Quantitative Microstructural Analysis of Exhumed Epidote-Amphibolites and Plate Interface Rheology in Warm Subduction Zones

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 during plate boundary formation and grain-scale processes that give rise to 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 (grain size ∼10–50 μm) and epidotes (grain size ∼5–20 μm) are strain-accommodating phases. Two-point correlation connectivity analysis demonstrates that amphiboles are well-connected regardless of phase proportions/distributions. Chemical analysis and electron backscatter diffraction reveals amphibole syn-kinematic metamorphic zonations, strong crystallographic and shape - preferred orientations (CPOs and SPOs), subgrain geometries indicating (hk0)[001] slip, and high average Grain Orientation Spreads (GOS; ∼6°), interpreted as coupled dissolution-precipitation creep (DPC) and dislocation glide. Epidotes record weak CPOs, low intragranular misorientations, moderate SPOs, and low GOS (∼0–2°), interpreted as deformation by DPC. Depending on phase distributions, epidote-amphibolite rheology can be approximated as interconnected weak layers of amphibole dissolution creep or a composite rheology of plasticity and fluid-assisted/diffusion-accommodated creep. We estimate stress from quartz piezometry (∼30–45 MPa) and strain rates from flow laws and geologic data (6 · 10−11 to 10−13 s−1), and calculate equivalent viscosities of <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 dislocation glide and diffusion creep exemplify a strain-hardening deformation state that could culminate in creep transients.