Anomalous Heat Flow Research Articles

Terrestrial Heat Flow and the Mantle Convection Hypothesis

Summary The paper aims to show that the distribution of terrestrial heat flow is consistent with the mantle convection hypothesis provided that certain restrictions are placed on allowable patterns of convection. Some objections to convection are first discussed. Numerical experiments have been carried out on the loss of heat from the upper surface of a mantle convection cell, assuming a simplified flow pattern. The results are compared with observed heat flow. Subject to the assumptions of the models, the results suggest that sub-oceanic mantle convection currents, if they exist, are overlain by a layer 50–100 km thick which is stationary or moves much less rapidly. This appears to rule out the mechanism of continental drift by ocean floor spreading as suggested by Dietz; continental drift affecting the overlying layer must occur at a much lower velocity than the convection current. If thermal diffusivity remains approximately constant with depth the velocity near the upper surface of the convection cell needs to be at least about 20 cm/y to explain the uniformity of heat flow, but if radiative conduction becomes dominant a lower velocity would be acceptable. The experiments have been extended to a convection cell flowing beneath a continental margin. The results suggest that the approximate equivalence of oceanic and continental mean heat flow can best be explained if convection currents are generally present beneath oceans but absent beneath continents, unless the continental crust has a much lower radioactivity than is normally supposed. The anomalous low heat flow of Pre-Cambrian shields suggests absence of convection in the mantle beneath since the Pre-Cambrian. Conditions particularly favourable to large-scale partial fusion of the upper mantle occur in the topmost section of a rising convection current. This should result in abundant igneous activity and local areas of anomalously high heat flow. This supports the view that ocean ridges overlie uprising convection currents. Belts of relatively low heat flow adjacent to the ocean ridges are difficult to explain if the convecting mantle behaves as a Newtonian viscous fluid, but are explicable on the non-Newtonian model of convection recently suggested by Orowan.

Mechanisms of low-flux intraplate volcanic fields—Basin and Range (North America) and northwest Pacific Ocean

We compare two intraplate, Pliocene-Pleistocene volcanic fields in different tectonic settings—the central Basin and Range and the northwest Pacific Ocean. Both fields are characterized by widely scattered, small-volume, alkali basaltic volcanoes; within the fields, each volcano apparently originates from a separate, volatile-enriched parental melt from the upper mantle. There is no evidence at either field for locally anomalous heat flow or ongoing introduction of new fluids into the upper mantle such as might occur above a subducting slab. We conclude that the volcanic fields reflect deformation-driven collection of already existing partial melts in a heterogeneous upper mantle. Deformation-driven melt collection may be an important mechanism for other diffuse intraplate volcanic fields, and this is consistent with a tectonically controlled, low-flux end member for intraplate fields where magmatism is a passive response to regional deformation. Differences in the degree of fractionation and contamination between the two fields are inferred to be related to flexure-induced vertical variations in the orientation of principal stresses in the northwest Pacific Ocean, which cause stalling of ascending dikes in the lithosphere.

Evidence for mantle plumes?

Geophysical hotspots have been attributed to partially molten asthenosphere, fertile blobs, small-scale convection and upwellings driven by core heat. Most are short-lived or too close together to be deeply seated, and do not have anomalous heat flow or temperature; many are related to tectonic features. Bourdon et al. investigate the dynamics of mantle plumes from uranium-series geochemistry and interpret their results as evidence for thermal plumes. Here we show why alternative mechanisms of upwelling and melting should be considered.

Thermal history of deep‐sea sediments as a record of recent changes in the deep circulation of the eastern Mediterranean

During three cruises of the MEDRIFF project in 1993 and 1994, 154 geothermal heat flow measurements and seven CTD profiles in the water column have been collected along a 300 km SW‐NE oriented transect traversing the Mediterranean Ridge accretionary complex. The original goal of the measurements was to identify areas of anomalous heat flow that could be interpreted as possible sites for fluid outflow. Contrary to expectations, the upper few meters of the temperature profiles in the sediments showed decreasing temperature from the seafloor down to 3 to 6 m depth indicating consistently transient temperature regimes. The only exception (positive heat flow) was found in the Sirte abyssal plain. Measurements collected in the same area at different times indicated that the thermal structure in the bottom water and sediments had changed significantly at weekly, monthly, and interannual timescales. One‐dimensional forward modeling of the conductive heat propagation into the sediment explains the observed thermal anomalies, assuming up to 0.5 K warming of the bottom waters that propagated south westward from the Matapan Trench to the crestal area of the Mediterranean Ridge. Analysis of nonsteady state thermal profiles in the upper sediment provided time information on the onset of bottom water warming, by what is now called the Eastern Mediterranean Transient (EMT). The sediment warming started in the Matapan Trench, between spring 1992 and spring 1993, and reached the Mediterranean Ridge crest in spring‐summer 1993. The average propagation velocity of the thermal perturbation along the profile is about 0.5–1.6 km d−1 (0.6–1.8 cm s−1).

