Thickness Of Thermal Lithosphere Research Articles

The state of the upper mantle beneath southern Africa

We present a new upper mantle seismic model for southern Africa based on the fitting of a large (3622 waveforms) multi-mode surface wave data set with propagation paths significantly shorter (≤ 6000 km) than those in globally-derived surface wave models. The seismic lithosphere beneath the cratonic region of southern Africa in this model is about 175 ± 25 km thick, consistent with other recent surface wave models, but significantly thinner than indicated by teleseismic body-wave tomography. We determine the in situ geotherm from kimberlite nodules from beneath the same region and find the thermal lithosphere model that best fits the nodule data has a mechanical boundary layer thickness of 186 km and a thermal lithosphere thickness of 204 km, in very good agreement with the seismic measurement. The shear wave velocity determined from analyzes of the kimberlite nodule compositions agree with the seismic shear wave velocity to a depth of ∼150 km. However, the shear wave velocity decrease at the base of the lid seen in the seismic model does not correspond to a change in mineralogy. Recent experimental studies of the shear wave velocity in olivine as a function of temperature and period of oscillation demonstrate that this wave speed decrease can result from grain boundary relaxation at high temperatures at the period of seismic waves. This decrease in velocity occurs where the mantle temperature is close to the melting temperature (within ∼100 °C).

Thermal-rheological structure of the lithosphere beneath Jiyang Depression: Its implications for geodynamics

Jiyang Depression, located in the southeast Bohai Bay Basin, has the geomorphologic framework of multiple uplifts intervening with sags. Combined the abundant geo-temperature data and thermo-physical parameters of rock samples derived from oil and gas exploration during the past years, with geothermal approaches, here we investigate the lithospheric thermal regime of this depression. Consequently, based on the obtained thermal structure of the lithosphere, along with rheological modeling, the lithospheric rheological profiles of Jiyang Depression are then determined. Our results show that the temperature at the bottom of sedimentary cover within depression varies from 129℃ to 298℃, accompanied with the basement heat flow ranging between 54.3 and 60.5 mW/m 2 ; and 406℃－436℃ for temperature at the bottom of the upper crust, along with heat flow varying from 47.7 to 52.6 mW/m 2 ; while the temperature at the bottom of the middle crust is between 537℃ and 572℃, as well as heat flow ranging from 41.3 to 56.3 mW/m 2 . The temperature at Moho ranges from 669℃ to 721℃, the heat flow derived from mantle is between 38.1 and 43.1 mW/m 2 , and calculated thickness of the thermal lithosphere beneath depression varies from 71 to 90 km. Lithospheric thermal regime is a close correlation with such factors as crustal thickness and surface heat flow, etc. Usually, the larger the surface heat flow, the larger the deep temperature and heat flow within lithosphere, and the thinner the thermal lithospheric thickness. This high thermal regime of the lithosphere in Jiyang Depression is thought to be related to Cenozoic back-arc spreading during the western Pacific plate subduction into Eurasian continent. Lithospheric rheological modeling shows that the lithosphere in Jiyang Depression is characterized by its distinct rheological stratification as follows: The upper and most part of the middle crust are of brittle, while the lower crust and the lower part of middle crust are all ductile, and lithospheric mantle is dominated by the ductile layer. Additionally, lateral rheological heterogeneities exist in the depression, and the lithospheric strength of sags within depression differs much from each other. The total lithospheric strength of Jiyang Depression is between 1.52 and 2.16×10 12 N/m, effective elastic thickness ( T e ) of the lithosphere in Jiyang Depression is about 24 km, approximating to the thickness of mechanically strong crust (MSC). We suggested that the dehydration of minerals in the subducting zone, along with upwelling of hot materials in the mantle wedge during subduction and back-arc spreading, results in the partial melting at the bottom of curst, which triggers magma intrusion and underplating. This geodynamics process maybe is the reason for the reduction of lower crustal viscosity for ductile flow. Lithospheric rheological stratification controlling the differential deformation styles of brittle fracture or frictional slide in the upper crust and ductile flow in the middle and lower crust, accounts for the basin formation and evolution of Jiyang Depression during Cenozoic.

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 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.

