Bollettino di Geofisica Teorica ed Applicata Vol. 61, n. 3, pp. 309-332; September 2020 DOI 10.4430/bgta0319
Quaternary tectonic activity in the north-eastern Friuli Plain (NE Italy)
G. Patricelli1,2 and M.E. Poli1 1 Department of Agricultural, Food, Environmental and Animal Sciences, University of Udine, Italy 2 Department of Life Sciences, University of Trieste, Italy
(Received: 4 June 2019; accepted: 21 January 2020)
ABSTRACT Interpretation of ENI industrial seismic lines, integrated with geophysical and morphotectonic data, allowed us to reconstruct the deep geometry, kinematics and Quaternary rates of the main blind thrusts in the north-eastern corner of the Friuli Piedmont Plain. The study area is located in the eastern Friuli close to western Slovenia, where the front of Late Cretaceous - Paleogene SW-verging External Dinarides joins with the front of south-verging Neogene eastern Southalpine Chain. Instrumental and historical earthquakes show that both the Alpine and Prealpine areas bordering the plain are seismically active. Nevertheless, the seismogenic role of the main buried tectonic structures of this sector of the Friuli Plain is still not completely clear. In this study, we present four geological cross-sections which depict the buried setting of the eastern Friuli area. Through the implementation with well logs and geoelectrical data, the 3D Plio-Quaternary bottom surface was also reconstructed. By merging these new data with morphotectonic observations, the 2D geometry and kinematics of two main buried active fault systems were investigated. Quaternary and late LGM (Last Glacial Maximum) slip rates (ranging between 0.14 and 0.27 mm/yr and 0.20 and 0.38 mm/yr, respectively) were estimated.
Key words: active faults, Quaternary activity rates, Friuli Plain, NE Italy.
1. Introduction
Friuli is the most seismic area in north-eastern Italy. Located at the northernmost tip of the Adria microplate, the complex structural framework of the present Friuli region is the result of the indentation and counterclockwise rotation of Adria microplate with respect to Eurasia (Márton et al., 2003; Vrabec and Fodor, 2006). Since late Mesozoic, the paleogeography of Tethyan extensional domain underwent compressional tectonics; in particular, north-eastern Italy has been affected by different tectonic processes due to the variation of σ1 in time. Within this context, Friuli represents the junction area between the external fronts of the SW-verging Paleogene External Dinarides and S-SE-verging Neogene Southalpine Chain. Moreover, since Pliocene NE Italy was influenced by the NE propagation of the northern Apennines (Castellarin and Cantelli, 2000; Caputo et al., 2010; Toscani et al., 2016). With geodetic velocity vectors in the order of 2-3 mm/yr (D’Agostino et al., 2008; Devoti et al., 2011) and a ca. N-S oriented σ1, the Friuli area is characterised by a
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complex stress field, changing from west to east (Serpelloni et al., 2016; Bressan et al., 2018).
Seismic catalogues show that at least four MW ≥ 6.0 seismic events struck this area in historical times (Rovida et al., 2016), whereas the inversion of focal mechanisms and a comparison with structural maps allow us to distinguish two different deformational domains: a western mostly compressional sector and an eastern area dominated by dextral strike-slip tectonics (Poli and Renner, 2004; Burrato et al., 2008; Moulin et al., 2014). Despite the medium to high seismicity, knowledge regarding the active tectonic structures that presently accommodate deformation is still not complete. This study deals with the 2D reconstruction of the main fault systems in the NE piedmont plain of Friuli region, with the aim to detect the active tectonic structures and to characterise their geometry, kinematics, and Quaternary activity rates. The parametrisation of active faults and the quantification of their activity are essential to better understand how the accumulating deformation is released in such an active area. These data play a fundamental role in the assessment of seismic hazard, in a highly urbanised region, strongly affected by the damage of the 1976 earthquakes.
2. Geological setting
The structural setting of Friuli region (Fig. 1) comes from the superimposition of different tectonic phases on pre-existing structural inheritances.
2.1. Mesozoic evolution Since Triassic, as part of the African passive margin, northern Italy was subjected to the extensional forces of the Tethyan realm. In particular, during late Triassic - early to middle Jurassic, the rifting phase (whose geometry was controlled by normal NW-SE and transcurrent/ transtensive NE-SW faults) caused the collapse of the carbonate platform (Tunis and Venturini, 1992; Sartorio et al., 1997). As a consequence, the structural high on which the middle-late Mesozoic Friuli carbonate platform developed was surrounded by the Slovenian Basin to the NE (Buser, 1989), the Carnian Basin to the north (Podda and Ponton, 1997), and the Belluno Basin to the west (Bosellini et al., 1981; Masetti et al., 2012). Note that NW-SE and NE-SW tectonic lineaments will represent fundamental inherited structures during the Dinaric and neo-Alpine tectonic phases.
