Variations In Surface Heat Flow Research Articles

Thermal State of the Guaymas Basin Derived From Gas Hydrate Bottom Simulating Reflections and Heat Flow Measurements

AbstractSeafloor heat flow provides information about the thermal evolution of the lithosphere, the magnitude and timing of volcanic activity, and hydrothermal circulation patterns. In the central Gulf of California, the Guaymas Basin is part of a young marginal spreading rift system that experiences high sedimentation (1–5 km/Myr) and widespread magmatic intrusions in the axial troughs and the off‐axis regions. Heat flow variations record magmatic and sedimentary processes affecting the thermal evolution of the basin. Here, we present new seismic evidence of a widespread bottom‐simulating reflection (BSR) in the northwestern Guaymas Basin. Using the BSR depths and thermal conductivity measurements, we determine geothermal gradient and surface heat flow variations. The BSR‐derived heat flow values are less than the conductive lithospheric heat flow predictions for mid‐oceanic ridges. They suggest that high sedimentation (0.3–1 km/Myr) suppresses the lithospheric heat flow. In the central and southeastern regions of the basin, the BSR‐derived geothermal gradient increases as the intruded magmatic units reach shallower subsurface depths. Thermal modeling shows that recent (<5,000 years) igneous intrusions (<500 m below the seafloor) and associated fluid flow elevate the surface heat flow up to five times. BSR‐derived geothermal gradients correlate little with the depth of the shallowest magmatic emplacements to the north, where the intrusions have already cooled for some time, and the associated hydrothermal activity is about to shut down.

Spatiotemporal Variations in Surface Heat Loss Imply a Heterogeneous Mantle Cooling History

AbstractEarth's heat budget is strongly influenced by spatial and temporal variations in surface heat flow caused by plate tectonic cycles. Here, we use a novel set of paleo‐seafloor age grids extending back to the mid‐Paleozoic to infer spatiotemporal variations in surface heat loss. The time‐averaged oceanic heat flow is 36.6 TW, or ∼25% greater than at present‐day. Our thermal budget for the mantle indicates that 149 K/Gyr of cooling occurred over this period, consistent with geochemical estimates of mantle cooling for the past 1 Gyr. Our analysis also suggests sustained rapid cooling of the Pacific mantle hemisphere, which may have cooled ∼50 K more than its African counterpart since 400 Ma. The extra heat released from the Pacific mantle may have been trapped there by the earlier long‐lived supercontinent Rodinia (∼1.1–0.7 Ga), and the Pacific mantle may still be hotter than the African mantle today.

Strength variations of the Australian continent: Effects of temperature, strain rate, and rheological changes

The Australian continent is composed of several geologic provinces, showing a general age progression from Archean in the west to Phanerozoic in the east. The lithospheric heterogeneity and complex tectonic history of this region make it a key area for studying the thermal and rheological structure of the geological provinces and testing the influence of different conditions, such as temperature, rheology, and strain rate on the plate strength. In a previous study, temperature and compositional variations of the Australian upper mantle have been determined based on a joint interpretation of the seismic tomography and gravity data. In this study, we further implement a thermal model of the crust, based on available surface heat flow data from regional and global database. The crustal and upper mantle thermal models show different anomalies distribution, indicating a significant variation of the thermal conditions with depth. The new thermal models are used to estimate strength and effective elastic thickness ( Te ) distribution in the lithosphere. For this aim, we assigned the rheology of the crust based on the seismic velocities provided by the AuSREM model and used the strain rate values obtained from a global mantle flow model, constrained by seismic and gravity data. The maximal strength and Te are found in the West Australian Craton, on account of the low temperatures in the lithospheric mantle. We found that locations of the intraplate earthquakes attend to sharp changes in the lithospheric strength. Comparison of the results with those obtained for uniform rheology and strain rate, indicate that in the Officer basin the variations of the crustal rheology enhance the effect of temperature changes, while in the Yilgarn craton they reduce it. On the other hand, the lower values of the strain rate in the cratons than in the Phanerozoic regions influence the strength/ Te in the opposite way with respect to temperatures. Integrated strength (Pa m) and strength (Pa) variations along three cross-sections. Horizontal and vertical axes of the cross-sections display the distance and depth (both in km), respectively. (a) Integrated strength of the lithosphere. The three black lines show the location of the cross-sections; (b) Integrated strength of the crust. White circles show the earthquakes location, occurred from 1990 to present-day, with moment magnitudes (Mw) >2.5, taken from the USGS seismic catalogue ( https://earthquake.usgs.gov/earthquakes/search/ ); (c) Integrated strength of the mantle lithosphere; (d) Percentage of crustal strength; (e) Strength variations along three cross-sections (A-A′, B-B′, and C-C′, respectively). • A crustal themal and rheological model of the Australian continent is set up. • Crustal thermal anomalies correlate with surface heat flow variations. • Upper mantle thermal anomalies correlate with age of the tectonic features. • The strength distribution is strongly influenced by the temperature variations.

