calculated by geophysical analyses (140–160 km). This discrepancy may have resulted from locally upwelling hot asthenosphere. Low‐temperature (800–900°C) peridotites from the Nuomin and Aershan are chemically fertile with low olivine Mg# values and high modal clinopyroxene contents. By contrast, high‐temperature (1050–1350°C) peridotites consist mainly of refractory harzburgites or clinopyroxene‐poor lherzolites. These facts reveal that the lithospheric mantle beneath the Nuomin and Aershan regions is characterized by juvenile mantle dominating shallow levels and ancient mantle occurring at greater depths. To the east of the NSGL, some Keluo mantle xenoliths yield higher REE‐based temperatures (839–863°C) than major‐element‐based temperatures (<800°C), indicating cooling by thermal relaxation. This cooling process produced a low geothermal gradient (~45 mW/m2) in the lithosphere beneath the Keluo region. The mantle xenoliths comprise both fertile lherzolites and refractory harzburgites without correlation between mineral compositions and temperatures, suggesting that the lithospheric mantle beneath the Keluo region is characterized by juvenile mantle mixed with ancient mantle at all depths. The results show contrasting thermal state and structure of the lithospheric mantle on either side of the NSGL within the XM, which are attributed to a previous cooling process in the Keluo region and local upwelling of hot asthenosphere beneath the Aershan and Nuomin regions.

Effects of the Karacadag Volcanic Complex on the thermal structure and geothermal potential of southeast Anatolia

The Karacadag Volcanic Complex (KVC) is the largest volcanic unit in SE Turkey. It is also defined as a shield volcano on the northernmost part of the Arabian Plate. The main goal of this study is to investigate the geothermal potential of this region associated with the magnetic signature of this volcanic complex and surrounding area. Besides this primary objective, the possibility of there being volcanic intrusion into the buried fault zones under the volcanic cover are also investigated to determine the interrelations between the active tectonics and heat flow in the area. A spectral analysis method is applied to the magnetic anomalies of the volcanic rocks to identify the Curie point depth (CPD) and geothermal gradient, as well as to estimate heat flow and radiogenic heat production of radioactive minerals in the complex. A tilt angle map is also presented, in correlation with instrumentally recorded earthquake magnitudes, to indicate tectonic trends that are consistent with the maps of the thermal parameters in this study. In contrast with expectations for the KVC area, the region around Akcakale and Suruc Grabens is the most prolific zone for geothermal potential, despite them not showing strong magnetic anomalies. Curie point depths are shallow, down to 18 km, around the Akcakale Graben, and deeper, down to 22 km, around the Bitlis-Zagros Suture Zone where the geothermal gradients increase from 26 to 32 °C km−1 through the graben area. Heat flows in this zone are in the range from 75 to 90 mW m−2 depending on the thermal conductivity coefficient (2.3, 2.5, 2.7, and 3.0 W m−1 K−1) used. Radiogenic heat production values also indicate slightly changing spectra in the range 0.19 to 0.25 μW m−3). None of these parameters are focused around Mt. Karacadag. However, the earthquake epicenters (generally M ≤ 4) are aligned with the boundary faults of the Akcakale Graben where the CPD, geothermal gradient, and heat flow maps indicate relatively high potential. We thus suggest that this graben area would be good for future geothermal exploration. On the contrary, considering the low geothermal gradient and heat flow values, Mt. Karacadag can be accepted as being an extinct volcano, despite its apparent, high, magnetic anomalies.

Triponzo: a thermal system in a cold area of the Apennines (Italy)

Bagni di Triponzo thermal springs, characterised by a Ca-SO4 composition and temperatures up to 30°C, are located in the eastern sector of Umbria region in the Umbria-Marche Apennine (central Italy). The region is characterised by a low geothermal gradient and low conductive heat flux and the composition of Triponzo thermal waters significantly differs with respect to the cold waters circulating in the surrounding areas. The origin of the heat transported by the waters of the Triponzo springs is mainly due to a deep component, characterised by high CO2 and He contents, coming from a deeper reservoir, rising along normal faults and mixing with infiltrating waters of meteoric origin. The total amount of thermal water discharged by the system is about 34 L s-1. According to the ternary SO4-2-F--HCO3- geoindicator for carbonate-evaporite reservoirs, the fluids at reservoir condition are charcterised by a partial pressure of CO2 about 0.5 bar and a temperature between 70-75°C whereas the Silica geothermometers give a temperature about 62°C. The computed thermal energy transported by advection and discharged at the surface by Triponzo springs is about 3.71×1011 ± 0.56×1011 J/day.