2.2 Late Cretaceous - Paleogene evolution This phase was marked by the instauration of an about N60° oriented compressional regime (Doglioni and Bosellini, 1987; Castellarin et al., 1992; Caputo, 1996). Starting from Coniacian - Santonian the SW-ward External Dinarides propagation affected the Carnian - Slovenian Basin, where a foredeep deposition started. From late Cretaceous (Campanian p.p.) until Eocene, the SW-ward migration of the chain-foredeep system occurred, inverting NW-SE Mesozoic normal to reverse faults and forming new NE-dipping normal faults. These latter caused the tectonic erosion of the north-eastern border of the Friuli platform, and its diachronous east to west drowning. During Lutetian, the Friuli platform was completely drown (Sartorio et al., 1997; Venturini, 2002; Placer et al., 2010) and in Paleocene - late Eocene it was interested by compressional structures. The development of the outer thrust fronts caused the instauration of a piggy-back
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Fig. 1 - Structural sketch map of Friuli and western Slovenia (from Tentor et al., 1994; Zanferrari et al., 2013; Accaino et al., 2019). Red box: study area with the structural model proposed in this work. Legend: AR: Arba - Ragogna Thrust; BFC: Borgo Faris - Cividale Fault; CN: Colle Nero Fault; DIV: Divača Fault; FS: Fella - Sava Fault; GK: Gemona - Kobarid Thrust; IA: Idrija - Ampezzo Fault; MC: Monte Cosici Fault; MT: Montello Thrust; PM: Polcenigo - Montereale Thrust; POZ1: Pozzuolo1 Thrust; PP: Palmanova - Panzano Thrust; PRJ: Predjama Fault; RA: Ravne Fault; RS: Raša Fault; SAV: Savogna Fault; ST: Susans - Tricesimo Thrust; TN1: Trnovo Thrust; UB: Udine - Buttrio Thrust.
basin to the NE and a foreland basin to the SW. About 4000 m of SW-thinning clastic turbiditic succession deposited in Julian Prealps, characterised by carbonate megabeds (seismoturbidites) interbedded with calciturbidites and siliciclastic turbidites (Tunis and Venturini 1992). The SW- ward propagation of the External Dinaric Chain ceased in late Eocene, when the Friuli region was subjected to subaerial exposure until the end of Oligocene.
2.3 Neogene - Quaternary evolution Starting from latest Oligocene a new contractional event, with a variable NNE-SSW to NNW- SSE oriented compressive regime affected the present Friuli region. The formation and the progressive (S)SE-ward migration of the eastern Southalpine Chain involved the older Dinaric structures by folding, displacing, and re-activating them, depending on their orientation with respect to the compressional field (Zanferrariet al., 2013). During the Neogene - Quaternary, a complex sequence of tectonic events can be recognised:
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a) Chattian - Burdigalian event (also known as “Insubric event”). In response to a N20°-30° oriented compressional regime (Castellarin and Cantelli, 2000), the uplifting of the Alpine Chain s.s. slightly affected north-eastern Italy. In particular, the Friuli area represented a distal foreland basin, gently north dipping, where terrigenous-carbonate sediments deposited. The Aquitanian - Langhian sequence (Cavanella Group, sensu ENI), very well visible in the seismic lines, is characterised by thicknesses from hundreds to tens of metres, spanning from the Friuli foothills to coastal areas, respectively. The SSE-thinning of Cavanella Group, together with its petrographic composition, reveals a northern provenance of sediments, consistent with the Austroalpine units of Alpine Chain s.s. (Stefani, 1987; Fantoni et al., 2002; Monegato et al., 2010); b) Serravallian - Messinian event (Massari et al., 1986a; Zanferrari et al., 2008b). Starting from the Serravallian, the NNW oriented compressional regime caused a rapid uplift and exhumation of the Southalpine Chain (Castellarin and Cantelli, 2000; Caputo et al., 2010). The (S)SE-ward migration of the Southalpine front occurred through the activation of new ENE-WSW oriented structures characterised by ramp-flat geometries and frontal splays (Castellarin et al., 1992, 2006; Castellarin and Cantelli, 2000; Zanferrari et al., 2013). Because of the increasing tectonic load, a strongly subsiding foredeep basin developed in the present Friuli area, filled by an about 3000 m thick shallowing upward sequence, with outer terrigenous platform to delta and alluvial facies deposition. The NNW-thickening of the clastic wedge testifies that the source area was located to the north (Fantoni et al., 2002). Since Miocene - Pliocene transition, the indentation of Adria microplate under the Alpine Chain and its counterclockwise rotation (Márton et al., 2003) was accommodated by: • high angle NW-SE striking dextral strike-slip fault systems in Slovenia (Vrabec and Fodor, 2006). In particular, at the Italian - Slovenian border region four main sub-parallel fault systems (Ravne, Idrija, Predjama, and Raša Faults) developed, displacing both the Dinaric and Southalpine structures (Vrabec and Fodor, 2006; Kastelic et al., 2008; Moulin et al., 2014, 2016); • (W)SW-(E)NE striking, (S) SE-verging fold and thrust belt of the eastern Southern Alps external front (Castellarin and Cantelli, 2000). This new structural system gave rise to the activation of the more frontal segments and fault splays, as well as the segmentation and partial reactivation of inherited structures (Galadini et al., 2005). However, despite the (S) SE-ward propagation of Southalpine fronts, no foredeep area can be recognised (Caputo et al., 2010). In the Veneto - Friuli Plain, the formation of new accommodation space was inhibited by the influence on the NE-verging northern Apennines. Despite the source area for sediments was still the Southalpine Chain (Stefani, 1987; Monegato and Stefani, 2011), the thin continental Plio- Pleistocene sequence deposited in Veneto - Friuli Plain shows only a slight flexure towards NE, while a prominent thickening towards the Apenninic Chain is clear (Fantoni et al., 2002; Caputo et al., 2010; Toscani et al., 2016). Besides, the late Miocene - Holocene geological evolution of the Friuli Plain was strongly influenced by the interaction between tectonics and climate. During late Miocene, continental conditions established in the Veneto - Friuli Prealpine area [Montello Conglomerate: Massari et al. (1986) and Zanferrari et al. (2008a)]. During the Messinian salinity crisis, deep incisions
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of the fluvial valleys developed. The following marine ingression (Ghielmi et al., 2010) did not reach the north-eastern Friuli Plain (Mancin et al., 2016; Toscani et al., 2016), where continental conditions persisted, but affected the Messinian paleovalleys up to the present pre-Alpine border (Zanferrari et al., 2013). In the Tagliamento paleovalley near Osoppo, a Gilbert delta of late Zanclean age developed: the Osoppo conglomerate (Monegato, 2006; Monegato and Vezzoli, 2011). The basal conglomeratic Ambiesta Unit, dated to 4.0-4.3 Myr (Monegato and Stefani, 2010; Monegato and Vezzoli, 2011) is coeval to the Pliocene marine deposits cropping out along the Veneto Prealpine area (Venzo, 1977; Cousin, 1981; Favero and Grandesso, 1982; Viaggi and Venturini, 1996). Starting from late Calabrian, the advance and retreat phases of glacial bodies strongly controlled the sedimentation, which was characterised by a discontinuous succession of alternating glacial and interglacial depositional units (Zanferrari et al., 2008b; Fontana et al., 2010). Moreover, the tectonic activity of Friuli Plain buried thrusts caused the uplift of isolated relieves (like Orgnano, Variano, Pasian di Prato, and Pozzuolo carved in Friuli Supersynthem conglomerates), which influenced the drainage pattern acting as water divides (Zanferrariet al., 2008b).