Three-dimensional inverse modeling of the refractive heat-flow anomaly associated with salt diapirism

This article introduces a technique for three-dimensional inverse modeling of geothermal heat conduction through heterogeneous media. The technique is used to determine the basal geometry of a diapiric salt structure found on the continental slope offshore Texas. Salt is two to four times more thermally conductive than other sedimentary rocks. The geothermal field is perturbed by the presence of salt and results in an anomaly in the heat flow through the seafloor. The spatial variation pattern of the anomalous heat flow reflects the geometry of the salt body. The inverse modeling obtains a model for the thermal-conductivity structure that causes the heat-flow anomaly observed on the seafloor. The inversion algorithm systematically searches for an optimal thermal-conductivity model by iteratively minimizing the misfit between the model-predicted and the observed heat flow. To reduce the problem of nonuniqueness, the inversion incorporates a priori information constrained independently, such as the upper surface geometry of the salt and the lateral extent of the salt body, which can be delineated by a limited coverage of two-dimensional seismic data. In addition, it is assumed that the thermal conductivity of the sedimentary strata surrounding the salt body is well constrained. This inversion method is applied to a heat-flow data set obtained over a salt structure on the Texas continental slope. The salt structure was first surveyed with the single-channel seismic reflection, which yielded the a priori information necessary. The base of the salt was not imaged seismically. Then, three dozen heat-flow measurements were obtained on the seafloor over and off the salt feature. The inverse heat-flow modeling performed here shows that this structure is a salt tongue, which has a diapiric root on one side. According to the most optimal thermal-conductivity model obtained, the root seems to extend to 6 km below the seafloor. Refinement in the model geometry and additional constraints on thermal conductivities of the surrounding strata should yield a model that is more detailed and would allow more thorough geological interpretation of the salt structure.

Petroleum system and production characteristics of the Muddy (J) Sandstone (Lower Cretaceous) Wattenberg continuous gas field, Denver basin, Colorado

Wattenberg field is a continuous-type gas accumulation. Estimated ultimate recovery from current wells is 1.27 tcf of gas from the Lower Cretaceous Muddy (J) Sandstone. Mean gas resources that have the potential to be added to these reserves in the next 30 yr are 1.09 tcf; this will be primarily through infill drilling to recover a greater percentage of gas in place and to drain areas that are isolated because of geologic compartmentalization. Greatest gas production from the Muddy (J) Sandstone in Wattenberg field occurs (1) from within the most permeable and thickest intervals of Fort Collins Member delta-front and nearshore-marine sandstones, (2) to a lesser extent from the Horsetooth Member valley-fill channel sandstones, (3) in association with a large thermal anomaly that is delineated by measured temperatures in wells and by vitrinite reflectance contours of 0.9% and greater, (4) in proximity to the bounding Mowry, Graneros, and Skull Creek shales that are the hydrocarbon source rocks and reservoir seals, and (5) between the Lafayette and Longmont right-lateral wrench fault zones (WFZs) with secondary faults that act as conduits in areas of the field. The axis of greatest gas production is north 25 to 35 degrees northeast, which parallels the basin axis. Recurrent movement along five right-lateral WFZs that crosscut Wattenberg field shifted the Denver basin axis to the northeast and influenced depositional and erosional patterns of the reservoir and seal intervals. Levels of thermal maturity within the Wattenberg field are anomalously high compared to other areas of the Denver basin. The Wattenberg field thermal anomaly may be due to upward movement of fluids along faults associated with probable igneous intrusions. Areas of anomalous high heat flow within the field correlate with an increased and variable gas-oil ratio.

