Mountain building in NW Sicily from the superimposition of subsequent thrusting and folding events during Neogene: structural setting and tectonic evolution of the Kumeta and Pizzuta ridges

ABSTRACT We present a 1:25.000 scale geological map of the Kumeta-Pizzuta ridge in north-western Sicily (Italy), achieved by integrating stratigraphic, structural and geophysical data. In this area, the tectonic edifice results from the piling-up of deep-water-, carbonate platform- and pelagic platform-derived tectonic units (Imerese and Sicilide, Panormide and Trapanese domains, respectively) resulting from deformations of the former southern Tethyan continental margin. The structural setting shows interference of tectonic events, different types of structural styles and different scales of deformational patterns. Early overthrust of the Imerese on the Trapanese units (since the late Serravallian) was followed by wedging at depth of the Trapanese units (after the Tortonian). The wedging implied re-embrication and shortening into the overlying Imerese tectonic units and so produced the main folding and compressive to transpressive structures along the Kumeta-Pizzuta Ridge. Seismic reflection profiles integrated with field data reveal that the main E-W-trending anticlines have been offset by high-angle reverse faults flattening at depth until they connect with low-angle, regionally widespread, decollement surfaces with a northward tectonic transport. This setting supports backthrusting along transpressional faults in the study area, ruling out that the Kumeta ridge is a positive flower structure related to a near-vertical deep, crustal, shear zone as formerly suggested.


Introduction
We present an updated 1:25,000 scale geological map of the Kumeta-Pizzuta ridges located in western Sicily, Italy ( Figure 1). This area represents a key sector of the Sicilian Fold and Thrust Belt (FTB) where the superposition of the deep-water Imerese units over the carbonate platform Trapanese units produces tectonic structures and interference pattern inherited from subsequent tectonic events.
The geological complexity of this sector has caused different authors to propose alternative interpretations about the cropping-out tectonic structure. Ghisetti and Vezzani (1984), Giunta, Nigro, Renda, and Giorgianni (2000), Renda, Tavarnelli, Tramutoli, and Gueguen (2000), Nigro and Renda (2001) interpreted the E-Wtrending 20 km long main lineament along the Kumeta ridge as part of the southern boundary of a regional crustal shear zone in Sicily (Kumeta-Alcantara alignment), whose northern boundary should be the Ustica-Eolie line in the southern Tyrrhenian (Gueguen, Tavarnelli, Renda, & Tramutoli, 2010). Catalano, Franchino, Merlini, and Sulli (2000) and Catalano, Valenti, et al. (2013), based on seismic reflection profile analysis and field surveys, reconstructed the deep structural features in central-western Sicily down to the crystalline basement emphasizing the role exerted by the collision-related contractional events.
This paper describes the stratigraphic and structural settings of the area and the relationships among the distinct carbonate tectonic units, forming the main bulk of the chain. Furthermore, by integrating field data with seismostratigraphic interpretation, we attempt to correlate outcropping and buried contractional features to reconstruct the Tertiary compressional tectonic geometries. We also discuss the role of the strike-slip features with respect to the compressional structures in the studied sector of the Sicilian orogen. framework of the CARG project (Italian official Geological Cartography; www.isprambiente.gov.it).
The geological mapping was accomplished through the following steps: (1) detailed geological field survey (at 1:10,000 scale) to collect stratigraphic and structural data in order to characterize the tectonic features and define the kinematic evolution. Good exposure and high lateral continuity of the outcropping successions have been the chosen criteria to select the sites of structural measurements.
(2) stratigraphic analysis, aimed at describing the geometry and lateral to vertical evolution of the lithostratigraphic sequences; (3) statistical fold and fault analysis, which allow definition of fault kinematics, fold-trending and stress-field orientations. The collected meso-structural data have been summarized in steroplots (Table 1) using Daisy 3.0 (Salvini, 2001).
The recognized tectonic units and their structural relationships are illustrated in the geological cross sections on the Main Map.
For a better lateral continuity of the outcrops, we mapped the inferred-lithostratigraphic units beneath urban areas using new confidential borehole data recently acquired from engineering projects.
The topographic basemap on the geological map is the Carta Tecnica Regionale (vector file format .dwg, plane coordinates referred to the East Zone of the Gauss-Boaga Italian national system projection, datum Roma 1940, Monte Mario) with a contour interval of 50 m. The drainage system has been simplified cleaning the first-and second-order tributaries. For better representation, the geological map has been draped on a shaded-relief map available for the area (http://map.sitr.regione.sicilia.it/ArcGIS/services/DTM _2 m/MapServer/WMSServer).

