EM122 Water Column Sonar Data Collected During AT26-04

Thermal Structure of the Cascadia Subduction Zone on the Washington Margin (AT26-04, EM122). We propose to conduct a comprehensive study of the thermal environment of the Cascadia Subduction Zone (CSZ) within the NSF GeoPRISM Corridor off the Washington margin. The primary goal of this study is to determine the temperature structure of Juan de Fuca basement and overlying accretionary prism sediments that comprise the CSZ deformation zone with a transect of systematic heat flow and fluid flux profiles off the Washington Coast. Temperature is a primary controlling factor of many subduction zone processes, particularly at active margins that are subject to large magnitude, megathrust earthquakes as the CSZ offshore WA, and a comprehensive heat flow study of the Cascadia Corridor is a fundamental parameter required by the Scientific Plan of the GeoPRISM Program. Compilation of existing data demonstrate that the segment of the CSZ off-shore WA is dramatically under-measured, with only sparse heat flow stations made during programs focused on other scientific goals. We are proposing to acquire a systematic profile of heat flow and fluid flux measurements along a corridor of the accretionary prism on the Washington margin at 47N, from west of the deformation front on the abyssal plain to just below the shelf edge at 500 m depth in order to make the first quantitative estimates of the thermal structure of the 'locked zone' of the Cascadia megathrust. To obtain an accurate estimate of conductive heat flux from the challenging environment of the accretionary prism, we plan redundant methods to obtain both thermal and fluid flux measurements along a 2.5 D profile of the margin at a single latitude. In this profile we would use thermal blankets suitable for impenetrable sub-stratum, continuous fluid flow meters, multi-core deployments for sediment pore water chemistry and thermal gradient measurements, and Jason-II heat flow probes.