SARDINIA, CALABRIA AND THE FAULT OROCLINE OF THE GULF OF OROSEI

The Alpine belt of NE Corsica projects southward toward the east coast of Sardinia. Alpine blueschist occurs in Calabria and serpentinized peridotite has been recovered from fault-block seamounts in the western Tyrrhenian Sea, supporting a former Alpine continuation between Sardinia and palinspastically restored Calabria. The rifting that led to opening of the Tyrrhenian Sea thus took place along the line of an older Alpine suture. However, there has been almost no terrane transfer, as usually happens when rifting does not exactly coincide with an earlier suture. The only possible case is the Nebbio terrane, an apparent analogue of Calabria, now at the west edge of the Alpine blueschists of NE Corsica. Near the middle of the east coast of Sardinia, the indentation of the Gulf of Orosei is paralleled by an array of concave-east faults. These faults displace Jurassic and Lower Cretaceous limestones, and thus were active during the Mesozoic and/or Cenozoic, but it has been difficult to determine the tectonic rôle they have played. In 1974, Alvarez and Cocozza could only find evidence for strike-slip motion on the faults of the Orosei array, and recent compilation maps also mark them as strike-slip. However, strike-slip motion alone does not well explain the east-dip and offset of the Jurassic-Cretaceous limestone blocks, and it is possible that the strike-slip motion is a reactivation of faults with a different original significance. One possibility is that these were originally normal faults related to Miocene rifting between Sardinia and Calabria. However that would require faults with strong convex-upward curvature, opposite to the usual concave-upward geometry of listric normal faults. A more promising possibility is that these were originally west-directed thrust faults cutting both the Hercynian basement of Sardinia and its Mesozoic carbonate cover. We will show how this interpretation of the Orosei fault array can explain the otherwise mystifying map pattern. If this is the correct explanation of the Orosei faults, it suggests that a promontory on the front of the colliding Calabrian microcontinent may have indented the eastern margin of Sardinia, producing basement thrusts.