Thorsten W. Becker

Shaping mobile belts by small-scale convection

Mobile belts are long-lived deformation zones composed of an ensemble of crustal fragments, distributed over hundreds of kilometres inside continental convergent margins. The Mediterranean represents a remarkable example of this tectonic setting: the region hosts a diffuse boundary between the Nubia and Eurasia plates comprised of a mosaic of microplates that move and deform independently from the overall plate convergence. Surface expressions of Mediterranean tectonics include deep, subsiding backarc basins, intraplate plateaux and uplifting orogenic belts. Although the kinematics of the area are now fairly well defined, the dynamical origins of many of these active features are controversial and usually attributed to crustal and lithospheric interactions. However, the effects of mantle convection, well established for continental interiors, should be particularly relevant in a mobile belt, and modelling may constrain important parameters such as slab coherence and lithospheric strength. Here we compute global mantle flow on the basis of recent, high-resolution seismic tomography to investigate the role of buoyancy-driven and plate-motion-induced mantle circulation for the Mediterranean. We show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region. More generally, vigorous small-scale convection in the uppermost mantle may also underpin other complex mobile belts such as the North American Cordillera or the Himalayan–Tibetan collision zone.

Mantle dynamics in the Mediterranean

The Mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt. Lithospheric dynamics are affected by slab rollback and collision of two large, slowly moving plates, forcing fragments of continental and oceanic lithosphere to interact. This paper reviews the rich and growing set of constraints from geological reconstructions, geodetic data, and crustal and upper mantle heterogeneity imaged by structural seismology. We proceed to discuss a conceptual and quantitative framework for the causes of surface deformation. Exploring existing and newly developed tectonic and numerical geodynamic models, we illustrate the role of mantle convection on surface geology. A coherent picture emerges which can be outlined by two, almost symmetric, upper mantle convection cells. The downwellings are found in the center of the Mediterranean and are associated with the descent of the Tyrrhenian and the Hellenic slabs. During plate convergence, these slabs migrated backward with respect to the Eurasian upper plate, inducing a return flow of the asthenosphere from the backarc regions towards the subduction zones. This flow can be found at large distance from the subduction zones, and is at present expressed in two upwellings beneath Anatolia and eastern Iberia. This convection system provides an explanation for the general pattern of seismic anisotropy in the Mediterranean, first-order Anatolia and Adria microplate kinematics, and may contribute to the high elevation of scarcely deformed areas such as Anatolia and Eastern Iberia. More generally, the Mediterranean is an illustration of how upper mantle, small-scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the Tethyan collision.

The development of slabs in the upper mantle: Insights from numerical and laboratory experiments

We have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab. A two-dimensional finite element code is applied to model Newtonian creep in the numerical experiments. Scaled analog media are used in the laboratory, a sand mixture models the brittle crust, silicone putty simulates creep in the lower crust and mantle lithosphere, and glucose syrup is the asthenosphere analog. Both model approaches show similar results and reproduce first-order observations of the subduction process in nature based on density and viscosity heterogeneities in a Stokes flow model. Subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold. We find that the subduction velocity and angle are time-dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670-km discontinuity. The style of subduction is controlled by the prescribed velocity of convergence, the density contrast between the plates, and the viscosity contrast between the oceanic plate and the mantle. These factors can be combined in the buoyancy number F which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate. Variations in F control the stress in the plates, the speed and the dip of subduction, and the rate of trench retreat, reproducing the contrasting styles of subduction observed in nature. The subduction rate is strongly influenced by the work of bending the lithosphere as it subducts.

