GNGTS 2009 SESSIONE 1.2

NEOGENE-QUATERNARY TECTONIC STRATIGRAPHY OF THE EASTERN SOUTHERN , NE R. Caputo1, M.E. Poli2, A. Zanferrari2 1 Department of Earth Sciences, University of Ferrara 2 Department of Georesources and , University of Udine

Based on the fact that the available chronology and stratigraphy of the Neogene-Quaternary sed- imentary succession within the Eastern Southern Alps is very detailed (Zanferrari et al., 2008; and references therein), we investigated the external sector of the Eastern Southern Alps and performed a detailed structural and stratigraphic mapping of the Tortonian-Quaternary sedimentary units out- cropping within a 120 km-long zone along the foothills of the mountain chain in and regions (Fig. 1). The Eastern Southern Alps is a major strucural subdivision of the broader Alpine Chain and it is conventionally delimited to the north by the Periadriatic Lineament. From a tecton- ic point of view, the Eastern Southern Alps correspond to a distinct late Oligocene-Quaternary oro- gene (Castellarin et al., 2006). This south-verging fold-and-thrust belt was generated during the complex crustal collision and indentation of the Adria promontory underneath the Alpine chain. Due to the relatively large distribution of conglomeratic bodies within the considered strati- graphic interval, we focused our attention on numerous deformed pebbles characterised by pitted surfaces. These structural features are classically related to small-scale pressure-solution processes mainly occurring along those particle contacts that are roughly oriented perpendicular to the direc- tion of maximum compression. Based on the careful analysis of the shape and orientation of the indented features on the pebbles’ surface and following a statistical approach based on as many as possible measurements for each site, the mean orientation of the maximum compressive stress axis (σ1) is obtained by contouring the data on a stereonet and calculating the density peak. We focus on

Fig. 1: Geological map of the investigated area (dashed line). 1: pre-Tortonian successions; 2: Tortonian-Messinian sedimentary units; 3: Pliocene-Pleistocene sedimentary units. Dots represent the sites of meso-structural investigations. TB = Thiene-Bassano Thrust, BC = Bassano- Thrust, MC = Thrust, CA = Cansiglio Thrust, PM = Polcenigo-Maniago Thrust, AR: Arba-Ragogna Thrust, ST: Susans-Tricesimo Thrust.

149 GNGTS 2009 SESSIONE 1.2 the Tortonian-Quaternary deposits outcropping along the foothill belt facing the Veneto-Friuli plain between the Brenta River, to the west, and the Tagliamento River, to the east (Fig. 1). As a whole, the investigated stratigraphic units show a typically clastic shallowing upwards sequence represent- ing the infilling of the Eastern Southalpine foredeep-foreland basins, which developed from Late Oligocene to Pleistocene (Massari et al., 1986; Fantoni et al., 2002). The investigated stratigraphic succession can be separated into three principal units. 1) Tortonian to lower Messinian conglomer- ates. These deposits correspond to the upper part of the Sandstone (VVE) and to the Montello Conglomerate (MON). They largely crop out along the whole study area. 2) Lower Pliocene conglomerates. They correspond to the Osoppo Conglomerate (OSP) exclusively cropping out in the easternmost sector of the investigated area. 3) Late Pliocene - Early (to Middle?) Pleistocene con- glomerates of the Coneglaino Unit (CON) that includes the pelitic-conglomeratic layers of the , Refrontolo and Caneva area. These continental clastic deposits overlie the marine Pliocene sediments of Cornuda (COR). Tectonic stratigraphy. Using the above methodological approach, the occurrence of four distinct deformational events has been determined (Fig. 2). 1) Late Tortonian event. Five localities belonging to the Montello Conglomerate and par- ticularly to its lower and middle members or to the Vittorio Veneto Sandstone represent a first group of sites showing a cluster of mean compressional axes, which are all parallel to the strongly dipping

Fig. 2: Synthetic diagram showing the Tortonian p.p.-Pleistocene sedimentary stratigraphy (units), the period of activ- ity for the major tectonic structures (thrusts), the direction of maximum compression estimated from the meso- structural analyses (?1), the tectonic stratigraphy of the Eastern Southern Alps with the principal deformational events (dark sectors) and the corre- sponding table including all datasets (event), the direction of relative motion of Africa with respect to Europe (AFR/EUR; numbers refer to normal magnetic anomalies, while arrows thickness is proportional to the amount of relative convergence; Mazzoli and Helman, 1994).

