The mechanics of slip at the updip limit of the seismogenic zone

The world’s largest earthquakes occur within the seismogenic zones of subduction thrusts, which lock between large magnitude earthquakes. At the updip limit of seismogenic zones, a transitional zone occurs if the subduction interface is not locked to the trench. Within this transitional zone a range of slip behaviors including seismic slip events propagating to the trench, slow slip events (SSEs), tremor, low frequency events (LFEs) and very low frequency events (VLFEs) are observed using geodetic and seismological data sets. Slip in transitional zones is believed to load the seismogenic zone, and may precede large magnitude earthquakes. Understanding the mechanics of slip in transitional zones is therefore important for characterizing the earthquake cycle.Exhumed subduction zone fault rocks preserve information about the composition, structure, and behavior of subduction zones. This thesis examines deformation structures from an exhumed, shallow (T = 190°C), subduction mélange (the Mugi Mélange, Japan) to provide insights into the mechanics of deformation in shallow transitional zones. Mapping, microstructural observations, experiments, and models are used to characterize the distribution of slip surfaces, and relate the observed surfaces to geophysical observations. Localized slip in subduction zones is expected to occur in velocity-weakening materials; however, most subducting materials (gabbro, basalt, calcite, and shale) have previously been shown to be velocity-strengthening at the updip limit of seismogenic zones. Field mapping in this study of basaltic blocks and slabs (embedded within a shale matrix) demonstrates that unstable slip occurred along their altered margins, where we document well-developed cataclasites and slip surfaces. We report the first documented occurrence of pseudotachylyte (quenched frictional melt) in a basaltic host, identified through microstructural criteria along the upper altered contact of a basaltic slab. Deformation in altered basalt is shown to have occurred through cataclasis and frictional sliding, while the surrounding shale matrix deformed through distributed pressure solution accommodated processes.Triaxial friction experiments were performed on crushed natural altered basalt and shale samples from the Mugi Mélange, at the in situ conditions of deformation, to characterize their frictional strengths and rate-and-state friction parameters. The shale is frictionally weaker (μ = 0.4) than altered basalt (μ = 0.6). and exhibits velocity-strengthening (a-b = ~0.01) behavior. Altered basalt exhibits velocity weakening behavior (a-b = ~-0.005), indicating that unstable slip may nucleate and propagate along altered basaltic margins. Numerical models of stress and strain-rates around basaltic blocks embedded in a shale matrix demonstrate that the stress threshold required for frictional failure in altered basalt is reached before the shale with increasing slip-rate. Experimental results are used to calculate critical nucleation lengths for dynamic slip in basaltic blocks with altered margins. At the conditions required to produce LFEs and VLFEs, basaltic blocks on the m- to 10’s of m-scale are required to produce dynamic failure. Blocks sitting close to this threshold are hypothesized to produce LFEs and VLFEs.Altered basalt is a ubiquitous lithology in subduction zones, and may provide a source of velocity-weakening material along which earthquakes can preferentially nucleate and propagate. Incorporation of velocity weakening altered basalt into a velocity strengthening matrix may provide a source for LFEs and VLFEs at near lithostatic pore fluid pressure conditions. Incorporation of blocks into the surrounding matrix may occur more readily around subducted seamounts and ridges

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