3. Seismotectonic setting
The M > 5.5 historical seismicity of north-eastern Friuli - western Slovenia, extracted from the Parametric Catalogue of Italian Earthquakes CPTI15 (Rovida et al., 2016), is reported in Fig. 2. In particular, four seismic events with M ≥ 6.0 affected the study area.
The 1348 Julian Alps event (MW = 6.6) caused heavy damage in Carinthia region (Austria), where IX MCS intensity was assigned; the quake was felt even in western Slovenia and northern
Italy where VIII - IX MCS intensity points are reported. The 1511 (MW = 6.3) Friuli - Slovenia earthquake affected a very wide area, spanning from Alessandria (NW Italy) to Ljubljana as far as northern Apennines. Recently, Camassi et al. (2011) indicate that the most damaged area was located between Gemona and Cividale del Friuli (I0 = IX), but VII - VIII MCS intensity points were assigned even to Slovenian localities. The 1928 (MW = 6.0) Carnia event hit the Tolmezzo area (I0 = IX). The 6 May 1976 devastating seismic event (MW = 6.4, I0 = IX-X) hit central Friuli.
The main shock was followed by a second destructive sequence on 11 September (MW = 5.6) and
15 September (MW = 5.9). Concerning instrumental seismicity (Fig. 2), the Seismometric Network of Friuli Venezia Giulia Region (managed by the National Institute of Oceanography and Applied Geophysics - OGS) register the seismicity of north-eastern Italy and neighbouring regions since 1977. The analysis of instrumental seismic events, together with the stress and strain tensor inversion from focal mechanisms, depicts the strain pattern of NE Friuli - western Slovenia area. Two different deformational sectors can be recognised: i) central Friuli. This area is characterised by a medium to high seismicity concentrated between 5-12 km depth (Peruzza et al., 2002). Middle-to-low angle E-W and WNW-ESE reverse faults accommodate compressive stress regime. In this sector, where the 1976 main shock is located, the maximum compressional axis is about N-S (Slejko et al., 1999);
ii) NE Friuli - western Slovenia. This area is characterised by deeper events (1998, MW =
5.6 Bovec and 2004, MW = 5.1 Tolmin) and shows a more complex deformational field,
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Fig. 2 - Instrumental seismicity of the study area (red box) extracted from OGS-CRS (2018). Red stars: M > 5.5 historical seismicity (from Rovida et al., 2016). Green arrows: GPS velocity vectors from Serpelloni et al. (2016).
dominated by strike-slip tectonics (Bernardis et al., 2000; Kastelic et al., 2008; Moulin et al., 2014) with transtensional (Poli and Renner, 2004) and/or transpressional features (Falcucci et al., 2018). The orientation of the P-T axes obtained from the analysis of focal mechanisms are in good agreement with the orientation of the principal strain-rate axes estimated from GPS velocities (ca. NW-SE in the western area and ca. NNE-SSW in the eastern area). The study area corresponds to a relatively high strain-rate patch of the Alpine Chain, characterised by maximum values of 0.36×10-7 yr-1 in the Montello - Cansiglio region and decreasing rates towards the east, up to western Slovenia (Serpelloni et al., 2016). The Friuli crustal velocity field illustrates the NNW moving of northern Adria with respect to the stable Alps (Bechtold et al., 2009), with velocity values spanning from 0.8-1.1 mm/yr for the Southalpine area, to 2.5-3.5 mm/yr for the plain and Prealpine area. These values suggest an about 2 mm/yr NNW-shortening (D’Agostino et al., 2005; Bechtold et al., 2009; Devoti et al., 2011). GPS velocity profiles across Eastern Southern Alps (ESA) and Dinaric-Slovenian deformation belt define an about 1 mm/yr shortening accommodating across the ESA front partitioned between a major dip-slip and a minor strike-slip component. Even though the GPS coverage is poor, an about 1 mm/yr shortening and a maximum 1.2 mm/yr right-lateral motion rate are estimated for the Dinaric strike-slip faults (Cheloni et al., 2012; Serpelloni et al., 2016).
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4. Methodology
4.1. Seismic lines interpretation Thanks to ENI S.p.A. (ENI) cooperation, enabled by the Geological Survey of the Friuli Venezia Giulia Region, industrial seismic lines covering an area of more than one thousand square kilometres were examined, with the aim to reconstruct the buried setting of the Friuli Plain and to detect the main active faults of the study area. Particularly, the interpretation of four SW-NE seismic sections (for location see Fig. 3) provided by ENI was realised through the Software 3D-Move™ by Petroleum Experts. The main stratigraphic horizons were detected and, then, digitalised in the 2D sections, also using ENI exploration well logs (Buttrio, Cargnacco 1, Lavariano 1, and Terenzano 1, Fig. 3). The deepest interpretable and well recognisable horizon is the Carnian unconformity, located between
Fig. 3 - Geological map of the NE Friuli Plain (modified after Carulli, 2006; Geo-CGT, 2008; Zanferrari et al., 2013). Legend: A, B, C, D: SW - NE geological cross-sections. B1: Buttrio well; “del Bosco”: del Bosco well; C1: Cargnacco 1 well; L1: Lavariano 1 well; S2-CARG: S2-CARG-FVG well; T1: Terenzano 1 well. Green thrusts: Pozzuolo Thrust System (POTS); blue thrusts: Trnovo Thrust System (TNTS), as reconstructed in this study. Black faults: traces derived from bibliography. AR: Arba - Ragogna Thrust; BFC: Borgo Faris - Cividale Fault; BTZ: Terenzano Backthrust; CN: Colle Nero Fault; CV: Colle Villano Thrust; LAV: Lavariano Thrust; DIV: Divača Fault; MC: Monte Cosici Fault; MD: Medea Thrust; POZ1-2: Pozzuolo 1 and Pozzuolo 2 thrusts; PP: Palmanova - Panzano Thrust; PRJ: Predjama Fault; PRM: Premariacco Thrust; SAV: Savogna Fault; ST: Susans - Tricesimo Thrust; TN1-2: Trnovo 1 and Trnovo 2 thrusts; TV: Trivignano Thrust; TZ: Terenzano Thrust; UB: Udine - Buttrio Thrust.
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the Carnian Dolomitic and evaporitic unit [Raibl Formation, now Travenanzes Formation: Neri et al. (2007)] and the above Carnian - Norian carbonate platform of the Dolomia Principale. Moving upwards, we mapped: the top of the Dolomia Principale, the top of the Jurassic - Cretaceous - Paleocene carbonate platforms, the bottom and top of Cavanella Group (early middle Miocene), coinciding with the top of the turbiditic upper Paleocene - Eocene succession and the bottom of the middle to upper Miocene “molassa”, respectively. The base of the Plio-Quaternary succession is marked by the Messinian unconformity, eroding increasingly older and uplifted units towards the NE (from upper Miocene molasse to lower Eocene turbiditic units). With the aim to convert the digitalised stratigraphic horizons from time to depth, a velocity model based on the velocity well logs provided by ENI was constructed and reported in Table 1. The velocity logs of two ENI exploratory wells are compared in Fig. 4: the Amanda 1bis (A1) located in the undeformed foreland and the Cargnacco 1 (C1) which shows two tectonic repetitions of the stratigraphic series at 2575 and 2945 m depth, respectively. Venturini (2002) interprets these two tectonic reverse structures as the evidence of the compressive events connected to Paleogene (Dinaric) tectonic phase. It is worth noting that stratigraphic units involved in the deformation (Cargnacco 1 well) are generally characterised by higher velocities with respect to the same units in undeformed areas (Amanda 1bis well). The study area corresponds to the active deforming front, and, then, higher velocity values have been adopted in the velocity model. Since no velocities are available for
Fig. 4 - Comparison between ENI velocity logs and stratigraphic columns of Amanda 1bis and Cargnacco 1 wells (Nicolich et al., 2004). The top right map shows the wells location.
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Table 1 - Velocity model used for 2D depth conversion. Velocities from ENI wells, Zanferrari et al. (2008a) and Toscani et al. (2016).
GEOLOGICAL UNIT VELOCITY [m/s]  Plio-Quaternary succession 2000 Molasse (middle - late- Miocene) 2600 Cavanella Group (early – middle Miocene) 4000 Turbidite Units (late Paleocene-early Eocene) 3600 Carbonate Platforms (Jurassic-Cretaceous-Paleocene) 6100 Dolomia Principale (late Triassic) 7000 Travenanzes Formation (late Triassic) 6000
Cavanella Group and middle-late Miocene molasse in the reported well logs, the adopted velocity values of 4000 and 2600 m/s were extracted from the literature (Zanferrari et al., 2008a, 2008b; Toscani et al., 2016). Moreover, in order to better constrain the velocity model, the Dolomia Principale was distinguished from the carbonate platform. The top of the Dolomia was mapped by considering a constant thickness of 875 m, derived from Cargnacco 1 stratigraphy, and a velocity of 7000 m/s was used. A lower velocity value of 6100 m/s was adopted for the carbonate platform succession, comprising the Jurassic - Cretaceous and Paleogene units.
4.2. Base of Plio-Quaternary surface interpolation With the aim to investigate the Quaternary activity of the detected tectonic structures, the Plio- Quaternary base horizon derived from seismic interpretation was implemented with the database of wells supplied by the Friuli Venezia Giulia Region (Fig. 5a). Concerning the area of Buttrio, bedrock depth information derives mainly from geoelectrical surveys by Vecchia and De Wrachien (1968). The depth value of pre-Plio-Quaternary bedrock (in terms of turbidites or molasse) was extracted from every well log, normalised for the datum plane and interpolated through Ordinary Kriging mode of 3D Move software. However, the elaborated gridding surface did not properly represent the Plio-Quaternary bottom since it did not fit with the numerous well exceeding the 60 m drilling depth that do not cut through the pre-Plio-Quaternary bedrock. Therefore, in order to better constrain the interpolated surface, the bottom well depth values of the latter were considered as minimum values of the base-Plio-Quaternary surface depth, then included within the interpolation phase. Furthermore, the north-eastern border of the reconstructed surface was constrained by the outcropping bedrock limit, included in the interpolation process as minimum elevation points of the top of the pre-Plio-Quaternary bedrock. The interpolation process used, the Ordinary Kriging, is a method based on a generalised least squares regression algorithm, which interpolates values by minimising the error variance of the estimate. The spatial distribution of data is automatically analysed by 3D Move by calculating the omnidirectional sample variogram. Moreover, a variogram surface is elaborated in order to analyse the anisotropy of the variance (see supplementary material). The parametric variogram function adopted by the software in the interpolation procedure is the exponential variogram function, since it best fits the Omnidirectional Variogram calculated. Regarding the anisotropy of the variance, it was not considered in the interpolation process. Nevertheless, the variogram
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Fig. 5 - a) Distribution of geophysical surveys and geological data used for the reconstruction of the Plio-Quaternary base surface; b) Quaternary base surface computed from the interpolation of geophysical and geological data. surface elaborated (supplementary material) clearly shows a central NW-SE elongated well defined region, which corresponds to the area where most of the bedrock wells are located. Conversely, moving SW-wards, where the Plio-Quaternary bottom reaches -300 m a.s.l. depths, the interpolation is less constrained, both because fewer data are present and because it mostly deals with no-bedrock wells exceeding the 60 m depth. The interpolated bottom Plio-Quaternary surface is illustrated in Fig. 5b. It shows that the thickness of the Plio-Quaternary succession in NE Friuli gets thicker from the Prealpine border,
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where the bedrock crops out, moving towards S-SW where it exceeds the -300 m a.s.l. depth. It is worth to emphasise that the Plio-Quaternary succession includes Pliocene marine sediments and Quaternary continental deposits out of the front of the Southalpine Chain (Mancin et al., 2016; Toscani et al., 2016), while in the north-easternmost portion of the Friuli Plain the exploratory wells did not record Pliocene sedimentation. Nevertheless, near Osoppo a continental conglomerate dated back to late Zanclean (Monegato and Stefani, 2010) is present. In the same way we cannot exclude the presence of Pliocene continental deposits filling the carved Messinian canyons even at the north-eastern border of the Friuli Plain. Therefore, even knowing that locally the Quaternary deposits directly overly the pre-Plio-Quaternary bedrock, the erosional top of the pre-Plio-Quaternary surface here reconstructed will be uniformly considered as Plio-Quaternary surface.
Fig. 6 - Section A: seismic interpretation of the main seismo-stratigraphic horizons (top) and related geological cross- section (bottom) representing the buried setting of the NE Friuli Plain and highlighting the progressive SE-ward increase of the geometric complexity of the tectonic structures. Section traces are reported in Fig. 3. Acronyms as in Figs. 1 and 3.
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Fig. 7 - Sections B and C: seismic interpretation of the main seismo-stratigraphic horizons and related geological cross- sections representing the buried setting of the NE Friuli Plain and highlighting the progressive SE-ward increase of the geometric complexity of the tectonic structures. Section traces are reported in Fig. 3. Acronyms as in Figs. 1 and 3.
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5. NE Friuli fault systems
The north-eastern corner of the Friuli Plain is interested by the NW-SE Pozzuolo Thrust System (POTS) and Trnovo Thrust System (TNTS) (Figs. 3, 6, 7). We devised four detailed geological cross-sections across the main tectonic structures with the aim to quantify their Quaternary tectonic activity.
5.1. Pozzuolo Thrust System The NW-SE striking POTS extends for about 30 km from San Daniele del Friuli to Trivignano (Fig. 3). Detaching from the Carnian unconformity (at a depth of about 7 km, Figs. 6 and 7) it cuts through the entire succession causing a wide ramp anticline in the carbonates. Lower Eocene turbiditic sequence onlaps the anticline and, locally, the overlying Cavanella Group directly lies on top of the carbonate succession (Venturini, 1987, 2002; Fantoni et al., 2002; Merlini et al., 2002; Nicolich et al., 2004).
Fig. 8 - Clippings of seismic lines and the corresponding line drawings across the POTS. Note the displacement of the Plio-Quaternary units.
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Geometric complexity of the thrust system increases towards the SE (Figs. 6 and 7): in its northern portion (Section A, Fig. 6), POTS shows a single thrust fault [Pozzuolo 1 Thrust, (POZ1)] which gives rise to a wide Paleogene anticline in the carbonates and strongly displaces the top of carbonate platform. The anticline is sealed by the Cavanella Group (Fig. 8), even if tectonic activity of POZ1 weakly involves Neogene - Quaternary deposits. In the central sector (Section B, Fig. 7a) POTS is split into two fault strands: POZ1 and POZ2 (Pozzuolo 2 Thrust). The anticline in the carbonates is evident, but in this sector, the compressional deformation strongly involves Cavanella Group, molasse and the Plio-Quaternary deposits. In the hanging-wall of the POZ1, the Plio-Quaternary unconformity partially eroded the molasse anticline, drilled by “del Bosco” well (Fig. 8b). Here the deformation of the Plio-Quaternary surface is clear: the “del Bosco” well, located near Variano (UD), drilled the molasse pronounced anticline at 26.74 m a.s.l. (Venturini, 1987). Conversely, the S2-CARG-FVG well (Zanferrari et al., 2008b) located at Colloredo di Prato (UD), about 4 km NE from the first one, cut through 157 m of Quaternary deposits from datum plane, reaching a depth of -44.7 m a.s.l. and, then, highlighting the SW-verging anticline geometry. Moving SE-wards, in the most deformed area (Section C, Fig. 7b), stress is accommodated by at least three more reverse structures: POZ1, POZ2 and a frontal buried thrust (Lavariano Thrust: LAV), which displaces carbonates and slightly deforms Cavanella Group (Fig. 8c). Moreover, in the hanging-wall of POZ1, Terenzano Thrust (TZ) with its backthrust (BTZ) and Trivignano Thrust (TV) develop, cutting through the entire stratigraphic succession up to the Quaternary units. The along length profile (Fig. 9) reveals a complex tectonic history of POTS: it records an inherited Paleogene tectonic phase as external SW-verging thrust of the External Dinarides with a 1000 m vertical displacement on the Jurassic - Cretaceous - Paleocene carbonate platform. Nevertheless, it shows also a clear Neogene activity involving both the Cavanella Group and the Plio-Quaternary bottom. The graph highlights a strong displacement of the Friuli carbonate platform during the Dinaric tectonic phase and the SE-ward increasing of throw values on the Neogene - Quaternary
Fig. 9 - Along length throw profile of POTS, elaborated by collecting for each section A, B, C the vertical displacement of the top carbonatic platform, top Cavanella Group and Plio-Quaternary bottom horizons.
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stratigraphic units, thus suggesting a SE-ward greater activity during the neo-Alpine tectonic phase, probably linked to the reactivation of a NE-SW trending lateral ramp of the Dinaric thrust system as frontal one. Neogene - Quaternary tectonics is partitioned along POZ between a strike- slip component (on the NW-SE striking portion) and a dip-slip component (on the WSW-ENE striking Terenzano and Trivignano frontal ramp) as suggested by Venturini (1987). Seismic sections provided by ENI, implemented with well logs and geologic field data, show that POTS cuts the succession in the lower stratigraphic units (carbonate platforms, turbiditic succession and Cavanella Group) by means of a well recognisable fault plane. Conversely, upwards, the more recent and shallow units (molasse and Plio-Quaternary deposits) are characterised by a pervasive deformation, which appears to be spread in a wider dipping zone. Morphological evidence of POTS tectonic activity is represented by surface anomalies in the Last Glacial Maximum (LGM) Friuli Plain south of Udine (Comel, 1947; Zanferrari et al., 2008b; Fontana et al., 2014a, 2014b). In particular, along the Pozzuolo Thrust the pre-LGM Friuli Supersynthem Unit [Fella-Sava Fault (FS), Zanferrari et al. (2008b)] crops out on the LGM surface, giving rise to the Variano, Orgnano and Pozzuolo highs (Fig. 3). Moreover, in the proximity of Pozzuolo del Friuli locality an outcrop of Cavanella Group is documented (Comel, 1955; Fontana et al., 2004, 2014a, 2014b; Zanferrari et al., 2008b). By correlating the stratigraphic logs of ENI exploratory wells Lavariano 1 (L1) and C1 (Fig. 10), located on the most deforming portion of POTS, a throw of 318 m affecting the Plio- Quaternary base was calculated for the Pozzuolo Thrust. The Quaternary units directly lie on the pre-Quaternary bedrock in both stratigraphies [Cavanella Group in C1 and molasse in L1 wells, according to Nicolich at al. (2004)], but no better constrains regarding the age of the oldest Quaternary units are available. Then, a maximum Quaternary age can be inferred for the vertical displacement, estimating a 0.12 mm/yr minimum Quaternary vertical deformation rate. By correcting the vertical displacement for the dip value of 65° of the shallow ramp fault of the POZ1, a slip rate value of 0.14 mm/yr can be estimated.
Fig. 10 - Detail of section C showing the displacement of the bottom of the Plio-Quaternary succession (see Fig. 7b for location).
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Downstream, the Cormor stream is confined between terrace slopes, up to 10 m high, until the hill of Pozzuolo. This is a structural high corresponding to the southernmost thrust of the chain (Galadini et al., 2005). The top of this relief was not affected by fluvioglacial sedimentation during the LGM because it had already been uplifted; thus, its surface is characterised by well- developed soils, likely polycyclic, and developing since the middle Pleistocene (Feruglio, 1925; Fontana, 1999; Fontana et al., 2014a). During the LGM this relief acted as an obstacle, forcing the outwash streams to pass around it and forming two fans on each side of the uplifted terrace (Fontana et al., 2014a). Furthermore, near Pozzuolo del Friuli, Fontana (1999) measured an about 4 m scarp carved on pre-LGM Friuli Supersynthem. At the base of the scarp the pre-LGM units are in contact with the outcropping Remanzacco Subsynthem, dated at 22.0-19.5 kyr cal B.P. (Fontana et al., 2014a). Thus, a minimum late Quaternary slip rate of 0.2-0.22 mm/yr can be estimated for the Pozzuolo Thrust. The slip rate values estimated for the different time intervals are summarised in Table 2.
Table 2 - Slip rate values estimated for Pozzuolo and Udine-Buttrio Thrusts, considering different late LGM and Quaternary time intervals.
Pozzuolo Thrust Udine-Buttrio Thrust_NW Udine-Buttrio Thrust_SE FAULT slip rate (mm/yr) slip rate (mm/yr) slip rate (mm/yr) (dip 65°) (dip 30°) (dip 65°) Late LGM (Remanzacco 0.20 - 0.23 subsynthem) Pozzuolo scarp (4 m) (22.0-19.5 kyr cal BP, Fontana et al., 2014) Late LGM (Canodusso 0.35 - 0.38 subsynthem) Pasian di Prato (23.0-21.0 kyr cal BP, scarp (4 m) Monegato et al., 2007) Quaternary (2.58 Myr) 0.14 0.27
5.2. Trnovo Thrust System The TNTS is composed of three main thrust faults [Udine - Buttrio Thrust (UB), Trnovo Thrust (TN), Premariacco Thrust (PRM)] (Fig. 3) probably originating deeply on Permo - Trias detachment levels, not detectable on seismic lines because of the elevated depth, and causing the south-western overlapping of the carbonate platform and turbiditic units on the Cavanella and upper molasse (Fig 7b). Placer et al. (2010) identify the TN as the outer front of the External Dinarides. Falcucci et al. (2018) segmented it in two portions: the Susans - Tricesimo Thrust (ST) and the TN. In detail, within the TNTS both TN and PRM deform the Plio-Quaternary bottom surface and local outcropping of lower Eocene turbiditic sequence along the Natisone River bed (Fig. 3) are documented (GEO-CGT, 2008).
5.2.1 Udine - Buttrio Thrust The UB extends for a total length of about 25 km under Udine plain and it is segmented in two distinct portions (Fig. 3): a north-western segment (sections A, B and C, Figs. 6 and 7) and a south-eastern segment, (sections D, Fig. 11). • The north-western segment (Fig. 3) runs as a flat on the top of the carbonate platform rising with a frontal ramp causing a ramp-anticline. It deforms Paleocene - Eocene turbidites,
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Fig. 11 - Geological cross-section D (see Fig. 3 for location), showing the displacement of the Quaternary units along the southern segment of the Udine - Buttrio Thrust.
Cavanella Group and molasse, involving the Plio-Quaternary deposition, as shown in section B (Fig. 7a). The following morphotectonic pieces of evidence testify the UB Quaternary activity. i) The presence of a monumental burial (the “Sant’Osvaldo Tumulus”) near Udine city centre, dated back to the Bronze Age (about 2000 B.C.). It was built on
Fig. 12 - Clippings of seismic lines and line drawings across the Udine - Buttrio Thrust, showing the displacement of the Plio-Quaternary units (see Fig. 7b for location of Fig. 12a and Fig. 11 for location of Fig. 12b).
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top of a conglomeratic Friuli Supersynthem relief, just because of the elevated position (Càssola Guida and Calosi, 2002). ii) The Udine hill, where the pre-LGM units of the Friuli Supersynthem crops out on LGM Friuli Plain surface (Zanferrari et al., 2008b). iii) The Pasian di Prato anticline where an about 4 m high scarp carved on pre-LGM deposits (Friuli Supersynthem) crops out from the late LGM Canodusso Subsynthem (Zanferrari et al., 2008b). Knowing that an age of 23-21 kyr cal B.P. was estimated for the Canodusso Subsynthem (Monegato et al., 2007), a throw rate of 0.17-0.19 mm/yr can be calculated, which gives a slip rate of the order of 0.35-0.38 mm/yr (Table 2). Conversely, the southern segment of the UB (sections C and D, Figs. 7b and 11), probably runs in a flat along the Cretaceous - Paleocene platform interface, rupturing the Paleocene thin succession, rising with a final ramp in Eocene turbidites (section D, Fig. 1).1 In the proximity of the surface UB heavily displaces the Quaternary deposition (Fig. 12). Near Buttrio and Collio hills, UB is responsible of the outcropping of Eocene turbiditic units. On top of the hills (i.e. in its hanging-wall), at altitudes between 100 and 130 m a.s.l., a late-to-middle Pleistocene unit crops out [Poggiobello Unit (PGB) in Cividale del Friuli Sheet (GEO-CGT, 2008)]. It is characterised by intensely altered and partly cemented coarse gravels, with abundant sandy-silty matrix, partly related to weathering processes. On top of these deposits, a very well evolved soil crops out (5 YR). By correlating the bottom of PGB with the Plio-Quaternary base surface interpolated from geophysical data (Fig. 11) a vertical displacement of 344 m and throw rate of 0.13 mm/yr was estimated, thus revealing a minimum Quaternary slip rate of 0.27 mm/yr (Table 2).
6. Concluding remarks
The north-eastern Friuli Plain experienced an articulated tectonic evolution with distinct successive phases superimposed on previously inherited structures. Thanks to ENI industrial seismic lines interpretation, implemented with additional well logs, morphological and geological data, two buried main active fault systems in the north-eastern Friuli Plain were identified and discussed. The first is the POTS. It represents an inherited SW-verging imbricate Dinaric structure, active since Paleogene and reactivated during the neo-Alpine phase under a NNW-SSE compressive stress regime. The Pozzuolo Thrust represents the main structure of this thrust system, which reveals a multiphase tectonic evolution. A kinematic variation from pure reverse to oblique can be hypothesised for POZ, during Dinaric (σ1 about NE-SW) and neo-Alpine (σ1 about NNW- SSE) tectonic phases, respectively. In the present stress field, deformation is accommodated by a right-lateral transpressive motion on the NW-SE striking POTS. An almost pure dip-slip kinematics under the present σ1 characterises the TZ, which probably represents a Paleogene (Dinaric) NE-SW striking lateral ramp, reactivated as a frontal one in the Neogene - Quaternary stress field. From industrial seismic lines and from geological and morphological evidence, both POZ and TZ thrusts interest the Quaternary deposition. Particularly a Quaternary slip rate of 0.14 mm/yr and a minimum late Pleistocene rate of about 0.2 mm/yr have been estimated for the Pozzuolo Thrust (Table 2).
