Increase In Geothermal Gradient Research Articles

Crustal processes sustain Arctic abiotic gas hydrate and fluid flow systems

The Svyatogor Ridge and surroundings, located on the sediment-covered western flank of the Northern Knipovich Ridge, host extensive gas hydrate and related fluid flow systems. The fluid flow system here manifests in the upper sedimentary sequence as gas hydrates and free gas, indicated by bottom simulating reflections (BSRs) and amplitude anomalies. Using 2D seismic lines and bathymetric data, we map tectonic features such as faults, crustal highs, and indicators of fluid flow processes. Results indicate a strong correlation between crustal faults, crustal highs and fluid accumulations in the overlying sediments, as well as an increase in geothermal gradient over crustal faults. We conclude here that gas generated during the serpentinization of exhumed mantle rocks drive the extensive occurrence of gas hydrate and fluid flow systems in the region and transform faults act as an additional major pathway for fluid circulation.

The initiation and tectonic regimes of the Cenozoic extension in the Bohai Bay Basin, North China revealed by numerical modelling

In this study the dynamic aspects of the Cenozoic extension in the Bohai Bay Basin are considered in the context of initial thickness of the crust and lithosphere, tectonic force, strain rate and thermal rheology, which are directly or indirectly estimated from a pure shear extensional model. It is accordingly reasonable to expect that, in the Bohai Bay Basin, the thickness variation could be present prior to the initiation of extension. The extensional deformation is localized by a thickness variation of the crust and lithosphere and the heterogeneity of the initial thickness plays an important role in rifting dynamics. The onset of rifting requires a critical tectonic force (initial tectonic force) to be applied, which then immediately begins to decay gradually. Rifting will only occur when the total effective buoyancy force of the subducting slab reaches a critical level, after a certain amount of subduction taking place. The magnitude of the tectonic force decreases with time in the early phase of rifting, which indicates the weakening due to the increase in geothermal gradient. In order to deform the continental lithosphere within the currently accepted maximum magnitude of the force derived from subducted slab roll-back, the following conditions should be satisfied: (1) the thickness of the continental lithosphere is significantly thin and less than 125km and (2) the lithosphere has a wet and hot rheology, which provides implications for rheological layering in continental lithosphere. Our results are strongly supported by the “crème brûlée” model, in which the lower crust and mantle are relatively ductile.

Effect of thermal non-equilibrium, seafloor topography and fluid advection on BSR-derived geothermal gradient

The seafloor and bottom simulating reflectors (BSRs) are interpreted from the 3D seismic data acquired in Krishna–Godavari (KG) offshore basin in the vicinity of sites drilled/cored during National Gas Hydrate Program (NGHP) Expedition-01. The shallow structures such as inner toe-thrust fault system, regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles. The geothermal gradient is estimated from the depths and temperatures of the seafloor and the BSR. The temperature at the BSR depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the BSR represents the base of the gas hydrate stability zone.The spatial variations in geothermal gradient (GTG) show a strong correlation with seafloor topography in the KG basin. The GTG decreases by ∼13–30% over the topographic mounds formed due to inner toe-thrust faults and recent mass transport deposits. The GTG decreases by only 5–10% over the mounds, likely due to defocusing of heat flux based on one-dimensional topographic modeling. Hence, the GTG perturbation due to topography alone cannot explain the observed GTG anomaly. The temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe-thrust faults or rapid deposition of sediments due to slumping/sliding. In contrast, an increase in GTG by 10–15% is observed in the vicinity of major fault systems. We presume that the likely mechanism for the increase in GTG is fluid advection from a deeper part of the basin. A detailed thermal modeling involving the effect of surface topography, high sedimentation rates, fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in KG offshore basin.

Design and optimization of a downhole coaxial heat exchanger for an enhanced geothermal system (EGS)

