Peter Lindquist, UW-Madison, presents:
Dehydration and deformation of subducted oceanic crust in ancient and modern subduction zones
The environment in which deep slow slip and tremor occur in subduction zones is fluid-rich and structurally and chemically complex. Fluids play a large role in mediating the chemical and mechanical evolution of these regions, altering the mechanical properties of rocks at the plate interface by metasomatizing rocks and stabilizing new minerals and by impacting both frictional and viscous deformation mechanisms. The fluids responsible for these changes are sourced from dehydration metamorphic reactions in the downgoing oceanic plate, but our understanding of what reactions are occurring, how much fluid is released, and how it impacts rocks at the plate interface is limited. I present several case studies exploring the sources and impacts of metamorphic fluids at the conditions of deep slow slip in a modern subduction zone (at Guerrero, Mexico) and in the geologic record (recorded by the Catalina Schist in southern California). Through thermodynamic modeling of the metamorphism and metasomatism of rocks in the modern subduction zone at Guerrero, Mexico, I demonstrate that major dehydration reactions in subducting metabasalts have the potential to produce fluid fluxes that result in the production of rheologically significant volumes of talc along the slow-slip-hosting flat-slab segment. In southern California, the Catalina Schist comprises slivers of oceanic crust that were subducted and underplated to North America during the Cretaceous. This exhumed subduction complex provides an opportunity to look for evidence of dehydration reactions in the geologic record and observe the impact of liberated fluids. Petrographic and geochemical (major-, trace-element, and isotopic) analyses on Catalina Schist metabasalts help to constrain their metamorphic history and reconstruct the conditions at which these basalts likely dehydrated, and, in some cases, deformed during subduction and underplating. In both modern and ancient subduction zones, pulses of dehydration from subducting basalts likely provide a significant fluid source at the conditions of deep slow slip and tremor.
