Mantle Heat Loss Research Articles

Evidence for a Global Slowdown in Seafloor Spreading Since 15 Ma

AbstractThe rate of ocean‐crust production exerts control over sea level, mantle heat loss, and climate. Different strategies to account for incomplete seafloor preservation have led to differing conclusions about how much production rates have changed since the Cretaceous, if at all. We construct a new global synthesis of crust production along 18 mid‐ocean ridges for the past 19 Myr at high temporal resolution. We find that the global production rate during 6–5 Ma was only 69%–75% of the 16–15 Ma interval. The reduction in crust production is mostly due to slower seafloor spreading along almost all ridge systems. While the total ridge length has varied little since 19 Ma, some fast‐spreading ridges have grown shorter and slow‐spreading ridges grown longer, amplifying the spreading‐rate changes. Our production curves represent a new data set for investigating the forces driving plate motions and the role of tectonic degassing on climate.

A tracer-based algorithm for automatic generation of seafloor age grids from plate tectonic reconstructions

The age of the ocean floor and its time-dependent age distribution control fundamental features of the Earth, such as bathymetry, sea level and mantle heat loss. Recently, the development of increasingly sophisticated reconstructions of past plate motions has provided models for plate kinematics and plate boundary evolution back in geological time. These models implicitly include the information necessary to determine the age of ocean floor that has since been lost to subduction. However, due to the lack of an automated and efficient method for generating global seafloor age grids, many tectonic models, most notably those extending back into the Paleozoic, are published without an accompanying set of age models for oceanic lithosphere. Here we present an automatic, tracer-based algorithm that generates seafloor age grids from global plate tectonic reconstructions with defined plate boundaries. Our method enables us to produce novel seafloor age models for the Paleozoic’s lost ocean basins. Estimated changes in sea level based on bathymetry inferred from our new age grids show good agreement with sea level record estimations from proxies, providing a possible explanation for the peak in sea level during the assembly phase of Pangea. This demonstrates how our seafloor age models can be directly compared with observables from the geologic record that extend further back in time than the constraints from preserved seafloor. Thus, our new algorithm may also aid the further development of plate tectonic reconstructions by strengthening the links between geological observations and tectonic reconstructions of deeper time.

Petrological, mineralogical and geochemical peculiarities of Archaean cratons

The most outstanding features of Archaean cratons are their extraordinary thickness and enduring longevity. Seismically, Archaean cratonic fragments are sharply-bounded deep roots of mechanically strong, cold lithospheric mantle, clearly distinguishable from non-cratonic lithosphere. Rhenium-depletion of deep cratonic xenolith whole rocks and sulphide inclusions in diamond indicate that melting was broadly coeval with formation of the overlying proto-cratonic crust, which was of limited mechanical strength.A very important process of proto-cratonic development was vertical crustal reorganisation that eventually yielded a thermally stable, cratonised crust with a highly K-U-Th-rich uppermost crust and much more depleted deeper crust. Clastic sedimentary rocks available for geochemical study are predominantly found in the youngest parts of supracrustal stratigraphies and over-represent the highly evolved rocks that appeared during cratonisation. Vertical crustal reorganisation was driven by crustal radiogenic heat and emplacement of proto-craton-wide, incubating and dense supracrustal mafic and ultramafic volcanic rocks. Statistical analysis of these cover sequences shows a preponderance of basalt and a high abundance of ultramafic lavas with a dearth of picrite. The ultramafic lavas can be grouped into Ti-enriched and Ti-depleted types and high pressure and temperature experimental data indicate that the latter formed from previously depleted mantle at temperatures in excess of 1700 °C.Most mantle harzburgite xenoliths from cratonic roots are highly refractory, containing very magnesian olivine and many have a high modal abundance of orthopyroxene. High orthopyroxene mode is commonly attributed to metasomatic silica-enrichment or a non-pyrolitic mantle source but much of the excess silica requirement disappears if melting occurred at high pressures of 4–6 GPa. Analysis of experimental data demonstrates that melting of previously depleted harzburgite can yield liquids with highly variable Si/Mg ratios and low Al2O3 and FeO contents, as found in komatiites, and complementary high Cr/Al residues. In many harzburgites, there is an intimate spatial association of garnet and spinel with orthopyroxene, which indicates formation of the Al-phase by exsolution upon cooling and decompression. New and published rare earth element (REE) data for garnet and orthopyroxene show that garnet has inherited its sinusoidal REE pattern from the orthopyroxene. The lack of middle-REE depletion in these refractory residues is consistent with the lack of middle- over heavy-REE fractionation in most komatiites. This suggests that such pyroxene or garnet (or precursor phases) were present during komatiite melting. In the Kaapvaal craton, garnet exsolution upon significant cooling occurred as early as 3.2 Ga and geobarometry of diamond inclusions from ancient kimberlites also supports cool Archaean cratonic geotherms. This requires that some mantle roots have extended to 300 to possibly 400 km and that early cratons must have been much larger than 500 km in diameter.We maintain that the Archaean-Proterozoic boundary continues to be of geological significance, despite the recognition that upper crustal chemistry, as sampled by sedimentary rocks, became more evolved from ca. 3 Ga onwards. The boundary coincides with the disappearance of widespread komatiite and marks the end of formation of typical refractory cratonic lithosphere. This may signify a fundamental change in the thermal structure of the mantle after which upwellings no longer resulted in very high temperature perturbations. One school of thought is that the thermal re-ordering occurred at the core-mantle boundary whereas others envisage Archaean plumes to have originated at the base of the upper mantle. Here we speculate that Archaean cratonic roots may contain remnants of older domains of non-convecting mantle. These domains are potential carriers of isotope anomalies and their base could have constituted a mechanical and thermal boundary layer. Above laterally extensive barriers, emerging proto-cratons were protected from the main mantle heat loss. The eventual collapse of these mechanical barriers terminated very high temperature upwellings and dismembered portions of the barrier were incorporated into the cratonic mantle during the final Neoarchaean ‘superplume’ event. The surviving cratons may therefore preserve biased evidence of geological processes that operated during the Archaean.

