Stress History of the Alpine Fault Using Rock Deformation Experiments and Numerical Modeling

Cuny City College, New York NY

Investigators

Abstract

The state of stress in the crust and its spatial and temporal variability is an active and longstanding research area in geodynamics, earthquake mechanics, structural geology and rock mechanics. Stress levels on plate boundary faults are the subject of ongoing debate and also a subject of particular societal importance because it is the buildup of stresses along these structures that is responsible for the most devastating earthquakes. This study examines stresses on the Alpine fault, New Zealand, using two complimentary approaches. First, a two-dimensional numerical model of the Alpine fault that incorporates shear heating will be developed to provide an independent constraint on long-term stress levels on the Alpine fault. Second, a set of rock deformation experiments will be carried out to explore the new idea that significant variations in the size of deformed grains in rock samples from the fault indicate that the large grains formed in the middle crust under low stress conditions and smaller grains formed during brief, seismically induced high-stress pulses. The project would advance desired societal outcomes through: (1) full participation of women and underrepresented minorities in STEM; (2) development of a diverse, globally competitive STEM workforce through training of graduate and undergraduate students from underrepresented groups in the Earth sciences and support of an early career scientist; and (3) increased partnerships through international collaboration with New Zealand scientists. Recrystallized quartz grain size piezometric data from Alpine fault mid-crustal rocks show two main features: (1) a significant lateral variation along strike of the fault such that peak stresses are significantly reduced in the central portion of the fault; and (2) grain size populations are strongly bimodal. When stresses based on recrystallized grain size are compared to available independent constraints on stress on the Alpine fault, a striking inconsistency emerges. Crustal strengths indicated by numerical models based on force balance, calculations based on potential energy, and earthquake focal mechanism data suggest integrated crustal strengths two to three times weaker than seemingly indicated by the grain size data. To reconcile these observations, this study investigates the hypothesis the coarse-grained quartz population formed at the brittle-ductile transition at a peak long-term stress of only ~50 MPa. The fine-grained quartz formed during brief, seismically induced high-stress pulse. The project delves deeper into the stress state(s) of the Alpine fault and the interpretation of quartz microstructures in potentially non-steady state settings using two complimentary approaches. First, a two-dimensional numerical model of the Alpine fault that incorporates shear heating will be developed. Two linked finite element models will be run together, one pertaining to the Southern Alpine fault where exhumation rates are low, and a second from the central Alpine fault where exhumation rates are extreme. The numerical model will be tested using an inversion against multiple thermal constraints such as metamorphic histories, thermochronologic data, and heat flow. As modeled stresses increase, it becomes impossible at a certain point to recreate observed thermal histories. The modeling thus provides an independent constraint on long-term stress levels on the Alpine fault and may provide some additional insights into its tectonic history. Second, a series of rock deformation experiments on quartzites in a Griggs rig will be carried out to explore the effects of stress variations on recrystallized grain size distributions and fabrics. Experiments will be conducted using both gradual stress increases (simulated exhumation paths toward the brittle ductile transition), and stress pulses followed by stress relaxation (simulated postseismic deformation). Fabrics from characteristic Alpine fault samples will be examined for purposes of comparison.