Metamorphic thermobarometry captures a rheology-controlled subduction depth limit in exhumed high-pressure/low-temperature rocks exposed on Syros, Greece

Abstract:

Deep underplating is a prerequisite for exhumation of coherent sheets of subducted rock. The dynamics of underplating are controlled by interface geometry, geothermal gradient, and rheological properties, each of which can be difficult to quantify with coarse-resolution geophysical imaging and thermal modeling. Structural and petrologic observations of exhumed rocks can refine our understanding of deep underplating. The subduction complex exposed on Syros, Greece comprises three, ~3 km thick, underplated meta-sedimentary nappes containing ~100’s m thick lenses of blueschist-to-eclogite facies oceanic-affinity rock. Estimates of peak P-T conditions (12-22 kbar, 450-580˚C) are poorly constrained; it is unclear whether each nappe reached similar or different P-T, and what rheological controls influenced underplating depth(s).

Here we demonstrate that all three nappes on Syros subducted to similar peak P-T conditions, and as rocks approached peak P-T, rheological properties and meta-sedimentary rock viscosity allowed for nappe underplating. Garnet growth P from the quartz-in-garnet solid inclusion barometer yielded consistent estimates, ~14-16 kbar (n=191, 25 samples, 10 localities), in different nappes and rock types across the island. Titanium-in-Quartz trace element thermobarometry targeting quartz inclusion trails in prograde-zoned garnets revealed homogeneous [Ti] of 0.2-0.6 ppm for samples from all nappes (n=28, 4 samples, 4 localities) corresponding to T ~420-480˚C. Meta-sedimentary rocks that captured deformation at the subduction-to-exhumation transition record solution creep textures and distributed ductile flow through peak P-T conditions. Nappe viscosity at peak depths, which is a function of nappe thickness, rock type(s), deformation mechanism, and temperature, was sufficient for decoupling and buoyancy-driven exhumation.

Novel thermobarometry, interpreted in the nappe-stack structural context, demonstrates that the entire complex reached a subduction depth limit of ~45-55 km, albeit at different times, before exhumation. The subduction depth limit may reflect conditions at which nappes achieve favorable viscosities such that buoyancy of subducting material and shear forces exerted by the down-going slab are balanced, thus halting sliver subduction.