SESS 2_16022011_TPR3.pub Il Quaternario Italian Journal of Quaternary Sciences 24, (Abstract AIQUA, Roma 02/2011), Congresso AIQUA Il Quaternario Italiano: conoscenze e prospettive Roma 24 e 25 febbraio 2011 NEW MORPHOLOGICAL, STRATIGRAPHIC, STRUCTURAL AND PEDOLOGICAL DATA ON THE T. TRAVERSOLA DEFORMATION ZONE RECENT EVOLUTION (ASTI, PIEDMONT) Alfredo Doglione1, M. Gabriella Forno2 & Marco Gattiglio2 1 Freelance professional, Via Cernaia 28, Torino 2Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, Torino Corresponding author: M. G. Forno ABSTRACT: Doglione A., Forno M. G. & Gattiglio M., New morphological, stratigraphic, structural and pedological data on the T. Traversola deformation Zone recent evolution (Asti, Piedmont). (IT ISSN 0394-3356, 2011). The new morphological, stratigraphic, structural and pedological data on the type-area Villafranchian Succession suggest the southern extension of the T. Traversola deformation Zone. This structure corresponds to the evident scarp that divides the Poirino Plateau by the Asti Reliefs. In the previous works the northern sector of the structure was, exclusively, recognized. RIASSUNTO: Doglione A., Forno M. G. & Gattiglio M., Nuovi dati morfologici, stratigrafici, strutturali e pedologici sull’evoluzione recente della Zona di deformazione del T. Traversola. (IT ISSN 0394-3356, 2011) I nuovi dati morfologici, stratigrafici, strutturali e pedologici riguardanti la Successione Villafranchiana dell’area-tipo suggeriscono la prosecuzione verso sud della Zona di deformazione del T. Traversola, di cui in lavori precedenti era già segnalato il tratto settentrionale, corrispondente alla evidente scarpata che separa i Rilievi dell’Astigiano dall’Altopiano di Poirino. Key words: Asti Reliefs, deformation, Villafranchian Succession. Parole chiave: Rilievi dell’Astigiano, deformazione, Successione Villafranchiana The morphological evidence of the Tertiary Piedmont Basin (BTP) consists of hilly reliefs: the Turin Hill and the Monferrato Reliefs on the northern side; the Poirino Plateau, the Asti Reliefs and the Alessandria Plateau in the centre; and the Langhe Reliefs on the southern side. The new research focuses on the central part of the BTP, which corresponds geologically to the Asti Syncline (4 in Fig. 1). This structure is made up of the littoral marine Asti Sands (Middle Pliocene) and the deltaic and fluvial type-area Villafranchian Succession (Middle Pliocene-Lower Pleistocene), together with a thin cover of subsequent fluvial sediments (Middle-Late Pleistocene), connected to the ancient courses of the Po and Tanaro rivers (5 and 6 in Fig. 1). The pronounced N-S scarp breaking up the central part is one of the most significant geological features of the hilly area: it is 30 km long and 100 metres high on average, running along a generally straight line, with local-level segmentation. The scarp divides a west sector plain (Poirino Plateau) from a hilly east sector (Asti Reliefs). The scarp, described by numerous authors, has been attributed to different geneses. In the early geological literature (CASTIGLIONI, 1934; SACCO, 1917) the scarp was linked exclusively to the erosional phenomenon produced by the present course of the Tanaro River (8 in Fig. 1) and its tributary hydrographical network, following the deviation of the ancient course (6 in Fig. 1) during the Late Pleistocene. In subsequent studies (FORNO, 1982; ALESSIO et al., 1982; CARRARO & VALPREDA, 1991) the different elevation of the Middle-Late Pleistocene fluvial sediments on the two sides of the scarp led to the presumption of the presence of a vertical d i sp lacement s t ruc tu ra l d iscon t inu i t y corresponding to the scarp, explained as a “flexure”. In the recent literature (CARRARO ed., 1996; BOANO et al., 1997; BOANO & FORNO, 1999) a detailed geological survey of the Villafranchian Succession type-area gave rise to the interpretation that the northern segment of the scarp was connected to a structural discontinuity named “T. Traversola