Aspetti morfologici ed evidenze neotettoniche della Linea Schio-Vicenza

Morphological and neotectonic evidences along the Schio-Vicenza Line

Abstract

This study considers the morphological and neotectonic evidence of the Schio-Vicenza Line at the point where it cuts the Marana-Piovene Line and along part of the tectonic scarp bounded to the east by the relief of the Lessini Mountains. In this area, the Permo-Triassic, Jurassic, Cretaceous, and Tertiary sedimentary formations lie over the outcropping South-Alpine crystalline basement. Eruptive products from two magmatic cycles cross or are intercalated with the various rocks of the stratigraphic series: the Middle Triassic cycle is characterized by acid materials, the Paleogene cycle by basaltic ones. In the plain sector, deep drillings have shown that Quaternary marine deposits are lacking; in the uppermost part, fluvial deposits about 100 m thick are found. The most important structures of the area are the Marana-Piovene and Schio-Vicenza Lines. The former, running ENE-WSW, corresponds to a fold-fault overthrusting towards the east. The latter runs straight NW-SE and cuts the Marana-Piovene Line perpendicularly near Poleo. According to the bibliography, the Schio-Vicenza Line has the following features: Strike: NW-SE, generally subvertical or, in the plain, dipping NE sharply. Length: between Besenello in Val Lagarina and near Conselve, to SE of the Euganean Hills. Displacements and types of movement: both throws and strike-slips. Age of formation: probably Triassic; it definitely existed in the Paleogene. Phases of activity: various, from Triassic to Quaternary. In particular, the activity of the line during the Quaternary is proven only for some parts buried under the plain. These bibliographical data formed the basis of a detailed morphological examination carried out in three areas believed to be of importance in verifying the hypothesis of uplift in the sector west of the Schio-Vicenza Line, maximum movement occurring near to the piedmont belt along the Marana-Piovene Line and, for a short stretch north of Schio, along the Schio-Vicenza Line itself. A similar detailed morphotectonic survey, carried out near S. Vito di Leguzzano, revealed that the eastern margin of the Lessini Mountains is uplifted with respect to the center of the Venetian part of the Po Plain, which is lowered, as stated by FINETTI (1972). In particular, in the Val Gogna, terraces were identified cut both on rocks and on landslide deposits, deeply dissected but closely linked altimetrically until they reach the plain, where their links with the present valley bottom suddenly seem to be interrupted. A comparison of the trends of the present and the ancient (pre-Quaternary) longitudinal profiles clearly shows a divergence attributed to a Pleistocene (?) uplift of this sector of the pre-Alpine margin, at the intersection between the Schio-Vicenza fault and the Marana-Piovene Line. In this area of interference of the two large structures, other evidence may confirm the differential uplift of the western sector of the Schio-Vicenza Line, e.g., eastward displacement of the Gogna seasonal stream, which now runs along a gorge excavated in the rock at the foot of M. Guizza-Faedo, and the clear SE deviation of the stream descending from the Val Mara. Near S. Vito di Leguzzano, paleosurfaces and a paleo-riverbed were also identified, both interrupted at the Schio-Vicenza Line. Here too, the longitudinal profiles diverge with respect to the present valley bottom, supporting the hypothesis that the western sector of the Schio tectonic system was uplifted, with consequent lowering of the eastern sector. The trend of the Leogra seasonal stream, which in the past ran at the foot of the eastern scarp of the Lessini Mountains, as shown by a paleo-riverbed near Magré Vicentino, underwent the uplift effects of the western sector and runs initially eastwards as far as the hills NE of Schio. It then returns (as MOLON had reported as far back as 1883) towards the eastern flank of the Lessini Mountains, along the fault angle depression caused by the Schio-Vicenza Line.