Tectonic setting of the Jurassic ophiolites in the Southern Apuseni Mts (Western Romania)

The Apuseni Mts. are located in the internal zone of the Carpathian belt. They are characterized by outcrops of metamorphic and sedimentary rocks representative of the Tisza microplate derived from the northernmost edge of the Adria Plate. The geological setting of the Apuseni Mts includes a stack of continental tectonic units mainly cropping out in the North Apuseni Mts. They are represented, by the Bihor, Codru, and Biharia tectonic complexes; all consisting of a polydeformed crystalline basement covered by Permian-Lower Cretaceous sedimentary sequences with Austroalpine affinity. In the Southern Apuseni Mts, the Bihor, Codru and Biharia complexes units are overlain by the Mures nappe as well as the Cretaceous Flysch units. The Mures nappe is characterized by the presence of MOR ophiolites, that includes a gabbroic intrusive section, followed by a sheeted dike complex showing transition to a volcanic cover topped by few meters of radiolarian cherts. Mantle rocks are absent, thus these ophiolites represent only the oceanic crustal portion. The ophiolites are overlain by an Upper Jurassic island arc orogenic magmatic association both intruded by calc-alkaline granitoids and covered by Upper Jurassic – Lower Cretaceous shallow-water carbonates. In turn, the Cretaceous Flysch units are represented by Early to Late Cretaceous clastic sedimentary succession, showing slide blocks and/or fragments of ophiolites and others lithologies derived by the Mures nappe. According to the geological data, the Cretaceous Flysch units and the overlying Mures nappe are thrust over the Bihor, Codru and Biharia complexes during the late Maastrichtian (“Laramian”) phase. However, evidences of older tectonic events can be also recognized, probably, connected with a phase (“Austrian”) occurred from the Aptian to early Albian time span. The subsequent Maastrichtian to Paleocene “banatitic” intrusions and related volcanites sealed the relationships between all the tectonic units of the Apuseni Mts.