Crustal Heat Production Research Articles

Thermal budget of the Transylvanian lithosphere. Reasons for a low surface heat-flux anomaly in a Neogene intra-Carpathian basin

SUMMARY This paper investigates the heat-flux budget of the Transylvanian Basin along a 240 km long WNW‐ESE geological profile. The thermal effects of sedimentation, erosion, crustal heat production and mantle heat flux on the thermal structure are studied. The thermal modelling is based on a 2-D finite-element model, which includes the effects of changing sedimentation rates, vertical and lateral variations in sediment thermal parameters, sediment compaction, as well as lateral variations in the crustal heat production and mantle heat flux. Even though the sedimentation history of the Transylvanian Basin began in Senonian time (90 Ma) and the total sediment thickness in the centre amounts to 8 km, half of the sediment cover was deposited in Neogene times from 16.2 to 5.6 Myr. The uplift and erosion of the basin began 5.6 Ma, resulting in a present-day average elevation of ∼600 m. The surface heat-flux pattern exhibits low values of ∼30 mW m −2 in the middle of the basin and higher values of ∼60 mW m −2 towards the margins. The low heat flux is explained by a combination of a depletion in crustal heat production, contributing with ∼15 mW m −2 , and a relatively low mantle heat flux. The present-day transient heat-flux depression caused by the rapid Neogene depositional history and the subsequent thermal relaxation (5.6‐0 Ma) amounts to ∼2.5 mW m −2 . Lateral variation of the heat flux at the base of the lithosphere cannot produce the surface heat-flux anomaly.

Geothermal modeling along a two-dimensional crustal profile in Southern Portugal

Crustal temperatures along a S–N trending profile crossing the main geological units of Southern Portugal, the South Portuguese Zone and the Ossa Morena Zone, were calculated by solving numerically the heat conduction equation in a 2-D geothermal model of the crust. Use of observed surface heat flow and heat production based on the empirical relationship between radiogenic heat sources and observed seismic velocities, yields temperatures at the crust/mantle boundary, which are higher than the upper limit (about 700 °C) of crustal temperatures based on magnetotelluric surveys. The most uncertain parameter of the model is the upper crust heat production, whose low value implies a large non-crustal component of surface heat flow, which must be conducted through the whole crust and increases the vertical temperature gradient. To overcome this uncertainty in heat sources, the Moho heat flow was estimated using Vitorello and Pollack’s [J. Geophys. Res. (1980) 983] estimate of the background heat flow of 27 mW m −2 arising from below the zone of crustal enrichment (identified in our model with the upper crust) and transient thermal perturbation, represented in Variscan units by 5–10 mW m −2. Taking into account the heat production of the middle and lower crust, the mean value of the Moho heat flow should be 28–33 mW m −2. The Moho heat flow was allowed to vary (with variation restricted to 0.2–1.0 mW m −2 per 5 km) around 28 or 33 mW m −2 along the profile in order to improve the fit between the calculated and measured surface heat flows. Because this approach yields a surface heat flow lower than observed, the residual differences were minimized by increasing the heat production of the upper crust, where the production was allowed to vary along the profile (with variation restricted to 0.02–0.2 μW m −3 per 5 km). If Moho heat flow is allowed to vary by a total of about 9–25 mW m −2, the upper crustal heat production necessary for the calculated surface heat flow to match the observed one is in the range 2–3 μW/m −3, which indicates a mostly granitic upper crust. The resulting Moho temperatures are below 700 °C everywhere along the profile in all model configurations considered.

Simultaneous inversion of gravity and heat flow data: constraints on thermal regime, rheology and evolution of the Canadian Shield crust ☆

Heat flow and gravity are sensitive to crustal composition and thickness. In this paper, we show that heat flow and gravity data, combined with constraints from seismic refraction, can be inverted to obtain the gross composition of the crust. The inversion method is used to determine the crustal structure in different parts of the Canadian Shield. The long wavelength variations in heat flow imply a difference in thermal regime between and within provinces and subprovinces of the Canadian Shield. The crustal heat production determined by inversion is used to construct temperature and rheological profiles for the lithosphere in different parts of the Canadian Shield. Regions enriched in radioelements (the western part of the Abitibi subprovince and the Thompson Belt) had a weak rheology after they stabilized and could not withstand stresses for geologically long times. In contrast, subprovinces that are depleted in radioelements (the eastern part of the Abitibi and the Lynn Lake Belt) had a stronger rheology and might have preserved crustal roots after their formation.