Neoproterozoic reworking of the Ubendian Belt crust: Implication for an orogenic cycle between the Tanzania Craton and Bangweulu Block during the assembly of Gondwana

The Paleoproterozoic pelitic gneisses from the Ubendian Belt host Neoproterozoic eclogites (590–520 Ma) that formed in a subduction zone. The Neoproterozoic subduction event in the Ubendian Belt was followed by a collision between the Tanzania Craton and the Bangweulu Block and tectono-metamorphic reworking of the Ubendian Belt rocks. This study focuses to characterize the P-T-t conditions of the Neoproterozoic collisional event in the Ubendian Belt and its regional geodynamic significance.Despite hosting Neoproterozoic eclogite, the Ubendian Belt pelitic gneisses have zircons that yield concordant metamorphic ages between 1949 ± 16 and 1901 ± 37 Ma (U-Pb zircon, 2σ errors) having neither clear younger metamorphic ages nor Neoproterozoic rim overgrowth. However, monazite crystals from the same rocks yield clear younger rims with ages between 566 ± 8 and 556 ± 5 Ma overgrowing old cores.Monazite inclusions in garnet yields an age of 566 ± 8 Ma, which implies garnet growth during the Neoproterozoic tectonometamorphic event. Garnet and the associated minerals yield high-pressure metamorphic P-T conditions (P = 9–12 kbar; T = 760–820°C) which indicates a low geothermal gradient of 17–22 °C/km. A clockwise P-T-t path (kyanite to sillimanite reactions) with nearly isothermal decompression (growth of symplectites) after the peak of metamorphism was deduced.The subduction of an oceanic basin between the Tanzania Craton and the Bangweulu Block started at about 590 Ma and resulted in a major collisional event between the Tanzania Craton and Bangweulu Block between 570 and 550 Ma. The collision ended at about 500 Ma during the final stages of Gondwana Supercontinent amalgamation. For this collisional event we therefore propose the term Ufipa orogeny.During the collision, the Ubendian Belt gneisses and the margins of the Bangweulu Block formed part of a passive continental margins that subducted to attain high-pressure metamorphic conditions. The absence of zircons with Neoproterozoic metamorphic ages from older reworked crusts, the lack of pre-collisional volcanic arc, and low geothermal gradient support the metacratonitization process of the Ubendian Belt and the Bangweulu Block.

Phase state control model of supercritical CO2 fracturing by temperature control

A phase state control model of supercritical CO2 (SC-CO2) fracturing by temperature control has been developed on the basis of CO2 physical properties, fluid filtration characteristics, internal energy, and flow work variation in the fractures. A considerable amount of analysis focuses on the effects of the CO2 injection temperature and pressure, geothermal gradient, pumping rate of CO2 fracturing fluid temperature, pressure field, and phase behavior in the wellbore and fractures. In this study, the phase control method of the fractures during SC-CO2 fracturing in addition to its chart is obtained. The results indicate that the temperature of the SC-CO2 fracturing model in the wellbore and fracture is less than that when not considering the flow work model at the same location. During the process of fluid flow, a transition occurs from the liquid to supercritical state in the wellbore or fractures. The phase transformation point differs in the fractures such that a lower injection temperature relates to a high pumping rate, lower geothermal gradient and closer location of transformation point to the end of fractures. Thus, to obtain the optimal stimulation effect of the supercritical CO2 fracturing, the phase behavior of CO2 should be controlled according to the reservoir conditions through surface equipment by optimizing the injection temperature, pressure, and pumping rate of the fracturing fluid.

