Surface Heat Flow Pattern Research Articles

Gravity field and geothermal structure of the Corsica‐Sardinia Block

AbstractThis paper presents a finite‐differences 3D numerical model that simulates the gravity and thermal structure of the Corsica‐Sardinia Block (CSB), an apparently stable lithospheric domain characterized by cryptic tectonic activity. In the experiments, we change the density and heat production rate of the model crust within a range of geologically realistic values to fit the measured Bouguer gravity anomaly and surface heat flow pattern. The discrepancy between the observed geophysical structure and the outcomes of numerical modelling are discussed in relation to the composition of the CSB crust and finally recast in the geodynamic framework of the western Mediterranean region.

Transient Measurement of Temperature Distribution Using Thin Film Thermocouple Array on Turbine Blade Surface

Smart sensors are needed to achieve a more intelligent aero-engine. The thin-film sensor array is thereby fabricated on the turbine blade surface using MEMS technology to measure the surface temperature distribution for the first time. A series of PtRh/Pt thin film thermocouple (TFTC) is directly integrated in-situ on blade surface by dry film photolithography and magnetron sputtering technology. A series of high temperature experiments are performed to test its annealing effect, stability and repeatability, responsiveness, and temperature distribution. The experimental results show that after annealing at 800 °C for 1 hour, TFTCs exhibit good stability and consistency. This TFTC array can capture the temperature distribution caused by the heat flame on the blade surface, and the transient temperature responsiveness can reach at least 10 μs under a dynamic heat flow. The TFTC is only 1 μm thick thin film coated on the blade surface and the thin film routing can direct the joint connection to the edge of the blade, hence causing minimum surface heat flow patterns and structure modulation. Therefore such TFTC array is very useful for accurate transient temperature measurements such as on turbine blade surface, comparing to the wired thermocouple method.

Thermal structure of the Kanto region, Japan

AbstractUsing a 3‐D numerical thermal model, we investigate the thermal structure of the Kanto region of Japan where two oceanic plates subduct. In a typical subduction setting with one subducting slab, the motion of the slab drives solid‐state mantle flow in the overlying mantle wedge, bringing in hot mantle from the back‐arc toward the forearc. Beneath Kanto, however, the presence of the subducting Philippine Sea plate between the overlying North American plate and the subducting Pacific plate prevents a typical mantle wedge flow pattern, resulting in a cooler condition. Further, frictional heating and the along‐margin variation in the maximum depth of slab‐mantle decoupling along the Pacific slab surface affect the thermal structure significantly. The model provides quantitative estimates of spatial variations in the temperature condition that are consistent with the observed surface heat flow pattern and distributions of interplate seismicity and arc volcanoes in Kanto.

Mantle wedge flow pattern and thermal structure in Northeast Japan: Effects of oblique subduction and 3-D slab geometry

We develop a 3-D thermal model for the Northeast Japan subduction margin, using a realistic slab geometry for the subducting Pacific plate, and investigate the effects of oblique subduction and 3-D slab geometry on the mantle wedge flow pattern and the thermal structure. In the Tohoku region, the mantle wedge flow pattern is nearly two-dimensional resulting in a thermal structure similar to those obtained by a 2-D model, owing to the simple slab geometry and subduction nearly perpendicular to the margin. However, in Hokkaido, oblique subduction leads to 3-D mantle wedge flow with northerly inflow and west–northwestward outflow and also results in lower temperatures in the shallow part of the mantle wedge than in Tohoku due to lower sinking rate of the slab. Between Hokkaido and Tohoku, the slab has a hinge-like shape due to a relatively sharp change in the dip direction. In this hinge zone, northerly mantle inflow from Hokkaido and westerly mantle inflow from Tohoku converge, discouraging inflow from northwest and resulting in a cooler mantle wedge. The model-predicted mantle wedge flow patterns are consistent with observed seismic anisotropy and may explain the orientations of volcanic cross-chains. The predicted 3-D thermal structure correlates well with the along-arc variations in the location of the frontal arc volcanoes and help to provide new insights into the surface heat flow pattern and the down-dip extent of interplate earthquakes.

