STRUCTURES ALONG THE OROBIC THRUST, CENTRAL OROBIC ALPS, ITALY

A series of regional deformation phases is described for the metamorph ic basement and the Permian cover in an area in the central Orobic Alp s, northern Italy. In the basement deformation under low-grade amphibo lite metamorphic conditions is followed by a second phase during retro grade greenschist conditions. These two phases predate the deposition of the Permian cover and are of probable Variscan age. An extensional basin formed on the eroded basement during the Late Carboniferous, fil led with fan conglomerates and sandstones, and rhyolitic volcanic rock s. Well-preserved brittle extensional faults bound these basins. Furth er extension deformed basement and cover before the onset of Alpine co mpressional tectonics. Cover and basement were deformed together durin g two phases of compressional deformation of post-Triassic age, the fi rst giving rise to tectonic inversion of the older extensional faults, the second to new thrust faults, both associated with south-directed nappe emplacement and regional folding. Foliations develop in the cove r only during the first phase of deformation as part of the activity o n ''shortening faults''. Main activity on the Orobic thrust actually p ostdates the first phase of thrusting and foliation development in the cover.