Hydrous oceanic crust hosts megathrust creep at low shear stresses

Rheology of Naturally Deformed Antigorite Serpentinite: Strain and Strain-Rate Dependence at Mantle-Wedge Conditions

Antigorite serpentinite is expected to occur in parts of subduction plate boundaries, and may suppress earthquake slip, but the dominant deformation mechanisms and resultant rheology of antigorite are unclear. An exhumed plate boundary shear zone exposed near Nagasaki, Japan, contains antigorite deformed at 474°C ± 30°C. Observations indicate that a foliation defined by (001) crystal facets developed during plate-boundary shear. Microstructures indicating grain-scale dissolution at high-stress interfaces and precipitation in low-stress regions suggest that dissolution-precipitation creep contributed to foliation development. Analysis of crystal orientations indicate a small contribution from dislocation activity. We suggest a frictional-viscous rheology for antigorite, where dissolution-precipitation produces a foliation defined by (001) crystal facets and acts to resolve strain incompatibilities, allowing for efficient face-to-face sliding between facets. This rheology can not only explain aseismic behavior at ambient plate boundary conditions, but also some of the contrasting behaviors shown by previous field and laboratory studies.

Is complex fault zone behaviour a reflection of rheological heterogeneity?

Fault slip speeds range from steady plate boundary creep through to earthquake slip. Geological descriptions of faults range from localized displacement on one or more discrete planes, through to distributed shearing flow in tabular zones of finite thickness, indicating a large range of possible strain rates in natural faults. We review geological observations and analyse numerical models of two-phase shear zones to discuss the degree and distribution of fault zone heterogeneity and effects on active fault slip style. There must be certain conditions that produce earthquakes, creep and slip at intermediate velocities. Because intermediate slip styles occur over large ranges in temperature, the controlling conditions must be effects of fault properties and/or other dynamic variables. We suggest that the ratio of bulk driving stress to frictional yield strength, and viscosity contrasts within the fault zone, are critical factors. While earthquake nucleation requires the frictional yield to be reached, steady viscous flow requires conditions far from the frictional yield. Intermediate slip speeds may arise when driving stress is sufficient to nucleate local frictional failure by stress amplification, or local frictional yield is lowered by fluid pressure, but such failure is spatially limited by surrounding shear zone stress heterogeneity. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.

What's down there? The structures, materials and environment of deep-seated slow slip and tremor

Deep-seated slow slip and tremor (SST), including slow slip events, episodic tremor and slip, and low-frequency earthquakes, occur downdip of the seismogenic zone of numerous subduction megathrusts and plate boundary strike-slip faults. These events represent a fascinating and perplexing mode of fault failure that has greatly broadened our view of earthquake dynamics. In this contribution, we review constraints on SST deformation processes from both geophysical observations of active subduction zones and geological observations of exhumed field analogues. We first provide an overview of what has been learned about the environment, kinematics and dynamics of SST from geodetic and seismologic data. We then describe the materials, deformation mechanisms, and metamorphic and fluid pressure conditions that characterize exhumed rocks from SST source depths. Both the geophysical and geological records strongly suggest the importance of a fluid-rich and high fluid pressure habitat for the SST source region. Additionally, transient deformation features preserved in the rock record, involving combined frictional-viscous shear in regions of mixed lithology and near-lithostatic fluid pressures, may scale with the tremor component of SST. While several open questions remain, it is clear that improved constraints on the materials, environment, structure, and conditions of the plate interface from geophysical imaging and geologic observations will enhance model representations of the boundary conditions and geometry of the SST deformation process. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.

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

The architecture and mechanical properties of the subduction interface impact large-scale subduction processes, including mass and volatile recycling, upper-plate orogenesis, and seismic behavior. The nature of the deep subduction interface, where a dominantly frictional megathrust likely transitions to a distributed ductile shear zone, is poorly understood, due to a lack of constraints on rock types, strain distribution, and interface thickness in this depth range. We characterized these factors in the Condrey Mountain Schist, a Late Jurassic to Early Cretaceous subduction complex in northern California that consists of an upper and lower unit. The Lower Condrey unit is predominantly pelagic and hemipelagic metasediment with m-to km-scale metamafic and metaserpentinitic ultramafic lenses all deformed at epidote blueschist facies (0.7-1.1 GPa, 450°C). Major and trace element geochemistry suggest tectonic erosion of the overriding plate sourced all ultramafic and some mafic lenses. We identified two major ductile thrust zones responsible for Lower Condrey unit assembly, with earlier strain distributed across the structural thickness between the ductile thrusts. The Lower Condrey unit records distributed deformation across a sediment-dominated, 2+ km thick shear zone, possibly consistent with low velocity zones observed in modern subduction zones, despite subducting along a sediment poor, tectonically erosive margin. Periodic strain localization occurred when rheological heterogeneities (i.e., km-scale ultramafic lenses) entered the interface, facilitating underplating that preserved 10%-60% of the incoming sediment. Modern mass and volatile budgets do not account for erosive margin underplating, so improved quantification is crucial for predicting mass and volatile net flux to Earth's interior.