Heat Flow Anomalies Research Articles

Leakage of mantle helium from the Liaodong Peninsula, China

The isotopic cumpositions of helium have been investigated in hot spring gases sampled from the Liaodong Peninsula. China. The3He/4He ratios range from 0.14 Ra (0.14 times the atmospheric3He/4He ratio of 1.40 × 10-6 to 0.72 Ra.3He/4He versus4He/20Ne ratios and He contents suggest that a small amount of mantle-derived helium reaches the earth’s surface. Helium iwtope ratios of the hot spring gases show a distribution pattern similar to the regional heat flow values, indicating that the positive thermal anomaly occurring in the rtrea is closely associated with the presence of mantle-derived helium. The observations described above provide some significant evidence that the molten magma originating from partial melting of the upper mantle may have intruded into the continental crust along the activity fault. The coupling of the presence of mantle He and heat flow anomalies to seismic activity in studied region seem to imply that there is an internal relationship between mantle degassing and seismic activity.

Beyond surface heat flow: An example from a tectonically active sedimentary basin

Thermal anomalies that have important geodynamic implications may not always be recognizable in present-day surface heat-flow patterns. The masking occurs because surface heat flow responds to mantle heat, crustal radioactivity, magmatism, crustal deformation, burial and/or exhumation, and fluid movement, any of which may offset the thermal effects of the others. Sedimentary basins are particularly suited to partitioning heat flow into its various components. We use Taranaki basin, New Zealand, as an example. It has a relatively undeformed (since the Miocene) western region that is used as a control against which the tectonically active eastern region can be compared. Although surface heat flow is roughly constant across Taranaki basin, basal heat flow modeled at lower crustal–upper mantle depths varies by a factor of two or more. A combination of low heat-producing crust and the heat sink effects of crustal thickening in the eastern region can account for the basal heat-flow anomalies. The tectonic thermal anomaly would have gone unnoticed without the aid of detailed basin analysis and thermal modeling.

Thermal History of the Lower and Middle Cretaceous Source Rocks in Kuwait

ABSTRACT The thermal history of Kuwait was reconstructed with Shell’s “Cauldron” basin modeling software and calibrated with maturity indicators from four wells. Maturity measurements were carried out on source rock samples from wells in the Ash-Shaham, Minagish, Raudhatain and Riqua (offshore) fields. The analytical data indicate that ‘top oil window’ is located at a depth of around 2,500 meters (8,250 feet). Due to the low structural dips of the sequences in Kuwait, thermal modeling in one-dimension was found to be adequate. The Cretaceous source rocks subsided without major anomalies in burial rate or heatflow. Modeling results indicate that the Cretaceous Sulaiy, Minagish and Zubair formations entered the oil window during the Late Cretaceous and Early Tertiary, whereas oil expulsion occurred throughout Tertiary time. Quantification of expelled oil volumes suggests that the Sulaiy Formation is the most productive source rock in Kuwait, whereas the Upper Cretaceous Burgan and Maudud formations are presently at the top of the oil window.

Indian Intraplate and Continental Margin Rifting, Lithospheric Extension, and Mantle Upwelling in Deccan Flood Basalt Volcanism near the K/T Boundary: Evidence from Mafic Dike Swarms

