A new geological model for the Lefka Ori Massif in the accretionary wedge of the Hellenic Subduction Zone.

Here we present the results of surface and subsurface (caves) geological mapping in the Lefka Ori Massif. The structural field data collection and the stratigraphy analysis were supported by the IGCP-715 project in collaboration of the Speleological Association of Crete (SPOK) and the Gourgouthakas 2023 exploration team. The overall structure of the Lefka Ori Massif consists of a roughly E-W trending open anticline. Regional uplift is related to compressional and extensional events during the Late Oligocene to present. The primary lithologies of the Lefka Ori Massif are comprised of the Plattenkalk Unit and the Trypali Unit. The Plattenkalk Unit contains (1) black brecciated karstified marbles/dolostones with interbedded stromatolites (Triassic), (2) white to gray marbles/dolostones (Triassic-Lias), and (3) thin bedded marbles (Dogger-Oligocene). Black dolostones and brecciated marbles (with stromatolite fragments) are building up the overlying Trypali Unit (Late Triassic-Lias).

The results from the structural analysis in the Lefka Ori Massif highlight 3 sets of faults, which are of (A) E-W striking thrust faults, (B) dextral NNW-SSE strike-slip faults and (C) E-W striking steep dip-slip faults. A regional thrust (hereafter "Pachnes Thrust") has a vertical displacement of 1000 m and duplicates the Plattenkalk Unit stratigraphy (sections 1 and 2) in the Lefka Ori Massif. The hanging wall consists of Plattenkalk units 1 and 2, while the footwall comprises the Plattenkalk unit 3. The Pachnes Thrust has an apparent horizontal displacement of approximately 12 km. The cross-cutting relationship of the thrust with the strike-slip faults indicate that the faults are coeval. The steep dip-slip faults are still of an unknown relative age but certainly most of them appear as normal, younger faults. The Pachnes Thrust is folded around a roughly E-W trending open fold axis. The Late Oligocene green and red, calcschist sediments in the footwall of the Pachnes Thrust indicates the maximum age of thrusting.

The third deepest cave in Lefka Ori Massif (Sternes Cave, depth -616 m) contains a system of galleries reaching a total length of 5.6 km. This karst system of subhorizontal galleries is well explained by the Pachnes Thrust as it parallels the thrust. Likewise, the Pachnes Thrust has been delineated in the deepest cave (Gourgouthakas) at a depth of -700 m. In addition, the accepted hydrology in the Lefka Ori Massif is defining an upper fast flowing reservoir and a lower slow flowing reservoir. The physical model for these two reservoirs is explained by the hanging wall 1 and 2 lithologies (fast flowing reservoir) and the footwall lithology 3 (slow flowing reservoir) of the Pachnes Thrust.

The Pachnes Thrust in Lefka Ori Massif probably correlates with the recorded southward thrusting and nappe stacking due to convergence between the African and Eurasian plates (Oligocene-Early Miocene). The deformed Pachnes Thrust plane is coeval with the southward thrusting, due to the uplift and exhumation phases during extension described in the literature. The present findings are offering new evidence of the tectonic evolution and the exhumation of the high pressure metamorphosed rocks in the Hellenic Subduction Channel.