Bruno Tomljenović

Geodynamic and petrogenetic evolution of Alpine ophiolites from the central and NW Dinarides: an overview

Abstract Dismembered ophiolites occur in the Dinaride Ophiolite Zone (DOZ) that is related to the open-ocean Tethyan realm, whereas highly dismembered ophiolites occur in the Vardar Zone (VZ) related to a back-arc basin. The ophiolites of DOZ are associated with a Jurassic olistostrome melange (DOZM), the youngest component of which are Tithonian limestone exotics and with the Mesozoic bed-to-bed Radiolarite Formation. Late Jurassic/Early Cretaceous to Late Cretaceous clastic sequences, comprising redeposited fragments of ophiolites, disconformably overlie the DOZM. Ophiolites of VZ are associated with tectonized ophiolite melange (VZM), the youngest component of which are Late Cretaceous–Paleogene limestone exotics. The VZM is associated with the Late Cretaceous–Paleogene flysch formation. Ophiolites of both the DOZ and the VZ are predominantly peridotite tectonites, represented mainly by fertile spinel lherzolite in the western and central part of DOZ and VZ, and by depleted harzburgites in their southeastern parts. Cumulate ultramafics and gabbros are subordinate and are in some places overlain by massive or sheeted dyke complexes, capped by metabasaltic pillow lavas. Metamorphic soles of ophiolites are represented by varieties of amphibolites with subordinate pyroxenite schists and scarce eclogites with ultramafic interlayers, which were progressively metamorphosed under P–T conditions of eclogite (?), granulite, amphibolite and greenschist facies. The according protoliths are cumulate gabbros in the DOZ, medium-grade bimineralic epidote–amphibolite facies amphibolites derived from diabase–dolerites, and low- to medium-grade metapelites and metapsammites. K–Ar and Sm–Nd measurements yield ages of 174±14–136±15 Ma on ophiolites from DOZ and 109.6±6.6–62.2±2.5 Ma on ophiolites from VZ. Basic petrological and geochemical features for all Dinaridic ophiolites and associated amphibolites are correlatively presented both for DOZ and VZ. Dinaridic ophiolites were generated in the Dinaridic Tethys over the period of about 150 Ma. The bulk of oceanic crust was generated during the Late Triassic to pre-Late Jurassic/Early Cretaceous when oceanic subduction processes, accompanied by DOZ ophiolite obduction onto the Apulian margin, started. Generation of the oceanic crust continued during the Cretaceous–Early Paleogene in a reduced Dinaridic Tethys under back-arc setting. Eocene closure of the Dinaridic Tethys was accompanied by the second emplacement of VZ ophiolites and the final structuration of the Dinarides and their uplift. At the end, geological and petrological similarities and dissimilarities of ophiolites from both DOZ and VZ are presented.

Tectonic evolution of the northwestern Internal Dinarides as constrained by structures and rotation of Medvednica Mountains, North Croatia

Abstract This paper attempts to explain the tectonic history and possible reasons for the change of trend of the northwestern part of the Internal Dinarides in a transitional area between the Southeastern Alps, central Dinarides and Tisia, north of Zagreb. Structural and palaeomagnetic data collected in pre-Neogene rocks at Medvednica Mountains, combined with palaeomagnetic data available from Neogene rocks in the surrounding area, point to the following conclusions: (1) The reason for dramatic deflection in structural trend of the Internal Dinarides in the area north of Zagreb is a 130° clockwise rotation and eastward escape of a tectonic block comprising Medvednica Mountains and the surrounding inselbergs, bounded to the north by the easternmost tip of the Periadratic Lineament. In Medvednica Mountains, the main period of tectonic escape and associated clockwise rotation occurred in the Late Palaeogene, possibly in the Oligocene–earliest Miocene. (2) When rotated into the original position, the trend of observed pre-Neogene structures of Medvednica Mountains becomes parallel to the major structural trend of the central Dinarides. In view of their original orientation, these structures are interpreted in the following way: (a) The first D1 deformational event is attributed to the Aptian–Albian nappe stacking in the central–northern Dinarides that was accommodated by a top-to-the-north directed shearing and northward propagation of already obducted ophiolites of the Central Dinaridic ophiolite zone. This nappe stacking, which resulted in a weak regional metamorphism in tectonic units underlying the ophiolites, was orogen-parallel or at a very acute angle to known structural (and possibly palaeogeographic) trends. This implies a major left-lateral shear component along the former Adriatic margin and obducted Dinaridic ophiolite zone. (b) This was followed by Early Albian orogen-perpendicular shortening (D2) that was accommodated by folding and top-to-the-west thrusting. This deformation resulted in gradual cooling of the metamorphic stack and also in uplift and erosion of the higher structural units. (c) The D3 deformational event was driven by renewed E–W shortening that took place after the Paleocene, most probably during the Middle Eocene–Oligocene, i.e. synchronous with the main Dinaridic tectonic phase of the External Dinarides. This shortening was probably triggered by collision and thrusting of Tisia over the northern segment of the Internal Dinarides. (d) This was finally followed by D4 pervasive, right-lateral N–S shearing that is tentatively interpreted as being related to the right-lateral shearing of the Sava zone during the Eocene–Oligocene. (e) Following the main period of tectonic escape and induced clockwise rotation along the Periadriatic fault, possibly in the Oligocene–earliest Miocene, the Medvednica Mountains and the surrounding area were affected by repeated extensions and inversions since the Early Miocene to recent times. Palaeomagnetic data suggest that in the Early Miocene (but probably before the Karpatian) this area was part of a regional block that shifted northwards and rotated in a counter-clockwise sense. A second episode of counter-clockwise rotation occurred at the present latitude in post-Pontian times (since c. 5 Ma), driven by the counter-clockwise rotating Adriatic Plate.

Magnetic fabric of Late Miocene clay-rich sediments from the southern Pannonian basin

The magnetic fabric of Late Miocene sediments from the southern Pannonian basin was studied on oriented samples collected from 19 geographically distributed localities. All of them are characterized by near-horizontal magnetic foliation plane after tilt correction, indicating weak deformation. Well-developed lineations were observed for 16 localities, which are interpreted as due to compressional/transpressional deformation, except from three localities, where the fabric must have been formed in an extensional setting. Comparison between the orientation of the map-scale folds and faults and magnetic lineation directions shows that magnetic lineation is either related to NNE-SSW directed compression, leading to the formation of folds or it can be connected to NW–SE or NNE-SSW trending dextral faults.