Crustal heat production and style of metamorphism: a comparison between two Archean high grade provinces in the Limpopo Belt, southern Africa

Different characteristics of late Archean metamorphism in the Northern and Southern Marginal Zones of the Limpopo Belt in southern Africa are examined in the light of differences between these provinces in contents of heat generating elements. In the Northern Marginal Zone intrusion of charnoenderbitic magmas lasted from 2.75 to 2.58 Ga, accompanied by repeated low to medium pressure granulite facies metamorphism with some anticlockwise P–T paths. In contrast, the medium pressure granulites of the Southern Marginal Zone experienced peak metamorphism at 2.69 Ga without much evidence of plutonism at that time, followed by near isobaric cooling at approximately 6.5 kbar. As a basis for our approach we used steady state geotherms, calculated for the deeply eroded Marginal Zones for late Archean times on the basis of their average K, Th and U concentrations, and assuming low basal heat flow values of 20 mW m −2. The Southern Marginal Zone and the adjacent northern Kaapvaal Craton are both characterised by low average Th and U concentrations (Th: 1.1 and 1.5 ppm, U: 0.3 and 0.65 ppm), which are not a consequence of high grade metamorphism. The average Th and U concentrations in the Zimbabwe Craton (8.8 and 2.2 ppm) are much higher than those in the Kaapvaal Craton, and those found in the Northern Marginal Zone are even higher (13.8 and 2.6 ppm). In the Southern Marginal zone, peak metamorphic temperatures are much above reasonable steady state geotherms and peak conditions could only have been attained through transient very high basal heat flux, which, in the context of a subduction zone, might have resulted from slab breakoff or slab windows. The Northern Marginal Zone in contrast has such a large internal heat generation that high grade metamorphic conditions plot close to steady state geotherms even with basal heat flux values of 20 mW m −2, and minor crustal thickening or basal heat flux increase would have sufficed to exceed fluid absent tonalite solidus temperatures in the lower crust. The repeated high grade metamorphism and charnoenderbite intrusions over a period of 1.7×10 8 years in the province can be explained by this observation. Both marginal zones of the Limpopo Belt can be understood as Archean active continental margins (of the Kaapvaal and Zimbabwe Craton, respectively). While the problems of explaining the metamorphism in the Southern Marginal Zone (i.e. the source of the required heat) are similar to those faced in many present day margins or orogens, the Northern Marginal Zone is non-uniformitarian in the sense that its extreme heat generation rate could not be a property of any geological province today.

Lithosphere thermal structure and evolution of the Transylvanian Depression — insights from new geothermal measurements and modelling results

The surface heat flow density pattern of the Transylvanian Depression (TD) represents a marked high amplitude short wavelength low in a region of elevated heat flow. Detailed temperature–depth profiles obtained by continuous temperature logging, combined with a finite element modelling of topographic and fluid flow effects, support the conclusion that the observed thermal gradient in the TD truly represents the rate of heat loss of the subsurface. The surface heat flux is 30 mW m −2 in the centre of the depression, increasing to about 60 mW m −2 at margins. Climatic correction is of the order of 2–10 mW m −2 depending on the investigated depth interval. The measured temperatures are compatible with the cooling of the Earth surface during the Weichselian glaciation followed by the climate warming which started 10–11,000 years ago. The transient effects of Neogene sedimentation and erosion mean an overall 2–3 mW m −2 reduction in surface heat flux. A low mantle heat flux (30 mW m −2) and a low crustal heat production rate in the TD (0.5 μW m −3 in the centre) are necessary to explain the heat flux anomaly. The surface heat flux pattern of the TD and the extreme low value in the centre of the depression point to an exceptionally cold and strong lithosphere in the Transylvanian region, as shown by strength envelopes characteristic to the entire depression. The evolution of the TD generally can be understood in terms of an exceptionally strong lithosphere block caught in a compressive tectonic environment.

