The geotectonic evolution of the Dinaridic part of Western Tethys from Early Permian to Late Triassic

Investigating the Middle Triassic volcanic and volcaniclastic rocks in the External Dinarides (Croatia and Bosnia and Herzegovina) an undeveloped rift system has been determined. This rift system was formed as a back-arc extension in the north-western edge of Palaeotethys Ocean (Stampfli & Borel, 2002 ; Schmidt et al., 2008). Geochemical data (whole rock REE and trace element composition and EMP analysis on pyroxene and feldspar crystals and crystaloclasts of different volcanic and volcaniclastic facies) indicates calc-alkaline magma composition, characteristic for subduction zones and contamination by the crustal material. Therefore we interpret the development of Middle Triassic rift in the Dinarides as a back-arc rift area that was strongly influenced by the subduction of the Palaeotethys toward north that stared in Early Permian and lasted throughout the whole Triassic. The geotectonic history of this part of Tethys is described as follows: In the Upper Permian and Early Triassic the Dinarides were located in the western Palaeotethys. According to the palinspastic reconstructions (Haas et al., 1995 ; Scotese, 2002 ; Stampfli & Borel, 2002 ; Schmidt et al., 2008 ; Stampfli et al., 2013) this Palaeotethyan area represented a shallow bay resembling a vast epicontinental sea, in the Dinarides, interpreted as a broad epeiric ramp (Aljinović et al., 2015). In the southern part of this ocean rifting started in the Permian between the Cimmerian block and the southwest coast of Pangea. This rift system (Neotethys) advanced to the north and finally formed the Tethyan Ocean in the Jurassic. From afore mentioned palaeogeographic reconstructions it is notable that the subduction of Pelaeotethys persisted throughout the Triassic. This subduction process could have influenced the magma formation in the Middle Triassic. The former subduction, which caused the formation of Pangea could have also left the imprint on the Middle Triassic magma composition (Crisci et al., 1984). Palinspastic reconstructions place the area of the External Dinarides, and surrounding areas in the vicinity of earlier Gondwana and Laurasia suture line, the similar direction of the newly formed Tethyan rift. This caused the subduction of the Gondwana crustal material, and its partial melting that had great influence on the Middle Triassic magma in the western Tethys. Except the primary rifting direction there were additional extensional/rift systems in the continental part of Pangea. There were three recorded back-arc rift zones in the Middle Triassic: Meliata and Meliac Oceans, and the rift system directed across the Dinaridic and Alpine area. The area of the External Dinarides was bordered by the opening of the Tethyan Ocean from the southwest and opening of the Meliata and Meliac Oceans in the north. The External Dinaridic aborted rifting direction did not evolve because of these two compressive movements that forced its closure.