Variations in Moho and Curie depths and heat flow in Eastern and Southeastern Asia

The Eastern and Southeastern Asian regions witness the strongest land–ocean and lithosphere–asthenosphere interactions. The extreme diversity of geological features warrants a unified study for a better understanding of their geodynamic uniqueness and/or ubiquity from a regional perspective. In this paper we have explored a large coverage of potential field data and have detected high resolution Moho and Curie depths in the aforementioned regions. The oldest continental and oceanic domains, i.e. the North China craton and the Pacific and Indian Ocean have been found thermally perturbed by events probably linked to small-scale convection or serpentinization in the mantle and to numerous volcanic seamounts and ridges. The thermal perturbation has also been observed in proximity of the fossil ridge of the western Philippine Sea Basin, which shows anomalously small Curie depths. The western Pacific marginal seas have the lowest Moho temperature, with Curie depths generally larger than Moho depths. The contrary is true in most parts of easternmost Eurasian continent. Magmatic processes feeding the Permian Emeishan large igneous province could have also been genetically linked to deep mantle/crustal processes beneath the Sichuan Basin. The regionally elongated magnetic features and small Curie depths along the Triassic Yangtze-Indochina plate boundary suggest that the igneous province could be caused by tectonic processes along plate margins, rather than by a deep mantle plume. At the same time, we interpret the Caroline Ridge, the boundary between the Pacific and the Caroline Sea, as a structure having a continental origin, rather than as hotspot or arc volcanism. The surface heat flow is primarily modulated by a deep isotherm through thermal conduction. This concordance is emphasized along many subduction trenches, where zones of large Curie depths often correspond with low heat flow. Local or regional surface heat flow variations cannot be faithfully used in inferring deep thermal structures, which can be better constrained overall through Curie depths detected from surface magnetic anomalies.

Thermal regime of the lithosphere in the Canadian ShieldThis article is one of a series of papers published in this Special Issue on the themeLithoprobe — parameters, processes, and the evolution of a continent.

Geothermal studies were conducted within the framework of Lithoprobe to systematically document variations of heat flow and surface heat production in the major geological provinces of the Canadian Shield. One of the main conclusions is that in the Shield the variations in surface heat flow are dominated by the crustal heat generation. Horizontal variations in mantle heat flow are too small to be resolved by heat flow measurements. Different methods constrain the mantle heat flow to be in the range of 12–18 mW·m–2. Most of the heat flow anomalies (high and low) are due to variations in crustal composition and structure. The vertical distribution of radioelements is characterized by a differentiation index (DI) that measures the ratio of the surface to the average crustal heat generation in a province. Determination of mantle temperatures requires the knowledge of both the surface heat flow and DI. Mantle temperatures increase with an increase in surface heat flow but decrease with an increase in DI. Stabilization of the crust is achieved by crustal differentiation that results in decreasing temperatures in the lower crust. Present mantle temperatures inferred from xenolith studies and variations in mantle seismic P-wave velocity (Pn) from seismic refraction surveys are consistent with geotherms calculated from heat flow. These results emphasize that deep lithospheric temperatures do not always increase with an increase in the surface heat flow. The dense data coverage that has been achieved in the Canadian Shield allows some discrimination between temperature and composition effects on seismic velocities in the lithospheric mantle.

