engleski

A kinematic forward model across the Velebit Mountains in the External Dinarides, validated by the structural position of various types of carbonate breccia

The Dinarides fold-and-thrust belt on the Balkan Peninsula records the long-lasting history of convergence between the European and the Adriatic plates. The Internal Dinarides are composed of composite ophiolite-bearing nappes, whereas the External Dinarides largely consist of Mesozoic and Paleogene shallow marine Adriatic Carbonate Platform (AdCP) rocks. Due to numerous paleontological studies the chronostratigraphic evolution of the AdCP is well known. However, kinematics, timing, and in some places even the geometry of Cenozoic deformation of the AdCP (i. e. the formation of the External Dinarides) are poorly constrained due to the lack of siliciclastic deposits suitable for dating of deformational events/phases. Consequently, neither exhumation rates nor deformation rates based on low-temperature thermochronology are yet known in this part of the Dinarides. However, the presence of different types of syn-depositional and tectonic breccia, in particular those outcropping over large areas on both sides of the Velebit Mts., could help in constraining the timing of Cenozoic deformational events. For this purpose, besides the definition of age and formation mechanisms of these breccia, their structural position is of crucial importance. To show the different structural postions we described the different breccia in the field and integrated them into a kinematic forward model of the Velebit Mts, an up to 1800 m high mountain range along the Adriatic coast. The geologically most viable, kinematically forward modelled cross-section is based on geological maps, dip data, four boreholes, an unpublished cross-section and fault kinematic data ; the latter revealing that the Velebit structure is controlled by a major NE vergent back thrust. In the northwestern part of the Velebit hinterland, a mostly monomict breccia conformably overlaying Jurassic and Cretaceous carbonates is widely exposed. Based on its structural position that is confined to the footwall of the major NE vergent back thrust and its petrographic characteristics, we interpret this breccia to represent tectonically reworked material mostly derived from lithologies in the footwall of the back-thrust. In contrast, the foreland-facing SW slope of the Velebit Mts. is characterised by the presence of mostly polymict breccia, known as Velebit breccia. Taking its structural position and its petrographic composition into account, this breccia is interpreted as consisting of prevailingly talus and fill-up deposits of a flexural foreland basin that formed during late Paleogene times due to the SW advancing nappe stacking. Our kinematic forward model suggests that both the cataclastic and the talus breccia must have formed at the same time, because their formation is directly related to shortening and uplift of the Velebit structure. Additionally, we found two types of much older intraformational breccia: Triassic intraformational breccia located in the hanging wall of Triassic normal faults and Cretaceous intraformational breccia, which is not associated to any local structure and hence only reflects pre-contractional emersion phase(s). Our study underlines that a precise distinction of the structural position and the formation mechanisms of seemingly monotonous, but genetically different carbonate breccia with respect to major km-scale faults and fault-related folds, are crucially important to gain an improved understanding of the kinematics and timing of deformation in carbonate-dominated thrust belt portions.

Kinematic forward model, Carbonate breccia, Velebit Mts., External Dinarides

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