Colorado Plateau: Geoid and Means of Isostatic Support

The Colorado Plateau underwent 1.5 to 2 km of surface uplift from near sea level during Late Cretaceous time. Neither the mechanism nor the timing of the uplift are well constrained. Knowledge of how the current topography of the Plateau is isostatically compensated could provide direct constraints on the mechanism, and perhaps indirect constraints on the timing, of uplift. Crustal thickening and/or decreases in mantle density must provide the buoyancy required for the uplift. These two modes have very different tectonic implications, and result in quite different average depths of the compensating mass deficits, potentially detectable with geoid anomalies. Geoid anomalies caused by long-wavelength continental topography are proportional to the elevation multiplied by the mean depth of compensation. Thus, for a particular elevation, the greater the average depth of the isostatic "root," the larger the geoid anomaly. The main complication in analyzing geoid data for this kind of problem is separating lithospheric geoid anomalies from those with deeper sources, especially those that arise in the lower mantle. The anomalies shown here were obtained by high-pass spherical harmonic filtering of the order-360 EGM96 geoid, with a cosine taper between orders 7 and 11. Taking the point-by-point ratio of filtered geoid to topography at 0.25° resolution, the Colorado Plateau (and the rest of the western United States, except for Yellowstone and the Snake River Plain) shows a moderate geoid/topography ratio of 6-8 m/km. The small amplitude of these geoid anomalies favors isostatic compensation of Colorado Plateau topography at depths of around 50 km, matching approximate Moho depths, and certainly shallower than asthenospheric depths of 80 km or greater. Mechanical or thermal lithospheric thickness changes may also contribute to the buoyancy that drove the uplift, but models involving strictly thermal thinning of the lithosphere still require crustal thickening. Some models that involve crustal thickening acting alone succeed in matching the elevation and geoid anomalies over the Colorado Plateau. Most plausibly, such thickening would have been Laramide in age. The only viable alternative to shallow compensation is mechanical loss of an ancient dense, deep lithospheric root in Late Cretaceous time, because the Plateau's elevation was near sea level for all of the Phanerozoic before the end of the Mesozoic.

Thermal and rheological structures of the Xisha Trough, South China Sea

The Xisha Trough, located in the northwest of the South China Sea (SCS) mainly rifted 30 Ma ago, has been a failed rift since the cessation of the seafloor spreading of the NW subbasin. Based on the velocity–depth model along Profile OBH-4 across the Xisha Trough, a seven-layer density–depth model is used to estimate density structure for the profile. The relationship between seismic velocity and radiogenic heat production is used to estimate the vertical distribution of heat sources in the lower crust. The 2-D temperature field is calculated by applying a 2-D numerical solution of the heat conduction equation and the thermal lithosphere thickness is obtained from the basalt dry solidus (BDS). The rheology of the profile is estimated on the basis of frictional failure in the brittle regime and power-law steady-state creep in the ductile regime. Rheological model is constructed for a three-layer model involving a granitic upper crust, a quartz diorite lower crust and an olivine upper mantle. Gravity modeling supports basically the velocity–depth model. The Moho along Profile OBH-4 is of relatively high heat flow ranging from 46 to 60 mW/m 2 and the Moho heat flow is higher in the trough than on the flanks. The depth of the “thermal” lithospheric lower boundary is about 54 km in the center, deepens toward two sides, and is about 75 km at the northern slope area and about 70 km at the southern Xisha–Zhongsha Block. Rheological calculation indicates that the two thinnest ductile layers in the crust and the thickest brittle layer in the uppermost mantle lie in the central region, showing that the Xisha Trough has been rheologically strengthened, which are mainly due to later thermal relaxation. In addition, the strengthening in rheology during rifting was not the main factor in hampering the breakup of the Xisha Trough.

Thermal thickness and evolution of Precambrian lithosphere: A global study

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat‐producing elements is estimated using regional models for ∼300 blocks with sizes varying from 1°×1° to about 5°×5° in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300°C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200–220 km and 300–350 km. In general, thin (∼220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200±50 km in early Proterozoic lithosphere, to about 140±50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ∼0.6–0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5–1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

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.

