Development of Anisotropic Fabric and Associated Anisotropic Viscosity within Lithospheric and Asthenospheric Shear Zones

Tuesday, 16 December 2014
Lars N Hansen1, Clinton P Conrad2, Jessica M Warren3, Svetlana Natarov2 and David L Kohlstedt4, (1)University of Oxford, Department of Earth Sciences, Oxford, United Kingdom, (2)University of Hawaii at Manoa, Dept. Geology and Geophysics, Honolulu, HI, United States, (3)Stanford University, Stanford, CA, United States, (4)University of Minnesota Twin Cities, Minneapolis, MN, United States
Shearing of mantle rocks within Earth’s lithosphere and asthenosphere causes mantle minerals, particularly olivine, to develop anisotropic fabrics that can be detected seismically. In laboratory studies, such anisotropic fabrics have also been associated with dependence of the viscosity of a mineral on the orientation of an applied stress. This anisotropic viscosity may affect tectonic plate motions and the stability of the lithospheric base, but it is highly dependent on 1) the rate of fabric development and 2) the poorly constrained viscosity tensor, which relates the stress applied to an olivine crystal of known orientation to the resulting strain-rate. Here we constrain the importance of viscous anisotropy in the asthenosphere using two steps. First, we developed a stochastic model of olivine fabric development in which we assume that the a-axes of olivine grains rotate into the direction of shear with continual random readjustments to the orientation of the crystals. We constrain parameters in the model by quantitatively comparing calculated fabrics to those developed in recent laboratory experiments on olivine aggregates. The resulting model is then compared to observations of naturally deformed peridotites to confirm its applicability to natural systems. Second, we use laboratory observations of strain-rates that result from stresses applied to aggregates of olivine with known crystallographic fabrics to constrain the relevant parameters of the anisotropic viscosity tensor. Together, this effort allows us to constrain both the rate of anisotropic fabric development in an arbitrary deformation geometry and also the magnitude of anisotropic viscosity that should result from these fabrics. These constraints can be applied usefully to time-dependent models of shearing within the asthenosphere or lithospheric shear zones to understand the governing viscosity structures of these regions and their associated geodynamic complexity.