Extremely high heat flow anomaly in the middle part of the Nankai Trough

Detailed heat flow measurements were carried out in the central part of the Nankai Trough off eastern Shikoku, where the fossil spreading center of the Shikoku Basin is subducting beneath the southwest Japan arc. The quality of temperature gradient values was examined for all the existing and new heat flow data and the average of new thermal conductivity data was used at all the stations. Reliable heat flow on the floor of the trough was found to be very high and rather uniform, with an average of about 200 ± 20 mW/m 2. Taking account of the effect of recent rapid sedimentation on the surface heat flow, the heat flow from the deeper part is estimated to be even higher by 20–30%, almost twice as high as the value expected from the age of the subducting Shikoku Basin, about 15 m.y. Such a large heat flow anomaly cannot be attributed solely to advective heat transfer by pore fluid flows associated with subduction and accretion of the sediments brought into the trough, suggesting that the underlying Shikoku Basin lithosphere is anomalously warm in the study area. A possible cause of the anomaly is reheating of the lithosphere by post-spreading thermal activities along the fossil spreading center, which produced the Kinan Seamount Chain and continued until about 7 Ma. However, such reheating even at 6 Ma cannot produce a heat flow high enough to explain the observed values. A steep landward decrease in the heat flow is observed at the toe of the accretionary prism. The scale of this decrease indicates that its origin is not deep seated, and thus a part of the anomalous heat flow on the trough floor may result from some processes in relatively shallow layers. The thermal structure of the Shikoku Basin is a critical boundary condition for thermal models of the Nankai subduction zone and its seismogenic zone for large thrust earthquakes. More heat flow measurements need to be made around the study area in order to clarify the extent and origin of the high heat flow anomaly.

Regional geochemistry and continental heat flow: implications for the origin of the South Australian heat flow anomaly

Existing measurements from South Australia define a broad (>250 km wide) zone of anomalously high surface heat flow (92±10 mW m −2). This zone is centred on the western margin of the Adelaide Fold Belt (Neoproterozoic to early Phanerozoic cover floored by Palaeoproterozoic to Mesoproterozoic basement), where it borders the eastern Gawler Craton and Stuart Shelf (Palaeoproterozoic–Mesoproterozoic). To the west, in the western Gawler Craton (Archaean to Palaeoproterozoic), heat flow averages ∼54 mW m −2 while to the east in the Willyama Inliers (Palaeoproterozoic) heat flow averages ∼75 mW m −2. We use a regional geochemical dataset comprising >2500 analyses to show that the anomalous heat flow zone correlates with exceptional surface heat production values, mainly hosted in Palaeoproterozoic to Mesoproterozoic granites. The median heat production of Precambrian ‘basement’ rocks increases from <3 μW m −3 west of the anomalous zone to ∼6 μW m −3 within the anomalous zone. In the highest known part of the heat flow anomaly, Mesoproterozoic gneisses and granites of the Mount Painter Province in the northern Adelaide Fold Belt yield an area-integrated mean heat production of 9.9 μW m −3. These data suggest that the anomalous heat flow reflects an unusual enrichment in U and Th in this part of the Proterozoic crust, with the total complement of these elements some 2–3 times greater than would be expected for Proterozoic crust on the basis of the global heat flow database. This extraordinary enrichment has played an important role in modulating the thermal regime of the crust in this region, and particularly its response to tectonic activity.

Geotectonic models and geologic evolution of the high-grade gneiss terranes of Juiz de Fora (MG), Brazil

