PYTHIA'S OASIS: A HIGH FLUX, FLUID-DOMINATED SEAFLOOR SEEP ON THE OREGON SECTOR OF THE CASCADIA MARGIN

Fluid production within the forearc of subduction zones is dominated by the compaction-driven dewatering of sediment pore fluids and by the release of mineral-bound water during sediment diagenesis and low-temperature metamorphic reactions. Fluid production is accompanied by an increase in in situ pore fluid pressure, thereby counteracting the lithostatic load and decreasing effective stress. These overpressures also drive fluid flow through high-permeability horizons (e.g. faults) and is manifested at the seafloor as cold seeps. Given the fundamental role that fluids play in controlling fault mechanics and modes of slip within subduction zones, a key question is the distribution of fluid sources, fluid flow, and pore pressures across the forearc. Seafloor seeps provide access to deep fluids, and the composition of these fluids informs on their source and the depth range of production. However, the ability to sample pristine deep-sourced fluids is often hampered by mixing between compaction-derived fluids and fluid-rock reactions that occur during transport at typical low flow velocities, as well as microbially-mediated alteration reactions. Fortunately, a fast-flowing seafloor seep has recently been discovered within the Oregon sector of the Cascadia Subduction Zone, providing an unprecedented opportunity to sample pristine deep sourced fluids to evaluate fluid sources and where overpressures are being generated. The seep emits low-salinity fluids at flow rates of 10-30 cm s^-1, which exceeds previous subduction zone fluid flow estimates by several orders of magnitude. Here, we describe this new seep site, present hypotheses for the sources of fluid flow at Pythia’s Oasis, and outline the objectives of an upcoming NSF-OCE-funded cruise to characterize fluid composition and transport pathways.