Crustal Heat Generation Research Articles

From 3.4 Ga TTG generation to 2.9 Ga crustal anatexis: The Archean crustal evolution of Porteirinha Complex (SE, Brazil)

The Porteirinha Block (PB) provides an important record of the origin and evolution of South American crust. The PB records the generation of Paleoarchean TTG, Mesoarchean anatexis, and Paleoproterozoic crustal reworking. It is exposed as a basement inlier in the Araçuaí Orogen as a result of a tectonic involvement of the continental margins of São Francisco Congo Paleocontinent (PSFC) during the collisional processes of Gondwana amalgamation at the Neoproterozoic. The PB basement unit is the Porteirinha Complex (PC), which consists of TTG orthogneisses, migmatites, amphibolites, metaultramafics rocks and Neoarchean granitoids. In this paper, we discuss the evolution of the TTG orthogneisses and migmatites from PC based on field, geochronological and whole-rock geochemistry data. Zircon U–Pb ages (LA-ICP-MS) show a complex and polycyclic evolution of these rocks. Concordant ages were obtained for crystallization of TTG magma at ca. 3.4 Ga and the geochemical signature supports the idea of TTG generation by partial melting of hydrated mafic crust under garnet-amphibolite facies conditions. Zircon U–Pb ages also suggest crustal reworking events at 3.1 Ga. Metatexite and diatexite domains related to partial melting of TTG orthogneisses are individualized. The migmatites record processes of crustal anatexis and melt generation that occurred between 2.8 and 2.9 Ga (LA-ICP-MS, U–Pb in zircon) and are linked to crustal reworking and heat generation events recognized regionally. The new data presented in this work allow us to establish correlations between Archean and Paleoproterozoic units of the PB and those that occur regionally, particularly those related to Gavião block, where a series of stages and events of early crustal evolution are shared.

A Geothermal Heat Flux Map of Antarctica Empirically Constrained by Seismic Structure

AbstractThe geothermal heat flux (GHF) is an important boundary condition for modeling the movement of the Antarctic ice sheet but is difficult to measure systematically at a continental scale. Earlier GHF maps suffer from low resolution and possibly biased assumptions in tectonism and crustal heat generation, resulting in significant uncertainty. We present a new GHF map for Antarctica constructed by empirically relating the upper mantle structure to known GHF in the continental United States. The new map, compared with previously seismologically determined one, has improved resolution and lower uncertainties. New features in this map include high GHF in the southern Transantarctic Mountains where warmer uppermost mantle is introduced by lithospheric removal and in the Thwaites Glacier region. Additionally, a modest GHF in the central West Antarctic Rift system near the Siple Coast and an absence of large‐scale regions with GHF greater than 90 mW/m2 are found.

Thermal modelling of very long-lived (>140 Myr) high thermal gradient metamorphism as a result of radiogenic heating in the Reynolds Range, central Australia

The Reynolds Range in the Paleo-Mesoproterozoic Aileron Province of central Australia exposes a differentially exhumed metamorphic rock system varying from greenschist-to granulite-facies. Calculated P–T pseudosections indicate that peak metamorphic conditions in greenschist-facies areas reached temperatures of ∼480 °C and pressures of 3.2–5 kbar. Peak conditions at amphibolite-facies are constrained to 500–620 °C and 1.3–3.8 kbar. P–T modelling of two petrologically different granulite-facies samples indicate that peak conditions in the exhumed granulite-facies rocks reached 780–920 °C and 3.5–5.4 kbar, corresponding to high apparent thermal gradients of 152–218 °C/kbar. In the amphibolite-facies rocks, in situ LA–ICP–MS U–Pb monazite geochronology records ages that range between 1680 and 1550 Ma. In contrast, in the granulite-facies rocks, monazite records ages of ca. 1560 Ma. The terrane contains significantly elevated rates of crustal heat production compared to global norms, and when incorporated into lithospheric thermal models with reasonable boundary conditions and average thermal properties, can plausibly account for the long-lasting metamorphic conditions. The thermal modelling shows that the long lasting (>100 Myr) record of monazite U–Pb ages, supported by previous studies, reflects a continuous duration of elevated crustal temperatures, an interpretation consistent with texturally simple mineral assemblages that lack any evidence for polymetamorphism. In high-crustal heat production terranes, high thermal gradients and high temperatures will persist until exhumation removes the upper crust. This physical reality provides a basis for the existence of arbitrarily long-lived metamorphic systems, even under high thermal gradients that are generally considered to be thermally anomalous. However, such thermal anomalism is not remarkable, as there is abundant evidence for elevated rates of crustal heat production in the Reynolds Range and worldwide. In terranes where accessory minerals record large continuous ranges of age, robust volumetric measurements of crustal heat production rates are a necessary step in evaluating the potential thermal significance of such geochronological records. Failure to document rates of crustal heat generation invalidates many interpretations for drivers of metamorphism.

