New Insights on the Role of the Strike-Slip Tectonics in the Late Miocene-Quaternary Evolution of Sicily

Understanding the influence of the Late Miocene-Quaternary opening of the Tyrrhenian Basin on the evolution of the external sectors of the Africa paleo-margin in Sicily, actually, represents a hard challenge, even though several, and contrasting, models have been proposed in the last decades. One open problem refers to localizing the main regional scale tectonic lineaments of Sicily that accommodated the hundreds of kilometers of lateral displacement, due to the Tyrrhenian Basin opening. In the present work, we present the results rising from a detailed field mapping carried out in relevant vast areas of central Sicily. These data enabled to reconstruct a Neogene-Quarternary kinematic evolutionary model of the collision belt of Sicily. We analyzed the northern tectonic margin of the Caltanissetta Through, which represents a structural depression hosting a thick allochthonous tectonic wedge, on which lay different unconformable thrust-top basin deposits. In more details, our study aims to reconstruct the tectono-sedimentary Late Tortonian-to-Quaternary evolution of this tectonic wedge, revealing that regional E-W-trending dextral shear zones deform and cut the NE-oriented, SE-verging, thrust-and-fold belt. The strike-slip tectonics thus controlled the deposition of different sedimentary cycles on the thrust-top basins and governed the tectonic inversion of the external sectors of the Africa paleo-margin.


Introduction
The Sicily collision belt represents the eastern termination of the east-west striking collision belt, which overthrust the Africa continental margin of the western Mediterranean [1] [2] (see inset in Figure 1). This orogenic segment has been classically described as a SE-verging thrust and fold edifice, resulting from a complete orogenic cycle that involved the Europe and the Africa continental margins and the intermediate Neo-Tethyan accretionary wedge terrains, sandwiched between them [2]- [8]. Recent studies on the Neogene tectonic evolution of Sicily [9] [10] evidenced the primary role of the Neogene-Quaternary E-W trending dextral shear zones (e.g. the Sicanian-Etna Line of [10]) that border the main tectonic domains of the region. The E-W-oriented Mt. Kumeta-Alcantara Line [9] [11] separates the Madonie-Nebrodi Axial Zone from the Pedemountain Zone (Figure 1). The Sicanian-Etna Line (SEL; [10]) is the northern tectonic border of the Caltanissetta Through (Figure 1), hosting a huge allochthonous tectonic wedge (Caltanissetta Thrust Wedge; CTW), responsible for the impressive negative gravimetric anomaly of central Sicily ( Figure 1).
In this paper, we describe and discuss the results of 1:10,000 scale geological and structural field mapping of relevant vast areas of the Caltanissetta Through  [36]. SEL: Sicanian-Etna Line; KAL: Mt. Kumeta-Alcantera Line; TL: Taormina Line. The inset shows the E-W-trending collision belt along the Nubia-Eurasia plate boundary [37]. International Journal of Geosciences (see Figure 2 for location), also using several available seismic lines and boreholes [8] [12] [13] [14] [15] (Figure 2) that show the subsurface regional scale tectonic structures across the orogenic belt. The new collected geological and structural field data completes the dataset of available previous geological maps and stratigraphic surface data [16] [17] [18] [19].
