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Both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western Pacific subduction zones. The goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting. We calculated three-dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates. Shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow-induced strain. Predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain. A parameter study shows that both finite strain and textural anisotropy developed within three-dimensional, plate-coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate. Therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere. Flow models incorporating plate motion boundary conditions for the Tonga, southern Kuril, and eastern Aleutian subduction zones produce splitting parameters consistent with observations from each region. Trench-parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge.

The goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions, the Tonga subduction zone in particular, can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate. We calculated two-dimensional mantle flow models using known Tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures. Shear wave splitting was predicted for the anisotropy due to lattice preferred orientation (LPO) of olivine and orthopyroxene in the flow model finite strain fields. The predicted shear wave splitting provides a good match to the fast directions (parallel to the azimuth of subducting plate motion) and splitting times (0.5–1.5 s) observed in Tonga, both for models where LPO anisotropy develops everywhere above 410 km and for models where LPO anisotropy is confined to regions of relatively high stress. If LPO anisotropy does develop over the entire upper 410 km of the mantle, the strength of anistropy induced by a given amount of shear strain must be relatively weak (∼4% for shear strains of 1.5, with a maximum value of ∼6% for very large strains). The splitting observations are comparably fit by a wide range of different viscosity models. Anisotropy due to melt-filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting.

In this study we investigated the origin of seismic anisotropy in the mantle beneath North America. In particular, we evaluated whether shear wave splitting patterns in eastern North America are better explained by anisotropy caused by lithospheric deformation, anisotropy due to mantle flow beneath the lithosphere, or a combination of both. We examined new measurements of shear wave splitting from the Missouri to Massachusetts broadband seismometer array (MOMA), the North American Mantle Anisotropy and Discontinuity experiment (NOMAD), as well as splitting parameters from several previous studies. We developed a simple finite difference model that approximates mantle flow around a complex, three-dimensional continental lithospheric keel. To evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern North America, we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model. Our results indicate that a significant portion of observed shear wave splitting in eastern North America can be explained by mantle flow around the continental keel. However, shear wave splitting patterns in a few regions of eastern North America indicate that a component of lithospheric anisotropy must exist, particularly in regions containing the largest keel thicknesses. For eastern North America, as well as for splitting observations in Australia, Europe, and South America, we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow.

Parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere-convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales. Recent experimental results argue against this assumption. We have investigated both the definition of the lithosphere-convecting mantle boundary and the power law relation describing convecting heat transfer, based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature-dependent viscosity. Other recent studies have treated only the heating from below, but volumetric heating is likely to be the dominant mode of heating in planetary mantles, either as a consequence of radioactive heating or as a proxy for secular cooling. Convection can occur either in the whole box or be located under a stagnant lid. In the lid regime, convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature. This result, in agreement with experimental studies, indicates that boundary between the stagnant lid and the convecting layer (similar to the lithosphere-convecting mantle boundary) cannot be defined as a fixed isotherm. During thermal evolution of planets, the viscosity contrast in the convecting mantle remains constant, not the temperature at the bottom of the lithosphere. We present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere-convecting mantle boundary. For reasonable values of the activation energy for thermally activated creep, the temperature defining the lithosphere-convecting mantle boundary, the mantle temperature, and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm.

Bathymetry and gravity data from four geophysical surveys along the 15°–19°S superfast spreading section of the East Pacific Rise are used to study formation of the axial topographic high. After removing a best fitting thermal model the averaged, residual, axial topography is ∼265 m high and ∼15 km wide. The residual gravity shows no evidence for shallow isostatic compensation of this topography suggesting deep-rooted compensation or dynamic support. Previous models postulate that the axial high is compensated by low densities in a zone of partial melting within the upper mantle. We model the physical properties of the lithosphere and asthenosphere required to match the residual gravity and bathymetry using a nonlinear inversion, assuming the high is uplifted by buoyancy forces from low-density mantle. Deep-rooted buoyancy forces cannot create a narrow axial high unless they occur in a low-viscosity conduit surrounded by high-viscosity mantle. This high-viscosity region could be created by depletion and extraction of all water during the melting process. We find that the viscosity of the asthenosphere beneath the melt conduit must also be lower than the viscosity of the residual mantle surrounding the conduit. Our most reasonable model has a viscosity of 1020 Pa s in the residual mantle surrounding a melt conduit which extends 50 km below the seafloor and a viscosity of 1016.6 Pa s in the asthenosphere below the melt conduit. The viscosity of the melt conduit must be about 1015 Pa s, or about 105 times lower than the surrounding residual mantle, with a density contrast equivalent to the retention of a small fraction of melt (5%). Because higher melt concentrations are probably required to create even small changes in viscosity and the viscosity of the undepleted mantle is probably higher than 1016.6 Pa s, we conclude that it is unlikely that the axial topographic high is formed by buoyancy forces in the crust and mantle.