Petrology and Geochemistry of A-type granites from Khanak and Devsar Areas of Bhiwani District, Southwestern Haryana

ABSTRACT Petrological and geochemical characteristics of the granites from the Khanak and Devsar areas have been discussed in this paper. Based on field, petrographical and geochemical observations, three types of granites (grey, green and pink granite) have been identified in these areas. Grey granites consist of quartz, plagioclase, biotite, hornblende as essential minerals and hematite, zircon, annite, monazite & rutile as accessory minerals. Petrographically, green granites are same as grey granites including perthite and zircon as accessory minerals. Pink granites consist of quartz, k-feldspar and biotite in Khanak whereas in Devsar granites alike as Khanak granites, but plagioclase is replaced by perthite and occurs as dominantly. Microscopically, granites of both areas show porphyritic, hypidiomorphic, granophyric, perthitic and micro granophyric textures. Geochemically, major oxide elements (except alkalies) and trace elements (Ba, Sr, Cr, Ni, V, Cu, Zn, Ga, Pb, Th and Zr) are more in green and grey granites of Khanak and Devsar areas than pink granites. Generally, they show enrichments in SiO2, Na2O+K2O, Fe/Mg, Rb, Zr, Y and and AI (Agpaitic Index) (ranges from 0.10 to 1.18) and depletion in MgO, CaO, P, Ti, Ni, Cr and V indicate their A-type affinity which is very similar to the A-type granites of MIS (Malani igneous suite) in northwestern peninsular India. Green and grey granites of Devsar area show high concentrations of Heat production (HP) 9.68 & 11.70 μWm−3 and total Heat Generation Unit (HGU) i.e 23.04 & 27.86 respectively. On the other hand, pink granites of Khanak area display a higher enrichment of HP (16.53 μWm−3) and HGU (39.37) than those granites of Devsar area. Overall, they have much a higher values of HP and HGU than the average value of continental crust (3.8 HGU), which imply a possible linear relationship with the surface heat flow and crustal heat generation in the rocks of MIS. From the petrography as well as the chemistry of Khanak and Devsar granites, it is suggested that they might have derived from the different degree of partial melting from the similar source of magma.

Estimates of [formula omitted] for continental regions using GOCE gravity

Satellite-only gravity fields and surface gravity obtained from altimetric measurements now agree well at wavelengths greater than ∼180 km. Satellite gravity fields can therefore be used to estimate the elastic thickness Te in regions where surface observations are sparse. They are used for this purpose in a number of continental regions, of India, Africa, and Antarctica, where the topography is sufficiently rough, and also in regions of the USA, China, Australia and Siberia, where there are surface measurements. Estimates of Te for Antarctica depend on measurements of ice thickness, which are now available for much of the continent. Values of Te are obtained using two methods: from the admittance between the free air gravity and the topography, and from the coherence between Bouguer gravity anomalies and the topography. The first, but not the second, gives values of Te that are everywhere less than the seismogenic thickness. Where there is sufficient topography, estimates of Te from PreCambrian shields are all greater than 10 km and do not correlate with the lithospheric thickness. They are probably are governed by variations in crustal heat generation rates. Values for regions strongly affected by Phanerozoic tectonics are all less than 7 km, and all such regions are underlain by thin lithosphere.

