Paper No. 6
Presentation Time: 10:15 AM


PROFETA, Lucia R.1, DUCEA, Mihai N.1 and JIANU, Denisa L.2, (1)Department of Geosciences, University of Arizona, 1040 E 4th St, Tucson, AZ 85721, (2)Department of Mineralogy, University of Bucharest, 1 Nicolae Balcescu Blvd, Bucharest, 010041, Romania,

The Sibisel Shear zone (SSZ) is a high-angle, 1-5 km wide ductile shear zone located in the South Carpathian Mountains (Romania). Equivalent shear zones are discontinuously exposed throughout various segments of the Romanian Carpathians; they probably represent the remnants of a single megashear that was fragmented by younger tectonic events.

We mapped a segment of the SSZ near Rasinari. At Rasinari, the SSZ juxtaposes amphibolite facies rocks of the regionally extensive Sebes-Lotru group (SLG) against the greenschist facies rocks of the Rausorul Cisnadioarei Series (RCS). The SLG represents the remnant of a Cambro-Ordovician island arc metamorphosed during the Variscan (330±20 Ma), whereas the RCS represents a low-grade volcanic arc, possibly equivalent in age with the Sebes-Lotru. Amphibolite grade ultra-mylonites occur within the central part of the shear zone: they could be deformed members of the Sebes Lotru or a distinct, highly attenuated series of rocks caught in the shear zone. We favor the second hypothesis, based on the petrographic diversity within the shear zone and the distinct radiogenic isotopic (Sr-Nd) systematics within the shear zone compared to the SLG. We determined a zircon U-Pb age of 462.8±6.4 Ma on a granitoid cross-cutting the metamorphic fabric within the RGS, which constrains its metamorphism to be Ordovician. Detrital zircon U-Pb ages measured on two meta-sedimentary rocks from the RCS suggest a late Cambrian age of the protolith. Mica Rb-Sr chronology documents that the two domains were juxtaposed during the Permian (270-290 Ma), and although the structure was clearly reactivated as a brittle fault during the Alpine there is no evidence for ductile deformation (>350ºC) in that period.

These results document that the ductile fabric is late- to post-Variscan. Based on the fabric present in the shear zone, we interpret the structure to be a fossil transform tectonic system that has been releasing tectonic stresses associated with the rupture of Pangea.