Small-scale Convection Research Articles

Operational Convective-Scale Numerical Weather Prediction with the COSMO Model: Description and Sensitivities

Abstract Since April 2007, the numerical weather prediction model, COSMO (Consortium for Small Scale Modelling), has been used operationally in a convection-permitting configuration, named COSMO-DE, at the Deutscher Wetterdienst (DWD; German weather service). Here the authors discuss the model changes that were necessary for the convective scale, and report on the experience from the first years of operational application of the model. For COSMO-DE the ability of the numerical solver to treat small-scale structures has been improved by using a Runge–Kutta method, which allows for the use of higher-order upwind advection schemes. The one-moment cloud microphysics parameterization has been extended by a graupel class, and adaptations for describing evaporation of rain and stratiform precipitation processes were made. Comparisons with a much more sophisticated two-moment scheme showed only minor differences in most cases with the exception of strong squall-line situations. Whereas the deep convection parameterization was switched off completely, small-scale shallow convection was still parameterized by the appropriate part of the Tiedtke scheme. During the first year of operational use, convective events in synoptically driven situations were satisfactorily simulated. Also the daily cycles of summertime 10-m wind and 1-h precipitation sums were well captured. However, it became evident that the boundary layer description had to be adapted to enhance convection initiation in airmass convection situations. Here the asymptotic Blackadar length scale l∞ had proven to be a sensitive parameter.

Tropical convection

Tropical convection

Elevated mantle temperature beneath East Africa

Research Article| January 01, 2012 Elevated mantle temperature beneath East Africa Tyrone O. Rooney; Tyrone O. Rooney * 1Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824, USA *E-mail: rooneyt@msu.edu. Search for other works by this author on: GSW Google Scholar Claude Herzberg; Claude Herzberg 2Department of Geological Sciences, Rutgers University, Piscataway, New Jersey 08855-1179, USA Search for other works by this author on: GSW Google Scholar Ian D. Bastow Ian D. Bastow 3Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (1): 27–30. https://doi.org/10.1130/G32382.1 Article history received: 22 Apr 2011 rev-recd: 28 Jul 2011 accepted: 18 Aug 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Tyrone O. Rooney, Claude Herzberg, Ian D. Bastow; Elevated mantle temperature beneath East Africa. Geology 2012;; 40 (1): 27–30. doi: https://doi.org/10.1130/G32382.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The causes of magmatism at magmatic rifted margins and large igneous provinces (LIPs) are uncertain because the condition of the mantle that underlay them during formation can no longer be directly observed. Therefore, whether the mantle was characterized by elevated potential temperatures (TP), small-scale convection, or anomalously fertile composition is debated. East Africa is an ideal area in which to address this problem because it contains both the young African-Arabian LIP and the tectonically and magmatically active East African Rift system. Here we present mantle TP estimates for 53 primitive magmas from throughout the region to reveal that thermal anomalies currently peak in Djibouti (140 °C above ambient upper mantle). Slightly warmer conditions accompanied the Oligocene African-Arabian LIP, when the TP anomaly was 170 °C. These values are toward the low end of the global temperature range of LIPs, despite the markedly slow seismic velocity mantle that underlies the region. Mantle seismic velocity anomalies in East Africa cannot, therefore, as is often assumed, be attributed simply to elevated mantle temperatures. We conclude that CO2-assisted melt production in the African superplume contributes to the markedly slow seismic velocities below East Africa. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Planform of small-scale convection under the island arc

We have proposed an existence of small-scale convection in the mantle wedge in order to explain the alignment of group of volcanoes in the NE Honshu subduction zone, Japan. The roll type thermal convection whose axis is normal to the plate boundaries and “flip-flopping” of rolls (warmer and colder parts exchange their positions, as time passes) are reported. However, the subsequent study shows an existence of non-flip-flopping rolls, also. Thus, in this study, I investigate the possible causes of these differences systematically. I found that along-arc wavelength of small-scale convection is controlled by two length scales which may be associated with the inclined nature of the bottom of the low viscosity mantle wedge where the small-scale convection emerges. When the convection is in early stage of evolvement or the speed of subducting slab is small, the long-wavelength rolls become prominent. As the convection evolves or the speed of subduction increases, short-wavelength rolls tend to take over the long-wavelength rolls. The transition from the long-wavelength to the short-wavelength rolls occurs in a several way. It may occur through the splitting of rolls, adjustment of roll patterns and flip-flopping. If the pattern becomes short-wavelength roll, it becomes stable. The range of existence of flip-flopping is limited and the existence of viscosity jump in the top thermal boundary layer may be required.

