APPLYING THE FRANCISCAN MODEL TO A NON-COLLISIONAL ALPINE SEGMENT IN THE CALABRIAN ARC OF SOUTHERN ITALY

The Calabrian Arc is an Alpine-aged, blueschist facies subduction complex exposed in southern Italy and comprised of dismembered ophiolites and continental crust fragments. Although it is part of the much larger Alpine-Apenninic belt, the local tectonic history here is quite different than in the rest of the orogen, most notably due to lack of evidence for continental collision. This makes a comparison to the non-collisional Franciscan complex useful to help understand some of its features. Metamorphic histories in the Calabrian orogen are similar to those in the Franciscan belt. Counterclockwise P-T paths with cool exhumation are present in some units, evidenced by late lawsonite overprinting epidote blueschist assemblages. This can be seen in the Diamante-Terranova Unit where lawsonite blueschist metabasalt does not have the usual greenschist facies re-equilibration usually present in Tethyan orogens. This anomalous history may be due to the fact that a small region of oceanic crust, or thinned continental crust, continued to subduct under the Calabrian accretionary prism, while areas to the north and south underwent continental collision along with the accompanying heating event. An inverted metamorphic gradient may also be present within the Calabrian belt, similar to that in the Franciscan. The Zangarona Unit, at the roof of the subduction complex, preserves a high-grade garnet amphibolite overprinted by blueschist; this may represent either an inherited Hercynian high pressure unit later overprinted with Alpine blueschist facies conditions, or it may have formed during initiation of subduction beneath a relatively high temperature unit. Structurally, Calabria has generally been interpreted in terms of nappes. However, many of the units are similar to mélange units within the Franciscan and represent discontinuous scraps of ocean crust which have been successively added to an accretionary complex. Along-strike continuity of units is a poor working assumption in Calabria, as in the Franciscan. A comparison between Calabria and the Franciscan has its limits: in Calabria the upper plate of the subduction system is continental crust, eclogite is not exposed, and the vergence of units within the orogen is still uncertain. Nevertheless, the Franciscan has provided a good template for comparison of Calabria.