SCEC Project Details
| SCEC Award Number | 17063 | View PDF | |||||
| Proposal Category | Individual Proposal (Data Gathering and Products) | ||||||
| Proposal Title | Constraining the rheology of brittle-ductile transition rocks during the seismic stress cycle | ||||||
| Investigator(s) |
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| Other Participants | Craig Stewart (current MS Geology student) and 1 future MS student | ||||||
| SCEC Priorities | 3b, 3d, 3f | SCEC Groups | Geology, FARM, SDOT | ||||
| Report Due Date | 06/15/2018 | Date Report Submitted | 11/14/2018 | ||||
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Project Abstract |
The objectives of our project are to 1) evaluate the constitutive relationships that describe the behavior of quartz-rich BDT rocks during the seismic stress cycle, and 2) determine if the rheology of the quartz-rich rocks are best described by ‘wet’ or ‘dry’ flow laws. Our methods involve 1) electron backscatter diffraction (EBSD) analysis to evaluate the constitutive relationships that describe the deformation of pseudotachylyte-bearing mylonites during the seismic cycle, and 2) atom probe tomography to determine the water content of quartz grains in the pseudotachylyte survivor clasts and in the host mylonites. The intellectual merit of this work is that it advances our knowledge and understanding in the field of fault mechanics because we show that pseudotachylyte-generating earthquakes can nucleate from ductile precursors within deforming mylonites at the brittle-ductile transition, and that this is likely promoted by hydrous fluids that enhance deformation by diffusion creep. These results are important to the SCEC community because they are directly related to the priorities of FARM. The products produced over the course of the project include 1 MS thesis and 1 student first-authored peer-reviewed publication. The broader impacts of the project include the increased participation of underrepresented groups in the geosciences, specifically 1) a female, Hispanic scientist (PI Miranda) at a federally designated Hispanic-serving Institution (PI’s home institution, CSUN), 2) a female undergraduate student at CSUN (Kelly Lourcey). |
| Intellectual Merit | The project contributes to the overall intellectual merit of SCEC because we explore details of the earthquake cycle at the brittle-ductile transition as recorded by the rock record. The research advances our knowledge and understanding in the field of fault mechanics because we show that pseudotachylyte-generating earthquakes can nucleate from ductile precursors within deforming mylonites at the brittle-ductile transition, and that this is likely promoted by hydrous fluids that enhance deformation by diffusion creep. This is important in the context of SCEC because the priorities of FARM include constraining the rheology of BDT rocks and evaluating the role of fluid flow on faulting and earthquake occurrence. The work is original and creative in that it involves the use of a new method (electron backscatter diffraction, or EBSD) to directly analyze coeval pseudotachylytes and their host mylonites. Although coeval mylonite and pseudotachylytes have been investigated with light microscopy and TEM analyses (e.g., White 1996; White 2012), our study is the first to use EBSD in the investigation of coeval mylonites and pseudotachylytes that record details of the seismic cycle. Our study is also unique in that we interpret a ductile, in situ precursor to these pseudotachylytes, in contrast to the conventional model of pseudotachylyte generation where a frictional instability in the brittle regime leads to the local generation of melt. Although another study of pseudotachylyte-bearing rocks involves the use of EBSD (Bestmann et al., 2011), their study emphasizes that pseudotachylytes develop along precursor plastic shear zones such that the shear zones and pseudotachylytes are not coeval, which is in striking contrast to our results. Thus, the intellectual merit of our work also includes the documentation of a lesser-known mechanism of pseudotachylyte generation. |
| Broader Impacts | The project contributes to the broader impacts of SCEC as a whole by having both graduate and undergraduate students conduct the primary research that informs our understanding of fundamental aspects of the earthquake cycle. The project has promoted teaching, training, and learning in my institution by providing financial support and time for PI Miranda to 1) teach students how to conduct research, 2) train students to collect quality EBSD data in the lab, and 3) facilitate student learning through their direct participation in data acquisition, data processing, interpretation of results, and writing of results in both thesis and manuscript form. The resulting products include 1 MS thesis by Craig Stewart, and 1 student co-authored, peer-reviewed paper (i.e., Stewart and Miranda, 2017). 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 (PI’s home institution, CSUN), 2) providing a mentored research opportunity for a female undergraduate student at CSUN (Kelly Lourcey), and 3) facilitating the recruitment of a Hispanic male CSUN undergraduate student (Miguel Zamora-Tamayo) to work on the project in the future. 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 EBSD-enabled Scanning Electron Microscopy lab. The possible benefits of the activity to society are that underrepresented students have received valuable training as scientists that will allow them to be competitive applicants for graduate school in the future, which in turn will lead to their future employment as geoscientists. 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 | The figure is Figure 6 in Stewart and Miranda (2017). This will be incorporated into the final report in November 2018. |
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Linked Publications
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