Internal Structure and Geochronology of the Gerrei Unit in the Flumendosa Area, Variscan External Nappe Zone, Sardinia, Italy

To investigate the stratigraphy and structural evolution of the Variscan Orogen of the External Nappe Zone, Sardinia, Italy, field work and geochronologic analyses were combined to assess both regional and orogen scale issues. Geologic mapping, structural analysis, and sampling in the Flumendosa area coupled with high-precision U-Pb zircon geochronology were used to determine the duration of Ordovician magmatic activity and the structural evolution of the Variscan Orogen. To constrain the duration of magmatism for the Flumendosa area, U-Pb ID/TIMS zircon analyses were performed on samples from a lower volcanic flow of the Ordovician Volcanic Unit and an upper, syn to post-magmatic porphyry. The analyses yielded ages of 457 and 452 Ma respectively. The start of magmatic activity at 457 Ma also marks the end of an unconformity found throughout the External Nappe Zone and provides evidence for correlation with the Sardic Unconformity of the Foreland Zone. Associations of the data with similar-aged Ordovician volcanic rocks and metagranitoids from the Mediterranean realm suggest either: (1) a Rheic wide magmatic event assuming the Posada-Asinara Line (PAL) represents the suture between Laurentia and Gondwana, or (2) the Southern Variscides collisional system of Europe occurred between Gondwana-derived terranes that composed the Hun Superterrane and Laurentia, correlating microterranes throughout the Mediterranean with the northern Gondwanan margin Within the study area, field evidence exists for four phases of deformation related to the Variscan Orogeny. The initial deformational phase (D1) is evidenced by a south-directed thrust fault related to the amalgamation of the composite Variscan nappe stack, indicating a N-S directed contractional event. The second phase of deformation (D2) is defined by the folding of the D1 thrust by the Flumendosa antiform, indicating a NE-SW directed contractional event. The third phase (D3) is expressed by top-to-the-east-southeast movement of the upper thrust sheet evidenced by penetrative deformation, shear, and a domino style series of N-S striking, high-angle faults within the lower sheet. This phase is interpreted to represent a transition from contraction to extension involving orogen-parallel transport. The evidence for the top-to-the-east D3 allows for a reinterpretation of the nature of the contact between the Sarrabus Unit, the last nappe sheet emplaced, and the Gerrei Unit as a strike-slip fault on the side flank of the indenting nappe. The fourth deformational event (D4) produced brittle fractures within the shale that cross-cut the penetrative deformation. Slickensides indicate a transport direction of 014°, 13°, consistent with extensional reactivation of the N-dipping D1 thrust surface during late stages of orogenic evolution. A post-D3 dike that contains the brittle fractures of D4 yielded a U-Pb ID/TIMS age of 302 Ma that constrains the transition between D3 and D4. A post-D4 granitoid of the Sardic Batholith yielded a U-Pb ID/TIMS age of 287 Ma that constrains the end of deformation for the Variscan Orogeny within the External Nappe Zone. These new age constraints are consistent with existing geochronologic data for the Variscan Orogen of Sardinia