Collaborative Research: Roles of lithology and water on deep continental crustal rheology from a natural setting and laboratory experiments

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

Earthquakes are generally observed at shallow depths (<15 km) in Earth’s upper continental crust or at greater depths (50-660 km) in subducting oceanic plates. Recent discoveries of rocks that are only formed by melting on fault surfaces during seismic slip events in the deep (20-50 km) continental crust indicate that earthquakes can occur in zones previously thought to be aseismic. The mechanisms that can cause earthquakes to occur in aseismic shear zones are not understood. However, recent observations indicate that the strength of many minerals in these rocks is sensitive to the amount of water entrained inside the mineral grains and that water loss may strengthen them. Researchers from the University of Colorado Boulder and the University of Akron will perform an integrated series of modeling, field, and laboratory-based investigations to quantify how water loss affects the strength of a deep crustal fault zone. They will perform field-based investigations of the processes operating in the Cora Lake Fault Zone, Saskatchewan, Canada, measure water contents in all of the rocks in the fault zone, and perform experiments to characterize the effect of water content on the strength of quartz, one of the dominant phases in the fault zone. The results of these studies will be used to create a predictive model of the processes that caused the deep crustal earthquakes. Desired societal outcomes will include advancement of our understanding of the processes that cause earthquakes, as well as the support of an early-career post-doctoral researcher, graduate students, and undergraduate students. This project will advance our understanding of the rheology of “dry” deep continental crust using three complementary approaches. First, the researchers will conduct field-based and microstructural characterization of a kilometer-scale shear zone exhumed from 35–25 km paleodepths in the Canadian shield. The structure hosts synkinematic pseudotachylyte that developed under nominally anhydrous granulite- to upper amphibolite-facies conditions. The work will include water content measurements on nominally anhydrous minerals, particularly quartz and feldspar, to correlate their rheology to dry, damp, or wet deformation experiments and rheologic modeling. Second, pressure-stepping deformation experiments will determine the effects of water content and fugacity on quartz creep strength. Shear experiments on low water content quartzites will also be conducted to compare microstructure and deformation/recovery mechanisms with observations from the shear zone. Third, viscoelastic numerical modeling using dry and wet mineral flow laws, with refined parameters from the above experiments, will test the hypothesis that stress enhancement due to lithologic and water content variations led to episodic brittle failure and pseudotachylyte generation in the shear zone. The models will also guide field work by highlighting areas in the shear zone where more accurate characterization of field relationships is needed. 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.