Seismic tomography, surface uplift, and the breakup of Gondwanaland: Integrating mantle convection backwards in time

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95171/1/ggge225.pd

Mantle convection with strong subduction zones

Because mantle viscosity is temperature‐dependent, cold subducting lithosphere should be strong, which implies that the rapid, localized deformation associated with subduction should strongly resist plate motions. Due to computational constraints, the deformation of a subducting plate cannot be accurately resolved in mantle‐scale convection models, so its effect on convection is difficult to investigate. We have developed a new method for implementing subduction that parametrizes the deformation of the oceanic lithosphere within a small region of a finite element grid. By imposing velocity boundary conditions in the vicinity of the subduction zone, we enforce a geometry for subduction, producing a slab with a realistic thermal structure. To make the model dynamically consistent, we specify a rate for subduction that balances the energy budget for convection, which includes an expression for the energy needed to deform the oceanic lithosphere as it subducts. This expression is determined here from a local model of bending for a strong viscous lithosphere. By implementing subduction in this way, we have demonstrated convection with plates and slabs that resemble those observed on Earth, but in which up to 40 per cent of the mantle's total convective resistance is associated with deformation occurring within the subduction zone. This additional resistance slows plate velocities by nearly a factor of two compared to models with a weak slab. For sufficiently strong lithosphere, the bending deformation slows surface plates sufficiently that they no longer actively participate in global‐scale convection, which occurs instead beneath a ‘sluggish lid’. By introducing a low‐viscosity asthenosphere beneath the oceanic plate, we demonstrate that small‐scale convection at the base of oceanic lithosphere may limit plate thickness, and thus the resistance to bending, and cause plate velocities to depend on the strength of the bending lithosphere rather than on the viscosity of the underlying mantle. For a cooling Earth, the effective lithosphere viscosity should be nearly constant, but the mantle viscosity should increase with time. Thus, subduction‐resisted convection should produce nearly constant plate velocities and heat flow over time, which has implications for the Earth's thermal evolution. We estimate that this style of convection should apply if the effective viscosity of the bending lithosphere is greater than about 10^(23) Pa s, but only if some mechanism, such as small‐scale convection, prevents the bending resistance from stopping plates altogether. Such a mechanism could be fundamental to plate tectonics and Earth's thermal history

Plate motions, Andean orogeny, and volcanism above the South Atlantic convection cell

International audienceThe geometric and kinematic evolution of the Andes provides insight onto the nature of the force balance beneath the South American plate. While the Andean load is opposed on its western edge by the force induced by subduction of the Nazca plate, its more elusive eastern counterpart, which we explore herein, requires some contribution from the mantle beneath the South Atlantic. Using a mantle flow model, we show that the Andes owe their existence to basal drag beneath South America caused by a cylindrical convection cell under the South Atlantic. We find that the observed Andean uplift requires both westward push fromactive upwelling beneath Africa andwestward drag toward the downgoing Nazca slab. These mutually-reinforcing downwellings and upwellings amount to 38% and 23% of the total driving force, respectively. Further decomposition reveals that the South Atlantic cell is most vigorous near its center, rendering the net drag force higher where the Andes also reach their highest elevation. Kinematic reconstructions suggest that the South Atlantic cell could have grown owing to the migration of the Nazca slab until ~50 Ma. We propose that from 50 Ma onwards, the cell may have ceased growing westward because (i) it had reached an optimal aspect ratio and (ii) the Nazca slab became anchored into the lower mantle. Continued westward motion of the plates, however, moved the surface expressions of spreading and convergence away from the upwelling and downwelling arms of this cell. Evidence for this scenario comes from the coeval tectonic, morphologic, and magmatic events in Africa and South America during the Tertiary

Convective instability of thickening mantle lithosphere

Mantle lithosphere, being colder and therefore denser than the underlying mantle, is prone to convective instability that can be induced by horizontal shortening. Numerical experiments on a cold layer with imposed horizontal shortening are carried out to examine the relative importance of mechanical thickening, thermal diffusion and gravitational instability in deforming the layer. This analysis is then used to develop a method for determining which of these styles dominates for a layer thickening at a given rate. If viscosity is non-Newtonian, the imposition of shortening decreases the lithospheric strength, which causes perturbations to the lithosphere’s temperature structure to grow exponentially with time. Once these perturbations become sufficiently large, they then grow super-exponentially with time, eventually removing the lithospheric base. Because lithospheric viscosity is highly temperature-dependent, at most only the lower 30 per cent of the lithosphere participates in the downwelling associated with this initial super-exponential growth event. After this event, however, a downwelling develops that removes material advected into the region of downwelling by horizontal shortening. The magnitude of this persistent downwelling depends on the rate and duration of shortening. If the total amount of shortening does not exceed 50 per cent (doubling of crustal thickness), then this downwelling extends to a depth three to four times the thickness of undeformed lithosphere and forms a sheet significantly thinner than the width of the region undergoing shortening. Once shortening stops, this downwelling is no longer replenished by the shortening process, and should then detach due to its inherent gravitational instability. The hottest 60 per cent of the mantle lithosphere could be removed in such an event, which would be followed by an influx of hot, buoyant asthenosphere that causes rapid surface uplift. Because more cold material is removed after the cessation of shortening than by the initial gravitational instability, the former has a potentially greater influence on surface uplift. The Tibetan interior is thought to have been shortened by about 50 per cent in ∼30Myr and afterwards, at ∼8Ma, experienced a period of rapid uplift that may have resulted from the removal of a large downwelling ‘finger’ of cold lithosphere generated by shortening

