EAGER: Collaborative Research: Can Low-Angle Normal Faults Produce Earthquakes? A Paleoseismic Perspective

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

The identification of low-angle normal faults, i.e. faults that have dips less than 30 degrees, are problematic with respect to our current understanding of the mechanics by which faults form. According to classic Andersonian fault mechanics, such faults should not form in most geologic settings. Despite this problem, they are observed in many geologic settings but the means by which they form remain controversial. Hypotheses explaining these faults fall into two categories: those that offer explanations (e.g., fault weakness, reduced effective normal stress) for how the faults slip in suboptimal orientations, and those that suggest they move at higher angles before rotating into their current orientations. A key question is whether or not these relatively faults can generate earthquakes. Of fundamental importance is the observation that low-angle normal faults commonly display an unequivocal - yet poorly studied - record of past seismic activity in the form of pseudotachylyte (frictional melt) veins. The existence of these ?fossilized earthquakes? places an important constraint on structural models that seek to explain the origin of low-angle normal faults: they were clearly seismogenic at some point in their history. Our goal is to determine the orientations at which low-angle normal faults from the southwestern U.S. and New Zealand produced earthquakes by using the magnetic remanence preserved in pseudotachylyte to quantify the potential effects of subsequent tilting since seismogenesis. The degree of tilt (if any) will be determined by comparing the magnetic vector of the sample with the expected reference direction for the study area based on well defined apparent polar wander paths. The age of pseudotachylite formation (and hence the age of seismogenesis) will be determined using 40Ar/39Ar dating using a combination of incremental heating analyses and UV laser-based in situ methods that will target areas of neocrystalline material and avoid the deleterious effects of glass and relict clasts on 40Ar/39Ar ages. This research will contribute to the resolution of a long-standing controversy in the structural geology and tectonics community. If our results show that these faults can only produce earthquakes at higher angles prior to tilting, they will confirm a long-established theory of the mechanics of earthquakes and faulting. If, however, our results show that the pseudotachylites have resulted from faults that formed at angles 30 degrees or less, our current concepts of earthquake mechanics must be either incomplete or flawed, and the data will require a reconsideration of the fundamental controls on earthquake mechanics. Regardless of outcome, our research will contribute a much greater understanding of how much information can be gleaned from pseudotachylytes, the sole rock record of paleoearthquakes. In addition to the research objectives, the project will support the training of two female graduate students at the University of Wisconsin-Madison. The research results will also be used co-design and evaluate "earthquake in a box" educational activities with a focus group of 6th grade girls. The activities will be used in both classroom and formal outreach programs of University of Wisconsin-Madison?s Geology Museum in an effort to both disseminate results to the public and encourage 10- to 14-year-old girls to remain involved in classes and enterprises that will allow them the flexibility to follow career paths in STEM fields in later years. The girls in our focus group will be full partners in our outreach effort, and each girl will be included as a co-author on a resulting paper to be submitted to a peer-reviewed journal in geosciences education. This project is a collaborative effort between the University of Wisconsin-Madison and the University of Minnesota.