Tectonic evolution of ophiolite mélanges in southern Albania

The Middle Jurassic ophiolites in the Dinaride-Albanide-Hellenide mountain belt in the Balkan Peninsula comprise upper mantle peridotites and crustal units with lateral and vertical variations. Large parts of these ophiolites have supra-subduction zone (SSZ) affinities, whereas other parts show mid ocean ridge basalt (MORB) characteristics. During the last two decades many studies have shown that SSZ forearc settings, beside intraoceanic subduction zones, are tectonically the most predestinated sites for ophiolite emplacement.

One feature of SSZ forearc settings is the local occurrence of serpentinite mud volcanism. Active serpentinite mud volcanoes located in the forearc region of, for example, the Izu-Bonin-Mariana system consist of serpentinite mud containing lithic clasts from the underlying forearc crust and mantle as well as from the subducting Pacific plate. These serpentinite seamounts are covered by pelagic sediments. Recycled materials from the subducted slab consist of metavolcanic rocks, metamorphosed pelagic sediments including cherty limestone as well as fault rocks. Recovered ultramafic clasts reveal various degrees of alteration and serpentinization. Ancient occurrences of such serpentinite mud seamounts are rare as the serpentinite muds have very little capability of being preserved due to their susceptibility to strong deformational overprint and tectonic obliteration.

Ophiolitic mélanges of Tithonian to Lower Cretaceous age are widespread in the Eastern Albanides. They lie transgressively or normally on top of an ophiolitic sequence formed by radiolarian cherts of Kimmeridgian-Tithonian age, or on top of the carbonate sequences along the periphery of the ophiolites formed by Middle Liassic to Malmian pelagic limestones with manganese nodules and radiolarian cherts. They are overlain by conglomerates or neritic limestones of Lower Cretaceous age.

The ophiolitic mélanges consists of ophiolitic conglomerates or breccias, often of homogenous composition with clasts of (serpentinized) ultrabasites, gabbros and basalts embedded within a serpentinite matrix. Less commonly they have a heterogeneous composition with a fine-grained serpentinite matrix and partly exotic clasts and blocks of marl with calpionellids, sandstone, radiolarite, limestone and amphibolite, together with serpentinite, ophicalcite, gabbro, plagiogranite, diabase, basalt, and dacite. These mélanges are overlain by flysch-like deposits.

The homogeneously composed mélanges widely form thrust- or strike-slip-fault related tectonic breccias, whereas the heterogeneously composed mélanges, and in particular the incorporated exotic blocks, indicate a different origin. These polymictic mélanges containing clasts of peridotites, basalts, dacites, amphibolite, etc., being covered by pelagic sediments, might be a good candidate to represent one of the rather rare fossil remnants of serpentinite seamounts bearing potential additional evidence for a SSZ forearc setting during their formation.