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Speaker: Evan Solomon, Professor, School of Oceanography, University of Washington

Host: Shuoshuo Han

Title: Geochemical and thermal constraints on forearc dewatering and megathrust pore fluid pressure at the Hikurangi and Cascadia subduction zones

Abstract: The balance of fluid inputs, storage, and drainage is thought to play an important role in the nature of fault slip at subduction zones. However, significant uncertainties exist in the distribution and rates of forearc dewatering due to the challenges of making in situ observations of pore fluid pressure and fluid flow rates. Models show that fault zones within the outer forearc are important pathways for fluid flow and enhance drainage of the plate boundary as faults often exhibit higher permeabilities than the matrix sediments. Indeed, long-range transport of chemically distinct fluids has been observed along the plate boundary and splay faults sampled through scientific ocean drilling at several subduction zones. However, only a few fault zones have been sampled, and direct measurements of the rates of fault-hosted fluid flow in the outer forearc of subduction zones are sparse. Seafloor seep sites often coincide with faults that extend to the megathrust, and, as such, are accessible locations to investigate both background rates and transients in fault-hosted fluid flow. In this talk, I will present the results from three recent offshore field campaigns where we cored, collected heat flow measurements, and deployed long-term continuous benthic fluid flow meters at fault-hosted seep sites and off-fault locations at the northern Hikurangi and central Cascadia subduction zones. The combined results of these expeditions highlight the important role of the forearc permeability structure in governing the distribution and rates of fluid flow, megathrust effective stress, fault behavior, and geochemical cycling.