A petrologic examination and detailed mapping of the tectonic margin between the Jurassic Pindos-Vourinos ophiolite and metamorphic Triassic-Jurassic Pelagonian platform carbonates in West Macedonia, Greece

At the end of this decade, the Ilariona hydroelectric dam will be completed in west Macedonia (Greece). The Aliakmon, the longest river in Greece, will be dammed and its valley will be flooded. Within the area to be flooded the Aliakmon has cut a valley deep into the contact zone between the mid Jurassic Pindos-Vourinos oceanic lithosphere complex and the Paleozoic-mid Jurassic continental Pelagonian margin. In order to preserve as much information as possible, the Aliakmon Legacy Project aims to gather geological data from the future flooded areas. In the framework of this project, a detailed geological map has been prepared and rock samples have been analyzed in the lab.
   The Pindos-Vourinos Ophiolite is a spoon-shaped ophiolitic nappe that was obducted in Jurassic time. The Vourinos Ophiolite is located at the leading edge of the nappe whereas the Pindos Ophiolite is located at the trailing edge. According to the fracture pattern with a common dip of 40° to the SW, the mapped area can be located in a central position at the leading edge of the Pindos-Vourinos Ophiolite.
   Various techniques have been used to analyze collected rock samples. In addition to petrographic and a reflected light microscope techniques, data has been acquired using electron microprobe analysis (EMPA). For the ophiolitic section, it could be shown that the degree of serpentinisation increases with proximity to the contact zone. Relicts of olivine and pyroxenes can be found together with chromite and magnetite. EMPA displays high content of oxygen, magnesium and silica for the peridotites. Furthermore, a relative accumulation of chromium relative to magnesium (10:1) can be found in the altered outer zones of chromite minerals compared to less altered inner zones (3:1). Towards the thrust sole, increasing shear strain causes tectonic brecciation (deformed host rock remnants in highly sheared matrix) as well as mylonitic occurrences of peridotite.
   The Pelagonian carbonates of the Vounassa vary from microcrystalline over sugarygrained to granular crystalline rocks. Intermittently, brownish, reddish or white bands cut through the carbonates. Close to the shear zone, the carbonates have been metamorphosed to marble. Thin sections of the granular crystalline carbonates display angled crystals, partly with twin lamellae distorted by kink bands. The mylonitic marble contains larger-grained calcite crystals with kink bands and smaller included quartz grains. Overall, the Pelagonian carbonates have been subjected to an early ductile deformation that is crosscut by a subsequent brittle deformation.
   Between the ophiolitic nappe and the Pelagonian carbonates lies a 400 m – 600 m thick wedge of phyllitic to schistose sediments with intercalated fault wedges, the Zavordas Mélange (ZM). The main part of the ZM is formed by the Agios Nikolaos Formation. This formation is predominantly composed of phyllitic, pebbly mudstones and carbonate mylonites. The carbonate mylonites are very soft and easy to erode. Thus, the Aliakmon mainly cuts its valley into this unit. It contains microcrystalline as well as granular crystalline calcite with kink bands running through twin lamellae. Furthermore, a minor amount of quartz can be found. The fault wedges, intercalated in the ZM, consist of autochthonous as well as allochthonous rocks. The allochthonous rocks are meta-diabase, pillow lavas and the so-called “rainbow rocks“. The meta-diabases contain twinned plagioclases and grain size grades to gabbroic. The pillow lavas exhibit intersertal mineral laths and few epidotes. The “rainbow rocks” include interbedded strata of quartz-bearing micritic carbonates, volcanic ashes and tuffs and detrital silts.
   Large parts of the mapped area are covered by young conglomerates, breccias and rock slides either from Vounassa or Vourinos mountains.