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

Abstract

Deformation and fluid flow in subduction zone forearcs are dynamically coupled, but our quantitative understanding of their coupling is incomplete. In this work, we investigate the hydrological and mechanical coupling in shallow forearcs, using a Lagrangian‐Eulerian finite element model that incorporates constitutive and transport properties of sediments and faults constrained by laboratory and field measurements. Wide‐ranging observations show that sediment thickness and composition, plate convergence rate, basement strength and roughness, and subducting slab dip angle vary between subduction zones. We therefore systematically study their effects on forearc stress and pore fluid pressure states, consolidation and dewatering patterns, and margin morphology. Our models, with the incorporation of a simple description of permeability enhancement along fault damage zones, yield a range of fault permeability (10−13–10−17 m2) consistent with previous estimates and describe the important role of upper plate splay faults in causing heterogeneous dewatering and consolidation patterns and in modulating effective normal stress on the plate interface. Spatial variations in tectonic loading and sediment consolidation can also be caused by subducting basement roughness such as a horst‐and‐graben structure. For typically observed relief and spacing, our models predict locally enhanced porosity reduction by up to 50% at the downdip edge of the horsts and anomalously high sediment porosity above the geometrical highs. At the margin scale, our results demonstrate that sediment permeability and thickness are dominant controls on fluid overpressure, sediment compaction, and megathrust strength. Rough and frictionally strong megathrusts produce similar effects in driving high wedge tapers.

Subduction earthquake cycles controlled by episodic fluid pressure cycling

“…This controversy poses a major challenge in our understanding of earthquake physics, with severe implications for seismic hazard. This problem is particularly challenging because the transient evolution of pore-fluid in active faults is controlled by depth-dependent variations in hydraulic properties over a broad range of timescales (Saffer and Tobin, 2011;Sun et al, 2020).…”

Crustal Structure of the Northern Hikurangi Margin, New Zealand: Variable Accretion and Overthrusting Plate Strength Influenced by Rough Subduction

“…In addition to its potential for causing tectonic erosion, subducting relief has been proposed to contribute to a full spectrum of slip rates including creep, slow‐slip transients, and fast earthquakes (Barnes et al., 2020; Bassett & Watts, 2015; Bell et al., 2014; Collot et al., 2017; Wang & Bilek, 2014). Rough topography on the incoming plate may lead to coupled deformation and fluid flow in the outer forearc (Sun, Ellis, & Saffer, 2020) by affecting compaction (Martínez‐Loriente et al., 2019; Sun, Saffer, & Ellis, 2020), fault activation (Dominguez et al., 1998; Morgan & Bangs, 2017), and hydraulic conductivity near the megathrust (Ellis et al., 2015). Volcanic fragments of subducting seamounts, may lead to greater frictional and mechanical heterogeneity along the plate boundary (Barnes et al., 2020; Fagereng & Sibson, 2010; Saffer & Wallace, 2015).…”

The Impact of Grain Size on the Hydromechanical Behavior of Mudstones

“…I characterize the geomechanical behavior of subduction zone inputs, which is critical in order to relate input sediments to those at frontal thrust regions and understand a wide range of subduction zone processes. For example, the consolidation test and model results presented here can be used to inform geomechanical and hydrogeological forward models that attempt to describe the evolution and/or distribution of pressure, deformation, and fluid flow in convergent margins (e.g., Gao et al., 2018; Sun et al., 2020). My compression results can also be used to inform models that infer in situ effective stress and dewatering processes from field porosity measurements (e.g., Saffer, 2003).…”