Tracing the aborted Middle Triassic Neotethyan rift along the Dinarides

Abstract

Volcano-sedimentary successions of Middle Triassic age can be traced along the Dinaridic mountain chain from northern Croatia in the northwest to Montenegro in the southeast. Their deposition is related to a regionally well-marked rifting phase linked to the opening of the Neotethys Ocean. The successions locally vary in thickness and display great facies variability of sedimentary and volcanic/volcaniclastic rocks, reflecting complex geodynamic settings in which they were formed.Sedimentary rocks in these successions range from the outer margin and upper slope deposits, characterized by coarse- and fine-grained resedimented shallow-marine carbonate material, to open-marine deposits including pelagic limestone and radiolarian chert. Locally, specific low-diversity fossil assemblages and the high proportion of organic matter in sediments indicate deposition in a restricted environment where primary production was limited to the upper part of the water column (Goričan et al. 2015).Stratigraphically stacked, volcanic and volcaniclastic lithologies range in composition from basaltic to rhyolitic and are present as doleritic subvolcanic intrusions, basaltic effusions and several volcaniclastic facies. Autoclastic basaltic facies and resedimented autoclastic facies are formed by the fragmentation caused by the rapid cooling of lava in contact with cold seawater and subsequent redeposition of the newly formed clasts. Rhyolitic pietra verde tuffs are interpreted as products of explosive volcanic eruptions distributed in pelagic environments by gravitational mechanisms, including air-fall and pyroclastic density currents. Locally, medium- to fine-grained volcanogenic turbidites represent unconsolidated pyroclastic detritus redeposited during or shortly after eruptions. Geochemical data show that parental magmas responsible for generating these volcanics/volcaniclastics had a calc-alkaline to shoshonitic composition and are interpreted to have formed during continental rifting in ensialic and mature arc settings.Biostratigraphic data indicate pelagic conditions generally lasted from the Middle Anisian to the Late Ladinian, with local variations. Late Anisian age (Reitziites reitzi Ammonoid Zone) is inferred for the oldest pietra verde tuffs with radiolarians from intercalated chert. Basaltic volcanic/volcaniclastic rocks yielded K-Ar and Ar-Ar ages of 241.1±5.2 and 244.5±2.8 Ma. These ages indicate the coeval existence of bimodal volcanism comparable to the modern East African Rift System.  Middle Triassic volcano-sedimentary successions of the Dinarides testify about an active rift in the Dinarides between the Middle Anisian and the Late Ladinian. Extensional movements of the lithospheric blocks created horst-and-graben topography with pelagic sediments and volcaniclastic detritus accumulating on the subsided blocks. Laterally still active carbonate platform areas supplied carbonate detritus to these pelagic environments. Deep-rooted normal faults served as conduits for the ascent of basaltic magma and the formation of associated volcaniclastic facies. However, with the main Neotethyan rift located further to the east, extension in this area of the Dinarides ceased in the Ladinian, resulting in the filling of pelagic areas and the reestablishment of shallow-marine carbonate sedimentation.