Implications of a simple mantle transition zone beneath cratonic North America

Many areas of old continental interiors have thick roots that extend to depths of 250 km or more, in contrast to the oceans and younger continents, whose lithospheric thickness is less than 100 km. These cratonic roots might perturb temperatures in their surrounding mantle, though the net result could be either cooling or heating; both of which may lead to small-scale convective flow around the root. We show here, using new data from a study of the seismic structure of the Canadian Shield, that the relative positions of the 410 km and 660 km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the Earth. Differential arrival times and internal discontinuity structure are remarkably uniform and simple, varying by little more than ± 0.5 s over the root's 3500 km lateral extent. This implies that the root has no significant thermal effect on the underlying mantle (< 50 K), and any small-scale convection or cold mantle downwelling associated with the large free-air gravity anomaly beneath the shield must be confined to the upper mantle. Our observations are also consistent with phase changes solely in the olivine system ((Mg,Fe) 2SiO 4), with the caveat that our method is dominantly sensitive to S-wave velocity jumps alone.

Seismic rays and traveltime tomography of strongly heterogeneous mantle structure: application to the Central Mediterranean

SUMMARY The structure of the Earth’s upper mantle near convergent plate margins, such as along the Nubia‐Eurasia collision zone in the Mediterranean, involves strong seismic wave speed contrasts associated with subducting lithospheric slabs and opening backarc basins. In this environment, seismic wave propagation is strongly influenced by heterogeneity, and requires appropriate modelling practice. Although accurate numerical methods are often used to model seismic traveltimes in the crust, only approximate techniques have been used for the mantle, on the assumption that speed contrasts are weaker. We devise, optimize, and test a method aimed at recovering strongly heterogeneous mantle structures using a finite-difference scheme to calculate first-arrival traveltimes and trace seismic rays with high accuracy even in the presence of strong gradients. We adapt this forward scheme—successfully used in local-scale tomography—to spherical geometry through source-specific Earth flattening approximations, and we split calculations in meshes with different step size to model optimally the crust (with a 2 km step) and the mantle (6 km step). We then use an iterative non-linear inversion approach, starting from a simple 1-D prior model. We test the ability of this procedure to reconstruct sample structures, devised to be illustrative for the Mediterranean region, using synthetic data calculated on the real distribution of sources and stations reported by the Bulletins of the International Seismological Centre (ISC). Besides regular checkerboard patterns, we also reconstruct a more representative model. Different strategies are used and compared in linear and non-linear inversion. We find that a linear approach, by which rays are only traced once in the background model, may result in an illusory fit to data. Realistic upper-mantle structures strongly deflect seismic rays, and correct paths can only be found after a few iterations. Although linear inversion seems able to identify the main features quite well, we verify how non-linear inversion and 3-D ray tracing significantly improve the results, especially when we attempt to reconstruct a realistic structure. We also apply the finite-difference, non-linear, traveltime tomography to data from the ISC to retrieve upper-mantle structure in the Central Mediterranean. We verify that the non-linear inversion is able to reveal sharpened velocity contrasts and thinner bodies than linear inversion. Clear differentiation found in the nonlinear result, between signatures of northern and southern Dinarides—showing lithosphere subducting only beneath the southern sector—is more coherent with the regional geodynamic framework. Such improvements due to non-linear mantle tomography may contribute to the general picture of slab detachment and small-scale mantle convection in the Mediterranean region, and therefore, significantly impact on geodynamic implications of resulting models.

Mantle heterogeneity and off axis volcanism on young Pacific lithosphere

Plate tectonics and mantle plumes explain most volcanism on earth, but there are numerous actively forming linear volcanic chains in the middle of tectonic plates that are not explained by these theories. Using the multidisciplinary geophysical dataset of the MELT and GLIMPSE experiments, we show that associated with 3 volcanic chains west of the East Pacific Rise there are low seismic velocities and densities in the asthenosphere that extend to the East Pacific Rise spreading center. Analogous to the Hawaiian swell, the low-density anomalies produce swells beneath the volcanoes on young seafloor. The associated gravity anomalies are part of a set of gravity lineaments that have been previously interpreted as being due to thermo-elastic cracking of the lithosphere or small-scale convection. The correlation between the surface volcanism and subsurface density and velocity anomalies and their extension to the spreading center suggest that pre-existing, buoyant or fertile asthenospheric mantle heterogeneities are stretched in the direction of plate motion by shear between the plate and the underlying mantle. These heterogeneities seed small-scale convection, producing upwelling and pressure release melting, forming volcanic chains that extend nearly to the ridge axis.