Three-dimensional inverse modelling of the thermal structure and implications for lithospheric strength in Denmark and adjacent areas of Northwest Europe

SUMMARY The subsurface thermal structure of Denmark and adjacent areas and lateral variations in lithospheric strength are investigated using a 3-D finite element solution of the steady-state heat equation and the equations of motion of a continuum. The optimum thermal model is determined by least-squares inversion of surface heat flow with prior information on the distribution of crustal radiogenic heat production rate, sediment and crustal thermal conductivity values, and the heat flow from the asthenosphere into the base of the lithosphere. The mechanical model invokes elastic, plastic and viscous deformation, depending on rock type, temperature, confining pressure and deviatoric stress. The thermal structure derived by the inversion is non-unique. A Monte Carlo-based resolution analysis shows that data and parameter uncertainties result in standard deviations in the Moho temperature of 75‐160 uC. The standard deviation increases almost linearly with Moho depth. All admissible thermal structures are characterized by laterally relatively homogeneous temperatures, and it is a robust conclusion that the lowest Moho temperatures occur in areas where the Moho is the shallowest, and the highest temperatures in areas where it is the deepest. Therefore, in the absence of lateral variations of the rheology of the lithospheric layers, and particularly for the upper mantle, the strength of the lithosphere in Denmark and the bordering areas is greatest in the basins where the Moho is the shallowest. This shows that in transition zones from cratons to younger sedimentary basins, lateral variations in lithospheric rheological properties are significant and should be considered in any attempt to understand the geodynamics of such areas.

Thermal controls on flexure of underthrust continental lithosphere

SUMMARY We model the thermal and mechanical structure of Precambrian foreland lithosphere subject to a range of initial thermal states and evolving thermal perturbations beneath an orogen. Temperatures in the lower crust exert a critical control over the flexural response by regulating crust‐mantle decoupling. By careful consideration of crustal heat production and temperature-dependent thermal conductivity, we verify that at least half the surface heat flow in Archean terranes is supplied by the crust and find that Moho temperatures in Proterozoic lithosphere may be as high as 800 K (525 uC). Such ‘hot’ Precambrian forelands may have an effective elastic thickness of less than 10 km. The ability of crustal heating beneath the orogen to enhance weakening of continental lithosphere is strongly dependent on the convergence rate. A rapidly underthrust foreland is insufficiently heated by thermal diffusion into the zone of maximum lower crustal deformation to significantly modify the flexural signature. This process has a minor effect at the 5 mm year x1 or greater convergence rates characteristic of most active orogens, but it may cause a marked reduction in lithospheric strength following the cessation of convergence. We propose that the large, y10‐100 km range of observed mountain belt effective elastic thicknesses can be entirely explained by the known variability of Precambrian surface heat flow, crustal heat generation, rock thermal conductivity, and lithospheric rheology without invoking any other mechanisms.

Is there any relation between helium isotope composition of underground fluids and heat flow in continental areas?

The regression formula between3He/4He ratio of underground fluids and terrestrial heat flow in continental areas is tested by data sets from the former Soviet Union and the mainland of China. The results show that there is no close relation between the two values. The heat-He relation might estimate the regional heat flow value with ±25% accuracy at best. We propose that the ratio of crust/mantle component of continental heat flow (qc/qm) be inversely related to the3He/4He ratio of underground fluids. Based on data sets of3He/4He ratio andqc/qm in the Eurasia and Canadian Shield, we obtain the regression relation betweenqc/qm and3He/4He:qc/qm=0.815−0.300*loge (3He/4He), in which the unit of3He/4He is Ra (atmospheric3He/4He ratio). The crust and mantle heat flow components can be taken from surface heat flow andqc/qm ratio. Based on this formula and heat flow data in major basins of China, the crustal, mantle heat flow values and the average crustal heat production rates were estimated. The estimated crustal chemical composition of China is in agreement with the result inferred by deep seismic sounding survey. Helium isotope ratio (3He/4He) of underground fluids may be a useful parameter for separating crust and mantle components of continental heat flow.

