Slope sedimentation, syn-orogenic stretching and post-orogenic collapse in a tectonic window along the Vlora–Elbasan–Dibra transfer zone, Albania

Introduction

The poorly studied Okshtuni tectonic window is located along a major transverse zone that affects the Hellenic orogen. It exposes Late Jurassic to Eocene siliciclastic sediments in two thrust sheets attributed to the Krasta nappe. Reconnaissance field work was carried out in remote eastern Albania in order to (1) describe the sedimentary characteristics of the flysch successions in the two Krasta thrust sheets, and (2) constrain the evolution of the orogen and the exhumation of the tectonic window.

Geological setting

The Krasta nappe consists of the Okshtuni and Ostreni thrust sheets. The tectonically lower Okshtuni thrust sheet makes up the core and most of the tectonic window, and consists of Maastrichtian – Eocene siliciclastic sediments. The overlying Ostreni thrust sheet occurs on the flanks of the window, and consists of Tithonian – Cenomanian slope sediments. The Krasta nappe is overlain by the Korab and West Vardar Ophiolite nappes around the Okshtuni window, and, in turn, it overlies the unexposed Kruja and Ionian nappes.

The Okshtuni window forms a dome that is subperpendicular to the general NNW-SSE structural trend of the Hellenides. The window is part of the Vlora–Elbasan–Dibra transfer zone that is interpreted as a major dextral accommodation zone during Neogene thrusting. The NE-SW oriented zone is seismically active with oblique normal kinematics.

Sedimentology

Sedimentary facies within selected intervals of the Okshtuni and Ostreni thrust sheets were examined during field work. Both thrust sheets consist of siliciclastic-dominated turbidites and hemipelagic mudstones. Both, high-density, very thick- to thick-bedded conglomerate and sandstone, and low-density, thick- to thin-bedded sandstone and siltstone turbidite units are present. These most likely represent confined (slope canyon), and unconfined levee or base of slope deposits, respectively. Soft-sediment deformation is present within the succession, indicating rapid deposition and dewatering. A deep-water Nereites ichnofauna is present. Palaeoflow was largely to the south to southwest (i.e. transverse to the current structural grain), although northwest-directed axially draining systems have also been identified. The succession of the Ostreni thrust sheet contains rare calciturbidite intercalations.

Well-cemented sandstones form the main clast type within the Okshtuni thrust sheet. These may have been derived from the Ostreni flysch unit prior to its emplacement onto the younger Okshtuni thrust sheet. Hundreds of metres to kilometre-scale carbonate olitholiths are also present within the Okshtuni thrust sheet. These are likely to be derived from the contemporaneous Kruja platform. Volcanic and ophiolitic clasts are rare suggesting that the advance of the ophiolitic nappe occurred after the termination of deposition in the Eocene.

Structural geology

The tectonically lowest Okshtuni thrust sheet is moderately deformed. Open folds are characteristic, but tight to almost isoclinal folds occur in the stratigraphically, and also tectonically,deepest parts of the window. Thrust faults, semibrittle shear zones with S-C fabrics, and asymmetrical folds indicate a generally western-southwestern vergence. This D1 phase reflects nappe stacking within the Hellenic orogen. A weak cleavage is usually developed in the mudstones of the Okshtuni thrust sheet, but does not affect the sandstones. The relation of cleavage to regional folding is questionable as the often shallow cleavage planes do not always align with the axial planes of upright or inclined folds.

Low angle normal faults, asymmetrical boudinage structures, and shear bands indicate dominantly SW-directed extension at semibrittle conditions. We interpret this D2 phase to have been synchronous with orogenic deformation. Asymmetrical flattening might have occurred in a SW-dipping, i.e. foreland-dipping, tectonic setting.

Steep normal and/or strike-slip faults affect the flysch of the Okshtuni thrust sheet, but also occur in cemented coarse Quaternary debris near the margins of the tectonic window. These faults crosscut the D1 and D2 structures, and are most likely related to late, at least partly Quaternary, oblique dextral extension along the Vlora–Elbasan–Dibra transfer zone (D3 phase). The exhumation of the tectonic window is attributed to this phase. In the central and south-western part of the window, steep slopes are often covered with cemented coarse talus breccias, and probably indicate active uplift.

The tectonically overlying Ostreni thrust sheet is much more deformed than the Okshtuni thrust sheet. Tight to isoclinal folds with ubiquitous, locally penetrative, low angle axial planar cleavage are common. This thrust sheet is weakly metamorphosed and resembles the Vermoshi–Beotian flysch unit in character and tectonic position.

Conclusions

The tectonic evolution of the Hellenic orogen around the Okshtuni window is summarised as follows. Initial nappe stacking (D1) was followed by still syn-orogenic stretching that probably reflects flattening associated with a foreland-dipping duplex or an antiformal stack (D2). Finally, the extensional collapse of the orogen, and the exhumation of the tectonic window, occurred in conjuction with oblique dextral shear along the Vlora–Elbasan–Dibra transfer zone (D3).