Heat flow anomalies in the Gulf of Cadiz and off Cape San Vincente, Portugal

Heat flow anomalies provide critical information in active tectonic environments. The Gulf of Cadiz and adjacent areas are affected by the plate convergence between Africa and Europe, causing widespread deformation and faulting. Active thrust faults cause lateral movement and advection of heat that produces systematic variations in surface heat flow. In December 2003 new heat flow data were collected during the research vessel Sonne cruise SO175 in the Gulf of Cadiz over two sites of recent focused research activity: (i) the Gulf of Cadiz sedimentary prism and (ii) the Marques de Pombal escarpment. Both features have also been discussed as potential source areas of the Great Lisbon earthquake and tsunami of 1755. Background heat flow at the eastern terminus of the Horseshoe abyssal plain is about 52–59 mW/m 2. Over the Gulf of Cadiz prism, heat flow decreases from ∼57 mW/m 2 to unusually low values of 45 mW/m 2 roughly 120 km eastward. Such low values and the heat flow trend are typical for active thrusting, supporting the idea of an east-dipping thrust fault. Slip rates are 10 ± 5 mm per year, assuming that the fault dips at 2°. A fault dipping at 5°, however, would result into slip rates of 1.5–5 mm per year, suggesting that subduction has largely ceased. Based on seismic data, the Marques de Pombal fault is interpreted as part of an active fault system located ∼100 km westward of Cape San Vincente. Heat flow over the fault is affected by refraction of heat caused by the 1 km high escarpment. Thermal models suggest that the slip rate along the fault must either be small or shear stresses acting on the fault are rather high. With respect to other fault zones, however, it is reasonable to assume that the fault's slip rate is small.

The thermal regime of South African continental margins

We present thermal data from 100 oil exploration boreholes on the continental margins of South Africa. Near surface temperature profiles are constrained with temperatures acquired during reservoir tests, corrected bottom-hole temperatures and seafloor temperatures. The thermal conductivity profiles are estimated from geophysical well logs using a neural network method. Available data allow us to determine 41 new heat flow values computed from depth ranges greater than 1000 m, located on the western and southern continental margins of South Africa. We believe that the variations in surface heat flow are not controlled by the contribution of the sediments or the basement, but rather by the mantle heat flow. An upper bound of ~30 mW m−2 for the mantle heat flow on the interior of the continent can be inferred from the lowest available estimates, located near the coastline on the southern continental margin. On both continental margins, heat flow values reach a maximum of about 60 mW m−2, 100 km away from the shore, and consistently decrease further away towards values typically encountered in old oceanic domains (~50 mW m−2). This indicates that (1) the mantle heat flow increases over a short distance at the edge of the continent, which is consistent with studies on the effect of continents on mantle convection, and (2) it may increase to a value greater than heat flow on the adjacent ocean, as a possible consequence of the effect of the transition between thin oceanic lithosphere and thick and cold continental lithosphere on mantle convection.

