Extensional tectonics on Sardinia (Italy): insights into the arc–back-arc transitional regime

Abstract

Although the tectonic features and stress regime typical for accretionary complexes and back-arc domains have been widely documented so far, few are known on the transitional zone separating these two systems. Here we report on structural analysis and anisotropy of magnetic susceptibility (AMS) results from Eocene–Pliocene sediments exposed in western Sardinia. From late Oligocene to middle Miocene, the studied area was located between the Alpine–Apennine wedge to the east, which was undergoing shortening and accretion, and the Liguro–Provençal basin, undergoing extension and spreading. We find that, prior to the formation of the Liguro–Provençal basin, the middle Eocene–lower Oligocene sediments cropping out at the southwesternmost edge of Sardinia were subjected to NE–SW shortening (in present-day coordinates), in agreement with recently reported geological information. Conversely, the upper Oligocene–Pliocene sedimentary sequences record a different evolutionary stage of extensional processes. Upper Oligocene–middle–upper Burdigalian sediments clearly show a N–S-oriented magnetic lineation that can be related to extensional direction along the prevalent E–W-oriented normal faults. On the other hand, no magnetic lineation has been detected in upper Burdigalian–Serravallian sediments, which mark the end of the first rifting process in Sardinia, which likely coincides with the rift-to-drift transition at the core of the Liguro–Provençal basin. Finally, a NE–SW extension is observed in two Tortonian–Pliocene sites at the northwestern margin of the NNW–SSE-oriented Campidano graben. Our study confirms that AMS may represent a valuable strain-trajectory proxy and significantly help to unravel the characters of temporally superimposed tectonic events.