The present study considers the design, performance analysis and optimization of a downhole coaxial heat exchanger for an enhanced geothermal system (EGS). The optimum mass flow rate of the geothermal fluid for minimum pumping power and maximum extracted heat energy was determined. In addition, the coaxial pipes of the downhole heat exchanger were sized based on the optimum geothermal mass flow rate and steady-state operation. Transient effect or time-dependent cooling of the Earth underground, and the optimum amount and size of perforations at the inner pipe entrance region to regulate the flow of the geothermal fluid were disregarded to simplify the analysis. The paper consists of an analytical and numerical thermodynamic optimization of a downhole coaxial heat exchanger used to extract the maximum possible energy from the Earth's deep underground (2 km and deeper below the surface) for direct usage, and subject to a nearly linear increase in geothermal gradient with depth. The thermodynamic optimization process and entropy generation minimization (EGM) analysis were performed to minimize heat transfer and fluid friction irreversibilities. An optimum diameter ratio of the coaxial pipes for minimum pressure drop in both limits of the fully turbulent and laminar fully-developed flow regime was determined and observed to be nearly the same irrespective of the flow regime. Furthermore, an optimum geothermal mass flow rate and an optimum geometry of the downhole coaxial heat exchanger were determined for maximum net power output. Conducting an energetic and exergetic analysis to evaluate the performance of binary power cycle, higher Earth's temperature gradient and lower geofluid rejection temperatures were observed to yield maximum first- and second-law efficiencies.

Some analytical methods for converting thermochronometric age to erosion rate

An analytical method is presented for converting thermochronometric ages to surface erosion or, equivalently, exhumation rate. The method incorporates the two most important thermal processes during cooling by erosion: the dependence of closure temperature on cooling rate and the advection of heat by rock motion toward the Earth's surface. Two thermal models are considered: (1) a steady state model, valid for low erosion rates; and (2) an eroding half‐space model, which has no steady state, but captures the transient increase of geothermal gradient with erosion. In each case, it is assumed that data consist of one or more thermochronometric ages, present‐day surface geothermal gradient, and topographic information including the elevation at which the age was obtained. Analytical solutions are provided to derive the erosion rate from these data either as an explicit expression for the steady case or as a root‐finding problem for the transient case. A graphical method for plotting age against erosion rate and geothermal gradient is presented as a method for solving the root finding problem and for tracking analytical errors in observations of age and surface geothermal gradient. The graphical method is also appropriate for comparing data from different elevations or from different thermochronometric systems. Examples are provided using synthetic data or published data from the literature.

Fault system and thermal regime in the vicinity of site NGHP-01-10, Krishna–Godavari basin, Bay of Bengal

Drilling/coring activities onboard JOIDES Resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of Krishna-Godavari (KG) basin, Bay of Bengal and the expedition recovered fracture filled gas hydrate at the site NGHP-01-10. In this paper we analyze high resolution multi-channel seismic (MCS), high resolution sparker (HRS), bathymetry, and sub-bottom profiler data in the vicinity of site NGHP-01-10 to understand the fault system and thermal regime. We interpreted the large-scale fault system (>5 km) predominantly oriented in NNW-SSE direction near NGHP-01-10 site, which plays an important role in gas hydrate formation and its distribution. The increase in interval velocity from the baseline velocity of 1600 m/s to 1750–1800 m/s within the gas hydrate stability zone (GHSZ) is considered as a proxy for the gas hydrate occurrence, whereas the drop in interval velocity to 1400 m/s suggest the presence of free gas below the GHSZ. The analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large-scale fault system. We conclude that the gas hydrate concentration near site NGHP-01-10, and likely in the entire KG Basin, is controlled primarily by the faults and therefore has high spatial variability. We also estimated the heat flow and geothermal gradient (GTG) in the vicinity of NGHP-01-10 site using depth and temperature of the seafloor and the BSR. We observed an abnormal GTG increase from 38 °C/km to 45 °C/km at the top of the mound, which remarkably agrees with the measured temperature gradient at the mound (NGHP-01-10) and away from the mound (NGHP-01-03). We analyze various geological scenarios such as topography, salinity, thermal non-equilibrium of BSR and fluid/gas advection along the fault system to explain the observed increase in GTG. The geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in GTG. The approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm/yr (0.37–0.6 mm/yr).

Back-arc rifting initiated with a hot and wet continental lithosphere

How back-arc rifting is initiated is studied by inferring the tectonic force and strain rate regimes from published subsidence data for nine back-arc basins in the European Tethyan belt and in the western Pacific. The results show that rifting only begins after some particular magnitude of tectonic force is reached. This implies that subduction has already progressed to a point such that the total negative buoyancy of the slab becomes so large that the relevant tectonic force exceeds its bending strength. Once back-arc rifting has been initiated, the force required to maintain it gradually decays. This indicates that the processes governing back-arc extension are fast enough that lithosphere weakening due to an increase in geothermal gradient exceeds strengthening due to the thermal relaxation and crustal thinning. This is achieved, given currently accepted estimates for the maximum force (≤7TN/m), if the thickness of the thermal lithosphere is significantly less than 125km and if the lithosphere has a wet rheology, meaning that a weak continental lithosphere is required for the initiation of back-arc rifting. The presence of significant strength in the uppermost mantle is not precluded, but it is not required.

