Middle Miocene-Early Pliocene wedge-top basins of NW Sicily (Italy): constraints for the tectonic evolution of a 'non-conventional' thrust belt, affected by transpression

Journal of the Geological Society Online First

10.1144/jgs2013-009, first published January 30, 2014; doiJournal of the Geological Society

 V. Valenti and R. CatalanoC. Gugliotta, M. Gasparo Morticelli, G. Avellone, M. Agate, M. R. Barchi, C. Albanese, affected by transpressionconstraints for the tectonic evolution of a 'non-conventional' thrust belt,

Early Pliocene wedge-top basins of NW Sicily (Italy):−Middle Miocene

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1

During a foreland-thrust belt development (Davis et al. 1983; Dahlen et al. 1984) the deformation propagates ‘in sequence’ from the internal to the external zone involving progressively deeper structural levels (Bally et al. 1966, 1985; Boyer & Elliott 1982; Roure et al. 1990; for a complete review, see Roeder 2013). When deeper structural levels are reached by deformation, the latter will propagate both towards the foreland, through the activation of new main thrusts (forward-braking sequence), as well as upwards (vertically), passively deforming the already emplaced and shallower structural levels (the ‘in-sequence’ mode, also called a piggyback thrust sequence; McClay 1992). As a consequence, looking at any point of a fold-and-thrust belt, the struc-tural arrangement derives from several superimposed deformation events developed at different structural levels where shallower thrusts are older than deeper thrusts. It is well known, however, that surface transport processes (both erosional and depositional) can influence the evolution of a thrust belt, disturbing the regular (in-sequence) emplace-ment of the thrust sheets, as well as the evolution of the syntectonic basins (e.g. Simpson 2006). The effects of tectonic inheritance (fault reactivation and/or inversion) also affect the localization and/or ampli-fication of the single thrusts (e.g. Butler et al. 2006).

In this context the Sicilian fold-and-thrust belt (Fig. 1) repre-sents an interesting ‘in-sequence’ and ‘non-conventional’ case study as the contractional deformation was accompanied by large clockwise rotations (about 120° in the upper structural units) accommodated by the emplacement of thrust sheets (Catalano et al. 1976; Grasso et al. 1987; Channell et al. 1990; Oldow et al. 1990; Speranza et al. 2003; Monaco & De Guidi 2006). As a conse-quence, the structures formed during successive tectonic events are superimposed on each other non-coaxially, at an angle of up to about 90° in the upper structural levels (Avellone et al. 2010).

Large clockwise rotations of the tectonic units yielded continuous variation of the ‘apparent’ transport direction during their emplace-

ment (thrust-sheet rotation) and probably at the same time a continu-ous variation of palaeoslope trend of the wedge-top depozone (basin rotation). In this context, the wedge-top basins are passively carried and rotated (piggyback mode; Allerton 1998). The effects of this non-coaxial superimposition of tectonic events can be recognized through the analysis of both structural (Roure et al. 1990; Avellone & Barchi 2003; Avellone et al. 2010) and depositional interferences (Gugliotta & Gasparo Morticelli 2012). In this paper the latter term has been used to describe the effects of a complex (i.e. multiphase and non-coaxial) tectonic evolution on the related depositional processes (e.g. the changes in topography affecting the sediment transport pathways, the rates and spatial distribution of erosion and sediment deposition, the progressive migration of depocentres, the growth of stratal pat-terns and occurrence of widespread intraformational unconformities). In the case of multiphase and non-coaxial tectonics, the direction of depositional features changes upwards in the basin infill.

Depositional interferences, in particular, are widespread within the upper Serravallian–lower Pliocene sedimentary infill remnant of wedge-top basins of the Sicilian fold-and-thrust belt (Gugliotta 2013; Avellone et al. 2011; Gugliotta & Gasparo Morticelli 2012). These ‘basins’, at present preserved corresponding to structural depressions of variable size, are from east to west (Fig. 2a): (1) the Early Pliocene Lascari Basin (northernmost Sicily); (2) the late Serravallian–late Tortonian Scillato Basin; (3) the late Serravallian–Early Pliocene Ciminna–Bosco Basin; (4) the late Serravallian–early Messinian Camporeale Basin.

The main aim of this paper is to document that the late Neogene tectonosedimentary evolution of the NW Sicily fold-and-thrust belt was characterized by the development of a wedge-top depozone within a transpressive tectonic regime. We will pay special atten-tion to characterizing the deformation style of the region and to constraining the timing of the basin infill evolution.

