SCEC Award Number 19023 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Building the Community Rheology Model: geologic investigation of ductile shear zone rheology
Name Organization
Elena Miranda California State University, Northridge Sinan Akciz California State University, Fullerton Joshua Schwartz California State University, Northridge
Other Participants 3 undergraduate students: Jennifer Bautista (CSUN), Miguel Zamora-Tamayo (CSUN), Jeremy Torres (CSUF)
SCEC Priorities 3b, 3d, 3f SCEC Groups CXM, FARM, Geology
Report Due Date 04/30/2020 Date Report Submitted 04/22/2020
Project Abstract
Building a rheology model requires consideration of whether strain is strongly localized on or near faults and their downward continuation as shear zones, or if strain is broadly distributed across the plate boundary in Southern California (SCEC Community Rheology Workshop, 2018). We proposed a field- and lab-based study of an exposed shear zone that is an ideal analog for the deep crustal shear zone associated with the Coachella Valley segment of the SAF system. Only reconnaissance work was completed by the end of summer 2019. Preliminary data analysis indicate the shear zone had a short life and was likely contemporaneous during the arc magmatism (~85-90 Ma). From the microstructural observations and CPO data, we interpret that both minerals primarily deform via dislocation creep at amphibolite-grade deformation temperatures (about 500-600˚C), but quartz deforms more easily by dislocation creep due to enhanced dislocation climb relative to plagioclase. In addition, phase mixing indicates that grain size sensitive deformation also contributes to shear zone development, and may significantly enhance strain localization. Our results indicate that the rheology of the Borrego Springs Shear Zone is characterized by both dislocation creep of quartz and grain size sensitive phase mixing, and that CRM models should incorporate a component of grain size sensitivity in order to avoid over-estimating shear zone strength. More detailed mapping and analysis of samples from different parts of the shear zone will be conducted when CSUF allows for field and lab research activities to continue.
Intellectual Merit The first component of the CRM is the 3D geologic framework, which is described by the Lithotectonic Block Model (Crouch and Suppe, 1993; Oskin et al., 2016; Hauksson and Meier, 2018), in which geophysical data, surface geology, and basement geology are used to define 16 rheologically distinctive terranes in Southern California. The second component is the Community Thermal Model (CTM), which provides constraints on temperature variation with depth in the terranes. The CTM is a crucial parameter for evaluating the third component of the CRM: whole-rock and shear zone rheology within the terranes.

In this proposal, we outline a study in which we will place constraints on shear zone rheology by studying a natural exposure of a mid-crustal shear zone that is an analog for the downward continuation of the southernmost SAF. We focus our work in the southeastern part of the Southern California geologic framework, near the Salton Sea, at the contact between the Eastern Peninsular Ranges and the Rifted Margin blocks. Here, the EPRMZ is developed within the Eastern Peninsular Ranges batholith, which is the same crystalline rock thought to underlie the Coachella Valley in the Rifted Margin block (Han et al., 2016).
Broader Impacts Preliminary field data was collected by a minority student who started to work on this project as a senior thesis. Due to limited field time, focus of the CSUF portion of the study went into constraining the duration of the shearing along the Borrego Shear Zone. Project will hopefully be completed by another minority student as soon as field research activities proceed.
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