Fluid pressure and shear zone development over the locked to slow slip region in Cascadia

Characterizing the porosity structure and gas hydrate distribution at the southern Hikurangi Margin, New Zealand from offshore electromagnetic data

The dynamics of accretionary prisms and the processes that take place along subduction interfaces are controlled, in part, by the porosity and fluid overpressure of both the forearc wedge and the sediments transported to the system by the subducting plate. The Hikurangi Margin, located offshore the North Island of New Zealand, is a particularly relevant area to investigate the interplay between the consolidation state of incoming plate sediments, dewatering and fluid flow in the accretionary wedge and observed geodetic coupling and megathrust slip behaviour along the plate interface. In its short geographic extent, the margin hosts a diversity of properties that impact subduction processes and that transition from north to south. Its southernmost limit is characterized by frontal accretion, thick sediment subduction, the absence of seafloor roughness, strong interseismic coupling and deep slow slip events. Here we use seafloor magnetotelluric (MT) and controlled-source electromagnetic (CSEM) data collected along a profile through the southern Hikurangi Margin to image the electrical resistivity of the forearc and incoming plate. Resistive anomalies in the shallow forearc likely indicate the presence of gas hydrates, and we relate deeper forerarc resistors to thrust faulting imaged in colocated seismic reflection data. Because MT and CSEM data are highly sensitive to fluid phases in the pore spaces of seafloor sediments and oceanic crust, we convert resistivity to porosity to obtain a representation of fluid distribution along the profile. We show that porosity predicted by the resistivity data can be well fit by an exponential sediment compaction model. By removing this compaction trend from the porosity model, we are able to evaluate the second-order, lateral changes in porosity, an approach that can be applied to EM data sets from other sedimentary basins. Using this porosity anomaly model, we examine the consolidation state of the incoming plate and accretionary wedge sediments. A decrease in porosity observed in the sediments approaching the trench suggests that a protothrust zone is developing ∼25 km seaward of the frontal thrust. Our data also imply that sediments deeper in the accretionary wedge are slightly underconsolidated, which may indicate incomplete drainage and elevated fluid overpressures of the deep wedge.

Constraints From Exhumed Rocks on the Seismic Signature of the Deep Subduction Interface

Low Velocity Zones (LVZs) with anomalously high V _p-V _s ratios occur along the downdip extents of subduction megathrusts in most modern subduction zones and are collocated with complex seismic and transient deformation patterns. LVZs are attributed to high pore fluid pressures, but the spatial correlation between the LVZ and the subduction interface, as well as the rock types that define them, remain unclear. We characterize the seismic signature of a fossil subduction interface shear zone in northern California that is sourced from the same depth range as modern LVZs. Deformation was distributed across 3 km of dominantly metasedimentary rocks, with periodic strain localization to km-scale ultramafic lenses. We estimate seismic velocities accounting for mineral and fracture anisotropy, constrained by microstructural observations and field measurements, resulting in a V _p/V_s of 2.0. Comparable thicknesses and velocities suggest that LVZs represent, at least in part, the subduction interface shear zone.

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.

Excitation of San Andreas tremors by thermal instabilities below the seismogenic zone

The relative motion of tectonic plates is accommodated at boundary faults through slow and fast ruptures that encompass a wide range of source properties. Near the Parkfield segment of the San Andreas fault, low-frequency earthquakes and slow-slip events take place deeper than most seismicity, at temperature conditions typically associated with stable sliding. However, laboratory experiments indicate that the strength of granitic gouge decreases with increasing temperature above 350°C, providing a possible mechanism for weakening if temperature is to vary dynamically. Here, we argue that recurring low-frequency earthquakes and slow-slip transients at these depths may arise because of shear heating and the temperature dependence of frictional resistance. Recurring thermal instabilities can explain the recurrence pattern of the mid-crustal low-frequency earthquakes and their correlative slip distribution. Shear heating associated with slow slip is sufficient to generate pseudotachylyte veins in host rocks even when fault slip is dominantly aseismic.