Dehydration and deformation: the sources and impacts of metamorphic fluids from oceanic crust in subduction zone forearcs

Abstract

Subduction zones are host to major seismic and volcanic hazards, drive plate tectonics, and are an critical force in global geochemical cycling. The mechanical properties of the interface between the subducting and overriding plates are important controls on how these processes progress, but those properties and how they are moderated by chemical processes like metamorphism and metasomatism are complex and poorly understood. This thesis focus on providing a better understanding of the environment in which deep slow slip and tremor occurs in subduction zones and the interacting chemical and mechanical processes that control rock deformation. Specifically, this works is motivated by wanting to understand the sources of metamorphic fluids in this region and the impacts that they can have. I apply a range of thermodynamic modeling, field geology, petrographic, and geochemical analyses tools to explore this question in modern and ancient subduction zones. Phase equilibrium modeling of oceanic crust and mantle wedge lithologies in the subduction zone at Mexico combined with geophysically-constrained thermal models and slow slip and tremor distributions give us insights into the metasomatic impacts of fluids and their possible relation to facilitating slow slip and tremor. We show how and where talc can be produced via metasomatism of mantle wedge serpentinites by local advection of Si-bearing fluids from the subducting Cocos plate. Major dehydration reactions in subducting metabasalts have the potential to produce fluid fluxes that result in the production of rheologically significant volumes of talc at the plat interface. In the geologic record, we search for evidence of dehydration reactions similar to the ones predicted by our phase equilibrium modeling to occur beneath Mexico. We apply petrologic and geochemical analyses to metabasalts of the Epidote Amphibolite Unit in the Catalina Schist to constrain their metamorphic evolution, and use phase equilibrium models to estimate what that entails for their dehydration history. Epidote-rich zones in these metabasalts have geochemical signatures and textures that suggest that they could represent channel-forming reactive porosity networks that developed as a result of pumpellyite breakdown at ~300°C. Epidote textures and compositions further record prograde, peak, and retrograde metamorphism—the latter stage associated with underplating and refrigeration in a cooling subduction zone. This work demonstrates the utility of epidote as a tracer of metamorphic processes during low- and medium-grade metamorphism. We have also reconstructed the metamorphic evolution of the Epidote Blueschist Unit of the Catalina Schist, which followed a similar P-T path to the Epidote Amphibolite Unit, but exhibits much stronger deformation fabrics. We therefore pair petrologic and microstructural analyses using electron backscattered diffraction to place the deformation of the Epidote Blueschist Unit in the context of its subduction and underplating metamorphic story. We show that these rocks deformed early in subduction while still at seismogenic zone conditions (<300°C), but that the dominant deformation fabrics in these rocks likely represent underplating. Amphibole and quartz deformed by dissolution precipitation creep and dislocation creep, respectively, during underplating, and quartz recrystallized grain sizes allow us to estimate differential stresses of ~50–180 MPa. This work highlights the importance of contextualizing structural observations within a metamorphic framework, and provides additional context for the environment at the base of the seismogenic zone where slow slip and tremor may occur.