Geodynamics of complex plate boundary regions: Insights from numerical models of convergent eastern Mediterranean and divergent east African plate tectonics

Abstract

GPS satellite observations indicate that in the tectonically complex eastern Mediterranean and east African regions microplates rotate counterclockwise with respect to the neighboring African plate. Using 3D numerical models, Glerum relates these observations of crustal deformation to the dynamics of the lithosphere and the underlying mantle that may cause this deformation. Glerum first describes her additions to the ASPECT software necessary for numerically modeling the upper mantle and lithosphere dynamics of convergent and divergent plate boundaries. These additions include the tracking of multiple materials with different physical properties and nonlinear viscous as well as viscoplastic rheologies. The implementations of complex, multi-material rheologies are verified with well-known 2D benchmarks and multi-material viscoplasticity is applied in 3D time-dependent thermomechanical models of oceanic subduction. Subsequently, Glerum uses ASPECT to investigate the sensitivity of horizontal surface motions to individual geodynamic processes in the eastern Mediterranean. Identification of all mantle drivers that should participate in modeling attempts to explain observations of crustal flow is essential to fully exploit the information contained by surface motions about their driving processes. Glerum therefore employs 3D data-driven instantaneous dynamics models of compressible flow including a complete set of possible mantle drivers of surface deformation. The reference instantaneous flow model results indicate that mantle processes can explain a large part of the crustal motion of the Aegean-Anatolian microplate. Subsequent systematic perturbations of model properties with respect to this reference model help estimate the individual contributions of tectonic plate motions, slab pull and trench suction, and density-induced mantle flow interacting with the slab and overlying plates while moderated by the mantle’s bulk viscosity. In order of regional importance, the predicted crustal flow of the Aegean-Anatolian region is most sensitive to slab pull, followed by slab-mantle interaction and basal drag, mantle rheology, and the absolute plate motion reference frame. Lastly, Glerum demonstrates a possible mechanism for the counterclockwise rotation of the Victoria microplate in the East African Rift System, which is in striking contrast to the clockwise motion of the surrounding plates. 3D models of the divergent system show that Victoria’s rotation can be caused by the drag of the African and Somalian plates along the strong edges of the microplate, while the rift segments along inherited lithospheric weaknesses facilitate Victoria’s rotation. The amount of rotation is therefore primarily controlled by the distribution of preexisting stronger regions and the weaker Precambrian mobile belts that surround Victoria. The induced counterclockwise rotation of the microplate leads to a clockwise shift of the local extension direction from E-W to more WNW-ESE along the overlapping rift branches. Comparison of the resulting predicted stress field and tectonic regimes to observations helps to elucidate the interpretation of local stress and strain indicators and to reconcile different opening models used to interpret the East African Rift System.