THE JURASSIC LIGURIAN TETHYS OCEAN
Ophiolites exposed along the Western Alpine - Northern Apennine (WA-NA) and Corsica orogenic system are thought to represent the oceanic lithosphere of the Ligurian Tethys (or Ligure-Piemontese basin). The Ligurian Tethys is believed to have developed by progressive divergence of the Europe and Adria blocks (Fig. 1), in relation to the pre-Jurassic rifting and Middle Jurassic opening of the Northern Atlantic (Dewey et al., 1973; Lemoine et al., 1987; Bill et al., 2001).
In the last decades, numerous contributions concerning the WA-NA ophiolites evidenced that: i) rather fertile, cpx-rich lherzolites are dominant (Bezzi and Piccardo, 1971; Nicolas and Jackson, 1972), while depleted, cpx-poor peridotites are subordinate; ii) both gabbroic intrusives and basaltic volcanites have MORB affinity (Serri, 1980; Beccaluva et al., 1980).
The main lithostratigraphic features of the WA-NA ophiolites are the following:
1) gabbroic rocks show intrusive relationships with mantle peridotites;
2) basaltic rocks mostly occur as pillowed or massive lava flows and as dykes and sills intruding gabbros and mantle peridotites. A true sheeted dyke complex is lacking and the basaltic layer is thin and discontinuous; 3) sedimentary breccias are locally widespread, commonly interlayered with basaltic lavas; 4) the stratigraphic juxtaposition of pillow lavas, ophiolitic breccias and pelagic sediments over mantle peridotites and gabbros are characterized by the occurrence of fault rocks (e.g., the ophicalcites).
Thus, a general consensus exists on the idea that the Ligurian Tethys was floored by a peridotite-gabbro basement (Decandia and Elter, 1969; Bezzi and Piccardo, 1971a; Piccardo, 1976,1983; Lemoine et al., 1987), subsequently covered by extrusion of a discontinuous layer of pillowed basaltic flows and by pelagic sediments. The oldest pelagic sediments of the Ligurian Tethys are radiolarian cherts, which are coheval to the basaltic extrusions and have Middle to Late Jurassic age (De Wever and Caby, 1981; Marcucci and Passerini, 1991). In particular, the oldest radiolarian cherts were dated as Bathonian (Bill et al., 2001). Therefore, the inception of the oceanic stage, following the continental breakup, is considered not older than ca. 165 Ma.