Middle Miocene–Early Pliocene wedge-top basins of NW Sicily (Italy): constraints for the tectonic evolution of a ‘non-conventional’ thrust belt, affected by

transpression

C. GuGLIOTTA1, M. GASPARO MORTICELLI2*, G. AvELLONE2, M. AGATE2, M. R. BARChI1, C. ALBANESE2, v. vALENTI2 & R. CATALANO2

1Department of Earth Sciences, University of Perugia, Piazza dell’Università 1, 06100 Perugia, Italy2Department of Earth and Marine Sciences, University of Palermo, Via Archirafi 22, 90123, Palermo, Italy

*Corresponding author (e-mail: [email protected])

Abstract: The study of geological evolution of a multiphase orogenic belt is complex, expecially when the tectonic events are superimposed in a coaxial fashion. The Sicilian fold-and-thrust belt represents an interest-ing case study, as a non-coaxial superimposition of structures is recognizable, owing to large synkinematic clockwise rotations during each of two subsequent compressional events. These rotations involved also the syntectonic basins that developed in the wedge-top depozone. This study aims to constrain the tectono-depo-sitional evolution of the NW Sicily fold-and-thrust belt and the associated wedge-top depozone between the middle Miocene and the early Pliocene. Integrated analyses of stratigraphic, sedimentological and structural field data allow us to better constrain the transition between the two tectonic events. The syntectonic basins developed during the first (late Serravallian–early Tortonian) tectonic event were relatively wide and char-acterized by marine sedimentation. The onset of the second (latest Tortonian) transpressional event induced localized deformation into the wedge-top depozone and the syntectonic successions were accommodated as basin fill in progressively narrower and laterally discontinuous basins, bounded by transpressional structures. The lateral correlation of the wedge-top successions suggests a latest Miocene regional palaeoslope with a present-day WSW dip, which fits well with the tectonic transport calculated for the early compressional event.

research-articleResearch ArticleXXX10.1144/jgs2013-009Gugliotta et al.Wedge-Top Basins and Thrust Beltc.2014

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Geological framework

The Sicilian fold-and-thrust belt represents a segment of the Neogene to Present Apennine–Maghrebian orogen (Fig. 1a). Basically it is a collisional foreland fold-and-thrust belt (Bello et al. 2000; Catalano et al. 2000) developed in the framework of a slow African–European convergence (Gueguen et al. 1998; Goes et al. 2004). Since the Tortonian the southward radial migration of the orogenic belt has been synchronous with the opening of the Tyrrhenian back-arc basin (Tyrrhenian spreading; Fig. 1b; Malinverno & Ryan 1986) and relates to a typical west-dipping subduction zone with asthenosphere that replaces the retreating lithosphere (Doglioni 1991; Doglioni et al. 1999a,b).

The evolution of the subduction beneath the Apennines–Calabria–Sicily has been described by two geodynamic models: (1) a continu-ous laterally bent slab underlying the whole Apennine to Maghrebide salient (Fig. 1b; Malinverno & Ryan 1986; Catalano et al. 2001; Carminati & Doglioni 2005, 2012); (2) a seismically active Calabrian slab solely driving the whole back-arc zone evolution, with the Southern Apennines and Sicily being only lateral rootless belts (e.g. Faccenna et al. 2004; Neri et al. 2012). In this latter geodynamic model a lithospheric tear fault (‘slab transfer edge propagator’ or STEP), as defined by Govers & Wortel (2005), can be predicted, bounding the southeastward rollback of the Ionian subduction zone.

A recently acquired crustal seismic profile (SI.RI.PRO. Project; Accaino et al. 2011; Catalano et al. 2013) reveals that the Sicilian fold-and-thrust belt consists of a SE-verging orogenic wedge, 20–25 km thick, showing a blend of thick- and thin-skinned deforma-tion (Fig. 1c), which accreted above an inclined NW-plunging regional monocline imaged by the inflection of the Iblean foreland crust (Fig. 1c). Catalano et al. (2013) constrained the above-men-tioned setting in a continental lithosphere subduction process. A mantle wedge is predicted above the subducting tectonic plate and below the overriding plate, which could be responsible (Doglioni 1991) for the uplift of the central sector of the belt. Catalano et al. (2013) highlighted the occurrence of a regionally extensive wedge-shaped body of high-density, sub-crustal material, several kilome-tres thick, interpreted as the southern wedge-edge of the Tyrrhenian mantle that splits the subducting Iblean–Pelagian (African) conti-nental slab from an overlying stack of allochthonous thrust sheets (Fig. 1c). In this view, the upper mantle wedge-edge and its Moho, rejuvenating southeastwards, progressively replaced the roll-back of the subducting Iblean–Pelagian slab (Fig. 1c).

