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.
Earthquakes (seismic events) put populations in tectonically active regions at risk. They are not predictable. However, we can assess the probability that an earthquake of a given size will occur in a location of interest by piecing together information gleaned from instruments that measure ground shaking during recent earthquakes (seismometers); the historical record of earthquakes; and the geologic record of prehistoric earthquakes (paleoseismicity). Our study falls in the latter category. The paleoseismic record we investigated is preserved in a rock called pseudotachylyte, formed when the sides of a fault slide past one another at rates fast enough to produce earthquakes. The sliding also generates sufficient frictional heat to melt the rock. The cooled melt forms a distinctive, fine-grained rock: pseudotachylyte. Faults slide in response to forces produced by the movement of tectonic plates. The geologic community has evolved a conceptual model to explain fault motion, in which the forces cause shear stress to build on a fault plane until it overcomes the frictional resistance to sliding and the fault fails. Slip occurs most efficiently when the direction in which maximum stress is applied is oriented ~30° from the fault plane. If the maximum stress is oriented >60° from the fault, it should not slide at all, as a force applied at such a high angle effectively ‘clamps’ the fault. However, some faults in the latter category still appear to move, or to have moved in the past. One subset of these faults that are not optimally oriented for failure, low angle normal faults, is the focus of our study. These faults dip less than 30° from horizontal, though they occur in places where the inferred maximum stress is vertical. Faults lined by pseudotachylyte veins were exposed by erosion and uplift along a low-angle normal fault in the South Mountains of Arizona. At this study site, veins dip 14° on average, 76° from the inferred maximum stress. Our research evaluated the feasibility of a test to determine whether these veins record seismic failure at their current low dips, in which case the conceptual model of fault mechanics must be revised, or failure at mechanically reasonable high angles followed by rotation. The test exploits the fact that rocks formed from a melt will record the orientation of the Earth’s magnetic pole when the melt ‘freezes’, as long as magnetic minerals crystallized from the melt. The test involves (1) measuring the ‘paleomagnetic’ record of pseudotachylyte veins and (2) knowing when the melt quenched, to compare results to the documented record of the position of the Earth’s magnetic pole over time. We used 40Ar/39Ar analysis to determine when pseudotachylyte veins cooled. Our data show that the host rock cooled below 150°C by 21.8 million years ago, but two veins from the same outcrop quenched 17.44±0.20 and 16.24±0.23 million years ago. Pseudotachylytes therefore record the timing of ancient earthquakes, and show that the site records a minimum million-year history of paleoseismicity. Previous research shows that the Earth’s magnetic pole did not change orientation during this time period. Because the pseudotachylyte veins we studied contain the magnetic mineral magnetite, we were able to measure their remanent magnetization. Some of the veins we analyzed either record, or are close to, the known magnetic pole orientation within confidence limits. Others are not, but cannot be explained by rotation from originally steeper dips. Ongoing research is designed to determine what factor(s) affect pseudotachylyte’s ability to accurately record the Earth’s magnetic field. Intellectual merit: The results of this work demonstrate that the approach we have taken is feasible. Further, the data collected indicate that a subset of the veins studied formed in their current low-angle orientations. Additional research must both estimate earthquake magnitudes from vein thicknesses and address the enigmatic paleomagnetic record of some of the veins. Collectively, these data will indicate whether or not the earthquake mechanics paradigm needs to be revised. Broader Impacts: Our results provide a foundation for further study, which will ultimately improve the geologic community’s ability to accurately assess seismic hazards. We have also, through an after-school club called ROCK-It Girls, identified an approach to both communicate research results more effectively and engage a demographic at high risk of dropping out of math and science: middle-school girls. The 6th and 7th grade girls who participated in our club produced both a flash mob earthquake dance and a jeopardy game linked to a series of highly creative training videos. Their audience – 46 members of the Madison Gem & Mineral Club and family – raved about the work. End-of-year evaluations show that ROCK-It Girls felt that their knowledge of the topics covered increased substantially. Both students and parents wanted the club to continue. Two members of the Madison Gem and Mineral Club independently contacted us to ask how they could support future ROCK-It Girls activities.