Geological background
The Sicilian orogen (Figure 1), located in the centre of the Mediterranean Sea at the NE corner of the Tunisian-Sicilian promontory (northern African continental margin, Figure 1(a)), links the Southern Apennines and the Calabrian Arc to the Tellian and Atlas systems of North Africa (Figure 1(a)).
The Sicilian FTB (Figure 1(b)) is a complex stack of S-and SE-verging imbricates locally more than 15 km thick, whose building up is linked to both the postcollisional convergence between Africa and a complex 'European' crust (Bonardi, Cavazza, Perrone, & Rossi, 2001), and to the coeval roll-back of the subduction hinge of the Ionian lithosphere (Catalano, Valenti, et al., 2013;Doglioni, Gueguen, Harabaglia, & Mongelli, 1999 and references therein).
The Sicilian FTB is formed by Meso-Cenozoic deepwater carbonate and siliciclastic units, overriding a more than 10 km thick carbonate platform thrust wedge, detached from the crystalline basement (Catalano, Valenti, et al., 2013) and covered by upper Miocene to middle Pleistocene clastics, pelagites and evaporites, which unconformably seal the whole underlying tectonic stack, filling thrust-top basins (Butler & Grasso, 1993;Gasparo Morticelli et al., 2015;Gugliotta et al., 2014;. The study area is located in the southernmost edge of the Palermo Mts (Figure 1; northernmost sector of the emerged Sicilian FTB). In this area, the tectonic edifice results from the piling-up of deep-water-, carbonate platform-and pelagic platform-derived tectonic units (Imerese and Sicilide, Panormide and Trapanese domains, respectively) originating from deformations of the former northern African continental margin (Catalano et al., 2000).
The tectonic units, each one 900-1200 m, 2000-3000 m and 3500-5000 m thick, respectively, are stacked to form an S-and SW-verging thrust pile characterized by ramp and flat geometry. The distance between subsequent culminations of the structures (ramp anticlines or thrust fronts) is 4-6 km for the Imerese tectonic units (Event 1), and 9-12 km for the Trapanese tectonic units (Event 2). The Neogene deposits lying at the top of the footwall block, postdate the respective tectonic emplacement (Abate, Catalano, & Renda, 1978).
The resulting tectonic structures are not coaxials and their present-day setting can be explained by the occurrence of progressive, vertical-axis clockwise rotations during their emplacement (Oldow, Channell, Catalano, & D'Argenio, 1990;Speranza, Maniscalco, & Grasso, 2003). The rotation amount decreases stepwise from internal to external tectonic units and the high value of rotation measured in the internal tectonic unit (e.g. Imesere tectonic unit) comes from adding the contribution of distinct rotation stages during forward orogenic propagation.

Stratigraphy and facies analysis
The lithostratigraphic units recognized in the study area pertain to the well-known facies domain of the Sicilian sector of the African continental margin (Catalano & D'Argenio, 1978).
The Sicilide unit is composed of mainly pelagic shale, marls and limestone (Varicoloured clays and Polizzi fm, the AVF and POZ in the Main Map, respectively) detached from their substrate.
The Numidian succession is mainly composed by turbidites (Numidian flysch, FYN) separated into two distinct members Late Oligocene-Early Miocene in age, followed by Burdigalian-Langhian marls and shales interlayered with a mega-bank of siliceous sandstones rich in glauconite (Tavernola fm., TAV).
The Imerese slope-to-basin succession (stratigraphic columns on Main Map), spanning from the middle Carnian to the early Miocene, is composed of carbonate and silico-carbonate pelagites interbedded with cyclically arranged carbonate platform-derived resedimented deposits (Basilone, Frixa, Trincianti, & Valenti, 2016 and references therein). The Trapanese pelagic platform succession (stratigraphic columns on Main Map) is made up of Lower Jurassic shallow-water limestone, unconformably followed by Jurassic-Eocene carbonate pelagites and Burdigalian-to-lower Tortonian clastic carbonates and outer shelf marls.
These units are unconformably covered by upper Miocene clastic and terrigenous rocks (Castellana and Terravecchia Fms, SIC and TRV, respectively), filling syn-tectonic sedimentary basins (Gugliotta et al., 2014), and by Quaternary continental deposits.
A complete description of the sedimentary succession is reported in the legend of the Main Map.

