Microstructural analysis of epidote-amphibolites and plate interface rheology in warm subduction zones

Abstract

A primary factor driving tectonic plate movement is the subduction of oceanic plates into the lithosphere. To initiate a subduction zone, a shear zone between the descending oceanic crust and overriding plate mantle should form, which requires substantial mechanical weakening for strain localization to occur. The metamorphic sole is the ideal place to study subduction initiation, in which the High-Temperature (HT) metamorphic sole records distinct time-temperature conditions from the Low-Temperature (LT) metamorphic sole. Geochronology points out that the HT sole forms before and the LT sole forms during and immediately after subduction becomes self-sustaining. However, the grain-scale deformation mechanisms and rheology of the structurally lower sole unit are poorly constrained, which hinders predictive insight into what deformation-metamorphic processes lead to strain localization during subduction infancy. Here, we apply optical and electron microscopy to naturally-deformed epidote-amphibolites from the Semail Ophiolite (Oman) to infer their rheological behaviors during prograde subduction to P-T conditions of ~7 – 10 kbar, 400 – 550 °C. Epidote-amphibolites are very fine-grained, strongly foliated, and lineated, and commonly exhibit polyphase fabrics in which amphiboles (~10 – 50 μm), and epidotes (~5 – 20 μm) are key strain-accommodating phases. We define three end-member fabric types defined by the relative proportion of strain-accommodating phases, amphibole-rich, epidote-rich, and mixed. Two-point correlation connectivity analysis demonstrates that amphiboles are always interconnected regardless of fabric type, while epidotes are less well-connected in epidote-rich fabrics, but non-connected in mixed-phase fabrics. Electron Backscatter Diffraction reveals Crystallographic Preferred Orientations (CPOs) in amphiboles, high intragranular misorientation angles with subgrain and tilt walls indicative of the easy slip system (hk0)[001], strong Shape Preferred Orientations (SPOs), and High Mean Orientations Spreads (MOS) (~0 – 6°), interpreted as evidence for coupled rigid rotation and dislocation glide. Epidotes, in contrast, record weak CPOs defined by [010] directions aligned with the lineation, low intragranular misorientation angles, moderate SPOs, small grain sizes (~5 – 20 μm), and low MOS (~0 – 2°), which we interpret as deformation by dissolution-precipitation creep. Our observations demonstrate that depending on the phase distributions, bulk epidote-amphibolite rheology could be approximated either as interconnected weak layers defined by amphibole dislocation glide or low-temperature plasticity, or a composite rheology defined by equivalent contribution from plasticity and fluid-assisted, diffusion-accommodated creep. We estimate strain rates from geologic and geochronological data (6 · 10-11 to 10-12 s-1) and stress estimates from quartz piezometry (11 – 75 MPa), which we use to calculate viscosities in the order of 1016 – 1018 Pa s. On long-term tectonic timescales (~Myrs), such low viscosities are consistent with these rocks serving as key strain localizing agents during subduction infancy and are one of the parameters that are consistent with the dynamics transition towards self-sustaining subduction. On shorter, seismic to decadal timescales, coupled glide- and diffusional deformation of epidote-amphibolites provide a grain-scale strain-hardening deformation that could lead to cyclic/recurring creep transients in warm subduction zones.