Setting the Sequence of Slicing Events Along Deep Subduction Interfaces: 2. P-T Conditions and Timing of Accretion and Exhumation in Western Crete (Hellenic Margin)

Abstract

To understand basal-accretion dynamics in subduction zones and forearc crustal response, it is crucial to constrain the timing of slicing events forming high pressure-low temperature accretionary duplexes. This second contribution investigates the pressure-temperature-time history of tectono-metamorphic units in the paleo-duplex of western Crete, accreted along the Hellenic subduction zone during the late Oligocene-Miocene. Petrological characterization, thermodynamic modeling and a data review reveal peak metamorphic conditions evolving from 17-18 kbar and 410-430 °C to 7-8.5 kbar and 310-360 °C from top to base of the nappe stack. These results suggest a decrease in basal-accretion depth from 55-60 km to 25-30 km, likely linked to an increase in the subduction-related geothermal gradient. New Rb/Sr multi-mineral ages show a consistent decrease toward the base of the duplex, except for the lowermost Plattenkalk Unit. These ages, along with the down-stepping of peak conditions, reveal two slicing episodes between ~26 Ma and ~15 Ma, and likely three additional events from the late Oligocene to middle Miocene. (U-Th-Sm)/He thermochronology on zircon indicates rapid exhumation during the middle Miocene, with rates of ~3-11 mm/yr, decreasing to ~2-4 mm/yr at shallow levels. This dynamics was driven by the accelerating southward retreat of the Hellenic subduction, enhanced by slab tearing from ~15 Ma, contributing to the geothermal gradient increase. This study further suggests a sequence of ~2-3-Myr-long deep slicing events, providing a critical timescale for monitoring the tectonic and topographic signatures of deep mass fluxes along active margins worldwide. It also supports ongoing basal-accretion events beneath Crete, contributing to the island’s emergence.