CAREER: Connecting metamorphic reactions, fluid production, and deep slow slip in subduction zones

Subduction zone faults, places where one tectonic plate dives beneath another, have the largest and most deadly earthquakes on Earth. However, the basic science leading to these events and the relationship to other types of slip along these faults is still not well understood. One of these different types of slip is called slow earthquakes (or slow slip events). Slow slip occurs along the subduction fault at depths of 25-65 km, below where most major earthquakes nucleate. These events have likely triggered these catastrophic megathrust earthquakes in the past. Thus, understanding what causes and controls slow earthquake formation is vital to understand the seismic cycle of subduction zone faults. How and why these slow earthquakes form remains debated; geophysical and geological observations suggest that high pore fluid pressures likely critically stress these faults and may trigger slow earthquakes. Yet, we do not know where the fluids that create these fluid pressure conditions come from. Making it impossible to understand the relationship between these fluids and slow earthquakes. Previous modeling work showed chemical reactions in the subducting plate, called dehydration reactions, may release the needed water. Yet, these models have not been ground-truthed in the geologic (rock) record. This proposal seeks out geologic evidence of the fluids that may lead to slow earthquakes and influence earthquake occurrence in subduction zones. The Education Plan increases subduction zone hazard literacy in the Pacific Northwest. It does this through curriculum development for the Rockin’ Out program at the University of Washington. It will also host the RESESS Satellite program at the University of Washington for three summers. This will provide ~18 geoscientist undergraduates with intensive summer research projects in subduction zone science. Together, the science and education plans will increase knowledge of subduction zone earthquakes and raise awareness of subduction zone hazards in the general public. The goals of this proposal are to look for geologic evidence of dehydration reactions within previously subducted rocks and see if there is a link between fluid production and the occurrence of deep slow earthquakes. One of the main hypotheses for slow earthquakes in subduction zones is that they are produced by high pore fluid pressures (Pf) along the megathrust which lowers the effective normal stress, triggering slow frictional deformation below the base of the subduction seismogenic zone. For this hypothesis to be valid and to explain the observations of slow earthquakes in these regions, fluid sources are required to provide the elevated pore fluid pressures. Petrologic modeling work, currently unconfirmed in the geologic record, shows that fluids liberated during prograde metamorphic dehydration reactions may produce these high Pf, particularly through the breakdown of the minerals lawsonite and chlorite to form epidote within subducting oceanic crust. The project will investigate three exhumed fossil subduction terranes for evidence of these modelled metamorphic reactions. It will use (1) detailed field characterization and petrology to document which of the modeled reactions occur naturally and the volumes of associated fluid production, (2) isotope geochemistry to fingerprint fluid sources and activities, and (3) develop diffusion chronometry tools to test if these metamorphic reactions play a causal role in triggering slow earthquakes. The Education Plan will create new outreach materials through the Rockin’ Out program at the University of Washington which visits K-12 classrooms around the state. This will increase subduction zone hazard literacy in the Pacific Northwest, which is one of two active subduction zones within the USA. It will also provide research experiences for ~18 geoscience undergraduates from a range of backgrounds by hosting the summer RESESS Satellite program for three summers, where each student working on a subduction zone science related project will present their research at national conferences. Overall, the outcomes of this project will increase the collective understanding of the seismic cycle in subduction zones, the role of chemical processes in producing earthquakes, will increase public knowledge of subduction zone science and hazards in WA state, and provide research opportunities for a host of undergraduate students, thus increasing participation in the geosciences and contributing to workforce development. This award will be funded from the Petrology and Geochemistry (CH), Tectonics (TE) and Education and Human Resources (EHR) programs in the Division of Earth 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.