Variations of surface heat flow and lithospheric thermal structure beneath the North American craton

Two end-member models have been proposed to explain the variations in surface heat flow in stable continents, calling for changes of either crustal heat production or heat flow at the base of the lithosphere. The scale of the surface heat flow variations controls how these variations affect the thermal structure and thickness of the lithosphere and provides constraints on these models. Data in the Canadian Shield and the Appalachians are now extensive enough to address problems of scale and relationship between average heat flow and heat production. We analyze the global data set as well as data from five compositionally distinctive subprovinces. Within each province, on scales <500 km, observed heat flow variations are linked to changes of local crustal structure. For the five subprovinces, the average values of heat flow ( Q̄) and heat production ( Ā) conform to the simple relationship Q̄= Q o+ HĀ, where H≈9 km and Q o≈33 mW m −2. This shows that, on scales larger than the dimensions of these provinces (>500 km), variations in crustal heat production dominate and hence that variations of mantle (Moho) heat flow must be small. The large heat flow step at the Grenville–Appalachian boundary (≈16 mW m −2) may be accounted for by a change in crustal heat generation only. In that case, the lithosphere is ≈40 km thinner in the Appalachians than in the Shield. At wavelengths of 500 km or more, mantle (Moho) heat flow variations are constrained to be smaller than the detection limit of heat flow studies, or about ±2 mW m −2, and may not be correlated with surface geology. Downward continued to the base of the lithosphere, the amplitude of these variations depends on wavelength and must be smaller than ±7 mW m −2. Such variations imply that temperature differences must be smaller than 400 K at 150 km depth. These bounds are consistent with seismic shear wave velocity variations and geothermobarometry studies on mantle xenoliths.

Amplitude of heat flow variations on Mars from possible shoreline topography

Analyses of the effective elastic thickness of the Martian lithosphere have been previously used to calculate surface heat flow on Mars at different places and times. In this work, I use elevation differences in a putative Late Hesperian shoreline, named Deuteronilus shoreline, and the relation between thermal state and buoyancy of the lithosphere, in order to estimate the amplitude of the variations of surface heat flow on Mars, probably related to the time in which this feature was formed. The results suggest that, if the Deuteronilus shoreline is a true paleo‐equipotential surface, the relative amplitude of surface heat flow variations on the shoreline regions in the Late Hesperian were less than present‐day ones in terrestrial continental areas. The results are also roughly valid for the outer contact of the Late Hesperian Vastitas Borealis Formation, if this contact is related to the limits of an ancient ocean. These results could imply that large areas of the Martian lithosphere have been tectonothermally stable since at least that time.

Heat flow and deep thermal structure near the southeastern edge of the Canadian Shield

Five new heat-flow and heat-production measurements in the Archean Superior Province are presented. These measurements include the first heat-flow values to be reported for the Opatica subprovince and the Otish basin. These new data complete the data set acquired in the eastern Canadian Shield during the Abitibi-Grenville Lithoprobe transect. The data set now available in eastern Canada, covering geological provinces ranging in age from 2700 to 400 Ma, achieves sufficient sampling to define the deep thermal structure of a continent near the edge of the craton. It shows that, for the Canadian Shield, there is no simple relation between heat flow and the age of tectonic provinces. The map of heat flow in eastern Canada demonstrates that there is no significant difference in heat flow between the Abitibi subprovince and the Grenville Province (including the Adirondacks) where the area-weighted average heat flow is the same (39 vs. 38 mW·m-2, respectively). Outside the Abitibi, the Superior Province is characterized by a higher heat flow (45 mW·m-2). Heat-flow and gravity data are used together to determine changes in crustal composition and thickness. The analysis of these data and constraints from seismology support the view that the variations in surface heat flow can be entirely accounted for by changes in crustal composition. Heat-flow variations across the Abitibi subprovince indicate that there are significant differences in crustal composition that reflect the complex assemblages that make up the Archean crust. The heat-flow map shows a sharp transition between the Grenville Province and the Appalachians, where the average heat flow is significantly higher (57 mW·m-2). This difference is due to higher heat production in the Appalachian upper crust with the same mantle heat flow as in the shield (~12 mW·m-2 throughout eastern Canada). Lower crustal and upper mantle temperatures are typically low, which might explain the preservation of irregular crustal thickness over several billion years.

