Structural petrology and petrochronology record subduction, underplating, and return flow in the Cycladic Blueschist Unit exposed on Syros Island, Greece

Abstract:

Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide fundamental insight into the dynamics of the deep (~20-70 km) subduction interface, as well as rates and mechanisms of HP rock exhumation. On Syros Island (Cyclades, Greece), an exhumed Eocene subduction complex -- the Cycladic Blueschist Unit (CBU) -- is a spectacularly preserved and exposed sequence of blueschist-to-eclogite facies oceanic- and continental-affinity rocks that are deformed during subduction and exhumation, providing a rare snapshot of the structural development, thermal evolution, and interface geometry and accretion dynamics. Despite decades of research on Syros, the Pressure-Temperature-Deformation evolution, and the timing of subduction vs. exhumation, are poorly constrained.

Here, we show that the CBU is a structurally coherent, 3-part nappe stack, that records protracted subduction, underplating, and return flow in a subduction channel configuration. At the subduction-to-exhumation transition, deformation kinematics and ductile fold axes of each nappe rotated from top-to-the-SSW (prograde-to-peak subduction) to top-to-the-NE (blueschist-facies exhumation) and then E-W, with coaxial stretching (greenschist-facies exhumation). Mineral replacement textures and dynamic amphibole zonations developed during exhumation as rocks cooled during decompression, indicating return flow along the top of a cold subducting slab. New multi-mineral Rb-Sr isochron petrochronology, supplemented by existing metamorphic geochronology, demonstrates for the first time that Syros nappes recorded multiple distinct stages of prograde subduction (~53 Ma, 50? Ma, and ~47 Ma), younging with structural depth, and capturing underplating of three lithologically distinct but genetically-related (with respect to paleogeographic context), nappe slices. Each nappe records similar PTD histories, despite subducting at different times. Subduction and exhumation occurred contemporaneously, in a planar interface geometry (i.e. non-downward tapering). Distributed, ductile, coaxial return flow in the Eocene subduction channel occurred at rates of ~1.5-5 mm/yr (an order of magnitude slower than subduction), and accommodated ~80% of the vertical exhumation of this HP/LT complex.