The evolution of the Piemonte-Ligurian ocean: a structural study of ophiolite complexes in Liguria (NW Italy)

Abstract

In this thesis, the geodynamic evolution of the Piemonte-Ligurian ocean is investigated by means ofcombined structural and petrological methods. It aims to assess the most important processes involved in [I] incipient oceanic rifting and opening of the basin, and [2] subduction of oceanic lithosphere. The conclusions drawn from this study primarily apply to the situation in the Piemonte-Ligurian ocean, and the dynamics of orogeny in the Alps and Apennines. However, strong analogies exist between the Piemonte-Ligurian ophiolites and oceanic lithosphere exposed in presentday oceanic rifts (Le Red Sea), along margins of slow-spreading "Atlantic-type" oceans, and in marginal basins (Sea of Japan, Tyrrhenian Sea, Cayman basin). Since the development of these basins is a topic ofdebate (e.g. Moores 1982; Bonani 1988; Jolivet et al. 1989; Tatsumi et al. 1989), it is suggested that the processes, inferred here for the case of the Piemonte Ligurian ocean, may also playa role in the geodynamic evolution of these modem basins. The data underlying this thesis are derived from a number of ophiolite complexes exposed in the Ligurian Alps and Apennines (Fig. 1.1, 1.2). In this region, a sequence oflow-grade (prehnite-purnpellyite facies) to high-pressure low-temperature (eclogite facies) metamorphic ophiolites and related sediments allows a study of the structural and metamorphic evolution ofoceanic lithosphere in a subduction zone which evolved into a zone of continental collision. In addition, a structural study of the low-grade ophiolites in the Apennines (Bracco Massif) and of a large body of mantle peridotites in the Ligurian Alps (Erro-Tobbio peridotites in the Voltri Massif) enables the identificationof the main processes during incipient oceanic rifting and opening ofthe Piemonte-Ligurian ocean.