Viscosity structure of the oceanic lithosphere inferred from the differential late Quaternary sea-level variations for the southern Cook Islands

SUMMARY Late Quaternary sea-level variations for the southern Cook Islands such as Rarotonga and Mangaia provide information on the time-dependent crustal movement due to viscoelastic arching in response to loading by the Pleistocene volcanic island of Rarotonga. The lithospheric responses to both external and internal loads have been investigated to estimate the viscosity of the lower part of the lithosphere and to examine the initial stage of swell formation. Detailed observations of sea-level variations for the past 125 kyr indicate that the crustal uplift for Mangaia is greater than 10 m, while Rarotonga was apparently stable for this period. The following geophysical implications for the lithospheric rheology and loading model are derived from these observations. The observed differential crustal movement implies that the viscous relaxation associated with this volcanic loading is still proceeding in the lithosphere. The layer supporting stresses has therefore been migrating with time from weaker lower zones into the stronger upper zones for a lithosphere with a depth-dependent viscosity structure. This fact provides an important constraint on the viscosity of the lower part of lithosphere. The observation that Rarotonga has been apparently stable for this period is indicative of a local buoyant internal load in the upper mantle. This load may be related to small-scale and secondary convection in the asthenosphere. Surface uplift due to an internal load is therefore required to cancel the subsidence by volcanic loading. This problem has been examined for two simplified background density models. One is a model in which the density of the lithosphere is equal to that of the asthenosphere. For this model, very large mass anomalies which are 10 times larger than the external load are required beneath the lithosphere in order to explain the observed differential crustal movement of the islands. For an earth model for which the density of the lithosphere is greater than that of the asthenosphere, which is possible for mature oceanic lithosphere, the observed differential crustal movement is explained for an internal-load model with density anomalies of less than 20 kg m-j. The volume of the internal load is at most twice the volume of the external load. A high-viscosity layer with an effective viscosity of Pa s and with a thickness of greater than 60 km is required beneath the top elastic layer with a thickness of 10-15 km. The thickness of thermal lithosphere estimated by the plate age of this region is approximately 80-90 km, regardless of the age-thickness relationship adopted. It is therefore suggested that the major part of the thermal lithosphere is composed of a viscoelastic layer with an effective viscosity of

The interaction of mantle plumes with surface thermal and chemical boundary layers: Applications to hotspots on Venus

Large volcanic swells on Venus are believed to be a manifestation of mantle upwelling, or hotspots. The study of these regions provides important information on the interior of the planet. Numerical experiments are carried out to examine the interaction of mantle plumes with the thermal lithosphere and a layer of depleted mantle, a product of pressure‐release melting, using an axisymmetric finite element and finite difference code that incorporates temperature‐dependent viscosity and pressure‐release melting. The lithosphere is defined as a high‐viscosity lid; plumes are initiated and maintained by a prescribed temperature at the base of the computational domain. The topographic uplift, the geoid‐to‐topography ratio, and the volume of pressure‐release melt are compared to estimated values for possible hotspots on Venus to constrain the properties of plumes and the lithosphere. The effects of lithospheric thickness, depleted layer viscosity and thickness, mantle temperature, and plume temperature and duration on the surface observables are predicted. Models with a thermal lithospheric thickness of approximately 100–150 km are consistent with observations, assuming a mantle temperature of 1300°C, a maximum plume temperature of approximately 1500°C, and mantle plume durations of 150–250 m.y. To be consistent with estimated volumes of volcanics at Venusian hotspots, a significantly thinner thermal lithosphere requires a much cooler mantle; a thicker lithosphere requires a hotter mantle. Models with a depleted layer thickness of 100–250 km, in addition to a 100‐km thick thermal lithosphere, predict the range of parameters found on Venus. Interpretation of large volcanic swells on Venus based on the evolutionary sequence predicted here implies that Beta, Atla, Western Eistla, and Imdr Regiones overlie either active or recently active mantle plumes. The geoid‐to‐topography ratio at Beta Regio is much larger than values found at other hotspots and may indicate that it is in an early stage of evolution or that the chemical or thermal boundary layers are significantly thicker. Bell, Dione, and Themis Regiones appear to represent very late stage, possibly now extinct, hotspots. An estimate of the total plume buoyancy flux for Venus is much lower than the value obtained for Earth. The small areal distribution of volcanism is consistent with a low level of ongoing resurfacing at a few active hotspots.