Garnet amphibolites from the Ganzi–Litang fault zone, eastern Tibetan Plateau: mineralogy, geochemistry, and implications for evolution of the eastern Palaeo-Tethys Realm

ABSTRACTThe Ganzi–Litang fault zone, an outstanding tectonic element in the eastern Tibetan Plateau has been intensively debated as an in-situ suture zone marking relict of a subducted Palaeo-Tethyan oceanic crust or a failed intracontinental rift. This paper reports the garnet amphibolites discovered along the Ganzi–Litang fault zone, eastern Tibetan Plateau. These garnet amphibolites are characterized by the garnet–hornblende–rutile–sphene–plagioclase–quartz assemblage. Conventional geothermobarometry figures out the metamorphic temperature and pressure conditions at 582–626°C and 1.61–1.82 GPa, respectively. Geochemical analysis (no Nb–Ta deletions and left-inclined to flat patterns of rare-earth elements) indicates that the garnet amphibolites could represent metamorphic product of the mid-ocean-ridge (MORB)-type mafic rocks that were contaminated by a mantle plume. The protolith of the garnet amphibolites was dated at 236 Ma using in-situ U–Pb zircon method, and the retrograde metamorphism was dated at 218 Ma using in-situ U–Pb sphene method. A comprehensive analysis combined with the development of the Palaeo-Tethys Ocean and the Yidun arc through geologic time indicates a Triassic to Early Jurassic age (236–195 Ma) for the metamorphism of the garnet amphibolites. The low geothermal gradient of 9.8ºC/km and the N-MORB nature of the garnet amphibolites suggest a subduction-zone environment for the high-pressure metamorphism. Therefore, the Ganzi–Litang fault zone is a Palaeo-Tethyan suture separating the Yidun arc and the Songpan–Ganzi terrane, representing the relics of a branch of the Palaeo-Tethys Ocean that was contaminated by a mantle plume.

Non-cracked oil in ultra-deep high-temperature reservoirs in the Tarim basin, China

Thermal stability of liquid petroleum in the subsurface is closely linked to reservoir temperature. Most oil accumulations occur at temperatures <120 °C. Oil cracks into gas at temperatures >150–160 °C leading to the dominance of gas condensate and free gas accumulations in ultra-deep high-temperature reservoirs. The recently drilled Fuyuan-1 exploration well (northern Tarim basin) produced high-quality non-cracked single-phase (black) oil from a carbonate reservoir located at maximum depth 7711 m and temperature 172 °C. This is the deepest oil discovery in China to date and among the deepest in the world. The oil density (0.825 g/cm3 at 20 °C or API gravity 40°), relatively low gas/oil ratio (135 m3/m3 or 758 scf/bbl), low variety and abundance of adamantanes as well as lack of thiaadamantanes and dibenzothiophenes indicate that the oil was expelled from a source rock at moderate thermal maturity and has not been cracked. The molecular and isotopic composition of oil-associated gases are consistent with this interpretation. We suggest that the oil remained uncracked because the residence time at temperatures >150-160°C was relatively short (<5 my based on 1D modeling) and apparently insufficient for cracking. We conclude that there is potential for finding unaltered liquid petroleum in other high-temperature reservoirs with relatively low geothermal gradient and recent burial in the Tarim basin and around the world.

Felsic granulite with layers of eclogite facies rocks in the Bohemian Massif; did they share a common metamorphic history?

High pressure granulite and granulite gneiss from the Rychleby Mountains in the East Sudetes form an approximately 7km long and 0.8km wide body, which is enclosed by amphibolite facies orthogneiss with a steep foliation. Well preserved felsic granulite is located in the central part of the body, where several small bodies of mafic granulite are also present. In comparison to other high pressure granulites in the Bohemian Massif, which show strong mineral and textural re-equilibration under granulite facies conditions, the mafic granulite samples preserve eclogite facies minerals (garnet, omphacite, kyanite, rutile and phengite) and their field and textural relations indicate that both mafic and felsic granulites shared common metamorphic history during prograde eclogite facies and subsequent granulite facies events. Garnet from both granulite varieties shows prograde compositional zoning and contains inclusions of phengite. Yttrium and REEs in garnet show typical bell-shaped distributions with no annular peaks near the grain rims. Investigation of major and trace elements zoning, including REEs distribution in garnet, was combined with thermodynamic modelling to constrain the early eclogite facies metamorphism and to estimate pressure-temperature conditions of the subsequent granulite facies overprint. The first (U)HP metamorphism occurred along a low geothermal gradient in a subduction-related environment from its initial stage at 0.8GPa/460°C and reached pressures up to 2.5GPa at 550°C. The subsequent granulite facies overprint (1.6–1.8GPa/800–880°C) affected the rocks only partially; by replacement of omphacite into diopside+plagioclase symplectite and by compositional modification of garnet rims. The mineral textures and the preservation of the eclogite facies prograde compositional zoning in garnet cores confirm that the granulite facies overprint was either too short or too faint to cause recrystallisation and homogenisation of the eclogite facies mineral assemblage. The results of this study are compared with other granulite massifs in the Moldanubian Zone. In addition, a possible scenario for the Variscan eclogite and subsequent granulite facies metamorphism in the Bohemian Massif is discussed.