A Late Pliocene–Quaternary foredeep is buried beneath the frontal sector of the orogen in southern Sicily (Fig. 1a) and in the Gela Basin (Catalano et al. 2000, and references therein).

Compressional deformation started in the latest Oligocene follow-ing the collision of the Sardinia Block with the African continental

Fig. 1. (a) Tectonic sketch of the central Mediterranean area (from valenti 2010; Catalano et al. 2013). Inset map shows the location of the study area in the regional tectonic map (after Jolivet & Faccenna 2000). Foreland basins are shaded dark grey. (b) Main tectonic features of the western Mediterranean realm, which has been shaped during the last 45 myr by the Apennines subduction zone and related back-arc basins (from Carminati & Doglioni 2005). (c) Geological transect derived from crustal seismic profile extending from northern Sicilian offshore to the Iblean foreland, highlighting the subduction of African continental lithosphere beneath the Sicilian fold-and-thrust belt (SFTB) (from Catalano et al. 2013).

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palaeomargin (Oldow et al. 1990; Catalano et al. 1996, 2000). With the continuous forward migration of the deformation the compres-sional structures involved Palaeozoic to Cenozoic successions origi-nally deposited in distinct palaeogeographical domains belonging to the Ionian Tethys (Sicilide domain, Ogniben 1960) and its prolonga-tion toward the African continental palaeomargin (Maghrebian units; Catalano & D’Argenio 1982; Roure et al. 1990; Lentini et al. 1994; Catalano et al. 1996, 2000; Finetti 2005). The main construction of the tectonic edifice resulted from the stacking of a thick (about

4–6 km) wedge of external carbonate platform units below thin (about 1.5 km) deep-water carbonate units (Figs 1c and 2b). First, deformation involved internal deep-water carbonate rock bodies with duplex geometry, resulting in their detachment from the base-ment and their progressive stacking and rotation above a still unde-formed outer carbonate platform (shallow-seated thrusts during Event I, Fig. 3; Avellone et al. 2010, and references therein). Later, southward-verging thrusts and backthrusts involved the carbonate platform (deep-seated thrust during Event II; Fig. 3). The deep-seated

Fig. 2. (a) Schematic geological map of western Sicily (after Catalano et al. 2000). The rectangles indicate the location of the studied basins. (b) Geological cross-section showing the main structural setting of the NW Sicily thrust belt (after Catalano et al. 2000).

Fig. 3. Schematic model (not scale) showing the propagation of the two compressive tectonic events of the Sicilian thrust belt (from Gugliotta & Gasparo Morticelli 2012).

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faults are transpressional and often reactivate pre-existing Mesozoic extensional faults (Avellone et al. 2010, and references therein).

In the framework of a piggyback thrust sequence (McClay, 1992), during which the rotation (see the section ‘vertical-axis rotations’, below) of the thrust sheets occurred (Oldow et al. 1990), the onset of the transpressive fault system follows the progressively increasing strike-slip motion within an obliquely converging orogen with non-parallel margins (McKenzie & Jackson 1983; Allerton 1998).

The deep-seated thrusting process transferred part of the defor-mation to the overlying thrust pile, producing (Figs 2b and 3) short-ening of the already emplaced thrust sheets (Albanese & Sulli 2012), uplift and arcing. Both the timing and the modalities of the transition from Event I to Event II across the Sicily fold-and-thrust belt are still not well constrained or clearly understood, mainly owing to migration of each structural set from internal to external areas of the thrust belt (Avellone et al. 2010).

Alternatively, the origin of transpression is explained by the pre-sumed occurrence of a crustal-scale, dextral shear zone, between the so-called ustica–Eolie line (Tyrrhenian basin; e.g. Renda et al. 2000) and the Kumeta–Alcantara line to the south (northern Sicily; e.g. Ghisetti & vezzani 1984), developed in the geodynamic framework of a STEP-like plate boundary, controlling the southern Tyrrhenian margin (Govers & Wortel 2005; Neri et al. 2012).

