Structural architecture and tectonic evolution of the Mt. Velebit in the central part of the External Dinarides in Croatia

Mt. Velebit is the most prominent geomorphological structure in the central part of the External Dinarides fold-thrust belt formed by contraction along the eastern margin of the Adriatic plate during Cenozoic convergence between the Adriatic and European plates. Detailed outcrop-scale analysis of fault-slip data were used in evaluation of existing tectonic models, reconstruction of the present-day structural architecture and tectonic evolution of the mountain. Our analysis shows the prevalence of dip-slip and top-to-NE motions along major Velebit’s faults, verifying that these faults are NE-verging thrusts and not NE-dipping normal faults as previously thought. Consequently, we also challenge earlier tectonic models that interpret the Mt. Velebit as a SW-vergent antiformal stack or thrust duplex formed above the major NE-dipping thrust system. Our data rather validate the concept of a passive roof duplex recently tested by Balling et al. (2017) by the construction of forward modelled balanced cross sections. Based on orientation and kinematic compatibility criteria of recorded fault-slip data, we separated six groups of conjugate fault sets used for assessing paleostress directions. Relative chronology between analysed sets and separated groups is partly defined. Presumably the oldest two groups contain fault sets with prevailing dip-slip and reverse motions: a first one is composed of antithetic NNW-striking sets that indicate the ENE–WSW trending contraction ; the second group contains a WNW-dipping set, which indicates WNW-ESE trending contraction. Structures of both groups strike parallel and slightly oblique to the structural grain and the mountain front. They are thus supposed to have formed during the major contractional tectonic phase in this part of the External Dinarides. Most of the recorded fault-slip data comprise structures with a normal sense of shear. These are separated into three groups: the most robust subgroup is composed of NW-striking antithetic fault sets indicating NE-SW trending extension ; a second group comprises NNE-striking antithetic sets, which show WNW–ESE extension, while a third group includes W-striking antithetic sets related to a N–S extension. Although separated into three kinematically homogeneous subgroups, these structures possibly resulted from one and the same state of stress, characterised by radial extension that according to good preservation and frequency of occurrence might play an important role in the formation of extremely voluminous carbonate breccia exposed along the SW slope of the mountain. The remaining group of faults comprises sets of subvertical, conjugate NW-striking dextral and NE-striking sinistral faults, which indicate N–S trending contraction. Outcrop- to map-scale overprinting relationships, suggest these to be the youngest brittle deformation structures in the study area. Acknowledgements. This research was financially supported by the Croatian Science Foundation, Grant no. IP-2014-09-9666. REFERENCES Balling, P., Tomljenović, B., Schmid, S.M., Ustaszewski, K. 2017. Contrasting deformation styles across the Split-Karlovac Fault (External Dinarides) induced by reactivation of pre-existing structures. In: Šarić, K., Prelević, D., Sudar, M. & Cvetković, V. (eds.): 13th Workshop on Alpine Geological Studies, Abstracts, Uni. Belgrade, Belgrade.