Decrease in oceanic crustal thickness since the breakup of Pangaea

Earth’s mantle has cooled by 6–11 °C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7 km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15–20 °C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation. Thicker oceanic crust forms from a hot mantle. Observations of unusually thick oceanic crust that formed 170 million years ago in the Atlantic and Indian oceans suggest that the ancient supercontinent Pangaea helped insulate and warm the mantle.

Seafloor spreading evolution in response to continental growth

The growth of the continental crust has shaped the evolution of the Earth from its interior to its fluid envelopes. Continents have played a major role in the evolution of global tectonics through their interaction with mantle convection. The feedback between continents and mantle convection has been studied for the past 25 years, but it is only recently that the dynamic influence of continents on seafloor spreading can be explored thanks to progress in convection modeling. In this work, we investigate how continental size impacts seafloor spreading activity with state-of-the-art three-dimensional spherical convection models. We show that increasing the continental area forces higher production rates of new seafloor with stronger fluctuations. As a consequence, the average age of the seafloor decreases with increasing continental area. This study suggests that mantle heat loss experienced significant fluctuations through continental growth and reinforces the estimate of

Multiple seismic reflectors in Earth’s lowermost mantle

The modern view of Earth's lowermost mantle considers a D″ region of enhanced (seismologically inferred) heterogeneity bounded by the core-mantle boundary and an interface some 150-300 km above it, with the latter often attributed to the postperovskite phase transition (in MgSiO3). Seismic exploration of Earth's deep interior suggests, however, that this view needs modification. So-called ScS and SKKS waves, which probe the lowermost mantle from above and below, respectively, reveal multiple reflectors beneath Central America and East Asia, two areas known for subduction of oceanic plates deep into Earth's mantle. This observation is inconsistent with expectations from a thermal response of a single isochemical postperovskite transition, but some of the newly observed structures can be explained with postperovskite transitions in differentiated slab materials. Our results imply that the lowermost mantle is more complex than hitherto thought and that interfaces and compositional heterogeneity occur beyond the D″ region sensu stricto.

Effects of continental configuration on mantle heat loss

AbstractTwo‐dimensional (2D) and three‐dimensional (3D) simulations are used to explore the effects of continental distribution on mantle convection. In both 2D and 3D, at total surface areas < 50%, internal temperature is weakly sensitive to continental configuration. Mantle heat flux values show mild variations with changing configuration. In 3D, at total continental area > 50%, the dependence of mantle heat loss on continental configuration becomes stronger. Mantle temperature continues to increase with total continental area but now varies by 5%–7% with changing configurations. When distributed, continents can cause flow patterns to become locked. This leads to significant variations in mantle temperature below continental and oceanic regions. This differs from the expectation that supercontinents would preferentially lead to large lateral variations in mantle temperatures and suggests that insulation‐induced thermal anomalies could exist below continents today if they have remained fixed relative to mantle flow (e.g., Africa).

