Lithospheric Deformation With Mechanical Anisotropy: A Numerical Model and Application to Continental Rifting

Rocks at various lithospheric depths commonly display a fabric, resulting in mechanical anisotropy. The mechanical response of such anisotropic rocks depends on both the intensity of the anisotropy and the orientation of the fabric relative to the applied stress. Despite its potential significance, the role of mechanical anisotropy in governing lithospheric strength and deformation style during extension remains poorly constrained. Here, we investigate how mechanical anisotropy influences the deformation of the lithosphere under tectonic extension. We use two-dimensional numerical models of lithospheric deformation that incorporate a non-linear, transversely isotropic model. Both viscous and plastic rheologies are direction-dependent, and fabric orientations evolve using the director-vector approach. We perform simulations of continental extension and show that mechanical anisotropy is a major factor in the development of continental rifts. It influences the architecture of rift basins and reduces the driving force required for rifting. We explore the role of extensional velocity and find that it has only a second-order influence on the evolution of rift systems. Furthermore, we investigate the relative contributions of crustal and mantle anisotropy, and highlight that mantle anisotropy plays a more significant role. The driving forces required for continental rifting are quantified and systematically analyzed. Compared to isotropic models, the required driving force is reduced by up to a factor of three when mechanical anisotropy is included. As a result, forces below 10 TN/m can be achieved, which is consistent with estimates from the geological record.