Crustal Radiogenic Heat Production Research Articles

Crustal Heterogeneity of Antarctica Signals Spatially Variable Radiogenic Heat Production

AbstractGeothermal heat flow (GHF) is a key basal boundary condition for Antarctic ice‐sheet flow. Large‐scale variations are resolved by several recent models but knowledge of the smaller‐scale variations, crucial for ice sheet dynamics, is limited by unresolved variations in crustal radiogenic heat production. To define this at continent‐scale we use 3D gravity inversion constrained by seismic Moho estimates to identify variations in crustal composition and geometry beneath thick ice. Geochemically‐defined empirical relationships between density and heat production capture the global average trend and its variability, and allow to estimate from upper‐crust density spatial variations in radiogenic heat production. Significant variations are observed typically 1.2–1.6 μW/m3, and as high as 2 μW/m3 in West Antarctica. The contribution to GHF from these heat‐production variations is similarly variable, typically 16–24 mW/m2 and up to 60 mW/m2. The mapped variations are significant for correctly representing GHF in Antarctica.

The thermal state of Volgo–Uralia from Bayesian inversion of surface heat flow and temperature

SUMMARY Volgo–Uralia is the easternmost segment of the East European Craton. It accommodates the Volga–Ural petroleum province where the maturity of source rocks is tightly related to the temperature distribution in the crust. Numerous heat flow and temperature measurements have been reported for this region. However, no consistent geothermal model was presented for the Volgo–Uralian crustal block so far. In this study, we present a novel model of the Volgo–Uralian geothermal field where we aim to reconcile the reported heat flow and temperature data. The main goal of the study is to explore lateral variations of the unknown thermal parameters within Volgo–Uralia. For this purpose, we applied a Bayesian Markov Chain Monte Carlo approach where we used the known surface heat flow, surface temperature, lithosphere–asthenosphere boundary temperature and thicknesses of the Earth's lithospheric and crustal layers as input and investigated the possible lateral variations of crustal and lithospheric mantle thermal conductivities, crustal heat production and mantle heat flow. We implemented this methodology for a single-layer and multilayer crust and validated the obtained geothermal models with existing subsurface temperature measurements for the region. The results show that the Volgo–Uralian subcraton is characterized by significant lateral variations of crustal radiogenic heat production (RHP) and mantle heat flow. The variations of crustal and lithospheric mantle thermal conductivities are less pronounced. According to our model, the surface heat flow distribution is controlled mostly by crustal RHP which accounts for more than half of Volgo–Uralian surface heat flow. Validation of the models shows that single-layer and multilayer crustal models give roughly the same fit of measured and modelled temperatures. This implies that a single-layer crust with constant RHP can be considered a sufficient approximation for regional-scale geothermal modelling.

The transition from ancient to modern-style tectonics: Insights from lithosphere dynamics modelling in compressional regimes

Orogens are traditionally classified according to their tectonic style. Paleoproterozoic tectonics is referred to as “ancient-style tectonics” while Proterozoic tectonics is referred to as “modern-style tectonics”. Ancient-style tectonics is characterised by distributed vertical structures and low topography gradients, often associated with diapirism and partial melting. In contrast, modern-style tectonics involve prominent strain localisation and the formation of thrusts, nappes and high topographic gradients. However, the parameters controlling the transition from ancient to modern-style tectonics are poorly understood. To quantify this transition, a combination of 1D and 2D high resolution lithospheric-scale thermo-mechanical models was conducted. The parameters controlling the strength of the lithosphere (i.e., Moho temperature, strain rate, crustal rheology, crustal radiogenic heat production and role of shear heating) were investigated in detail. Our results show that tectonic style is controlled by the maximum of crustal strength (shear stress). Modern-style tectonics is observed to occur when the maximum of crustal strength is greater than 300 MPa. At the opposite, a maximum crustal strength lower than 300 MPa leads to ancient style tectonic structures. Therefore, crustal rheology, temperature and background strain rate significantly influence the transition from ancient to modern-style tectonics. Shear heating remains a key factor in promoting strain localisation in modern-style tectonics. Crustal radiogenic heat production has a moderate influence by increasing/decreasing the tendency for faulting within the crust. This crustal strength criterion also provides an excellent fit for a second potential proxy: a localisation criterion of ca. 225°C. These two proposed proxies can be used interchangeably to predict the transition from ancient to modern-style tectonics.