Heat flow and thermal structure of the lithosphere across the Baltic Shield and northern Tornquist Zone

Terrestrial heat-flow data from Archean and Proterozoic Provinces of the Baltic Shield, its southwestern margin, and the tectonic transition to the younger tectonic units of Central Europe (northern segment of the European Geotraverse) are compiled and examined for local and regional variations. A 2-D numerical lithospheric temperature and heat-flux model is presented covering 1800 km of the deep seismic Fennolora profile across the Baltic Shield and 500 km of Eugeno-s/ northern Eugemi seismic lines across the northern Tornquist Zone and the Danish and North German basins. Thermal lithosphere, defined as the outer layer with relatively low temperatures ( T < 1200–1400° C) in which heat transfer is dominantly by conduction, is found to vary markedly in thickness from 85–110 km beneath basins and Tornquist Zone to about 150 km in central shield areas, and 200–250 km in areas of low heat flow along the northern shield profile. These results are in very good agreement with seismological information from surface wave dispersion analysis and the refraction / wide-angle seismic interpretation of Fennolora data. The combined thermal and seismologic evidence indicates that the uppermost mantle region stratified in seismic velocity is restricted to the upper, relatively cold thermal layer, which is interpreted to constitute shield lithosphere. Local and to some extent regional variations in surface heat flow are found to be closely related to variations in near-surface radiogenic heat production. For the southern Baltic Shield a heat-flow / heat-production relationship of q 0 = 32.8 + 7.6 H 0 is derived. Regional increase in heat flow from northern and central shield provinces to the shield margin and the Tornquist Zone is interpreted to be related partly to differences in upper-crustal heat generation, partly, and in southernmost areas mainly, to an increased heat flux from the upper mantle. Upper-mantle heat flux is estimated at 20–25 mW m −2 along the northern and central shield transect, increasing to 30–40 mW m −2 beneath the shield margin, basins, and Tornquist Zone. Lithospheric thickness variations in this region of northern Europe are found to be closely associated with variations in heat flux from the uppermost mantle and perhaps to a large extent controlled by geodynamically determined lateral differences in upper-mantle heat flux. Earthquake focal depth data and information on crustal strength show Baltic Shield seismicity to be limited generally to the upper 20–25 km brittle part of the crust at temperatures less than 300–400°C. Maximum source depth of magnetic anomalies at temperatures near 600°C is in the deepest part of the crust near the crust-mantle boundary in the southern region (about 25 km depth along the Tornquist Zone and about 35–40 km in southern shield areas) and in the uppermost mantle (60–70 km depth) in the northern shield areas. Some petrologic implications are discussed. The present deep crust is supposed to be at lower temperature-pressure conditions than at early stages of formation. However, also the present deepest parts of the shield crust (> 35–40 km) are within the stability field of eclogites. The general absence of significant gravity anomalies in areas of marked crustal thickness variations indicate high-density eclogites to be present in significant amounts in the deep shield crust.

An upper limit to palaeo heat flow: theory and examples from the Danish Central Graben

The surface heat flow yields a condensed expression of the subsurface thermal regime. While the present heat flow can be measured as the product of thermal conductivity and temperature gradient, resolution of the past surface heat-flow variations calls for indirect methods such as the modelling of vitrinite reflectance and other organic maturity indicators, which represent integrals of the past temperature history. This paper argues that regarding the reflectance of vitrinite, which is the most common maturity indicator, the feature of the past thermal regime which can survive the effects of measuring and modelling uncertainties is the maximum temperature, present or past. How the maximum temperature was obtained is only rarely resolved. Given maximum temperature, burial history, thermal conductivity structure, and surface temperature history, the one-dimensional steady-state heat equation yields an approximate upper limit to the surface heat flow. Artificial examples show that the past heat flow may be similar to the maximum heat-flow constraint when the past was significantly warmer than the present. However, this requires a delicate balancing of burial history and heat-flow history to occur: the thermal relaxation rate of the hot past must compensate for heating due to burial. Generally, the past heat flow may be well below the maximum constraint. Examples from the Central Graben show that vitrinite reflectance samples of Upper Jurassic age constrain the maximum palaeo heat flow back to 150 Ma. This is due to the very rapid and significant sedimentation of Upper Jurassic sediments resulting in immediate deep burial of vitrinite. The data of the present study are consistent with a Early Cretaceous heat flow elevated by about 20–30% above the present, and with a heat flow throughout the Cenzoic only marginally higher than the present. However, the data are also fully consistent with a time independent background heat-flow history. This illustrates the limits of resolution in palaeo thermal reconstruction based on vitrinite reflectance and temperature data. The problem is one of finding the space of admissible solutions rather than one of finding a single solution.

