Modelled mantle texture evolution during periodic geodynamic cycles

Many geodynamic processes on Earth occur with a certain periodicity, which can range from decades to centuries for earthquake cycles to hundreds of millennia for glacial cycles and up to hundreds of millions of years for plate tectonic cycles. From the simplified view of a rock, all of these geodynamic cycles induce a deformation during a loading/opening phase followed by deformation in the opposite direction during the unloading/closing phase. These periodic cycles thus produce deformation without any net strain on the rock.

In this work, we use simple models to determine the types of rock texture that can develop within mantle rocks after multiple cycles of dynamic processes, and to understand how such textures can influence the effective viscosity of the mantle.

Our simplified setup consists of an olivine polycrystal aggregate ( = our mantle rock) that has an initial (either isotropic or anisotropic) texture at the start of the model. We impose a velocity gradient representing either simple or pure shear in a given direction. The aggregate is sheared with the given velocity gradient for a prescribed amount of strain and then the deformation is reversed. To be impartial, we test the same setting with multiple texture evolution models, including the MDM, the D-REX, the SpecFab and the VPSC models.

Our results show that the frequency of deformation cycling and the magnitude of the deformation (in the measure of strain) can dramatically impact both the stability and the type of texture that forms after a few or many deformation cycles.  Because these textures are viscously anisotropic, the strain achieved in a deformation cycle thus greatly influences how the mechanical anisotropy of the mantle evolves, and in turn, influences different geodynamic processes.

 

As an example, during glacial cycles one expects small amounts of strain in the mantle ranging from 0.0001 to a maximum of 0.01 units of strain during loading and unloading of ice on the surface. Our results suggest that i) a given piece of mantle needs to experience the same glacial-interglacial cycles hundreds to thousands of times to experience enough strain cycles to develop a significant texture, and ii) when this happens the developed texture is very different than what one would get for continuously deforming the mantle in the same direction. Instead of developing a point maximum in the shear direction, we observe a girdle-type texture with a small maxima normal to the shear plane that remains stable once developed. At lower frequencies, for which shear direction reversals occur less frequently and with larger amounts of strain, the texture does not stabilize. Instead, the texture initially develops toward a point maxima that becomes partially destroyed by the subsequent reverse deformation.

 

Given these trends, we conclude that periodic geodynamic processes may significantly influence the formation of upper mantle rock textures, and that the deformation frequency exerts a particularly important control on the eventual rock texture.