Subsurface thermal regime to delineate the paleo-groundwater flow system in an arid area, Al Kufra, Libya

The purpose of this study was to understand the groundwater flow system in Al Kufra basin, Libya, as a case study of arid areas using subsurface temperature. The temperature-depth profiles and water levels were measured in eight boreholes in the area. Well 6 is considered a recharge type profile with low geothermal gradient (0.0068°C/m) and an estimated paleo-temperature around 19.5°C. The other profiles are of discharge type with higher geothermal gradient (0.0133 to 0.0166°C/m). The constructed horizontal 2D distribution maps of the hydraulic heads and the subsurface temperature measurements reveal that the main recharge area is located to the south with low temperature while the main discharge area is located to the north with higher temperature. Vertical 2D distribution maps show that location of well 4 has low hydraulic heads and higher temperature indicating that the fault defined in the area may have affected the groundwater flow system. The estimated groundwater flux ranges from 0.001 to 0.1mm/day for the recharge area and from −0.3 to −0.7mm/day in average in the discharge area.

Curie point depth and heat flow from spectral analysis of aeromagnetic data over the northern part of Western Desert, Egypt

The present work aims to estimate the Curie point depth and the surface heat flow for the northern part of the Western Desert using aeromagnetic data. Applying spectral analysis to aeromagnetic anomalies has provided equitable promising geological results, useful for further geothermal or petroleum exploration. The total intensity aeromagnetic map was first reduced to the north magnetic pole to correct the shape and position of different magnetic anomalies over their causative bodies. Secondly, the short wavelengths were removed to enhance the deeper long wavelengths related to the deep sources. Spectral analysis indicates that the area is underlined by an average Curie point depth of about 27km. This implies an average thermal heat flow (53mW/m2) lower than the average global heat flow. The investigated area was divided into eighteen blocks, where the average depths to centroid and top of the magnetic source were estimated for each block. The results of this work show a general depth increase of the magnetic boundaries from 24.5km in the southern area to 33km at the northern part. The calculated surface heat flow decreases from about 56 to 42mW/m2 in the same direction. Consequently, this area is characterized by its low geothermal gradient and surface heat flow. This low geothermal gradient indicates that the upper mantle contributes to the magnetic features at the northern offshore parts. This work also recommends by deep drilling for petroleum exploration and production within the Egyptian Mediterranean Sea exploration strip.

Discovery of eclogite in the Bangong Co–Nujiang ophiolitic mélange, central Tibet, and tectonic implications

An eclogite has been recently identified within ophiolitic mélange in the western segment of the Bangong Co–Nujiang suture zone, at Shemalagou in the Gaize area of central Tibet. The eclogite consists of garnet, omphacite, phengite, rutile, quartz, diopside, and amphibole. The omphacite, which has not been recognized in the suture zone until this study, occurs as rare relics within diopside grains in the eclogite. Phase equilibria modeling shows that the eclogite formed under P–T conditions of 22–28kbar and 600–650°C with a low geothermal gradient of ca. 8°C/km, suggesting that it formed during the subduction of oceanic crust. The protoliths of the eclogite and coexisting garnet amphibolites have geochemical characteristics similar to those of normal mid-ocean ridge basalt (N-MORB), confirming that the eclogites formed from oceanic crust. The presence of high-pressure (HP) eclogite indicates that the ophiolitic mélange in the Bangong Co–Nujiang suture zone underwent oceanic subduction and was subsequently exhumed. We conclude that this ophiolitic belt represents a newly identified HP metamorphic belt in the Tibetan Plateau, adding to the previously recognized Songduo and Longmucuo–Shuanghu eclogite belts. This discovery will result in an improved understanding of the tectonic evolution of the Bangong Co–Nujiang suture zone and the Tibetan Plateau as a whole.