Tectonic features
The geological-structural surveys allowed better constraint of the deformational pattern and the lateral continuity of the structures, and to age-date the wedging of the Imerese above the Trapanese tectonic units. These units are superimposed along a main thrust, whose cutoff line outcrops at M. Leardo. Here the Upper Triassic cherty limestone of the Imerese unit tectonically overlaps the Upper Cretaceous pelagic limestone of the Trapanese unit.
As a consequence of the compressive deformation that duplicates the Imerese succession, we can distinguish three stacked thrust sheets, each one forming a minor tectonic unit topped by Numidian flysch deposits. From the top of the wedge, we distinguished Gradara, Pizzuta and Leardo tectonic units, respectively. Conversely, the Kumeta tectonic unit is formed by Trapanese pelagic platform succession on the whole that does not show duplication on outcrop.
The meso-structural data (Tables 1-3) collected at 28 sites ( Figure 2) provided the kinematics of the tectonic structures (Table 4). We have classified threefold systems with different age and trends, here named h1, h2, h3.
In the following, we will describe the most relevant tectonic features and outcrop location of these tectonic units.

Gradara tectonic unit
A small portion of this unit crops-out in the NW corner of the investigated area (Structural map on Main Map) where it forms a north-east dipping monocline tectonically overriding the Pizzuta tectonic unit along a WNW-ESE-oriented thrust (Gradara thrust in Figure 2). This thrust is shifted towards the SE by a right-lateral fault. To the east, the Gradara tectonic unit is bounded by a main NE-SW-oriented left-lateral fault (Cannavera Fault in Figure 2).

Pizzuta tectonic unit
In this widely extended unit, cropping-out along the northern sector of the area (Structural map on Main Map), three main large-scale folds can be recognized ( Figure 2): (a) the WNW-ESE-trending Mirabella anticline (h1-system), located in the north-western sector of the map, at the hanging wall of the Mirabella-Pizzuta lineament, with a wavelength of hundreds of metres. At the core, the Mufara Fm. outcrops, while the Scillato Fm. forms the flanks at Pizzo della Nespola and Mt. Matassaro Renna (Figure 3(a)). The outer flank is overturned along the southern slope of Pizzo Mirabella (Main Map); (b) the N-S-trending Pizzuta anticline (h3-system) crops-out between Serre del Frassino and Portella del Garrone, characterized by two hinge zones: (i) N-S to NNE-SSW oriented with E-vergence along the Garrone-Pizzuta ridge, and (ii) NNW-SSE oriented with WSW-vergence along Serre del Frassino ( Figure 3(b,c)). Toward the S, the Pizzuta anticline is abruptly interrupted along an SSE-dipping backthrust. Above the latter fault, a ENE-WSW-trending anticline occurs (Maja e Pelavet anticline in Figure 2; h2-system). The structural analysis (see Table 1) reveals that the Imerese succession is affected by the NE-SW-trending h2-fold system (see also Avellone & Barchi, 2003).
This feature displays a complex tectonic style due to the interference of two folding events. In detail, the superimposition of the h2-fold system on the h1fold-system produces type 1 and 2 interference patterns ( Figure 4) (sensu Ramsay & Huber, 1987) and locally rotation of the h1-fold system along the N-S-trend Gasparo Morticelli, 2008).
We also recognized two sets of pressure solution cleavages (C1 and C2 in Figure 4), with extensional veins (V1 and V2 in Figure 4). In agreement with the relative chronology indicated by the interference figures, the cross-cutting relationships indicate that C1-V1 is older than C2-V2 ( Figure 4).
The Piana degli Albanesi syncline (h1-system) extends in a wide sag between the M. Pizzuta and M. Kumeta ridges, where the Numidian flysch deposits crop out (Main Map); the syncline axis shows a bended trend varying from NW-SE to W-E.
The Pizzuta tectonic unit is affected by some main fault systems (Figure 2 Figure 2). This thrust is also inferred as being buried in the sector between the Pizzuta and Kumeta ridges ( Figure 5). (b) strike-slip and transpressive faults, with offset of a few kilometres, displacing the Mirabella-Pizzuta lineament (Figure 2): the NW-dipping Cannavera  (Figure 3(c)); the NNE-SSWtrending left-lateral strike-slip fault that displaces the Mirabella-Pizzuta lineament in the eastern sector of the map. (c) In the northeastern sector, the Pizzuta tectonic unit forms a large SW-dipping monocline interpreted as the forelimb of a ramp anticline Barreca, Maesano, & Carbone, 2010). This monocline is affected by reverse faults with SW and NE tectonic transport direction, left-lateral strike-slip NE-SW-trending faults, SW-and SE-verging minor folds, and NW-SE-trending normal faults. As concerns these latter faults, we observed that the Jurassic interval of the hangingwall succession displays a thickness higher than the coeval footwall succession. These features highlight Mesozoic syn-sedimentary extensional tectonics that affect the Imerese succession. Along the Eleuterio River valley, an NE-SW-trending left-lateral strike-slip fault (Eleuterio lineament in Figure 2) breaks off the monocline, bounding the Pizzuta tectonic unit eastward.

