T34A-01 INVITED 16:00h
Constraining the Extent of Crust-Mantle Coupling in Central Asia Using GPS, Geologic, and Shear-Wave Splitting Data
We have obtained constraints on mechanical crust-mantle coupling for Tibet and Yunnan/Indo China, by comparing the observed surface deformation field inferred from GPS and Quaternary fault slip rate data, with the mantle deformation field inferred from several SKS shear wave splitting data sets. It was first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction, which is assumed to be parallel to the direction of maximum shear. For asthenospheric flow, we solved for a best-fitting uniform sub-asthenospheric velocity model for Eurasia. The fit, however, was not satisfactory. Solving for separate uniform flow fields in each region improves the fit, although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely. We then considered lithospheric models. For Tibet, vertically coherent deformation (i.e., maximum shear direction from surface deformation is parallel to the fast polarization direction) yields an improved match for left-lateral shear. Both the goodness of fit and the dominance of left-lateral surface faulting in Tibet, argue for a lithospheric source of anisotropy. The misfit for Yunnan is large for either right- or left-lateral shear, which requires a complete crust-mantle de-coupling. Dynamic modeling of Eurasia deformation shows that boundary conditions and topographically induced body forces contribute roughly equally. Because these body forces are applied to the crust, they will only contribute to mantle deformation if crust-mantle coupling is strong. The observed vertical coherence of lithospheric deformation in Tibet thus argues for strong crust-mantle coupling there. Conversely, crust-mantle de-coupling within Yunnan lithosphere makes the specific prediction that mantle deformation would be controlled by boundary conditions alone. To test this, we determined the mantle deformation field generated only by boundary conditions, and found that the fit for Yunnan is much improved for left-lateral shear. The fit is not good for Tibet, further illustrating the need for crust-mantle coupling there. Finally we constructed a "hybrid" lithospheric mantle model, where crustal body-forces were applied in Tibet, but not in Yunnan. The resulting model provides an excellent fit to the entire data set, and is our preferred model. These results have the following implications. First, they imply coherence and strong mechanical coupling for Tibet. This coherence is driven by the dynamics of the upper crust, not the dynamics of the mantle. An upper crust that is substantially weaker than the mantle is not compatible with our results. In addition, they are incompatible with the popular "jelly-sandwich" rheology, and preclude behavior such as large-scale lower crustal flow or mantle delamination. Second, the decoupling in Yunnan implies that the crust is moving south with respect to the mantle at rates as high as ~30mm/yr. Third, there is a fundamental rheological lithospheric transition between Tibet and Yunnan that may provide a key to understanding this significant orogen.
T34A-02 16:20h
Crust-Mantle Interaction and Tectonic Stress: A Multi-Data Analysis and Modeling to Investigate Large Earthquakes in the SE Carpathians
The principal purpose of the study is to understand the interplay between intermediate-depth large earthquakes in the SE-Carpathians (Vrancea) and tectonic stress within a high-velocity body (lithospheric slab) descending into the mantle. To analyze processes of stress generation and localization in and around the descending slab, we develop a 3D numerical model of contemporary mantle flow and stress beneath the Vrancea region. The input data of the model consist of (i) temperatures derived from seismic P-wave velocity anomalies and surface heat flow, (ii) crustal and uppermost mantle densities converted from P-wave velocities obtained from seismic refraction studies, (iii) geometry of the Vrancea crust and slab from tomography and refraction seismic data, and (iv) the estimated strain rate in the slab (as a result of earthquakes) to constrain the model viscosity. Major crustal uplifts predicted by the model coincide with the East Carpathian orogen and surround the Transylvanian basin, and predicted areas of subsidence are associated with the Moesian and East European platforms. The location of intermediate-depth earthquakes coincides with the predicted localization of maximum shear stress. Modeled tectonic stresses predict large horizontal compression at depths of about 70 to 220 km beneath the Vrancea region, which coincides with the stress regime derived from fault-plane solutions for the intermediate-depth earthquakes. This implies that the buoyancy-driven descent of the lithospheric slab beneath the Vrancea region is directly linked to intermediate-depth seismicity.