ContrastingP-Tpaths within the Barchi-Kol UHP terrain (Kokchetav Complex): Implications for subduction and exhumation of continental crust

![Figure][1] The Barchi-Kol terrain is a classic locality of ultrahigh-pressure (UHP) metamorphism within the Kokchetav metamorphic belt. We provide a detailed and systematic characterization of four metasedimentary samples using dominant mineral assemblages, mineral inclusions in zircon and monazite, garnet zonation with respect to major and trace elements, and Zr-in-rutile and Ti-in-zircon temperatures. A typical diamond-bearing gneiss records peak conditions of 49 ± 4 kbar and 950–1000 °C. Near isothermal decompression of this rock resulted in the breakdown of phengite associated with a pervasive recrystallization of the rock. The same terrain also contains mica schists that experienced peak conditions close to those of the diamond-bearing rocks, but they were exhumed along a cooler path where phengite remained stable. In these rocks, major and trace element zoning in garnet has been completely equilibrated. A layered gneiss was metamorphosed at UHP conditions in the coesite field, but did not reach diamond-facies conditions (peak conditions: 30 kbar and 800–900 °C). In this sample, garnet records retrograde zonation in major elements and also retains prograde zoning in trace elements. A garnet-kyanite-micaschist that reached significantly lower pressures (24 ± 2 kbar, 710 ± 20 °C) contains garnet with major and trace element zoning. The diverse garnet zoning in samples that experienced different metamorphic conditions allows to establish that diffusional equilibration of rare earth element in garnet likely occurs at ~900–950 °C. Different metamorphic conditions in the four investigated samples are also documented in zircon trace element zonation and mineral inclusions in zircon and monazite. U-Pb geochronology of metamorphic zircon and monazite domains demonstrates that prograde (528–521 Ma), peak (528–522 Ma), and peak to retrograde metamorphism (503–532 Ma) occurred over a relatively short time interval that is indistinguishable from metamorphism of other UHP rocks within the Kokchetav metamorphic belt. Therefore, the assembly of rocks with contrasting P-T trajectories must have occurred in a single subduction-exhumation cycle, providing a snapshot of the thermal structure of a subducted continental margin prior to collision. The rocks were initially buried along a low geothermal gradient. At 20–25 kbar they underwent near isobaric heating of 200 °C, which was followed by continued burial along a low geothermal gradient. Such a step-wise geotherm is in good agreement with predictions from subduction zone thermal models. [1]: pending:yes

Phase relations of lawsonite-blueschists and their role as a water-budget monitor: a case study from the Hakoishi sub-unit of the Kurosegawa belt, SW Japan

This study investigated progressive changes in the mineral assemblage of a Palaeozoic lawsonite-blueschist (LBS) unit of the Kurosegawa belt, Kyushu, Japan. The LBS unit is mainly composed of intercalated metachert and metabasaltic rocks surrounded by serpentinite. Based on the spatial distribution of mineral assemblages in the metabasaltic rocks, three mineral zones can be identified in a ~10 km-long east–west oriented unit: (1) a pumpellyite (Pmp) + Na amphibole (Namp) assemblage dominated area (Zone 1); (2) an intermixed area of lawsonite (Lws) + Na/Ca-Na pyroxenes +(Napx) + Pmp and Lws + Namp + Pmp assemblages (transition zone); and (3) an area dominated by Lws + Namp + Napx assemblages (Zone 2); all contain excess chlorite (Chl), albite (Ab), quartz (Qz), titanite (Ttn), and goethite. Napx varies in composition from jadeite(Jd) 20 -diopside(Di) 25 of Zone 1 to Jd 35 Di 15 in Zone 2, with a similar range of aegirine (Aeg) content from 40 to 55 mol%. The Al/(Al + Fe 3+ ) ratio of Na amphibole (= Y Amp ) associated with Lws and Pmp increases from Y Amp = 0.2 in Zone 1 to Y Amp = 0.8 in Zone 2. The Jd contents and the lack of epidote and zeolite-group minerals suggest that the pressure ( P ) conditions of the BS increase from Zone 1 (0.50 GPa) to Zone 2 (0.75 GPa) between 200 and 300 °C, suggesting a low geothermal gradient (~5 to 10 °C/km). Schreinemakers’ analyses in a NaCaMgFe 3+ AlSiH (NCMF3ASH) system, using mean mineral compositions of representative samples and excess Chl, Ab, Qz and H 2 O, suggest that, (1) observed variation in mineral assemblages can be explained by the hydration reaction Pmp + Napx + Chl + H 2 O = Lws + Namp, which was driven by a P increase from Zone 1 to Zone 2, and represents the transition from the Pmp-BS to the Lws-BS sub-facies; (2) the total H 2 O content stored in hydrous minerals increases from 3.6 wt% in the Pmp-Namp assemblage of Zone 1 to 5.0–6.4 wt% in the Lws + Namp + Napx assemblages in Zone 2; and (3) the Napx + Chl assemblage can retain a significant amount of H 2 O, along with the Lws-Namp and Pmp-Namp assemblages, and its stability field can expand to higher P conditions by decrease of the Di and increase of the Jd and Aeg contents in Napx.

The transport of water in subduction zones

Abstract The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low (≤5°C/km), lawsonite may carry water into great depths of ≤300 km. In the hot subduction zone where the geothermal gradient is high (>25°C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of T /low- P hydrous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of ≤60–80 km to release large amounts of water. In contrast, the low- T /low- P hydrous minerals are replaced by low- T /high- P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80–160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultrahigh-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.

Geothermal gradients of the northern continental shelf of the Gulf of Mexico

A wide, systematic variation of sedimentary geothermal gradients has been previously observed along the northern continental shelf of the Gulf of Mexico. From east to west, geothermal gradients change from 25 to 30 °C/km off Alabama to lower values (15–25 °C/km) off eastern Louisiana and to higher values (30–60 °C/km) off Texas. In order to assess the mechanism responsible for this variation, the present study first compiled an extensive bottom-hole temperature database from over 6000 wells in the northern continental shelf and constructed a more detailed geothermal gradient map than those published previously. Second, basin models were then constructed for three areas within the continental shelf (off Texas, Louisiana, and Alabama) that show differing geothermal gradients. A basin model is a mathematical model that simulates the heat transport through the crust and the sediments of a basin in the context of its geologic evolution. Previous researchers proposed two possible causes for the observed geothermal gradient variation in the northern continental shelf. The first was the thermal effect of sedimentation: areas with faster sediment accumulation result in low geothermal gradients, and vice versa. The second was that basal heat flow (heat flow that enters from the igneous crust to the bottom of the sediments) varied across the continental shelf. The present study finds that sedimentary geothermal gradients in these areas are primarily impacted by two competing mechanisms associated with sediment accumulation. One is the radiogenic heat production within the sediment that adds to the total heat budget upward through the sedimentary column. The other is the transient effect of fast sediment accumulation, which results in reduction in the upward heat flow. Off Louisiana, the transient effect prevails, and hence the area shows the lowest geothermal gradients. Off Texas, due to slower sedimentation, the positive contribution by radiogenic heat is most significant. Off Alabama, because the sediments there are not as thick, the overall contribution of radiogenic heat is less. The models show that the thermal effects of sedimentation are large enough to explain the observed variation in geothermal gradients. Therefore, corresponding variation in basal heat flow is not required.

Potential map of gas hydrate formation in Caspian Sea based on physicochemical stability evaluation of methane hydrate

ABSTRACTPreliminary studies of Caspian Sea have shown the possibility of gas hydrate accumulations, because of suitable physicochemical conditions, existence of clayey deposits, and high concentrations of organic matter. Studies have indicated that gas hydrates are mainly composed of methane. Therefore, based on physicochemical equations for methane hydrate stability in different pressure, temperature, and salinity, this study was designed to calculate the potential of gas hydrate formation in the Caspian Sea basin. For this, data of more than 600 locations were analyzed and in each location, upper and lower limits of methane hydrate formation zone were calculated. Then, the zoning maps of upper and lower limits were prepared which can be useful for exploring the gas hydrate as an energy source or predicting gas hydrate hazards. According to the calculations and maps, methane hydrate formation in Caspian Sea, theoretically, can take place from near the seabed to 4000 and 2500 m beneath the sea surface when low and high geothermal gradient are supposed, respectively. By comparing the results with gas hydrate zones revealed in geophysical profiles, it has been shown that, in Caspian Sea, gas hydrates probably accumulate near the lower limit when a high geothermal gradient is assumed.