Modeling petroleum expulsion in sedimentary basins: The importance of igneous intrusion timing and basement composition

Abstract The concept of a critical moment in a petroleum system (the time of highest probability of entrapment and preservation of oil and gas) has underlain petroleum exploration for over 25 years. However, one area where understanding the critical moment is challenging is the Faroe-Shetland Basin (FSB; offshore UK). Isotopic dating of oils suggests that petroleum generation began between ca. 68 and 90 Ma; however, most basin models invoke an earlier generation beginning in the mid-Cretaceous at ca. 100 Ma, predating deposition of Paleocene and Eocene reservoirs. This time discrepancy has previously been explained by remigration from intermediary accumulations (“motel” hypothesis) and/or overpressure retardation of kerogen maturation. The FSB is characterized by a thick Cretaceous stratigraphic package (up to 5 km) that includes a large net thickness (up to 2 km) of Paleogene igneous material. In our model, separating sedimentary and igneous material and adding the igneous material at the correct time between ca. 58 and 55 Ma shallows the modeled burial depth of the Upper Jurassic source rocks during the Cretaceous sufficiently to delay maturation by 17 m.y. in comparison to results of previous studies. Additionally, previous studies have invoked crustal radiogenic heat production (RHP) based on the Phanerozoic crust averaging ∼2.8 µW/m3 in the North Sea (300 km to the east). However, the FSB basement is composed of significantly older, colder Neoarchean orthogneisses (ca. 2.7–2.9 Ga), reducing RHP by up to 50% to ∼1.6 µW/m3 (σ = 0.74). For the first time, our model unifies geological, geochronological, and geochemical observations, delaying the onset of petroleum expulsion by up to 40 m.y. in comparison to previous models.

Present-day thermal field and Mesozoic-Cenozoic thermal evolution of the Western Bredasdorp Basin (South Africa): An integrated 3D numerical forward modelling approach

Abstract We analyse a crust-scale 3D structural model of the Western Bredasdorp Basin (WBB) located offshore southern South Africa to assess both the spatial and temporal development of geothermal gradients in the Southern African ‘passive-transform’ continental margin. The results of simulations of the present-day conductive thermal field were used to constrain the heat flow evolution during 3D numerical modelling of the paleo-thermal regime. Besides conforming to the regional geotectonic framework, the ensuing models are largely consistent with measured temperatures. The largest control on present-day temperatures is exerted by the basin's present-day geometry and distribution of thermal properties which, in turn, trace back to the earliest tectonic event involving crustal thinning and significant syn-rift sedimentation. Implying from the modelled shallow and deep geotherms, we find that the variations of present-day surface heat fluxes in the WBB are much more dictated by the long-term crustal radiogenic heat production in concert with the interaction of low thermally conductive sediments and high thermally conductive crustal rocks rather than by the present-day mantle heat flow. In addition, model calibration with measured vitrinite reflectance trends suggests that three major phases of thermal disequilibrium resulting in high heat flow during syn-rift and post-rift times characterise the distribution of maximum paleo-temperatures and paleo-geotherms. In terms of subsidence, the syn-rift tectonic event plays the most significant role in basin evolution. The height and width of the syn-rift heat flow anomalies and their lateral variations reflect a direct consequence of instantaneous crustal stretching leading to rapid burial and lateral differences in crustal thicknesses and amounts of radiogenic heat produced. The post-rift heat flow peaks are interpreted to be related to a Late Cretaceous mantle-related hotspot event and a late Miocene episode of margin uplift. As indicated by validated vitrinite trends and the modelled distribution of eroded sediments, the hotspot event represents a more localised thermal event, while the late Cenozoic erosion and inferred uplift depict a regional character. The modelled evolution of deep heat flow further suggests that ancient thermal perturbations have little influence on the present-day thermal field.