THERMAL CONSTRAINTS ON THE SURVIVAL OF PRIMITIVE BLOBS IN THE LOWER MANTLE

Geochemical models have frequently divided the mantle into depleted upper and undepleted lower mantle reservoirs, usually taken as indication for a layered style of convection. This is difficult to reconcile with seismological and geodynamical evidence for substantial mass flux between lower and upper mantle. Various models have been proposed to jointly interpret the evidence, including that of G.F. Davies [J. Geophys. Res. 89 (1984) 6017‐6040] in which the author suggested that lumps of primitive material may exist in the lower mantle, representing reservoirs for undepleted basalts. Mixing calculations have suggested, however, that such blobs could not survive 4 Ga of convection. Calculations by M. Manga [Geophys. Res. Lett. 23 (1996) 403‐406] on the other hand showed that high-viscosity blobs could persist in convective cells for geologically long times without being substantially deformed and mixed with the surrounding flow. We investigate a blob model of convection based on these ideas and consider dynamical, thermal, geochemical and rheological consequences. The radiogenic heat production in the primitive blobs would lead to higher temperatures. However, these would be modest (1T < 300 K) for sufficiently small blobs (radius<800 km). The resulting thermal buoyancy can be offset by a small intrinsic density excess (<1%) so that blob material is hidden from the ridges but sampled by rising plumes. To satisfy geochemical constraints, blobs would have to fill 30% to 65% of the mantle (less if they are taken to be enriched rather than primitive). Thermal considerations require that they be surrounded by depleted material of lower viscosity that would convectively transport heat to the surface. The thermal decrease in blob viscosity would be about one order of magnitude but constrained to the interior; the stiffer ‘shell’ can then be expected to control the dynamical mixing behavior. On average, the viscosity of the lower mantle would be increased by the presence of the blobs; if they were 100 times more viscous than the surrounding mantle the net effect would be to increase the effective viscosity approximately

Dehydration melting at the top of the lower mantle

Cycling water through the transition zone The water cycle involves more than just the water that circulates between the atmosphere, oceans, and surface waters. It extends deep into Earths interior as the oceanic crust subducts, or slides, under adjoining plates of crust and sinks into the mantle, carrying water with it. Schmandt et al. combined seismological observations beneath North America with geodynamical modeling and high-pressure and -temperature melting experiments. They conclude that the mantle transition zone—410 to 660 km below Earths surface—acts as a large reservoir of water. Science, this issue p. 1265 Downwelling of hydrous minerals may cause partial melting of Earth’s lower mantle. The high water storage capacity of minerals in Earth’s mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir, which could cause dehydration melting of vertically flowing mantle. We examined the effects of downwelling from the transition zone into the lower mantle with high-pressure laboratory experiments, numerical modeling, and seismic P-to-S conversions recorded by a dense seismic array in North America. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt. Detections of abrupt decreases in seismic velocity where downwelling mantle is inferred are consistent with partial melt below 660 kilometers. These results suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H2O in the transition zone.

Savani: A variable resolution whole‐mantle model of anisotropic shear velocity variations based on multiple data sets

We present a tomographic model of radially anisotropic shear velocity variations in the Earths mantle based on a new compilation of previously published data sets and a variable block parameterization, adapted to local raypath density. We employ ray-theoretical sensitivity functions to relate surface wave and body wave data with radially anisotropic velocity perturbations. Our database includes surface wave phase delays from fundamental modes up to the sixth overtone, measured at periods between 25 and 350 s, as well as cross-correlation traveltimes of major body wave phases. Before inversion, we apply crustal corrections using the crustal model CRUST2.0, and we account for azimuthal anisotropy in the upper mantle using ray-theoretical corrections based on a global model of azimuthal anisotropy. While being well correlated with earlier models at long spatial wavelength, our preferred solution, savani, additionally delineates a number of previously unidentified structures due to its improved resolution in areas of dense coverage. This is because the density of the inverse grid ranges between 1.25° in well-sampled and 5° in poorly sampled regions, allowing us to resolve regional structure better than it is typically the case in global S wave tomography. Our model highlights (i) a distinct ocean-continent anisotropic signature in the uppermost mantle, (ii) an oceanic peak in above average ξ<1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness, and (iii) a long-wavelength pattern of ξ<1 associated with the large low-shear velocity provinces in the lowermost mantle.

