The Thermal and Fluid Environment of the Cascadia Subduction Zone, Southern Washington State

Thesis (Ph.D.)--University of Washington, 2018Subduction zones are areas of tectonic plate collision that produce associated earthquakes and destructive tsunamis and, in addition, provide a link between the atmosphere and ocean. Mapping the thermal and fluid circulation of an active subduction zone can advance our understanding of subduction zone dynamics and fluid budgets, important in understanding global chemical fluxes and the carbon cycle. By exploring the intertwined nature of both the thermal and fluid environment, a more complete picture of the governing processes can be established. The overarching goal of this thesis is to map the thermal and fluid environment within the Cascadia Subduction zone offshore Southern Washington State. Eleven recently collected multi-channel seismic (MCS) profiles from the 2012 Cascadia Open-Access Seismic Transect (COAST) experiment offshore Washington State were used to characterize the distribution of bottom simulating reflectors (BSRs) from seaward of the deformation front onto the continental shelf of the Cascadia Subduction Zone. From these MCS profiles, we generated a 3-D view of the Cascadia margin thermal structure by interpreting 40,232 individual BSR data points in terms of temperature and heat flow. Localized differences between BSR heat flow and numerical models reflect an estimated regional mean upward fluid flow of 0.53 cm yr-1 for the survey area, with localized fluid flow approaching a maximum of 3.8 cm yr-1. At the deformation front, the incoming oceanic sediment/crust interface temperatures vary between 164°C to 179°C, indicating the up-dip limit of the Cascadia seismogenic zone. Seafloor heat flow data provides valuable insight into seafloor and subseafloor fluid and geological processes. Thus, understanding the uncertainties associated with heat flow instruments is essential to the interpretation of acquired data. While there have been studies of various instruments’ ability to accurately capture the necessary elements of a heat flow measurement, a need remains to examine how well individual sensors perform in-situ and to make quantitative comparisons between different instruments and their corresponding methodologies. Four different heat flow instruments were compared: the Violin-bow Probe, Alvin Probe, Thermal Blanket, and a modified Multi-core system, in terms of individual instrument uncertainty and the accuracy of each instrument in estimating the local heat flow. On an east-west profile of the Cascadia accretionary prism, a total of 251 heat flow measurements over water depths from 550 to 2600 meters captured a wide range of seafloor thermal environments. The deployment chronology illuminated large differences in measured temperature profiles due to variable near bottom water temperatures that propagated into the sediment column. While near seafloor water temperature variability generally decreases with water depth, bathymetry and localized oceanic currents are also important because even sites at similar depth may be exposed to vastly different seafloor thermal environments that impact all instrument types. The collected seafloor heat flow measurements were co-located with the COAST MCS profiles that run perpendicular to the megathrust fault strike, providing additional contextual data in the form of BSR heat flow and internal geological structure. Additional context comes from in-situ thermal conductivity measurements, multibeam derived backscatter, acoustic imaged bubble streams, and ROV video of carbonate deposits and pockmarks. Fluid emission sites differ across-strike from the actively deforming and faulted accretionary toe, with highly variable and shallow sourced fluid, to long-lived point sources of methane gas from deeper within the accretionary wedge. These surface heat flow measurements throughout the survey proved useful for detecting fluid emissions from the seafloor sediment, especially in the absence of methane gas and carbonate