Downward continuation of heat flow data

Abstract Downward continuation operators are derived and used to calculate the subsurface temperature from surface heat flow data. If the distribution of the shallow heat sources were known, their effect on the temperature field could be included in the downward continuation operator. The distribution of heat sources with depth can be estimated from the Fourier spectral content of the heat flow data, but the relationship is non-unique. Several operators have been derived with different assumptions regarding the relationship between the wavelength of the surface heat flow variations and the depth of the corresponding heat sources. These downward continuation operators were applied to determine the subsurface temperature and the deep heat flow beneath the Colorado Plateau; the calculations show that the temperature in the crust beneath the plateau is lower than in the surrounding Basin and Range and Rio Grande Rift Provinces. However, the temperature is locally higher in regions of recent tectonic or magmatic activity. The same downward continuation operators can also be used to model the effect of hydrothermal convection on the temperature field. An example is presented where closely spaced heat flow data from the Juan de Fuca Ridge have been used to calculate the crustal temperature and to estimate the heat flow below the convecting layer.

Episodic volcanism of tidally heated satellites with application to Io

A simple model of the coupled thermal and orbital evolution of a tidally heated satellite in an orbital resonance is presented and applied specifically to Io. The model quantitatively demonstrates how a feedback mechanism between the orbital and thermal energy of such a satellite can lead to periodic variations in surface heatflow and orbital eccentricity. The convective heatflow and ( k/ Q) of the satellite are parameterized as local power laws of the temperature, where Q is the quality factor and k is the second-degree tidal potential Love number. The time evolution of the model is determined by two nonlinear equations: an equation governing the orbital eccentricity, and a simple heat-balance equation determining the temperature. A linear stability analysis reveals that the time-independent solution is unstable if n > m + p, where n and m are the exponents in the power laws for ( k/ Q) and convective heatflow, respectively, and p is the ratio of the convective cooling time scale to the time scale for equilibration of the eccentricity. Numerical integration of the nonlinear equations reveals behavior in qualitative agreement with this relation. Laboratory data on near-solidus peridotites suggest 20 ≲ n ≲ 30 and parameterized convection schemes suggest m ∼ 10. Since p is of order unity, it follows that tidally heated satellites are probably in the unstable regime if they are operating near the solidus. It is thus probable that Io has no thermal steady state. The model is made more realistic by (1) arresting the reduction of ( k/ Q) at low temperature, and (2) arresting the growth of temperature at the mantle solidus and allowing volcanism to remove the excess heat. When the second modification is included, the unstable regime becomes periodic. In addition, a global k substantially larger than the elastic value is possible for a mostly solid Io because the body may begin to behave viscously when the tidal period is longer than or comparable to the Maxwell time. This requires a solid-state viscosity of ≲4 × 10 15 Pa sec, which may be achievable with a small amount of partial melt. The model can easily be adjusted to pass through Io's current observed heatflow (1–2 W m −2) and eccentricity (∼0.004) for reasonable choices of parameters ( Q/ k) min ∼ 100, ( Q/ k) max ∼ few × 10 3, solidus viscosity ∼10 15–10 17 Pa sec, and Q J within the required dynamical bounds. The periods of high heatflow and acceptable eccentricity typically have durations of ∼20–30 myr, separated in time by ∼80–100 myr. Spatial heterogeneities in Io's thermal structure are likely to make the behavior more complicated. The model predicts that Io's mean motion may be currently increasing, a possibility suggested by recent estimates of n dot 1 from eclipse data. Since Europa's eccentricity mimics that of Io, the model also implies that the tidal stresses in Europa's ice shell may have recently been large enough to produce the observed fracturing. The episodic heating mechanism may be responsible for the resurfacing of Enceladus < 10 9 years ago.