Describes the characteristics of possible hotspots on Venus, the approach used to simulate mantle upwelling, model results, and presents the implications for the properties of plumes and the lithosphere, hotspot evolution, and resurfacing on Venus.

We study the development of finite amplitude folding of the lithosphere through the application of finite element models characterized by both discontinuous (layer over half-space) .and continuous (strength envelope) viscosity distributions. Both models include strongly strain rate-dependent rheologies that approximate slip on pervasive faults. Folding with layers of form, contrasting viscosities is driven solely bye magnitudes of discontinuities in .viscosity at layer interfaces; however, folds may also develop in a medium with a continuously varying viscosity distribution. The net driving force for folding in a region of continuous viscosity variation is the same as the driving force across a discontinuity with the same net viscosity contrast. Continuous and discontinuous viscosity variations give essentially identical fold growth rates if the depth over which the viscosity varies is small compared to the wavelength of folding or the depth of the viscosity variation. In the uniform viscosity layer model, flexural bending of the layer results in a state of extension on topographic highs and compression in topographic lows. In contrast, the strength envelope model is characterized by significant penetrative shortening near the surface that results in a state of compression everywhere along the fold. This model predicts layer thickening beneath fold troughs, as observed in the intmpladeformation zone in the central Indian Ocean. Buocy forces retard the rate of fold amplification but are inadequato prevent folding for a range of realistic lithospheric theological structures.

Melting of the upper mantle and extraction of melt result in the formation of a less dense depleted mantle. This paper describes series of two-dimensional models that investigate the effects of chemical buoyancy induced by these density variations. A tracer particles method has been set up to follow as closely as possible the chemical state of the mantle and to model the chemical buoyant force at each grid point. Each series of models provides the evolution with time of magma production, crustal thickness, surface heat flux, and thermal and chemical state of the mantle. First, models that do not take into account the displacement of plates at the surface of Earth demonstrate that chemical buoyancy has an important effect on the geometry of convection. Then models include horizontal motion of plates 5000 km wide. Recycling of crust is taken into account. For a sufficiently high plate velocity which depends on the thermal Rayleigh number, the cell's size is strongly coupled with the plate's size. Plate motion forces chemically buoyant material to sink into the mantle. Then the positive chemical buoyancy yields upwelling as depleted mantle reaches the interface between the upper and the lower mantle. This process is very efficient in mixing the depleted and undepleted mantle at the scale of the grid spacing since these zones of upwelling disrupt the large convective flow. At low spreading rates, zones of upwelling develop quickly, melting occurs, and the model predicts intraplate volcanism by melting of subducted crust. At fast spreading rates, depleted mantle also favors the formation of these zones of upwelling, but they are not strong enough to yield partial melting. Their rapid displacement toward the ridge contributes to faster large-scale homogenization.

Segmentation and along-axis variations within individual segments indicate the inherently three-dimensional nature of mantle up welling and melting beneath oceanic spreading centers. Numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center. The experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher-viscosity mantle half-space. A periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets. Buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt. The relative magnitudes of the buoyant and plate-driven components of mantle flow are controlled by the spreading rate and mantle viscosity, with buoyant flow more important at lower spreading rates and viscosities. Buoyant flow beneath the spreading axis amplifies along-axis variations in upwelling near a ridge-transform intersection, and distributes the variations along the entire spreading axis. Buoyant flow may thus be responsible for the more three-dimensional character of slow spreading centers. Away from the spreading axis, thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along-axis wavelength controlled by the prescribed thickness of the asthenosphere. However, the position and stability of rolls are influenced by the segmentation geometry. In cases where the spreading center geometry does not allow a stable configuration of rolls, the flow is time-dependent. Along-axis variations in upwelling cause variations in melt production, which imply large variations in crustal thickness that dominate the surface gravity signal. The crustal thickness distributions implied by these numerical experiments produce bulls-eye-shaped negative mantle Bouguer anomalies centered over spreading segments, as observed at several spreading centers. The amplitude of the anomaly increases with decreasing spreading rate.

