GNGTS 2016 - Atti del 35° Convegno Nazionale

132 GNGTS 2016 S essione 1.1 Pedostratigraphy and micromorphology of soil thin sections as a tool in paleoseismology: deciphering past processes interacting with tectonics C. Frigerio 1 , A. Zerboni 2 , F. Livio 1 , M.F. Ferrario 1 , A.M. Michetti 1 , L. Bonadeo 3 , F. Brunamonte 1 1 Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, Como, Italy 2 Dipartimento di Scienze della Terra A. Desio, Università degli Studi di Milano, Italy 0 INGV, Roma, Italy Introduction. Paleoseismology aims at identifying and characterizing past earthquakes using geological evidence (e.g., McCalpin, 2009). In particular, relevant information can be gathered thanks to the crosscutting relationship existing between pedosedimentary deposits and tectonic structures, usually investigated through trenching. In such approach, the detailed study of soils/paleosols and pedostratigraphic sequences has been extensively included. Nevertheless, soils have been usually considered as simple marker horizons and in very few cases the interplay between tectonic deformation and typical soil microfeatures has been investigated (e.g., Nelson 1992; Previtali 1992;Amit et al. , 1996; Livio et al. , 2014). Soil microfeatures (e.g., clay coatings, nodules, papulae ) can be analysed and identified through the micromorphology of soil thin sections, allowing including additional constraints into the paleoseismological reconstruction built up on the basis of crosscutting relations. Previtali (1992), applied the study of soils in these terms, stating that “seismic events could disturb the internal arrangement of many soils, leaving behind macroturbations” . He used the term “seismipedoturbations” (introduced by Boul et al. 1973) for indicating “upsets, deformations and collapses in soil profile due to tectonic activity” . This issue was no longer discussed afterwards, except in describing stress features in the glaciotectonic context (Khatwa and Tulaczyk, 2001; Larson et al. , 2016; Menzies and Reitner, 2016). In some recent works, we take up this topic (e.g., Livio et al. , 2014; Zerboni et al. , 2015; Frigerio et al. , in press), integrating paleoseismology, structural analysis and pedostratigraphy, in order to disclose the earthquake-related evolution of two sites in northern Italy, located in compressional tectonic environment and characterized by complex sequences of Late Pleistocene loess-paleosols. In the following we briefly summarize the main results and observations related to this topic. Monte Netto site (northern Italy - BS). The site is located in the central portion of the Po Plain foredeep. Monte Netto is an isolated hill, deriving from a tectonic uplift, induced by an outcropping secondary anticline with some related faults (Figs. 1b, 1c). In a quarry exposed at the top of this hill, we identified a thick and complex loess-paleosols sequence, resting upon fluvial/fluvioglacial deposits. The analyses of thin sections of undisturbed soil blocks, allow deciphering at least two deformation event. Fig. 2 shows a thin section of sediment sampled in a system of subvertical fractures deforming fine sand fluvial sediments. At the macroscale (Fig. 2a), the slide shows planar voids, ascribable to fractures, filled by a clay-rich plasma (visible at the microscale, Fig. 2b-d’), which displays different generations of clay infillings characterized by different colours and textures. These differences indicate several infillings, whose formation required a multistep process of fissures opening and infilling by pedogenetic clay (illuviation); these events are related to subsequent coseismic movements. Pecetto di Valenza site (northern Italy – AL). At the Pecetto di Valenza site (Figs. 1d, 1e) we analyzed a 10 m long and up to 4 m deep stratigraphic section, located in the vicinity of the southern margin of the Valenza Plateau, in the eastern sector of the Monferrato Arc (western Po Plain). This site is characterized by a recent paleoseismic reverse surface faulting affecting a complex pedosedimentary sequence, characterized by, from the bottom, a marly bedrock, followed by a colluvial deposit cover with two loess covers at its top. Also here, the paleoseismological history has been constrained thanks to the study of some soil thin sections, obtained from undisturbed blocks taken in correspondence with damage features.