Tectonics of the Okić area
The area of Plešivica, the Karlovac Depression and the Samobor Hills lies on the boundary of major geotectonic units — the Alps, the Dinarides and the Pannonian Basin — and has consequently been subject to numerous tectonic movements throughout its geological past. The result of this tectonic activity is a highly disturbed succession of deposits and a complex tectonic framework. All the changes in sedimentation described above were triggered by tectonic movements. The movements during the Palaeozoic and Mesozoic were global in character, while younger movements disrupted the older structures and influenced the development of the present-day relief.
The oldest — and at the same time the most powerful — tectonic movements whose effects were directly expressed in the Okić area occurred at the beginning of the Eocene, approximately 50 million years ago. It was then, as a result of the overthrusting of Triassic dolomites onto Cretaceous clastics, that the foundations of present-day Plešivica were created. The precise mechanism of overthrusting is difficult to explain, as it is not entirely clear whether this involves a large-scale nappe or a thrust that developed from a recumbent fold.
Further tectonic activity takes the form of oscillations along major vertical faults extending along the southern and northern slopes of Plešivica. The initial shaping of Plešivica as a distinct geomorphological unit took place in the middle of the Badenian, approximately 14 million years ago. At that time it formed an island in the Pannonian Sea, but was very quickly submerged. Towards the end of the Pontian, some 7 million years ago, the gradual uplift of Plešivica and the Samobor Hills began again, accompanied by the subsidence of the Karlovac Depression. The differences in relief elevation triggered erosion of the uplifted areas and the redeposition of material in the sunken parts of the terrain.
At the beginning of the Pleistocene, approximately 2 million years ago, uplift of the Karlovac Depression began, causing a large lake to dry up. Uplift continued throughout the Quaternary, so that a large proportion of the lake sediments was eroded. By examining the type and size of redeposited fragments, it is possible to determine the progress of erosion and hence the rate of uplift. Over the last 7 million years, Plešivica has been uplifted by at least 780 m — equivalent to its present height — but if the material that has been eroded from it is added to its current elevation, it becomes clear that the total uplift was considerably greater.
The frequent earthquakes that affect the Plešivica and Samobor Hills area indicate that tectonic movements in the region have not yet ceased.
