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.