Low viscosity of the bottom of the Earth’s mantle inferred from the analysis of Chandler wobble and tidal deformation

Viscosity of the D″ layer of the Earth’s mantle, the lowermost layer in the Earth’s mantle, controls a number of geodynamic processes, but a robust estimate of its viscosity has been hampered by the lack of relevant observations. A commonly used analysis of geophysical signals in terms of heterogeneity in seismic wave velocities suffers from major uncertainties in the velocity-to-density conversion factor, and the glacial rebound observations have little sensitivity to the D″ layer viscosity. We show that the decay of Chandler wobble and semi-diurnal to 18.6years tidal deformation combined with the constraints from the postglacial isostatic adjustment observations suggest that the effective viscosity in the bottom ∼300km layer is 1019–1020Pas, and also the effective viscosity of the bottom part of the D″ layer (∼100km thickness) is less than 1018Pas. Such a viscosity structure of the D″ layer would be a natural consequence of a steep temperature gradient in the D″ layer, and will facilitate small scale convection and melt segregation in the D″ layer.

Destruction of the North China Craton: Lithosphere folding-induced removal of lithospheric mantle?

High heat flow, high surface topography, and widespread volcanism indicate that the lithospheric mantle of typical cratonic character of the North China Craton has been seriously destroyed in its eastern half. However, the mechanism of this process remains open to intense debate. Here lithosphere folding-induced lithospheric mantle removal is proposed as a new mechanism for the destruction of the craton. Four main NNE–SSW-striking lithospheric-scale anticlines and synclines are recognized within North China east of the Helan fold-and-thrust belt. The lithosphere folding occurred possibly during the Late Triassic through Jurassic when the Yangzi Craton collided with the North China Craton. It was accompanied or followed by lithospheric dripping, and could have possibly induced the lithosphere foundering of the North China Craton. The lithosphere folding would have modified the lithosphere morphology, creating significant undulation in the lithospheric base and thus causing variations of the patterns of the small-scale convection. It also could have provoked the formation of new shear zones liable to impregnation of magma, producing linear incisions at the cratonic base and resulting in foundering of lithospheric mantle blocks. Furthermore, it generated thickening of the lithosphere or the lower crust and initiated the destabilization and subsequent removal of the lithospheric mantle.

A sporadic low-velocity layer atop the western U.S. mantle transition zone and short-wavelength variations in transition zone discontinuities

Teleseismic receiver function analysis of data from six dense arrays in the western U.S. is used to investigate mantle transition zone (MTZ) discontinuities and the prevalence of a low-velocity layer atop the 410 km discontinuity (410-LVL). Negative polarity Ps arrivals indicative of a low-velocity layer with a top 25–60 km above the 410 are identified in 8–11 out of 18 stacks of receiver functions from highly sampled back azimuth corridors. The 410-LVL is interpreted as partial melt resulting from upwelling of hydrated mantle across a water solubility contrast at the 410. The 669 km mean depth of the 660 km discontinuity (660) and the magnitude of 660 topography suggest variable hydration, locally approaching saturation, in addition to >150 K lateral temperature variations beneath five arrays. Mean amplitudes of P410s and P660s increase monotonically with period from 2 to 10 s; however, greater variations are observed in the frequency dependence of P410s compared to P660s implying 410 thickness is more heterogeneous. Variable 410 thickness is attributed to changes in hydration modulating the width of the olivine-to-wadsleyite transition interval. Frequency dependence of P660s amplitudes suggests a broad velocity gradient consistent with multivariate phase changes in the olivine and garnet systems. Sporadic detection of the 410-LVL, the magnitude and length scales of MTZ discontinuity topography, and inferred variations in hydration support the occurrence of vigorous small-scale convection in the western U.S. mantle. Comparison of receiver functions with body wave tomography suggests small-scale convection driven by sinking slab segments and lithospheric instabilities contributes to the intermittent nature of the 410-LVL.

