engleski

Detachment and parasitic folds on the island of Kornat (central Adriatic, Croatia)

During the detailed geological mapping for the purposes of a new Basic geological map of National Park Kornati in scale 1:50, 000 (BRČIĆ et al., 2019), so far unknown tectonic deformations of the Adriatic carbonate platform succession are observed. The western part of the island of Kornat is built almost entirely of >600 m thick succession of well-bedded, lithologically monotonous, mostly micritic peritidal limestones. The competent carbonate rock succession is intensively folded in form of kilometers long folds characterized by the typical Dinaridic strike (NW-SE), and a clear SW vergence (Figure 1A). These folds are symmetrical, asymmetrical, and overturned, up to a kilometer high (anticlines and synclines) and thus are considered as the first-order folds characterized by a different amount of compaction during orogenic shortening.. These huge folds are associated with decametric second-order asymmetric parasitic folds intercalated within the successions of the Gornji Humac formation, in moderately inclined limbs of the first-order folds. The zones of parasitic folding strike continuously along many kilometers, and the estimated thickness between concordant underlying and overlying limestone succession is 100-200 m. The parasitic folds have typical „Z“ and „S“ geometry (Figure 1B), although in places deformations seem rather chaotic (without any uniform vergence). The parasitic folds are in places dissected by indistinct internal faults. It should be noted that parasitic folds are not observed in the subvertical and overturned limbs of the first-order tight folds. The genesis of the first-order folds on the island of Kornat is probably related to a detachment (décollement) folding as described e.g. in DAVIS & REYNOLDS (1996). The lack of the parasitic folding within the steep (and overturned) limbs of the tight first-order folds can be explained by the simultaneous formation of the both, the tight and open first-order folds. Parasitic folds (the second- order folds) are likely formed simultaneously with the formation of the first-order open folds (anticlines), because of a lateral thickness compensation within the folded thrust sheet during the tectonic thickening, since the tight folds are generally higher than open folds. According to the exclusive surface appearance of the Upper Cretaceous and Paleogene deposits in the wider region, we assume that detachment has been activated within older Mesozoic deposits during the early-orogenic thin-skinned tectonic phase in the area (KORBAR, 2009). However, considering a few kilometers thick deformed Upper Cretaceous carbonates in the boreholes on the neighbouring island of Dugi otok (cf. FUČEK et al., 2016), the tectonic structure in this part of the External Dinarides was probably formed as a multiple thrust structure. The frontal thrust of the External Dinarides, previously interpreted on the seismic profiles (GRANDIĆ et al., 2002 ; KORBAR et al., 2010), is located beneath the seabed about 20 kilometers southward of the island of Kornat, and is probably related to the latest activity of once active main décollement (detachment) at the base of the External Dinarides. According to our knowledge, this is the first report on parasitic folding in the External Dinarides.

External Dinarides, shallow-water carbonates, décollement folding, parasitic folds

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