The b-spacing values of white micas and their metamorphic implications in the Lesser Himalaya, central Nepal

The Lesser Himalaya in central Nepal exposes a very wide section (>80 km) of very low- to low-grade metamorphic rocks. Metamorphic data from these rocks are relatively rare because of fine-grained nature of the rocks and lack of suitable assemblage for themobarometric calculations. In the present study, b-spacing measurements and compositional analyses were carried out on white micas in very low- to low-grade metapelites collected along the Kali Gandaki river valley and Tansen–Pokhara road sections to determine evidence of polymetamorphism and P–T conditions of metamorphism. The study showed that the Lesser Himalaya experienced at least two metamorphic events, one prior to the Upper Main Central Thrust (Upper MCT) activity (pre-Himalayan? or M 0) and the other during the Upper MCT activity (Neohimalayan or M 2). The M 0 produced celadonite-rich white micas defining the S 1(=S 0) foliation which pre-dates the Upper MCT event. The M 0 white micas were partially or completely re-equilibrated during M 2. The white micas recrystallized during M 2 are relatively celadonite-poor and define S 2 foliation. The M 2 was of an intermediate-P type (Barrovian-type) in the south of the Phalebas Thrust while it was of relatively high-T type in the north near the Upper MCT. The geothermal gradient varied from 22 °C km −1 in the south to 26 °C km −1 in the north. The northward increase in geothermal gradient is thought to be due to the heat transfer from hot Higher Himalaya or shear heating along the Upper MCT as suggested by Le Fort [Le Fort, P., 1975. Himalaya, the collided range: present knowledge of the continental arc. American Journal of Science 275A, 1–44. Arita, K., 1983. Origin of the inverted metamorphism of the Lower Himalayas, central Nepal. Tectonophysics 95, 43–60].

Mineral inclusions in diamonds from the Panda kimberlite, Slave Province, Canada

Ninety diamonds from the Eocene Panda kimberlite (Ekati Mine, Northwest Territories, Canada) were analyzed for the major and trace element compositions of their mineral inclusions using electron microprobe techniques (EPMA) and secondary ion mass spectrometry (SIMS). Additionally, nitrogen aggregation characteristics of the host diamonds were measured using Fourier-transform infrared spectroscopy (FTIRS). Within the cratonic lithosphere, Panda diamonds are principally derived from peridotitic sources (85 %) with a minor content of eclogitic diamonds (10 %). Ferropericlase bearing diamonds (5 %) contain combinations of ferropericlase with either Mg-Al spinel plus olivine or with olivine or with a pure silica phase. The chemical char- acteristics of these inclusions are in accordance with a lithospheric origin from ferropericlase-bearing dunites. Ferropericlase coexisting with CaSiO3 (most likely originally included as Ca-silicate perovskite), however, is regarded as evidence for a lower mantle origin of the host diamond. Major element compositions show that the peridotitic diamonds formed in a moderately depleted environment, indicated by the presence of harzburgitic garnet inclusions with calcium contents generally > 2.5 wt% CaO and olivines with Mg numbers (100*Mg/(Mg+Fe)) of 92-93.5. Rare earth element (REE) concentrations in peridotitic garnets largely follow subdivisions based on major elements with lherzolitic garnets showing middle REE to heavy REE enriched, slightly sinusoidal patterns, whilst harzburgitic garnets have distinctly sinusoidal REEN. Inclusion geothermobarometry indicates formation of peridotitic diamonds in the temperature range 1100-1250°C, following a geothermal gradient of 40-42 mW/m2, in accordance with similar observations world-wide. Touching garnet-olivine and garnet-orthopyroxene inclusion pairs equilibrated at lower temperatures of 1000-1100°C, corresponding to a geothermal gradient around 37 mW/m2. The higher temperatures are considered to be those prevailing during diamond formation. Nitrogen contents in Panda diamonds vary strongly from below detection (< 10 ppm) to 2700 atomic ppm. Nitrogen aggrega- tion ranges from poorly aggregated (Type IaA diamond) to highly aggregated (Type IaB diamond). If all diamonds that show signs of plastic deformation during mantle residence are excluded from the dataset, then a diamond subset becomes apparent with an overall low nitrogen aggregation state of < 30 % B-center. This result may indicate that plastic deformation increases the aggre- gation of nitrogen in Panda diamonds. Taking the Early Archean Re-Os isochron date for sulfide inclusions in Panda diamonds (Westerlund et al., 2003b) at face value, the low aggregation states of undeformed diamonds may indicate mantle residence at rela- tively low temperatures (< 1100°C). If this is the case, the decrease in temperature inferred from the comparison of touching and non-touching inclusion pairs must have occurred soon after diamond formation. Thus diamond formation beneath the central Slave may be restricted to short lived and localized thermal events. An apparent increase in geothermal gradient with depth in the litho- spheric mantle beneath the Central Slave for the time of kimberlite eruptions (Upper Cretaceous to Eocene) may have a similar cause and reflect transient heating of the deep lithosphere during melt infiltration.

