Consistent detachment of supracrustal rocks from a fixed subduction depth in the Cyclades

Abstract

While subduction zones recycle Earth's crust back into the mantle, high pressure metamorphic rocks exposed at the surface testify to occasional crustal subduction failure. Balancing the budget of Earth's recycling factory requires a fundamental comprehension of how and when some crustal rocks are returned to the surface. Here, we constrain the timing and conditions for rocks that were subducted, exhumed and now exposed on Syros and Sifnos islands (Cyclades, Greece). Detailed petrochronology on domains of supracrustal oceanic, passive margin and continental provenance shows that progressive subduction and exhumation has resulted in imbrication of a former continental margin. The consistency of age groups in 2–4 Myr intervals across two islands suggests episodic triggering of exhumation of coherent slices, similar to nappe stacking in the upper levels of orogens. Similar maximum burial conditions across the domains indicate that their highly variable material properties have not exerted a control on the depth from which they were recovered, and supracrustal exhumation was instead triggered by a steady state mechanism operating at ∼65–70km depth in the subduction zone. We propose that for mechanically strong rocks, consistent exhumation may have been triggered by thermal weakening at the depth of coupling between the subducting slab and mantle wedge corner flow in an extended back-arc. For weakly coupled supracrustal rocks, consistent exhumation may have been triggered by a stepped buoyancy increase for subducting rocks at the crust mantle boundary of a thickened overlying plate. These models help to understand the mechanisms and conditions by which supracrustal rocks may be systematically detached from a downgoing plate.