Lithospheric Decoupling and Rotations: Hints from Ethiopian Rift

Abstract:

Plates move relative to the mantle because some torques are acting on them. The shear in the low-velocity zone (LVZ) at the base of the lithosphere is the expression of these torques. The decoupling is allowed by the low viscosity in the LVZ, which is likely few orders of magnitudes lower than previously estimated. The viscosity value in the LVZ controls the degree of coupling/decoupling between the lithosphere and the underlying mantle. Lateral variations in viscosity within the LVZ may explain the velocity gradient among tectonic plates as the one determining the Ethiopian Rift (ER) separating Africa from Somalia. While it remains not fully understood the mechanisms of the torques acting on the lithosphere (thermally driven mantle convection or the combination of mantle convection with astronomical forces such as the Earth's rotation and tidal drag), the stresses are transmitted across the different mechanical layers (e.g., the brittle upper crust, down to the viscous-plastic ductile lower crust and upper mantle). Differential basal shear traction at the base of the lithosphere beneath the two sides of the East African Rift System (EARS) is assumed to drive and sustain rifting. In our analysis, the differential torques acting on the lithospheric/crustal blocks drive kinematics and block rotations. Since, ER involves the whole lithosphere, we do not expect large amount of rotation. Rotation can be the result of the whole plate motion on the sphere moving along the tectonic equator, or the second order sub-rotation of a single plate. Further rotation may occur along oblique plate boundaries (e.g., left lateral transtensional setting at the ER). Small amount of vertical axis rotation of blocks in northern ER could be related to the presence of local, shallower decollement layers. Shallow brittle-ductile transition (BDT) zone and differential tilting of crustal blocks in the northern ER could hint a possibility of detachment surface between the flow in the lower crust relative to the brittle crust above. Our study suggests that kinematics of crustal blocks in the ER is controlled by Africa and Somalia plates interaction at different scale and layers.