Elucidating lithosphere -mantle coupling by modeling the lithospheric stress field and predicting plate motions

Even after the establishment of the plate tectonic theory nearly four decades ago, some fundamental questions have still not been satisfactorily answered. What drives the Earth’s plates? Are plates and mantle coupled, and if so, what is the nature of that coupling? What is the role of density buoyancy-driven flow in driving the plates? These are some of the questions we try to address in our study through a joint modeling of lithosphere dynamics and mantle convection. If the initial coupling model is correct, the predicted stresses will match the observed deformation along the plate boundary zones and the predicted velocities will match the observed plate motions. We model the lithospheric deviatoric stress field from gravitational potential energy (GPE) differences and compare our modeled stress tensor field with velocity gradient tensor field along the Earth’s deforming plate boundary zones (from GSRM). The deviatoric stresses due to active basal tractions acting at the base of the lithosphere, arising from density buoyancy-driven mantle convection, are also compared with the strain rate tensor dataset from GSRM. We find that the combined stresses from lithosphere and mantle buoyancies yield the best fit to the deformation indicators, especially in areas of continental deformation. This is most likely due to driving shear tractions induced by the surrounding mantle, related to the history of subduction in those areas. We also generate plate motions in our convection models by incorporating lateral viscosity variations generated by major geological features of the Earth, such as the continent-ocean divide, the presence of cratonic roots as well as age differences in the oceanic lithosphere. For each structure, we predict the deviatoric stress field, the pattern of poloidal and toroidal flow and the partitioning ratio between toroidal/poloidal velocities. The predicted deviatoric stress field is added to the deviatoric stresses generated by lithosphere buoyancies and the total stress field is compared with strain rate tensor information from GSRM. The best-fit model has to satisfy both the constraints of matching the plate motions and the deviatoric stress field simultaneously. By using both these constraints, we are able to eliminate several types of models and narrow down significantly the set of models that fit the observations

Surface Motions and Continental Deformation in the Indian Plate and the India-Eurasia Collision Zone

The collision of the Indian plate with Eurasia has played a major role in controlling the dynamics of central Asia leading to the world's largest continental deformation zone. In order to study the deformation within the Indian plate as well as the India-Eurasia collision zone, we model the lithospheric stress field by calculating the two primary sources of stress, one arising due to topography and shallow lithospheric structure estimated by gravitational potential energy differences and the other arising from basal tractions derived from density-driven mantle convection. We use several tomography models to calculate horizontal tractions using the convection code HC for two radially varying viscosity structures. We also take into account lateral viscosity variations in the lithosphere model arising from stiff cratons, weak plate boundaries, and strength variations due to old and young oceanic lithosphere. We do a quantitative comparison of our predicted deviatoric stresses, strain rates, and plate velocities with surface observables and find that the regional tomography model of Singh et al. (2014) embedded in the global S wave model S40RTS does a remarkable job of fitting the observations of GPS velocities and strain rates as well as intraplate stress field from the World Stress Map

How the Indian Ocean Geoid Low Was Formed

Abstract The origin of the Earth's lowest geoid, the Indian Ocean geoid low (IOGL) has been controversial. The geoid predicted from present‐day tomography models has shown that mid to upper mantle hot anomalies are integral in generating the IOGL. Here we assimilate plate reconstruction in global mantle convection models starting from 140 Ma and show that sinking Tethyan slabs perturbed the African Large Low Shear Velocity province and generated plumes beneath the Indian Ocean, which led to the formation of this negative geoid anomaly. We also show that this low can be reproduced by surrounding mantle density anomalies, without having them present directly beneath the geoid low. We tune the density and viscosity of thermochemical piles at core‐mantle boundary, Clapeyron slope and density jump at 660 km discontinuity, and the strength of slabs, to control the rise of plumes, which in turn determine the shape and amplitude of the geoid low

Understanding deep earth dynamics: a numerical modelling approach

Enhancement in computing power and better data availability have paved the way for deciphering the earth's deeper dynamics and have provided viable explanations for various surface phenomena. Tools such as seismic tomography, numerical modelling and geophysical observations such as stresses, gravity anomalies, heat flow, etc. have helped us in addressing the mechanisms of plate driving forces, anomalous geoid variations, cratonic stability, topographic support, in-traplate earthquakes and similar outstanding issues in geodynamics. Due to lack of direct observations from deep earth, numerical modelling has aided considerably in learning about subsurface processes. With better algorithms being developed everyday, it is the right time to tap their potential to push the frontiers of human knowledge

Joint modeling of lithosphere and mantle dynamics: Sensitivity to viscosities within the lithosphere, asthenosphere, transition zone, and D `' layers

