Solomon, Evan AAylward, Irita2025-01-232025-01-232024Aylward_washington_0250E_27699.pdfhttps://hdl.handle.net/1773/52849Thesis (Ph.D.)--University of Washington, 2024Subduction zone faults host a range of modes of slip spanning the spectrum from aseismic creep to devastating earthquakes. Hydrogeologic processes and conditions at the plate boundary and in surrounding sediments of the outer forearc have profound impacts on geomechanical processes in these convergent margins. Specifically, fluid production, permeability and fluid migration pathways, and pore fluid pressures, are thought to impact the nature and timing of slip behavior. While direct measurements of these properties are rare and limited to depths and locations only accessible by scientific ocean drilling, seafloor seeps hosted by deep-reaching faults provide a unique opportunity to assess the nature and efficiency of fluid flow through the system, informing on the state of pore fluid overpressure, the permeability structure, and the overall water budget of the outer forearc. This dissertation comprises three studies in which visual, geophysical, and chiefly geochemical methods are employed at sites of focused and diffuse flow (i.e., seeps and non-seeps, respectively) to investigate and advance our understanding of the interrelationships between mechanical and hydrogeologic processes at subduction zones.Chapters 2 and 3 target active seafloor seeps and diffuse flow sites spanning the continental slope at the northern and southern Hikurangi margin, offshore of the North Island of New Zealand. Using pore water from sediment cores and heat flow measurements, Chapter 2 provides direct constraints on the source and flow rate of fluids traveling through the accretionary wedge. Results suggest that fluids are not efficiently draining from the plate boundary or surrounding sediments to the seafloor along fault zones. This implies that splay fault permeability at depth is low in the offshore forearc, likely contributing to the accumulation of pore fluid overpressure in the region of slow slip at northern Hikurangi. Chapter 3 reports continuous fluid flow rates measured by benthic fluid flow meters deployed at seeps and non-seep locations over a 2-year period. Early in the deployment period, a large slow slip event (SSE) occurred beneath the instrument array offshore northern Hikurangi. Net flow polarities at non-seep locations are consistent with creeping on the shallow megathrust and locking in the slow slip source region in the inter-SSE period; flow transients reflect local volumetric strain induced by the slow slip event. At seep sites, flow transients additionally reflect gas-driven processes and possibly a shallow permeability response to the SSE, but convincing evidence of wide-spread fault-valving is not observed. Chapter 4 presents a detailed examination of a single seep site called Pythia’s Oasis, which is located ~20 km landward of the deformation front on the central Cascadia margin. The composition of the venting fluid suggests a fluid source within the subducting sediment section, at source temperatures of ~170 ˚C – 250 ˚C. Highly altered fluid compositions, along with anomalously high heat flow and rapid flow rates observed at Pythia’s Oasis, are consistent with focused, long-range water flow along the Alvin Canyon fault in the mid-slope region. High permeability conditions along the Alvin Canyon fault and/or high pore fluid pressures in the central Cascadia outer wedge likely permit sustained, high-volume discharge at Pythia’s Oasis. These hydrogeologic conditions may play an important role in the reduced locking, increased seismicity, and other changes in margin characteristics that occur in this region. Solute fluxes at Pythia’s Oasis resulting from the rapid, long-distance fluid transport are comparable to those at both high- and low-temperature hydrothermal discharge sites, indicating that subduction zone seeps may be more important for marine geochemical cycles than previously considered.application/pdfen-USCC BY-SAMarine geologyGeochemistryPlate tectonicsOceanographyThesis