Multiagent simulation of evolutive plate tectonics applied to the thermal evolution of the Earth

The feedback between plate tectonics and mantle convection controls the Earth's thermal evolution via the seafloor age distribution. We therefore designed the MACMA model to simulate time‐dependent plate tectonics in a 2D cylindrical geometry with evolutive plate boundaries, based on multiagent systems that express thermal and mechanical interactions. We compute plate velocities using a local force balance and use explicit parameterizations to treat tectonic processes such as trench migration, subduction initiation, continental breakup and plate suturing. These implementations allow the model to update its geometry and thermal state at all times. Our approach has two goals: (1) to test how empirically‐ and analytically‐determined rules for surface processes affect mantle and plate dynamics, and (2) to investigate how plate tectonics impact the thermal regime. Our predictions for driving forces, plate velocities and heat flux are in agreement with independent observations. Two time scales arise for the evolution of the heat flux: a linear long‐term decrease and high‐amplitude short‐term fluctuations due to surface tectonics. We also obtain a plausible thermal history, with mantle temperature decreasing by less than 200 K over the last 3 Gyr. In addition, we show that on the long term, mantle viscosity is less thermally influential than tectonic processes such as continental breakup or subduction initiation, because Earth's cooling rate depends mainly on its ability to replace old insulating seafloor by young thin oceanic lithosphere. We infer that simple convective considerations alone cannot account for the nature of mantle heat loss and that tectonic processes dictate the thermal evolution of the Earth.

The effects of deep water cycling on planetary thermal evolution

[1] We use a parameterized convection model to investigate the effects of deep water cycling on the thermal evolution of an Earth-like planet. The model incorporates two water reservoirs, a surface and an interior mantle reservoir. Exchange between the two is calculated using a mantle convection parameterization that allows for temperature- and water-dependent mantle viscosity together with internally self-consistent degassing and regassing parameterizations. The balance between degassing and regassing depends on the average spreading rate of tectonic plates, the amount of water partitioned into melt, the thickness of a mantle melt zone, and of a hydrated layer at the top of subducting plates. Degassing scales with melt zone thickness such that an early period of extensive melting would create a drier and more viscous mantle, shifting the solidus line in a direction that would reduce the melt zone thickness and the rate of mantle heat loss. Coupling a hydrated zone thickness-dependent regassing factor to the model, to mimic water delivery to the mantle via a serpentinized layer, allows for the potential of a reversing point where the overall water flow direction switches from degassing to regassing as the mantle cools. The water effect on viscosity creates a negative feedback that tends to regulate the final amount of water in the mantle so it is not strongly dependent on the initial amount of planetary water. The final amount of water in the surface reservoir is then determined by this feedback effect together with the initial water budget of the entire planet. This implies that if the initial water budget of a planet can be estimated, from planetary formation models, then the volume of surface water can be used to estimate the volume of water in the mantle of an Earth-like planet. Applying this methodology to the Earth leads to predictions for water concentration in the Earth's mantle that are in line with geochemical and petrological constraints.

The Kalahari Epeirogeny and climate change: differentiating cause and effect from core to space

