Crustal Heat Research Articles

Continental thermal isostasy: 2. Application to North America

Continental elevations can be partitioned into contributions from thermal and compositional buoyancy and geodynamic forces. In order to isolate the thermal component, elevations of 36 North American tectonic provinces are adjusted for the effects of compositional thickness and density variations by computing an isostatic adjustment relative to a standard crustal section (2850 kg m−3 density and 39 km thickness). Crustal densities are estimated using an empirical velocity‐density relationship. Compositional elevation adjustments applied to North American provinces range from ∼−1100 m in the southern Rocky Mountains to 2300 m in the Gulf of California, with uncertainties ranging from ∼200 m to >600 m. Theoretical thermal buoyancy is estimated by integrating the difference between a geotherm derived from observed values of heat flow and a reference geotherm. The best fitting continental heat flow–elevation model has a reference heat flow of 46.6 mW m−2 at 0 km and a 60:40 partitioning of surface heat flow between reduced and upper crustal radiogenic heat flow. Raw elevations of continental provinces show little correlation with heat flow, while compositionally adjusted elevations show a clear trend with ∼3 km difference between hot and cold provinces. A continental heat flow–elevation plot is used to identify outliers in adjusted province elevations. Anomalous elevations may reflect a nonsteady state thermal regime, dynamically supported elevation, anomalous mantle, or some combination of these states. Discriminating between these elevation sources provides insight into the geodynamics of North America, demonstrating the usefulness of this approach in continental geodynamic studies.

Transient geotherms in Archean continental lithosphere: New constraints on thickness and heat production of the subcontinental lithospheric mantle

Because of its large thickness and thermal relaxation time, Archean lithosphere cannot be in thermal equilibrium with the instantaneous rate of heat production in the lithospheric mantle and heat supplied to its base. Comparison of xenolith (P,T) data with time‐dependent thermal models allows constraints on lithosphere thickness, in situ heat production in the lithospheric mantle and time changes of basal heat flux. In this paper, the lithosphere is defined as the rigid part of the thermal boundary layer where heat transport occurs by conduction only. A Monte Carlo procedure is used to determine the full range of thermal models consistent with xenolith (P,T) arrays from Newlands and Finsch orangeites, Kaapvaal craton, South Africa. These xenolith suites from an early phase of kimberlite magmatism in the Kaapvaal record thermal conditions in unperturbed lithosphere. Together with constraints on surface heat flow and crustal heat production, these data require the lithosphere thickness to be between 200 and 270 km, with most values between 210 and 250 km. Additional constraints, including the condition that lithospheric temperatures have remained below the solidus, estimates of the cooling rate in mantle xenoliths, and the depth extent of seismic anomalies beneath cratons tighten the solution range. Present‐day values of basal heat flow and heat production in the Kaapvaal lithospheric mantle lie between 12–16 mW m−2 and 0–0.02 μW m−3 respectively. At 240‐km depth, lithospheric material undergoes secular cooling at a rate of 40 to 110 K/Ga due solely to in situ radioactive decay. Changes of basal heat flow must be less than 5% per Ga, suggesting that thermal conditions in the underlying mantle have not changed much since the Archean.

Controls of stable continental lithospheric thickness: the role of basal drag

The lithosphere is the earth's strong outer layer that remains a coherent unit for long times during plate motions. The motions lead to shearing of its hot and ductile lower part. The base of the lithosphere is placed where the shearing is large enough to effectively decouple the coherent plates from the underlying mantle. Here the depth and temperature where this occurs under stable continental areas were modeled by using the experimentally determined creep properties of olivine (which depend on temperature, pressure and fluid content) for different geotherms and different magnitudes of the basal drag. It is found that the shearing increases rapidly downward in a ca. 20–25 km thick decoupling zone where the bottom of the lithospherecan can be placed. This zone is deeper for cooler geotherms and for smaller a basal drag, while the presence of fluids makes it shallower. In dry rock this zone is at the mantle adiabat or cooler when the basal drag is > 0.5 MPa. Under Archean cratons whose temperature is represented by xenolith geotherms the decoupling zone is 180–240 km deep. Higher crustal heat generation than in cratons (common in post-Archean terranes) by itself causes the decoupling zone to be a few tens of kilometers shallower, and it will be even shallower if the mantle heat flow is larger than under cratons. The base of the lithosphere need not follow an isotherm. As the lithospheric thickness depends on several factors it will change when any of these changes, which is expected to have happened many times in earth history. To persist the lithosphere should escape being sheared off laterally, regardless of its buoyancy.