T34A-03 16:35h
Thermal structure of the oceanic and continental lithosphere
Recent studies of the focal depths of earthquakes in old continental lithosphere have shown that they are almost entirely confined to the crust. Except where recent subduction of oceanic lithosphere is likely to have occurred, no earthquakes with a magnitude of $> 5.5$ have yet been located beneath the Moho. In contrast, in oceanic lithosphere earthquakes commonly occur within the mantle. The principal control on whether or not deformation occurs by brittle failure has long been believed to be temperature. We re-examine the thermal models of both oceans and shields. Taking account of the temperature dependence of the thermal conductivity lowers the temperature within the oceanic lithosphere. Except beneath the outer rises of trenches, where the strain rates are large, intraplate oceanic earthquakes are confined to regions cooler than $600^\circ$C. In continental regions most earthquakes occur in the mobile belts that surround Archaean cratons, where the crust is as thick as 50-60~km. Recent studies, of the Canadian Shield in particular, have shown that radiogenic heating is not as concentrated at shallow depths as was previously believed. Taking account of both these effects and the temperature dependence of the thermal conductivity increases the Moho temperatures, which can exceed $600^\circ$C, and produces geotherms that agree well with pressure and temperature estimates from nodule suites from kimberlites. Therefore the mechanical behaviour of oceanic and continental upper mantle appears to depend on temperature alone, and there is as yet no convincing evidence for any compositional effects.
T34A-04 16:50h
Geoid Constraints on Density Varations Between Continental and Oceanic Lithosphere and on Associated Geodynamic Forces
The Earth's Geoid directly reflects density variations within the Earth, and therefore may be used to constrain density variations within the Earth. One important horizontal density variation with important geodynamic implications is the average density change between continental and oceanic lithosphere. Long wavelength density variations are associated with variations in gravitational potential energy, and hence tectonic forces. In the present study we focused on passive continental margins where density and geoid variations can best be used to constrain differences in lithospheric density. A total of 167 geoid profiles were generated perpendicular to passive continental margins on six continents globally. The geoid profiles were created using the EGM96 data set, but were also tested against the newer but not fully released GRACE geoid coefficients. The profiles, each just over 1000 km in length, were centered on the continental margin, and taken approximately every three degrees along the passive continental margins of North America, South America, Africa, Australia, India, and Antarctica. The geoid was tapered from degrees 11 to 85 in an attempt to eliminate long wavelength geoid features commonly assumed to originate in the lower mantle, and to use only those shorter wavelengths for which local isostasy is a good assumption. The average geoid step up from old oceanic lithosphere to continental lithosphere is 6 meters, based on all 167 profiles. The step up for each continental margin varied from 5 to 7 m. While the averages are very robust, the individual profiles are fairly noisy, in part because the edge of the continent is often rather arbitrarily defined by a particular bathymetric level. To overcome this, we allowed the center point of each profile to move up to 100-200 km if it increased the coherence of the profiles. Using these adjusted profiles, the average geoid step was 9m. This is likely an upper bound for the geoid step from oceanic to continental lithosphere. The geoid step found in this study has geodynamic implications. In the absence on any other forces acting on the system, the continents would have a tendency to fail in extension due to the excess gravitational energy associated with the geoid step. The so-called ridge force also arises from horizontal density variations within oceanic lithosphere, and has previously been associated with a 15-20m geoid step, leading to compression in the lithosphere. The 6-9m geoid step found in this study from oceanic to continental lithosphere indicates that the ridge force is reduced by one third to one half on the continents. This is consistent with the increase of strike slip and extensional deformation on continents compared to oceanic lithosphere.
T34A-05 INVITED 17:05h
Origin and evolution of lithospheric stresses in the Cenozoic
The tectonic stress field is geophysically important because it is the agent that preserves in the crust a memory of dynamical processes. We use a finite element model of the lithosphere to calculate stresses induced by mantle flow, crustal heterogeneity and topography, in an attempt to elucidate the origin of the present-day state of stress. We compare all models qualitatively and quantitatively to observations of intraplate stresses as given by the World Stress Map. We explore the effects of varying assumptions for the mechanism of crustal compensation, for the viscosity structure of the mantle, and the effects of lateral variations in viscosity in the form of weak plate boundaries. We find that a combined model that includes both mantle and lithospheric sources of stress yields the best match to the observed present-day stress field (60% variance reduction) although there are many regions where agreement between observed and predicted stresses is poor. The stress field produced by mantle tractions alone shows a greater degree of long-wavelength structure than is apparent in the stress observations, but agrees very well with observations in some areas where radial mantle tractions (dynamic topography) are particularly strong such as in southeast Asia and the western Pacific. We observe strong spatial variability in the relative contributions of lithospheric and mantle sources of stress to the observed stress field. Hence, we conclude that lateral variations in the viscosity of the mantle, and in the rheology of the lithosphere leads to variable amounts of decoupling between lithosphere and mantle, allowing the mantle signature to dominate in some areas, and the crustal signature to dominate in others. We propose that comparison between predicted and observed stress fields may lead to constraints on the spatial variability of rheology in the lithosphere and mantle. We also examine the evolution of the stress field in the Cenozoic by considering the evolution of the mantle flow field in the last 64 my. Insofar as the variations in a mantle flow field dominated by subduction are small, we expect relatively small changes in the mantle contribution through time, except in areas of subduction initiation and cessation.
