Fluid budgets of subduction zone forearcs: The contribution of splay faults

Abstract

[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.

The impact of splay faults on fluid flow, solute transport, and pore pressure distribution in subduction zones: A case study offshore the Nicoya Peninsula, Costa Rica

“…In cases where k fault ≤ k décollement , pressures are lower, and increase systematically with increased splay fault permeability. The systematic variations in pressure translation with splay fault permeability are consistent with previous work showing that increased splay fault permeability results in greater capture of fluids at depth that would otherwise be channeled along the décollement or seep out of the system diffusely [ Lauer and Saffer , ], and with the idea that higher permeability should lead to smaller head losses along the conduit. The translation and trapping of pressure along permeable structures or stratigraphic conduits has also been documented in other environments, and is known as the “centroid effect” [e.g., Dugan and Flemings , ; Flemings et al ., ].…”

“…The results are consistent with observations at fluid seeps and regions of focused flow on the upper slope [e.g., Hensen et al ., ; Ranero et al ., ; Lauer and Saffer , ]. Splay faults effectively tap into the zone of peak freshening, especially where the hydraulic impedance is less than that along the décollement to the trench.…”

“…To explore the role of splay faults in partitioning of deeply sourced fluids and translating fluid pressures, we include seven faults that connect the décollement to the seafloor and cut across the upper plate [ Lauer and Saffer , ] (Figure ). Based on seismic reflection surveys of the margin [ Ranero et al ., ; Shipley et al ., ], we specify that the splay faults dip 30° landward and are spaced 5 km apart.…”

Coupled Evolution of Deformation, Pore Fluid Pressure, and Fluid Flow in Shallow Subduction Forearcs

“…Deformation and faulting, in turn, influence fault and sediment permeability—and therefore affect drainage and pore pressure—as well as the evolution of stress and mechanical loading. For example, interconnected fractures near fault zones form effective pathways that accommodate volatile fluxes and focus fluid seepage (drainage), enhancing the rates of consolidation and dewatering at depth (e.g., Lauer & Saffer, 2012, 2015) and potentially affecting megathrust slip behavior within the seismogenic and tremor zones (Halpaap et al, 2019; Ranero et al, 2008; Wells et al, 2017). In this study, we systematically investigate these processes and their fundamental feedbacks, using a numerical approach that fully couples mechanical loading, deformation, and fluid drainage.…”

“…Some studies suggest the likely widespread presence of fluid overpressure at many subduction margins (e.g., Gamage & Screaton, 2006; Saffer & Bekins, 1998; Screaton et al, 1990), as an outcome of rapid mechanical loading that outpaces drainage, together with fluid release from low‐temperature diagenetic and metamorphic reactions (Saffer & Tobin, 2011). Other studies have highlighted the role of interplate and intraplate faults as permeable conduits in governing distributions of fluid pressure and surface seepage (e.g., Ellis et al, 2015; Lauer & Saffer, 2012, 2015). These existing studies typically did not fully couple mechanical and hydrological processes; instead, they either prescribed a steady‐state porosity field and fixed sediment flux trajectories (e.g., Bekins & Dreiss, 1992; Saffer & Bekins, 1998; Screaton et al, 1990; Wang, 1994) or used a simplified representation of in situ stress state to approximately account for mechanical loading (e.g., Screaton & Saffer, 2005; Shi & Wang, 1988; Skarbek & Saffer, 2009; Stauffer & Bekins, 2001) (see Text S1 in the supporting information for a summary of previous modeling studies).…”