GNGTS 2016 - Atti del 35° Convegno Nazionale

140 GNGTS 2016 S essione 1.1 TEPHROCHRONOLOGY AS A TOOL FOR ACTIVE TECTONICS STUDIES IN PENINSULAR ITALY B. Giaccio 1 , P. Galli 1, 2 , P. Messina 1 , E. Peronace 1 1 Istituto di Geologia Ambientale e Geoingegneria, CNR, Rome, Italy 2 Dipartimento della Protezione Civile, Roma Introduction. The study of the active tectonics typically requires the application and integration of a number methodological approaches enabling us to acquire data and information on a series of spatial, temporal, and physical parameters, fundamental for understanding the architecture and the behavior of the active and capable faults. Among others, the chronological constraints are essential for evaluating: i) the general status of activity of a given structure; ii) its short- to long-term slip rate and its changes trough time; iii) the time recurrence of the fault ruptures. In the framework of the Italian active tectonics, most of the active and seismogenetic faults are located along the axis of the Apennine chain (Galadini and Galli, 2000; Galli et al. , 2008). Usually, active faults bound and drive since early Quaternary the evolution of continental sedimentary basins hosting thick alluvial-fluvial-lacustrine successions. The investigation and dating of this sediments is thus pivotal to define the Pleistocene-Holocene tectonics. However, until few years ago these sediments were hardly datable, whereas beyond the applicability limit of the radiocarbon (i.e., ~40 ka) the chronological framework of the continental deposits were essentially relied on assumptions and qualitative-speculative regional cross correlations. In this framework, the study of distal volcanic ash layers or tephra - ejected into atmosphere during large explosive eruptions and simultaneously deposited in different stratigraphic settings up to thousands of kilometers from the source - is changing this frustrating limits, assuming a strong relevance as reliable geochronological tool. Indeed, several volcanic centers are located along the western side of the backbone of the Italian peninsula, almost all characterized by an intense explosive volcanic activity; therefore, tephra layers are a common features of the sedimentary successions of the Apennine (Fig. 1), making this region an ideal setting for applying tephrochronology. The tephrochronological method. The tephrochronological method essentially consists in determining the peculiar geochemical features of tephra - i.e., the so called chemical fingerprinting - that allow to unambiguously recognize and trace ash layers in different sedimentary settings, providing thus an effective way through which associated deposits can be reliably dated and correlated over wide regions. As any other comparative method, to be effective, tephrochronology require a reliable and possibly complete reference geochemical and geochronological dataset. Unfortunately, up to few years ago, this was really limited and incomplete, making the application of tephrochronology highly hazardous, weak and misleading. Today, due to recent and continuously enhancing development of tephra studies in central Mediterranean area (e.g. Wulf et al. , 2004, 2012; Munno and Petrosino, 2007; Giaccio et al. , 2012a, 2013, 2014; Paterne et al. , 2008; Zanchetta et al. , 2008; Bourne et al. , 2010; 2015; Sulpizio et al. , 2010; Smith et al. , 2011; Tamburrino et al. , 2012; Tomlinson et al. , 2012; Insinga et al. , 2014; Petrosino et al. , 2014, 2016; Leicher et al. , 2016), we can access to a satisfactory reference dataset for the Upper Pleistocene and partly for the Middle-late Early Pleistocene. The primary geochemical fingerprinting, on which tephra recognition is based, is the major element composition of the glass. This is determined by microprobe analyses (wavelength dispersive spectroscopy; WDS) or SEM (energy dispersive spectroscopy; EDS) which provide the composition of the glass in terms of oxides of a dozen of elements. These data, together with those of the potential correlatives, are then plotted in classification diagrams (e.g. Total Alkali Silica, TAS) and in other covariant diagrams which, highlighting analogies and differences among tephra, allow to correlate the investigated layer to a specific proximal or distal dated counterpart. Moreover, further integrative geochemical analyses that help in making more