Foreland basin system

Reflecting the transport direction of the thrust belt, a wide foreland basin system (sensu De Celles & Giles 1996) developed from latest Oligocene time (Fig. 4) accommodating a thick succession of upper-most Oligocene–Lower Miocene flysch (Catalano et al. 1989, 2001; Oldow et al. 1990; Nigro & Renda 2000; Grasso 2001; Gugliotta 2011). A considerable part of these deposits crop out widely in the study area (Numidian Flysch sensu lato) represented by a coarsening-upward succession consisting, from the bottom to the top (Fig. 4), of Chattian–lower Aquitanian hemipelagic marls, argillites and thin-bedded turbidites (Portella Colla member in Fig. 4; ramp mud of Ricci Lucchi 1986) unconformably or paraconformably overlain by Aquitanian–Burdigalian sandstone turbidites and quartzarenites (Geraci Siculo member in Fig. 4; main clastic wedge of Ricci Lucchi 1986). Starting from Burdigalian time the Numidian Flysch foredeep began to be deformed, as suggested by unconformable sedimentation of ‘piggyback’ upper Burdigalian–Langhian, brown–yellowish sandy marls with interbedded glauconitic sandstone (Tavernola formation in Fig. 4). At almost the same time, the innermost Sicilide units were progressively thrust onto both the Numidian Flysch and the Tavernola formation, marking the definitive closure of the Numidian Flysch sensu lato basin. Sedimentation followed both above (wedge-top depozone) and at the front of the advancing fold-and-thrust belt (fore-deep depozone) during the Middle Miocene and the Early Pliocene, as recorded by the syntectonic successions studied here.

Vertical-axis rotations

vertical-axis rotations suggested by palaeomagnetic data have played an important role in the development of the Sicilian fold-

and-thrust belt (Channell et al. 1990; Duermeijer & Langereis 1998; Speranza et al. 2003). Integrated palaeomagnetic and structural analyses reveal a large (in some cases more than 120°) clockwise rotation that took place in Sicily, mostly related to the contractional phase generating the fold-and-thrust belt (Oldow et al. 1990; Avellone et al. 2010). Amounts of vertical-axis rotations do not show a random distribution: the uppermost thrust sheets in the stack exhibit the greatest rotation, recording up to 120° (e.g. the Panormide unit in northern Sicily is rotated clockwise about 134° and south-ward the Trapanese unit records about 60° of rotation); the rotation amount progressively decreases towards the unrotated foreland (Channell et al. 1990; Speranza et al. 1999, 2000). Palaeomagnetic data from the Trubi Formation cropping out in the investigated area (Lascari Basin; Fig. 2a) reveal that only 25° clockwise rotation took place after the Early Pliocene (Grasso et al. 1987).

Two main mechanisms for thrust-sheet rotation have been extensively discussed in the literature: (1) propagation of thrusting;

Fig. 5. Simplified stratigraphic column (not to scale) showing the main syntectonic units that crop out in the study area.

Fig. 4. Simplified scheme (not to scale) showing the depositional setting of the Numidian Flysch basin between the Chattian and the Langhian. FYNpc, Numidian Flysch Portella Colla member; FYNgs, Numidian Flysch Geraci Siculo member; TAv, Tavernola formation.

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(2) block rotation in the context of transpressive tectonics. We sup-port the concept that the latter mechanism plays a subordinate role because rotation associated with transpressive tectonics produces effects only at a local scale close to the strike-slipe fault and the rotation amount rapidly decreases away from the fault.

With reference to the Sicily fold-and-thrust belt, the rotation of the allochthonous Sicilian units derives from a process of ‘thrust propagation and piggyback rotation’ (Bates 1989; Oldow et al. 1990). This mechanism can account for a large amount of rotation, occurring on a regional scale; for example, up to 120° has been documented in the eastern Subbetic Zone of southern Spain (Allerton 1994) as a consequence of large rotations accumulated by progressive piggyback faulting (Allerton 1998).

Geological setting of the study area

upper Serravallian to Lower Pliocene wedge-top sedimentation is recorded in the study area (Fig. 2a) by an often incomplete suc-cession (several hundred metres thick) of lithostratigraphic units. These are locally known, from oldest to youngest, as the follow-ing (Fig. 5): (1) Castellana Sicula Formation marine clays, silt-stones and sandstones (upper Serravallian–lower Tortonian); (2) Terravecchia Formation continental conglomerates, fan delta sandstones and marine clays (upper Tortonian–lower Messinian); (3) lower Messinian carbonates, gypsum turbidites and turborotalita-bearing marls unconformably followed by lower to upper Messinian evaporites and continental conglomerates (for details, see Lo Cicero et al. 1997; Catalano et al. 2010b); (4) Lower Pliocene Trubi Formation chalks and calcarenites, the uppermost unit.

The syntectonic units overlie a deformed substrate consisting of three main thrust wedges, superimposed through regional, sub-planar décollements; from the top these are as follows (Fig. 6).

(1) A thrust wedge made up of thin slices of Sicilide units over-thrusting Oligo-Miocene siliciclastic successions (Numidian Flysch and Tavernola formation), often detached from their origi-nal Mesozoic carbonate substrate.

