SCEC Award Number 17114 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Characterization of fault-rocks from the West Salton Detachment Fault (WSDF) at sites that show evidence of paleo-seismic versus paleo-creep (aseismic) slip
Name Organization
Gary Axen New Mexico Institute of Mining and Technology
Other Participants Katrina Soundy (MS student)
SCEC Priorities 3d, 3e, 3c SCEC Groups FARM, Seismology, SDOT
Report Due Date 06/15/2018 Date Report Submitted 11/16/2018
Project Abstract
Fault rocks were studied in detail at two locations along the West Salton detachment fault, a low-angle normal fault active from ~8-1 Ma. Fault rocks from the top of the footwall were the focus: they formed in (and above?) the upper seismogenic zone. Protoliths at both sites are quartzo-feldspathic plutonic rocks or orthogneiss, and alteration minerals (zeolites, clays) form a very small fraction of the rocks. Presence of catclastic foliation at one site, plus textural evidence for rounding of large grains by rolling and chipping of small grains from corners of large grains support that part of the fault having moved mainly (or finally) by creep, in spite of a general lack of velocity-strengthening minerals (e.g., clays). Abundant pseudotachylyte at and near the other site suggest that fault rocks there formed during seismogenic slip events. More angular fault-rock grains at this site along with pulverization textures support this conclusion. Grain shapes were measured quantitatively using shareware developed by Rene Heilbronner, applied to BSE images of matrix material. At both sites, differences in fault-rock textures were also studied as a function of distance below the fault. We conclude that fault-rock textures at the grain scale are useful records of fault paleoslip rate.
Intellectual Merit This research contributes to our understanding of the processes of formation of catclastic fault rocks and how to interpret such textures, particularly in quartzo-feldspathic host rocks typical of "average continental crust" and in the absence of significant syn-tectonic alteration.
Broader Impacts This research has funded the M.S. thesis research of a female graduate student, who has a job offer from Chevron (beginning in March, 2019).

We have recognized a problem related to measuring the perimeter length of very small grains using grain-shape software applied to bitmap images (software developed by Rene Heilbronner, and available by request). The problem results from pixelation of grain boundaries of very small grains causing a "stairstep" pattern that can artificially increase the measured perimeter length by up to a factor of square root of two. This can be mitigated somewhat by choosing fewer pixels then using trig to calculate a more-accurate perimeter length, but cannot be entirely negated.

Ms. Soundy also has written a user's guide to the Heilbronner grain-shape software that will help any future students at NMT who perform this sort of grain-shape analysis. This will be included as an appendix in her thesis and possibly as an electronic supplement in the final publication (depending upon editorial decisions beyond our control).

Ms. Soundy's thesis will be given a SCEC publication number, as will the planned final journal article.
Exemplary Figure Not available at this time.