Leardo tectonic unit
It crops out along an E-W-trending belt from the Piana degli Albanesi lake up to Marineo village, and at Pizzo Chiarastella. It overrides the Trapanese tectonic unit along the WNW-ESE-trending Leardo thrust (see Table 2. Structural data related to fault planes. Along the northern slope of M. Leardo, the tectonic unit is bounded by a transpressive right-lateral fault (Jato-Balatelle lineament in Figure 2 and cross section C-C' in Main Map). At Pizzo Chiarastella, the Leardo tectonic unit crops out in an NW-SE-trending anticline bounded by right-lateral transpressive faults pertaining to the E-W Jato-Balatelle lineament (here NW-SEtrending).
Regarding the whole Imerese tectonic unit, crosscutting relationships revealed that the h2-system as well as the NW-SE and NE-SW transpressive fault planes postdate the h1-system and the NW-SE thrust. The h3-system results from the interference between the h1-and h2-systems.

Trapanese tectonic units
The Trapanese tectonic unit outcrops along the E-Wtrending Kumeta ridge from M. Jato (West) to M. Balatelle (East). Seismic reflection data show that the Kumeta tectonic unit is up to 3000 m thick . The Kumeta ridge forms a main   (Figures 2 and 3(e,f)). Close to the eastern end of the ridge, the anticline axis, as well as the transpressive faults, turn taking on a NW-SE trend (Balatelle anticline; Figures 2 and  3(g)). The meso-structural data collected along the Kumeta tectonic unit (Tables 1-3) revealed three main minor-fold systems: (i) E-W-trending S-vergent folds (h1-system); (ii) NE-SW-trending symmetric folds (h2-system); (iii) NW-SE-trending SW-vergent folds (h3-system).

Seismic reflection data
The interpretations of the seismic reflection profiles, crossing the Kumeta ridge (NW-SE and N-S striking), reveal the deep geometry of the tectonic stack (Figure 7 (c)), allowing the fault systems, already recognized by field work, to be correlated down to the deep base of the tectonic units. The high-angle, S-ward dipping transpressive fault, bounding N-wards the Kumeta ridge (Jato-Balatelle lineament), belongs to the thrust system imaged in   . Cross-cutting relationships between deformation features that affected the Imerese tectonic units: (a,b) hinge and limb tracers, respectively, of a fold where cross-cutting relationship between two pressure solution-cleavage and extensional vein systems occur (C-V system; St. 6 in Figure 2 and Table 1; see Figure 3 for ubication); (c) fold systems and related interpretative sketch showing type 2 fold interference pattern (sensu Ramsay & Huber, 1987). the seismic sections of Figure 7, whose geometries have been highlighted through the displacement and tilting of the seismic horizons attributed to the Trapanese succession. The divergent horizons of the lower portion of the Trapanese succession correspond to the contact between the low-angle ramp and the flat thrust, pointing out, the progressive decreasing of the thrust-dip at depth. At a depth of 4 s/TWT, the thrust planes flatten (Figure 7(c)), clearly showing that the thrusting propagated following sedimentary packages not involving the deep crustal layers.
By comparing the geometries coming from the seismic interpretation with the present morphostructural setting (Figure 7(a)), the arching of the Kumeta tectonic unit can be noted as having a wavelength larger than that one imaged by the outcropping ridge.
The seismic profiles (Figure 7(c)) indicate that the Jato-Balatelle lineament is not the actual northern boundary of the Kumeta tectonic unit which can be recognized up to some kilometres N-ward, where it is buried beneath the Imerese tectonic units (see cross sections on Main Map).

