Collaborative Research: Can Low-Angle Normal Faults Produce Earthquakes? Reading a Pseudotachylyte 'Rosetta Stone'

The physics of earthquake-producing fault rupture has been studied for over a hundred years, a period marked by dramatic advances in the instrumentation and analytical techniques required to understand both the ground shaking (seismicity) caused by fault failure and the physical properties of fault rocks. This research addresses a discrepancy between the long-term rock record of ancient earthquakes and the short-term historical seismic record. Specifically, it focuses on representatives of a class of faults that appear to be poorly oriented for breakage according to the current earthquake mechanics paradigm and Andersonian mechanical theory for the rupture of rocks. According to established theory, normal faults that form at low angles (i.e., less than 30 degrees) should not produce significant earthquakes because the tectonic forces that cause failure are oriented at a high angle to the fault surface. The historical seismicity record generally supports this argument, suggesting that such faults produce only microearthquakes. However, the geologic record contains numerous examples of normal faults that appeared to have slipped at low (less than 30 degree) dips. The presence of pseudotachylite, an extremely quickly cooled, glassy melt rock that generally forms as the result of frictional heating during seismic slip, has been found to be associated with some of these low angle faults, and thus preserves a record of 'fossil earthquakes.' In this study, the principal investigators have identified an unusually rich record of 'fossil earthquakes' on a low angle normal fault in the South Mountains, Arizona. This research is exploring this fossil record with the goal of determining whether or not such faults produce significant earthquakes in non-Andersonian orientations, thus addressing a first-order question in fault mechanics. The research will not only result in a deeper understanding of the earthquake record and the potential seismic hazard of 'misoriented' faults, but also has the potential to transform our understanding of fault mechanics. In addition to the scientific objectives of this research, the project will contribute to the training of graduate and undergraduate students in a STEM (science, technology, engineering, and mathematics) discipline, thus contributing to a more scientifically literate and vibrant society. It represents a collaborative effort between investigators from two research-intensive public universities. Research results will be disseminated by presentation at national geoscience meetings and through the peer-reviewed scientific literature. The results will also be used to engage and educate the public regarding geologic concepts and earthquake hazards through a partnership with the University of Wisconsin Geology Museum to produce participatory exercises and videos and online teaching modules.

Low-angle normal faults (LANFs) are poorly oriented for slip according to Andersonian fault mechanics. The geologic record generally supports slip at current low (less than 30 degrees) dips; however, the seismic record generally suggests such faults cannot produce earthquakes greater than magnitude 5.5. One explanation for the discrepancy between these data sets is that the large size and greater efficiency of LANFs result in recurrence intervals longer than the seismic record. A second is that LANFs were re-oriented by isostatic rebound as they were exhumed, allowing them to slip at steeper dips before rotating into their final, shallow orientations. A third explanation, one which reconciles the geologic and geophysical records, is that low angle normal faults fail by creep, producing microseismicity but not substantial earthquakes. Although a convincing explanation for the two active faults that demonstrably record creep, this does not account for the common occurrence of pseudotachylyte in exhumed low-angle normal fault zones worldwide. The purpose of the proposed research is to explore this fossil record of earthquakes using exposures from the South Mountains metamorphic core complex. This site was chosen because pseudotachylyte fault veins are plentiful and amenable to both rock magnetic and geochronologic analyses. This project will test the following hypotheses by integrated structural, thermochronologic, and paleomagnetic analyses and modeling: (1) Recurrence intervals of the largest earthquakes are sufficiently long that they can be distinguished within the error of 40Argon/39Argon isotopic ages (+/- ca. 0.25 million years). (2) Paleomagnetic remanence indicates that earthquakes occurred at current fault dips (less than 30 degrees). (3) Fault veins in the South Mountains record a range of earthquake sizes, the largest of which are greater than magnitude 5.5. The principal investigators will use the magnetic record preserved in pseudotachylyte to quantify fault rotation (tilting), if any, seismogenesis. The remanence vector recorded by each sample will be compared with the expected geomagnetic field direction by correlating the sample?s 40Argon/39Argon age with its concomitant geomagnetic north location and comparison with the North American apparent polar wander path. Any divergence between the two vectors will represent rotation of the system since seismic slip, ultimately allowing us to quantify the angle at which a LANF was active. The cooling history of the wall rock also will be determined using several thermochronometers. Modeling will incorporate these data and constrain earthquake magnitude based on pseudotachylyte fault vein thickness. The project addresses a first-order question in fault mechanics and will provide a deeper understanding of the record of ancient seismicity in the metamorphic core complexes of the western U.S.