At least three scales of convection in a mantle with strongly temperature-dependent viscosity

We studied the convective patterns developing at high Rayleigh numbers and intermediate viscosity ratios in a fluid with a strongly temperature-dependent viscosity. Within this sluggish lid regime, three different scales of convection develop. The largest convective scale is cellular, with cold downwelling sheets of viscous fluid encasing hotter, less viscous, parts of the tank. Within each of those cells develop several (typically 3–7) hot 3D upwelling plumes. Upon impinging under the cold thermal boundary layer, each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank. Applying those results to the Earth’s mantle, we suggest that the large-scale features of mantle convection and the co-existence of several scales of convection, i.e. slabs and plumes, are produced by thermal convection in a mantle with high Rayleigh number and a strongly temperature-dependent viscosity material. This generates a convective pattern in two large-scale cells : the Pacific and the Indo-Atlantic “boxes”. Our experiments further suggest that what has been named the two hot superplumes, i.e. the two seismically slow regions encased within the subduction rings, are in fact each constituted of several hot instabilities. Moreover, plumes impacts under the lithosphere should be surrounded by cold rings of small extent. The asthenosphere appears therefore as the graveyard of both small-scale convection and hot plumes generated in the system.

Impact of radiative heating, wind shear, temperature variability, and microphysical processes on the structure and evolution of thin cirrus in the tropical tropopause layer

[1] Thin cirrus that frequently form in the tropical tropopause layer (TTL) are important for vertical transport through the TTL, regulation of stratospheric humidity, and the Earth's radiation budget. Here, we use a three-dimensional cloud-resolving model to investigate the impact of circulations driven by TTL cirrus radiative heating on the cloud evolution. We use observations of TTL environmental conditions (thermal stability and wind shear) and TTL cirrus microphysical properties (ice crystal sizes and concentrations) to constrain the simulations. We show that with ice crystal sizes consistent with available observations (effective radii ≥ 12 μm), typical thermal stability, and moderate wind shear, the ice cloud sediments to lower levels before radiative heating can drive a circulation to maintain the cloud and before small-scale convection builds up. In this case, the cloud lifetime is controlled by sedimentation of ice crystals into subsaturated air below the initial cloud level, followed by sublimation. Strong wind shear (>10 m s−1 km−1) tends to hasten the cloud dissipation. With relatively weak thermal stability, small-scale convection builds up rapidly, resulting in mixing at cloud top and extension of the cloud lifetime. We also consider the impact of synoptic-scale and mesoscale temperature variability on cloud lifetime. Using TTL trajectories with small-scale wave temperature perturbations superimposed, we show that TTL cirrus will often dissipate within 12–24 h simply as a result of background temperature variability.

Spatial and temporal variability in Hawaiian hotspot volcanism induced by small-scale convection

be explained by the classical mantle plume concept. Here we present a numerical model of mantle plume upwelling beneath Hawaii. We find that small-scale convection in the ambient mantle can erode the base of the lithosphere, creating a washboard topography on the underside of the plate. As the plate migrates over the upwelling plume, the plume interacts with alternating thicker and thinner sections of lithosphere to generate temporal variations in the flux of erupted volcanic material. The pre-existing washboard topography also causes the plume to spread and melt asymmetrically. In our simulations, this asymmetry in mantle flow generates an asymmetry in the chemistry of the erupted lavas. Finally, a more vigorous type of small-scale convection develops within the spreading plume, generating localized zones of upwelling well away from the hotspot. The associated magmatism is fed by chemically distinct material originating from the edges of the plume conduit. Our results show that shallow processes have an important influence on the character of volcanism fed by deep-rooted mantle plumes.

Tectonic regionalization without a priori information: A cluster analysis of upper mantle tomography

Global mantle tomography can be improved through better use of data and application of more accurate wave propagation methods. However, few techniques have been developed for objective validation and exploration of the resulting tomographic models. We show that cluster analysis can be used to validate and explore the salient features across such models. We present a cluster analysis of a global upper mantle radially anisotropic model SEMum developed using full waveform tomography and the Spectral Element Method. Applied to SEMum down to 350 km depth, the cluster analysis reveals that absolute shear wave velocity (Vs) depth profiles naturally group into families that correspond with known surface tectonics. This allows us to construct a global tectonic regionalization based solely on tomography, without the help of any a priori information. We find that the profiles of stable platforms and shields consistently exhibit a mid-lithospheric low velocity zone (LVZ) between 80 and 130 km depth, while the asthenosphere is found at depths greater than 250 km in both regions. This global intra-continental-lithosphere low velocity zone agrees with recent receiver function studies and regional tomographic studies. Furthermore, we identify an anomalous oceanic region characterized by slow shear wave speeds at depths below 150 km. Hotspots are found preferentially in the vicinity of this anomalous region. In the Pacific Ocean, where plate velocities are largest, these regions have elongated shapes that align with absolute plate motion, suggesting a relationship between the location of hotspots and small-scale convection in the oceanic upper mantle.