Discussion and conclusion
Since the pioneering insight of Ruggieri (1966), many authors shared the idea the Kumeta Ridge was a rock body exhumed along prominent strike-slip faults. Indisputably, a lot of meso-structural data reveal strike-slip tectonic features that have contributed to build up the Kumeta Ridge. But do these transcurrent structures reflect an extensive, regional stress field or are they only minor secondary features?
All the authors (see the list below) agree that the shortening which affected this area during the Neogene occurred following two subsequent tectonic events: the former (middle-late Miocene) is responsible for the thrusting of the Imerese tectonic units over the Trapanese succession; the latter induced the Trapanese units to be folded, faulted and uplifted.
Except for Giunta (1991), who suggests a possibility that the orogenic process in the whole area developed in a transpressive mode, the interpretation that the first event is related to continental collision between the Sardinian Block and the northern African continental margin has been largely accepted. Whereas, two different explanations for the origin of the later deformational stage have been suggested.
According to Vezzani (1981, 1984), the Kumeta Ridge is a segment of a main tectonic lineament, the Kumeta-Alcantara fault system, crossing through the whole of northern Sicily. The authors interpreted this fault system as the main surface branch of a major crustal shear zone located between northern Sicily and the southern Tyrrhenian.
Other authors (Finetti, Lentini, Carbone, Catalano, & Del Ben, 1996;Giunta et al., 2000;Nigro & Renda, 1999) confirm the deep crustal character of the Kumeta-Alcantara fault system, speculating about a major shear zone along the southern Tyrrhenian margin bounded by the Ustica-Eolie line to the north and the Kumeta-Alcantara line to the south.
Our detailed geological and structural surveys, integrated with seismic reflection profiles, show that the buried, a few thousands metres thick, Trapanese unit has been deformed and displaced by an S-dipping thrust system subsequent to the former overthrusting of the Imerese tectonic units (see also Avellone et al., Figure 7. (a) Panoramic view of the eastern sector of the study area. On the right side, the Kumeta ridge and on the left side the main outcrop of the Imerese succession. In this area, we can observe the structural relationships between the Imerese and Trapanese tectonic units (IMU and TPU, respectively). On the central-left side, we drew the Mirabella-Pizzuta lineament along which the Pizzuta tectonic unit overrides the Leardo tectonic unit. From this observation point, we can depict the geometry of the bending coming from the Trapanese units thrusting and uplifting: the red line depicts the wavelength of folds involving the buried carbonate platform succession; otherwise, the white line outlines the shorter wavelength folding which characterize the Kumeta ridge. Interpreted seismic reflection profiles (traces in b) across the central (c) and the eastern (d) sectors of the Kumeta ridge (modified from Catalano et al., 2000).

2010
; Barreca et al., 2010;Barreca & Maesano, 2012;Catalano et al., 2000); the latter developed with an S and SW tectonic transport direction (Event 1). The wedging at depth of the Trapanese units (Event 2), implying passive deformations of the already deformed overlying Imerese tectonic units, produced main (higher wavelength) anticline culmination, outcropping along the Kumeta Ridge (see schematic model on Main Map). The structural data collected in the Imerese tectonic units (Figure 2; Tables 1-4) also highlight that the reconstructed stress-field orientation related to Cannavera, Dammusi, Serre del Frassino and Chiusa faults is associated with tectonic Event 2.
Seismic reflection profiles show the main E-Wtrending anticlines have been offset by high-angle reverse to transpressive faults that merge at depth with low-angle, regionally widespread, flat decollement surfaces that in this sector show an N-directed tectonic transport.
In the seismic sections, the Kumeta tectonic unit dips S-ward (Figure 7(c)), highlighting an enhanced asymmetry, not in agreement with characteristic strike-slip-fault structures, as previously suggested by the authors that described the so-called Kumeta-Alcantara line.
The displacement associated with the fault system imaged in the seismic sections (Figure 7), accounted for the almost 2 km uplift of the Kumeta tectonic unit, while tectonic transport along the main thrust is N-ward-directed. These features support a backthrusting along a transpressional fault in the Kumeta region, while meso-structural field data document that the minor-fold-vergence is S-directed.
Our field data, according to the previously mentioned authors, support that the main E-W-oriented high-angle faults bordering the Kumeta ridge display a dominant transpressive offset. Moreover, as illustrated before, a lot of Mesozoic normal faults and neptunian dykes ( Figure 6(a,b)), trending parallel to the carbonate ridge, have been detected in this sector. Normal faults produced small intra-platform basins ) that were, often, reactivated by transpressive faults . Thus, also taking into account the large-scale, sin-kinematic, clockwise rotations involving the thrusts during their emplacement (Oldow et al., 1990), the transpressive reactivation during the post-Tortonian shortening is compatible with the orientation of the syn-sedimentary (Mesozoic) normal faults (see also Avellone et al., 2010, p. 115).
According to the structural pattern here depicted, we assess the Kumeta ridge represents only an exhumed (by transpressive to reverse faults related to tectonic Event 2), narrow slice of a wider N-vergent tectonic unit, derived from deformation of the buried Trapanese carbonate platform (Figure 7) and the already thrusted Imerese tectonic units. As a whole, this structural setting rules out the Kumeta ridge as a positive flower structure related to a deep crustal shear zone as formerly suggested.

Software
The map was produced using Esri ArcGIS and Global Mapper. The geo-referenced data-files were then modified using Adobe Illustrator. Daisy 2.0 was used to produce the stereonet plots (Salvini, 2001).