New heat flow data from the immediate vicinity of the Kola super-deep borehole: Vertical variation in heat flow confirmed and attributed to advection

We present new heat flow values and other geothermal data in the upper crystalline crust in the immediate vicinity of the 12.4-km deep Kola super-deep borehole, NW Russia. Our results show a systematic vertical increase in geothermal gradient and heat flow density as deep as we could measure (1.6 km). Our results confirm earlier results on vertical heat flow trends of in the uppermost part of the Kola super-deep hole, and imply that the thermal regime is not in steady-state conductive conditions. In an area of 3-km × 5-km measurements were performed in 1–2-km deep boreholes surrounding the Kola super-deep hole and on core samples from these holes. Temperature logs are available from 36 holes. Core data exists from 23 boreholes with a total length of 11.5 km at a vertical resolution of 10 m. We carried out a very detailed study on thermal conductivity with regard to anisotropy, inhomogeneity and temperature dependence. Tensor components of thermal conductivity were determined on 1375 core samples from 21 boreholes in 3400 measurements. Additionally, we measured specific heat capacity, heat generation rate, density, porosity, and permeability on selected subsets of core samples. Heat flow from 19 boreholes varies between 31 and 45 mW m −2 with an average value of 38 mW m −2. In most boreholes the vertical heat flow profiles show a considerable variation with depth. This is consistent with observations in the upper part of the Kola super-deep borehole. We conclude that this variation is not caused by technical operations but reflects a natural process. It is considered to be due to a combination of advective, structural and paleoclimatic effects. Preliminary 3-D numerical modeling of heat and flow in the study area provides an indication of relative contributions of each of these factors: advective heat transfer turns out to have a major influence on the vertical variation of heat flow, although transient changes in surface temperature may also cause a significant variation. Heterogeneity of the rocks in the study area is less important.

Heat transport analysis and three‐dimensional thermo‐hydraulic simulation in the Ishikari basin, Hokkaido, Japan

AbstractTemperature profiles of 22 observation boreholes in the Ishikari basin were investigated in order to extract information on the ground thermal processes and the associated water flow disturbances. Data analyses methods were based on temperature profile typology, thermal gradient–temperature plots, and a thermo‐hydraulic model. The distributed data sets of measured temperature profiles and thermal conductivity show lateral heat flow variations from 34 to 162 mW m−2. These thermal conditions revealed the co‐existence of a highland geothermal field, a lowland central subnormal heat flow area, and a strong heat flow anomaly near the northeast Ishikari River. To explain this specific configuration of the thermal field, possible effects of petro‐physical properties, topography and groundwater movements were examined with a three‐dimensional thermo‐hydraulic simulation model. Laboratory measurements of thermal conductivity, porosity, density, and permeability from borehole core samples were used to constrain the model parameters. The main results suggest that: the lateral increase of geothermal gradient and heat flow density with elevation, i.e. in the southwest direction, are essentially due to hydro‐ and thermo‐physical differences between the hydrogeologically active Quaternary sedimentary system and the underlying Tertiary volcano–sedimentary formation; the low heat area in the central lowland may be explained by the effect of its local groundwater flow recharge, and then the positive heat anomaly near the Ishikari River would result from a basin‐wide heat flow redistribution by water flow. In general, the thermal regime in the Ishikari basin appears dominated by conduction processes in the southwest area and strongly influenced by advection phenomena in the northeast. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Implications of middle Eocene epizonal plutonism for the unroofing history of the Bitterroot metamorphic core complex, Idaho-Montana