The orthogneisses of the Ribeira Belt, in the Juiz de Fora area, underwent a sequence of geologic processes during times previous to the Brasiliano orogeny that generated this belt. The present study intends to recover and to reconstitute the geological evolution related to these processes. The units are distributed in two main tectonic domains (JFTD and ANDTD) that were subdivided in three structural domains with specific characteristics. The metamorphic studies point to the existence of two high-grade metamorphic peaks associated to two different tectonic events. The last event is related to the Brasiliano orogeny, with ages around 610-490 Ma and this affected the orthogneisses of the basement as well as Andrelândia Group supracrustal rocks, deposited between 1.0 and 0.6 Ga. The metamorphism related to the Transamazonian orogeny, dated at 2.2-1.9 Ga, affected only the basement orthogneisses. Peak P-T conditions for these events are considered to be 6-8 kb/750-800°C and 4-6kb/800-850°C respectively. It is proposed that the tectonic evolution for the Transamazonian orogeny could result in crustal thickening by continuous subcontinental accretion (vertical) and consequent thinning of the lithospheric mantle, and generating this way a continental magmatic are. The resulting anomalous heat flow would elevate the continental geotherms causing extensive partial melting and differentiation of the lower crust generating preferentially tonalitic magma, and later passing on to more granitic compositions. These magmas would ascend in the crust, reworking older segments (Archean?) in upper crustal leveis. The proposed geological evolution involves extensional processes with magmatic intra and underplating, during the Transamazonian orogeny, that provided COyrich fluids and the necessary heat for the granulite metamorphism. The final stage of the Transamazonian orogeny witnessed crustal thinning aided by erosion, which resulted in rifting of the upper crust, recorded by the intracratonic Sao Joao dei Rei basin metasedimentary rocks. After a long period of quiescence, compressive continental collision processes took place during the Brasiliano orogeny and this juxtaposed the supracrustals and basement rocks. Intense anatetic processes in this period, removed large amounts of H. 2 O from the system, enabling the metasedimentary rocks to attain granulite facies.

GEOTECTONIC MODELS AND GEOLOGIC EVOLUTION OF THE HIGH-GRADE GNEISS TERRANES OF JUIZ DE FORA (MG), BRAZIL

The orthogneisses of the Ribeira Belt, in the Juiz de Fora area, underwent a sequence of geologic processes during times previous to the Brasiliano orogeny that generated this belt. The present study intends to recover and to reconstitute the geological evolution related to these processes. The units are distributed in two main tectonic domains (JFTD and ANDTD) that were subdivided in three structural domains with specific characteristics. The metamorphic studies point to the existence of two high-grade metamorphic peaks associated to two different tectonic events. The last event is related to the Brasiliano orogeny, with ages around 610-490 Ma and this affected the orthogneisses of the basement as well as Andrelândia Group supracrustal rocks, deposited between 1.0 and 0.6 Ga. The metamorphism related to the Transamazonian orogeny, dated at 2.2-1.9 Ga, affected only the basement orthogneisses. Peak P-T conditions for these events are considered to be 6-8 kb/750-800o C and 4-6kb/800-850oC respectively. It is proposed that the tectonic evolution for the Transamazonian orogeny could result in crustal thickening by continuous subcontinental accretion (vertical) and consequent thinning of the lithospheric mantle, and generating this way a continental magmatic arc. The resulting anomalous heat flow would elevate the continental geotherms causing extensive partial melting and differentiation of the lower crust generating preferentially tonalitic magma, and later passing on to more granitic compositions. These magmas would ascend in the crust, reworking older segments (Archean?) in upper crustal levels. The proposed geological evolution involves extensional processes with magmatic intra and underplating, during the Transamazonian orogeny, that provided CO 2 -rich fluids and the necessary heat for the granulite metamorphism. The final stage of the Transamazonian orogeny witnessed crustal thinning aided by erosion, which resulted in rifting of the upper crust, recorded by the intracratonic São João del Rei basin metasedimentary rocks. After a long period of quiescence, compressive continental collision processes took place during the Brasiliano orogeny and this juxtaposed the supracrustals and basement rocks. Intense anatetic processes in this period, removed large amounts of H 2 O from the system, enabling the metasedimentary rocks to attain granulite facies.

Geochemistry and petrogenesis of Lower Paleozoic meta-granites of Lahaul-Spiti region, Himachal Pradesh, India.

In the Lahaul-Spiti region of Himachal Pradesh two meta-granite bodies (Jispa granite and Ratilaman granite) emplaced in the low grade Haimanta Formation which is tectonically overlying the Central Crystallines. Field, petrographical and geochemical features of these meta-granites are compared with the orthogneisses of the Central Crystallines. Geochemical signatures of these meta-granites and orthogneisses suggest that they are products of peraluminous melt generated by partial melting of older crustal rock, with high A/CNK values and higher normative corundum. Decrease in Fe2O3, MgO, CaO, TiO2, Sr and Zr and enhancement in K2O, SiO2 and Rb as well as Rb/Sr ratios from melanocratic (margin) to leucocratic (core) portion of these bodies indicate that they are consistent with fractional crystallization processes. Decrease of total REE, moderate LREE/HREE ratios and more pronounced negative Eu anomalies are also in accord with fractional crystallization. The heat source for these lower Paleozoic granite magmas may be due to the lithospheric extension which resulted in mantle melting beneath the crust which led to anomalous heat flow, and triggered crustal melting. Felsic magmas thus produced have intruded into shallow crustal levels.