Mantle plumes: Dynamic models and seismic images

Different theories on the origin of hot spots have been debated for a long time by many authors from different fields, and global-scale seismic tomography is probably the most effective tool at our disposal to substantiate, modify, or abandon the mantle-plume hypothesis. We attempt to identify coherent, approximately vertical slow/hot anomalies in recently published maps of P and S velocity heterogeneity throughout the mantle, combining the following independent quantitative approaches: (1) development and application of a “plume-detection” algorithm, which allows us to identify a variety of vertically coherent features, with similar properties, in all considered tomographic models, and (2) quantification of the similarity between patterns of various tomographic versus dynamic plume-conduit models. Experiment 2 is complicated by the inherent dependence of plume conduit tilt on mantle flow and by the dependence of the latter on the lateral structure of the Earths mantle, which can only be extrapolated from seismic tomography itself: it is inherently difficult to disentangle the role of upwellings in “attracting” plumes versus plumes being defined as relatively slow, and thus located in regions of upwellings. Our results favor the idea that only a small subset of known hot spots have a lower-mantle origin. Most of those that do can be associated geographically with a few well-defined slow/hot regions of very large scale in the lowermost mantle. We find evidence for both secondary plumes originating from the mentioned slow/hot regions and deep plumes whose conduits remain narrow all the way to the lowermost mantle. To best agree with tomographic results, modeled plume conduits must take into account the effects of advection and the associated displacement of plume sources at the base of the mantle.

A Review of the Role of Subduction Dynamics for Regional and Global Plate Motions

Subduction of oceanic lithosphere and deep slabs control several aspects of plate tectonics. We review models of subduction dynamics that have been studied over the last decade by means of numerical and analog experiments. Regional models indicate that trench rollback, trench curvature, and back-arc deformation may be explained by fl uid slabs that are 250–500 times stiffer than the upper mantle. Slab width and, more importantly, rheology determine the role of viscous bending, poloidal-sinking fl ow and toroidal-rollback stirring, and interactions of the slab with the higher viscosity lower mantle. Several of these contributions can be represented by a local sinking veloCity. Back-arc deformation may then result from an imbalance if larger-scale plate forcing leads to deviations of the convergence rate from the local equilibrium. Lateral viscosity variations (LVVs) are also key for understanding plate driving forces. The realism of global circulation computations has advanced and such models with weak zones and other LVVs have lead to an improved match to observed plate tectonic scores. Those include the correlation with plate motions, the magnitude of intraplate deformation, and oceanic to continental plate veloCity ratios. Net rotation of the lithosphere with respect to the lower mantle may be caused jointly by regional slab forcing and the stirring effect of cratonic keels. However, slab models have so far only produced net rotations that are small compared to recent hotspot reference-frame models. Progress in the next years will likely come from a better understanding of slab strength, which is still uncertain since large-scale subduction zone observables and laboratory results do not put strong constraints on slab rheology. Importantly, circulation models with an improved representation of convergent margins will help to close the gap between regional and global approaches to subduction, and to better understand the potential role of the overriding plate.

Numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow

[1] The development of Lattice Preferred Orientations (LPO) within olivine aggregates under flow in the upper mantle leads to seismic and rheological (or viscoplastic) anisotropies. We compare predictions from different micromechanical models, applying several commonly used theoretical descriptions to an upwelling flow scenario representing a typical oceanic spreading center. Significant differences are obtained between models in terms of LPO and associated rheology, in particular in regions where the flow direction changes rapidly, with superior predictions for the recently proposed Second-Order approach. This highlights the limitations of ad hoc formulations. LPO-induced rheological anisotropy may have a large effect on actual flow patterns with 1–2 orders of magnitude variation in directional viscosities compared to the isotropic case. Citation: Castelnau, O., D. K. Blackman, and T. W. Becker (2009), Numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow, Geophys. Res. Lett., 36, L12304,

On the role of slab pull in the Cenozoic motion of the Pacific plate

We analyze the role of slab pull acting on the Pacific plate during its early Tertiary change in motion. Slab pull forces are estimated by integrating the negative buoyancy of a 700 km long slab along a revised subduction boundary model adopting the Muller et al. (2008) seafloor age reconstructions. Our results indicate that torques predicted from a simple slab pull model match the Pacific plate Euler vectors during the Tertiary fairly well. The change of the Pacific motion at similar to 50-40 Ma appears to be driven by the onset of the Izu-Bonin-Mariana system and, soon afterwards, by the Tonga-Kermadec subduction zones.