The deformation history of an accreted ophiolite sequence: the Internal Liguride units (Northern apennines, Italy)

Abstract

The Mt. Gottero, Bracco/Val Graveglia and Colli/Tavarone units from the Northern Apennines, Italy, are remnants of the Ligure-Piemontese oceanic lithosphere that were involved in the tectonic events related to convergence between the European and Adria plates. These events included late Paleocene – middle Eocene subduction followed by late Eocene continental collision.These units, referred to as Internal Liguride units, consist of Jurassic ophiolites covered by a thick sedimentary sequence, which includes late Jurassic-Santonian pelagic deposits (chert, Calpionella limestone and Palombini shale) and Campanian-Early Paleocene trench deposits (Val Lavagna shale and Mt. Gottero sandstone) that are overlain by Early Paleocene lower slope deposits (Bocco shale). This sequence is interpreted as recording trenchward motion of the oceanic lithosphere.The structural analysis of the Internal Liguride units reveals a pre-Oligocene deformation history, which includes the following stages:– stage 1: pre-folding, bedding-parallel veins developed during burial and dewatering of a thick sedimentary sequence;– stage 2: D1 phase distinguished by non cylindrical, isoclinal folds with approximately similar geometries. The associated slaty cleavage is characterized by a mineral assemblage related to P/T metamorphic climax of 0.3–0.4 GPa and 180°-300°C. The asymmetric recrystallized tails around detrital minerals and framboidal pyrites indicate a non-coaxial deformation history characterized by top-to-west sense of shear. Cataclastic shear zones developed in the latest stage of D1;– stage 3: D2 phase marked by overturned, parallel folds associated with a crenulation cleavage. The following D3 phase is characterized by gentle folds with a steep axial plane.The sedimentary records and the structural features support the interpretation of the Internal Liguride units as slices of oceanic lithosphere deformed in a subduction setting. Their deformation history probably developed during underthrusting, cohereht underplating and later exhumation in an accretionary wedge related to an east-dipping, low-rate subduction zone.