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

Ultrahigh temperature (UHT) metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900 °C. UHT crustal metamorphism is recognised in more than 50 localities globally in the metamorphic rock record and is accepted as ‘normal’ in the spectrum of regional crustal processes. UHT metamorphism is typically identified on the basis of diagnostic mineral assemblages such as sapphirine + quartz, orthopyroxene + sillimanite ± quartz and osumilite in Mg–Al-rich rock compositions, now usually coupled with pseudosection-based thermobarometry using internally-consistent thermodynamic data sets and/or Al-in-Orthopyroxene and ternary feldspar thermobarometry. Significant progress in the understanding of regional UHT metamorphism in recent years includes: (1) development of a ferric iron activity–composition thermodynamic model for sapphirine, allowing phase diagram calculations for oxidised rock compositions; (2) quantification of UHT conditions via trace element thermometry, with Zr-in-rutile more commonly recording higher temperatures than Ti-in-zircon. Rutile is likely to be stable at peak UHT conditions whereas zircon may only grow as UHT rocks are cooling. In addition, the extent to which Zr diffuses out of rutile is controlled by chemical communication with zircon; (3) more fully recognising and utilising temperature-dependent thermal properties of the crust, and the possible range of heat sources causing metamorphism in geodynamic modelling studies; (4) recognising that crust partially melted either in a previous event or earlier in a long-duration event has greater capacity than fertile, unmelted crust to achieve UHT conditions due to the heat energy consumed by partial melting reactions; (5) more strongly linking U–Pb geochronological data from zircon and monazite to P–T points or path segments through using Y + REE partitioning between accessory and major phases, as well as phase diagrams incorporating Zr and REE; and (6) improved insight into the settings and factors responsible for UHT metamorphism via geodynamic forward models. These models suggest that regional UHT metamorphism is, principally, geodynamically related to subduction, coupled with elevated crustal radiogenic heat generation rates.

Petrological and Geochemical Constraints in the Origin and Associated Mineralization of A-Type Granite Suite of the Dhiran Area, Northwestern Peninsular India

Neoproterzoic, anorogenic, A-type granites of Dhiran area of Malani Igneous Suites is made up wholly of peralkaline-peraluminous alkali feldspar granites and is composed of K-feldspar, quartz, alkali amphibole, plagioclase, biotite and accessory minerals are iron oxides, monozite, zircon, apatite, and annite. This granitoid is associated with acid volcanic and minor basic volcanic-plutonic and dykes. Petrographically, they show cloudy, patchy perthitic texture and graphic texture. They are highly evolved ferroan, alkaline, A-type granites, displaying the typical geochemical characteristics of A-type granites with high SiO2, Na2O + K2O, FeO*/MgO, Ga/Al, Zr, Nb, Ga, Y, Ce and rare earth elements (REE) and low CaO, MgO, Ba and Sr. Their trace and REE characteristics along with the use of various discrimination schemes revealed their correspondence to magmas derived from crustal origin. The extremely high Rb/Sr ratios combined with the obvious Sr, Ba, P, Ti and Eu depletions clearly indicate that these A-type granites were highly evolved and require advanced fractional crystallization in upper crustal conditions. These granites are high to medium content of radioactive element (U, Th, K) together Heat generation and Heat Production (HP). Leucocratic granites show higher Heat Production (4.84-8.75 µWm-3) and total Heat Generation Unit (11.52-20.84 HGU) than pink granite (2.09-2.94 µWm-3 (HP), 4.98-7.07 HGU). The granites of the Dhiran area show higher average value of total Heat Generation Unit (16.51 HGU and 5.79 HGU) than the average value of 3.8 HGU for the continental crust. The high Heat Generation Unit values (4.98-20.84) of the Dhiran granites indicate ‘hot crust’ category and a possible linear relationship between the surface heat flow and crustal heat generation in the Malani Igneous Suite.

3D crustal-scale heat-flow regimes at a developing active margin (Taranaki Basin, New Zealand)

The Taranaki Basin in the west of New Zealand's North Island has evolved from a rifted Mesozoic Gondwana margin to a basin straddling the Neogene convergent Australian–Pacific plate margin. However, given its proximity to the modern subduction front, Taranaki Basin is surprisingly cold when compared to other convergent margins. To investigate the effects of active margin evolution on the thermal regime of the Taranaki Basin we developed a 3D crustal-scale forward model using the petroleum industry-standard basin-modelling software Petromod™.The crustal structure inherited from Mesozoic Gondwana margin breakup and processes related to modern Hikurangi convergent margin initiation are identified to be the main controls on the thermal regime of the Taranaki Basin. Present-day surface heat flow across Taranaki on average is 59mW/m2, but varies by as much as 30mW/m2 due to the difference in crustal heat generation between mafic and felsic basement terranes alone. In addition, changes in mantle heat advection, tectonic subsidence, crustal thickening and basin inversion, together with related sedimentary processes result in variability of up to 10mW/m2. Modelling suggests that increased heating of the upper crust due to additional mantle heat advection following the onset of subduction is an ongoing process and heating has only recently begun to reach the surface, explaining the relatively low surface heat flow. We propose that the depth of the subducted slab and related mantle convection processes control the thermal and structural regimes in the Taranaki Basin. The thermal effects of the subduction initiation process are modified and overprinted by the thickness, structure and composition of the lithosphere.