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

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

The Volcanic and Tectonic History of Enceladus

Enceladus has a protracted history of impact cratering, cryovolcanism, and extensional, compressional, and probable strike–slip faulting. It is unique in having some of the outer Solar System's least and most heavily cratered surfaces. Enceladus' cratering record, tectonic features, and relief elements have been analyzed more comprehensively than done previously. Like few other icy satellites, Enceladus seems to have experienced major lateral lithospheric motions; it may be the only icy satellite with global features indicating probable lithospheric convergence and folding. Ridged plains, 500 km across, consist of a central labyrinthine ridge complex atop a broad dome surrounded by smooth plains and peripheral sinuous ridge belts. The ridged plains have few if any signs of extension, almost no craters, and an average age of just 107to 108years. Ridge belts have local relief ranging from 500 to 2000 m and tend to occur near the bottoms of broad regional troughs between swells. Our reanalysis of Peter Thomas' (Dermott, S. F., and P. C. Thomas, 1994, The determination of the mass and mean density of Enceladus from its observed shape,Icarus,109, 241–257) limb profiles indicates that high peaks, probably ridge belts, also occur in unmapped areas. Sinuous ridges appear foldlike and are similar to terrestrial fold belts such as the Appalachians. If they are indeed folds, it may require that the ridged plains are mechanically (perhaps volcanically) layered. Regional topography suggests that folding may have occurred along zones of convective downwelling. The cratered plains, in contrast to the ridged plains, are heavily cratered and exhibit extensional structures but no obvious signs of compression. Cratered plains contain a possible strike–slip fault (Isbanir Fossa), along which two pairs of fractures seem to have 15km of right-lateral offset. The oldest cratered plains might date from shortly after the formation of the saturnian system or the impact disruption and reaccretion of Enceladus. Another area of cratered plains has modified craters (e.g., Ali Baba and Aladdin), which some workers have explained by anomalous heat flow and viscous relaxation; lateral shear and shield-building volcanism also may have been important. A young rift-like structure (northern Samarkand Sulci) has few craters and a concentration of cracks or grabens and flattened, flooded, and rifted craters. Pit chains and cratered domes suggest explosive volcanism. Smooth plains may have formed by cryovolcanic equivalents of flood-basalt volcanism. Pure H2O would be difficult to extrude through an icy crust and is cosmochemically improbable as a cryovolcanic agent. Density relations rule out eutectic brine lavas on Enceladus, but NH3–H2O volcanism is possible. Current steady-state tidal dissipation may cause melting of ammonia hydrate at a depth of ∼25 km if the crust is made of ammonia hydrate or ∼100 km if it is made of water ice.

Development of Sediment Overpressure and Its Effect on Thermal Maturation: Application to the Gulf of Mexico Basin