(2) A thrust wedge consisting of deep-water carbonate imbri-cates (locally Imerese units). These rocks are widely exposed in the mountain ranges of the study area (Palermo Mts and Madonie Mts; Fig. 2a) and crop out as major ramp–anticlinal structures (e.g. Mt. Dei Cervi, Mt. Cane and Mt. San Calogero; see Fig. 2a) that formed during the compressional Event I. Both the anticlines and the related thrusts show a present-day NW–SE direction with a mean southwestward vergence, believed to originate from the large-scale rotation of the thrust sheets during their emplacement.

(3) A thrust wedge consisting of carbonate platform imbricates (Trapanese units). This lowermost structural level, clearly imaged by seismic reflection profiles (Catalano et al. 2000; Albanese & Sulli 2012), is exposed in the study area along narrow, east–west-elongated structural highs, raised along major, double-verging deep-seated ramps (e.g. Busambra and Kumeta ridges; Fig. 2b) that developed during the transpressional Event II. As shown in Figure 6, the structural arrangement of the deformed substrate units changes across the thrust belt, from one basin to another.

Dataset

New sedimentological, stratigraphic and structural data collected also within some multidisciplinary research projects (such as CARG, Catalano et al. 2010a, b, and SI.RI.PRO, Accaino et al. 2011; Catalano et al. 2013) have been integrated to constrain the tectono-depositional evolution of the late Neogene foreland basin system in western Sicily

Methods

Detailed field geology as well as facies and sedimentological anal-yses of several measured and correlated stratigraphic sections accompanied by mesoscopic structural analyses have been used to characterize the main tectonic structures. The deformation patterns regarding both folding and faulting were determined at many meas-uring stations located in the basins and their surroundings. The ste-reographic projection of the data has been performed by means of Daisy 3 software (Salvini 2001).

A detailed set of field analyses has been performed concerning the stratigraphic and structural features of the above-mentioned successions because of the good exposures and the excellent timing relations owing to syntectonic deposition. According to their pre-sent-day geographical location, a synthesis (from east to west) of the main geological character of the studied basins is presented in the following sections.

the Lascari Basin. The Lascari Basin is the smallest and eastern-most basin studied here, and it is partly preserved along the north-ern coast of Sicily, close to the Lascari village (western Madonie Mts, Figs 2a and 7a). There, an up to 100 m thick succession of Lower Pliocene whitish marls, limestones and interbedded gravity-flow yellowish calcarenites crops out overlying an angular uncon-formity carved into the deformed substrate (Figs 5 and 6). The latter consists of a Sicilide nappe wedge overthrusting the Numid-ian Flysch units along a NE-dipping thrust plane (structural data shown in Fig. 7d, site 1). The S–C fabrics along the thrust fault zone (Platt, 1984) indicate a southwestward tectonic transport of the Sicilide nappes. The thrust plane is in its turn involved in a roughly east–west-trending, northward-verging synform. The stratal pattern recorded in the Lascari Basin infill (Fig. 7b and c) reveals a syntectonic deposition controlled by limb rotations of the basin (Rafini & Mercier 2002) and constrains an Early Pliocene timing of deformation (Avellone et al. 2011).

the Scillato Basin. The Scillato Basin is located in the central–northern sector of the Sicily fold-and-thrust belt, along the western edge of the Madonie Mts (Figs 2a and 8a). The stratigraphic suc-cession of the Scillato Basin consists of about 50 m of Castellana Sicula Formation deposits unconformably covered by about 1200 m of upper Tortonian Terravecchia Formation clastic deposits. The whole succession unconformably overlies a deformed substrate made by Sicilide, Numidian Flysch and Imerese thrust sheets (Figs 5 and 6). The Scillato Basin consists of an approximately NE–SW-oriented structural depression, bounded to the SE by major carbon-ate structural highs that have been interpreted as partially outcropping, NW–SE-trending ramp anticlines associated with major SW-verging thrusts involving Meso-Cenozoic Imerese units (Fig. 8b–e). The statistical analysis of folds hinges (Fig. 8g) revealed for these structures a strain field compatible with the com-pressional Event I. Both major anticlines and thrusts are cut and displaced by a superimposed NE–SW-striking, SE-dipping major transpressive, left-lateral fault (Cervi Fault in Fig. 8; Gugliotta & Gasparo Morticelli 2012). The statistical analysis of the striated fault planes reveals a N170°-oriented maximum palaeo-stress (σ1) compatible with tectonic Event II (Fig. 8g).