Mafic dikes and dike swarms associated with tholeiitic flows of the continental Deccan flood basalt province are studied to understand the timing of lithospheric extension, rifting, and rift activation related to the eruption of the Deccan basalts. Integrated geophysical, tectonic, and geochemical studies and and K-Ar ages of these mafic dikes and associated Deccan flows along the intraplate Narmada-Tapti rift and western continental margin rift point to flood basalt eruptions between 67 and 64 Ma (peaking at ~65 Ma) with concurrent lineament reactivation into rifts and dominant N-S and E-W extension of the Indian lithosphere. Correlation of Bouger gravity, shallow and deep seismic, and heat flow anomalies show penetration of elongated and discontinuous mafic bodies between 5 and 6 km deep along the central axis and flanks of the intraplate rift and a long, elongated, and anomalous mantle body into the upper crust along the continental margin rift with upward inflexion of the Moho toward the intersection of the two rifts. Crustal thinning along the rifts ranges between 8 and 25 km. Correlated geophysical and field-structural data show listric faulting and high concentrations of mafic dikes over rift-oriented geophysical anomalies. Field and geochemical relations, and age (67-64 Ma) similarities of many mafic dikes and basal flows, indicate their comagmatic nature and establish many rift-oriented mafic dikes as primary feeders. Geochemical and petrological evidence indicates that the majority of the lower Deccan tholeiites evolved in local and multiple magma chambers close to the surface up to a depth of 7 km, which is consistent with the geophysical evidence. An asthenospheric plume origin under a thinned lithosphere for the parental olivine-tholeiites is possible. Liquid line of descent calculations shown on clinopyroxene-olivine-silica and clinopyroxene-plagioclase-olivine pseudoternary plots and variation diagrams involving MgO and various oxides and elements suggest that the tholeiites of dikes and flows are related by fractional crystallization. The dikes and flows also show evidence of contamination (mainly crustal), which diminishes from the lower Deccan to the Poladpur and the Ambenali flows. Geochemical-stratigraphic relations indicate that the basal flows and dikes in Narmada-Tapti rift region and basal flows in the Gujarat region are older than the Igatpuri flows in the Western Margin rift region. A mantle plume model of laterally spreading magma with propagation of dikes from magma chambers and formation of magma chambers and eruptive centers along lithospheric weak trends is presented to explain copious and rapid eruptive activity over a widely spread area, crustal contamination during ascent, and magma evolution in shallow crustal chambers in the Deccan province.

Fracture‐induced hydrothermal convection in the oceanic crust and the interpretation of heat‐flow data

Small‐scale seafloor heat‐flow variations with a characteristic wavelength of about 1200 m have been observed on a profile over a sediment‐sealed ridge flank. Two theories have been advanced to explain them‐low aspect ratio hydrothermal convection in a highly permeable basement layer 600 m thick or high aspect ratio convection in an anisotropic permeable layer 200 m thick, induced by 20 m topographic variations with a 1000 m half wavelength. Neither theory is totally satisfactory. The first requires a very thick highly permeable zone while the absence of any resolvable basement relief on a complementary seismic survey limits the credibility of the second.We hypothesize that sub‐critical hydrothermal convection in normal upper oceanic crust, driven by discrete fractures, can cause the observed heat flow anomalies. We test the hypothesis by using a finite‐element code to solve the coupled time‐dependent heat transport and fluid flow differential equations. Discrete fractures are incorporated explicitly. We show that the inclusion of fractures in layers 2A and 2B promotes convection. Fluid flow through fractures causes horizontal thermal gradients, and initiates and maintains sub‐critical convection within the upper basalts. The predicted heat flow variations are comparable with the observed data.

Seafloor structural observations, Costa Rica Accretionary Prism

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

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

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

Basin‐scale hydrogeologic modeling

Mathematical modeling of coupled groundwater flow, heat transfer, and chemical mass transport at the sedimentary basin scale has been increasingly used by Earth scientists studying a wide range of geologic processes including the formation of excess pore pressures, infiltration‐driven metamorphism, heat flow anomalies, nuclear waste isolation, hydrothermal ore genesis, sediment diagenesis, basin tectonics, and petroleum generation and migration. These models have provided important insights into the rates and pathways of groundwater migration through basins, the relative importance of different driving mechanisms for fluid flow, and the nature of coupling between the hydraulic, thermal, chemical, and stress regimes. The mathematical descriptions of basin transport processes, the analytical and numerical solution methods employed, and the application of modeling to sedimentary basins around the world are the subject of this review paper. The special considerations made to represent coupled transport processes at the basin scale are emphasized. Future modeling efforts will probably utilize three‐dimensional descriptions of transport processes, incorporate greater information regarding natural geological heterogeneity, further explore coupled processes, and involve greater field applications.

Hydrogeothermal studies across the Pecos River Valley, southeastern New Mexico

Heat-flow values calculated along a profile in southeast New Mexico are used to study the regional ground-water flow pattern. The west-east profile extends from the Sacramento Mountains, across the southern part of the Roswell artesian basin, to the New Mexico–Texas border. The heat-flow data are generally calculated from bottom-hole temperatures estimated at various depths, and are therefore thought to reflect depth-dependent anomalies (principally ground-water flow). Two major positive heat-flow anomalies, interpreted to indicate upward ground-water flow, are present along the profile. A narrow anomaly in the western part of the profile is consistent with upward ground-water movement along the western boundary of a buried Precambrian horst. A rather broad anomaly in the middle of the profile appears to be associated with cross-formational flow and discharge to the Pecos River. Both anomalies may be associated with upward flow from depths of 915 or 1220 m and possibly 1524 m in the western region, but probably not deeper than 1829 m. Vertical ground-water flow appears to be absent just east of the Mescalero Ridge; farther east on the High Plains, the data are generally consistent with downward ground-water flow at depths of 610–1220 or 1524 m.