Petrologic and geochronologic limits on the progression of east-directed extensional unroofing of the Bitterroot metamorphic core complex provide a means to compare rates of denudation across the footwall during the earlystages of extensional denudation. U-Pb and 4 0 Ar/ 3 9 Ar data from epizonal plutons in the northwestern and southwestern Bitterroot metamorphic core complex demonstrate that much of the western footwall was exhumed to shallow levels (1-5 km) by ca. 49 Ma, while eastern footwall rocks remained at high temperatures following an episode of isothermal decompression. U-Pb ages of epizonal plutons along the western margins of the footwall suggest that as much as 13 km of unroofing occurred between anatectic melting of Belt Supergroup metasedimentary rocks in the western footwall no later than 56 Ma and emplacement of the adjacent epizonal Lolo Hot Springs batholith at ca. 49 Ma in the northwestern Bitterroot Range. Tectonic denudation during this interval removed roughly the same amount of overburden from eastern footwall rocks, suggesting that the initial stages of unroofing may have been accommodated by a planar, moderately east-dipping (30°-50°) shear zone. 4 0 Ar/ 3 9 Ar cooling data show that cooling in the western footwall was synchronous with unroofing. In contrast, we suggest that a significant delay in cooling of eastern footwall rocks may have resulted from a slight increase in geothermal gradient following this unroofing stage. Exposure of a west-dipping mylonitic shear zone along the western limb of the Bitterroot complex may provide a key for interpreting the postmylonitic structural evolution of the core complex. If this shear zone can be correlated to the Bitterroot mylonite zone to the east, then strong evidence for flexural uplift consistent with postmylonitic passage of a rolling hinge emerges.

Subsurface temperature variations and heat flow in the Anambra Basin, Nigeria

Data from sixteen deep walls drilled for oil exploration purposes in the Anambra Basin of southeastern Nigeria indicate large variations in temperature gradients and heat flow within the basin. Geothermal gradients vary between 25 and 49 ± 1°C km −1, while heat flow estimates are in the range 48 to 76 ± 3 mW m −2. The highest geothermal gradients and heat flow values were computed for the wells located in the southwestern part of the basin north of Onitsha and Asaba. This part of the basin coincides with zones of thick, low conductivity sediments, low ground surface elevation, and hydraulic discharge zones. The general direction of increase in geothermal gradient, originally projected as south to north by earlier workers dealing with the Niger Delta data and the very limited well data from the Anambra Basin, is inconsistent with the results of the present study. The distribution of subsurface temperatures, geothermal gradients and heat flow is found to be directly related to the basin hydrodynamics - higher geothermal gradients and heat flow in areas of low hydraulic head distribution. Hydrocarbon metamorphism and migration appear to have been greatly influenced by the movements of circulating meteoric waters. A higher level of organic maturity of sediments should be expected in the southwestern zone, where the thermal anomaly exists. However, owing to hydrodynamic activities, tertiary migration would have taken place leaving many traces of residual hydrocarbons. The several cases of fluorescence noticed in wells in the southwestern zone of the Anambra Basin are taken as evidence that this process may indeed have taken place in the geological past of the basin.

The origin and genesis of major Jurassic unconformities within the triple junction area of the North Sea, UK

Three Jurassic unconformities of intra-Aallenian, intra-Callovian and intra-middle Oxfordian age are consistently identified within the literature for the North Sea region but despite their common recognition these unconformities remain poorly understood and they are essentially spatially and temporally unconstrained. The recent development of a sequence stratigraphic framework for the Jurassic of the North Sea allows for a thorough investigation of the spatial and temporal characteristics of the intra-Callovian and intra-Oxfordian unconformities and these data have important implications for Middle and Late Jurassic rifting within the North Sea region. The intra-Aalenian unconformity is believed to have been generated as a result of an increase in geothermal gradient within the area and subsequent thermal uplift that preceded rifting. The stratigraphic gap associated with this unconformity increases towards the proposed location of a Middle Jurassic, triple junction centred dome. In contrast the stratigraphic break associated with the intra-Callovian and the intra-middle Oxfordian unconformities is found to increase onto what are now intra-rift highs and rift margin areas. These unconformities occur at the same time as distinct changes in the geometry of the fill of the triple junction and also occur at the same time as seismic scale extension. They do not appear to correlate to any supposed global fall in relative sea-level. Stratigraphic data indicate that three phases of extension occurred during the Callovian, middle and late Oxfordian and lastly during the Kimmeridgian and Volgian. Although the intra-Callovian and intra-middle Oxfordian unconformities can be confused with the effects of the Middle Jurassic North Sea dome unconformity, they are in fact the result of extension during the initial Callovian and Middle/late Oxfordian rift phases. The recognition of these regional unconformities suggests that the Middle and Late Jurassic rifting process was punctuated rather than being continuous.

