Symmetry versus asymmetry of rifted margins: The role of mechanical anisotropy

Abstract Continental lithosphere extension leads to necking and breakup, forming conjugate rifted margins that vary from symmetric to asymmetric morphologies. Although such differences are commonly linked to variations in initial lithospheric rheology, along-strike transitions between symmetric and asymmetric margins within single rift systems point to additional controlling factors. Structural inheritance, causing mechanical anisotropy in the continental lithosphere, is commonly suggested to influence rift evolution. Here we present novel geodynamic models of lithospheric extension incorporating inherited mechanical anisotropy using a transversely isotropic visco-plastic rheology coupled to the director vector approach. By systematically varying anisotropy strength and initial fabric orientation, we demonstrate that mechanical anisotropy alone can explain the transition from symmetric to asymmetric rifting. In our models, isotropic materials favor symmetric rifting dominated by pure shear deformation, while anisotropic materials promote asymmetric rifting driven primarily by simple shear. This transition occurs at low initial fabric angles and moderate anisotropy strengths. Our results offer a novel and robust mechanism for the formation of both symmetric and asymmetric conjugate margins and suggest that along-strike variations in structural inheritance—and thus mechanical anisotropy—can produce contrasting deformation styles within a single rift system. These findings highlight the critical role of mechanical anisotropy in shaping rifted margins and influencing the tectonic evolution of continental lithosphere.