HR: 0800h
AN: T21B-0582    [Abstracts]
TI: A Best-fit Lithosphere-Mantle Coupling Model Constrained by Plate Motions and the Velocity Gradient Tensor Field in the Plate Boundary Zones
AU: * Ghosh, A
EM: aghosh@mantle.geo.sunysb.edu
AF: Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, United States
AU: Wen, L
EM: lwen@notes.cc.sunysb.edu
AF: Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, United States
AU: Holt, W E
EM: wholt@mantle.geo.sunysb.edu
AF: Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, United States
AU: Haines, J
EM: ajh50@cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, United Kingdom
AU: Flesch, L M
EM: lmflesch@purdue.edu
AF: Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907, United States
AB: Lateral variation in lithosphere-mantle coupling is caused by lateral variation in asthenosphere viscosity and distribution of density buoyancies in the mantle. The goal of the present study is to achieve a best-fit lithosphere- mantle coupling model for the Earth. That is, a model with appropriate radial and lateral viscosity variations that would successfully predict not only plate moions, but also deformation indicators along the Earth's plate boundaries. The convection model used is a whole mantle model driven by density buoyancies within the mantle with free slip boundary conditions at the surface and at the core-mantle boundary. We test viscosity structures by varying the thickness and viscosity of the asthenosphere layer and 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 toroidal-poloidal partitioning ratio. The predicted deviatoric stress field is added to the deviatoric stresses generated by lithosphere buoyancies (gravitational potential energy or GPE differences due to density buoyancies within the lithosphere), calculated based on the Crust 2.0 dataset, and the total stress field is compared with strain rate tensor information along the Earth's deforming plate boundary zones from the Global Strain Rate Map (GSRM). The best-fit model has to satisfy both the constraints of matching the plate motions and the deviatoric stress field simultaneously. We find that combined models with 1-2 orders of lateral viscosity variations within the lithosphere and 2-3 orders of lateral viscosity variations within the asthenosphere are able to match plate motions, the observed toroidal-poloidal ratio and the strain rate tensor data from GSRM. The viscosity variation in the lithosphere arises because of the higher viscosity continents as well as because of higher viscosity of old oceanic lithosphere compared to young oceanic lithosphere. The viscosity variations in the asthenosphere are mostly due to the presence of high viscosity keels below the Archean cratons. A weak (low viscosity) asthenosphere is crucial in matching the plate motions and giving rise to sufficient toroidal velocity. However, the weakness, if uniform, would decouple the deeper density buoyancies and would prevent the stresses from these deeper density buoyancies to be transmitted up to the lithosphere. Presence of strong asthenosphere below the Archean cratons enables transmission of these stresses to the overlying lithosphere and hence controls the match to the strain rate tensor data. These imply that major geological features play an important role in explaining the first order features of plate tectonics.
DE: 1038 Mantle processes (3621)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8162 Rheology: mantle (8033)
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: 2007 Fall Meeting