A new heat flux model for the Antarctic Peninsula incorporating spatially variable upper crustal radiogenic heat production

AbstractA new method for modeling heat flux shows that the upper crust contributes up to 70% of the Antarctic Peninsula's subglacial heat flux and that heat flux values are more variable at smaller spatial resolutions than geophysical methods can resolve. Results indicate a higher heat flux on the east and south of the Peninsula (mean 81 mW m−2) where silicic rocks predominate, than on the west and north (mean 67 mW m−2) where volcanic arc and quartzose sediments are dominant. While the data supports the contribution of heat‐producing element‐enriched granitic rocks to high heat flux values, sedimentary rocks can be of comparative importance dependent on their provenance and petrography. Models of subglacial heat flux must utilize a heterogeneous upper crust with variable radioactive heat production if they are to accurately predict basal conditions of the ice sheet. Our new methodology and data set facilitate improved numerical model simulations of ice sheet dynamics.

Subduction styles in the Precambrian: Insight from numerical experiments

Plate tectonics is a self-organizing global system driven by the negative buoyancy of the thermal boundary layer resulting in subduction. Although the signature of plate tectonics is recognized with some confidence in the Phanerozoic geological record of the continents, evidence for plate tectonics becomes less certain further back in time. To improve our understanding of plate tectonics on the Earth in the Precambrian we have to combine knowledge derived from the geological record with results from well-constrained numerical modeling. In a series of experiments using a 2D petrological–thermomechanical numerical model of oceanic subduction we have systematically investigated the dependence of tectono-metamorphic and magmatic regimes at an active plate margin on upper-mantle temperature, crustal radiogenic heat production, degree of lithospheric weakening and other parameters. We have identified a first-order transition from a “no-subduction” tectonic regime through a “pre-subduction” tectonic regime to the modern style of subduction. The first transition is gradual and occurs at upper-mantle temperatures between 250 and 200 K above the present-day values, whereas the second transition is more abrupt and occurs at 175–160 K. The link between geological observations and model results suggests that the transition to the modern plate tectonic regime might have occurred during the Mesoarchean–Neoarchean time (ca. 3.2–2.5 Ga). In the case of the “pre-subduction” tectonic regime (upper-mantle temperature 175–250 K above the present) the plates are weakened by intense percolation of melts derived from the underlying hot melt-bearing sub-lithospheric mantle. In such cases, convergence does not produce self-sustaining one-sided subduction, but rather results in shallow underthrusting of the oceanic plate under the continental plate. Further increase in the upper-mantle temperature (> 250 K above the present) causes a transition to a “no-subduction” regime where horizontal movements of small deformable plate fragments are accommodated by internal strain and even shallow underthrusts do not form under the imposed convergence. Thus, based on the results of the numerical modeling, we suggest that the crucial parameter controlling the tectonic regime is the degree of lithospheric weakening induced by emplacement of sub-lithospheric melts into the lithosphere. A lower melt flux at upper-mantle temperatures < 175–160 K results in a lesser degree of melt-related weakening leading to stronger plates, which stabilizes modern style subduction even at high mantle temperatures.

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 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.