Convective instability of thickening mantle lithosphere

Mantle lithosphere, being colder and therefore denser than the underlying mantle, is prone to convective instability that can be induced by horizontal shortening. Numerical experiments on a cold layer with imposed horizontal shortening are carried out to examine the relative importance of mechanical thickening, thermal diffusion and gravitational instability in deforming the layer. This analysis is then used to develop a method for determining which of these styles dominates for a layer thickening at a given rate. If viscosity is non-Newtonian, the imposition of shortening decreases the lithospheric strength, which causes perturbations to the lithosphere’s temperature structure to grow exponentially with time. Once these perturbations become sufficiently large, they then grow super-exponentially with time, eventually removing the lithospheric base. Because lithospheric viscosity is highly temperature-dependent, at most only the lower 30 per cent of the lithosphere participates in the downwelling associated with this initial super-exponential growth event. After this event, however, a downwelling develops that removes material advected into the region of downwelling by horizontal shortening. The magnitude of this persistent downwelling depends on the rate and duration of shortening. If the total amount of shortening does not exceed 50 per cent (doubling of crustal thickness), then this downwelling extends to a depth three to four times the thickness of undeformed lithosphere and forms a sheet significantly thinner than the width of the region undergoing shortening. Once shortening stops, this downwelling is no longer replenished by the shortening process, and should then detach due to its inherent gravitational instability. The hottest 60 per cent of the mantle lithosphere could be removed in such an event, which would be followed by an influx of hot, buoyant asthenosphere that causes rapid surface uplift. Because more cold material is removed after the cessation of shortening than by the initial gravitational instability, the former has a potentially greater influence on surface uplift. The Tibetan interior is thought to have been shortened by about 50 per cent in ∼30Myr and afterwards, at ∼8Ma, experienced a period of rapid uplift that may have resulted from the removal of a large downwelling ‘finger’ of cold lithosphere generated by shortening

The Elastic Response of the Earth to Interannual Variations in Antarctic Precipitation

Measurements of elastic displacements of the bedrock surrounding large ice sheets have been proposed as a means to detect mass changes in these ice sheets. However, accumulation of glacial mass on the ice sheets is a noisy process, subject to large spatial and temporal variations in precipitation. We simulated the response of the Antarctic continent to a stochastic model of interannual precipitation variations and found that interannual variations in the elastic response of the earth are large when compared to the long-term mean of displacements produced by an assumed average ice mass imbalance of 10%. If, as some scientists predict, Antarctic ice mass changes in the future become dramatic, the long-term signal should be large enough to be detected by a few years of geodetic measurements, despite climatic noise

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 the first 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

Iceland, the Farallon slab, and dynamic topography of the North Atlantic

ABSTRACT Upwelling or downwelling flow in Earth's mantle is thought to elevate or depress Earth's surface on a continental scale. Direct observation of this ''dynamic topography'' on the seafloor, however, has remained elusive because it is obscured by isostatically supported topography caused by near-surface density variations. We calculate the nonisostatic topography of the North Atlantic by correcting seafloor depths for lithospheric cooling and sediment loading, and find that seafloor west of the Mid-Atlantic Ridge is an average of 0.5 km deeper than it is to the east. We are able to reproduce this basic observation in a model of mantle flow driven by tomographically inferred mantle densities. This model shows that the Farallon slab, currently in the lower mantle beneath the east coast of North America, induces downwelling flow that deepens the western North Atlantic relative to the east. Our model also predicts dynamic support of observed topographic highs near Iceland and the Azores, but suggests that the Icelandic high is due to local upper-mantle upwelling, while the Azores high is part of a plate-scale lower-mantle upwelling to the south. An anomalously deep area off the coast of Nova Scotia may be associated with the downwelling component of edge-driven convection at the continental boundary. Thus, several of the seafloor's topographic features can only be understood in terms of dynamic support from flow in Earth's mantle