Anisotropic viscosity and time-evolving lithospheric instabilities due to aligned igneous intrusions

Abstract

Anisotropic viscosity is often discussed in the context of flow-induced crystallographic alignment, also known as lattice-preferred orientation, believed to exist throughout the mantle. However, other geophysical systems can produce bulk anisotropic rheology. One example is so-called shape-preferred orientation, such as that imparted by repeated, non-randomly oriented igneous intrusions. Dikes and sills tend to intrude perpendicular to least compressive tectonic stress, and therefore are typically aligned with other coeval intrusions. Intrusions provide distinct planes of weakness, producing, in a bulk sense, an anisotropic fabric to the rocks in which they are intruded. We parametrize this style of anisotropic viscosity with an effective-medium approximation, based on analytically derived characteristics of layered media. We find that the magnitude of anisotropy is highly sensitive to the presence of weak layers, even if they are relatively thin and/or sparsely distributed. We test the effects of our constitutive law on Rayleigh-Taylor instability growth, representing lithospheric instabilities. Our results indicate dramatically decreased stability of lithosphere intruded by dikes and/or sills.