Correlation between seafloor heat flow and basement relief: Observational and numerical examples and implications for upper crustal permeability

There is a strong positive correlation between bathymetric and well‐navigated heat flow data from the Galapagos Mounds hydrothermal area and the east flank of the Juan de Fuca Ridge. A similar correlation is apparent at Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) site 504, where it provides a constraint on the intensity and geometry of ridge flank hydrothermal circulation. We test the implications of this correlation with a hydrogeological model of the shallow crust that contains significant permeability mainly within a few thin layers (tens of meters thick) in the upper few hundred meters of basement, consistent with observations in DSDP/ODP holes into upper oceanic crust. These simulations are compared directly to others in which the same bulk permeability is represented more homogeneously within the upper crust. A model with thin permeable zones and subtle relief beneath a flat seafloor can explain the variations in seafloor heat flow observed on one part of the Juan de Fuca Ridge east flank, as can a model with a flat seafloor and basement top but relief within the most permeable zone. We explore the importance of layered heterogeneities versus explicit permeability anisotropy in these simulations through a series of parametric tests. Some form of permeability anisotropy appears to be required in order to achieve efficient lateral heat transport within the upper volcanic crust so as to produce the common seafloor heat flow‐basement relief correlation. This permeability anisotropy may result directly from the primary architecture of the volcanic crust. Numerical results suggest that within an anisotropic system such as the upper oceanic crust, the length scale of heat flow variations (and therefore the length scale of underlying hydrothermal convection cells) is not an indication of the depth extent of fluid flow. Rather, the length scale of heat flow anomalies may reflect the length scale of relief along aquifer boundaries, while the depth of vigorous flow is constrained by the depth extent of significant permeability, and may be unresolvable with surface heat flow measurements.

Heat flow and gravity responses over salt bodies: A comparative model analysis

Two‐dimensional numerical modeling of sea‐floor heat flow and water‐bottom gravity responses to systematic variations in simple subsurface salt body geometries provides insight on the relative usefulness of these two data types for extracting salt geometry information. For a given salt body geometry, diffusion of heat through overlying sediments results in a dramatic decrease in the amplitude of heat flow anomalies as the depth to the top of the salt body increases. For top‐of‐salt depths greater than about 1 km, the heat flow response is insensitive to the length of salt feeder stocks and to the thickness of salt tongues/sheets. This shallow depth‐to‐top‐of‐salt sensitivity range, in addition to a number of environmental factors that can adversely affect interpretation of heat flow anomalies in terms of heat refraction towards and through salt bodies, severely limits the usefulness of sea‐floor heat flow data for constraining aspects of salt body geometry. For gravity data, the critical factor for addressing salt body geometry is the distribution of salt relative to the sediment‐salt density crossover depth (above and below which salt is more and less dense, respectively, than the surrounding sediment). Except when the relevant geometry information being sought (presence and/or length of feeder stock, thickness of salt tongue or sheet) is near the density crossover depth, the geometry‐related information content of the gravity field is greater than that of the heat flow field. Based on these model results, measurement uncertainty considerations, and data limitations, we conclude that gravity data generally offer an order of magnitude greater resolution capability than sea‐floor heat flow data for addressing salt body geometry issues of exploration interest.

Thermal regimes of the Southern Rocky Mountains and Wyoming Basin in Colorado and Wyoming in the United States