Interpretation of heat-flow density in the Central Andes

This study focuses on the interpretation of surface heat-flow density data in the central Andes to examine possible lithospheric thermal structures. Along a profile at about 21°S from the Peru–Chile trench in the west to the Andean foreland in the east, surface heat-flow density is used to constrain quantitative models that are employed to investigate the influence of various effects on the thermal field. Negligible changes in surface heat-flow density along strike of the Andean orogen, in contrast to major variations across the orogen, seem to justify the application of two-dimensional (2-D) cross-section models. The region of the Andean orogen between the trench and the volcanic front is described by 2-D subduction models, the back-arc region, characterised by crustal doubling, is analysed by 2-D models describing simple underthrust phenomena and the Andean foreland is investigated by one-dimensional temperature estimates. The influence of various geometrical and petrophysical parameters on the thermal structure is investigated. Model calculations show that the low heat-flow density values in the fore-arc region suggest low shear stresses on the order of 15 MPa along the plate contact. Variations of the subduction angle influence mainly the W–E extent of the surface heat-flow density anomaly. The distribution of the crustal radiogenic heat-production rate influences the surface heat-flow density significantly, but has only a minor influence on crustal temperatures. Modelling results show that temperatures at the maximum depth of seismic coupling between the oceanic Nazca plate and the overriding South American continental plate (∼45 km) are on the order of 250 to 300°C, whereas at the maximum depth of the plate contact (∼60 km) temperatures are on the order of 300 to 350°C. The lithospheric temperature structure in the active magmatic arc region is strongly sensitive to temperature changes caused by the occurrence of an assumed asthenospheric mantle wedge at shallow depth (∼70 km). To match conditions for melting at subcrustal levels, the minimum extent of an asthenospheric wedge towards the west has to coincide with the position of the volcanic front. The thermally thinned lithosphere results in a surface heat-flow density on the order of 60–70 mW m −2. Locally higher heat-flow density in the volcanic arc is interpreted as the effects of shallow magma chambers. The doubling of the crust in the back-arc region affects the lithospheric thermal structure considerably, but models do not show reverse temperature–depth distributions in the continental crust. In the back-arc region the transition from a thermally thinned lithosphere to a `normal' lithospheric thickness for continental shields may occur. The observed surface heat-flow density in the Andean foreland suggests lithospheric thermal conditions as typical for shield areas.

Gravity, heat flow, and seismic constraints on the processes of crustal extension: Northern margin of the South China Sea

Multichannel seismic data and gravity data have been used to construct crustal thickness profiles for three transects (eastern, central, western) across the rifted northern margin of the South China Sea. The present‐day crustal configuration of the margin is then interpreted by modeling the effects of two end‐member classes of extension processes, pure shear and simple shear. The applicability of each of these processes to the extension of the south China margin has been evaluated by comparing model predictions of subsidence and heat flow with observations across the margin. Neither of these end‐member models satisfactorily fits the observed data on the eastern and central transects across the south China margin when typical values for standard input parameters are used; the resulting heat flow is significantly underestimated by both models. In the case of a pure shear model, heat flow observations may be matched either by assuming an uncommonly thin initial steady state lithospheric thickness (∼60 km) or by assuming an unusually large crustal radiogenic heat production within the original, unextended continental crust. A perhaps more reasonable alternative scenario presumes the existence of an initially slightly thinner than “normal” steady state lithosphere (thicknesses of ∼90–100 km) in conjunction with a significant amount of upper crustal radiogenic heat production. Such heat production could be accommodated by the presence of Cretaceous granitic bodies (hypothesized) within the basement beneath the south China margin. In the case of a simple shear model, however, the observed high heat flow on the rifted south China margin may only be matched if the steady state lithospheric thickness is assumed to be uncommonly thin (∼60 km). Because the observed geophysical data characterizing lithospheric extension may be matched using more realistic input parameters in the pure shear case, pure shear extension is preferred over simple shear extension as the dominant mechanism for explaining the large‐scale rifting of the south China margin. For extension within the crust, however, combinations of both processes are not only possible, but probable, given published seismic evidence for through‐going crustal faults on the south China margin.