Southern Africa’s high Kalahari plateau and its flanking mountain ranges formed over an extended period of ∼200 million years through vertical tectonic processes different from those at convergent plate boundaries. We refer to this as the Kalahari epeirogeny. Episodic uplift and erosion during the Kalahari epeirogeny is inferred from thermo-chronology and from stratigraphy of sediment accumulated around its continental margin. A total thickness of 2 to 7 km of rock (of which basalt was a major component) was eroded from the subcontinent’s surface during two punctuated episodes of exhumation, in the early-Cretaceous and in the mid-Cretaceous. Major exhumation was over by the end of the Cretaceous, and the rate of erosion decreased by more than an order of magnitude to remove less than 1 km thickness of rock during the Cenozoic. Cosmogenic dating shows that erosion rates today are almost an order of magnitude less still. The cause for the Kalahari epeirogeny remains elusive, but three striking observations stand out: (i) major exhumation occurred during the late Mesozoic break-up of Gondwana; (ii) large scale basaltic magmatism closely match two accelerated episodes of exhumation: first along the west coast (∼132 Ma, Etendeka large igneous province [LIP], associated with vast amounts of underplating along this entire passive margin), and the second flanking the sheared south coast margin (∼90 Ma, Agulhas oceanic LIP). Yet exhumation that might have accompanied the vast Karoo LIP (∼180 Ma) is not readily detected; (iii) the two main regional episodes of accelerated exhumation and coastal sediment accumulation are near synchronous with two regional ‘spikes’ of kimberlite intrusions: >450 kimberlites at ∼90 Ma, and >200 kimberlites at ∼120 Ma. Southern Africa is underlain by an anomalous warm region in the lowermost ∼1500 km of the mantle that is linked to core to mantle heat loss. This warm region was inherited from a large Cretaceous thermo-chemical anomaly in the mantle created during long-lived subduction beneath Gondwana. Associated Mesozoic kimberlite genesis and basaltic magmatism may have been sufficient to cause volatile/heat-induced density changes in the lithospheric mantle of southern Africa to sustain the Kalahari plateau. In addition, such changes may have been induced by tectonic uplift and decompressional melting resulting from far-field collision processes between Africa and Europe, and/or final continental lithosphere decoupling between the Falkland plateau and southern Africa. CO 2 released into the ocean-atmosphere system during the Kalahari epeirogeny contributed to the global mid-Cretaceous hot-house conditions. However, because the CO 2 -consumption rate associated with basalt weathering is about eight times that of granite, atmospheric CO 2 was also efficiently sequestrated during the rapid erosion of Karoo basalt. Thus, the Kalahari epeirogeny also may have catalysed the onset of long-term global Cenozoic cooling.

Convection under a lid of finite conductivity: Heat flux scaling and application to continents

A scaling law for the heat flux out of a convective fluid covered totally or partially by a finitely conducting lid is proposed. This scaling is constructed in order to quantify the heat transfer out of the Earth's mantle, taking into account the effect of the dichotomy between oceans and continents, which imposes heterogeneous thermal boundary conditions at the surface of the mantle. The effect of these heterogeneous boundary conditions is studied here using simple two‐dimensional models, with the mantle represented by an isoviscous fluid heated from below and continents represented by nondeformable lids of finite thermal conductivity set above the surface of the model. We use free‐slip boundary conditions under the oceanic and continental zones in order to study in an isolated way the possible thermal effect of continents, independently of all mechanical effect. A systematic study of the heat transfer as a function of the Rayleigh number of the fluid, of the width of the lid, and of its thermal properties is carried out. We show that estimates of continental lithosphere thickness imply a strong insulating effect from continents on mantle heat loss, at least locally. The heat flux below continents was low in the past and of the order of the present one if the continental thickness has remained broadly constant over the Earth's history.

Continental insulation, mantle cooling, and the surface area of oceans and continents

It is generally assumed that continents, acting as thermal insulation above the convecting mantle, inhibit the Earth's internal heat loss. We present theory, numerical simulations, and laboratory experiments to test the validity of this intuitive and commonly used assumption. A scaling theory is developed to predict heat flow from a convecting mantle partially covered by stable continental lithosphere. The theory predicts that parameter regimes exist for which increased continental insulation has no effect on mantle heat flow and can even enhance it. Partial insulation leads to increased internal mantle temperature and decreased viscosity. This, in turn, allows for the more rapid overturn of oceanic lithosphere and increased oceanic heat flux. Depending on the ratio of continental to oceanic surface area, global mantle heat flow can remain constant or even increase as a result. Theoretical scaling analyses are consistent with results from numerical simulations and laboratory experiments. Applying our results to the Earth we find, in contrast to conventional understanding, that continental insulation does not generally reduce global heat flow. Such insulation can have a negligible effect or even enhance mantle cooling, depending on the magnitude of the temperature dependence of mantle viscosity. The theory also suggests a potential constraint on continental surface area. Increased surface area enhances the subduction rate of oceanic lithosphere. If continents are produced in subduction settings this could enhance continental growth up to a critical point where increased insulation causes convective stress levels to drop to values approaching the lithospheric yield stress. This condition makes weak plate margins difficult to maintain which, in turn, lowers subduction rates and limits the further growth of continents. The theory is used to predict the critical point as a function of mantle heat flow. For the Earth's rate of mantle heat loss, the predicted continental surface area is in accord with the observed value.