The role of radiogenic heat production in the thermal evolution of a Proterozoic granulite-facies orogenic belt: Eastern Ghats, Indian Shield

The Eastern Ghats Belt (EGB) is a deeply eroded Proterozoic orogenic belt juxtaposed against the Archaean Dharwar–Bastar–Singhbhum cratons in the Indian Shield. The tectono-metamorphic history of this belt is broadly similar to the Grenvillian orogenic belts. The EGB exposes dominantly the granulite-facies rocks such as charnockites, khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites. The granulites are intruded by small plutons of granites, syenites and anorthosites. The northern and southern parts of the belt (NEGB and SEGB) have distinct crustal histories, but 1600–1000–550 Ma old tectono-metamorphic events are common to both the belts. In this research, we attempt to understand the present-day and past thermal state of this orogen on the basis of our radioelement (K, U and Th) measurements at 562 sites using in-situ gamma-ray spectrometry, with other geological and geophysical constraints. The dominant rock types of the NEGB and SEGB have similar radioelement abundances (>3% K, 3 ppm U and 30 ppm Th) and heat production. Heat production of charnockites, gneisses, khondalites and intermediate granulites of the NEGB is 2.9, 2.8, 2.9 and 1.1 μW m−3, respectively. In the SEGB, they are 2.7, 2.5, 2.8 and 0.6 μW m−3, respectively, and the mafic granulites are the lowest in heat production (0.3 μW m−3). On the basis of the heat production data and crustal petrologic models, we show that the crustal contribution of the NEGB and SEGB is 54 and 45 mW m−2, respectively, which are broadly in good agreement with the available surface heat flow data. Thermal models of the present-day EGB crust indicate that the temperature at the Moho is ∼550 °C. Radioelements and heat production of the Eastern Ghats granulites are higher than the other granulite belts in the Indian shield. The crustal radiogenic heat contribution and Moho temperatures of the EGB are higher than the adjoining Archaean cratons. This could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the EGB and the adjoining cratons during a collisional orogenic event at about 550 Ma ago.

The transition from diffuse to focused extension: Modeled evolution of the West Antarctic Rift system

Two distinct stages of extension are recognized in the West Antarctic Rift system (WARS). During the first stage, beginning in the Late Cretaceous, extension was broadly distributed throughout much of West Antarctica. A second stage of extension in the late Paleogene was focused primarily in the Victoria Land Basin, near the boundary with the East Antarctic craton. The transition to focused extension was roughly coeval with volcanic activity and strike–slip faulting in the adjacent Transantarctic Mountains. This spatial and temporal correspondence suggests that the transition in extensional style could be the result of a change in plate motions or impingement of a plume. Here we use finite element models to study the processes and conditions responsible for the two-stage evolution of rifting in the WARS. Model results indicate that the transition from a prolonged period of broadly distributed extension to a later period of focused rifting did not require a change in the regional stress regime (changes in plate motion), or deep mantle thermal state (impingement of a plume). Instead, we attribute the transition from diffuse to focused extension to an early stage dominated by the initially weak accreted lithosphere of West Antarctica, and a later stage that concentrated around a secondary weakness located at the boundary between the juvenile West Antarctica lithosphere and Precambrian East Antarctic craton. The modeled transition in extension from the initially weak West Antarctica region to the secondary weakness at the West Antarctic–East Antarctic boundary is precipitated by strengthening of the West Antarctica lithosphere during syn-extensional thinning and cooling. The modeled syn-extensional strengthening of the WARS lithosphere promotes a wide-rift mode of extension between 105 and ∼ 65 Ma. By ∼ 65 Ma most of the extending WARS region becomes stronger than the area immediately adjacent to the East Antarctic craton and extension becomes concentrated near the East Antarctic/West Antarctic boundary, forming the Victoria Land Basin region. Mantle necking in this region leads to syn-extensional weakening that promotes a narrow-rift mode of extension that becomes progressively more focused with time, resulting in formation of the Terror Rift in the western Victoria Land Basin. The geodynamic models demonstrate that the transition from diffuse to focused extension occurs only under a limited set of initial and boundary conditions, and is particularly sensitive to the pre-rift thermal state of the crust and upper mantle. Models that predict diffuse extension in West Antarctica followed by localization of rifting near the boundary between East and West Antarctica require upper mantle temperatures of 730 ± 50 °C and sufficient concentration of heat producing elements in the crust to account for ∼ 50% of the upper mantle temperature. Models with upper mantle temperatures < ca. 680 °C and/or less crustal heat production initially undergo diffuse extension in West Antarctica, and quickly develop a lithospheric neck at the model edge furthest from East Antarctica. Models with upper mantle temperatures > ca. 780 °C do not develop focused rifts, and predict indefinite diffuse extension in West Antarctica.