On the thermo‐mechanical evolution of continental lithosphere

Elastic thickness of continental lithosphere is modified by both mechanical (thickening and thinning) events and thermal evolution. A simple thermo‐mechanical model can account for the processes of tectonic alteration and subsequent thermal reequilibration of lithospheric thermal structure. Mechanical thinning or thickening of the lithosphere directly affects its thickness and flexural rigidity. After a tectonic event, thermal reequilibration proceeds at a rate dependent upon the amount of thinning or thickening and total lithospheric thickness, previous tectonic history, and the heat flux at the base of the lithosphere. Heat flux at the base of the lithosphere is controlled by the efficiency of convective mantle heat delivery. Rapid heat delivery to the base of the lithosphere retards lithospheric thickening, whereas slow conductive heat delivery allows lithospheric thickening at the maximum rate. The relative importance of thermal effects to mechanical alterations of lithospheric elastic thickness depends on the time scale of concern. Very thin lithosphere undergoes rapid thermal evolution such that cooling between tectonic events significantly thickens the lithosphere. For thick lithosphere, conductive heat transport is so slow that only minor changes in thermal structure and no significant changes in elastic thickness take place between tectonic events. Model results with infinite stretching agree with observations of oceanic lithospheric thickness evolution, showing a divergence from a cooling half‐space at about 70 m.y. Model results also agree with independent estimates of continental thermal evolution in the absence of tectonic events. On the longest time scales approaching the age of the Earth, thermal reequilibration is attained, and any tectonic effects are overprinted. The time scale at which the transition between mechanically and thermally dominated elastic thickness evolution occurs depends on the heat transport efficiency of the convecting mantle. For high Nusselt numbers, thermal effects take longer to become important Even for lower Nusselt numbers, mechanical effects still dominate on time scales of 108 years. Crustal radiogenic heat production reduces the equilibrium lithospheric thermal and elastic thickness. It also introduces a dependence of equilibrium thickness on tectonic crustal thinning or thickening. Model results indicate that on time scales of thrust sheet and sedimentary loading in foreland basins, mechanical effects dominate the evolution of elastic thickness except for very thin lithosphere. The model can be applied to the lithosphere beneath foreland basins so that observed basin geometry can be used to constrain tectonic history prior to loading by thrust sheets and basin sediments.

Depth of seismic coupling along subduction zones

Underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth. The maximum depth of the seismically coupled zone (or seismogenic zone) is a transition from unstable to stable sliding along the plate interface. We have determined the depth of this stability transition for the circurn‐Pacific subduction zones of: Honshu, Kuriles, Kamchatka, Aleutians, Alaska, Mexico, and Chile. These subduction zones have experienced great interplate earthquakes and the aftershock regions are well‐located. Depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling. For an average P wave velocity of 6.7 km s−1 above the plate interface, we find that for most subduction zones the stability transition occurs at 40 ± 5 km depth. There are, however, several exceptions. At the Hokkaido trench junction, where the Japan trench and the Kurile trench intersect, seismic coupling is deep and extends down to 52–55 km. Deep coupling was also found in the Coquimbo region in central Chile. The Mexico subduction zone has shallow coupling: the transition occurs at 20–30 km depth. Previous studies of micro‐earthquakes in Honshu, Hokkaido, the Aleutians, and Alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find. Given our measurements of seismic coupling depth, we then explore the mechanism that may determine coupling depth. The concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments, thus we first test whether a critical temperature can explain our results. Temperatures at the plate interface are dependent on many variables; but two that are poorly determined are shear stress and radiogenic heat generation. Shear stress has been constrained by inversion of heat flow data. Assuming a crustal radiogenic heat production rate of 3.1 exp−z/8.5 μWm−3 and a constant coefficient of friction, we find two critical temperatures of about 400 ° C and 550 ° C. The lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust. On the other hand, one single critical temperature of about 250 ° C can explain the coupling depths if shear stresses are constant with depth.

Downward continuation of heat flow density data and thermal regime in Eastern Canada

Heat flow density (HFD) data from Eastern Canada were analyzed by downward continuation. The data cover parts of the Superior and Grenville provinces of the Canadian Shield, and parts of the Appalachian orogen. The downward continuation eliminates the effect of shallow crustal radiogenic heat production and better reveals differences in the thermal regime of deeper origin. The analysis shows that the deep heat flow beneath the Superior and Grenville provinces is low and the temperature in the lower crust is also low. The thermal regime of the crust changes markedly across a boundary that follows the St Lawrence River. Surface heat flow and heat production are markedly higher in the Appalachian Province. With much higher temperature and heat flow, the thermal regime in the Appalachian lower crust is also distinctly different from that of the shield. Temperature and heat flow increase eastward toward the Atlantic margin.