Tectonic feedback, intraplate orogeny and the geochemical structure of the crust: a central Australian perspective

Abstract The geological record of intraplate deformation in central Australia implies that past tectonic activity (basin formation, deformation and erosion) has modulated the response of the lithosphere during subsequent tectonic activity. In particular, there is a correspondence between the localization of deformation during intraplate orogeny and the presence of thick sedimentary successions in the preserved remnants of a formerly widespread intracratonic basin. This behaviour can be understood as a kind of ‘tectonic feedback’, effected by the long-term thermal and mechanical consequences of changes in the distribution of heat producing elements induced by earlier tectonism. From a geochemical point of view, one of the most dramatic effects of intraplate orogeny in central Australia has been the exposure, in the cores of the orogens, of deep crustal rocks largely depleted in the heat producing elements. The geochemical structuring of the crust associated with the erosion of the heat-producing upper crust resulted in long-term cooling of the deep crust and upper mantle with associated lithospheric strengthening. This is illustrated here by mapping the consequences of deformation and associated tectonic responses onto the h-q c plane, where h is the characteristic length-scale for heat production distribution, and q c is the total crustal heat production. Because rates of intraplate deformation in central Australia appear to be much slower than that typical of plate margin orogens, it is possible that the ongoing geochemical structuring of the crust has played an important role in terminating intraplate orogeny in central Australia by providing a ‘thermal lock’. The diagnostic geophysical signature of this lock may be the extraordinary gravity anomalies of the central Australian intraplate orogens.

Crustal composition and mantle heat flow: Implications from surface heat flow and radiogenic heat production in the Variscan Erzgebirge (Germany)

From an enlarged data set of temperature logs and thermal conductivity measurements, surface heat flow (qs) in the Erzgebirge was determined to range from 61 to 112 mW m−2. U‐Th‐K2O data show that the heat flow pattern is controlled to first order by the occurrence of high heat production Variscan granites within a metamorphic basement. Highest heat flow correlates with granite plutons and sharply decreases off granite. These granites display variable but typically high radiogenic heat production (A), usually between 4 and 10 μW m−3, depending on their chemical type and degree of fractionation. U accounts for 40–90% of the total heat production in the granites, whereas U and Th contribute equally to the radioactivity in the metamorphic basement. Compositional heterogeneity in the upper crust, owing to variation of conductive heat transfer, required corrections of measured surface heat flow up to 20 mW m−2, depending on location of the heat flow site. Heat budget calculations, considering crustal models derived from seismic and gravimetric surveys, define the thickness (D) of crust enriched in radioactive elements to 15 km, the rate of reduced heat flow (q*) to 30–34 mW m−2, and the mantle heat flow to 20–30 mW m−2, in contradiction to what is implied from qs ‐ A plots. In fact, D (5–8 km) and q* (45–52 mW m−2) from these plots represent the thickness of, and the heat flow beneath, the (granite) layer that is most enriched in heat‐ producing elements. For a crustal section strongly heterogeneous in upper crustal heat production, traditional interpretation of qs ‐ A plots is misleading and should be supplemented routinely by heat budget calculations.

Distribution of crustal extension and regional basin architecture of the Albertine rift system, East Africa

By applying a kinematic and flexural model for the extensional deformation of the lithosphere, and using a recently available EROS Data Center topography DEM of Africa in conjunction with new and previous gravity data from Lakes Albert, Edward and George, we have determined the distribution, amplitude, and style of deformation responsible for the formation of the Albertine rift system, East Africa. Further, we have been able to approximate the three-dimensional architecture of the Albertine rift basin by analyzing a series of profiles across and along the rift system for which we also estimate the flexural strength of the rifted continental lithosphere and its along-strike variation. Previous modeling studies of the Lake Albert basin either overestimated the flexural strength of the extended lithosphere and/or underestimated the crustal extension. The single most important factor that compromised the success of these modeling efforts was the assumption that crustal extension was limited to the present-day distribution of the rift lakes. The style of deformation appears to have changed with time, beginning with a regionally distributed brittle deformation across the region that lead progressively to the preferential growth and development of the major border faults and antithetic/synthetic faults within the collapsed hangingwall block. Minor fault reactivation within the footwall block appears to be related to the release of bending stresses associated by the flexural uplift of the rift flank topography. By simultaneously matching the observed and modeled topography and free-air gravity across the Albertine rift system, we have determined a cumulative extension ranging from 6 to 16 km with the maximum extension occurring in the central and northern segments of the basin. Crustal extension is not constrained to the lake proper, but extends significantly to the east within the hangingwall block. Effective elastic thickness, T e, varies between 24 and 30 km and is unrelated to either the amount of extension or the maximum sediment thickness. The variation of T e relates possibly to small changes in crustal thickness, heterogeneities in crustal composition, and/or variations in radiogenic crustal heat production. Maximum sediment thickness is predicted to be 4.6 km and occurs within the central region of Lake Albert. Low bulk sediment densities, correlating with the location of major lake deltas, may be indicative of present-day sediment overpressures. Our results show that basin geometry is strongly dependent on the cumulative (and distribution) of lithospheric extension and the flexural rigidity of the lithosphere. Thus, in order to determine the total amount of extension responsible for the formation of a basin system, it is necessary to independently constrain the flexural strength of the lithosphere both during and after extension. Conversely, in order to determine the rigidity of extended lithosphere using the stratigraphy and/or geometry of rift basins and passive margins, it is necessary to independently constrain the cumulative extension of the lithosphere.

