Rheology of Metasedimentary Rocks at the Base of the Subduction Seismogenic Zone

Abstract

Subduction plate boundary faults are seismogenic and host megathrust earthquakes to depths of 30-50 km, beyond which they slip aseismically (Heuret et al., 2011;Hyndman et al., 1997). The depth of the transition from seismogenic frictional slip to continuous viscous deformation influences societal hazards by controlling how far downdip earthquakes initiate and propagate, and therefore, how far landward ground shaking might occur (e.g., Hyndman, 2013). Because viscous deformation at the base of the seismogenic zone controls the state of stress and loads the seismogenic zone above it, constraining the rheology of this portion of the plate interface is an integral part of understanding deformation cycles at subduction plate boundaries. Modeling work has suggested that

Geologic Evidence of Lithostatic Pore Fluid Pressures at the Base of the Subduction Seismogenic Zone

“…We study the orientations and (micro)structures of veins within a metapelitic schist along the subduction interface and approximately 50 m structurally below the contact with the overriding Austroalpine plate (Figure 1b). This work is complemented by our previous constraints on the rheology of the viscous deformation in the schist and the chemistry of both the schist and veins (Condit et al, 2022). The metapelitic schist contains a penetrative foliation that is crosscut by multiple generations of crack-seal veins (Figure 2).…”

“…In our previous work, we used micro‐X‐ray fluorescence maps of veins and the surrounding schist to show that vein filling silica was not sourced by diffusion from the adjacent wall rock, but rather through fluid advection (Condit et al., 2022). This is supported by our measurements of triple oxygen isotopes of quartz within all veins types which indicate that silica‐precipitating fluids were sourced from dehydration of oceanic lithosphere (fluid δ 18 Ο value of 6.7–9.9 ‰ and Δ 17 Ο value if −0.058‰ to −0.072‰ (Condit et al., 2022). Thus, these veins formed from precipitation of silica delivered by water from outside the metapelite outcrop itself over many cyclic fracturing and precipitation events.…”

“…Using microstructures, rheologic flow laws, and phase mixing models, Condit et al. (2022) showed that deformation and foliation development of the schist occurred at differential stresses between ∼17 and 35 MPa corresponding to equivalent shear stresses of 10–20 MPa (Figure 4a). Our documentation of dislocation‐accommodated deformation within quartz‐dominated veins is consistent with the rheological estimates for quartz at these geologic conditions (Condit et al., 2022).…”

“…However, Condit et al. (2022) showed that by ∼30–35 km depth and 300–430°C, viscous deformation of the metasedimentary schists is sufficient to account for all aseismic deformation at a shear stress of <20 MPa. This is very similar to the stress estimated from exhumed subduction interfaces (Behr & Platt, 2013; Platt et al., 2018; Schmidt & Platt, 2022) and is near the lower bound of stress estimated for the base of the seismogenic zone using heat flow constraints (England, 2018).…”

“…The foliation strikes NE‐SW and dips to the NW, sub‐parallel to the plate boundary contact (Figures 1b, 2a and Table S1 in Supporting Information ). The foliation is defined by mm‐scale compositional layering of phengite with fine grained albite and quartz, actinolite, minor epidote, and late stilpnomelane, and is interpreted to record deformation by pressure‐solution creep of albite and quartz at shear stresses of <10–20 MPa and tectonic creep or slower strain rates (Condit et al., 2022). However, these deformation mechanisms could not facilitate slow slip deformation rates at such low stresses.…”