Isostatic Compensation of Equatorial Highlands on Venus

Spherical harmonic models for Venus' global topography and gravity incorporating Magellan data are used to test isostatic compensation models in five 30° × 30° regions representative of the main classes of equatorial highlands. The power spectral density for the harmonic models obeys a power-law scaling with spectral slope β∼2 (Brown noise) for the topography and β∼3 (Kaula's law) for the geoid, similar to what is observed for Earth. The Venus topography spectrum has lower amplitudes than Earth's which reflects the dominant lowland topography on Venus. Observed degree geoid to topography ratios (GTRs) on Venus are significantly smaller than degree GTRs for uncompensated topography, indicative of substantial compensation. Assuming a global Airy compensation, most of the topography is compensated at depths greater than 100 km, suggesting a thick lithosphere on Venus. For each region considered we obtain a regional degree of compensation C from a linear regression of Bouguer anomaly versus Bouguer gravity data. Geoid anomaly ( N) versus topography variation ( h) data for each sample were compared, in the least-squares sense, to theoretical correlations for Pratt, Airy, and thermal thinning isostasy models yielding regional GTR, zero-elevation crustal thickness ( H), and zero elevation thermal lithosphere thickness ( y L 0 ), respectively. We find the regional compensation to be substantial ( C ∼ 52-90%), and the h, N data correlations in the chosen areas can be explained by isostasy models applicable on the Earth and involving variations in crustal thickness (Airy) and/or lithospheric (thermal thinning) thickness. However, a thick crust and lithosphere ( y L 0 ∼ 300 km) must be assumed for Venus. Compensation mechanisms are distributed as follows: For the high plateaus of Ovda (GTR ∼ 9 m/km) and Thetis (GTR ∼ 11 m/km), mainly Airy isostasy with H ∼ 50-60 km; for the Chasmata area sample (GTR ∼ 15 m/km), Airy with perhaps a thermal component; finally, a very strong thermal component for the broad swells of Beta (GTR ∼ 32 m/km) and Atla (GTR ∼ 25 m/km), with the lithosphere thinned to ∼100 km. Aphrodite Terra, which shows a marked eastward increase in Pratt and Airy model parameter values, could be a candidate for thermal isostasy decay from east to west.

