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Microstructural analysis of exhumed epidote-amphibolites and plate interface rheology in warm subduction zones
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  • Laurens Hendrik Kleijbeuker,
  • Hamed Amiri,
  • Maartje Hamers,
  • Alissa Jeanne Kotowski
Laurens Hendrik Kleijbeuker
Universiteit Utrecht
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Hamed Amiri
Utrecht University
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Maartje Hamers
Utrecht University
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Alissa Jeanne Kotowski
Universiteit Utrecht

Corresponding Author:[email protected]

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Abstract

Epidote-amphibolites form along the plate interface during subduction infancy, and are stable in warm, mature subduction zones that generate slow earthquakes. Epidote-amphibolite rheology therefore likely influences plate-scale processes facilitating plate boundary formation, and grain-scale processes generating slip transients. We present optical and electron microscopy of naturally-deformed epidote-amphibolites from beneath the Oman ophiolite (~7–10 kbar, 400–550 °C) to characterize their deformation behavior. Epidote-amphibolites are fine-grained, strongly foliated and lineated, and exhibit polyphase fabrics in which amphiboles (~10–50 μm) and epidotes (~5–20 μm) are strain-accommodating phases. Two-point correlation connectivity analysis demonstrates that amphiboles are always well-connected, regardless of phase proportions/distributions. Electron Backscatter Diffraction reveals strong amphibole Crystallographic and Shape Preferred Orientations (CPOs and SPOs), subgrain geometries indicating (hk0)[001] slip, and high average Mean Orientation Spreads (MOS; ~6°), interpreted as coupled rigid rotation and dislocation glide. Epidotes, in contrast, record weak CPOs, low intragranular misorientations, moderate SPOs, and low MOS (~0–2°), interpreted as deformation by dissolution-precipitation creep. Depending on phase distributions, epidote-amphibolite rheology can be approximated as interconnected weak layers of amphibole dislocation glide, or a composite rheology of plasticity and fluid-assisted/diffusion-accommodated creep. We estimate strain rates from geologic and geochronological data (6 · 10-11 to 10-12 s-1), stress from quartz piezometry (11 – 45 MPa), and equivalent viscosities of 1016 – 1018 Pa-s. On tectonic timescales, such low viscosities are consistent with epidote-amphibolites serving as strain localizing agents during subduction infancy. On seismic timescales, coupled glide- and diffusion exemplify a strain-hardening deformation state that could culminate in creep transients.
13 Sep 2024Submitted to ESS Open Archive
17 Sep 2024Published in ESS Open Archive