Caratteri strutturali mesoscopici delle Argille Scagliose dell'Appennino bolognese

Several tectonic meso-structures have been observed within the Ligurian chaotic units (Argille Scagliose Auct.) included in the Pedeapennine deformed belt of the hills near Bologna. Similar structures have observed in nearly all the outcrops of the Argille Scagliose between the Reno and Sellustra Valleys, in several of which they are impressively exposed. The following are the most important observed meso-structures: — Slip planes. These structures affect the Argille Scagliose clayey matrix and develop an intense tectonic lamination. The slip-plane surfaces show striae, mechanical steps, grooves and other slip structures typical of faults in clayey rocks. — Lenticular bodies. There are clayey, marly and sandy bodies included in the Argille Scagliose matrix. They represent residual parts of previously more tabular and continuous lithozones which underwent strong flattening processes accompanied by boudinage. The strain behaviour of the various lithological types are very different. The more competent lenticular bodies display brittle systems of deformation. The observed brittle structures indicate that boudinage is the result of an extensive-flattening produced by simple shear. — Folding. Isoclinal, asymmetric and disarmonic folds have plastically deformed the slip planes. The folded zones are enclosed between unfolded levels. Two genetical interpretations are proposed based on drag mechanism and/or on pseudo-diapiric lateral injection. Owing to the presence of these structures the Argille Scagliose in the studied area may be considered as a «tectonic mélange» originated by a very pervasive deformation. This deformation is related to simple shear and pure shear stresses acting on brittle and/or visco-plastic materials. The observed structures can be mainly related to the neogenic polyphased tectonics which originated the thrust system recognized underneath the Ligurian units and across the Apennine border by subsurface geological data and reconstruction produced by AGIP.