Initiation of Continental Rifting by Flexure Over a Density Anomaly

Active rifting, such as rifting induced by a mantle density anomaly beneath continental lithosphere, is a foundational component of tectonic dynamics. However, the kinematics, force balance, and failure criteria for rift initiation and early evolution in this context are poorly constrained, since many numerical simulations of rifting rely on steady far-field divergent velocity boundary conditions and a priori crustal or lithospheric weakness that localizes deformation. To test whether a density anomaly alone can excite stresses large enough to break a homogeneous and isotropic lithosphere, we solve a numerical simulation of Stokes' flow around a spherical density anomaly exciting flexure of a thin elastic lid with a simplified two-step numerical simulation, first calculating pressure gradients in the fluid and then calculating elastic flexure excited by those gradients. The amplitude and wavelength of flexure, hence the stress in the elastic lid, depend on the distance between the lid and the anomaly, the geometry of the density anomaly, the rigidity of the lid, and the magnitude of the buoyancy force. Some geometric and material parameterizations result in flexural stresses that exceed the typical yield strength of lithospheric materials, allowing rift initiation solely from buoyancy-related flexure. The system should be considered conditionally stable, with small variations in elastic parameters (yield strength, thickness, depth of a mantle anomaly, or anomaly scaling) determining whether continental rifting initiates or not and then whether it stalls or propagates into a through-going structure. Numerical solutions are then compared to the Main Ethiopian Rift.