Deep water methane hydrates in the Arctic Ocean: Reassessing the significance of a shallow BSR on the Lomonosov Ridge

Recently published multichannel seismic data from the Lomonosov Ridge image a reversed polarity bottom‐simulating reflector (BSR) tentatively attributed to the presence of deepwater marine hydrates and recognized throughout a survey area exceeding 100,000 km2. In addition to the importance of these findings for estimating Arctic hydrate reserves, if shown to correspond to the base of the hydrate stability zone, this seismic marker could provide a means for expanding spatial cover of heat flow data in deepwater settings of the Amerasian Basin, where little is known about the tectonic origin and nature of plate boundaries. As an initial test on the validity of this assumption, we develop a petrophysical model using sediments collected from circumpolar regions of the Lomonosov Ridge to derive an estimate of surface heat flow patterns from the BSR. The results show that the BSR inferred geothermal gradient and surface heat flow are exceedingly high when compared to published regional measurements. Although potential errors in the analysis may explain some of this discrepancy, the observation that the BSR remains at a constant subbottom depth despite large variations in water depths (>2400 m) and relative sedimentation rates provides additional evidence that it cannot mark the base of the hydrate stability zone. A further understanding of its origin requires a more detailed investigation of the existing seismic data and highlights the need for renewed collection of heat flow data from the Arctic Ocean.

The thermal structure of stable continental lithosphere within a dynamic mantle

The thermal structure of stable continental lithosphere is determined by (1) the concentration and distribution of heat sources within the crust and (2) the amount of heat input from the convecting mantle. The self-consistent coupling of these two factors has not been included in thermal models of stable lithosphere to date. We conducted two suites of numerical simulations (one with variable crustal heat production and the other with a chemically distinct cratonic root) to explore the thermal coupling between stable continental lithosphere and the convecting mantle. The distribution of heat producing elements within the crustal column was found to play a significant role in determining the local thermal structure of the continental lithosphere. Concentrating heat producing elements in the lower crust lead to a thinner thermal lithosphere. Mantle heat flux into the base of stable continents was low relative to surface heat flux and did not vary significantly within the simulations regardless of the presence or absence of a thick cratonic root. A suite of simulations with variable root thickness indicated that although cratonic roots have a weak effect on surface heat flow patterns, relative to crustal heat source variations, they do have a pronounced effect on deeper thermal structure. Roots stabilized temporal variations of deep continental geotherms and were required to generate a thick thermal lithosphere. The ratio of thermal to chemical lithospheric thickness was found to decrease toward unity with increasing root thickness and thick cratonic roots limited small-scale mantle convection beneath themselves.

The thermal structure of stable continental lithosphere within a dynamic mantle

The thermal structure of stable continental lithosphere is determined by (1) the concentration and distribution of heat sources within the crust and (2) the amount of heat input from the convecting mantle. The self-consistent coupling of these two factors has not been included in thermal models of stable lithosphere to date. We conducted two suites of numerical simulations (one with variable crustal heat production and the other with a chemically distinct cratonic root) to explore the thermal coupling between stable continental lithosphere and the convecting mantle. The distribution of heat producing elements within the crustal column was found to play a significant role in determining the local thermal structure of the continental lithosphere. Concentrating heat producing elements in the lower crust lead to a thinner thermal lithosphere. Mantle heat flux into the base of stable continents was low relative to surface heat flux and did not vary significantly within the simulations regardless of the presence or absence of a thick cratonic root. A suite of simulations with variable root thickness indicated that although cratonic roots have a weak effect on surface heat flow patterns, relative to crustal heat source variations, they do have a pronounced effect on deeper thermal structure. Roots stabilized temporal variations of deep continental geotherms and were required to generate a thick thermal lithosphere. The ratio of thermal to chemical lithospheric thickness was found to decrease toward unity with increasing root thickness and thick cratonic roots limited small-scale mantle convection beneath themselves.