Heat flow in the eastern ranges of the Southern Rocky Mountains in Colorado substantially exceeds that in the Wyoming Basin-Southern Rocky Mountains area in southeastern Wyoming. The transitions between these areas are narrow (≤ 60 km wide), estimated near-surface crustal radiogenic heat productions are different, and there is no evidence for young magmatism in the easternmost mountains in northern Colorado and southern Wyoming. Therefore, radioactive heat contrasts in the upper crust are used to explain significant amounts of the heat-flow differences in these regions. In southern Wyoming, normal heat flow in Archean and Proterozoic terranes probably reflects a deeply eroded, thin (7–15 km) granitic layer that overlies low-radioactivity units in a 37–40 km thick crust. In the Colorado mountains to the south, silicic metamorphic and igneous rocks with relatively enriched radiogenic heat could comprise a 20–30 km thick granitic layer in the upper parts of the 50–52 km thick crust, and explain much of the high reduced heat flow and isostatic equilibrium. Areas of unusually high heat flow occur in the Rio Grande rift zone in the environs of the Colorado Mineral Belt in the Leadville-northern Sawatch Range region, eastern parts of the San Juan Mountains in southern Colorado, and in Park Range-Middle Mountain Park areas near the Colorado-Wyoming border. The flux in these areas implies unrealistically high equilibrium temperatures near the crust-mantle boundary, and the narrow borders (50–60 km wide) of the Leadville-northern Sawatch Range heat-flow anomaly must be caused by sources in the upper crust. Hence, young (10-1 Ma) intrusions in a late Tertiary rhyolitic complex in the upper crust are preferred to explain gravity lows, late Cenozoic uplift and igneous activity, and the excess heat flux in the Leadville-northern Sawatch Range area. If this interpretation is correct, magmatic thickening of the crust, not extensional-subsidence mechanisms, probably explains late Cenozoic uplift and extension of the northern Rio Grande rift-Southern Rocky Mountains system in Colorado.

Seafloor hydrothermal systems

The discovery of seafloor hydrothermal systems approximately two decades ago has led to a major reassessment of the Earth's thermal and geochemical budgets and has revolutionized our understanding of biological processes. This review traces the development of the study of seafloor hydrothermal systems from the indirect evidence provided by conductive heat flow anomalies to the discovery of ≈ 350°C black smoker vents on the East Pacific Rise at 21°N. Although the review focuses on physical characteristics and processes, it outlines some key characteristics of vent fluid chemistry that provide constraints on physical models. Ridge crest systems have thermal power outputs ranging from 10 to 104 MW. They are transient systems, driven by magmatic heat sources, but episodic events such as megaplumes, the interplay between focused and diffuse venting, and other aspects related to their thermal, chemical, and biological evolution remain poorly understood. Advances will be made by continuing exploration and discovery to determine the full range of possible phenomena both on and off axis and in different tectonic settings. In order to understand the complete, integrated ridge system, however, future studies must include long‐term monitoring of an active system, deep drilling into the reaction zone, and mathematical modeling that incorporates both physical and chemical constraints.

Heat flow in the Kenya rift zone

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

Seismicity in the Hellenic Volcanic Arc: relation between seismic parameters and the geophysical fields in the region

SUMMARY A study of the spatial distribution of seismicity and seismic hazard and geophysical parameters is undertaken along the Hellenic Volcanic Arc (HVA). Seismicity of the area is divided in a cellular manner allowing analysis of the localized seismicity parameters and representation of their regional variation as contour maps. Extreme value analysis locates a zone of highest seismicity just west of centre of the HVA, around the Cyclades Islands. Non-probabilistic modelling of the strain energy release process in terms of magnitudes equivalent to the average annual energy release rate (M,) and the maximum possible energy release in a seismic cycle (M3) enhances this picture. Maximum values of M, and M3 are more tightly localized in the central Cyclades west of Amorgos Island and also in the east of the HVA in the Sporadhes north-west of Rhodes. Isocyclic contours of the waiting times for M3 show values in excess of 70 years in the Cyclades and eastern Sporadhes, i.e. the zones of largest earthquakes show the longest strain accumulation periods. The waiting time for the HVA as a whole is about 54 years, and at present it appears to be about 75 per cent through to accumulation of the notional maximum strain energy. The b value of the magnitude-frequency distribution shows a consistent spatial variation along the HVA which is not sensitive to subdivision into different focal depth ranges. Regional contour maps of the b values, and maps of local values of heat flow, magnetic and electrical anomalies compared with the Bouguer gravity anomaly map, indicate correlations which reflect the deformation of the Aegean shallow crust in relation to the deep intrusive magmatic processes. The correlations between the different geophysical fields are displaced commensurate with an Aegean extension of approximately 45 km.