Thermal control on post-orogenic extension in collision belts

Intracontinental extension develops in two different ways. In the Rhine Graben, the Baikal or the East African Rifts, it is localized in a long and narrow zone, along a few normal faults which often delineate a single graben. In contrast, in the Basin and Range Province or the Tibetan Plateau, it is distributed over a much broader area, up to more than 1000 km across. This difference arises from differences in rheology and applied forces. Widespread extension probably reflects the thinning of a previously thickened crust. Geological observations indicate that only wide orogenic belts undergo late or post-orogenic thinning. A related fact is that anatectic granites seem to be most abundant in wide orogenic belts. The general correlation between belt width and occurrence of both extension and anatectic granites suggest that it reflects in part the characteristics of horizontal heat transfer between thickened crust and surroundings. In wide orogenic plateaux, the large amount of heat produced in the crust by radioactive decay and lack of erosion leads to high temperatures. In narrow mountain belts, horizontal heat transfer and high erosion rates concur to prevent temperatures from rising to values at which the crust may yield in extension. We have tested this hypothesis using transient two-dimensional numerical models of orogenic belts, focussing on the effects of the shortening rate, the final width of the belt and the rate of crustal radiogenic heat production. In the range of geological values, the shortening rate has little influence on the results. Crustal temperatures increase as a function of belt width. We find that the sensitivity of crustal temperatures to belt width is most pronounced in the 100–300 km width range and for large values of crustal heat generation. The pressure-temperature paths followed by geological formations depend on the belt width, which is consistent with available data. The integrated shear strength of the lithosphere decreases with time as the belt is being built and then eroded. This evolution is sensitive to belt width. If extension occurs when the integrated shear strength drops below a critical value, our results yield a framework for comparing different orogenic belts. We predict that extension starts late in the history of wide belts and never occurs in narrow belts. A final implication of this study is that the average rate of crustal heat production is probably close to 1.1 × 10 −6 W/m 3, which is in the upper range of possible values. We present a simple and convenient diagram summarizing the thermal characteristics of orogenic belts as a function of width and shortening rate.

Heat flow and heat production in the Namaqua Mobile Belt, South Africa

Results of heat flow and heat production surveys at 13 localities in the Namaqua mobile belt, southern Africa, are presented. Twelve stations traverse the 1200–1100 m.y. old gneissic and metamorphic Bushmanland and Gordonia subprovinces. One station is situated in the Richtersveld subprovince, a 2000–1730 m.y. old volcano‐sedimentary domain mildly affected by the 1200–1100 m.y. old orogeny. Surface heat flow in the mobile belt ranges from 39 to 81 mW m−2 and averages at 61 ± 11 mW m−2. Surface heat production ranges from 0.8 μW m−3 in mafic rocks to extreme values of 6.9–12.3 μW m−3 in the Springbok district, western Namaqualand. High heat production in the Springbok district is associated with a thin, tabular granitic intrusive and is not representative of the basement. The mean heat production of the remaining stations in the mobile belt is 2.3 ± 1.1 μW m−3. Linear regression analysis of the heat flow‐heat production data yields a poor fit (correlation coefficient = 0.62) and imprecise estimates for the intercept (34 ± 6 mW m−2) and slope (11 ± 3 km). The eastern margin of the mobile belt has relatively low heat flow and heat production; geothermal, geological, and gravity data suggest a relative depletion of crustal radiogenic heat production in the region. However, the average Namaqua heat flow is higher than worldwide averages for Proterozoic terrains. There is no geological or geophysical evidence for recent mantle heating below the Namaqua mobile belt, suggesting that an elevated crustal radioactive component is largely responsible for the elevated heat flow. In this respect, the mobile belt is similar to several mid‐late Proterozoic heat flow provinces that are characterized by high average surface heat flow and probably have elevated crustal radiogenic heat input. These observations, and seismic data, suggest that the Namaqua crust consists of an upper layer that contributes 40–50%, on average, to the heat flow and a depleted lower layer; the inferred heat flow at midcrustal levels is 50–60% of the average surface heat flow. The mobile belt's average surface heat flow is 40–85% higher than that from the basement of the adjacent Archaean Kaapvaal craton, and its inferred midcrustal heat flow is similar to the lowest surface values in the craton. Heat production data from the Kaapvaal craton indicate that its upper crust is not depleted of heat sources. While enhanced radiogenic heat input from the Namaqua upper crust may account for part of the higher‐than‐craton heat flow, it cannot account for the full excess. Consequently, a larger equilibrium mantle heat flow, related to a reduced lithospheric thickness below Namaqualand, seems to be required by the new data.