Determining heat flows and radiogenic heat generation in the crust and lithosphere based on seismic data and surface heat flows

The balance of the Earth’s energy involves the generation of radiogenic, gravitational, chemical, and other types of energy and the heat loss from the Earth’s surface. In the quasi-stationary formulation, heat losses through the surface are equal to the sum of energy generated by all sources in the Earth’s interiors. The flow from the continental surface is estimated at 47‐ 49 mW/m 2 [1]. Estimates (accomplished by various researchers) of the mantle component of the heat flows range from 3 to 25 mW/m 2 [1]. Mantle flows are usually assayed on the basis of extensive geothermal measurements at numerous sites around the world. The heat flow through the Earth’s surface Q 0 can be expressed as the sum of the mantle flow Q M and the contribution of radiogenic energy released within the crust Q Cr . The value of Q Cr is determined based on geochemical models currently adopted for the distribution of radiogenic elements and the solution of stationary heat conduction [1, 2]. At the same time, the mantle flow Q M is, in fact, calculated as the difference between Q 0 and Q Cr . The distribution of K, U, and Th are still known relatively poorly, so that these estimates for both the crust and the mantle broadly differ, and this problem is actively debated in the literature [3, 4]. Thermal models for the mantle are the least definite, mostly because of the uncertainties in the crustal and lithospheric contributions to the overall heat generation [1‐4]. The solution to the problem of determining the temperature and/or the chemical composition of the mantle from the velocities of seismic waves was debated in [5‐9]. It is quite difficult to derive mantle geotherms from the results obtained by measuring the surface heat flow because of the lack of information on the mantle contribution. This publication proposes a method for assaying the conductive heat transfer of the lithosphere of both mantle and crustal heat flows on the basis of the data of seismic models and the heat flow through the Earth’s surface Q 0 . As an example of such a calculation, we used the profiles of velocities from the IASP91 [10] global reference model (for the “averaged” continental crust) and the BP11A regional model [11], which was developed for the mantle beneath the Kaapvaal craton in South Africa. The solution routine was provisionally subdivided into two stages. During the first of them, the temperature of the mantle ( T P and T S ) is determined from the velocities of seismic P and S waves. The procedure of the conversion of seismic profiles into thermal ones is conducted based on the equations of state for the material with regard for anharmonic and anelastic effects. The temperature profile T P , S was then made consistent with the conductive heat transfer model for the crust and mantle, and this enabled us to derive an analytical expression for temperature variations with depth, the values of crustal heat sources, and the components of heat flows in the crust and mantle. Determining temperature in the mantle from the velocities of seismic waves. Geochemical, seismic, and thermal models for the Earth’s upper mantle were made mutually consistent using the THERMOSEISM computer program complex and database, by methods of physicochemical simulations, which make it possible to convert compositional models into physical characteristics and velocity profiles into models for the temperature distribution [5, 9, 12, 13]. The temperature T P in the mantle is determined by solving the inverse problem for the velocities of primary (and/or secondary) seismic waves and the bulk composition of the rock at any point in a depth profile. The equilibrium mineral composition of the rock is determined from the bulk composition by minimizing the Gibbs free energy for the system Na 2 O–TiO 2 –eaO–FeO–MgO–Al 2 O 3 –SiO 2 with solid solutions of the following minerals: binary solutions of Fe‐Mg olivine, spinel, and ilmenite; garnet ( Gar of the pyrope‐almandine‐grossular series); orthopyroxene ( Opx — MgSiO 3 , FeSiO 3 , Ca 0.5 Mg 0.5 SiO 3 , Ca 0.5 Fe 0.5 SiO 3 , Al 2 O 3 ); and clinopyroxene ( Cpx —same components plus the jadeite end member) [12, 13]. The

