SCEC Award Number 21140 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Investigating the exhumed ductile roots of fault zones: can the rheology of shear zones tell us about fabric inheritance in brittle faults of the San Andreas Fault system?
Name Organization
Elena Miranda California State University, Northridge Joshua Schwartz California State University, Northridge
Other Participants 1 graduate student advised by PI Miranda (unknown, but two prospective students D. Flores and A. Stricker are being actively recruited by PI Miranda). 1 undergraduate student SURE intern will be recruited by PI Schwartz.
SCEC Priorities 3b, 3a, 1c SCEC Groups FARM, CXM, SDOT
Report Due Date 03/15/2022 Date Report Submitted 09/10/2022
Project Abstract
We proposed an investigation of mylonitic fabrics in the Cucamonga terrane (eastern San Gabriel Mountains) to constrain the geometry and rheology of Late Cretaceous shear zones with the goal of evaluating the possibility that it functioned as a stress guide for the formation of the Quaternary Cucamonga Fault. This is a critically important study in the context of SCEC goals because this shear zone is the closest natural analog to the ductile roots of the major faults of the SAF system. We use field geology, geochronology, and microstructural observations to document the following results: 1) mylonitic fabrics in the Cucamonga mylonites strike NNW and dip moderately (40-50º) to the NE, 2) brittle faults that cut the mylonites also strike NNW and dip more shallowly (30-40º) to the NE, 3) U-Pb zircon geochronology demonstrates that plutonic arc rocks crystallized from 172-86 Ma, and were subsequently metamorphosed at granulite-facies conditions from 88-74 Ma at 730-800ºC. U-Pb zircon geochronology of the Black Belt mylonites demonstrates that the hornblende tonalite host rocks crystallized from 88 - 85 Ma at 725-799ºC, and U-Pb titanite petrochronology shows that mylonitic fabric developed shortly thereafter at 83 Ma at temperatures 650-550ºC. The similarity in orientation of the ductile and brittle fabrics suggests the utility of foliation plane weakness in guiding the formation of brittle faults. In addition, a key outcrop in the Black Belt mylonites that contains parallel bands of cataclasites and coeval mylonites and pseudotachylytes that further support the genetic nature of the relationship between ductile and brittle fabric development.
Intellectual Merit The project contributes to the overall intellectual merit of SCEC by having students conduct the primary field- and laboratory-based research that helps inform how high-temperature ductile fabrics may act as stress guides for the brittle faulting that characterizes the major faults of the San Andreas Fault system. The orientation and age of those fabrics relative to major Cenozoic faults of the SAF fault system are particularly insightful for understanding how pre-existing fabrics influence the localization of brittle strain. The research contributes to advancing knowledge and understanding in the field by documenting the field relationships that demonstrate how ductile fabric orientations of the Cucamonga mylonites have been inherited and exploited by the younger Cucamonga Fault; this relationship between ductile fabrics and brittle faults has also been suggested for the Elsinore and San Jacinto Faults through geophysical methods (Schulte-Pelkum et al., 2020). In particular, our work advances SCEC research objectives by showing how this tectonic inheritance was instrumental in the development of a range-bounding fault that defines the southern edge of the San Gabriel lithotectonic block. In addition, similar shear zone rocks are likely to be present at depth beneath other major faults of the SAF system, offering powerful and realistic constraints on shear zone properties for SCEC’s Community Rheology Model. The activity involved the development of creative and original concepts through the discovery of a key outcrop of coeval mylonites and pseudotachylytes, which are extremely rare in the geologic rock record. In this outcrop, foliated, brittle cataclasites are reworked by co-planar foliated mylonitic fabrics to various degrees, and alternating parallel layers of these variably mylonitized cataclasites are separated by narrow seams of co-parallel pseudotachylytes. These relationships show that there is a continuum between co-planar brittle and ductile deformation, and that the continuum of this fabric development is cyclical in nature and punctuated by earthquakes in middle crustal shear zone rocks. These relationships push the boundary of what we know about deep crustal shear zone deformation and how it is punctuated by the earthquake cycle.
Broader Impacts The project contributes to the broader impacts of SCEC as a whole by having students play a prominent role in the implementation of the project; SCEC reports that 2/3 of the SCEC science budget goes to students and early-career researchers involved in investigator-initiated research. The project has promoted teaching, training, and learning in our institution by providing financial support and time for PIs Miranda and Schwartz to 1) teach students how to conduct research, 2) train students to collect quality data in the lab, and 3) facilitate student learning through their direct participation in field work, data acquisition, data processing, interpretation of results, and writing of results as part of presentations and theses. The resulting products include 1 MS thesis in progress by Francine Robles (expected Spring 2023), 2 undergraduate student thesis projects for Sarah Sengpiehl (BS 2022) and Omid Arzili (BS 2022), and 3 student co-authored poster presentations at the SCEC annual meeting (Sengpiehl et al., 2021; Robles et al., 2021; Robles et al., 2022). The two undergraduate students were SCEC SURE interns, and as part of their internship work, Ms. Sengpiehl worked on the petrochronology of titanite to date the timing of ductile fabric formation in the Black Belt mylonites at high temperatures, and Mr. Arzili worked on the petrochronology of apatite to date the timing of ductile fabric formation at lower temperatures.

The project has also broadened the participation of underrepresented groups by 1) providing funding for the research program of a female, Hispanic scientist (PI Miranda) at a federally designated Hispanic-serving Institution (PIs’ home institution, CSUN), 2) providing a mentored research opportunity for a female, Hispanic MS graduate student (Francine Robles) and a female, Hispanic undergraduate student (Sarah Sengpiehl) at CSUN, and 3) providing specialized funding and support for the female undergraduate student through a SCEC SURE internship. The project has enhanced the infrastructure for research and education by supporting the research training of both graduate and undergraduate students in the CSUN Dept. of Geology’s LA-ICPMS Laser Lab and the Scanning Electron Microscopy lab. The benefits of the activity to society are that underrepresented students have received valuable training as scientists, which is a critical and necessary part of diversifying the STEM fields. Society benefits from having an increasingly diverse population of scientists because this workforce needs to reflect the diversity of the society that is served by science.
Exemplary Figure Figure 2.