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

Glacial loading and unloading: a possible cause of rock salt dissolution in the Western Canada Basin

Each of the five main bedded Devonian rock salts in the Western Canada Basin has been leached in places more-or-less continuously since shortly after deposition. For the most part, salt dissolution has been self-sustaining and relatively slow. At various times however, leaching has been significantly accelerated by large-scale external processes such as regional uplift and erosion (during the pre-Cretaceous hiatus), and regional faulting (during the mid-Late Cretaceous). The most recent phase of accelerated leaching occurred during the Quaternary, probably as a result of glacial loading and unloading. Two principal lines of evidence support the thesis that accelerated rates of salt dissolution occurred during the Quaternary. First, there is an apparent correlation between surface drainage patterns and the active dissolution margins of the Devonian rock salts. A significant percentage of the lakes and rivers in proximity to active salt dissolution margins are situated along the near-zero edge of the respective salt bodies. Second, salt-related subsidence features at the pre-Quaternary subcrop level have been mapped on seismic data in the immediate proximity of active salt dissolution margins. The Quaternary subsidence features associated with the Prairie Formation rock salt for example, parallel the active salt margin and can be up to several kilometers wide. The areal extent of these shallow subsidence features and the high volume of rock salt dissovled at these sites suggest that accelerated rates of leaching occurred during the Quaternary (relative to the average rate of dissolution throughout the Cretaceous). The magnitude of these shallow structural features demonstrates clearly that subsidence associated with Recent salt dissolution can influence surficial drainage patterns to the extent suggested by the distribution of the lakes and rivers in proximity to the salt dissolution margins. The accelerated rates of salt dissolution during the Quaternary are attributed to glacial loading and unloading, and associated processes. Glacial loading could result in the compaction of subsurface strata, an increase in geothermal gradient, the thermal expansion of the deep aquifers, and the expulsion of interstitial water. These effects could increase both the rate of centrifugal fluid flow within a basin and the rate of salt dissolution. Glacial unloading could also affect the rate of salt dissolution. This rapid decrease in applied load is manifested as glacial rebound, a process which could increase the porosity and fracture permeability of the subsurface, and decrease the geothermal gradient. Potentially, the rate of centripetal fluid flow (at least around the basin margins) could be increased. Increased centripetal fluid flow, accompanied by an influx of understurated glacial melt waters would enhance the rate of salt dissolution.

Thermo‐tectonic history of Ryoke Basement in Hohi volcanic zone, northeast Kyushu, Japan: Constraints from fission track thermochronology

Abstract Apatite and zircon fission track ages from Ryoke Belt basement in northeast Kyushu show late Cretaceous, middle to late Eocene, middle Miocene and Quaternary groupings. The basement cooled through 240 ± 25°C, the closure temperature for fission tracks in zircon, mainly during the interval 74‐90 Ma as a result of uplift and denudation, the pattern being uniform across northeast Kyushu. In combination with published K‐Ar ages and the Turonian‐Santonian age of sedimentation in the Onogawa Basin, active suturing along the Median Tectonic Line from 100‐80 Ma, at least, is inferred. Ryoke Belt rocks along the northern margin of Hohi volcanic zone (HVZ) cooled rapidly through ∼100°C to less than 50°C during the middle Eocene to Oligocene, associated with 2.5‐3.5 km of denudation. The timing of this cooling follows peak heating in the Eocene‐Oligocene part (Murotohanto subbelt) of the Shimanto Belt in Muroto Peninsula (Shikoku) inferred previously, and coincides with the 43 Ma change in convergence direction of the Pacific‐Eurasian plate and the demise of the Kula‐Pacific spreading centre. Ryoke Belt rocks along the southern margin of HVZ have weighted mean apatite fission track ages of 15.3 ± 3.1 Ma. These reset ages are attributed to an increase in geothermal gradient in the middle Miocene combined with rapid denudation and uplift of at least 1.4 km. These ages indicate that heating of the overriding plate associated with the middle Miocene start of subduction of hot Shikoku Basin lithosphere extended into the Ryoke Belt in northeast Kyushu. Pleistocene apatite fission track ages from Ryoke Belt granites at depth in the centre of HVZ are due to modern annealing in a geothermal environment.