Palaeozoic Intraplate Crustal Anatexis in the Mount Painter Province, South Australia: Timing, Thermal Budgets and the Role of Crustal Heat Production

The effect of radiogenic heat production within the crust on thermal processes such as crustal anatexis is generally disregarded as bulk geochemical models suggest that crustal heat generation rates are too low to effect significant heating. However, the Mount Painter Province in northern South Australia is characterized by a total crustal contribution to surface heat flow of more than twice the global average. The province is composed dominantly of Proterozoic granites and granite gneisses with an area average heat production of 16·1 μW/m3; individual lithologies have heat production >60 μW/m3. These Proterozoic rocks are intruded by the British Empire Granite, a younger intrusive whose origin has remained enigmatic. Isotope geochemistry suggests crustal sources for the melt and it has a crystallization age of ∼440–450 Ma, which places the setting >750 km inboard of the nearest active plate boundary zone at this time. Phase equilibria calculations suggest that temperatures of at least 720–750°C are required to produce the granite but the intensity of crustal thickening during Palaeozoic deformation (∼12%) cannot account for these conditions. Here we describe a model for the generation of the British Empire Granite in which the primary thermal perturbation for mid-crustal anatexis was provided by the burial of the high heat-producing Mount Painter basement rocks beneath the known thickness of Neoproterozoic cover sediments. The high heat-producing rocks at Mount Painter imply that the natural range and variability of crustal heat production is much greater than previously believed, with important consequences for our understanding of temperature-dependent crustal processes including the exploitation of geothermal energy resources.

On the role of heat flow, lithosphere thickness and lithosphere density on gravitational potential stresses

Gravitational potential stresses (GPSt) are known to play a first-order role in the state of stress of the Earth's lithosphere. Previous studies focussed mainly on crust elevation and structure and little attention has been paid to modelling GPSt using realistic lithospheric structures. The aim of the present contribution is to quantify gravitational potential energies and stresses associated with stable lithospheric domains. In order to model realistic lithosphere structures, a wide variety of data are considered: surface heat flow, chemical depletion of mantle lithosphere, crustal thickness and elevation. A numerical method is presented which involves classical steady-state heat equations to derive lithosphere thickness, geotherm and density distribution, but additionally requires the studied lithosphere to be isostatically compensated at its base. The impact of varying surface and crustal heat flow, topography, Moho depth and crust density on the signs and magnitudes of predicted GPSt is systematically explored. In clear contrast with what is assumed in most previous studies, modelling results show that the density structure of the mantle lithosphere has a significant impact on the value of the predicted GPSt, in particular in the case of thick lithospheres. Using independent information from the literature, the method was applied to get insights in the state of stress of continental domains with contrasting tectono-thermal ages. The modelling results suggest that in the absence of tectonic stresses Phanerozoic and Proterozoic lithospheres are spontaneously submitted to compression whereas Archean lithospheres are in a neutral to slightly tensile stress state. These findings are in general in good agreement with global stress measurements and observed geoid undulations.

Variations of strength and localized deformation in cratons: The 1.9 Ga Kapuskasing uplift, Superior Province, Canada

The Kapuskasing Structural Zone was the site of local crustal thrusting and uplift within the Superior craton, presumably in response to stresses applied at the margin of the craton during the Paleoproterozoic ca. 1.9 Ga. We use the present-day heat flow data in the Superior Province to calculate the strength of the lithosphere and to show that the uplift took place in the weakest part of the craton. After eliminating heat flow data that were affected by the uplift and later events (Keweenawan rifting), we calculate the heat flow and the temperature field in the lithosphere at 1.9 Ga. We show that the Kapuskasing region was in the center of a “thermal anomaly” at this time. The strength of the lithosphere in this region may have been up to 2–3 orders of magnitude lower than in surrounding parts of the craton, depending on crustal heat source distribution. Uplift and the resulting erosion of the upper crust, as evidenced by geothermobarometry data from around Kapuskasing, led to a reduced crustal heat production compared to the average crust of the southern Superior Province. This reduced heat production resulted in low crustal temperatures and a strong lithosphere permitting the long term preservation of a crustal root.

