SCEC Award Number 19075 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title The role of shear zones on the rheology of Southern California
Name Organization
Greg Hirth Brown University Mark Behn Boston College
Other Participants Leif Tokle, graduate student at Brown
William Shinevar, graduate student at MIT/WHOI
SCEC Priorities 3b, 1b, 1c SCEC Groups CXM, SDOT, FARM
Report Due Date 04/30/2020 Date Report Submitted 11/03/2020
Project Abstract
The goal of the project is to use our estimates of effective rheology, which incorporate the influence of composition on the viscosity of an isotropic rock, to determine likely rheologies for shear zones. In 2019 we approached this problem with two approaches. In collaboration with Laurent Montesi (presented in his Plenary presentation at the 2019 Annual Meeting) and others working on the Community Rheology Model, we employed effective media mixing relationships to provide estimates for the effective viscosity of shear zones. This modeling complemented our prior work (Shinevar et al., 2018) defining effective viscosities for the different regions of the Geologic Framework based on a combination of the Community Thermal Model (CTM) and the Community Velocity model (CVM). In a second approach, we investigated ways to incorporate grain size evolution and grain size sensitive creep into estimates of the width and effective viscosity of shear zones. Our model, motivated by analyses of microstructures in natural shear zones, is based on the hypothesis that the shear zone viscosity is controlled by dislocation creep of the weakest phase in the rock, and that the processes that lead to interconnection of the weak phase are directly linked to grain size reduction, phase mixing, and grain size sensitive creep of the hard phases in the rock. We are now focusing on practical ways to introduce these effects into the CRM (presented as part of CRM discussion in our plenary talk at the 2019 annual meeting).
Intellectual Merit The development of the CRM in SCEC5 provides a platform to assess the role of lithosphere rheology on fault loading at time-scales relevant for post-seismic creep to time-scales much greater than the earthquake cycle. The CRM leverages (and integrates) community efforts involved with SCEC goals linked to the Community Stress Model (CSM), Community Geodetic Model (CGM), and the Stress and Deformation Over Time (SDOT) working group. With on-going efforts to refine the Community Thermal Model (CTM), as well as efforts to constrain pertinent rock types in the Geologic Framework (GF) using structural data and relationships between seismic velocity (Vp, Vs, Vp/Vs) and rock composition, we focused on the efficacy of calculating viscosity based on rock composition. The more detailed analyses of the role of grain size evolution on strain localization will (1) facilitate the analysis of processes responsible for lithospheric-scale strain localization, (2) improve the interpretation of post-seismic creep, and (3) potentially inform earthquake rupture dynamics near the brittle-plastic transition (through understanding processes that control the width of shear zones beneath the base of the seismogenic zone).
Broader Impacts Our research also includes broader impacts involved with educating undergraduates and graduate students. We strive to recruit undergrads from traditionally underrepresented groups, as engaging undergraduate research is a promising way to increase the number of underrepresented minorities in the “pipeline” to graduate school and academic positions in Earth sciences. Over the last several years, we developed collaborations with field geologists involving experimental work. This led to a number of publications and NSF funding for early-career faculty who primarily conduct field studies – including SCEC researcher Alexis Ault (Calzolari et al., 2020).
Exemplary Figure Figure 1 (Hirth & Behn, 2019). Rheological model for the effective viscosity and width of shear zones. The shear zone viscosity evolves to be controlled by dislocation creep of the weak phase (the red spot, for example plagioclase in a lower crustal granulite, shown above from Mehl & Hirth, 2008). The shear zone width, defined by the “volume fraction of shear zone”, is controlled by the ratio of the shear zone strain rate to the country rock strain rate (F), calculated with the assumption that the shear zone viscosity is initially set by grain size reduction and the promotion of grain size sensitive creep of the strong phases (the relationship outlined by the red box). The viscosity for grain size sensitive creep is modeled using relationships for grain size pinning, in which the pinning grain size is set by dynamic recrystallization of the hard phases at low strain.