YOUNG SCIENTIST AWARD (DONATH MEDAL): METAMORPHIC HETEROGENEITY AND TRANSIENT RHEOLOGY OF THE DEEP SUBDUCTION INTERFACE

We investigate the rheological properties of a deep subduction interface using exhumed metabasites exposed at Kini beach on Syros Island, Greece. Syros rocks were subducted to ~60 km in the Eocene, were exhumed part way along the top of the slab, and were ultimately exhumed by mid-crustal detachments in the mid-Miocene.

Blueschists at Kini exhibit a prominent foliation oriented parallel to isoclinal recumbent folds. Si-in-phengite and inclusion barometry indicate this fabric formed at ~16 kb. Local upright folds crenulate this fabric and realign minerals— mineral zonations suggest slightly higher P for realigned phases, consistent with progressive deformation under prograde conditions. Eclogites at Kini are identical in bulk composition to blueschists, indicating they are derived from the same protolith, but inclusion barometry suggests they formed at lower pressures (~12 kb). The eclogites exhibit a wide range of deformational styles, occurring as discontinuous pods and variably attenuated lenses within the blueschists. Some form rigid boudins up to m-scale— several exhibit a weak foliation that is at high angles to surrounding blueschist, whereas others are massive and show shear fractures and veins filled with high-P phases. Other eclogites show semi-ductile behavior, exhibiting pinch-and-swell structures that are ductile on their margins but brittle internally. Some appear entirely ductile at the outcrop scale and are folded tightly at similar wavelengths to surrounding blueschist, suggesting isoviscous behavior. At the microscale, both blueschists and ductile eclogites exhibit strong CPOs and evidence for dislocation creep.

These relationships highlight substantial complexity of the subduction interface that is driven by metamorphic disequilibrium within a single oceanic protolith. The observed heterogeneity may scale up to affect both the transient and steady-state rheology of the deep subduction zone. Mixed-mode brittle-viscous behavior among eclogites and blueschists could be a mechanism for deep episodic tremor and slow-slip; and isoviscous ductile flow among blueschists and eclogites, each deforming by dislocation creep, suggests that the bulk rheology is non-Newtonian and that it is a composite function of the relative percentages of blueschist vs. eclogite-facies material.