Crustal heat production in the Superior Province, Canadian Shield, and in North America inferred from heat flow data

Measurements of heat flow and U, Th, K concentrations are used to determine the amount of heat generated in various belts of the Superior Province, the largest Archean craton on Earth. These data allow estimates of the average crustal heat production and indicate compositional differences between upper and lower crustal assemblages. The bulk average heat production of the Superior Province crust is 0.64 μW m−3and is almost the same in different belts of slightly different ages, illustrating the remarkable uniformity of crust‐building mechanisms. In the wider context of the North American continent, the bulk crustal heat production decreases from 1.0 μW m−3in the oldest Slave Province to a minimum of 0.55 μW m−3in the Paleo‐Proterozoic Trans‐Hudson Orogen. It increases in younger provinces, culminating with a high value of 1.05 μW m−3in the Phanerozoic Appalachian Province. In all provinces, U and Th enrichment is systematically associated with sedimentary accumulations. A crustal differentiation index is obtained by calculating the ratio between the average values of heat production at the surface and in the bulk crust. The differentiation index is correlated with the bulk average heat production, which suggests that crustal differentiation processes are largely driven by internal radiogenic heat.

The mid-Norwegian margin: a discussion of crustal lineaments, mafic intrusions, and remnants of the Caledonian root by 3D density modelling and structural interpretation

The high-density lower crustal body (LCB) on the mid-Norwegian margin is almost universally interpreted to represent magmatically underplated material, added to the crust during Early Tertiary opening of the NE Atlantic. The thickness of the LCB is uneven, and its distribution along the margin is sharply limited by margin-perpendicular lineaments. Three-dimensional density modelling constrained by petrophysical and seismic data was performed to investigate the expression of the various lineaments (Bivrost, Jan Mayen, Gleipne, etc.) in the crustal architecture, with and without the LCB. The Bivrost Lineament, separating the Vøring and Lofoten margin segments, is clearly expressed and so is arguably a lineament in the SW Vøring margin. Notably, most of the lineaments can be interpreted as offshore prolongations of major onshore detachments stemming from Late Caledonian orogenic collapse. An alternative interpretation of the LCB is thus that it represents high-grade metamorphic rocks, remnant from the Caledonian root. Determining the nature of the LCB has profound effects for the volume estimate of magmatic rocks in the North Atlantic Igneous Province, and consequently also for the degree of crustal thinning and heat flow.

The early thermal and magnetic state of the cratered highlands of Mars

Surface heat flows are calculated from elastic lithosphere thicknesses for the heavy cratered highlands of Mars, in terms of the fraction of the surface heat flow derived from crustal heat sources. Previous heat flow estimations for Mars used linear thermal gradients, which is equivalent to ignoring the existence of heat sources within the crust. We compute surface heat flows following a methodology that relates effective thickness and curvature of an elastic plate with the strength envelope of the lithosphere, and assuming crustal heat sources homogeneously distributed in a radioactive element-rich layer 20 or 60 km thick. The obtained results show that the surface heat flow increases with the proportion of heat sources within the crust, and with the decrease of both radioactive element-rich layer thickness and surface temperature. Also, the results permit us to calculate representative temperatures for the crust base, rock strength for the upper mantle, and lower and upper limits to the crustal magnetization depth and intensity, respectively. For Terra Cimmeria, an effective elastic thickness of 12 km implies between 30% and 80% of heat sources located within the crust. In this case the uppermost mantle would be weak at the time of loading, and temperatures in the lower crust cold enough to favor unrelaxed crustal thickness variations and to permit deep Curie depths in the highlands, as suggested by the observational evidence.

Correction to “Martian megaflood‐triggered chaos formation, revealing groundwater depth, cryosphere thickness, and crustal heat flux”

Correction to “Martian megaflood‐triggered chaos formation, revealing groundwater depth, cryosphere thickness, and crustal heat flux”

Evidence for a differentiated crust in Solis Planum, Mars, from lithospheric strength and heat flow

Two independent sets of heat flow estimates provide constraints on the Hesperian-era surface and mantle heat flows, and the thickness of the heat-producing elements (HPE)-enriched upper crust, in the Solis Planum region of Mars. The calculations, which use the concentration of uppermost crust heat sources deduced from orbital gamma ray spectroscopy and soils geochemistry, are based on the effective elastic thickness of the lithosphere and the minimum depth of faults underlying winkle ridges. We find that, for the majority of analyzed settings, the HPE-enriched crust is thinner than the whole crust thickness in this region (∼65 km). Thus, our results strongly support a differentiated martian crust.

