Observations of seafloor seeps on the continental slope of many subduction zones illustrate that splay faults represent a primary hydraulic connection to the plate boundary at depth, carry deeply sourced fluids to the seafloor, and are in some cases associated with mud volcanoes. However, the role of these structures in forearc hydrogeology remains poorly quantified. We use a 2-D numerical model that simulates coupled fluid flow and solute transport driven by fluid sources from tectonically driven compaction and smectite transformation to investigate the effects of permeable splay faults on solute transport and pore pressure distribution. We focus on the Nicoya margin of Costa Rica as a case study, where previous modeling and field studies constrain flow rates, thermal structure, and margin geology. In our simulations, splay faults accommodate up to 33% of the total dewatering flux, primarily along faults that outcrop within 25 km of the trench. The distribution and fate of dehydration-derived fluids is strongly dependent on thermal structure, which determines the locus of smectite transformation. In simulations of a cold end-member margin, smectite transformation initiates 30 km from the trench, and 64% of the dehydration-derived fluids are intercepted by splay faults and carried to the middle and upper slope, rather than exiting at the trench. For a warm end-member, smectite transformation initiates 7 km from the trench, and the associated fluids are primarily transmitted to the trench via the decollement (50%), and faults intercept only 21% of these fluids. For a wide range of splay fault permeabilities, simulated fluid pressures are near lithostatic where the faults intersect overlying slope sediments, providing a viable mechanism for the formation of mud volcanoes.
[1] Geochemical and geophysical evidence indicate that splay faults cutting subduction zone forearcs are a key hydraulic connection between the plate boundary at depth and the seafloor. Existing modeling studies have generally not included these structures, and therefore a quantitative understanding of their role in overall fluid budgets, the distribution of fluid egress at the seafloor, and advection of heat and solutes has been lacking. Here, we use a two-dimensional numerical model to address these questions at non-accretionary subduction zones, using the well-studied Costa Rican margin as an example. We find that for a range of splay fault permeabilities from 10 À16 m 2 to 10 À13 m 2 , they capture between 6 and 35% of the total dewatering flux. Simulated flow rates of 0.1-17 cm/yr are highly consistent with those reported at seafloor seeps and along the décollement near the trench. Our results provide a quantitative link between permeability architecture, fluid budgets, and flow rates, and illustrate that these features play a fundamental role in forearc dewatering, and in efficiently channeling heat and solutes from depth.
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