THE MARITIME ALPS ARC IN THE LIGURIAN AND TYRRHENIAN SYSTEMS

After Late Eocene meso-alpine collision between the European and Adria plates, the Maritime Alps underwent further deformations linked to the Apennine orogenesis and consistent with a stress field developed during anticlockwise rotation. The western limit of the rotated sector is east of the Argentera massif and, to the southwest, represents the northwestern margin of the post-Eocene Ligurian sphenochasm. The sinistral rotation of the Maritime Alps initially occurred contemporaneous with the opening of the Ligurian basin (about 30-19 Ma) and probably continued during the subsequent opening of the Tyrrhenian basin (about 10-0 Ma). The rotating block comprises the northwestern part of the Adria indenter and a part of the paleo-European plate (namely, the Maritime Alps, Corsica and Sardinia). If a convenient configuration is assumed for the Moho of the Adria margin and suitable poles are chosen for subsequent rotations, then a simplified model of rigid body can explain the rotation structures in the Maritime Alps, the Apennines and Po Plain subsurface compressional arcs, the late foreland-directed transport of nappes in the Western Alps, the presence of extensional and dextral transcurrent structures, and the general uplift of the lithospheric mantle between the Ligurian sea and the Ivrea body. As to the causes of rotation, two alternatives (''subduction'' or ''asthenosphere uplift'') are discussed. Chronological and mechanical arguments seem to favour the latter.