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Episodic tremor and slow slip (ETS) downdip of the subduction seismogenic zone are poorly understood slip behaviors of the seismic cycle. Talc, a common metasomatic mineral at the subduction interface, is suggested to host slow slip but this hypothesis has not been tested in the rock record. We investigate actinolite microstructures from talc‐bearing and talc‐free rocks exhumed from the depths of modern ETS (Pimu'nga/Santa Catalina Island, California). Actinolite deformed by dissolution‐reprecipitation creep in the talc‐free rock and dislocation creep ± cataclasis in the talc‐bearing rock. This contrast results from stress amplification in the talc‐bearing rock produced by high strain rates in surrounding weak talc. We hypothesize that higher strain rates in the talc‐bearing sample represent episodic slow slip, while lower strain rates in the talc‐free sample represent intervening aseismic creep. This work highlights the need to consider fluid‐mediated chemical change in studies of subduction zone deformation and seismicity.
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This content will become publicly available on August 1, 2026
Fluid-mediated deformation leads to weakening, strengthening, and block-in-matrix structures during prograde subduction mélange formation
Mélange (or block-in-matrix structures) exerts a first-order control on both the mechanical and chemical evolution of subduction megathrusts. However, the timing and mechanisms that form mélanges are variable and debated. Field observations and (micro-) structural analyses from a metasedimentary mélange in the lawsonite blueschist unit of the Catalina Schist (Santa Catalina Island, California, USA) reveal that syn-subduction deformation and fluid-mediated processes led to mélange formation at the plate interface. Deposited as turbidites, early shear occurred parallel to bedding planes (S1 foliation). At near peak subduction conditions, at the base of the subduction seismogenic zone (∼1.0 GPa, 320 °C), the rocks were intensely deformed in recumbent open to tight folds (F2) with axial planar cleavages (S2). Fracturing, fluid flow, and quartz precipitation are preserved as extensional vein mesh networks in fold noses. Continued shearing led to boudinage of these strengthened noses and transformation into strong blocks within the weaker less-veined matrix composed of high-strain fold limbs (S1−2). Microstructures reveal viscous deformation in the high-strain fold limbs occurred by pressure-solution creep of fine-grained quartz ± albite. In contrast, the fold noses and/or blocks contain coarse-grained quartz veins with little evidence of deformation. These rocks record the development of syn-subduction block-in-matrix mélange structures through the interaction of deformation and mineral precipitation; pressure solution weakened fold limbs-turned-matrix and veining strengthened fold noses-turned-blocks. Although mélange structure is often invoked to explain tremor and slow slip, rheological analysis indicates that these metasedimentary rocks can host tectonic creeping but cannot accommodate slow-slip strain rates by the deformation mechanisms preserved in their microstructures.
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- PAR ID:
- 10633312
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geology
- ISSN:
- 0091-7613
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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