Global heat flow simulation based on a kinematic model of mantle flow

SUMMARY The global heat flow is the surface representation of thermal processes within the earth's mantle. The long-wavelength pattern of observed heat flow closely resembles the plate tectonics and its most prominent feature is higher values along ocean ridge systems. Theoretically, to determine the thermal state of the Earth's mantle, the heat transfer problem and the mantle convection problem have to be solved simultaneously since they are coupled with each other. However, the development of global seismic tomography provides us with a possibility that, at least under certain assumptions, these problems can be decoupled from each other and solved separately. This allows us to calculate mantle flow velocities first based on the internal loading theory and then use the velocity field as the input to solve the thermal problem. In addition to the internal density anomalies, surface plate movements also excite mantle circulations and, under certain circumstance, they may dominate the structure of the mantle flow. In this study, using a kinematic model of mantle convection and heat transfer, we investigated the underlying processes that generated the observed global heat flow. Buoyancy from the density anomalies and the coupling from the overlying plates are treated as the mantle flow driving force. Both advection and conduction heat transfers are included in the energy equation. Results show that calculated depth derivatives of the near surface temperature are closely correlated to the observed surface heat flow pattern. Higher heat flow values around midocean ridge systems can be reproduced very well. The predicted average temperature as a function of depth reveals that there are two thermal boundary layers, one is close to the surface and another is close to the core-mantle boundary. The rest of the mantle is nearly isothermal. Although, in most of the mantle, advection dominates the heat transfer, the conductive heat transfer is still locally important in the boundary layers and plays an important role for the surface heat flow pattern. The existence of surface plates is responsible for the long wavelength surface heat flow pattern.

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.

Heat flow modelling: a generalized two-dimensional diffusive and convective numerical model for the Exmouth Plateau, offshore Northwest Australia

The surface heat flow pattern for the Exmouth Plateau region offshore Northwest Australia has been defined. The average surface heat flow is 59 mW/m 2, which is the expected level for continental crust. The heat flow frequency has a Gaussian (bell shaped) distribution about this mean within the range of 19 mW/m 2 to 106 mW/m 2. Overall the areal heat flow variation is rather smooth, and there is a gradual trend from high heat flow, in the order of 100 mW/m 2, from a confined region in the southeast, to average heat flow values on the plateau. It is within this background (average) heat flow on the plateau that a single region of low heat flow is situated. The area of the heat flow low, which has levels below 40 mW/m 2, is about 4000 km 2 and the centre of the low is about 230 km from the heat flow high. Conduction alone cannot give rise to this low heat flow so a conduction and convection analysis needs to be undertaken. The mathematics for two-dimensional non-linear heat and mass transfer in a heterogeneous and anisotropic porous medium that is applicable to the geological environment has been developed. The equations governing fluid motion are coupled to the diffusive and convective heat equation, so enabling an emphasis on free or natural convection rather than forced convection. A numerical model, based on the Alternate Direction Implicit method, has been developed around the mathematical formulation to simulate heat and mass transfer in a porous medium. It is concluded that lateral variations in rock thermal conductivity give rise to horizontal temperature gradients that are the major driving force of natural convection at the Exmouth Plateau region.

Deep temperature distribution along three profiles crossing the Teisseyre-Tornquist tectonic zone in Poland

The Teisseyre-Tornquist tectonic zone is a large deep fracture zone separating the Precambrian East European craton from the Variscan system of Central Europe. With the use of ample deep seismic sounding data, the detailed crustal temperature distribution was calculated for three profiles crossing this zone in its northern, central and southern segments. The conversion technique of transforming the reported seismic velocities ( υ p ) into heat production values ( A ) was used together with the temperature-dependent thermal conductivity structure and the observed surface heat flow pattern. In addition to the obtained 2-D temperature distribution, this work allowed a more reliable estimate of the crustal contribution to the surface heat flow, and thus the characteristic Moho heat flow ( Q M ~ 26 mW m −2 ), to be assessed in this region. The results can also serve as an independent confirmation of the necessity of applying the υ p − A conversion separately for Phanerozoic and for Precambrian rocks.

3-D numerical modelling of the conductive heat flow of SW England

SUMMARY The effect of 3-D geology on conductive heat flow was investigated using the finite element technique on two models from SW England, one a regional model centred on the Comubian batholith as a whole and the other a more detailed model centred on the Cammenellis pluton. Only two rock types, granite and country rock, were used to represent the geology of the region and yet good agreement between modelled and observed data was obtained provided that the granite was assumed to be highly homogeneous, with surface heat production values persisting throughout its depth, and the base of the batholith was assumed to be flat at a depth of 15 km. The results show that the surface heat flow pattern is dominated by the 3-D shape of the batholith which controls the relative importance of the two opposing effects that operate at its margins: heat refraction due to conductivity contrasts enhancing heat flow in the granite, and lateral flow of heat caused by heat production contrasts diminishing it. The highest heat flows occur at the boundaries of outcrops facing the axis of the batholith, where there is significant heat refraction but little lateral loss. A high degree of correlation between mineralized zones and areas of very high heat flow, in both granite and country rock, is also noticeable. The temperature field at any depth displays wide-ranging lateral variations over relatively short distances, the highest values occurring in areas of granite which are covered by insulating country rock and which lie towards the centre of the batholith. Regions of highest heat flow are not necessarily the regions where the highest sub-surface temperatures are likely to be found and it is shown that the hot dry rock geothermal test project at Rosemanowes is not optimally sited from temperature considerations. When the constraints on the physical parameters of the model are sufficiently reliable, as they are in SW England, the heat flow and temperature predictions from 3-D numerical models can be of enormous value in the assessment of the geothermal potential of a region.