Petrology on Mars

Petrologic investigations of martian rocks have been accomplished by mineralogical, geochemical, and textural analyses from Mars rovers (with geologic context provided by orbiters), and by laboratory analyses of martian meteorites. Igneous rocks are primarily lavas and volcaniclastic rocks of basaltic composition, and ultramafic cumulates; alkaline rocks are common in ancient terranes and tholeiitic rocks occur in younger terranes, suggesting global magmatic evolution. Relatively uncommon feldspathic rocks represent the ultimate fractionation products, and granitic rocks are unknown. Sedimentary rocks are of both clastic (mudstone, sandstone, conglomerate, all containing significant igneous detritus) and chemical (evaporitic sulfate and less common carbonate) origin. High-silica sediments formed by hydrothermal activity. Sediments on Mars formed from different protoliths and were weathered under different environmental conditions from terrestrial sediments. Metamorphic rocks have only been inferred from orbital remote-sensing measurements. Metabasalt and serpentinite have mineral assemblages consistent with those predicted from low-pressure phase equilibria and likely formed in geothermal systems. Shock effects are common in martian meteorites, and impact breccias are probably widespread in the planet’s crustal rocks. The martian rock cycle during early periods was similar in many respects to that of Earth. However, without plate tectonics Mars did not experience the thermal metamorphism and flux melting associated with subduction, nor deposition in subsided basins and rapid erosion resulting from tectonic uplift. The rock cycle during more recent time has been truncated by desiccation of the planet’s surface and a lower geothermal gradient in its interior. The petrology of Mars is intriguingly different from Earth, but the tried-and-true methods of petrography and geochemistry are clearly translatable to another world.

The thermal history of the Sichuan Basin, SW China: Evidence from the deep boreholes

The Sichuan Basin is a superimposition basin composed of terrestrial and marine sediments that is well known for its abundant petroleum resources. Thermal history reconstruction using paleogeothermal indicators, including vitrinite reflectance and thermochronological data, shows that different structural subsections of the Sichuan Basin have experienced various paleogeothermal episodes since the Paleozoic. The lower structural subsection comprising the Lower Paleozoic to Middle Permian (Pz-P2 successions experienced a high paleogeothermal gradient (23.0–42.6°C/km) at the end of the Middle Permian (P2, whereas the upper structural subsection comprising Late Permian to Mesozoic strata underwent a relatively lower paleogeothermal gradient (13.2–26.9°C/km) at the beginning of the denudation (Late Cretaceous or Paleocene in the different regions). During the denudation period, the Sichuan Basin experienced a successive cooling episode. The high paleogeothermal gradient resulted from an intensive thermal event correlated to the Emeishan mantle plume. The heat flow value reached 124.0 mW/m2 in the southwestern basin near the center of the Emeishan large igneous province. The low geothermal gradient episode with heat flow ranging from 31.2 to 70.0 mW/m2 may be related to the foreland basin evolution. The cooling event is a result of the continuous uplift and denudation of the basin.

Ikaite pseudomorphs in Neoproterozoic Dalradian slates record Earth’s coldest metamorphism

Calcite pseudomorphs have replaced euhedral ikaite (CaCO 3 .6H 2 O) porphyroblasts in Dalradian calcareous slates and metadolostones of western Scotland, with a volume decrease of at least 47%. Porphyroblast–fabric relationships indicate that the initial growth of ikaite post-dates a penetrative tectonic fabric developed during upright folding. This is the first reported occurrence of metamorphic ikaite porphyroblasts and points towards growth within the slates during an ultra-low-temperature metamorphism with an exceptionally low geothermal gradient. This event is associated with the penetration of long-lived and extreme permafrost deep into subaerially exposed bedrock during Neoproterozoic glaciation. The presence of the well-preserved pseudomorphs within the Easdale slates of the Argyll Group implies that a Neoproterozoic orogenic unconformity exists above the stratigraphic position of these rocks.

Contrasting styles of (U)HP rock exhumation along the Cenozoic Adria‐Europe plate boundary (Western Alps, Calabria, Corsica)

AbstractSince the first discovery of ultrahigh pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel‐flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria‐Europe plate boundary, and along three different transects (Western Alps, Calabria‐Sardinia, and Corsica‐Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging toward the foreland. Both mechanisms were active to create boundary divergence along the Corsica‐Northern Apennines transect, where European southeastward subduction was progressively replaced along strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present‐day eastern Papua New Guinea, which is presented as a modern analog of the Paleogene Adria‐Europe plate boundary.