Martian megaflood‐triggered chaos formation, revealing groundwater depth, cryosphere thickness, and crustal heat flux

Ravi Vallis is an outflow channel complex that emerged from Aromatum Chaos, a region of collapsed ground in Xanthe Terra, Mars. I propose that the megaflood that carved Ravi Vallis incised the crust more than 700 m, penetrated the cryosphere, and triggered the release of groundwater and genesis of secondary chaos zones in the deepest parts of the channel system. Discharge rates of 4 × 106 to 5 × 107 m3 s−1 are estimated for early overland flooding. Assuming bank‐full flow would yield unrealistic discharge rates of &gt;3 × 108 m3 s−1. Waning stage flooding in an inner channel had a discharge rate on the order of ≲1 × 106 m3 s−1. The cryosphere thickness and groundwater depth are estimated as the sum of the channel incision depth and the thickness of residual cryosphere beneath the channel floor that could be disrupted by high artesian pressure. The surface elevation (−1231 m) of a transient water body in Aromatum Chaos would have controlled the maximum potentiometric heads in confined aquifers to the east. Where Iamuna Chaos was spawned, the floor of Ravi Vallis would have experienced upward hydraulic pressures of ≤2.4 MPa, enough to rupture a basaltic rock layer ≤255 m thick. At the time of the flood the cryosphere thickness and groundwater depth would have been 700 to 1000 m at this location on the equator, indicating a relatively cold, long‐term climate trend similar to that of present‐day Mars. The crustal heat flux likely exceeded 50 mW m−2 at Iamuna Chaos. If deep lakes existed at that time in the ancestral Valles Marineris, their ice covers are estimated to have been less than 3 km thick. The fluvial incision and groundwater breakout model described here can fully account for the inception of Iamuna Chaos, Oxia Chaos, and a smaller unnamed chaos. Therefore a locally enhanced crustal heat flux was not required to initiate the formation of these chaotes.

Relationship between recent heat flow and seismic properties: Some notes from crustal research in Germany

Deep seismic reflection profiling has been used extensively during the last 25 years in order to unravel crustal structure and evolution, complemented by deep drilling in crystalline rocks. Examples are presented from the Variscan/Hercynian crust in Germany showing moderate-to-high surface heat flow densities which can be studied for relationships with seismic reflections and velocities. Four main seismic features are identified which emphasise that heat transfer through the crust is a strong three-dimensional problem similar to that of sedimentary basins: (1) The top and thickness of the highly reflective lower crust as a specific dominant crustal entity correlate with high surface heat flow. The creation of lower crustal reflections is commonly attributed to a post-collisional extensional regime. The rheological character of top and bottom (crust–mantle boundary) of the lower crust indicates decoupling horizons at these levels. (2) Upper crustal reflectivity is frequently attributed to fault and fracture zones. These may delineate pathways for heat transport through fluid circulation due to higher (crack) porosity and possibly higher permeability. (3) Seismic anisotropy of large crustal volumes has been confirmed as a common feature and is particularly strong in gneisses due to their strain-related preferred orientation of the texture. Gneisses are also known for their anisotropy of heat conductivity having the same magnitude and preferred orientation. (4) Seismic in-situ velocities are always lower than laboratory-derived or composition-derived velocities. In consequence, crustal composition models inverted from seismic velocity models tend to be biased towards felsic composition. The reason for this is most likely the porosity of crystalline rocks including some hydrothermal activity which may also explain the occurrence of low-velocity zones often observed in the middle crust. Modelling of crustal temperatures, heat transfer and heat production should account for these phenomena.