High sedimentation rates can potentially lead to overpressuring and sediment undercompaction within basins. Sediments with anomalously high porosity, in turn, induce low thermal conductivities and so tend to act as a thermal insulator to the flow of heat. In the Gulf of Mexico basin (Gulf basin), the generation of overpressure is caused mainly by the inability of pore pressure fluids to escape at a rate commensurate with sedimentation. We modeled the generation and dissipation of abnormal sediment pore pressure due to variations in sedimentation rate, facies, formation porosity, and permeability within the Gulf basin using finite-element techniques to solve the differential equations of both heat and fluid transport within compacting sediments. We assume that the porosity effective stress relationship within the sediment follows a negative exponential steady-state form when the pore pressure is hydrostatic. An important feature of our modeling approach is the assumption that sediments are incapable of significant expansion in response to increasing pore pressure. Sediments are assumed to hydrofracture when the pore pressure approaches the lithostatic pressure, rather than a common assumption of porosity expansion even in lithified sediments. From our modeling, we conclude that significant overpressures have been created (and dissipated) at various times within the Gulf basin and track, in general, the west to east migration of sediment loads deposited since the Cretaceous. Although predicted overpressures of more than 0.75 kpsi (i.e., an equivalent excess hydraulic head of 500 m) of Campanian-Maastrichtian age remain to the present day, the main phase of overpressure development in the Gulf basin is predicted to have occured during the Miocene-Holocene. Maximum overpressures (~13.6 kpsi; excess hydraulic head of 9.4 km) are predicted for the present day. Overpressure development during the Miocene-Quaternary, a consequence of rapid sediment deposition associated with the Mississippi delta system, is also predicted to be associated with undercompaction. This undercompaction led to increased temperature gradients during the Miocene and Quaternary despite the fact that the anomalous basal heat flow engendered by extension had practically dissipated. We further predict that by the end of the Neogene, temperatures would have been approaching s eady state over broad regions of the Gulf basin implying that the highest temperatures occur in the deepest parts of the basin. In contrast, during the Quaternary, the rapid progradation of overpressured and undercompacted sediments resulted in a thick section that has yet to reach thermal equilibrium and thus is anomalously cold with respect to its present depth. The predicted vitrinite reflectance indicates that for most of the Gulf basin history, the depth to the top of the oil window remained at approximately 2.5±0.5 km below sea floor (bsf). Similarly, the depth to the base of the oil window ranged from 3.5 to 6.5 km bsf. This relatively constant position of the top of the oil window defines a maturation front that propagated from the offshore into the End_Page 1367------------------------------ onshore regions of the northern Gulf basin as a function of time. As such, hydrocarbon generation is predicted to have occurred continuously within the Jurassic and Cretaceous sections of the onshore region during the entire Cenozoic. Prior to this, maturation fronts within each of the onshore basins resulted in maturation of Upper Jurassic source rocks during the Early Cretaceous. In the offshore Gulf Coast area, pre-Tertiary source rocks are predicted to be overmature for liquid hydrocarbons at present. In the offshore regions affected by Quaternary sedimentation, the depth to the top of the oil window has been significantly depressed in response to sediment loading and subsidence.

Characterization and Evolution of Paleozoic Source Rock Organic Matter in Algerian Central Sahara: ABSTRACT

The objective of the proposed poster is to provide a knowledge of the evolution history of organic matter in an intracratonic basin. The Paleozoic source rocks (Ordovician - Silurian - Upper Devonian and Carboniferous) of the Algerian Central Sahara (Ahnet and Timimoun basins) experienced severe conditions of maturation during the geological history, therefore, the source rocks intervals are presently mature to overmature and only dry gas has been descovered throughout this zone. The several geochemical models (Genex, Basimod, Matoil) in addition to Afta and Zafta Data show that regionaly significant heating event occured with maximum palaeo-temperature and maximum gas generation at 300 +/- 30 My. However, high palaeotemperatures can not be explained only by the significant burial. An important anomalous heat flow is needed to explain the geothermal history. In this case, there has been no significant petroleum from the Paleozoic source rocks in this zone since this age; but following some assumptions, a certain hypothesis about a recent generation (-60 to -30 My) is now in discussion.

Contemporary state of stress and neotectonic deformation in the Carpathian‐Pannonian region

The Carpathian‐Pannonian region has been characterized by a two‐stage tectonic evolution since the beginning of the Neogene. During early and mid‐Miocene the lateral eastward escape of the Pannonian Fragment caused thrusting and folding in the Outer Carpathian Mountains, south‐ and westward directed subduction of the Eurasian lithosphere, calc‐alkaline volcanism along the Inner Carpathian bend, and localized fault‐controlled subsidence of basins in the Pannonian region. This style of tectonic deformation ceased by the end of the mid‐Miocene. The neotectonic stage began in the late Miocene. It is characterized by differential regional subsidence with maximum rates in the west and east and minimum rates in the centre of the Pannonian Basin. Further characteristics of the central Pannonian Basin are the anomalous high heat flow values, the thin crust and lithosphere.The neotectonic stress field has been determined by in situ stress measurements, the analyses of borehole breakouts and fault‐plane solutions of earthquakes. In situ stress measurements by the doorstopper and triaxial strain cell methods indicate high compressional stresses in the western Pannonian Basin with its maximum in WNW‐ESE to NW‐SE direction. In contrast to its western part, the central Pannonian Basin shows tensional stresses near the Earth's surface with maximum tension in the same WNW‐ESE direction. Borehole breakout data indicate a general WNW‐ESE orientation of maximum horizontal stress in the western part of the Pannonian Basin and in the eastern part as well, whereas in the central Pannonian Basin this direction is the preferred azimuth of minimum horizontal stress.It is suggested that the neotectonic deformations and stresses in the Carpathian‐Pannonian region have a sublithospheric origin.Asthenospheric convection with an upwelling mantle flow below the centre of the Pannonian Basin and downwelling flows along the cold lithospheric roots below the eastern Carpathians and the Alps seems to be most plausible. Accordingly, the relative uplift of the centralPannonian Basin, the high heat flow and also the tensional stresses are explained as the surface expression of an upstreaming branch of a localized convection cell below the Pannonian Basin.