Tourmalinites from the Golden Dyke Dome, Northern Australia

Tourmalinites occur at five stratigraphic levels within the low metamorphic grade multiply deformed Lower Proterozoic metasediments and metavolcanics of the Golden Dyke Dome. The geological setting, exhalite associations, sedimentary structures and tourmaline chemistry all preclude a granitic origin. Tourmalinite derived from the isochemical metamorphism of an exhalite comprising silica and tourmaline. Tourmalinite and other exhalites are hosted by carbonaceous pelitic metasediments and were deposited in deeper basinal areas. The ore fluid was probably dolerite-heated seawater which leached a thick pile of argillaceous sediments and acquired metals and boron by leaching. The first and second increase in geothermal gradient produced tourmalinite, the third produced auriferous tourmalinite and iron formations with tourmalinite and exhalative Pb-Zn deposits forming during the period of maximum geothermal gradient. More tourmalinite formed during the subsequent decline in geothermal gradient. Tourmalinites are regarded as B-rich iron formations which can be the host for or associated with exhalative deposits of Au and base metals.

Continental lithosphere strength: the critical role of lower crustal deformation

Summary A thermo-rheological model of lithosphere deformation, incorporating the elastic, ductile and brittle behaviour of lithosphere material, has been used to examine intraplate continental lithosphere strength, brittle-ductile transition depth and flexural rigidity. These parameters are critically dependent on crust and mantle rheology and consequently on geothermal gradient, crustal thickness and lower crustal composition. For lithosphere subjected to a lateral tectonic force, creep in the lower crust and mantle leads to stress release, and the subsequent stress redistribution generates stresses in the upper lithosphere sufficient to cause brittle fracture. The extent of creep in the lower crust and mantle and the degree of upper lithosphere stress amplification (which together determine bulk lithosphere strength) increase with geothermal gradient. For significant lithosphere extension, under maximum likely levels of available tectonic stress, a lithosphere surface heat flow of 60 mW m −2 or greater is required, while for compressive lithosphere deformation, heat flow must exceed 75 mW m −2 . Similarly flexural rigidity increases with increase in the thermal age of the lithosphere at the time of loading. The depth of the brittle-ductile transition decreases with increase in geothermal gradient. For a limited range of gradients (expressed by heat flow q = 50–55 mW m −2 ) multiple brittle-ductile transitions may exist in the middle and lower crust and upper mantle, with important tectonic implications (e.g. for intra-crustal detachments and crust-mantle decoupling). Lithosphere strength in extension and compression, and flexural rigidity, are both controlled by the quartzo-feldspathic rheology of the crust for thermally young lithosphere and by the olivine rheology of the mantle for older lithosphere. Lithosphere strength is therefore critically influenced by the thickness of the crust (decreasing with increase in crustal thickness) and by the composition of the lower crust, particularly for lithosphere with intermediate heat flows.

Variations in Gulf Coast Heat Flow Created by Ground-Water Flow: ABSTRACT

Two major regional trends in Gulf Coast heat flow have been noted: a decrease in temperature at given depths basinward and an increase in geothermal gradient with depth. These trends are complicated by local heat flow anomalies. Although basement heat input may have increased during a Mesozoic rifting episode, it has probably remained essentially constant since the Late Jurassic. Therefore, the regional differences in heat flow must be the result of intrinsic sediment properties, sediment deposition, and fluids moving through the sediments. Numerical modeling indicates that the major regional trends can be explained by a moving-boundary effect caused by continuous, long-term sediment deposition, coupled with forced convection caused by the compaction of the sediments. Dep sition of cool sediments depresses the isotherms until the sediments can be heated to normal levels. Marginward heat advection due to lateral and upward movement of fluids through the more permeable sediments also contributes to the regional trend of higher temperatures in the older, inland sediments. Local perturbations (abnormally high temperatures) are caused by fluid movement focused along fault zones or flanks of salt domes. Observed thermal anomalies near salt domes may be caused primarily by fluid movement, not by the greater thermal conductivity of salt. Finally, theoretical and field evidence indicates the possible occurrence of free convection in the Gulf Coast basin, which may be an important factor in the heat flux on both a regional and a local scale. End_of_Article - Last_Page 1418------------