T34A-06 17:25h
Lateral variations in mantle viscosity and the lithospheric stress field
For wavelengths longer than $\sim$ 200-500 km, the lithospheric stress field is controlled largely by tractions exerted on the lithospheric base by viscous mantle flow. The mantle flow field, and the tractions that it exerts on the lithosphere, are governed by the mantle's heterogeneous viscosity structure, which is not well constrained everywhere. Lateral variations in mantle viscosity in particular are poorly constrained, but significantly affect the mantle flow field and its coupling to the lithospheric base. For example, viscous flow in the upper mantle will exert tractions directly on the base, or even the sides, of deeply penetrating continental roots, but may be effectively decoupled from the oceanic lithosphere that overlies a low-viscosity asthenosphere. To characterize the effects of lateral viscosity viscosity variations on the lithospheric stress field, we developed models of mantle flow driven by either tomographically-inferred mantle density heterogeneity or imposed surface plate motions using a spherical 3-D finite element code. We then used a spherical model of the elastic lithosphere to transmit basal tractions to the surface. We examined the effects of several different lateral viscosity variations expected for the mantle, including those associated with strong, deeply-penetrating continental roots, ocean-continent and age-dependent variations in lithospheric thickness and strength, as well as depth- and temperature-dependent effects. For density-driven flow, we find that stronger lithosphere that penetrates more deeply into the upper mantle couples more effectively to mantle flow, increasing the magnitude of lithospheric surface stresses. We also find that large lateral variations in basal shear tractions are induced by flow associated with plate motions. These variations tend to resist plate motions, and show strong resisting stresses ahead of, and around the periphery of, a deeply-penetrating continental root moving through the mantle. The sum of the stress fields generated by density- and plate-driven flow provides a prediction of lithospheric stresses, which, when compared to the observed lithospheric stress field, can help constrain lateral variations in mantle viscosity, and their effect on mantle flow.
T34A-07 17:40h
Anomalous Topography and Heat Flow in the Western Atlantic Caused by Small-Scale Convection at the Passive Plate Margin
Although the topography of the ocean floor is dominated by cooling and subsidence of the plate as it moves away from the mid-ocean ridge, anomalous topographic features can be superimposed on this long-wavelength plate subsidence. As an example, we focus on the western North Atlantic region and derive `residual' topography which corrects observed bathymetry data for sediment loading, ocean plate cooling and isostatic compensation of the continental crust (using Crust2.0). The residual topography is characterised by long-wavelength intraplate anomalies that are defined by the NE-SW trending Bermuda Rise and adjacent lows. These features can not be reconciled with simple hot spot theory. We test the hypothesis that the observed topography signal reflects the existence of small-scale convective flow induced by the sharp lateral thermal gradient at the passive continent-ocean margin of North America, or `edge-driven convection'. A series of coupled lithosphere-mantle numerical experiments are conducted where an idealized plate margin is modeled by prescribing strong plates with a step-like thickness/temperature discontinuity at the continent-ocean boundary. A primary edge-driven convection cell and secondary flow circulation develops at the margin. This induces a lithospheric deflection that matches with the observed residual topography. Namely, both show subsidence at the continent-ocean margin, an off-shore peak/plateau of high topography on the ocean plate, and distal ocean plate subsidence. The small-scale mantle convection also manifests in anomalous surface heat flow, which is consistent with observations. Unlike hot spots, the edge-driven convection cell and associated topography and heat flow anomalies migrate with moving lithospheric plates. The flow cell and wavelength of the topography anomalies are broadened with continent-ward motion of the lithosphere relative to the mantle, whereas a migration in the ocean-ward direction suppresses the formation of the edge-driven convection cell and surface anomalies. We show that the coupled crust-mantle dynamics are consistent with the measured motion of the North American plate relative to a fixed hot spot reference frame.