deformation Zone” (1 in Fig. 1). In this segment the structure has traces of a dextral strike-slipe displacement and has different stratigraphic successions on the two sides. In the other segment of the scarp the structure was not recognized due to the scarcity of outcrops and the absence of structural evidence and stratigraphic elements. Indeed, recognition of strike-slipe displacement structures in a sub-horizontal succession is obviously very difficult and this difficulty essentially resulted in a delay in the identification of the T. Traversola deformation Zone, despite its strong morphological evidence (Fig. 2). Only through a detailed survey of the stratigraphic succession outcropping on both sides of the scarp and an interdisciplinary approach, comprising the collection of morphological, structural and pedological data, now being presented, has it been possible to identify a structure along the entire scarp. Specifically, this research has enabled the 101 - 103 102 the depressions aligned along the scarp and the numerous landscapes, are described. Meso-structural evidence suggests the horizontal displacement of the Asti Syncline axis (4 in Fig. 1) and detailed observations have permitted the identification of some metric fault surfaces, along the main structure. The survey of soils involving the sediments on both sides of the scarp, on the other hand, have enabled the correlation of the corresponding geological bodies and identification of the relative movements. Some evidence, already reported in the literature, supports the BTP uplift: the progressive transition from a marine to a continental environment, the deviation of the ancient rivers (5 and 6 in Fig. 1), recognition of the structure’s southern extension and the description of both its chronological reference and displacement type. The new stratigraphic data collected show no vertical displacement and, instead, suggest the horizontal displacement of the Villafranchian Succession body along the scarp. There is also a cut in the continuity of the fluvial sediments connected to the ancient courses of the Po and Tanaro rivers. The deviation of these rivers, responsible for their present courses (7 and 8 in Fig. 1), has essentially been caused by the Pleistocene deformation. The scarp’s morphological features, with deep valleys draining in the opposite direction and an asymmetric transverse cross section, as well as Fig. 1, Structural map of the studied area. 1) T. Traversola deformation Zone (prevailing dextral strike-slipe displacement); 2) C. Fagliaverde fault Zone (prevailing vertical displacement); 3) Castelnuovo fault Zone (prevailing vertical displacement); 4) Asti Syncline axis; 5) Po River ancient course; 6) Tanaro River ancient course; 7) Po River present course; 8) Tanaro River present course. Earth Observing System Data and Information System (EOSDIS). 2009. Earth Observing System ClearingHOuse (ECHO) / Warehouse Inventory Search Tool (WIST). Greenbelt, MD: EOSDIS, Goddard Space Flight Center (GSFC) National Aeronautics and Space Administration (NASA). Schema strutturale dell’area studiata. 1) Zona di deformazione del T. Traversola (con movimento prevalentemente trascorrente destro); 2) Zona di faglia di Cascina Fagliaverde (con movimento prevalentemente verticale); 3) Zona di faglia di Castelnuovo (con movimento prevalentemente verticale); 4) asse della Sinclinale di Asti; 5) antico percorso del F. Po; 6) antico percorso del F. Tanaro; 7) percorso attuale del F. Po; 8) percorso attuale del F. Tanaro. A. Doglione, M.G. Forno & M. 103 Ms. received: Testo ricevuto il the Monferrato. From middle Pliocene to present-day, the Torino Hill northward translation and uplift, induce a RFDZ dextral strike-slipe tectonic (LAUBSCHER et al., 1992; PIANA & POLINO, 1995). According to the new collected data, this tectonic activity can be responsible of the T. Traversola deformation Zone develop. REFERENCES ALESSIO M., ALLEGRI F., AMBROSETTI P., BARTOLOMEI G., BELLA F., BELLUOMINI