High Heat Producing Volcano-Plutonic Rocks of the Siner Area, Malani Igneous Suite, Western Rajasthan, India

he volcano-plutonic rocks of the Siner area of the Malani Igneous Suite (MIS) are characterized by high content of radioactive elements (U, Th, K) and are classified as high heat producing felsic volcano-plutonic rocks of A-type granitoid. Microgranite shows highly comparable heat production (26.07 HP) and total heat generation value (62.06 HGU in average) as compared to other granite (HP = 12.73; HGU = 26.57), rhyolite (HP = 4.98; HGU = 11.85) and trachyte (HP = 5.00; HGU = 11.91). The volcano-plutonic rocks of the present show higher average value of total HGU than the average value of 3.8 HGU for the continental crust, which suggests a possible linear relationship among the crustal heat generation of the MIS.

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.

Reply to comment by W. P. Schellart on “The thermal structure of subduction zone back arcs”

[1] Currie and Hyndman [2006] documented the evidence that the majority of continental back arcs are uniformly hot with thin lithospheres over regional extents of hundreds of kilometers, even if where is no thermally significant extension. Using a number of independent observations, we showed that eight such current subduction zones have similar thermal regimes, with Moho temperatures greater than 800 C (35–40 km depth) and lithosphere thicknesses of 60 km. The one exception, the Peru flat slab region of South America, supports our model explanation. We then presented the hypothesis that the elevated back-arc temperatures have a common mechanism: efficient transport of heat by vigorous thermal convection in hydrated, low-viscosity back-arc upper mantle. [2] The paper was structured such that the observations indicating regionally high back-arc temperatures were separate and independent from our proposed model. The evidence for high uniform temperatures seems secure; the discussion is on the explanation. Schellart [2007] appears to agree that the back arcs included in our study are characterized by hot, thin lithospheres but does not agree with our interpretation of the underlying cause. His main criticisms are that (1) local tectonics, in particular extension, may account for the high temperatures in some areas, and (2) three-dimensional mantle flow and complex slab dynamics may produce elevated temperatures. [3] We acknowledge, as stated in the original paper, that a number of site-specific processes may affect the thermal regime at each back arc. We also recognize that nearly all back-arc regions have undergone at least some past extension, as described by Schellart [2007]; notable exceptions are central Alaska and the northern Canadian Cordillera. The key question is whether or not this extension is the primary cause of the observed uniformly hot thermal regimes. [4] Schellart [2007] argues that high surface heat flow at some back arcs is due to recent extension, including parts of the Sunda (Borneo) and Ryukyu (Korea) back arcs. We note that our compilation used a number of independent observational constraints on lithosphere temperatures, not just surface heat flow. Mantle temperatures determined using only surface heat flow data are highly uncertain, due to the point sampling and measurement uncertainties, and uncertainties in the thermal parameters used to calculate the geotherm. Other temperature constraints on upper mantle thermal structure include seismic velocities from refraction Pn and tomography, mantle xenolith thermobarometry, effective elastic thickness Te, etc. For each back arc, the upper mantle temperatures from all constraints are in good agreement and show no significant lateral variation on a regional scale. This is especially illustrated by the uniform low upper mantle seismic velocities for different back arcs [Currie and Hyndman, 2006, and references therein]. [5] Extension affects the thermal regime by locally decreasing lithosphere thickness and by inducing upwelling of hot underlying mantle. Modeling studies have shown that such extension should only affect lithosphere temperatures and surface heat flow within 100 km of the extended region [Morgan, 1983; Alvarez et al., 1984], and for continental crust, extension may actually decrease surface heat flow due to thinning of the crustal radioactive heat generation [Waples, 2001]. In addition, limited local extension should not significantly perturb the deep thermal region. For the back arcs in our compilation there is no evident upper mantle temperature increase in more strongly extended regions, suggesting that a more regional phenomenon is required to produce regionally elevated temperatures. We also note that extension can only occur if the lithosphere is sufficiently hot that it is weak enough to fail under available extensional stresses. A normal cool lithosphere is too strong to be deformed by plate tectonic forces [e.g., Hyndman et al., 2005]. We suggest that back-arc extension is a consequence of high temperatures and associated weak lithosphere, and not the primary cause. [6] Schellart [2007] also suggests a thermal role for a number of upper mantle dynamical processes, including along-margin mantle flow associated with a slab edge, slab retreat/advance, and slab break-off. We agree that each of these factors may affect the back-arc thermal regime in some locations. However, these are local phenomena. If they are thermally important, they should be evident in lateral variations in mantle seismic velocity and other deep temperature indicators. Lateral variations are not apparent in the current observational data. [7] As stated in the original paper [Currie and Hyndman, 2006, paragraph 71], we do not rule out local factors in JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B11408, doi:10.1029/2007JB005415, 2007 Click Here for Full Article

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.