The Scillato Basin infill has been deformed as a roughly asym-metric synform, the SE limb of which is steeper than the NW limb (i.e. NW-verging in Fig. 8a) as a consequence of Event II (Gugliotta & Gasparo Morticelli 2012).

the Ciminna–Bosco Basin. The Ciminna–Bosco Basin is located in the central part of the study area, bounded by major SW-verging

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asymmetrical structures (Cane, San Calogero and Pipitone anti-clines; Figs 2a and 9a) formed by deep-water carbonates, Numidian Flysch and the Sicilide units (Lo Cicero et al. 1997; Del Ben & Guarnieri 2000; Guarnieri 2004; Catalano et al. 2010b). These structures are interpreted as having developed during tectonic Event I (Catalano et al. 2010a). An upper Serravallian–lower Plio-cene composite stratigraphic succession (up to 1700 m thick) crops out in the Ciminna–Bosco Basin (Fig. 9c–e) consisting, from bot-tom to top, of about 150 m of Castellana Sicula Formation deposits, about 1000 m of Terravecchia Formation clastic deposits, about 500 m of Messinian deposits, and about 30–50 m of Trubi lime-stones (Figs 5 and 6). The succession is deformed into two major, NW–SE- to east–west-trending synclines (Ciminna and Monte Bosco synclines). These major synclines appear separated and lat-erally shifted (with a displacement of some kilometres) along a major, NE–SW-striking, left-lateral transpressional fault (San Leonardo Fault, site 2 in Figure 9b–f; named ‘Fiume S. Leonardo line’ by Montanari 1968), which also cross-cuts and offsets the major structures related to tectonic Event I. A comparison between the western and eastern sector of the Ciminna–Bosco Basin (Fig. 10) reveals abrupt variations of thickness of the Messinian deposits near the transpressional fault: the stratigraphic succession is thicker in the footwall of the San Leonardo Fault (western Ciminna–Bosco Basin) than in the hanging wall (eastern Ciminna–Bosco Basin), suggesting a syndepositional activity at least during the Messinian.

the Camporeale Basin. The Camporeale Basin is a wide struc-tural depression located in NW Sicily that opens southwestward from the Palermo Mts (Figs 2a and 11a). The Camporeale Basin succession (about 1000 m thick) unconformably rests on the deformed substrate and is composed of up to 250 m of Castellana Sicula Formation deposits, about 700 m of Terravecchia

Formation, followed by lower Messinian bioclastic calcarenites (Figs 5 and 6). Interpreted deep seismic reflection profiles show that the Camporeale Basin development is related to the deforma-tion of the underlying multi-layered substrate mainly by means of deep-seated high-angle faults (Fig. 11d; Catalano et al. 2000, 2010b; Gugliotta 2011). These faults, which are north- and mostly south-dipping and cut the buried carbonate platform Trapanese units, induced deformation of the overlying sedimentary cover, along with the older thrusts (Fig. 11e), producing the develop-ment of localized east–west-trending, double-verging transpres-sional ridges (e.g. Pietroso Ridge; Fig. 11c), locally associated with north-verging asymmetric synclines (Gasparo Morticelli et al. 2008; Gugliotta 2011). Kinematic indicators, recognized along the fault planes and the folds affecting the syntectonic units, are consistent with a roughly N170°-oriented stress field, sug-gesting that the deformation of these units mostly occurred during Event II (stereoplots in Fig. 11b).

Middle Miocene–Early Pliocene wedge-top sedimentation and tectonics

The comparison between the described basins suggests some consid-erations about both the structure and the evolution of the wedge-top depozone and the underlying thrust belt, developed during the Middle Miocene (Serravallian)–Early Pliocene time interval.

Serravallian to early tortonian

This time span is recorded, in the study area, by the Castellana Sicula Formation deposits. This unit is of late Serravallian–early Tortonian age (pre-10.4 Ma, according to the informal subdivi-sion of the Tortonian of Catalano et al. 2010b; Fig. 5). The Sicula Formation consists of outer shelf to slope marls and siltstones

Fig. 6. Simplified stratigraphic columns (not to scale) from integrated field and subsurface data showing the main relationships between the syntectonic units that crop out in the study area and the deformed substrate units.

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WEDGE-TOP BASINS AND ThRuST BELTC. 7

with gravity flow sandstones and conglomerates. These deposits cover a regionally extended angular unconformity carved into the already deformed substrate units (Sicilide units and Numidian Flysch), suggesting deposition in a wedge-top setting. The angu-lar unconformity at the base of the Sicula Formation retains an interval of about 1.3 myr (uppermost Langhian and lower Serravallian). Comparison between several basins revealed that the outcrops of the Sicula Formation show homogeneous strati-graphic and sedimentological characteristics notwithstanding the variable geometric relationships with its substrate. From ENE to WSW, the Sicula Formation progressively onlaps more external structural units of the Sicilian thrust belt (Fig. 6). In the eastern basins (Scillato and Ciminna–Bosco basins) the Sicula Formation unconformably covers both the Sicilide and Numidian Flysch units, sealing the Sicilide units floor thrust. Toward the west, the Sicilide units suddenly disappear in the Camporeale Basin. The Sicula Formation covers detached tectonic slices of Numidian Flysch and deep-water carbonate units (Imerese units) as imaged by seismic reflection profiles (Catalano et al. 2010b). The Imerese units thrust over the carbonate platform units (Trapanese units) progressively wedge out southward, disappearing in the central Camporeale Basin. There the Sicula Formation unconformably covers thin slices of Numidian Flysch units directly thrust onto the Trapanese units (Fig. 6).