Nankai Trough: A hot trench?

Heat flow estimated from the gas hydrate layers on the landward slope of the Nankai Trough reveals that heat flow increases downslope toward the trench floor. This data plus six new heat flow values obtained by a conventional probe and two values available from DSDP drill holes give a fairly detailed heat flow distribution in the Nankai Trough area, when combined with the already existing data set. There appears to be a zonal pattern parallel to the trough axis, with a high heat flow zone on the floor of the trough that is quite anomalous for a subduction zone. It might be explained as a result of subduction of the hot portion of the Philippine Sea plate, i.e. the Shikoku Basin, and/or of more local effects such as heating due to intrusion of hot water from subducted sediments to shallow depth beneath the trough floor. Surface heat flow patterns landward of the trough were calculated for a simple thermal model of subduction. Perfect reproduction of the observed zonal pattern is difficult to achieve by the simple model, suggesting the necessity for further heat flow and other observations.

The Galapagos Spreading Center at 86°W: A detailed geothermal field study

Appendix is available with entire article on microfiche. Orderfrom American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, D.C. 20009. Document J80‐013; $01.00. Payment mustaccompany order.We report here measurements of the heat flow field of the Galapagos Spreading Center on crust of age less than 1.0 m.y. The 443 measurements in an area of about 570 km2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near‐axis conductive heat flux. The intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern, with circulation apparently continuing beyond the limits of our survey. The areal average of the conductive heat flux is 7.1 ± 0.8 HFU (295 ± 33 mW/m2), about one‐third the heat flux predicted by plate models. The remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis. The pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography. Sharp lateral gradients in the heat flow, greater than 10 HFU/km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys, may indicate a local concentration of the circulation by surface fault systems and/or variable sediment thickness.

Two-dimensional correlation of geophysical data applied to the heat flow and earth's crust thickness patterns in Czechoslovakia

A simple method for the mutual correlation of various geophysical fields is proposed to disclose their prospective inherent correlations. Some problems connected with the statistical and physical confidence of the resulting maps of the correlation coefficient are mentioned. The correlation between the surface heat flow pattern and the crustal thickness on the territory of Czechoslovakia is presented as an example and briefly discussed in terms of crustal structure.

Mantle heat flow

A map of the heat flux out of the earth's mantle has been prepared by subtracting the heat flow arising in the earth's crust from the surface heat flow. In continental areas the crustal contribution of the enriched zone is determined from the parameters of the linear heat flow-heat production relationship q o = q* + bA 0 in areas where such data exist. Where heat flow-heat production data are not available, a new empiricism relating reduced heat flow q* to the mean heat flow of a province q-O, q* ~ 0.6 qo, is used to estimate the reduced heat flow, and the depth parameter b is assigned an average value of 8.5 km. The oceanic crustal heat flow contribution includes both heat liberated by cooling, which is a function of the age of the ocean floor, and a small radiogenic component. A spherical harmonic analysis to degree 18 of the computed 5 ° X 5 ° mantle heat flow values yields a mean of 48 mW m-2; the degree variance spectrum has prominent strength at degrees 1,4 and 5. Continent-ocean differences are more apparent in the mantle heat flow than in the surface heat flow. However, contrasting surface heat flow patterns within continents, such as in central and western Australia, which arise from different surficial radioactivity distributions, do not appear in the mantle heat flow. Mantle heat flow is positively correlated with the geopotential and negatively correlated with the topography of the earth. However, correlation of heat flow with a normalized topography, in which the base line elevation difference between oceans and continents is suppressed is significantly positive, in accordance with observation that ocean floor topography is thermally controlled.