Associazioni micro e mesostrutturali nelle Argille Scagliose del Pedeappennino Bolognese: loro significato meccanico

Since some years the Chaotic Complex of the Northern Apennines (Bologna-Modena-Reggio Emilia area) was subdivided in different units: tectonites and olistostromes. Tectonites develop from Ligurian sequences of Cretaceous age, which were strongly deformed by pervasive tectonic lamination. Tectonic lamination caused thinning and boudinage of strata and tectonic duplication of the sequences. The structures which characterize tectonites are described and interpreted in previous works; here they are reconsidered by the comparison with fault rocks structures. The studied examples of fault rocks are linked to brittle tectonic deformation and develop along major tectonic lineaments in the Southern Alps and Northern Apennines. Flattening and extension of the tectonites are due to simple shear and not to gravitational spreading. Pervasive simple shear deformation can he linked to shear zone developed along thrusts in a accretionary prism, as shown by comparison with DSDP samples from actualistic examples. The mechanical behaviour of poor consolidated clays, under high pore pressure, observed in DSDP cores can easily explain the pervasive development of (argille scagliose fabric (scaly fabric) and the structural complexity which characterize the tectonites. Very thin lithons (microlithons), which are developed in the tectonite matrix, give high tectonic mobility producing flattening of the bedding and boudinage like structures. This first deformational phase probably occurs during Cretaceous (?) and Eocene age. Subsequent tectonic events (Oligocene and Early Miocene age) affect the Epiligurian sequences of Early Oligocene («peliti variegate» and olistostromes) and Late Oligocene-Early Miocene (Marne di Antognola) age. The «peliti variegate» and the olistostromes are frequently involved in tectonic duplications and laminations with and within tectonite units. Early Oligocene deposits suffer a severe, but less pervasive than tectonite deformation producing a spaced «argille scagliose» fabric (scaly fabric). The assemblage of tectonites and tectonized deposits of Early Oligocene age, both characterized by scaly fabric, is here defined as «Argille Scagliose» unit. Few, well developed, thrusts arc due to the last tectonic phases (Tortonian ?, Messinian and Plio—Pleistocene age); they testify the last stage of competence increase and a pore pressure decrease in Argille Scagliose, mechanical behavior of which become very similar to the one of the tectonic elements present both within ligurian nappe substratum and within Po Plain buried thrusts. The geometry and style of Neogene age thrusting is very similar inside Argille Scagliose as well as in Po Plain buried thrust system.