Heat flow in Eastern Egypt: The thermal signature of a continental breakup

The Red Sea is a modern example of continental fragmentation and incipient ocean formation. Heat flow data have been collected from eastern Egypt to provide information relating to the mode and mechanism of Red Sea opening. Preliminary heat flow data, including new data reported here, are now available from twenty-five sites in eastern Egypt and one site in western Sinai. A pattern of low to normal heat flow (35–55 mW m −2) inland with high heat flow (75–100 mW m −2) in a zone within 30 to 40 km of the coast is indicated. Moderately high heat flow (around 70 mW m −2) is indicated for the Gulf of Suez. The coastal zone thermal anomaly appears continuous with high heat flow previously reported for the Red Sea shelf. Heat production data indicate that the coastal thermal anomaly is not primarily related to crustal radiogenic heat production. The effects of rapid erosion may contribute to the anomaly, but are not thought to be the primary cause of the anomaly. If the anomaly is caused by lateral conduction from hot, extended, offshore lithosphere, the extension must have been active for the last 30 Ma or so, and a minimum of 100% extension is indicated. Alternatively, the anomaly is primarily caused by high mantle heat flow causing lithospheric thinning, centred beneath the Red Sea. The Red Sea is probably underlain by dominantly basic crust, formed either by intrusion into attenuated continental crust or sea-floor spreading, and for most purposes the crust formed in these two modes of extension may be essentially indistinguishable. Fission-track ages from eastern Egypt indicate that uplift started prior to, or at latest at the time of initial Red Sea opening, and this result, together with thermo-mechanical considerations, suggests an active asthenospheric upwelling beneath the Red Sea and high temperature in the lithosphere prior to extension.

Crustal radiogenic heat production and the selective survival of ancient continental crust

If the oldest terrestrial rocks are to be used as a representative sample of the early terrestrial crust, it is important to consider the possibility that they may represent a biased sample. Chemical data indicate that Archean crust is on average depleted in potassium, uranium, and thorium relative to younger crust. Radiogenic isotopes of these elements produce a significant portion of surface heat flow from continents, and low mean heat flow in Archean terrains relative to younger terrains also indicates that these elements are relatively depleted in Archean crust. In stable continental lithosphere the primary variable controlling variations in the geotherm is crustal radiogenic heat production. Qualitative evidence suggests that terrains with low radiogenic heat production, and thus a relatively cool geotherm, are less susceptible to magmatic and tectonic reactivation than terrains with higher heat production and a hotter geotherm. Since crustal radiogenic isotope abundances must have been greater in the past if the isotopes were immobile in stable crust, the relative effect of crustal heat production must have been greater, and thus the range in geotherms must have been greater for the same mantle heat flow. It is suggested that crust with low heat production is less susceptible to orogenic reworking than high heat production crust, and thus there may have been selective stabilization of low heat production (low potassium, uranium, and thorium) crust early in earth history. Either Archean crustal generation was by a mechanism that perhaps fortuitously produced only low heat production crust, or all high heat production crust was reworked by orogenesis during the Archean and did not stabilize in significant volumes until the earliest Proterozoic, perhaps in association with a reduction in mantle temperatures and mean heat flow. If selective crustal reworking did operate during the Archean, surviving Archean terrains may represent a biased sample of the early terrestrial continental crust.

