Accretion-controlled forearc deformation pulses recorded by high-pressure paleo-accretionary wedges: the example of the Hellenic subduction zone

<p>Subduction margins are the loci of a wide range of deformation processes occurring at different timescales along the plate interface and in the overriding forearc crust. Whereas long-term deformation is usually considered as stable over Myr-long periods, this vision is challenged by an increasing number of observations suggesting a long-term pulsing evolution of active margins. To appraise this emerging view of a highly dynamic subduction system and identify the driving mechanisms, detailed studies on high pressure-low temperature (HP-LT) exhumed accretionary complexes are crucial as they open a window on the deformation history affecting the whole forearc region.</p><p>In this study, we combine structural and petrological observations, Raman spectroscopy on carbonaceous material, Rb/Sr multi-mineral geochronology and thermo-mechanical numerical models to unravel with an unprecedented resolution the tectono-metamorphic evolution of the Late-Cenozoic HP-LT nappe stack cropping out in western Crete (Hellenic subduction zone). A consistent decrease of peak temperatures and deformation ages toward the base of the nappe pile allows us to identify a minimum of three basal accretion episodes between ca. 24 Ma and ca. 15 Ma. On the basis of structural evidences and pressure-temperature-time-strain predictions from numerical modeling, we argue that each of these mass-flux events triggered a pulse in the strain rate, sometimes associated with a switch of the stress regime (i.e., compressional/extensional). Such accretion-controlled transient deformation episodes last at most ca. 1-2 Myr and may explain the poly-phased structural records of exhumed rocks without involving changes in far-field stress conditions. This long-term background tectonic signal controlled by deep accretionary processes plays a part in active deformations monitored at subduction margins, though it may remain blind to most of geodetic methods because of superimposed shorter-timescale transients, such as seismic-cycle-related events.</p>

A new approach to the opening of the eastern Mediterranean Sea and the origin of the Hellenic subduction zone. Part 2: The Hellenic subduction zone

We discuss the structure of the present Hellenic subduction zone. We show that the present Hellenic subduction zone was formed at about 15 Ma when it started to consume the Mediterranean lithosphere and to form the large accretionary wedge that covers a large part of the eastern Mediterranean. We establish that there is independent evidence that the very large Hellenic Trough that it created was formed simultaneously. Shortly before, an 8–10 km thick backstop that extends 200 km southward, where it presently abuts the African margin, was put into place. We reconstruct the northern margin of the eastern Mediterranean Sea prior to the Hellenic subduction in a new and independent way. The faults recently identified by Sachpazi et al. (2016a . Geophysical Research Letters, 43: 651–658) and Sachpazi et al. (2016b . Geophysical Research Letters, 43: 9619–9626) within the Hellenic seismic slab are a key element of our reconstruction. This is because the slab, which is part of the Nubia plate, is rigid and the faults within it coincide with the lines of slip congruent with the relative motion of the Aegean block over it. These faults demonstrate that about 400 to 500 kilometers of eastern Mediterranean lithosphere have been subducted with essentially the same southwestward direction of motion during the last 15 Myr. Our reconstruction shows that before the onset of the Hellenic subduction, the northern margin of the eastern Mediterranean Sea coincided with a major Jurassic transform fault that limited the eastern Mediterranean to the north during its formation in the Jurassic and Early Cretaceous as proposed in part 1. We discuss the implications of this reconstruction on the Neogene evolution of the Anatolia–Aegea block and its geodynamics.