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.

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 (>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.

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.

Heat flow and deep lithospheric thermal structure at Lac de Gras, Slave Province, Canada

Temperature measurements in two drill holes near Lac de Gras in the Slave Province of the Canadian Shield yield heat flow values of 43 ± 5 and 48 ± 8 mW m−2 with an average of 46 ± 6 mW m−2. Heat production measured on samples from one of these holes is relatively high (1.9 μW m−3) confirming measurements made on outcrop samples. The surface heat flow and heat generation data imply that below a 10 km thick radioelement enriched layer, heat production is comparable to that in other Archean Provinces of the Canadian Shield. The lithospheric geotherm is very close to the geotherms derived from geothermobarometry on xenoliths. Variations in crustal heat generation account for ≈50 K differences in Moho temperatures within the Slave Province.

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.

Depth of faulting on Mercury: Implications for heat flux and crustal and effective elastic thickness

Topographic profiles across a lobate fault scarp on Mercury have been used to constrain the depth of faulting to 30–40 km. Here we use this depth to place constraints on the crustal thickness and heat flow into the base of the crust. With no crustal heat production, the mantle heat flux on Mercury at the time of scarp formation was 30–50 mWm−2. However, higher crustal heat production rates allow significantly lower mantle heat fluxes. In all cases the mantle heat flux exceeds the likely radiogenic heat flux 4 Gyr ago; it is likely that secular cooling accounted for the remainder. Irrespective of crustal heat generation, a crustal thickness of ≤140 km is required to satisfy both the faulting observations and the requirement that the base of the crust does not melt. The effective elastic thickness of the lithosphere at the time of faulting is predicted to be 25–30 km.

Heat flow models across the Trans-European Suture Zone in the area of the POLONAISE’97 seismic experiment

Heat flow data from the Polish basin show a sharp change in the transition from the East European Craton (EEC) and Teisseyre–Tornquist Zone (TTZ) in the north-east to the accreted terranes in the south west (Paleozoic Platform). The analysis of this data and numerical modelling of the crustal temperatures show evidence of extensive crustal–mantle warming in the area between the Sudetes to the south and the Trans-European Suture Zone to the north. The change in heat flow is 100% when compared with values for the EEC. Heat flow in the anomalous zone is also higher than in the Sudetes. The axis of the anomaly is aligned with the Dolsk Fault and Variscan deformation front. Low crustal/mantle temperatures derived from the relationship between temperature and P n velocities (more than 8.2 and as high as 8.4 km/s) are at odds with high crustal temperatures calculated from surface heat flow, seismic velocity based heat generation models and thermal conductivity. High heat flow (Variscan platform) and related high temperatures of the crust coincide with small crustal thickness (30–35 km). The opposite is the case for the low heat flow EEC (45–50 km). High heat flow above thin crust and low heat flow above thick crust with no major variation in elevation is supported by a simple isostatic balance model. Crustal heat generation explains part of the high heat flow within the zone with thick meta-sediments reaching down to 20 km depth, however, it is far from explaining high heat flow in Variscan crust and in the transition zone into a cold EEC. 2D numerical models of heat flow based on new seismic data require a contrast of 15 mW/m 2 in mantle heat flow. High mantle heat flow (35–40 mW/m 2) is likely to occur in the high heat flow zone while cold crust and cold and high-density mantle (mantle heat flow of 20–30 mW/m 2) is typical of the EEC. Thermal lithosphere thickness for the craton is 200 km while it is only 100 km in the accreted terranes to the southwest of the TTZ. The TTZ in Poland appears as a relatively cold area.

ESTIMATION OF HEAT SOURCES IN PLANETARY CRUSTS FROM ISOTHERM DEPTH, SURFACE HEAT FLOW, AND CRUSTAL THICKNESS

The depth to an isotherm provides clues on the intensity of heat sources within a planetary crust. In this work, we show that the depth to an isotherm and crustal thickness can be jointly used to give an approximate estimation of the fractionof the surface heat flow that is being originated from the crust of a terrestrial planet. Relationships between crustal heat generation rate and crustalthickness, and surface and mantle heat flow variations on a planet were also explored. The proposed methodology may serve to improve present descriptions of the crustaltemperature-depth profiles of terrestrial planets, and may also provide information on chemical and thermal evolution.