On the evolution of rifted continental margins: comparison of models and observations for the Nova Scotian margin

A model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed. Model predictions are then compared with geological and geophysical observations from a transect of the ∼185 Ma old rifted margin off Nova Scotia through the Scotian Basin. Three kinematic models of the rifting process are discussed. These are: (1) the uniform extension model, in which the amount of extension is uniform with depth but varies with position across the margin; (2) the uniform extension and melt segregation model which has similar properties, but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust, and; (3) the depth-dependent extension model in which the first-order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth, each of which undergoes differing amounts of extension. These rift models predict the form of crustal and lithospheric thinning, subsidence and temperature change once the amount of extension has been determined. This is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness, equal to that currently measured in the adjacent continental region, before rifting occurred. Rifting is also considered to occur instantaneously. A time-stepping thermo-mechanical model is used to predict the cooling of rifted margin, additional thermal contraction subsidence and its amplification by water and sediment loading. Thermal aspects are calculated using a finite difference model of time-dependent conductive heat transport, whereas regional isostatic response to loading is calculated by a finite element model. The models are coupled because the temperature distribution is used to define a rheological lithosphere (an elastic region with thickness that varies in time and space as the model evolves) for flexural calculations in the mechanical model. Secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment. The model predictions of: (1) sedimentary basin stratigraphy, (2) Moho position, (3) free air gravity anomaly, (4) age—depth relations for deep exploratory wells and (5) subsidence and temperature histories agree well with observations from the Scotian Basin. Additional effects due to sedimentary and crustal radiogenic heat production, lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered. It is concluded that a model in which rifting occurred by depth-dependent extension, and which includes a finite thickness for the rheological lithosphere, radiogenic heat production in the sediments and crust, and a brittle listric faulted crustal layer affords an accurate description of the first-order processes that occur during rifting and evolution of this margin.

Plate motion and structure of the continental asthenosphere: A realistic model of the upper mantle

A one-dimensional shear flow model of the upper mantle beneath continents is presented. The model includes a two-layer crust with values of thermal conductivity and radiogenic heat production distinct from those of the underlying mantle, in which the conductivity is generally a function of temperature and depth. Temperature, velocity, effective viscosity, and volumetric heat generation profiles are determined, accounting for viscous dissipation and using a temperature and pressure dependent nonlinear mantle rheology in which the rate of strain is directly proportional to a power n of the shear stress (n = 3 is generally assumed, although the Newtonian case is investigated). The solutions provide the values of lower crustal radiogenic heat production and shear stress, which are a priori unknown. The boundary conditions on the model are the values of velocity, temperature, and heat flux at the surface and the vanishing of velocity with a prescribed upward heat flux at great depth. The models produce a shear zone (asthenosphere), the region where the geotherm bends over, the velocity drops rapidly to zero, the viscosity has a minimum value, and the shear stress heating has a maximum value. The thickness and depth of the shear zone, the value of viscosity therein, the associated shear stress, the lower crustal radioactivity, and other characteristics of the model depend on parameters such as the activation energy and volume of the constitutive stress-rate of strain equation, plate velocity, mantle radioactivity, and heat influx from the lower mantle. These dependences are explicitly demonstrated through the presentation of a series of numerical calculations. A realistic model of the mantle beneath stable continental regions is found which satisfies the additional constraints of the temperature at and the depth to the shear zone inferred from the petrology of peridotite xenoliths in kimberlites from South Africa, the seismic shear wave structure beneath shields, and the value of radioactivity in gabbroic rocks likely to represent the lower crust. The activation volume V* is the most important constitutive parameter controlling the depth of the shear zone. The model for stable continental regions predicts that V* is perhaps twice as large as would be appropriate for diffusion of O− − ions. Viscous dissipation contributes about 0.1 HFU (μcal cm−2 s−1) to the continental surface heat flux. Through thermal feedback as a result of the pressure and temperature dependence of the viscosity, shear stress heating leads to the formation of an asthenosphere decoupling the plates from the underlying mantle.