Convective heat transfer as a function of wavelength: Implications for the cooling of the Earth

Attempting to reconstruct the thermal history of the Earth from a geophysical point of view has for a long time been in disagreement with geochemical data. The geophysical approach uses parameterized models of mantle cooling. The rate of cooling of the Earth at the beginning of its history obtained in these models is generally too rapid to allow a sufficient present‐day secular cooling rate. Geochemical estimates of radioactive element concentrations in the mantle then appear too low to explain the observed present mantle heat loss. Cooling models use scaling laws for the mean heat flux out of the mantle as a function of its Rayleigh number of the form Q ∝ Raβ. Recent studies have introduced very low values of the exponent β, which can help reduce the cooling rate of the mantle. The present study instead focuses on the coefficient C in the relation Q = C Raβ and, in particular, on its variation with the wavelength of convection. The heat transfer strongly depends on the wavelength of convection. The length scale of convection in Earth's mantle is that of plate tectonics, implying convective cells of wide aspect ratio. Taking into account the long wavelength of convection in Earth's mantle can significantly reduce the efficiency of heat transfer. The likely variations of this wavelength with the Wilson cycle thus imply important variations of the heat flow out of the Earth on a intermediate timescale of 100 Ma, which renders parameterized models of thermal evolution inaccurate for quantitative predictions.

Effects of continents on Earth cooling: Thermal blanketing and depletion in radioactive elements

Estimate of mantle heat flow under continental shields are very low, indicating a strong insulating effect of continents on mantle heat loss. This effect is investigated with a simple approach: continents are introduced in an Earth cooling model as perfect thermal insulators. Continental growth rate has then a strong influence on mantle cooling. Various continental growth models are tested and are used to compute the mantle depletion in radioactive elements as a function of continental crust extraction. Results show that the thermal blanketing effect of continents strongly affects mantle cooling, and that mantle depletion must be taken into account in order not to overestimate mantle heat loss. In order to obtain correct oceanic heat flow for present time, continental growth must begin at least 3 Gy ago and steady‐state for continental area must be reached for at least 1.5 Gy in our cooling model.

On the asymmetric distribution of heat loss from the Earth’s interior

Mean heat flows and heat Josses of the Northern and Southern hemispheres are calculated using degree 12 spherical harmonic representation of the global heat flow field (Pollacket al., 1993). Mean heat flows and heat losses of 0° hemisphere and 180° hemisphere, with median lines being 0° longitude and 180° longitude, are also calculated. The mean heat flow of the Southern Hemisphere is 99.3 mW·m¨, significantly higher than that of the Northern Hemisphere (74.0 mW·m¨). The mean heat flow of 0° hemisphere (94. 1 mW·m2) is also higher than the value of 180° hemisphere (79. 3 mW·m2). The mantle heat loss from the Southern Hemisphere is 22.1 × 1012 W, as twice as that from the Northern Hemisphere (10.8 × 1012 W). The 16.9 × 1012 W mantle heat loss from 0° hemisphere is close to 16.0 × 1012 W from 180° hemisphere. The hemispherical asymmetry of global heat loss is originated by the asymmetry of geographic distribution of continents and oceans. The asymmetric distribution of heat loss is a long-term phenomenon in the geological history.

On the partitioning of mantle heat loss below oceans and continents over time and its relationship to the Archaean paradox

Summary The Archaean paradox stems from the fact that while certain lines of evidence suggest that Archaean continental geotherms were similar to those at present, other lines of evidence suggest that the average mantle heat flux was considerably greater. The simplest qualitative solution, which holds that a greater proportion of heat loss was carried by the creation and subduction of oceanic lithosphere in the Earth’s past, has lacked a clear physical basis. At a fundamental level, it requires that the ratio of mantle heat loss below oceans to that below continents be an increasing function of overall convective vigour and, to date, no self-consistent model has been used to suggest a means by which this could be so. A simple means is suggested by models that allow accumulations of chemically light near-surface material, analogues to continents, to form within the upper thermal boundary layer of a thermally convecting and chemically dense fluid, an analogue to the mantle. The physical properties of continental crust cause the thermal coupling condition between a model continent and the convectively unstable mantle below to be different from that which exists at the interface between mantle and oceans. For the latter, a spatially constant temperature condition holds due to (1) the participation of oceanic crust in convective mantle overturn and (2) the large effective thermal conductivity of oceans relative to the mantle. For the former, a spatially near-constant equilibrium heat flux condition holds due to (1) the long-term stability of continental crust against remixing into the mantle and (2) the comparable thermal conductivities of distinct rock types. This leads to a laterally variable thermal condition at the surface of the convecting mantle, which, in turn, leads to different local mantle heat flux behaviour in continental versus oceanic regions. As a result, heat flux below continents can increase at a slower rate, with increasing convective vigour, than it does elsewhere. Model heat-flux scalings are used to assess the degree to which continental geotherms can be time-stabilized by this means. Results suggest that the thermal surface heterogeneity imposed on the mantle by the presence of continental crust can prevent wide-scale crustal melting in the Archaean by forcing oceans to carry a greater proportion of the Earth’s heat-loss load in the past.