In the following sections, after the description of the tectonic setting of Sicily, we will describe the architecture of the Caltanissetta Thrust Wedge, desumed from literature surface and sub-surface geological data. In the second part of the paper we will argue on the Neogene-Quaternary deformation of the Caltanissetta Thrust Wedge, based on original geological field mapping of two key areas (Butera and Centuripe areas), discussing about the relationships between the main tectonic features of central Sicily. Finally, our study provides new insights to better understand the relation between the thrust propagation within the CTW and the concurrent displacement along the main dextral SEL and their relation with the Tortonian-Quaternary tectono-sedimentary evolution of the region.
The thrust motion, driven by the NW-SE Nubia-Eurasia convergence [21], was markedly oblique to the strike of the collision belt, giving rise to impressive E-W trending dextral thrust ramps, the Mt. Kumeta-Alcantara Line (KAL in Figure   1; [9] [22] [23]) and the Sicani-Etna Line (SEL in Figure 1; [10]) that duplicate the entire Africa Crust, breaching the overlying allochthonous edifice, in the northern and the central sectors of Sicily, respectively. At the northeastern corner of the island, the collision zone is interrupted by the southern edge (Taormina Line; TL in Figure 1) of the Calabrian arc [24]- [31], which represents a portion of the crystalline European margin that migrated at the hangingwall of the Neogene-Quaternary Ionian subduction zone [32] [33].
At the hangingwall of the KAL, a wide ramp-anticline of the carbonate Afri-    [10]). Dashed boxes refer to the study areas (see reference to the corresponding figures). In the map the buried salt deposits (from [40]), the inner edge of Gela Nappe (from [39]), the depocenters of the Pasquasia (PA), Corvillo-Mandre (CV) and Centuripe (CE) basins, and borehole (from [15]) are also reported. The schematic geological profile are modified from [12].
depth by to a wide tectonic stack of the African foreland succession [8] (see profile in Figure 2). This deep-seated imbrication caused a re-organization of the physiography of the thrust-top basins perched on the Numidian Flysch, mainly due to the huge tectonic subsidence of the Caltanissetta Through area. In the thrust-top basin resting on the hangingwall of the imbricated stack, the Tortonian-Messinian deposition setting remained almost stable, with depth from 0 to 200 m b.s.l.. Conversely, in the adjacent subsiding footwall, the thrust-top basins International Journal of Geosciences sedimentary setting evolved from continental-shallow water to deep (-1000 m) marine conditions during the Early Messinian [10] [38]. Nevertheless, most of the accommodation space (about 7 km) due to the foreland down-bending was balanced the by imbrication of the Numidian Flysch units at the front of the CTW. The syn-tectonic deposition at the hangingwall of the SEL ended with the deposition of the the Lower Pliocene Trubi Fm., whereas it persisted during the Plio-Pleistocene on the subsided areas of the CTW ( Figure 2) [2] [7] [17].
To the east, the CTW is bordered by a culmination of the Africa paleo-margin sequences, exposed in the Mt. Judica area (Figure 1). To the south, the arc-shaped frontal portion of the CTW, described as the Gela Nappe [39], is now emplaced on Early Pleistocene deposits on top of the African foreland of the Hyblean Plateau region (southeastern Sicily) [6] [7] (Figure 2).