Using Helium Isotope Composition in Underground Fluid to Estimate the Ratio of Crust/mantle Component of Continental Heat Flow

AbstractThe helium isotope 3He in underground fluid comes from the mantle by degassing process, and 4He is the decay product of uranium and thorium. Because U and Th mainly concentrate in the continental crust, so 4He flux is approximately proportional to the heat flux of crust; but 3He flux is associated with mantle heat flux. So the ratio of crust/mantle component of continental heat flow (qc/qm) is inversely related to the 3He/4He ratio of underground fluid. Based on data sets of 3He/4He ratio and qc/qm in Eurasia and Canadian Shield, we obtain the regression relation between qc/qm and 3He/4He: qc/qm = 0:815 − 0:300 ln(3He/4He), in which the unit of 3He/4He is RA (atmospheric 3He/4He ratio). Furthermore, the crust and mantle heat flow components can be taken from surface heat flow and qc/qm ratio. On the basis of this formula and heat flow data in major sedimentary basins of China, the crustal and mantle heat flow values were estimated; and the thickness of thermal lithosphere and the average crustal heat production rate inferred from these values are consistent with the results by seismology. Helium isotope ratio (3He/4He) is a useful parameter for separating crust and mantle components from continental heat flow.

Thermal structure of continental upper mantle inferred from S-wave velocity and surface heat flow

Results from seismic tomography provide information on the thermal structure of the continental upper mantle. This is borne out by the good agreement between tectonic age, surface heat flow and a tomographic S-wave velocity model for depths less than 180 km. The velocity anomalies of tomographic layers deeper than 230 km have relatively small amplitudes and show little correlation with surface heat flow or shallow velocities. We associate the drop in correlation and amplitude of the velocity perturbations between 180 and 230 km depth with the maximum thickness of the thermal boundary layer (TBL), in which larger variations in temperature and possibly composition than in the underlying convecting mantle can be sustained. Velocity profiles for different tectonic provinces are converted to temperature using mineralogical data. Both anharmonic and anelastic effects on the wave speeds are taken into account. The resulting geotherms differ most at depths of 60–120 km with variations of up to 900°C. Below 230 km, differences do not exceed 300°C. These geotherms agree well with one-dimensional conductive geotherms for the observed range of continental heat flow values using the empirical relationship that 40% of the surface heat flux stems from upper crustal radiogenic heat production. The S-wave velocity in the continental upper mantle appears to be adequately explained (within the uncertainties of the tomography and the conversion to temperature) by a thermal signature. A compositional component can, however, not be ruled out as it may have only a minor effect on the velocity and the heat flow. The surface heat flow is controlled by the shallow heat production and the thickness of the TBL. Seismology helps to determine the relative importance of the two factors and our results confirm the similar importance of both factors. Variations of TBL thickness could be controlled by compositional differences and/or by the effect of temperature on the rheology.