Although mantle rheology is one of the most important properties of the Earth, how a radial mantle viscosity structure affects lithosphere dynamics is still poorly known, particularly the role of the lithosphere, asthenosphere, transition zone, and D `' layer viscosities. Using constraints from the geoid, plate motions, and strain rates within plate boundary zones, we provide important new refinements to the radial viscosity profile within the key layers of the lithosphere, asthenosphere, transition zone, and D `' layer. We follow the approach of the joint modeling of lithosphere and mantle dynamics (Ghosh and Holt, 2012; Ghosh et al., 2013b, 2019; Wang et al., 2015) to show how the viscosities within these key layers influence lithosphere dynamics. We use the viscosity structure SH08 (Steinberger and Holme, 2008) as a starting model. The density variations within the mantle are derived from the tomography models which, based on prior modeling, had provided a best fit to the surface observables (Wang et al., 2015). Our results show that narrow viscosity ranges of moderately strong lithosphere (2.6-5.6 x 10(22) Pa-s) and moderately weak transition zone (5-9.3 x 10(20) Pa-s), as well as slightly large ranges of moderately weak asthenosphere (5-34 x 10(19) Pa-s) and D `' layer (4.8-18 x 10(20) Pa-s), are necessary to match all the surface observables. We also find that a very strong lithosphere (> 8.6 x 10(22) Pa-s) along with a weak asthenosphere (< 5.4 x 10(19) Pa-s) fits well with the surface observables. Our results show that the geoid is very sensitive to the viscosities of both the transition zone and D `' layer, while plate motions are slightly sensitive to them, but the orientations of stresses are insensitive to them

Contribution of gravitational potential energy differences to the global stress field

Modelling the lithospheric stress field has proved to be an efficient means of determining the role of lithospheric versus sublithospheric buoyancies and also of constraining the driving forces behind plate tectonics. Both these sources of buoyancies are important in generating the lithospheric stress field. However, these sources and the contribution that they make are dependent on a number of variables, such as the role of lateral strength variation in the lithosphere, the reference level for computing the gravitational potential energy per unit area (GPE) of the lithosphere, and even the definition of deviatoric stress. For the mantle contribution, much depends on the mantle convection model, including the role of lateral and radial viscosity variations, the spatial distribution of density buoyancies, and the resolution of the convection model. GPE differences are influenced by both lithosphere density buoyancies and by radial basal tractions that produce dynamic topography. The global lithospheric stress field can thus be divided into (1) stresses associated with GPE differences (including the contribution from radial basal tractions) and (2) stresses associated with the contribution of horizontal basal tractions. In this paper, we investigate only the contribution of GPE differences, both with and without the inferred contribution of radial basal tractions. We use the Crust 2.0 model to compute GPE values and show that these GPE differences are not sufficient alone to match all the directions and relative magnitudes of principal strain rate axes, as inferred from the comparison of our depth integrated deviatoric stress tensor field with the velocity gradient tensor field within the Earth\u27s plate boundary zones. We argue that GPE differences calibrate the absolute magnitudes of depth integrated deviatoric stresses within the lithosphere; shortcomings of this contribution in matching the stress indicators within the plate boundary zones can be corrected by considering the contribution from horizontal tractions associated with density buoyancy driven mantle convection. Deviatoric stress magnitudes arising from GPE differences are in the range of 1–4 TN m−1, a part of which is contributed by dynamic topography. The EGM96 geoid data set is also used as a rough proxy for GPE values in the lithosphere. However, GPE differences from the geoid fail to yield depth integrated deviatoric stresses that can provide a good match to the deformation indicators. GPE values inferred from the geoid have significant shortcomings when used on a global scale due to the role of dynamically support of topography. Another important factor in estimating the depth integrated deviatoric stresses is the use of the correct level of reference in calculating GPE. We also elucidate the importance of understanding the reference pressure for calculating deviatoric stress and show that overestimates of deviatoric stress may result from either simplified 2-D approximations of the thin sheet equations or the assumption that the mean stress is equal to the vertical stress

Traction and strain-rate at the base of the lithosphere: An insight into cratonic survival

Cratons are the oldest parts of the lithosphere, some of them surviving since Archean. Their long-term survival has sometimes been attributed to high viscosity and low density. In our study, we use a numerical model to examine how shear tractions exerted by mantle convection work to deform cratons by convective shearing. We find that although tractions at the base of the lithosphere increase with increasing lithosphere thickness, the associated strain rates decrease. This inverse relationship between stress and strain-rate results from lateral viscosity variations along with the model’s free slip condition imposed at the Earth’s surface, which enables strain to accumulate along weak zones at plate boundaries. Additionally, we show that resistance to lithosphere deformation by means of convective shearing, which we express as an apparent viscosity, scales with the square of lithosphere thickness. This suggests that the enhanced thickness of the cratons protects them from convective shear, and allows them to survive as the least deformed areas of the lithosphere. Indeed, we show that the combination of a smaller asthenospheric viscosity drop and a larger cratonic viscosity, together with the excess thickness of cratons compared to the surrounding lithosphere, can explain their survival since Archean time