Architecture of the Caltanissetta Thrust Wedge
The seismic lines across the Caltanissetta Through [8] [12] evidence that the CTW form a thin-skinned regional scale pop-up structure, due to the impressive tectonic extrusion of the allochthonous Numidian Flysch units and the underlying Africa paleo-margin units, infilling the Caltanissetta Through. The impressive back-verging tectonic slices at the northern border of the CTW [10] ramped on the previous tectonic stack of the Africa-margin units along the SEL. The frontal S-verging tectonic slices of the CTW (Gela Nappe; Figure 2) are widely emplaced on the Early Pliocene horizons (Trubi Fm.) on top of the down-bended African foreland carbonate sequences (see boreholes Manfria 01, Settefarine 01, Ursitto 01, Caltagirone 01; Leone 01; Palma 02, Patti 01 in [15]), ramping even on the Late Pliocene-Early Pleistocene horizons of the present foredeep [39] (see boreholes Margi 01 and Mineo 02 in [15]).
The Numidian Flysch units of the CTW consist of Cretaceous up to Tortonian successions. They include basal chaotic varicolored clays [41] that show characters of a tectonic mélange with a typical block-in-matrix fabric. This unit locally preserves coherent layered intervals of red-to-greenish claystone with associated fine-grained siliciclastic and calcareous turbidites and wrapping blocks of various size, nature and origin (e.g. red-to-green radiolarites, Scaglia-type sediments, marly limestomes and calcareous turbidites, exotics of aphiric basalts and pillow lavas; [3] [42] [43]). Upward, the basal chaotic varicolored clays pass to the Upper Oligocene-Early Miocene Numidian Flysch turbidites, which are represented by almost regular arenaceous-pelitic alternations, otherwise by a broken formation [44] [45]. This latter shows extensive stratal disruption with locally preserved layered intervals represented by sequences of grey-to-green marls and quartzarenitic sandstones. The sequence ends upward with the overlying marly clay sequence of the Licata Formation [3] [43].
The CTW is unconformably covered by Late Tortonian to the Early Quaternary syn-tectonic deposits [17] [46], which filled thrust top basins perched on the allochthonous units ( Figure 2). The Late Tortonian to the Messinian syntectonic deposits widely cover the entire thrust wedge. Along the inner portion International Journal of Geosciences of the Caltanissetta Through, the Late Tortonian horizons unconformably cover the internal successions of the Numidian Flysch Nappe and consist of alluvial sandstones, clays, and patch reef sediments of the Terravecchia Formation [47]. In the more external domains of the Caltanissetta tectonic wedge, the Late Tortonian horizons indeed represent the top of the continuous Early Langhian to Late Tortonian hemipelagic clays sequence of the Licata Formation. The Messinian horizons on the CTW recorded a severe tectonic subsidence of the area associated with an impressive contractional deformation. They form two distinct sequences, separated by a regional unconformity [38] [40] [48] (Figure 2). The lower sequence is made of evaporitic limestones, gypsum and salts, while the upper one mainly consists of gypsum horizons interleaved within prevalent detrital evaporites and coarse-grained clastic deposits. Huge volumes of salts, capping the lower sequence, are preserved at the core of the main footwall synclines of the CTW [16], concealed by the unconformable clastic deposits of the second Messinian cycle. The analysis of available deep well logs provided to identify distinct deepest basin-depocenters that, from SE to NW, are represented by the Braemi (see borehole location on Figure 2), Pasquasia (PA in Figure 2) and Corvillo-Mandre (CV in Figure 2) footwall synclines.
The Early Pliocene sediments consist of the marls and the marly limestones of the Trubi Formation, spread all over the CTW and deposited during the Zanclean flooding, successive to the Messinian Salinity Crisis [48]. On the allochthonous units, Late Zanclean to Middle Pleistocene thrust-top basin successions are represented by distinct cycles of terraced regressive deltaic depositional sequences [46]. Each sequence consists of a proximal clastic and bioclastic wedge, onlapping onto the pro-delta deposits of the previous cycle and prograding onto the coeval distal pro-delta marls [16] [17] [18]. The basal unconformities of the proximal clastic wedges laterally correlate with conformities within the pro-delta succession, which is represented as undifferentiated pro-delta deposits in Figure  3, that are thus progressively younger towards the front of the orogen. These Plio-Pleistocene depositional cycles are confined, to the northeast, by a large NW-SE oriented antiform of the pre-Pliocene substratum (Angelo Antiform in

Neogene-Quaternary Polyphase Deformation in Central Sicily
In this section we present the main results of extensive field mapping carried out in a key area of the frontal sectors of the CTW (Butera area; Figure 4, see Figure   2 for location) and along the SEL (Centuripe area; Figure

The Butera Area
The geological map of the area of Butera (Figure 4; see Figure 2 for location) illustrates the geometry of the CTW and of the overriding Late Miocene-Quaternary thrust top basin deposits, at the front of the orogenic edifice (Gela Nappe, Figure 2). The map details on the impressive bend of the thrust front, from the NE-SW to the roughly E-W orientation that occurs immediately to the southeast of Butera. In this area, the units of the CTW are widely exposed

The Centuripe Basin
The Centuripe Basin is an E-W elongated depocenter, perched on the Numidian  The earlier and oldest structures of the thrust wedge ( Figure 6(a)) are evidenced by the location of the main depocenters of the Lower Messinian sediments (e.g. Corvillo and Pasquasia basins in Figure 2) [10]. The two depocenters rest on the opposite sides of a major pop-up structure, which developed during the breaching of the internal sectors of the CTW. The thrust ridge played a major role on the Messinian-Early Zanclean syn-tectonic deposition of the region, as it dammed the clastic deposits of the Messinian second cycle fed from the Madonie-Nebrodi axial zone [10] [13] and represented the physiographic boundary of the Late Zanclean transgression (I cycle in Figure 3; Enna cycle). At this stage, a depocenter of the Messinian-Early Zanclean deposits, controlled by NW-SE oriented tectonic alignments developed in the Centuripe area.

Discussion
The new collected field data provided precious information to complete the dataset on the Late Zanclean to Quaternary tectono-stratigraphic features of the CTW, useful to constrain the overall tectonic evolution of the CTW and its relation with the kinematics along the tectonic boundaries of the structural depression. Two different stages can be identified:

Late Zanclean-Calabrian Stage
This stage is characterized by the progressive emergence of the CTW that is evidenced by the distribution of the proximal delta deposits of the subsequent Plio-Pleistocene depositional cycles (Figure 3). The emersion started after the emplacement of the tectonic wedge onto the marls and marly limestones of the Trubi Formation that deposited on the margin of the Hyblean Foreland [6]. Figure 6(b) illustrates the approximate position of the coastal deposits of the four deltaic cycles that deposited in the Caltanissetta Through. Each of them developed at the forelimb of an anticline deforming the deposits of the previous sequence ( Figure 3). Four main anticlines, showing wavelength of about 10 km, deform the Plio-Pleistocene deposits in the external areas of the CTW. Combining the field evidence with seismic data the anticlines can be interpreted fault-propagation folds (see profile in Figure 3) related to blind ramps that propagated to the SE through the CTW. This imbricated fan was responsible for the huge thickening of the allochthonous wedge, emplaced on the regional monocline of the African Foreland. The ages of the different cycles, coherent with the foreward-younging of the ramp anticlines, is indicative of the propagation of the thrust system for about 60 km during this time interval.
In the Butera area (Figure 4), at the frontal sector of the CTW, a new generation of NE-SW oriented folds, affecting the Piacenzian-Gelasian thrust-top basin deposits, developed at the toe of the tectonic wedge, whose front can be considered as almost fixed. The superposition of NE-SW to N-S oriented fold axes on previous NW-SE trending folds determines a clear interference pattern. In the Centuripe area ( Figure 5), E-W trending dextral thrust ramps of the SEL with the associated NW-SE oriented dextral faults propagated from the Africa basement to the surface, through the Numidian Flysch units, after their emplacement [10]. These structures controlled the geometry of the basin, whose depocenter remained almost fixed during the entire considered time-interval.  Figure 5). According to the interpretation of the available seismic lines [6] and field data this dextral oblique main structure would represent the field evidence of Sicanian-Etna Line [10], representing a crustal wrench fault located just beneath the Centuripe Basin.

Middle Pleistocene Stage
This

Conclusions
The new field data, collected in the analyzed key areas of the Caltanissetta the lateral motion along the wrench faults remain the only mode to accommodate the regional shortenings.
In conclusion, the new field data point out several evidence of dextral shearing along E-W oriented belts that overprint the thrust and belt features of the Caltanissetta Thrust Wedge. The reconstructed shear zone seems to have played a major role in the tectonic picture of central Sicily, causing the dextral lateral shifting of the major tectonic domains of the region. In this new kinematic picture, the tectonic stacking of the Caltanissetta Thrust Wedge is the result of the tectonic inversion of the Africa paleomargin, trapped between the dextral shear zone, to the north, and the converging Hyblean plateau crustal buttress, to the southeast.
Finally, this paper represents a further tile for understanding the main role of the strike-slip tectonics in Sicily and could constitutes a relevant tectonic feature for the identification of the on-shore prolongation of the major strike-slip fault systems recognized in the Ionian offshore.