engleski

Structure of the Dinarides: a working model

During a series of field visits a model in the sense of a working hypothesis was developed for the Dinarides. This working model somewhat contradicts of modifies existing views. Our main points are given below. 1, Concerning a transect through Bosnia, there appears there are two belts with ophiolites (Central Dinaridic Ophiolite Belt and Eastern Dinaridic or Vardar Ophiolite Belt) but they resulted from obduction of just one ocean (Dinaridic Tethys) of Triassic-Jurassic age by the end of Jurassic – Early Cretaceous. No proof for the existence of a branch of "Vardar" that remained open up to Tertiary times was found in Bosnia of Central Serbia so far. Occurence of ophiolites in two and not only in one belt is due to out-of-sequence thrusting and later nappe re-folding. These subsequent structural events can be as late as Paleogene-Neogene. 2, Final collision and closure of the Dinaridic Tethys Ocean occurred in Middle Cretaceous times. Occurence of ophiolites in younger formations are due to reworking and redeposition from mélange and large ophiolite massifs already exposed on the surface since Late Jurassic – Early Cretaceous times. Late Jurassic obduction and mid-Cretaceous collision have to be separated into two different structural events. It is proposed that the Dinaric shelf was the lower plate and the Tisza (-Serbo-Macedonian) microcontinent was the upper one during Cretaceous collision. 3, The older part of the Bosnian flysch (Vranduk Fm) is of distal turbiditic-foredeep origin, grading eastwards- upwards into a more proximal facies. It was deposited on the active Dinaric margin and not on the passive margin as earlier proposed. Many Late Jurassic – Early Cretaceous dark turbidite formations can be correlated with the Vranduk Formation, including Rossfeld (Salzkammergut, Austria ; Gerecse, Hungary), Lök and Mónosbél (Bükk Mts), Ostrc (Croatia). The stratigraphic underlayer might be Jurassic Radiolarite upon a distal parautochthonous Dinaric margin, or it might be sheared off this margin. 4, We separate the Late Cretaceous Ugar Fm (younger part of the Bosnian flysch) from Vranduk Formation, because there is a marked unconformity between the two turbidites. This unconformity marks the end of the "pre-Gosau"deformation of Early Cretaceous age, an event that so far largely remained unrecognised in the Dinarides. Their deformation styles and clastic material are different (syn-cleavage tight folds vs. open, rarely cleaved folds ; basically clastic vs. marly-carbonatic material). The Ugar Formation represents a post tectonic formation, together with other similar deposits of the Late Cretaceous, widespread in the whole Dinarides. This formation was deformed only by Tertiary structural events. 5, Durmitor is an out-of-sequence nappe rethrusting the former nappe pile and its Senonian cover onto Senonian, therefore its age is younger. 6, The Senonian – Paleogene Posavina belt is a turbidite zone with coeval calcoalkaline acidic mafic magmatism. It is not necessarily related to any ocean closure. Instead, this belt might be related to intracontinental right lateral strike slip reactivation of an earlier collisional contact between Tisza and the Dinarides. This zone is tentatively correlated with similar rocks in Medvednica (Croatia) and the Szolnok Flysch Belt (norther margin of Tisza, Central Hungary). The former linear shear zone is now strongly disrupted by later microcontinent- and block-rotations. 7, The Tertiary evolution of the Dinarides is dominated by major rotations, right lateral strike slip faulting and southwestward thrusting. When reconstructed, the Dinaric structural belts and those of Tisza are aligned N-S. The External Dinarides are constantly shifted to the north, to indent into Europe at the Alpine sector. 500 km N-S shortening in the Alps is conveyed through right lateral N-S shear zones into subduction zone(s) in the External Hellenides. A Palaeogene westward thrusting is also a dominant feature. A first major period of CCW-rotation is in Late Palaeogene – Early Miocene times, followed by a second major period of CCW-rotation in Late Miocene – Pliocene.

Dinarides; geodynamic evolution; tectonics;

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