Abstract:
The Tyrrhenian-Apennine region consists of a "rift/fold-and-thrust belt" couple that has been developing since the middle-late Miocene. Rifting propagated eastward towards the Adriatic foreland to produce a series of offshore and onshore asymmetric graben and normal fault systems. Similarly, a compressive front some tens of kilometers eastward of the rift zone allowed the progressive emplacement of thrust sheets and the development of foredeep basins. We infer that the extensional deformation of the Tyrrhenian rift zone occurs by the discontinuous shear across a sequence of faults that basically detach at the base of the crust while lithospheric mantle thinning occurs by prevalent ductile deformation. Involvement of the entire crust in the extensional process, at least in the southern Tyrrhenian Sea is evidenced by the unroofing of mantle harzburgites sampled during the ODP Leg 107 drilling of the Vavilov basin (site 651). Lithospheric mantle thinning and crustal extension are not simply related in the Tyrrhenian-Apennine region because the locus of maximum crustal thinning (the Tuscan-Tyrrhenian zone) is separated spatially by some tens of kilometers westward from the region of present normal fault activity. However, the spatial form of the lithospheric mantle thinning can be mapped using the distribution, timing and chemical composition of the volcanics observed across the Tyrrhenian-Apennine region. For example, Plio-Pleistocene Na-alkaline rocks characterize the western border of the Tyrrhenian sea, Plio-Pleistocene transitional-MORB basalts are widespread within the zone of Tyrrhenian extension, Quaternary HK-alkaline rocks characterize the western border of the Italian peninsula (Roman-Campanian Province), while late Pleistocene melilititic and Ca-carbonatitic rocks occur within the Apennine mountain belt (Intramontane Ultra-alkaline Province). These magmatic patterns suggest that the present topography of the lithosphere-asthenosphere boundary is strongly asymmetric towards the east, with the step front located directly beneath the western boundary of the Apennines. Towards the west, the lithosphere-asthenosphere boundary gradually deepens towards the Corsica-Sardinia margin. Calculation of the horizontal forces implied by the existence of this asymmetric distribution of lithospheric mantle thinning defines a major zone of in-plane compression located at the eastern border of the thinned Tyrrhenian lithosphere. Considerations of the force required to support the present relief of the Apennines suggest that it is similar to the magnitude of the in-plane compression predicted from the inferred lithospheric mantle thinning using the volcanics. We conclude, therefore, that the contractional deformation responsible for the Apennines can be independent of subduction, per se, being instead a consequence of the compressional body forces created by the asymmetric thinning of the lithospheric mantle. With time, the Apennine thrust sheets will be "pulled apart" again as crustal extension continues to propagate eastward.