Moreover, the thickness of the Sicula Formation progressively increases southwestward: the unit is absent in the easternmost basin (Lascari Basin), about 50 m thick in the Scillato Basin, up to 150 m thick in the Ciminna–Bosco Basin, and more than 250 m thick in the Camporeale Basin. The Sicula Formation is partially time-equivalent to another lithostratigraphic unit cropping out in the study region as sedimentary cover of the Trapanese carbonate succession, locally known as the Marne di San Cipirello (step 1 in Fig. 12). The Marne di San Cipirello consists of deep-water to outer shelf clays, siltstones and marls deposited, differently from the Sicula Formation, above the still undeformed carbonate plat-form succession. The deposition of the Marne di San Cipirello occurred in continuity of sedimentation covering a relative time interval of about 3 myr, corresponding to the Serravallian and the lower part of the Tortonian. As suggested by both field and geo-physical data and as shown in Figure 6, the Marne di San Cipirello occurs below the floor thrust of the Numidian Flysch and Imerese units (e.g. Catalano & D’Argenio 1982; Catalano et al. 2010a,b; Albanese & Sulli 2012).

Late tortonian to Early Pliocene

The transition to the late Tortonian records an impressive change in both depositional and tectonic setting of the Sicily fold-and-thrust

Fig. 7. (a) Simplified geological map of the Lascari Basin (modified from Avellone et al. 2011). Geological section (b) and related line-drawing (c) show that the Early Pliocene Lascari Basin was hosted at the core of a syncline controlled by syndepositional transpressional tectonics (see growth geometries). (d) Stereographic projection (locations of structural sites are indicated on the map) of the collected data (faults, folds, pressure solution plane, extensional fractures). The associated stress axes are calculated from stress inversion applied to transpressive faults and related slickensides.

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GuGLIOTTA Et AL.8

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WEDGE-TOP BASINS AND ThRuST BELTC. 9

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belt. A regionally extended angular unconformity cuts the already emplaced tectonic units (Sicilide and Numidian Flysch units; Fig. 6) as well as the overlying, already deformed, upper Serravallian–lower Tortonian wedge-top successions, representing the main field evi-dence of this transition. The angular unconformity marks the first episode of subaerial exposure of the thrust belt and is overlain by continental to fan delta deposits of the Terravecchia Formation. The time span retained by the angular unconformity strongly varies across the study region and it is not easy to assess owing to the prox-imal sedimentary environment of the Terravecchia Formation depos-its. In some cases (such as in the Scillato and Ciminna–Bosco basins) the Terravecchia Formation directly covers the higher structural units (Sicilide or Numidian Flysch units) or small slices of Sicula Formation deposits (Fig. 6). In the westernmost Camporeale Basin the unconformity cuts the uppermost part of the Sicula Formation.

The Terravecchia Formation crops out widely in the study area with a wide range of lithologies and facies associations, which

change suddenly across a roughly ENE–WSW-oriented belt. The internal pattern of deformation and the relationships with the deformed substrate also change in the same direction. On the whole, these lateral variations account for the distinction between a late Tortonian–early Messinian ‘inner’ and ‘outer’ wedge-top depozone that overlaps the outermost sector of the orogenic wedge (Fig. 6; for details, see Gugliotta 2012).

