Assessing the Role of Lithospheric Buoyancy Forces and Mantle Flow in Driving Deformation across the East Africa Rift through 3D Geodynamic Modeling
Abstract
Constraining the origin of forces that drive continental rifting remains an unresolved question within geodynamics. The East African Rift (EAR) provides an ideal natural laboratory to examine the relative role of plate driving forces, as recent studies provide robust constraints on deformation rates and the structure of both the lithosphere and deeper convecting mantle. However, recent geodynamic investigations suggest widely varying estimates for the relative contribution of lithospheric buoyancy forces and horizontal mantle tractions to observed deformation patterns. Here, we employ high-resolution 3D thermal-mechanical models of the EAR, with the open source code ASPECT, to test the hypothesis that stress induced by variations in lithospheric buoyancy forces is the primary driver of the ~E-W extension across the EAR. First, we calculate surface deformation induced solely by lithospheric buoyancy forces arising from topography (ETOPO1) and internal variations of density (CRUST1.0) for the majority of the EAR centered on the Eastern Branch. Second, we calculate surface deformation driven by horizontal mantle tractions derived from regional tomography-based mantle flow. In both simulations, the lithospheric temperature structure is derived from estimates of regional lithospheric thickness and surface heat flow, which are used to calculate a steady-state conductive geotherm characteristic of the continental lithosphere. The rheological model combines non-linear viscous flow with plastic failure, with distinct viscous flow laws assigned to the crust, mantle lithosphere, and sub-lithospheric mantle. Calculated surface velocities driven only by variations in lithospheric buoyancy forces match kinematic predictions of rigid plate motions including the counter-clockwise rotation of the Victoria Block (with RMS misfit of 1.4 mm/yr and angular misfit of 8o ), but poorly fit with GPS velocities with deforming zones (with RMS misfit of 2.9 mm/yr and angular misfit of 70o ). Surface velocities driven by only mantle horizontal traction are small compared to the kinematic predictions indicating mantle flow alone is insufficient to drive observed deformation. Our results demonstrate that variations in lithospheric buoyancy forces dominate the force balance driving ~E-W extension across the EAR.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMT024.0011R
- Keywords:
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- 7205 Continental crust;
- SEISMOLOGY;
- 8110 Continental tectonics: general;
- TECTONOPHYSICS;
- 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS;
- 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS