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An Overview on The pre-Neogene Geology of Zagorje - Mid-Transdanubian Zone - Croatian Part

Zagorje - Mid-Transdanubian Zone (ZMTZ) is known in published literature of Hungarian geologists for more then 30 years under different names like "Igal – Bükk zone", "Mid-Hungarian zone", "Mid-Transdanubian shear zone" and many others. It has been recognized as a major tectonic zone within the Intra-Carpathian or Pannonian Basin area separating two regional-scale tectonic units, i.e. ALCAPA on the north and TISIA on the south, composed of different pre-Neogene basement units of Apulian-Adriatic and European paleogeographic origin, respectively. Structurally, ZMTZ is seen as a shear zone with a complex kinematic history of repeated inversions, composed of NE-striking fault sets, bounded to the north by the easternmost segment of the Periadriatic Line together with its eastern extension, the Balaton Line, and by the Mid-Hungarian or Zagreb-Zemplin Line to the south. On the regional scale, ZMTZ is recognized as a shear zone that accommodated ca. 400-500 km of eastward extrusion of tectonic units from the Eastern and Southern Alps-External Dinarides into their recent position in NNE Hungary and Slovakia (i.e. Transdanubian Range and Internal West Carpathian units. Simultaneously, this extrusion resulted with a regional displacement of ophiolite-bearing units of the Internal Dinarides, which are only sporadically exposed on the surface within ZMTZ at its southwestern and northeastern ends (i.e. ophiolites and associated sediments of Hrvatsko zagorje area and Bükk Mts., respectively). However, correlative rocks (e.g. ophiolites and radiolarites) are well known from the Meliata unit of the Internal West Carpathians and from deep boreholes in the area between the Balaton and Mid-Hungarian Lines. The time of this extrusion activity, although still a matter of debate, is most probably of Oligo– Miocene age. In this report a short overview on the major pre-Neogene tectonic and lithostratigraphic units of the Croatian part of ZMTZ is presented. These units crop out on a few isolated, up to thousand meter high mountains of Ravna Gora, Ivanščica, Kalnik, Medvednica and Žumberak. Four units are distinguished and described here. 1) Palaeozoic-Mesozoic, Adriatic margin formations comprise non-metamorphosed clastic - carbonate and volcanic-sedimentary formations of the Late Palaeozoic and Mesozoic age, derived from continental paleogeographic realms situated west of the Dinaridic Tethys - Meliata ocean in Triassic and Jurassic times. These formations predominantly crop out on Mts. Ravna Gora, Ivanščica and Žumberak, mostly strongly dismembered in the northern part of Zagorje area along a set of faults that branch off from the easternmost tip of the Periadriatic Line. 2) Metamorphic complex of Mt. Medvednica comprise Palaeo-Mesozoic clastic – carbonate and volcanic-sedimentary formations affected by a very low- to low-grade metamorphism of Early Cretaceous age (122-110 Ma). This complex crops out on the central, southeastern and northeastern part of Mt. Medvednica. Correlative rocks were also documented in several boreholes in northern Croatia and southwestern Hungary. 3) Ophiolitic mélange complex is a chaotic unit, characterized by a mixture of graywackes, mafic and ultramafic igneous rocks, radiolarites and limestones embeded in a dark shaly matrix. Besides the largest outcropping area on the northern slopes of Mt. Medvednica and Mt. Kalnik, this unit is well exposed on the southern slopes of Mt. Ivanščica and only scarcely on the easternmost edge of Mt. Žumberak. Correlative rock assemblages are penetrated by several deep boreholes in surrounding area of Croatia and Hungary. It shows the typical features of ophiolitic mélanges which are interpreted as being related to subduction and accretianary wedge settings. 4) The Late Cretaceous - Paleocene transgressive to basinal formations (i.e. Gosau formations) crop out along the main ridge of Mt. Medvednica, as a post-Early Cretaceous tectonic cover that unconformably rest on both the Metamorphic complex of Mt. Medvednica and the Ophiolitic mélange complex. These formations show great facies similarities with formations of the Lower and Upper Gosau Group of the Eastern Alps. Due to the post-Paleocene deformations, on the central part of Mt. Medvednica these formations are recently stacked between and imbricated within their original basement composed of rocks of both complexes. As a result, they are locally found as tightly folded, cleavaged and highly dismembered.

Lithostratigraphy, Tectonics, Palaeozoic, Mesozoic, Zagorje - Mid-Transdanubian Zone, Croatia

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