[1] Although the Pacific and Nazca plates share the East Pacific Rise (EPR) as a boundary, they exhibit many differing characteristics. The Pacific plate subsides more slowly and has more seamounts than the Nazca plate. Both the seismic and magnetotelluric components of the Mantle ELectromagnetic and Tomography Experiment (MELT) found pronounced asymmetry in mantle structure across the spreading axis near 17°S. The Pacific (west) side has lower S-wave velocities, exhibits greater shear wave splitting, and is more electrically conductive than the Nazca (east) side. These results suggest asymmetric mantle flow and melt distribution beneath the EPR. To better understand the causes for these asymmetric properties, we construct numerical models of melting and mantle flow beneath a midocean ridge migrating to the west over a fixed mantle. Although the ridge is migrating to the west, the migration has little effect on the upwelling rates, requiring a separate mechanism to create the asymmetry. Models that produce asymmetric melting with a temperature anomaly require large (>100°C) excess temperatures and may not be consistent with the observed subsidence and crustal thickness. A possible mechanism for creating asymmetry without a temperature anomaly is across-axis asthenospheric flow, possibly driven by pressures created by upwelling beneath the Pacific Superswell to the west. Pressure-driven asthenospheric flow follows the base of the lithosphere, extending the upwelling region to the west as it follows the thinning lithosphere toward the axis, and shutting off melting as it crosses the axis and encounters an increasingly thick lithosphere to the east.

[1] Volcanic activity away from plate boundaries occurs in a variety of settings. Linear, age-progressive volcanic chains have been explained as the manifestation of fixed hot spots, possibly generated by plumes originating deep in the mantle. However, important examples of oceanic intraplate volcanism cannot be explained in this way, including short-lived chains and chains violating the predicted age-distance behavior. Furthermore, a significant fraction of volcanism does not occur in linear chains, and observations suggest control by lithosphere structure in many cases. In this study, melting in mantle upwellings that result from the buoyancy associated with the melting itself is considered as a mechanism of intraplate volcanism. Numerical models of this “buoyant decompression melting” in a layer that is initially at its melting temperature over some depth range are used to determine the duration and amount of melting in upwellings which organize from an initially small, random perturbation for a range of model parameters. A predicted inverse correlation between the amount of melt produced and the duration of melting may be diagnostic of the buoyant melting process. Buoyant melting could occur in a number of geologic settings. Since the oceanic upper mantle may have previously melted beneath a spreading center and subsequently cooled, spontaneous buoyant decompression melting may be possible only in regions where a large-scale mantle upwelling can counteract conductive cooling, keeping the mantle at its solidus temperature over some depth range. Where the mantle is at, or sufficiently near, its solidus, buoyant decompression may spontaneously generate “convective storms” of melting. This may explain the abundance of volcanism associated with the South Pacific Superswell. Where mantle is slightly cooler than its melting temperature, buoyant melting may not occur spontaneously but may be triggered by some initial upwelling. This may provide a physical explanation for volcanism that is controlled by lithosphere structure.

All known propagating rifts (PR's) are growing away from shallow ridge axis seafloor. We examine a model for rift propagation in which the ridge axis conduit, like a crack in the lithosphere, propagates in response to a stress concentration at its tip. The stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity-spreading stresses associated with anomalously shallow ridge axis along the growing ridge. This stress concentration near the tip is characterized by a stress intensity factor K, which must exceed a threshold value to cause failure of the plate. A remotely applied tensile stress would make a positive contribution to K but would act equally on both ridge segments and not favor the propagation of either. The pressure in the crack may, depending on its distribution, make either a positive or negative contribution to K. For a high enough propagation rate, viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the PR tip would make K < 0. This simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well-studied 95.5°W PR of the Galapagos spreading center. Analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the PR axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 Pa s (1020 P). We also examine a more general model of a ridge-transform-ridge system on the basis of energy conservation. In terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis. However, the more general model also includes mechanisms that may stop ridge propagation. Propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis, i.e., if the stress intensity factors for both ridge axes are similar. Both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85°W in the Galapagos. Finally, in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation.

The axial valley along a slow spreading mid-ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis. Along a ridge segment, the axial valley floor deepens by as much as 2–3 km over distances of several tens of kilometers approaching a ridge-transform intersection. This deepening may be explained, in part, by a pressure gradient associated with horizontal flow in the ridge axis conduit. Vanishing vertical velocity on a conduit end wall at a ridge-transform intersection results in reduced vertical flow within about one conduit width of the intersection. Horizontal flow along the conduit, toward the intersection, must occur to form lithosphere at a uniform rate along the ridge axis. A simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge-transform intersections is considered. The model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls. This model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection. For reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection.

Models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate. However, there has so far been little direct evidence for the magnitude and distribution of thermal stresses. We present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones. Specifically, our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones. Including the effect of varying thermal bending moments, thin plate theory predicts vertical, nonisostatic displacements of the lithosphere by plate flexure. The predicted amplitude of the resulting geoid anomaly is large enough to be observed in Seasat altimeter profiles. Furthermore, the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable. The anomaly due to thermal stresses has been clearly identified in geoid profiles across the Clarion and the Udintsev fracture zones. The amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700°C, consistent with the maximum depth of seismicity in the oceanic lithosphere. The distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes.