Description:

_Journal of Structural Geology 32 (2010) 1009-1027_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Neogene-Quaternary tectonic stratigraphy of the eastern Southern Alps, NE Italy R. Caputo a,*, M.E. Poli b, A. Zanferrarib a Department of Earth Sciences, University of Ferrara, via Saragat 1, 44100 Ferrara, Italy b Department of Georesources and Territory, University of Udine, via Cotoni?cio, 114, Udine, Italy Article info Article history: Received 9 October 2009; Received in revised form 6 June 2010; Accepted 16 June 2010; Available online 23 June 2010 Keywords: Mesostructural analysis Conglomerates Stress field Geodynamics Collision Abstract To reconstruct the Neogene-Quaternary tectonic stratigraphy of the eastern Southern Alps, late Tortonian-lower Pleistocene foredeep clastic sequences cropping out in the Veneto and Friuli piedmont areas were extensively investigated focusing on contractional features observed in frequent conglomerate layers (pitted pebbles). The eastern Southern Alps is the result of a typical polyphase tectonic activity initiated in Late Oligocene, characterised by several noncoaxial stress regimes. Based on careful analyses of the pebbles’ surfaces (shape and orientation of indented features) and following a systematic and statistical approach, the mean orientation of the maximum compressive stress axis (s1) was obtained for more than 30 sites along the ca. 120 km-long investigated piedmont area. The affected lithostratigraphic units and the orientation (pre-versus post-tilting) of stereonets density peaks make it possible to recognize four distinct deformational events and characterize them in terms of mean s1 direction and timing: late Tortonian (s1 ? 313 x14 ± 00 x14), late Messinian-Early Pliocene (338 x14 ± 04 x14), Late Pliocene (314 x14 ± 03 x14), and Early-Middle Pleistocene (160 x14 ± 03 x14). The tectonic stratigraphy of the eastern Southern Alps during the last ca. 10 Ma shows the occurrence of several variations of the stress field characterized by a repeated oscillation of the s1 axis between an NNW-SSE and NW-SE directions (Twist Tectonics). Taking into account literature data about older deformational events (Chattian-Burdigalian and Serravallian-Tortonian), we analyse the Neogene-Quaternary tectonic evolution in the frame of the central Mediterranean realm and compare it with the convergence direction between the Adria-Africa and Europe lithospheric plates. We show that compressional directions within the eastern Southern Alps are basically governed by remote plate convergence, though since late Messinian, the approaching Northern Apennines started to play a crucial role within the investigated area. Alternating short-lived phases (1-2 Ma) of coupling and decoupling along the basal detachment of the Apennines accretionary wedge probably caused temporary perturbations of the ‘local’ stress field and complex accommodation structures in this region of ongoing crustal collision. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Every orogen is commonly characterized by several deformational events that affect the crust and sedimentary cover by successive and distinct tectonic phases. The Southern Alps, Northern Italy (Fig. 1), are not an exception and the first major goal of the present research is to reconstruct the ‘Tectonic Stratigraphy’ of the investigated area. In particular, the aims of Tectonic Stratigraphy (Caputo and Pavlides, 1993) are firstly, the recognition of how many tectonic events occurred; secondly, for each event, the reconstruction of stress trajectories, areal distribution of associated deformation as well as relative and absolute chronology. For simplicity, in the following sections we will uniformly use the term ‘tectonic (or deformational) event’ regardless the dimension of affected crustal volume and its duration. The Southern Alps is a major structural subdivision of the broader Alpine Chain and it is conventionally limited to the north by the Periadriatic Lineament (Fig. 1b). From a tectonic point of view, the Southern Alps correspond to a distinct late Cretaceous-Quaternary orogen (Doglioni and Bosellini, 1987; Massari, 1990; Castellarin et al., 1992, 2006a), while the Periadriatic Lineament represents its polyphased (though mainly oblique-slip) back-stop (Castellarin et al., 1992, 2006b; Schmid et al., 1996). This south-verging fold-and-thrust belt was generated during the complex crustal collision and indentation of the Adria promontory underneath the Alpine chain (Roeder, 1989; Polino et al., 1990; Schmid et al., 1996; Beaumont et al., 1996; Castellarin et al., 2006b). R. Caputo et al. Journal of Structural Geology 32 (2010) 1009-1027 Fig. 1. a) The North Italian geodynamic framework (modified from Bigi et al., 1990) showing the position of the Southern Alps within the Europe-Africa convergent system. b) Simplified tectonic map of the eastern Southern Alps. The thin dashed line represents the morphological hills-plain boundary, while the dotted line indicates the investigated area (Fig. 2). 1 - Austroalpine; 2 - eastern Southern Alps; 3 - Northern Dinarides; 4 - Apennines; TB - Thiene-Bassano Thrust; BC - Bassano-Cornuda Thrust; MC - Montello Thrust; CA - Cansiglio Thrust; PM - Polcenigo-Maniago Thrust; AR: Arba-Ragogna Thrust; ST: Susans-Tricesimo Thrust. Based on the fact that the available chronology and stratigraphy of the Neogene-Quaternary sedimentary succession within the eastern Southern Alps is now very detailed (Zanferrari et al., 2008a,b; and references therein), we investigated the external sector of the eastern Southern Alps and performed structural and stratigraphic mapping of Tortonian-Quaternary sedimentary units outcropping within a 120 km-long zone along the foothills of the mountain chain in Veneto and Friuli regions (Fig. 2). Therefore, it has been possible to reconstruct a similarly detailed tectonic stratigraphy for the study region. Moreover, the principal results obtained from mesoscopic structural analyses were correlated with