Structural Analysis of the Flumendosa Area, External Nappe Zone, Sardinia, Italy

Several collisional orogenic systems have documented expressions of both contractional and extensional deformation. In detail, the kinematic interactions and relative timing of these phases, however, are commonly unclear or not well constrained. The Variscan basement in Sardinia, Italy, was not strongly overprinted by later Alpine tectonics, so contains a well preserved geologic record of Variscan contraction and extension. New mapping and structural analysis of the Flumendosa region, located in the external nappe zone of the Variscan in southwestern Sardinia, suggest a complex deformational history with four stages.

The initial deformational phase (D1) is evident by the thrust faulting of the Cambrian San Vito Sandstone over Silurian shale and phyllite during S-directed amalgamation of the composite Variscan nappe stack. The thrust contact is sharp and visible in a number of outcrops in the mapped area. The second phase of deformation (D2) is defined by the folding of the D1 thrust by the Flumendosa antiform. Analysis of bedding orientation from both the upper and lower thrust sheets suggests a hinge line of 142°, 13°. The third phase of deformation (D3) involved penetrative deformation within the shale and domino style normal faulting in the competent arkosic sandstone within the lower sheet, consistent with top-to-the-east shear and tectonic transport of the upper sheet relative to the lower. This pattern of deformation suggests orogen parallel extension and may represent a transitional phase from early contraction to later extension in the orogen. The fourth deformational event (D4) is evident by brittle fractures within the shale that cross-cut the penetrative deformation. Several of these brittle fractures contain preserved slickensides with a transport direction of 014°, 13° consistent with extensional reactivation of the D1 thrust surface during late stages of orogenic evolution.