G., CALDERONI G., CARRARO F., CHARRIER G., CORTESI C., ESU D., FORNO M. G., IMPROTA S., MANFRA L. & PETRONE V. (1982) - Il giacimento fossilifero pleistocenico superiore di Moncucco Torinese. Geogr. Fis. Dinam. Quatern., 5, 219-239. BOANO P., FORNO M. G. & GIARDINO M. (1997) - Deformazioni sinsedimentarie nella successione villafranchiana presso Castelnuovo Don Bosco (Asti). Il Quaternario It. Journ. Quatern. Sc., 10 (2), 355-358. BOANO P. & FORNO M. G. (1999) - La successione “villafranchiana” nell’area di Castelnuovo don Bosco (Asti). Il Quaternario It. Journ. Quatern. Sc., 12 (2), 161 -194. CARRARO F. (ED.) (1996) - Revisione del Villafranchiano nell'area-tipo di Villafranca d'Asti. Atti del Convegno: "Il significato del Villafranchiano nella stratigrafia del Plio- Pleistocene" (Peveragno (CN)-Villafranca d'Asti (AT), 20-24 giugno 1994). Il Quaternario It. Journ. Quatern. Sc., 9 (1), 98-101. CARRARO F. & VALPREDA E. (1991) - The Middle-Upper Quaternary of the Asti Basin. Il Quaternario It. Journ. Quatern. Sc., 4 (1), 151-172. CASTIGLIONI B. (1934) - Sulle cause delle deviazioni dei fiumi. Zeitsch. Geomorph., 8/5. FORNO M. G. (1980) - Evidenza di un drenaggio abbandonato nel settore settentrionale dell'Altopiano di Poirino (Torino). Geogr. Fis. Dinam. Quatern., 3, 61-65. FORNO M. G. (1982) - Studio geologico dell'Altopiano di Poirino (Torino). Geogr. Fis. Dinam. Quatern., 5, 129- 162. PIANA F. & POLINO R. (1995) - Tertiary structural relationships between Alps and Apennines: the critical Torino Hill and Monferrato area, Nothwestern Italy. Terra Nova, 7, 138-143. LAUBSCHER H., BIELLA G.C., CASSINIS R., GELATI R., LOZEJ A., SCARASCIA S. & TABACCO I. (1992) -The collisional knot in Liguria. Geol. Rndsch., 81, 275-289. the entrenchment of the ancient rivers in the east sector, the dissection of the east sector and the genesis of numerous landslides. Other data also suggest some vertical displacement structures known as the C. Fagliaverde fault Zone and Castelnuovo fault Zone (2 and 3 in Fig. 1) (CARRARO ed., 1996; BOANO et al., 1997; BOANO & FORNO, 1999). The most significant are: the vertical displacement of the type-area Villafranchian Succession (Middle Pliocene-Lower Pleistocene), the stratigraphic interference along the structures and the local presence of re-sedimented bodies. Further data suggest, at least, the presence of a major strike-slipe displacement structure; only a short segment of which has been reported in the literature (op. cit.). The most significant data are: the horizontal displacement of the Asti Syncline axis (4 in Fig. 1), the horizontal displacement of the Villafranchian body and the lateral sliding of rivers (ancient courses of the Po and Tanaro rivers (5 and 6 in Fig. 1). In conclusion, the described scarp (Fig. 2) is the morphologic expression of an important and continuous deformation zone, characterized by main dextral strike-slipe displacement, presumably localized near the scarp. These new data explain an old observation (CASTIGLIONI, 1936) that reports the relative stability of the scarp over time: this observation was in conflict with the proposed genesis, connected exclusively to regressive fluvial erosion, and could not be explained. It was impossible to envisage how a high scarp, shaped in incoherent sediments, could not be affected by the regressive erosion. In view of the reported observations it is highly likely that the exceptional morphologic expression of the T. Traversola deformation Zone and its relatively stable position are connected to its probable recent activity. In the early Pliocene, the Apennine tectonic became to affect the Torino Hill inducing dextral activations of the Rio Freddo Deformation Zone (RFDZ) (PIANA & POLINO, 1995). This NNW-SSE deformation zone, developed a few kilometres NNE the studied area, divides the Torino Hill by Fig. 2, The described scarp evidence Evidenza della scarpata descritta Gattiglio New morphological, stratigraphic, structural ... January 14, 2011 14 gennaio 2011