Hydrous oceanic crust hosts megathrust creep at low shear stresses

Abstract

The rheology of the metamorphosed oceanic crust may be a critical control on megathrust strength and deformation style. However, little is known about the strength and deformation style of metamorphosed basalt. Exhumed megathrust shear zones exposed on Kyushu, SW Japan, contain hydrous metabasalts deformed at temperatures between ~300° and ~500°C, spanning the inferred temperature-controlled seismic-aseismic transition. Field and microstructural observations of these shear zones, combined with quartz grain-size piezometry, indicate that metabasalts creep at shear stresses <100 MPa at ~370°C and at shear stresses <30 MPa at ~500°C. These values are much lower than those suggested by viscous flow laws for basalt. The implication is that relatively weak, hydrous, metamorphosed oceanic crust can creep at low viscosities over a wide shear zone and have a critical influence on plate interface strength and deformation style around the seismic-aseismic transition.

Tracking Deep Sediment Underplating in a Fossil Subduction Margin: Implications for Interface Rheology and Mass and Volatile Recycling

“…Meter‐ to decameter‐scale mafic and ultramafic lenses (Figure 3) within this dominantly pelagic to hemipelagic metasedimentary shear zone have comparable rheology and do not perturb distributed deformation. Evidence from other field areas similarly supports a low rheologic contrast between epidote blueschists and mica schists (e.g., Kotowski & Behr, 2019; Tulley et al., 2020). Localized serpentinite shear zone .…”

“…formation by pressure solution, a linear-viscous mechanism (e.g., Passchier & Trouw, 2005;Den Brok, 1998). Meter-to decameter-scale mafic and ultramafic lenses ( and mica schists (e.g., Kotowski & Behr, 2019;Tulley et al, 2020).…”

Deep decoupling in subduction zones: Observations and temperature limits

“…Earthquakes rupture at seismogenic depths to account for plate motion in some but not all subduction zones (Scholz and Campos, 2012;Wang and Bilek, 2014), and slow-slip phenomena demonstrate that this localized deformation persists to depths of 40-80 km (e.g., Lay et al, 2012;Bürgmann, 2018). Geological observations of high-pressure metamorphic rocks document narrow shear zones along the plate interface to 2.8 GPa or ~80 km depth, exhibiting a variety of localized mechanisms of brittle and viscous deformation (Agard et al, 2018;Tulley et al, 2020). In this region, the overlying upper-plate mantle does not participate in large-scale mantle flow, forming a cold and highly viscous "nose" of stagnant mantle (e.g., Kincaid and Sacks, 1997;Stachnik et al, 2004;Abers et al, 2006).…”

Inferring Interseismic Coupling Along the Lesser Antilles Arc: A Bayesian Approach

“…A negative correlation between interplate coupling and high V p / V s ratios has indeed been observed before (Moreno et al., 2014), as well as a positive correlation between the amount of subducting fluids and the occurrence of intermediate‐depth earthquakes (Faccenda et al., 2012; Hacker et al., 2003). A hydrated oceanic crust has also been associated with creep along the deeper parts of the subduction interface (i.e., in the 370°–450° temperature range), because a weak phyllosicilate‐bearing mineralogy may allow the crust to creep at shear stresses low enough to accommodate significant plate interface displacement (Tulley et al., 2020). The subduction of fluid‐rich slow‐spread lithosphere is therefore an important candidate to explain the low coupling of the Lesser Antilles subduction inferred from GPS observations.…”