The impact of variability in the rheological activation parameters on lower-mantle viscosity stratification and its dynamics

The recently discovered spin-state crossover in iron in major mantle minerals at high pressures should exert dramatic influences on transport properties, such as the activation creep parameters in the deep mantle. Wentzcovitch et al. (2009) have computed the elastic parameter of ferropericlase that is affected by this crossover, the bulk modulus, at high realistic conditions, using first principles density functional theory plus Hubbard U. As a consequence of the spin crossover there is a significant softening of the bulk modulus and an attendant reduction of the activation energy in the creep law. Using a parameterized model capturing the basic physics, we have studied the dynamical consequences within the framework of a 2-D Cartesian convection model. Our models reveal a series of asthenospheres and are characterized first by a low-viscosity channel below the 670 km boundary (‘second asthenosphere’) caused by an increase of temperature and decrease in grain size reduction (superplascity) with the post-spinel transition, which results in partially layered convection. Further, the variability of the rheological parameters leads to the prevalent formation of viscosity minimum at a depth of about 1600 km (‘third asthenosphere’) caused by spin crossovers followed by a ‘viscosity hill’ in the bottom half of the lower mantle, the latter due to the increase of the activation energy in some low spin irons at the bottom of the mantle. Our numerical simulations reveal a tendency to the formation of small-scale convection below the 670 km boundary and bigger stabilized plumes – and/or plume clustering in the deep lower mantle. Other material properties at the bottom of the mantle (decrease of thermal expansivity, radiative thermal conductivity) also exert significant influence on the multi-scale lower-mantle plume dynamics.

Application of the Lorenz equations for studying thermal convection in the lithosphere

Stationary solutions including wave solutions with constant amplitudes are found for nonlinear equations of thermal convection in a layer with nonlinear rheology. The solution is based on the Fourier expansion of unknown velocities and temperatures with only the first and first two terms retained in the velocity and temperature series, respectively. This method, which can be regarded as the Lorenz method, yields the Lorenz equations that fairly well describe the thermal convection in a layer with Newtonian rheology if the Rayleigh number is not very large. The obtained generalization of the Lorenz equations to the case of an integral (having a memory) nonlinear rheology implies that only the first term is retained in the Fourier series for the stress components, i.e., the nonlinear rheological equation is harmonically linearized. However, in the Fourier series of temperature, it is essential to keep the second term: this term, which is independent of the horizontal coordinate, models the thermal boundary layer that characterizes the developed convection. We constructed the bifurcation curves that describe the stationary convection in the nonlinear integral medium simulating the rheology of the mantle, and analyzed the stability of stationary convective flows. The Lorenz method is applied to study small-scale thermal convection in the lithosphere of the Earth.

Small-scale convection beneath oceans and continents

Small-scale convection supplies heat flow of ∼17 mW m−2 to the base of stable continents where xenolith studies resolve the geotherm. However, effects of small-scale convection are difficult to resolve in ocean basins. On first pass, most seafloor appears to subside to an asymptote compatible with ∼40 mW m−2 convective heat flow. These common regions are tracked by hotspots so uplift associated with ponded mantle material is an attractive alternative. Unaffected seafloor in the North and South Atlantic continues to subside with the square root of age as expected from pure conduction. The theory of stagnant-lid convection provides good scaling relationships for heat flow. For linear viscosity, heat flow is proportional to the underlying “half-space” viscosity to the −1/3 power and the temperature to change viscosity by a factor of e to the 4/3 power. The formalism is easily modified to represent convection beneath a lid of highly viscous and buoyant cratonal lithosphere and to represent transient convection beneath thickening oceanic lithosphere. Asthenospheric mantle with linear, strongly temperature-dependent, and weakly depth-dependent viscosity is compatible with both oceanic and continental data. More complicated rheology may allow vigorous small-scale convection under most but not all old ocean basins. Still viable hypotheses require poorly understood global features, including lateral variations of asthenospheric temperature. Seismological studies have the potential to resolve the lithosphere-asthenosphere boundary, including local variations of its depth associated with small-scale convection.