The thermal structure and thickness of continental roots

We compare heat flow data from the Precambrian shields in North America and in South Africa. We also review data available in other less well-sampled Shield regions. Variations in crustal heat production account for most of the variability of the heat flow. Because of this variability, it is difficult to define a single average crustal model representative of a whole tectonic province. The average heat flow values of different Archean provinces in Canada, South Africa, Australia and India differ by significant amounts. This is also true for Proterozoic provinces. For example, the heat flow is significantly higher in the Proterozoic Namaqua–Natal Belt of South Africa than in the Grenville Province of the Canadian Shield (61 vs. 41 mW m −2 on average). These observations indicate that it is not possible to define single value of the average heat flow for all provinces of the same crustal age. Large amplitude short wavelength variations of the heat flow suggest that most of the difference between Proterozoic and Archean heat flow is of crustal origin. In eastern Canada, there is no good correlation between the local values of heat flow and heat production. In the Archean, Proterozoic and Paleozoic provinces of eastern Canada, heat flow values through rocks with the same heat production are not significantly different. There is therefore no evidence for variations of the mantle heat flow beneath these different provinces. After removing the local crustal heat production from the surface heat flow, the mantle (Moho) heat flow was estimated to be between 10–15 mW m −2 in the Archean, Proterozoic and Paleozoic provinces of eastern Canada. Estimates of the mantle heat flow in the Kaapvaal craton of South Africa may be slightly higher (≈17 mW m −2). Large-scale variations of bulk crustal heat production are well-documented in Canada and imply significant differences of deep lithospheric thermal structure. In thick lithosphere, surficial heat flow measurements record a time average of heat production in the lithospheric mantle and are not in equilibrium with the instantaneous heat production. The low mantle heat flow and current estimates of heat production in the lithospheric mantle do not support a mechanical (conductive) lithosphere thinner than 200 km and thicker than 330 km. Temperature anomalies with surrounding oceanic mantle extend to the convective boundary layer below the conductive layer, and hence to depths greater than these estimates. Mechanical and thermal stability of the lithosphere require the mantle part of the lithosphere to be chemically buoyant and depleted in radiogenic elements. Both characteristics are achieved simultaneously by partial melting and melt extraction.

Heat flow and the structure of Precambrian lithosphere

Studies of heat flow from Precambrian terrains have demonstrated three empirical relationships; a temporal relationship between heat flow and tectonic age, a spatial pattern between heat flow and the proximity of Archean cratons, and a temporal relationship between heat flow and the age of lithosphere stabilization. In the first relationship, heat flow is inversely related to tectonic age. The second pattern is characterized by low heat flow from Archean cratons and Proterozoic terrains adjacent to cratonic margins (pericratonic terrains), and higher heat flow from Proterozoic terrains that are more than a few hundred kilometers from a craton. In the third pattern, heat flow decreases as the age of stabilization of the lithosphere increases. A number of interpretations of Precambrian heat flow have been offered to explain one or more of these relationships. The simple cooling of a thermal boundary layer predicts essentially no change in heat flow in terrains older than ∼1.5 Ga, and therefore does not likely provide a comprehensive framework for the interpretation of Precambrian heat flow. By contrast, two other interpretations, (1) thicker lithosphere beneath Archean terrains than beneath Proterozoic terrains, and (2) greater heat production in Proterozoic crust than in Archean crust, when combined with the special structural configuration of sutures, can both contribute to the spatial and temporal heat flow distributions. Xenolith thermobarometry constraints on lithospheric temperatures, however, limit the contribution of age-dependent crustal heat production, and therefore at least part of the heat flow distributions derive from variations in lithosphere thickness.

Geothermal investigations in the 1993 Latur earthquake area, Deccan Volcanic Province, India

