Collaborative Research: An integrated evaluation of lower crustal rheology and localization processes in plagioclase-rich rocks

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Earth’s crust deforms by different processes at different depths. The lower part of the crust is difficult to access in the field and is therefore less understood than the upper crust. Although earthquakes typically occur in the upper crust, the lower crust affects earthquake activity by transferring stress to shallower faults. Using a combination of field studies, laboratory analyses, and numerical models, this project will evaluate which processes allowed deformation to occur in narrow zones in the lower crust. This project will use an exposure of lower crustal deformation that at the surface in Québec, Canada, allowing us to directly study the rocks and deformational processes. In particular, the researchers will determine if interaction with shallower earthquake-causing faults in the upper crust is sufficient to explain the characteristics of this region, or if additional rock weakening processes play a fundamental role. This relationship between upper and lower crustal deformation is important to understand the earthquake cycle and related hazards. In addition, undergraduate students will participate in both the field and analytical portions of this project as part of a capstone class, gaining valuable exposure to the benefits of scientific collaboration between numerical modelers (theoreticians) and field geologists (empiricists). The field site selected for this project is the Morin Shear Zone in Québec, Canada. Its western portion exclusively deforms anorthositic rock. The large proportion of plagioclase in this rock means that it can be treated it as effectively a single phase and take advantage of the abundance of laboratory constraints on the mechanical behavior of plagioclase. This project is based on tight integration between constraints from the field and laboratory and numerical geodynamic modeling. Specific tasks will be to: 1) document shear zone deformation conditions, structure, and evolution from the field; 2) numerically model and predict strain gradients in the lower crust near a brittle fault, absent of any weakening processes; 3) report from the field evidence of microstructural or chemical changes that may affect rheology and weaken rocks; and 4) numerically evaluate the impact of any suspected weakening processes on shear zone geometry and structure. At the conclusion of this project, the researchers will have determined whether the observed shear zone results from deformation imposed by a brittle fault higher in the strike-slip system, or whether weakening mechanisms are necessary to cause the observed pattern. The project will also demonstrate the power of integrating field-based studies and numerical modeling studies in structural geology. Furthermore, this project will engage undergraduate students in research internships and in a non-field camp capstone experience. These students will have direct exposure to our combined observational and theoretical approaches, and will gain a better appreciation for the range of expertise and interests of current research in the geological sciences. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.