Nicola Levi

Tectono-metamorphic history of the ophiolitic Lento unit (northern Corsica): evidences for the complexity of accretion-exhumation processes in a fossil subduction system

The Alpine Corsica (Corsica Island, France) is characterized by a stack of continent- and ocean-derived tectonic units, known as Schistes Lustrés complex. This complex is affected by deformation and metamorphic imprint achieved during Late Cretaceous – Early Tertiary subduction- related processes connected with the closure of the Ligure-Piemontese oceanic basin and subsequent continental collision. In the Schistes Lustrés complex, the Lento oceanic unit is characterized by four deformation phases, from D1 to D4 phase. The D1 phase, characterized by blueschist metamorphism, is regarded as related to coherent underplating in a subduction zone at a depth of about 25-30 km. The subsequent deformation phases can be referred to exhumation history, as suggested by the continuous decrease of metamorphic conditions. The transition from accretion to exhumation is represented by the D2 phase, achieved during the development of a duplex structure of accreted units. The D3 phase is in turn achieved by a further horizontal shortening, whereas the D4 phase is developed during an extensional event representing the final exhumation of the Lento unit. On the whole, the data collected for the Lento unit suggest an history that include an accretion by coherent underplating followed by exhumation, more complex than previous described.

THE POLYPHASE DEFORMATION HISTORY OF THE ANTOLA UNIT (NORTHERN APENNINE, ITALY)

The Antola Unit (AU) occurs at the top of the Northern Apenninic nappe pile, cropping out in the area regarded as the boundary between Alpine and Apenninic units. The relationships among all these units are sealed by the Tertiary Piedmont Basin (TPB) (Late Eocene-Miocene). The AU is characterized by a succession that consists of an Helmintoid Flysch, correlated to the External Ligurian successions. The first part of the AU deformation history represents the Early to Middle Eocene Meso-Alpine tectonics, related to closure of the Ligure-Piemontese oceanic basin and the subsequent continental collision; the second one is related to the reactivation of the Meso-alpine structures during the Late Oligocene-Early Miocene. The first deformation phase (D1) is characterized by structures, as for example folds and boudinage, affecting mostly the base of the AU and showing features indicative of soft-sediment conditions. All these structures are deformed by a folding phase (D2) characterized by the development of sub-isoclinal folds and extensional tectonic contacts. The structures related to the D1 and D2 tectonic phases are sealed by the TPB deposits. The third deformation phase (D3), affecting also the TPB deposits (Early Oligocene), is characterized by the development of open, asymmetric, overturned folds with sub-horizontal axial planes. The axes orientation is strongly variable, with foldfacing ranging from North to East. The D3 structures are deformed by a D4 folding phase, characterized by the development of open concentric folds with sub-vertical axial plane. The kinematics indicators of the D1 phase are indicative of an “Alpine” vergence during the early stage of its emplacement. The D2 is indicative of a NE-ward emplacement, probably related to a gravitational tectonic phase. The Alpine structural setting was subsequently reactivated during the Late Oligocene-Early Miocene age, as supported by the occurrence of the D3 phase folding structures affecting the TPB deposits.