Transient Permeability of a Deep‐Seated Subduction Interface Shear Zone

Abstract

Slow slip and tremor is observed along many subduction margins and is commonly linked to fluid pressure variations and migration. Accurate estimates of porosity and permeability around subduction megathrust shear zones are vital for understanding fluid‐seismicity interactions. We use high‐resolution digital outcrop data and (micro)structural analysis to assess transient permeability and porosity of a deep‐seated subduction interface exposed on Syros Island, Greece. We document the orientations, relative timing, and opening aperture (based on crack‐seal textures) of veins that were emplaced synkinematically with ductile deformation during early exhumation within the subduction channel. Our findings indicate high permeability through vein‐filled fractures amidst a lower permeability matrix, with transient, fracture‐controlled permeabilities ranging from 10 ⁻¹⁴ to 10 ⁻¹⁵ m ² and fracture porosities of 1%–10%. These estimates align with low‐end values from seismological/geodetic observations in active subduction zones, and are also consistent with fault‐valve‐like numerical models that suggest high background‐to‐transient permeability contrasts favor unstable slip.

Plain Language Summary

Understanding the physical mechanisms of fast and slow slip in subduction zones requires knowledge on how rocks respond when they are squeezed, fractured and deformed (rheology). Recent numerical studies show that fluid flow through rocks (permeability) is important in controlling their slip response. Geophysical and geological observations also suggest that fluids can cause mechanical instabilities, but quantitative data from natural observations is extremely scarce. To address this gap, we developed a new methodology to quantify the permeability of these rocks from natural observations. We study in detail the geometry, distribution and microstructures of vein systems (former paleo‐fractures filled with fluids) that opened and allowed for transient fluid circulation. From these data, we quantify the paleo‐permeability associated with these former fractures and compare our results to previous investigations and explore their implications in the frame of plate interface rheology and slip.

Key Points

• We present a new methodology to estimate subduction zone paleo‐permeability

• High permeability is associated with fracture‐controlled, channelized fluid flow at the base of the forearc

• Strong permeability contrast between fracture‐controlled (transient) and matrix‐related (background) fluid flow