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layers. Following the palinspastic restorations the pre-tilting sub-horizontal σ1 axes trend NW-SE with an average orientation of 313°/00°. The lower chronological constraint for this deformational event is provided by the age of the youngest affected deposits (i.e. late Tortonian), while the imprinting process certainly occurred before the beginning of the large-scale fault-propagation- folding, and the consequent tilting, associated with the Bassano- and Cansiglio- Maniago thrusts, whose paroxistic activity started in early Messinian (Castellarin et al., 2006). 2) Late Messinian-Early Pliocene event. Eleven sites belonging to the Vittorio Veneto Sandstone and both lower and upper members of the Montello Conglomerate represent a second group of measurements. In most sites the observed compressional features are parallel to layering or form a low intersecting angle. The palinspastic restoration provides a NNW-SSE trending compression with an average orientation of 338°/04°. As concern the timing, this is constrained i) by the age of the youngest affected rocks (early Messinian) and ii) by the full development of the Bassano- Valdobbiadene and Cansiglio-Maniago thrusts, as lower and upper chronological boundaries, respectively. 3) Late Pliocene event. Seven sites have been included in this group, which is again charac- terised by a mean NW-SE direction of compression (314°/03°). This value is similar and statistical- ly equivalent to that of the Late Tortonian event but there are two main reasons for separating the two datasets and inferring a distinct deformational event. Firstly, in four out of seven sites the youngest affected conglomerates belong to the Early Pliocene Osoppo Conglomerate or even to the Late Pliocene-Early Pleistocene Unit, therefore constraining the lower chronological boundary. Secondly, in the three remaining sites affecting the older Montello Conglomerate the layering is steeply dipping while the measured meso-scale contractional features are horizontal. This observa- tion suggests that they have been imprinted after the occurrence of the macro-scale tilting process associated with the fault-propagation folding of the Bassano-Valdobbiadene and Cansiglio-Maniago thrusts (Messinian to Early Pliocene in age accordingly to Castellarin et al., 1992). Thus confirm- ing the timing of this deformational event: Late Pliocene at the oldest. 4) Early-Middle Pleistocene event. The largest dataset obtained from the investigated area con- sists of seventeen sites forming a last group of mesostructural measurements. They are characterised by a renewed NNW-SSE-trending direction of compression with an average orientation of 160°/03°. Notwithstanding a perfectly matching mean σ1 direction with that of the Messinian event (338°/04°), the striking difference among the two deformational events and groups of data is due to the fact that the measured compressional directions are all horizontal to sub-horizontal, therefore lacking any significant post-deformation tilting. In conclusion, we can say that this last dataset rep- resents a distinct younger deformational event, which likely started in Early Pleistocene. Taking into account the elapsed time, say 1-1.5 Ma, and comparing the average compressional direction obtained from the meso-structural analyses with that inferred from the seismicity of the area (Bressan et al., 2003), it is likely that this tectonic regime is still active. Discussion. Our reconstruction of the late Tortonian-Quaternary Tectonic Stratigraphy in the Eastern Southern Alps has emphasized the occurrence of four distinct deformational events. It is noteworthy that these events do not represent 'simple' local variations of the stress field because for all datasets the corresponding sites of measurements are spread along the entire 120 km-long inves- tigated area and in many cases, the same localities and sedimentary units are affected by more than one dataset (viz. stress field). In particular, the four events could be recognized and distinguished based on i) a different mean direction of compression and ii) their timing of activity. Accordingly during Late Neogene-Quaternary the stress trajectories of the Eastern Southern Alps were affected by repeated rotations with rapid flipping of the principal horizontal stresses. We refer to this behav- iour as Twist Tectonics. The tectonic evolution of the Eastern Southern Alps has been compared with Neogene-Quaternary the convergence direction between Adria-Africa and Europe plates (Fig. 2) showing a good fitting with both timing and directions of compression of the major deformation- al events. However, since late Messinian, the Northern Apennines were close enough to the Eastern

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Southern Alps in order to perturb the stress field and generate short-lived (1-2 Ma) rotations of the stress trajectories (Twist Tectonics) possibly due to accomodation processes of the Adria indenter and overlapping effects of the two 'remote' engines (Africa and Apennines). References Bressan G., Bragato L. and Venturini C. (2003): Bull. Seism. Soc. Am., 93, 1280-97. Castellarin A., Vai G.B. and Cantelli L. (2006): Tectonophys., 414, 203-223. Fantoni R., Catellani D., Merlini S., Rogledi S. and Venturini S. (2002): Mem. Soc. Geol. It., 57, 301-313. Massari F., Grandesso P., Stefani C. and Zanferrari A.(1986a): Giornale di Geologia, 48(1-2), 235-255, . Mazzoli S. and Helman M. (1994): Geol. Rund., 83, 464-468. Zanferrari, A., Avigliano, R, Grandesso P., Monegato, G., Paiero G., Poli M.E., Stefani, C. (2008): Note illustrative della Carta geologica d’Italia alla scala 1:50,000 – Foglio 065 “Maniago”; 224 pp. Graphic Linea, Tavagnacco (UD).

THE LATE QUATERNARY CRUSTAL DEFORMATION OF NE SICILY: EVIDENCE FOR AN ACTIVE MANTLE DIAPIRISM S. Catalano, G. Romagnoli, G. Tortorici Dipartimento di Scienze Geologiche, Università di Catania

The origin and the nature of the deformation processes, affecting the volcanic regions of eastern Sicily, are still debated. These regions are, in fact, characterised by the coexistence of both exten- sional and compressional dynamics that have been evidenced by geological, seismological and geo- detic data. A dynamic and kinematic model of the Late Quaternary deformation of the NE Sicily, based on new structural and morphological data, is here discussed in order to provide new tectonic constraints for interpreting the volcanism and the seismicity of the region.

Fig. 1 – Late Quater- nary tectonics of NE-Sicily. The geo- detic data are from Hollestein et al. (2003). Uplift-rate measurements are from Catalano & Di Stefano (1997) and Catalano & De Gui- di (2003). Focal me- chanisms are from Cello et al. (1982) and Giammanco et al. (2008).

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