Chemical relationship between discharging fluids in the Siena-Radicofani graben and the deep fluids produced by the geothermal fields of Mt Amiata, Torre Alfina and Latera (Central Italy)

The thermal springs discharging in the Siena-Radicofani basin and the deep fluids within the geothermal systems of Piancastagnaio (Mt Amiata), Torre Alfina and Latera (Vulsini Mts) have a common origin. The chemical composition and evolution towards the low enthalpy of the springs as compared to the high enthalpy of the geothermal fluids are affected by both the structural setting of the region and the deep hydraulic conditions. Recharge of both the shallow thermal aquifer and the deep geothermal systems takes place in the outcrop areas of Mesozoic carbonate rocks, which constitute the main potential geothermal reservoir in central Italy. The waters of meteoric origin are heated at depth, as a consequence of anomalous heat flow in the region; these waters acquire a CO 2-rich rising gas phase, equilibrate equilibrate with the reservoir rocks and, finally, assume their CaHCO 3SO 4 composition. If these waters discharge rapidly from the border fault systems of the Siena-Radicofani basin they maintain their original composition. If, instead, they emerge from the inner faults of the graben, their temperature and dissolved solids increase so that they become NaCl with a high content of NH 4, H 3BO 3 and Li +. The chemical evolve towards high enthalpy. In this case the deep fluids again acquire an NaCl composition similar to carbonate reservoir is at a structural high, buried by both allochthonous flyschoid series (“Ligurids”) and by Neogenic pelitic formations, a slower recharge occurs in the reservoir, and consequently the fluids evolve towards high enthalpy. In this case the deep fluids again acquire an NaCl composition similar to that of the springs emerging inside the graben, since part of the reservoir recharge also occurs through the leakage of saline fossil waters stored within the Neogenic sedimentary formations at the border of the geothermal systems.

Heat flow from a scientific research well at Cajon Pass, California

The long‐standing “stress/heat flow paradox” was the primary scientific motivation for the Cajon Pass borehole. For nearly two decades, the absence of a fault‐centered heat flow anomaly from measurements to relatively shallow (∼200 m) depths had indicated low average shear stresses (≤20 MPa) on the San Andreas fault, while laboratory data on rock strength and in situ stress determinations to about a kilometer had indicated high stress (∼100 MPa). Initial results from an unsuccessful 1.7‐km‐deep oil well at the site gave a high heat flow (∼90 mW m−2) consistent with a strong San Andreas fault; however, the late Cenozoic geologic history of the Cajon Pass area suggested that the anomalous heat flow was the transient effect of rapid erosion. Theoretical studies predicted that the ∼30% surface anomaly would be substantially reduced at depths of 3–5 km. The research borehole reached a total depth of 3.5 km. Below a superficial covering of Tertiary sedimentary rocks, it penetrated gneissic rocks with composition ranging from gabbroic to granodioritic. Core recovery amounted to only about 3% of the total depth, necessitating the use of drill cuttings to characterize thermal conductivity. This, in turn, resulted in much higher uncertainties in average conductivity (±10–15%) than would have occurred with a continuously cored hole (±3–5%). From a time series of temperature logs, equilibrium temperature gradients were established over selected intervals of 250–500 m to within 95% confidence limits of 2%. These gradients were combined with harmonic mean thermal conductivities having larger uncertainties to give interval heat flows, which vary systematically from 100 ± 5 mW m−2 in the uppermost 400 m to 75 ± 3 mW m−2 in the lowermost 300 m. Thus, at the Cajon Pass site, heat flow is decreasing with depth at a mean rate of more than 7 mW m−2 per kilometer, consistent with a frictionless fault and with theoretical predictions based on local erosional history.