Downward continuation of heat flow data: Method and examples from the western United States

We outline a method to determine, directly from the data, a subsurface temperature distribution which is compatible with the surface heat flow and takes into consideration the effect of the shallow heat sources. The analysis assumes that (1) the crust is in conductive equilibrium, (2) the lateral changes in thermal conductivity can be neglected, and (3) the depth of the heat sources is proportional to the wavelength of the variations in surface heat flow. We apply the technique to determine the crustal temperatures in two tectonically active regions of the western United States. In the Rio Grande rift, temperatures higher than 1 100°C are predicted at a depth of 20 km. In the transition region between the Colorado Plateau and the Basin and Range, the variability in surface heat flow is partially accounted for by shallow heat sources; however, the increase in heat flow toward the Basin and Range seems associated with deep seated (i.e., lower crust or upper mantle) variations in the thermal regime.

Regional geotherms and lithospheric thickness

Research Article| May 01, 1977 Regional geotherms and lithospheric thickness David S. Chapman; David S. Chapman 1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Search for other works by this author on: GSW Google Scholar Henry N. Pollack Henry N. Pollack 2Department of Geology and Mineralogy, University of Michigan, Ann Arbor, Michigan 48109 Search for other works by this author on: GSW Google Scholar Geology (1977) 5 (5): 265–268. https://doi.org/10.1130/0091-7613(1977)5<265:RGALT>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David S. Chapman, Henry N. Pollack; Regional geotherms and lithospheric thickness. Geology 1977;; 5 (5): 265–268. doi: https://doi.org/10.1130/0091-7613(1977)5<265:RGALT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Continental and oceanic geotherm families parametric in surface heat flow intersect the mantle solidus at a depth coincident with the top of the seismic low-velocity zone, thus allowing surface heat-flow variations to be used to map the thickness of the lithosphere on a global scale. Our thermal models predict a lithospheric thickness of a few tens of kilometres in young oceans and continental orogenic provinces and more than 300 km in shield areas. The variable thermal structure of the mantle implies a greater viscosity beneath shields, which offers an explanation for the observed retarded motions of plates that bear shields. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geophysical data and long-wave heterogeneities of the Earth's mantle

To investigate the presence and nature of the lateral heterogeneities in the earth's mantle, geophysical data with global distribution are analyzed and correlated. Surface heat flow, seismic travel-time residuals, and crustal thickness data are expanded in spherical harmonics. Using these, and the coefficients of similar expansions of surface topography of the earth, and the gravitational potential variations as revealed by satellite data, both total and degree correlation coefficients are obtained. From the interpretation of these results, jointly with available laboratory data on velocity-density relationships and the temperature coefficient of velocity, the following are concluded: (1) Lateral mass anomalies and/or density variations in the earth's mantle are larger by at least an order of magnitude from what is required to explain the satellite data. Earth's surface topography and crustal structure introduce loads which are compensated in the upper, and perhaps lower, mantle giving rise to lateral density anomalies in the mantle. (2) Surface heat flow variations are controlled primarily by the shallow structure and tectonic features of the earth. (3) At long wavelengths (n ≤ 6), gravitational, heat flow, and seismic travel-time variations are not correlated with topographic elevations.