Model studies of advective heat flow associated with compaction and dehydration in accretionary prisms

Surface heat flow across the deformation front of subduction zones shows complex patterns, and fluid flow is often invoked to interpret these patterns. Here we estimate the maximum possible amount of fluid flow due to sediment compaction and dehydration in subduction zones, and compare these estimates with the required fluid flow to produce significant thermal disturbances across faults and vents in two‐ and three‐dimensions. The results show that lateral heat loss becomes significant for faults or vents with thickness or diameter less than about 1 km. For two‐dimensional channel flow in fault zones or permeable strata, subduction‐induced compaction alone can hardly provide enough steady state flow to cause surface heat flow anomalies such as that observed in the Barbados accretionary prism [Foucher et al., 1990; Langseth et al., 1990], Fluid flow in three‐dimensional pipes (vents or vent fields) of a few hundred meters to ∼1 km in diameter is efficient in producing thermal disturbances; but steady state flow in small isolated pipes of a few meters in diameter is incapable of producing measurable heat flow anomalies. Small‐scale heat flow anomalies which are observed in some accretionary prisms are probably best explained by intermittent, transient flow.

Preliminary report on an approach to testing for normality

The report is the introductory step to our investigation on joint probability distributions of crustal movements and geological parameters (basement depth, heat flow, seismicity and gravity anomalies) which will be carried out in the near future. Because of the remarkable role of the normal distribution, the working out of procedures testing for normality has been a continuing research subject of statisticians. Recently, Shapiro and Wilk (1965) and Shapiro et al. (1968) proposed a new test statistic and carried out a Monte Carlo comparative study with the known test statistics, and the result indicates superiority of their procedure from the test power point of view. The present report considers an approach to testing for normality which is related to the Shapiro-Wilk procedure. Also, the closed form of the test statistic is given. A brief historical outline of both the normal distribution theory and testing procedures is presented, making it of more methodological value, since the normal distribution is often involved in geodesy.

Heat flow from four new research drill holes in the Western Cascades, Oregon, U.S.A.

Conceptual models of the thermal structure of the Oregon Cascade Range propose either (1) a narrow zone of magmatic heat sources, flanked by shallow heat-flow anomalies caused by lateral ground-water flow; or (2) a wide zone of magmatic heat sources, with localized, generally negligible ground-water effects. The proposed narrow heat source coincides with the Quaternary volcanic arc, whereas the wider heat source would extend 10–30 km west of the arc. To test the models, four new heat-flow holes were sited west of the Quaternary arc but within the area of the proposed wide heat source. The sites are separated from the Quaternary arc by topographic divides and (or) major river valleys, so that heating by regional-scale ground-water flow seems unlikely. Measured heat flow (76±5 mW mm −2) was significantly lower than the values predicted by interpolation from an existing heat-flow contour map (95±7 mW m −2). The lower values are consistent with a narrow zone of magmatic heat sources. The complete heat-flow data set consists mostly of shallow (100–200 m) data and defines lobate highs around hot-spring groups in the Western Cascades. However, all of the deepest holes in the study area (44°–45°15′N) show hydrologic gradient disturbances extending to depths >200 m, so the shallow data must be extrapolated with caution.

P wave tomographic imaging of the crust and upper mantle beneath the Japan Islands

More than 50,000 high‐accuracy arrival times from 1200 shallow and deep earthquakes selected from the Japan University Network Earthquake Catalog are used to investigate the three‐dimensional (3‐D) seismic velocity structure beneath Japan. Travel times and ray paths are accurately calculated by using an efficient 3‐D ray‐tracing technique. P wave tomographic images of the crust and upper mantle to a depth of 500 km beneath the Japan Islands are determined with spatial resolutions of 25–50 km. The obtained P wave velocity in the crust and uppermost mantle is slightly higher and relatively homogeneous in western Japan, in contrast with an average lower velocity and a large heterogeneity in eastern Japan where many active volcanoes exist. This feature correlates well with the distributions of heat flow and gravity anomalies. Low‐velocity (low‐V) zones are imaged in the crust and uppermost mantle beneath active volcanoes. The low‐V zones in the mantle wedge dip toward the west and extend continuously to a depth of about 200 km. A high‐velocity (high‐V) zone corresponding to the subducted Pacific plate is delineated. The thickness of the high‐V zone is estimated to be 80–90 km. In southwestern Japan, the descending Philippine Sea plate is also delineated; its thickness is estimated to be 30 km.

Salt Geometry Constrained by Conductive Heat Flow Anomalies

Salt Geometry Constrained by Conductive Heat Flow Anomalies