Predicted Ductile Rheology of Textured and Non-textured Rocks in Southern California

Rheology controls how global and local stress sources are redistributed through the lithosphere to produce the observed deformation field and to load potentially seismogenic faults. Even though most geodynamical models assume uniform rheology, heterogeneities in lithology, which are commonplace in the continental crust, should lead to lateral variations in rheology. Rock fabric, especially the layering observed in ductile shear zones, is also expected to modify rheology, even without changes in composition and mineralogy. We discuss here what the effect of shear zone fabric is expected to be for the various lithologies suspected to be present in the Southern California crust. A detailed geological framework (GFM) has recently been developed for this region by the Southern Californias Earthquake Center. It defines the rock type and associated mineral assemblage at various depth intervals in each lithotectonic block. The Community Rheology Model (CRM, DOI: 10.5281/zenodo.4579626) prescribes the bulk rheology of each of the 19 rock types in the GFM by averaging the flow laws of each end-member mineral according to the MPG-e model of Huet et al. (2014). Here we discuss how to modify these rheologies to account for shear zone fabric. To the difference with the bulk rheologies included in the CRM, the mixing relation for a layered rock does not lead to a simple power-law relationship. We compare the viscosity of each bulk rock to those of a layered rock with the same mineralogy and of the weakest mineral at equal stress and temperature. If present, biotite controls weakening at stress larger than ~ 10 MPa. Biotite is essentially plastic, which forces the shear zone to deform at prescribed stress. At lower stress, feldspar and quartzite control the shear zone rheology at temperatures about and below ~800°C, respectively. Intense weakening is observed only in the quartz-dominated domain at the lowest temperature and in presence of biotite. Under these conditions, shear zone viscosity is ~6 times that of quartzite. The viscosity of shear zones with the composition of basalt is approximately 5 times that of feldspar regardless of deformation conditions. These correction factors are linked to the abundance of the corresponding mineral. Other lithologies show only minimal weakening when fabric is taken into consideration.