Heat flow measurements have been made in four boreholes in the epicentral area of the Latur earthquake ( M w 6.2, 1993) in the southeastern part of the Cretaceous–Eocene Deccan Volcanic Province (DVP), central India. Three of these, located 4 to 8 km from the surface rupture zone near Killari, were about 180 m deep and yielded heat flow values in the range 33 to 40 mW m −2. A hole was specially drilled in the surface rupture zone and penetrating the entire 338 m Deccan basalt cover and a further 271 m into the Archaean granite-gneiss basement. Temperature gradients over both the basaltic as well as the granite-gneiss sections, which have a large thermal conductivity contrast, yielded a consistent heat flow value of 43 mW m −2. Earlier measurements at Koyna, the site of another stable continental region (SCR) earthquake ( M w 6.3, 1967) in the western part of the DVP, and at another locality, Lonar, in the central part of the DVP had yielded heat flow values of 41 mW m −2 and 47 mW m −2, respectively. Thus the southern part of the DVP is characterised by a low heat flow regime, similar to that over the Archaean Dharwar province immediately to the south. The 617 m deep hole at Killari provided an opportunity for carrying out measurements of radioactive heat production on core samples of the 271 m long basement section. Thirty-nine core samples of the migmatitic gneisses and the granites encountered in the 271 m long basement section of KLR-1 were analysed for U, Th and K, and resulted in estimates of heat production of 0.5 and 2.6 μW m −3, respectively, for the two types. A crustal heat production model is envisaged, with the thicknesses of the gneisses and the granites constrained from available pressure estimates in the Dharwar craton, and also considering a 17 km `granitic' upper crust and a 20 km `granulitic' lower crust based on deep seismic sounding data. Using this crustal heat production model and the heat flow value of 43 mW m −2, the estimated crustal temperatures imply a deep (>30 km) brittle–ductile transition. Even though heat flow–crustal temperature considerations allow for deeper, crustal earthquake foci, most of the M w>6 events of the last three decades in the Australian and Indian SCRs show shallow foci (<10 km). This calls for seeking possible mechanisms for frictional failure to take place in the top part of a crust which is largely brittle.

Heat flow and thickness of the lithosphere in the Canadian Shield

Heat flow and radioactive heat production data were obtained in the Canadian Shield in order to estimate the crustal heat production and the mantle heat flow. Several methods have been used to determine radioactive heat production in the crust. The analysis yields values for the mantle heat flow in the craton that are consistently between 7 and 15 mW m−2. Assuming that the lithosphere is in thermal equilibrium, we investigate the conditions for small‐scale convection to supply the required heat flux through its base. For a given creep law, the thickness of the lithosphere, the temperature at the base of the lithosphere, and the effective viscosity of the mantle are determined from the value of the mantle heat flow beneath the shield. The viscosity of the mantle depends on the creep mechanism and on the fluid content. Wet diffusion creep implies a viscosity between 1020 and 1021Pa s, corresponding to a mantle temperature of 1620 K at a depth of 250 km. The other creep mechanisms can be ruled out because they imply values for viscosity and temperature inconsistent with geophysical data. For a given creep law, there is a minimum mantle temperature below which equilibrium cannot be reached. For wet diffusion creep, this minimum mantle temperature (1780 K at 280 km depth ) is close to that of the well‐mixed ( isentropic ) oceanic mantle at the same depth. For a thermally stable lithosphere, our model requires the mantle heat flow to be at least 13 mW m−2 and the compositional lithosphere to be less than 240 km.

Thermal structure, thickness and composition of continental lithosphere

Global compilations of surface heat flow data from stable, Precambrian terrains show a statistically significant secular change from 41±11 mW/m 2 in Archean to 55±17 mW/m 2 in Proterozoic regions far removed from Archean cratons. Using the tectonothermal age of the continents coupled with average heat flow for different age provinces yields a mean continental surface heat flow between 47 and 49 mW/m 2 (depending on the average, non-orogenic heat flow assumed for Phanerozoic regions). Compositional models for bulk continental crust that produce this much or more heat flow (i.e., K 2O>2.3–2.4 wt%) are not consistent with these observations. More rigorous constraints on crust composition cannot be had from heat flow data until the relative contributions to surface heat flow from crust and mantle are better determined and the non-orogenic component of heat flow in the areally extensive Phanerozoic regions (35% of the continents) is determined. We calculate conductive geotherms for 41 mW/m 2 surface heat flow to place limits on the heat production of Archean mantle roots and to evaluate the significance of the pressure–temperature ( P– T) array for cratonic mantle xenoliths. Widely variable geotherms exist for this surface heat flow, depending on the values of crustal and lithospheric mantle heat production that are adopted. Using the average K content of cratonic peridotite xenoliths (0.15 wt% K 2O, assuming Th/U=3.9 and K/U=10,000 to give a heat production of 0.093 μW/m 3) and a range of reasonable crustal heat production values (i.e., ≥0.5 μW/m 3), we calculate geotherms that are so strongly curved they never intersect the mantle adiabat. Thus the average cratonic peridotite is not representative of the heat production of Archean mantle roots. Using our preferred estimate of heat production in the cratonic mantle (0.03 wt% K 2O, or 0.019 μW/m 3) we find that the only geotherms that pass through the xenolith P–T data array are those corresponding to crust having very low heat production (<0.9 wt% K 2O). If the lithospheric mantle heat production is higher than our preferred values, the continental crust must have correspondingly lower heat production (i.e., bulk crustal K, Th and U contents lower than that of average Archean granulite facies terrains), which we consider unlikely. If the xenolith P–T data reflect equilibration to a conductive geotherm, then Archean lithosphere is relatively thin (150–200 km, based on intersection of the P–T array with the mantle adiabat) and the primary reason for the lower surface heat flow in Archean regions is decreased crustal heat production, rather than the insulating effects of thick lithospheric roots. On the other hand, if the xenolith P–T points result from frozen-in mineral equilibria or reflect perturbed geotherms associated with magmatism, then the Archean crust can have higher heat producing element concentrations, lithospheric thickness can range to greater depths and the low surface heat flow in Archean cratons may be due to the insulating effects of thick lithospheric roots. An uppermost limit for Archean crustal heat production of 0.77 μW/m 3 is determined from the heat flow systematics.