Seismic constraints on temperature of the Australian uppermost mantle

We derive estimates of temperature of the Australian continental mantle between 80 and 350 km depth from two published S-velocity models. Lithospheric temperatures range over about 1000 °C, with a large-scale correlation between temperature and tectonic age. In detail however, variations ranging from 200 to 700 °C occur within each tectonic province. At the current seismic resolution, strictly Proterozoic and Archean blocks do not have substantially different temperatures, nor does the Phanerozoic lithosphere east and west of the Tasman line. Temperatures close to an average (moist) MORB source mantle solidus characterize the eastern seaboard and its offshore. Differences between the temperatures derived from the two velocity models illustrate the importance of well-constrained absolute velocities and gradients for physical interpretation. The large range of lithospheric temperatures cannot be explained solely with documented variability in crustal heat production, but requires significant variations in mantle heat flow as well.

Heat flow, thermal regime, and elastic thickness of the lithosphere in the Trans-Hudson Orogen

Heat flow studies on the exposed part of the Trans-Hudson Orogen (THO) in northern Manitoba and Saskatchewan allow constraints on crustal composition and lithosphere structure. The average of all heat flow values in the THO is the same as in other geological provinces of the Canadian Shield. However, where juvenile crust is exposed, heat flow is on average lower than in the Superior and Grenville provinces (37 vs. 41 mW m–2). Heat flow increases towards the surrounding Archean provinces, Rae–Hearne to the west, Sask to the south, and Superior to the east. There are strong differences in heat flow within and between the belts of the THO. The poor correlation between heat flow and heat production in the rocks exposed at the surface implies that these differences involve a large fraction of the crustal column. One new heat flow determination confirms the existence of a ``cold spot'' around the town of Lynn Lake in the northern part of the THO. Heat flow data in the Kisseynew and Glennie domains remain sparse, but they indicate that this low heat flow region extends as far south as the Flin Flon – Snow Lake Belt. The Lynn Lake Belt is underlain by poorly radiogenic rocks, possibly Kisseynew-type crust with oceanic basement. Northward increase in heat flow along the Thompson Belt is consistent with the view that the belt is thrust over Kisseynew-type basement only in the south. Heat flow increases southward in the Paleozoic basin because of higher heat production in basement rocks, probably from the Sask craton. We used the low heat flow regions to obtain an upper bound of 15 mW m–2 for the mantle heat flow in the THO. The effective elastic thickness of the lithosphere can be determined from the coherence between the topography and the Bouguer gravity. The effective elastic thickness is high (&gt;40 km) thoughout the Canadian Shield and is highest in the central part of the shield, in particular in the Lynn Lake region. There seems to be a negative correlation between elastic thickness and heat flow in the central and western Canadian Shield. This indicates that, even in stable continents, the elastic thickness is largely controlled by the lithospheric temperatures that depend strongly on crustal heat generation and hence crustal structure.

本震と最大余震のマグニチュード差と地殻熱流量

本震と最大余震のマグニチュード差と地殻熱流量

Heat flow in the Vøring Basin, Mid-Norwegian Shelf

In situ temperature and heat flow were determined in 1994 at 159 sites, grouped into 66 clusters between latitude 65° N and 67°30’ N at water depths from 669 m to 1464 m. The mean of all cluster heat-flow measurements conducted in this survey was 58.5 mW m −2 , with a standard error of ±4.40 mW m −2 . The mean heat flow from IKU well data for the Trøndelag Platform is 56.2±6.65 mW m −2 . Shorter wavelength heat-flow variations appear to be controlled structurally and can be explained by sedimentation and thermal refraction effects. High heat flow associated with faulted structural highs such as the Nyk High and Vema Dome–Rym Fault Zone may also result from hydrothermal convection. Relatively isolated high (106.6 mW m −2 ) heat flow observed at 846 m water depth may be an artefact of bottom water disturbances; however, virtually identical deep-water heat-flow anomalies, believed to be of hydrothermal origin, also exist. While heat-flow measurements made at water depths less than 1000 m should be regarded with caution, there is presently no justification for eliminating those exhibiting linear heat flow with depth. Submarine avalanches seem unimportant in the survey area. Neither crustal thinning, underplating nor sill intrusion, within the last 50 Ma, would have a measurable effect on present-day heat flow. The net effect of crustal thinning may be a reduction of the crustal heat generation potential, depending on the degree of thinning of the upper crust, since the accumulating sediments cannot compensate fully for the lost heat generation from a crystalline basement.