the large-scale tectonic setting of the investigated area, while in a later section of this paper we eventually discuss the eastern Southern Alps orogenic event in the frame of the Late Tertiary geodynamic evolution of the broader Central Mediterranean region. The eastern Southern Alps is one of the most seismically active zones of the Central Mediterranean region as documented by both historical and instrumental seismicity (Slejko et al., 1989, 1999; Bressan et al., 2003; Working Group CPTI, 2004; Galadini et al., 2005; Burrato et al., 2008). In particular during historical times it has been struck by numerous earthquakes with magnitude between 6 and 7, as indicated in the catalogue by Working Group CPTI (2004) among which are the 1117 (Verona area), 1348 (Carnia region), 1695 (Asolo area), 1873 (Belluno area), 1936 (Cansiglio area) and 1976 (Friuli) events. Accordingly, a better understanding of the recent tectonic and geodynamic evolution of the broader investigated area could also contribute to improve the seismic hazard assessment of the region locally characterised by high vulnerability values. As a second principal aim of this research, we would like to show the potential benefit in analyzing and using pitted pebbles in conglomerates as palaeostress indicators when performing mesostructural investigations. As a final goal, we analyse and discuss the problem of separating different deformational events when performing mesoscopic and macroscopic structural analyses in broad regions. In particular, the main issue is how to distinguish between a major regional tectonic ‘phase’ from a second-order tectonic ‘pulse’ when investigating complex structural frameworks at the regional scale. Indeed, the question on the meaning of the space and time distribution of deformational events affecting wide areas is posed. 2. Regional tectonic framework The development of the eastern Southern Alps as a well distinct orogen with respect to the Alps s.s. occurred during the Neogene-Quaternary time interval (Doglioni and Bosellini, 1987; Doglioni, 1990a; Castellarin et al., 1992; Fantoni et al., 2002). As commonly observed during the orogenic processes within the peri-Tethyan realm, deformation of the sedimentary cover and uppermost crust was not homogeneous in terms of involved rock volumes and stress-strain field, therefore inducing researchers to distinguish and separate some major tectonic events or subevents (Massari, 1990; Castellarin et al., 1992, 2006a; Caputo, 1996; Caputo et al., 1999). 2.1. First event The first well-defined tectonic event probably started in late Chattian (Mancin et al., 2007), but mainly developed in Early Miocene (Aquitanian-Burdigalian). In the literature, it is commonly referred to as the “Insubric” (Massari, 1990; Castellarin et al., 2006a) or “Chattian-Burdigalian” event (Caputo, 1996; Castellarin and Cantelli, 2000). Based on systematic mesoscopic structural analyses, the mean orientation of the maximum compressional axis (s1) associated with this event was N20°-30°, while the s3 axis was sub-vertical thus implying a purely compressional Andersonian regime (Castellarin et al., 1992; Caputo, 1996). The affected area was mainly restricted to the northern sector of the eastern Southern Alps mountain chain. Ключевые слова: letters, external sector, event, italy, map representing, sub-horizontal setting, upper pliocene, mesoscopic, science, phase, sector, deformational, renard, deformation event, region, adria margin, activity, del, compression, twist tectonics, pitted feature, publ, ha, doglioni, result, plate convergence, dipping, calamita, scale, yellowish sandstone, journal structural, monegato, upper, mesoscopic structural, northern sector, cansiglio-maniago thrust, investigated, tagliamento river, grandesso, highly plunging, mantovani, foredeep evolution, succession, journal, period, bologna, selli, sandstone, international, axis, pitted pebble, based, northern, alps, orientation, venturini, relative motion, stratigraphy, site documenting, ?eld, poli, process, tectonic phase, principal result, nw-se, data, eastern southern, analysis, eastern southern alps, conegliano unit, worth mentioning, geological, nnw-sse, late cretaceous, eds geology, conegliano, della, cantelli zanferrari, tectonics, pleistocene, deformational event, tectonophysics, london, stress ?eld, rotation, africa, late, mcewen, measurement, sch?nborn, stereographic projection, layering, caputo, geol, tortonian, terra, montello hill, southern alps, thesis university, palinspastic restoration, abrupt change, early, feature, spec, italy spec, case, veneto, pressuresolution process, compression occurred, italian, eld, major, stratigraphic unit, compressional, structural, unit, setting, indented feature, park, montello, deformation, terra nova, fantoni, geology, conglomerate, massari, clastic succession, relative, stress trajectory, pebble, soc, density peak, thrust, ma, caputo caputo, compressional direction, dal piaz, stereonett, steeply dipping, thrusting, angular unconformity, mets, average orientation, messinian, crustal structure, wa, frontal thrust, maximum compression, tectonic regime, geodynamics, pliocene, subhor, central, montello conglomerate, tectonic evolution, stefani, trajectory, feel condent, dei, southalpine foredeep, caputo journal, structure, tectonic, zanferrari, dip angle, dolomites, alpine, doi, time, evolution, external, investigated area, stereographic view, adria, sedimentary, detailed, site, generally characterized, nw-se direction, direction, castellarin, group, di, stratigraphic, focal mechanism, tectonic stratigraphy, friuli, characterised, apennines, basal detachment, sedimentary cover, memorie, gratier, university, occurred, stress, southern, pitted, major segment, giulio, celles tozer, motion, site pedeguarda, geophysical, northern apennines, observed, plate, erosional surface, careful analysis, hierarchically classifying, geologia, generally, nova, area, structural geology, eastern, stress eld, table, maximum, venezia, cantelli