Estimating Lithospheric Properties at Atla Regio, Venus

Magellan spherical harmonic gravity and topography models are used to estimate lithospheric properties at Atla Regio, Venus, a proposed hotspot with dynamic support from mantle plume(s). Global spherical harmonic and local representations of the gravity field share common properties in the Atla region in terms of their spectral behavior over a wavelength band from ∼2100 to ∼700 km. The estimated free-air admittance spectrum displays a rather featureless long-wavelength portion followed by a sharp rise at wavelengths shorter than about 1000 km. This sharp rise requires significant flexural support of short-wavelength structures. The Bouguer coherence also displays a sharp drop in this wavelength band, indicating a finite flexural rigidity of the lithosphere. A simple model for lithospheric loading from above and below is introduced (D. W. Forsyth, 1985, J. Geophys. Res. 90, 12,623-12,632) with four parameters: f, the ratio of bottom loading to top loading; Z m , crustal thickness; Z l, depth to bottom loading source; and T e, elastic lithosphere thickness. No model is found that fits simultaneously the estimated free-air admittance and Bouguer coherence across the entire spectrum. A dual-mode compensation model is introduced in which the shorter wavelengths (λ ⪉ 1000 km) might be explained best by a predominance of top loading by the large shield volcanoes Maat Mons, Ozza Mons, and Sapas Mons, and the longer wavelengths (λ ⪊ 1500 km) might be explained best by a deep depth of compensation, possibly representing bottom loading by a dynamic source. A Monte Carlo inversion technique is introduced to thoroughly search out the four-space of the model parameters and to examine parameter correlations in the solutions. It is shown that simultaneous inversion on admittance and Bouguer coherence in the short-wavelength band constrains the elastic thickness to a range of approximately 40 to 50 km. Introduction of the free-air coherence as an observable in the inversion narrows the estimates of the other three parameters ( f = 0.10 ± 0.01, Z m = 30 ± 13 km, Z l = 7 ± 5 km) and provides a solution T e = 45 ± 3 km. In the long-wavelength band, Z l is found to be about 150 km, and this is the only parameter that can be estimated robustly. Crustal thickness, Z m , is unbounded, and the solutions are degenerate in that there is no discrimination between top and bottom loading. Acceptable elastic thicknesses range from 0 to 140 km. The elastic thickness results from both wavelength bands are corroborated in the spatial domain by examining residual standard deviations for a range of T e values. The results found here are consistent with the results of Smrekar (1994) for wavelengths longer than 600 km. Values of Z l are consistent with other analyses that exclude wavelengths longer than about 2,000 km. Inclusion of wavelengths longer than about 3000 km leads to compensation depths of approximately 200 km. Moment-curvature relationships based on yield strength envelopes are used to constrain temperature gradient. Average curvature is obtained from the flexurally deformed Moho surface by Fourier transform techniques. Surface temperature gradients are obtained in the range 7-10 K km -1, depending on the strain rate adopted. The relationship between elastic thickness and temperature gradient is calibrated by comparing elastic thickness estimates of two East African hotspots with measured heat flow. The excess heat flow at the East African hotspots over the African continental mean, if applicable to Venus, can be used to show that the globally averaged heat flow on Venus must be much lower than the Earth-scaled value predicts. A thick (∼350-km) ambient thermal lithosphere, as proposed by A. B. Kucinskas and D. L. Turcotte in this issue, and interpretable from the long-wavelength depth of compensation at Atla Regio, provides an acceptable background temperature gradient. When it is added to the excess heat flow or temperature gradient, the predicted heat flow at Atla Regio is consistent with the moment-curvature results. The thermal lithosphere beneath Atla Regio may be about 100 km thick and this answer can be obtained by two independent methods: from the ambient thermal lithosphere thickness and simple elevation scaling (result dependent on long gravity wavelengths) and from the elastic thickness estimate converted to temperature gradient (result dependent on short gravity wavelengths). Either Venus is considerably deficient in heat sources relative to Earth, or the thermal lithosphere is overthickened in response to an earlier episode of significant heat loss from the planet.

Heat flow constraints on the South Pacific Superswell

The South Pacific superswell has been defined as a large region of anomalously shallow bathymetry, low Love wave velocities, and low effective elastic thicknesses relative to those predicted for its age. These phenomena have been interpreted as reflecting a combination of lithospheric reheating and thinning, and dynamic uplift due to mantle flow. We use heat flow data to better constrain the thermal structure of this region and examine the predictions of various possible models. The average heat flow for the superswell region does not differ significantly from that for lithosphere of similar ages elsewhere on the Pacific plate. Given their uncertainties, the heat flow data imply that thermal lithospheric thickness exceeds 60 km, but cannot discriminate between greater thicknesses. The lack of observed high heat flow appears not to be explained by biases due to water circulation in the thin sediment cover, since the superswell heat flow is not higher than for sites elsewhere with similar sedimentary environments. The Darwin Rise has been proposed as a fossil superswell in the Cretaceous, on the basis of the many similar characteristics to the South Pacific superswell. We find that the Darwin Rise heat flow values do not exceed those for similar ages elsewhere in the Pacific and Atlantic. This observation suggests that any thermal effects associated with the formation of the Darwin Rise are no longer present, and it is consistent with the idea of a fossil superswell. The surface heat flow data thus provide no evidence that the temperatures in the uppermost portion of the lithosphere; are significantly higher in the entire superswell region than in other areas of comparable age. This observation is intriguing given the suggestion that the thin effective elastic thicknesses inferred from seamount loading may reflect reheating of the lithosphere. Models in which the plate thicknesses and/or the basal temperatures are increased to yield temperatures high enough to explain the low effective elastic thicknesses predict surface heat flow much higher than observed. Reheating the lithosphere, as is proposed for hot spots, yields temperatures adequate to explain the effective elastic thicknesses only if reheating occurs at very shallow depths, and again implies a surface heat flow much greater than observed. Hence, unless shallow reheating is somehow localized beneath the seamounts, the thinner elastic thicknesses may reflect mechanical, rather than thermal, weakening of the lithosphere.