Australian Proterozoic high-temperature, low-pressure metamorphism in the conductive limit

Abstract High-temperature, low-pressure (HTLP) metamorphism often reflects transient advection of heat due to magma ascent. However, the origin of HTLP metamorphism in a number of Australian Proterozoic terranes remains contentious either because of the deficiency of magmatic bodies in the terranes, or because the long time delay (&gt;100 Ma) between magmatism and metamorphism precludes heating by existing magmatic bodies. Furthermore, a number of Australian Proterozoic HTLP terranes (such as the Reynolds Range in central Australia) show evidence of an extended history ( c. 30 Ma) of HTLP mineral growth suggesting metamorphism during a thermal regime dominated by conduction at lithospheric length scales. Australian Proterozoic metamorphic terranes are characterized by both elevated modern-day heat flows (averaging c. 85 mW m −2 ) and granitic gneisses with anomalously high heat production rates (commonly &gt;5–10 μW m −3 ). We show that the conditions required for HTLP metamorphism may result from conduction if the crustal heat production responsible for modern-day heat flows is concentrated at mid-crustal levels (15–20 km). Importantly, for low-intermediate mantle heat fluxes (10–20 mW m −2 ) and moderate synmetamorphic crustal thicknesses ( c. 45 km), the conductive geotherms attendant with such HTLP metamorphism do not necessarily lead to significant melting of a refractory lower crust. Importantly, the thermal regimes are very sensitive to the depths at which crustal heat production is localized. The strong dependence of the resulting geotherms on the depth of the heat-producing layer has the important consequence that only minor burial may be required to induce HTLP metamorphism, while only minor erosion ( c. 5 km) is necessary to terminate the event.

On the heat flow variation from Archean cratons to Proterozoic mobile belts

Heat flow data from a number of continental shield regions show a trend of relatively low to high values from Archean cratons to bordering Proterozoic mobile belts. Of the two end‐member explanations for this trend, low heat production in Archean crust or a relatively thick cratonic lithosphere, the latter has come to be generally preferred. Such an explanation assumes a strong, one‐to‐one correspondence between the mantle component of surface heat flow and local lithospheric thickness. This assumption has been well validated below oceans and its applicability to continental lithosphere has gone largely unchallenged. It is, however, not fully valid. This is demonstrated through numerical models that allow continents to form over a convecting mantle. Model continents consist of a core region of thickened crust and mantle residuum and a peripheral region of thick crust, analogs to a craton and a mobile belt, respectively. Despite a thicker thermal lithosphere in the core relative to the periphery, the equilibrium surface heat flux across a continent shows little variation. The finite thermal conductivity of buoyant continental material is at the heart of this behavior as it allows continents to enforce a spatially near‐constant heat flux condition on the mantle below. Such a condition is associated with a weak correspondence between mantle heat flow and lithospheric thickness defined thermally or mechanically. This general result, together with specific modeling results applied to heat flow data, suggests that variable lithospheric thickness is most likely not the primary cause of heat flow variations near Archean cratons, leaving differing degrees of crustal heat production as the more likely candidate.