Shear zone rheology: Importance or fabric in the context of the CRM

Laurent G. Montesi, Greg Hirth, Mark Behn, William Shinevar, & Michael E. Oskin

Published August 14, 2020, SCEC Contribution #10554, 2020 SCEC Annual Meeting Poster #178

Loading of faults throughout Southern California depends on how stress is transferred from global mantle convection and plate tectonic processes to the regional and local scale. An important aspect of this transfer is the rheology of the rocks present in region, especially in the crust. The Community Rheology Model (CRM, see Hearn et al., this meeting) contains estimates of ductile flow laws appropriate for each rock type that appears in the Geological Framework Model (GFM). These rock-appropriate flow laws are constructed by averaging flow laws determined for each constituent mineral in the laboratory, using the MPGe model of Huet et al. (2014). An assumption in this averaging is that the rocks do not contain a fabric, so that all the mineral phases are randomly distributed throughout the rock and a geometrical average is appropriate. While acceptable for undeformed rocks, this assumption is not tenable in the ductile shear zones that are expected to be present below major seismogenic faults. Thus, it is important to develop alternative flow laws for highly deformed shear zone rocks.

The major difficulty in considering textured rocks is that the geometric average of MPGe (or MPGs) is no longer valid. Therefore the rheology cannot be expressed in a simple power law relationship. For foliation-parallel shear, strain rate is additive. It can be obtained by summing the strain rate in each phase when subjected to the overall stress, with summation weights given by the volume fraction of each phase. We describe how the extent to which strain rate is higher in a shear zone than undeformed rock for the each compositions in the GFM when subjected to the same stress. While results depends in detail on the specific stress selected, the following are generally true. Strain rate enhancement, and therefore the importance of considering shear zones, is highest at lower temperature. Enhancements of a factor higher than 1000 are expected for gabbroic rocks, because of the strength contrast between plagioclase and pyroxene, and in rocks that contain biotite at temperatures less than ~600°C, because of the highly nonlinear behavior of this mineral, especially when associated with plagioclase. No significant enhancement is observed in sediments or peridotite. Thus, considering shear zones can correspond to major rheological heterogeneities in most of Southern California in the middle crust.

Key Words
rheology, shear zone

Citation
Montesi, L. G., Hirth, G., Behn, M., Shinevar, W., & Oskin, M. E. (2020, 08). Shear zone rheology: Importance or fabric in the context of the CRM. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)