Subduction zone hydrogeology: quantifying fluid sources, pathways and pressure

Open Access
Author:
Lauer, Rachel Mollie
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 05, 2013
Committee Members:
  • Demian Saffer, Dissertation Advisor
  • Demian Saffer, Committee Chair
  • Chris Marone, Committee Member
  • Donald Myron Fisher, Committee Member
  • Derek Elsworth, Special Member
Keywords:
  • subduction zone
  • hydrogeology
  • earthquake mechanics
Abstract:
Subduction zone fluids and fluid pressure distribution exert fundamental controls on the transport of heat and volatiles, and their associated distribution throughout the forearc. Additionally, excess fluid pressures have been proposed as a mechanism governing fault slip behavior, including their association with slow-slip events (SSE), episodic tremor and slip (ETS) and very-low frequency earthquakes throughout the conditionally stable region of the outer forearc. Fluid pressures develop as subducted sediments compact during tectonic burial, and through the production of dehydration derived fluids at depths associated with higher temperatures, and low porosity. Due to their production in a low porosity environment, the dehydration sources have an enhanced potential for pressurization, and the location of these reactions is therefore critical to understanding the mechanical strength of the plate boundary décollement, as it is mediated by the fluid pressure distribution. In an effort to further our understanding of fluid distribution in the forearc, I first develop a fluid budget using a numerical model parameterized by a combination of data obtained through ocean drilling, and laboratory testing of recovered sediments, which estimate the compaction behavior during subduction, and associated permeability reduction. Previous efforts to quantify the fluid budget did not include permeable splay faults, despite geochemical and geophysical evidence that they represent regions of focused flow, and provide a hydraulic connection from the source of fluids at the plate boundary, to the seafloor. The model results suggest that faults capture up to 35% of the total flux, and modeled flow rates are highly consistent with studies of both seafloor seeps, and flow at the trench, suggesting a quantitative link between the underlying permeability architecture of the forearc and flow rates at the seafloor. The results highlight the importance of these features in efficiently channeling heat and solutes from depth. iii The second study examines the importance of faults in the determining the distribution of fluid pressures and deeply sourced fluids by coupling the results of dehydration modeling with a flow and transport model. The dehydration derived freshwater effectively acts as a tracer to consider the role of faults in distributing deeply sourced fluids, and translating fluid pressures away from the plate boundary. Model results indicate that faults are efficient translators of fluids, and capture deeply sourced fluids before they reach the trench. Overpressures develop at the base of the slope sediments, regardless of the permeability architecture employed, suggesting a potential mechanism for the formation of mud volcanoes, spatially correlated with faults that penetrate the forearc. The results of a comprehensive heat-flow campaign offshore Costa Rica suggests plate boundary temperatures are much lower than previously thought, with dehydration reactions shifted farther down-dip, into regions of further depleted porosity. The third project in Costa Rica uses this new thermal structure to evaluate the spatial distribution of dehydration reactions along a 500-km segment of the plate boundary, from North of Nicoya, to north of Osa peninsula. I find that peak dehydration sources are spatially correlated with the rupture patch of all major earthquakes since 1950, and LFEs and slow-slip occur immediately down-dip of the reaction midpoint where the mole fraction of Smectite-Illite in the mixed layer clays is 50% Smectite and 50% Illite. I find that the transition from aseismic to seismic behavior correlates well with the temperature range associated with the precipitation of silica produced through smectite transformation, which stiffens the sediment through chemical compaction, and suggests a potential chemical mechanism for the onset of seismicity.