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The second active thrust system is the inner TNTS. According to Placer et al. (2010), the ST and TN formed the same tectonic structure in the framework of the external Paleogene Dinaric front. Starting from early Miocene, when σ1 turned to NNW-SSE, this Dinaric structure was segmented (Zanferrari et al., 2008b; Falcucci et al., 2018). At present, ST shows clear evidence of Quaternary activity (Peruzza et al., 2002; Galadini et al., 2005; Poli and Zanferrari, 2018), because it is more favorably oriented with respect to the Quaternary σ1, as compared to the Trnovo Thrust. Within the TNTS, the UB is the structure that shows the clearest evidence of Quaternary activity. A minimum Quaternary slip rate of 0.27 mm/yr was calculated for UB. If we restrict the time interval to the age of Canodusso Subsynthem, the late Quaternary slip rate is of the order of 0.35-0.38 mm/yr. Late LGM to present slip rates of Pozzuolo and UB are consistent with the slip rate calculated for the Arba - Ragogna Thrust (AR) located in the eastern Carnian Prealps (about 0.2 mm/yr in Poli et al., 2009). Moving westwards, slip rates seem to be typically higher: the Polcenigo - Montereale Thrust System (western Carnian Prealps) ranges from 0.2 and 0.5 mm/yr (Poli et al., 2015) and the Meduno Thrust reaches 0.6 mm/yr (Monegato and Poli, 2015). In the western part of the eastern Southalpine front (i.e. western Carnic Prealps), thrusts show a pure dip slip movement because they are WSW-ENE striking and σ1 is about NW-SE oriented (Galadini et al., 2005). In the eastern part, tectonic structures progressively reach a WNW-ESE / NW-SE striking.
In this sector also σ1 rotates reaching N-S direction in the central Friuli area (Slejko et al., 1999) and NNE-SSW direction in the Julian Prealps (Poli and Renner, 2004; Bressan et al., 2018). This means that, on the NE-SW striking cross-sections here studied, we record the dip slip component of the transpressive motion. Assuming a constant deformation rate since Quaternary, the POZ and UB calculated activity rates can be compared to GPS data, highlighting that the estimated slip rate values are well below the total 2 mm/yr velocities of Bechtold et al. (2009) and 1 mm/yr shortening of Cheloni et al. (2012) and Serpelloni et al. (2016). Taking into account the articulated structural framework of the area, it is likely that part of the accumulated deformation is absorbed by the NW-SE strike- slip structures that certainly interact with the transpressive/reverse structures (Falcucci et al., 2018; Poli and Zanferrari, 2018). In the structural model proposed in this work a gradual SE-ward variation of the deformation regime highlights, which spans from a compressive/transpressive kinematics on medium-to-high angle structures to pure strike-slip motion on vertical faults deformation regimes (Accaino et al., 2019). Similarly to the interplay between strike-slip and reverse structures proposed by Venturini (1987) for the eastern Southalpine Chain, by Tentor et al. (1994) for the Isonzo Karst area and by Poli (1996) for the Medea area. Moreover, at the transition between the inner sector of north-eastern Friuli Plain towards the Julian Prealps, present tectonics seems to be ruled by transcurrent regime: the Colle Villano Thrust (CV), that borders the first reliefs of the hilly NE-Friuli area, shows a clear deformation of Quaternary deposition, causing the outcrop of the Eocenic turbiditic bedrock. Recent field and paleoseismological investigations suggest a combined activity for both Colle Villano and Borgo Faris - Cividale (BFC) NW-SE right-lateral strike-slip fault system. Particularly, CV would represent a shallow splay that connects at depth to the sub-vertical strike-slip fault. Thus, the transpressive slip would splits on a horizontal component absorbed by the BFC fault system and a contractional component accommodated by CV (Falcucci et al., 2018).
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So, in the light of this, further investigations are necessary to better understand how the reverse- to-oblique structures connect with the NW-ward propagation of NW-SE dextral strike-slip faults of western Slovenia.
Supplementary material related to this article is available online at the BGTA website www. bgta.eu.
Acknowledgements. Research developed in the framework of the project “Faglie Attive” promoted by the Regione Autonoma Friuli Venezia Giulia - Geological Survey, in cooperation with the National Institute of Oceanography and Applied Geophysics (OGS) and the University of Trieste. Many thanks to ENI for the consultation and supply of seismic data and well velocity logs. Petroleum Expert Ltd is acknowledged for making available the 3D-MOVE Software to the University of Udine. We are grateful to Daniela Croce for the elaboration and critical review of bibliographic geophysical survey in the Manzano - Buttrio - Cividale area. Many thanks to A. Zanferrari, G. Monegato, and G. Paiero for their useful reviews and comments. These data were presented at the 37th GNGTS National meeting (Bologna, 19-21 November 2018). The work has also benefited of the constructive suggestions and corrections by M. Busetti and an anonymous reviewer.
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Corresponding author: Giulia Patricelli Dipartimento di Scienze Agroalimentari, Ambientali e Animali, Università di Udine Via Cotonificio 114, 33100 Udine, Italy Phone: +39 0432 558740; e-mail: [email protected]
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