Deformation path of the Internal Liguride Units (Northern Apennine, Italy): record of shallow level underplating in the alpine accretionary wedge

In the Northern Apennines (Italy), Jurassic ophiolite sequences and related sedimentary covers belonging to the Internal Ligurid Units (Mt.Gottero, Bracco/Val Graveglia and Colli/Tavarone Units) are interpreted as remnants of oceanic lithosphere of the Western Tethys involved in Cretaceous/Eocene intraoceanic subduction and subsequent continental collision. The sedimentary cover of the Jurassic ophiolite sequence includes Chert (middle Callovian - Tithonian), Calpionella Limestone (Berriasian - Valanginian) and Palombini Shale (Valanginian - Santonian). The Palombini Shale grades upwards in the Val Lavagna Shale and Mt.Gottero Sandstone (Campanian - early Paleocene), represented by distal and proximal turbidites derived from a continental source. This sequence is completed by mass- gravity deposits (Bocco Shale - Colli/Tavarone Formation) derived from previously accreted slices of oceanic lithosphere. The sedimentary cover of the Internal Ligurids ophiolites is interpreted as a coarsening-upward sequence of pelagic, trench and lower-slope deposits resulting from the motion of the oceanic lithosphere toward a subduction zone. The pre-Oligocene deformation path of the IL Units includes: 1st stage - development of bedding-parallel calcite and/or quartz veins with a coarse-grained mosaic texture; 2nd stage - characterized by markedly non-cylindrical, isoclinal F1 folds with thickened hinges and boudinaged limbs. The S1 axial-plane foliation can be recognized in shaly lithologies as slaty cleavage characterized by an assemblage of metamorphic minerals consisting of quartz, albite, chlorite, white mica (illite), calcite and Fe oxides. The A1 axes show a scattered distribution clustering around N150E/30E, whereas the L1 mineral lineations, represented only by calcite fibers around pyrite framboids, show NW/SE to E/W strikes. The relationships between S0 and S1 surfaces and shear-sense criteria point to a W/SW vergence of the D1 deformations. Illite crystallinity and vitrinite reflectancy point to low-grade metamorphism with P/T conditions of 300-400 MPa and 200°-300° C. The values of finite strain are low with a prevalence of oblate-type ellipsoids. Associated with D1 deformation, a vein set with calcite antitaxial fiber infillings is recognized. All the D1-deformation structures are coherent, with simple shear as the main deformation mechanism. Large scale folds with ophiolites at the core and thrusts are developed during the D1 phase. 3rd stage - distinguished by generally overturned, parallel F2 folds with variable style related to a flexural-slip mechanism. The A2 axes are clustered around N130E/N160E. The S2 axial-plane foliation is recognizable as crenulation cleavage without syntectonic recrystallization. During the D2 phase, large scale folds associated with E/NE thrusting are developed. The features of the deformation path recognized in the IL Units indicate their involvement in the E/SE-dipping, low-rate subduction zone. By comparison with modern and ancient examples, the 1st and 2nd stages can be related to underthrusting and coherent underplating at shallow levels (8-12 km). The 3rd stage can be connected to exhumation related to extensional tectonics in the inner part of the alpine accretionary wedge.