Lithospheric thermal structure in the Baltic shield

SUMMARY Steady-state heat conduction modelling was carried out to calculate the crustal temperature field and thermal lithosphere thickness in the Baltic shield. The radiogenic sources at the surface were fixed depending on the age of the crust. Below the uppermost 10 kilometres, a relationship between heat generation and P-wave velocity was applied and seismic structure was used to define individual crustal blocks of specific thermal parameters. Small-scale surface heat-flow density anomalies are interpreted as lateral variations of heat generation within the upper part of the crust, whereas the large anomaly in the southern part of the shield is attributed to an anomalously high mantle heat-flow density. The results are shown on maps outlining mantle heat-flow density, Moho temperature and thermal lithosphere thickness. A relationship between subcrustal temperature and Moho depth was found, except for southern Sweden. The lithospheric thickness is found to exceed 200 km in the Bothnian Gulf-northern-central Finland and part of the Kola peninsula, where temperatures at the Moho are less than 500°C; the temperature gradient at the top of the mantle has, on the average, a value of 7.5 mK m-' and the mantle heat-flow density varies from 19 to 25mWm-2. Towards the south, the lithospheric thickness decreases until it attains a value lower than 100km under southern Sweden. In this latter area, the Moho temperature and mantle heat-flow density are remarkably high, 700-900 C and 30-45 mW m-2, respectively, as well as the temperature gradient, which amounts to 10 mK m-'.

Skagerrak evolution derived from tectonic subsidence

Stratigraphic data gathered from 32 points in the continental Skagerrak Basin have been analyzed by basin modelling methods. Non-tectonic effects of sediment loading were removed by backstripping, giving the “observed” tectonic subsidence. This was compared to synthetic subsidence curves constructed on the basis of a combined lithospheric extension and metamorphism model, where gabbroic magma, which was emplaced during the Permian, is transformed to granulits-eclogite facies. The overall picture is that prior to 230 Ma subsidence was roughly similar north and south of the Fennoscandian Border Zone (FBZ). After this time the northern area sagged due to the still-preserved lithospheric thermal anomaly and continued metamorphism, whereas the southern area was also affected by extension. A physical explanation for this contrast in evolution is given by considering the relative lithospheric strength history of the areas, the northern area becoming progressively stronger with time relative to the southern area as crustal thicknesses are reduced. Total post-570 Ma extension is approximately 1.4 (40%) north of the FBZ and 1.5 (50%) south of it. North of the FBZ the average thickness of the metamorphic layer is between 3 and 7 km, whereas south of the zone a thickness of 1–2 km is estimated. This difference may reflect variations in the degree of Permian magmatism. The temperature history of the sediments and the degree of organic maturation has been calculated by the TTI method. The youngest sediments to have experienced temperatures high enough for significant oil generation are estimated to be of Bathonian age. The present model heat flow of about 57 mW m −2 is in agreement with observations. Lateral displacements along the FBZ are of the order of 10–20 km, and have consequently only played a minor role in the development of the Skagerrak Basin. A thermal lithosphere thickness of about 125 km is inferred.

Observations of flexure and the state of stress in the oceanic lithosphere

Observations of flexure indicate the effective elastic thickness of the oceanic lithosphere is 2 to 3 times smaller than the seismic or thermal thickness of oceanic lithosphere. The effective elastic thickness is a function of temperature and hence age of the lithosphere at the time of loading. Recent results of experimental rock mechanics indicate that the strength of rocks is a strong function of temperature and that the oceanic lithosphere responds to loading by thinning rapidly from its seismic thickness to its rheologic thickness. We have used a yield stress envelope based on experimental rock mechanics to estimate the maximum bending stresses associated with the load of the Hawaiian Islands near Oahu. These results indicate that the oceanic lithosphere is capable of supporting stresses of at least 1 kbar for long periods of geological time (>50 m.y.).