Patterns of intraplate volcanism controlled by asthenospheric shear

Volcanism observed far from plate boundaries, in the interior of oceanic and continental plates, may result from flow in the underlying mantle. Comparison between a numerical model of mantle flow and the spatial distribution of intraplate volcanism indicates that rapid shear motion in the mantle may drive melting that causes intraplate eruptions. Most of Earth’s volcanism occurs at plate boundaries, in association with subduction or rifting. A few high-volume volcanic fields are observed both at plate boundaries and within plates, fed by plumes upwelling from the deep mantle1. The remaining volcanism is observed away from plate boundaries. It is typically basaltic, effusive and low volume, occurring within continental interiors2,3,4,5,6,7 or creating seamounts on the ocean floor8,9,10,11. This intraplate volcanism has been attributed to various localized processes12 such as cracking of the lithosphere8,13,14, small-scale convection in the mantle beneath the lithosphere15,16,17 or shear-induced melting of low-viscosity pockets of asthenospheric mantle that have become embedded along the base of the lithosphere18. Here we compare the locations of observed intraplate volcanism with global patterns of mantle flow from a numerical model. We find a correlation between recent continental and oceanic intraplate volcanism and areas of the asthenosphere that are experiencing rapid shear due to mantle convection. We detect particularly high correlations in the interior of the continents of western North America, eastern Australia, southern Europe and Antarctica, as well as west of the East Pacific Rise in the Pacific Ocean. We conclude that intraplate volcanism associated with mantle convection is best explained by melting caused by shear flow within the asthenosphere, whereas other localized processes are less important.

Lithosphere-asthenosphere interaction beneath Ireland from joint inversion of teleseismic P-wave delay times and GRACE gravity

The nature and extent of the regional lithosphere-asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signatures pervade the lithosphere, tertiary structure related to the Iceland plume has been inferred to dominate the asthenosphere. To address this apparent contradiction in the literature, we image the 3-D lithospheric and deeper upper-mantle structure beneath Ireland via non-linear, iterative joint teleseismic-gravity inversion using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments. The inversion combines teleseismic relative arrival time residuals with the GRACE long wavelength satellite derived gravity anomaly by assuming a depth-dependent quasilinear velocity-density relationship. We argue that anomalies imaged at lithospheric depths probably reflect compositional contrasts, either due to terrane accretion associated with Iapetus Ocean closure, frozen decompressional melt that was generated by plate stretching during the opening of the north Atlantic Ocean, frozen Iceland plume related magmatic intrusions, or a combination thereof. The continuation of the anomalous structure across the lithosphere-asthenosphere boundary is interpreted as possibly reflecting sub-lithospheric small-scale convection initiated by the lithospheric compositional contrasts. Our hypothesis thus reconciles the disparity which exists between lithospheric and asthenospheric structure beneath this region of the north Atlantic rifted margin.

Temperature and rheological properties of the mantle beneath the North American craton from an analysis of heat flux and seismic data

[1] We combine heat flux data and seismic velocity models for the North American lithosphere to derive constraints on thermal conditions and deformation mechanisms in the underlying convecting mantle. Local heat flux averages that are not affected by shallow crustal heat production contrasts allow calculation of reliable lithospheric geotherms and uncertainty ranges. For consistency with the seismic data, the mantle potential temperature beneath North America must lie within a 1290°C–1450°C range, close to that for the oceanic mantle sampled at mid-ocean ridges. The heat flux at the base of the lithosphere varies laterally from 11 ± 3 mW m−2 beneath the ∼250 km thick Archean core of the Superior province to 15 ± 3 mW m−2 beneath the thinner younger Appalachians province. It is shown that the most likely cause of such rates of heat supply into the North American continent is small-scale convection in an unstable boundary layer beneath the rigid mechanical lithosphere. This allows useful constraints on the mantle rheological properties. We show that the most likely deformation mechanism is dislocation creep in wet mantle rocks. Ranges for the mantle temperature, water content, and rheological parameters could be tightened very significantly once strong constraints are obtained on radiogenic heat production in the lithospheric mantle.