Abstract

This paper describes the structural setting of a segment of M. Orsaro-M. Cetona Ridge, representing an important regional morphotectonic feature, oriented NNW-SSE and over two hundred kilometres long. The Rapolano-M. Cetona tract separates the Siena-Radicofani Neogene Basin (to the west) from Valdichiana Neogene-Quaternary Basin (to the east). This ridge is mainly composed of Triassic-Oligocene stratigraphic units belonging to the Tuscan Succession, deformed during the tectonic evolution of the Northern Apennines (Losacco & Del Giudice, 1958; Passerini, 1964; Lazzarotto, 1972; Bertini et alii, 1991; Brogi et alii, 2000). Two significant areas south of the Chianti mountains have been examined in order to unravel the relationships between the Tuscan Succession and the Argille e Calcari Units, representing the subligurian Unit overthrusted above the Tuscan Succession during the Northern Apennines tectonic evolution. The first area is located between the villages of Sinalunga and Rapolano Terme, whereas the second is close to Civitella in Valdichiana (figs. 1, 2, 3 e 5). In the Sinalunga-Rapolano Terme area a Subligurian unit, referable to the Argille e Calcari Unit (Boccaletti et alii, 1987), rest on the Macigno Fm. This Unit is bounded to the west by the Macigno Fm. and to the east by Pliocene-Quaternary deposits of the Valdichiana Basin (Losacco, 1944). The Argille e Calcari Unit (Subligurian Unit and/or Canetolo Unit) is represented by the Successione argillitico-calcarea di Canetolo-Cirone (Boccaletti et alii, 1987). The Macigno Fm. and Argille e Calcari Unit also outcrop in the Civitella in Valdichiana area. The latter consists of the Successione argillitico-calcarea di Canetolo-Cirone with interbedded the Calcareniti di Groppo del Vescovo (Boccaletti et alii, 1987). Geological survey and structural analysis have allowed the structural setting of these areas to be defined. Three tectonic phases are detected by superimposition of structures: i) thrust of the Argille a Calcari Unit above the Tuscan Succession (D <sub>1</sub> phase), ii) over-turned east-verging folds, oriented NNW-SSE (D <sub>2</sub> phase) and iii) high-angle normal and strike-slip faults oriented N150 degrees -N170 degrees and N40 degrees -N70 degrees (D <sub>3</sub> phase). In the study areas F <sub>2</sub> folds are the main structures characterising the tectonic framework; their dimension ranges from kilometric to metric. In particular in the Sinalunga-Rapolano Terme area, three map-scale antiforms are present (fig. 2). These folds have been assigned to D <sub>2</sub> tectonic phase based on structural evidence from the Sinalunga-Farnetella area (fig. 3), where the thrust surface separating the Argille e Calcari Unit from the Macigno Fm. (Tuscan Succession) is involved in the easternmost map-scale overturned antiform. The folds have been later dissected by high angle normal and strike-slip faults (Phase D <sub>3</sub> ). Structural analysis carried out in the Civitella in Valdichiana area, have provided the same structural framework. The structural evidence, coupled with data from the literature, has led to improved knowledge of the structural evolution and tectonic setting of the M. Chianti-M. Cetona ridge. The folds found are referable to the D <sub>2</sub> tectonic phase. They post-date the thrusting phase (D <sub>1</sub> ) in the building of the Northern Apennines.

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