Slab reequilibration and slab roll back timing in the Cyclades: Evidence from garnet diffusion and P-T estimates (Naxos, Greece)

Abstract

Understanding the mechanisms of mountain building and its destruction poses significant challenges. Following subduction of a slab, an increase in the subduction rate and/or a decrease of the convergence rate, may lead to slab rollback. During slab rollback, segments of the middle-lower crust may undergo heating and partial melting due heating of thinned lithosphere by the asthenosphere and the migration of the magmatic arc. Alternatively, after accretion of radiogenic crust, the lower parts of the crust may relax through radiogenic decay. In both scenarios, the heating of the middle-lower crust reduces crustal strength, resulting in the development of metamorphic core complexes. The timing of lower crust heating is crucial for understanding the switchover from lithospheric shortening to extension. The Hellenides in the eastern Mediterranean constitute an arcuate orogen located north of the present-day active Hellenic margin, marking the site of NNE-ward subduction of the African plate beneath Eurasia. The Aegean Sea region in the Hellenide is a world-class example of large-scale continental extension above a retreating subduction zone. In the Cyclades, the Hellenide orogeny began in the early Cenozoic, causing subduction and sustained high-pressure (HP) metamorphism between approximately 53 and 30 Ma. The timing of slab rollback is subject of intense debate. A decrease in the convergence rate was interpreted to suggest that slab rollback initiated at around 35–30 Ma, during or even after the waning stages of HP metamorphism. In contrast, the formation of extensional sedimentary basins and radiometric dating of extension-related mylonite place rollback at approximately 23 Ma, distinctly after the final stages of HP metamorphism. Between about 30 Ma and 20 Ma, isobaric heating during the exhumation of some HP rocks has been proposed on some islands (Syros, Tinos, Andros, and Naxos). However, the precise timing and duration of this isobaric heating remains largely unconstrained. On Naxos, previous geothermobarometry estimates indicate isobaric heating occurred from 500 to 550°C from a middle segment of the nappe stack (Peillod et al., 2021). Garnet chemical zonation formed during exhumation allows for a heating timescale to be estimated via diffusion chronometry. We conducted Monte Carlo diffusion simulations to determine the best-fitting timescale based on Chi-square statistics, assuming three different heating paths. Diffusion model results for a heating path from 500 to 550°C indicate a 10 Myr timescale. A lower temperature path (from 400 to 450°C) requires heating over an unreasonable geological timescale (>100 Myr). Conversely, a higher temperature path (from 600 to 650°C) requires a timescale of <1 Myr. This higher temperature path corresponds to temperatures recorded near the migmatite core at the bottom of the Naxos nappe stack. At such temperatures and within a 10 Myr period, chemical heterogeneities in garnet would have relaxed, as observed in most garnets near the migmatite core. The results of this approach indicate that the heating of the lower crust occurs over a 10 Myr period, suggesting that the heating may result from radiogenic decay during the Oligocene before the slab roll during the Miocene.