Convective thermal evolution of the upper mantles of Earth and Venus

On Earth the present‐day rate of heat loss is about twice the heat generation rate; on Venus it is about half. Though this rough balance may be due to a feedback mechanism between mantle temperature and heat loss, it is difficult to see how such a mechanism can occur on timescales of 1 Ga or less when the upper mantle of the Earth is thought to be cooling at about 40°C Ga−1. On Venus a decrease in surface heat flux presumably occurred at the end of the catastrophic resurfacing event at ∼500 Ma. Parameterized convection models relate heat flux to Rayleigh number by the exponent β. Such models using a range of viscosities and values of β from 0.2 to 0.3 show that the effect of a sudden decrease in surface heat flux is to cause an independently convecting upper mantle to increase in temperature by 100–500°C over 1 Ga, whereas, if whole mantle convection occurs, the temperature change is less than 60°C. An increase in mantle temperature of 200°C or more will affect mantle viscosity, lithospheric thickness and melt generation rate, all of which may affect the feedback mechanism.

Mantle dynamics and the heat flow into the Earth's continents

REDUCEDcontinental heat flow, defined as the measured heat flow minus that due to radiogenic sources in the upper crust, initially decreases with time after the last thermotectonic event in a region1, but then tends to stabilize at a roughly constant value, which is higher than would be predicted by a conductive cooling model (Fig. 1). Here we argue that this behaviour results from the influence that continents have on mantle heat loss. The finite thermal conductivity of continental crust and the fact that it is not recycled into the mantle on a large scale, as is oceanic crust, allows stable continental blocks to limit the local heat flux out of the mantle. Numerical models that allow blocks of crust to form over a mantle layer demonstrate how thermal perturbations associated with mantle convection penetrate into continents, causing lateral temperature variations at their base that parallel those in the mantle just below. This tends to establish a constant flux of heat from the mantle into the continents, which accounts for the trend in reduced heat flow.

Some effects of a dry crustal flow law on numerical simulations of coupled crustal deformation and mantle convection on Venus

Models of coupled crustal deformation and mantle convection on Venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine. A previous model which assumed a relatively weak basalt flow law, more appropriate for hydrated crust, is compared to a similar model that assumes a strong basalt flow law, more appropriate for dry crust. A principal difference is the timescale for significant crustal thickening, of the order of 108 years for the weak basalt model and 109 years for the strong basalt model. These results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near‐surface weakness, of the type associated with plate margins on Earth, the lithospheric overturn time, which sets the timescale required for convective thickening of crust, scales dominantIy with the Rayleigh number defined by the average viscosity of the lithosphere. Theoretical scalings, as well as numerical models, suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle Rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103. Such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present‐day Earth. The large values inferred for the thickness of the mechanical lithosphere on Venus, if correct, would thus tend to favor timescales of the order of 109 years, if one assumes relatively low convective mantle heat loss over an equivalent time. This, in turn, would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows. If, instead, one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years, then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust.

Correction to “Processes involved in the formation of magnesian‐suite plutonic rocks from the highlands of the Earth's Moon” by Gregory A. Snyder, Clive R. Neal, and Lawrence A. Taylor

Models of coupled crustal deformation and mantle convection on Venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine. A previous model which assumed a relatively weak basalt flow law, more appropriate for hydrated crust, is compared to a similar model that assumes a strong basalt flow law, more appropriate for dry crust. A principal difference is the timescale for significant crustal thickening, of the order of 108 years for the weak basalt model and 109 years for the strong basalt model. These results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near‐surface weakness, of the type associated with plate margins on Earth, the lithospheric overturn time, which sets the timescale required for convective thickening of crust, scales dominantIy with the Rayleigh number defined by the average viscosity of the lithosphere. Theoretical scalings, as well as numerical models, suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle Rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103. Such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present‐day Earth. The large values inferred for the thickness of the mechanical lithosphere on Venus, if correct, would thus tend to favor timescales of the order of 109 years, if one assumes relatively low convective mantle heat loss over an equivalent time. This, in turn, would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows. If, instead, one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years, then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust.