Tectonite units and olistostrome bodies in the Argille Scagliose complex of the Northern Apennines (Bologna area, Emilia, Italy)

The Argille Scagliose complex of the Northern Apennines have been recently subdivided in tectonics and olistostromes, both characterized by a “block in clayey matrix” texture. Tectonite units derive from tectonic lamination of an lower Cretaceous Ligurian sequence (the so called “basal complexes” of the Ligurian Flysch). These units cannot be considered as a mélange at a regional scale, in spite of “block in matrix” fabric, because they outcrop in narrow (less than 1 km thick) elongated belts, separated by tectonic contacts and characterized by a high coherence of lithology and age (tectonite units on Neocomian-Baremian and Aptian-Albian stratigraphic intervals). A mixing of different lithotypes of the same age is therefore observable at the outcrop scale inside a single belt and was produced by a stratal disruption due to bed elongation, duplexing and interfingering, that occurred almost parallel to the tectonic boundary. This deformation was caused by associations of brittle tectonic discontinuities (shear planes and pressure solution cleavage) which are much most compatible with simple shear than with layer parallel extension by gravitational spreading. The high tectonic mobility of the clayey matrix is produced by a penetrative “scaly” structure dominated by millimetric spaced R, P, and D shear planes. This shear plane association is geometrically similar to the one recognized in fault jocks linked to thrusts and faults, but only in the tectonite units of Argille Scagliose such a penetrative and close spacing has developed in a quite similar way as the scaly fabric recognized in DSDP cores for the more external part of the subduction complex (see f.i. Legs 78A, 66,67 and 84). Olistostromes are sedimentary bodies due to mud and debris flow, that unconformably overlay the tectonite units. They rework the previously deformed tectonites, as shown by the presence of tectonic blocks as large clasts, and can be considered as a mélange at every scale. Scaly fabric is usually absent, except in the tectonite clasts; it only develops as an overprinting on the sedimentary structures close to the Neogene thrusting. Olistostromes unconformably overlay the tectonite belts and can be related to different emplacement events both before (Late Cretaceous (?)-middle Eocene) and in concomitance with the Epiligurian and foreland deposits (middle Eocene to Plio-Pleistocene). The tectonite generation can be considered as mainly predating the olistostrome emplacement and developing in an Late Cretaceous-middle Eocene subduction complex. The Late Cretaceous-middle Eocene olistostromes could have been deposited in the late stage of this tectonic structuration (eo- to meso-alpine compressional phases), while the Epiligurian olistostromes seem to be linked to the Neogene tectonic events producing the thrusts affecting the Ligurian and Epiligurian units as a whole.