Thrust tectonics in the External Hellenides: from a salt-bearing rifted margin to convergence in a retreating subduction zone

The Hellenides constitute a long-lived convergent system resulting from oceanic–continental subduction and subsequent continental collision between Apulia and Eurasia. Their external domain developed above inherited sectors of a hyperextended Mesozoic passive margin, composed of alternating thick carbonate platforms including the Apulian (and Pre-Apulian) and Gavrovo units, and thin basinal domains such as the Ionian. Such inherited structural and stratigraphic architecture exerted a first-order control on thrust localization, wedge geometry, and foreland basin evolution. Since the Late Cretaceous, convergence was accompanied by significant slab retreat, producing a strongly asymmetric orogen with outward thrust propagation in the prowedge and coeval extension in the Aegean region.

We present three E-W, regionally balanced cross sections across the External Hellenides, sequentially restored to constrain the pre-contractional configuration of the sedimentary cover, the kinematic evolution of the thrust belt, and its relationship with inherited rift-related domains and salt-related deformation. The cross sections run through the western Hellenides and are roughly parallel to the main transport direction. The northernmost section crosses the Corfu area, whereas the southernmost profile is located south of the Kefalonia Fault, where the tectonic regime transitions from continental collision to active oceanic–continental subduction.

The sections are based on detailed field surveys along the Ionian structural unit, integrated with published seismic profiles and exploration wells. In the eastern Ionian zone, synclines affecting Jurassic to Oligocene–Miocene flysch are generally broad, whereas toward the west, folding becomes tight to isoclinal, locally forming box-type folds with overturned limbs. Anticlines are tight, variably elongated, doubly plunging, and locally associated with breakthrough thrusts. This structural contrast reflects variations in pre-orogenic stratigraphic thickness and mechanical behaviour: tight folds involve a thin sedimentary cover detached on Triassic salt, while broader synclines record deformation of thicker, locally welded successions. The pre-contractional Ionian basin consisted of a salt-influenced deeper-water carbonate system with salt pillows and plateaus, and subsident areas receiving episodic carbonate debrites from adjacent shallow-water domains. The absence of halokinetic sequences in the pre-orogenic succession suggests that diapirism in the study area was exclusively syn- to post-shortening and controlled by shortening-related uplift and erosion.

During contraction, all the Ionian structural units show regionally consistent allochthonous behaviour, detached along Triassic evaporites and overthrusting the autochthonous structural units together with Aquitanian deposits, as documented by tectonic windows. Progressive Miocene deformation involved thicker sub-thrust units, producing broader structures that subsequently controlled deformation of the overlying thinner Ionian units.

Sequential restoration from the Oligocene to the present reveals forward-propagating thrusting consistent with a prowedge-dominated orogen above a retreating slab. This supports an evolution in which thin-skinned deformation above Triassic evaporites and subsequent reactivation of sub-thrust structural units was driven by underplating of Adriatic crust beneath the External Hellenides. Our balanced cross sections provide quantitative constraints on the relative proportions of accreted versus subducted continental crust of the former hyperextended margin, and allow prosing a tentative location for the transition between the Pre-Apulian ramp and the Ionian basinal domain, which acted as precursor for thrust nucleation.