An examination of atmospheric mechanisms that may be responsible for the annual reversal of the beaufort sea ice field

AbstractThe late‐summer reversal of the Beaufort Sea Gyre and the overlying sea ice cover is an intriguing phenomenon that may reveal some insight into atmosphere‐cryosphere interactions through a detailed diagnostic analysis.In this study we examine the atmospheric processes that may give rise to this reversal and maintain it through a case by case study of selected days during the late‐August‐early‐September reversal in 1980. We inspect the fields of total vertical motion and the contributions by advective, diabatic, and frictional mechanisms, as well as the isentropic potential vorticity, in order to identify the forcing functions.This episode is an example of the classic cold low with a central cold pool, the surface and 500‐mbar fields aligned vertically, yet indications in the ageostrophic field of significant potential for baroclinic development, and a depression that remains anchored in the region for a significant period (27 days).The episode occurs during a transition period from the summer regime with sluggish upper air anticyclonic circulation to the vigorous winter regime of a cyclonic vortex extending up through the stratosphere with concomitant increase of potential vorticity available for surface development. This transition, which appears to occur during a period that typically experiences rapid upper atmosphere cooling, may be the trigger for the creation of the low.The surface heat flux appears to be a significant factor in the maintenance of the low although there are two factors at work. The first is the anomalous heat flow from the ocean along the ice‐ocean boundary and from open water in the diverging ice pack, but this primarily appears to result in a regional modulation of the synoptic‐scale effect. At the synoptic scale, there is significant heat flux associated with the very cold core of the depression and the associated pattern of the lower atmospheric heating moves with the wandering migration of the system within the region. In addition, there is an important dynamic contribution to development owing to the ageostrophic advection and convergence of cold air out of the core. The long duration of this feature within the Canada Basin must be traced to a complex interplay of heat flow processes related to the nature of the surface as well as the formation of the cold core of the depression, dynamic properties of mid‐tropospheric air flow, and the increase of upper air potential vorticity.

Thermal anomalies in the central Indian Ocean: Evidence for dewatering of the Bengal Fan

Seafloor heat flow measurements in the central Indian Ocean are locally higher than predicted by lithospheric cooling models. These anomalously high heat flow values occur in a region affected by folding and faulting of oceanic crust and overlying distal Bengal Fan sediments. Three closely spaced sites drilled during Ocean Drilling Program Leg 116 dated the onset of deformation in this area at about 7–8 Ma. Here apatite fission track thermochronometric data from sites 717 and 718 and numerical simulations are used to (1) quantify the effects of Bengal Fan sedimentation on seafloor heat flow, (2) evaluate the thermal history of sediments penetrated at these two sites and (3) constrain mechanisms responsible for anomalous seafloor heat flow in this area. At the rates sediments were deposited along the distal fan (≤ 350 m/m.y.). numerical simulations predict a 6% to 9% reduction in present‐day heat flow at the seafloor relative to heat flow through the base of the lithosphere. At site 717, thermal history simulations assuming heat transfer dominated by conduction are consistent with available down‐hole temperature measurements and apatite fission track data. Site 718 is located 8 km south of Site 717 and 2 km south of a high‐angle fault that offsets oceanic crust by about 300 m. A previously published detailed heat flow survey conducted around site 718 documented seafloor heat flow ranging from 44 to 166 mW/m2 over kilometer‐scale distances. In addition, a monotonic 10 to 20% reduction in the mean length of fission tracks in apatite from 560 to 960 m below the seafloor at site 718 implies bottom‐hole temperatures of the order of 50°–60°C for a time period greater than a few million years. Both the seafloor heat flow and apatite fission track data argue for localized, possibly intermittent, advection of heat by lateral, updip migration of pore fluids in the vicinity of site 718. Taken together, the heat flow data and temperature range inferred from the apatite fission track data at site 718 imply a fluid source in the upper crust. Upward movement of pore fluids at velocities of less than 1 cm/yr through a basement‐sediment boundary maintained at temperatures ranging from 65° to 80°C is capable of explaining both sets of observations. As a hypothesis to reconcile existing observations, I suggest that the anomalous thermal structure in the central Indian Ocean represents a joint response to compaction‐driven dewatering of the Bengal Fan and local enhancement of the vertical permeability in the upper crust and sediment pile due to intraplate deformation.