The inner wedge-top depozone, now partially preserved in the Scillato and Ciminna–Bosco basins, was characterized by the dep-osition of fining- to coarsening-upward successions. here, the angular unconformity at the base of the Terravecchia Formation is overlain by upper Tortonian conglomerates. Sedimentological analyses (Gugliotta 2011) indicated out that these conglomerates were accommodated in NW–SE-trending incised valleys along major structural depressions with the same orientation. Both mean clast composition analysis and palaeocurrent patterns reveal an extrabasinal supply for the conglomerates, the clasts of which are derived from a source area to the north reflecting the composition of the surrounding substrate (prevalently Sicilide and Numidian Flysch units), with a conspicuous occurrence of metamorphic (mostly gneiss) and igneous fragments. Accordingly, these frag-ments have been interpreted (Ferla & Alaimo 1975; Catalano & D’Argenio 1990; Cirrincione et al. 1995) as derived from the dis-mantling of the Kabylo-Calabride crystalline units that crop out at present in NE Sicily (Peloritani Mts) and occur offshore from northern Sicily along a east–west continuous band (Fig. 1). Occurrence of both crystalline basement fragments and incised val-ley filling progressively decreases from the easternmost to the westernmost basins, where they are absent. In fact, no such clasts or incised valley deposits are present in the wider Camporeale Basin, where an outer wedge-top depozone succession can be envisaged. Fan delta deposits characterize the middle and upper part of Terravecchia Formation in the studied basins although in the inner wedge-top depozone deltaic deposition was mostly coarse-grained and shows facies tracts suggesting deposition by steep, high-energy flood-dominated fan delta systems probably sheltered from the open marine areas. In the outer wedge-top depozone del-taic sedimentation occurred prevalently across storm- and wave-dominated sandy shoreface delta systems, open to the main marine area (Gugliotta 2012).

Also, the basin-to-basin comparison of the Terravecchia Formation deformational pattern shows some interesting aspects. Local-scale, syndepositional tectonics was active in the inner wedge-top depozone during the latest Tortonian. This is suggested by the internal deformation of the Terravecchia Formation deposits exposed in the Scillato and Ciminna–Bosco basins (Gugliotta 2011, 2012). Syndepositional deformation occurred mainly in response to deep-seated, high-angle transpressive faults (e.g. Cervi Fault in Fig. 8a–c). These faults caused the uplift of localized structural highs, flanked by depressed areas, influencing the geometry of the stratal pattern (Fig. 8e–f), the drainage network of the basins, and the lateral variation of the thickness of basin infill (Gugliotta & Gasparo Morticelli 2012). No clear evidence of local-scale syn-sedimentary deformation has been observed in the Camporeale Basin (outer wedge-top depozone).

Transpressive structures produced prominent post-late Tortonian deformation by development of double-verging transpressional faults (Fig. 11c and d; Gasparo Morticelli et al. 2008; Gugliotta 2012). The deformed substrate varies toward the SW and the Terravecchia Formation overlies progressively deeper structural units, suggesting that during Terravecchia Formation deposition the orogenic wedge was thinning toward the present SW (Fig. 6). The Terravecchia Formation is unconformably overlain by Messinian turbidites and evaporites. These rocks are well exposed

Deformed substrate units

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TRB

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Fig. 10. Changes into the Ciminna–Bosco Basin stratigraphy across the San Leonardo Fault. The main differences concerning both the abrupt thickness variation of the succession and the lithofacies of the Messinian deposits suggest a syndepositional activity of the fault at least during the Messinian. The synsedimentary tectonics induced the progressive uplift and erosion in the eastern Ciminna–Bosco Basin.

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Fig. 11. (a) Simplified geological map showing the stratigraphic and structural setting of the Camporeale Basin (modified from Gugliotta 2011; (b) stereographic projection (locations of structural sites are indicated on the map) of the structural data (faults and folds) and associated stress axes calculated from stress inversion applied to transpressive faults; (c) panoramic overview of the vertical bedding along the Pietroso Ridge (white lines); (d) interpreted seismic reflection profile (the trace is marked in (a)) showing the structural significance of the Pietroso Ridge (modified after Gasparo Morticelli et al. 2008). The scheme in (e) is inspired by Roure et al. (1990).

in the Ciminna–Bosco Basin. Evidence of latest Tortonian and intra-Messinian transpressional tectonics in the Ciminna–Bosco Basin has been already noted by some researchers (Lo Cicero et al. 1997; Del Ben & Guarnieri 2000; Guarnieri 2004; Gugliotta 2012). Our data suggest that Messinian syndepositional tectonics has been mainly produced by the activity of east–west reverse faults and folds (site 1 in Fig. 9a) and NW–SE- to NE–SW-striking transpres-sive faults (such as the San Leonardo Fault, Fig. 9a and b). The kinematics of this faults is consistent with a roughly north–south-oriented maximum compressional axis (stereoplots in Fig. 9f). The San Leonardo Fault led to the uplift of localized structural highs bounding the southern side of the basin (Cozzo Pipitone and vicari

ridge in Fig. 9a and b). Seismic reflection profiles crossing this region (Catalano et al. 2000; 2010a,b) show that this tranpressional fault is rooted at depth, involving the underlying deep structural level (carbonate platform units). Based on both cross-cutting rela-tionships and kinematics this fault has been related to the deep-seated transpressional Event II (e.g. Avellone et al. 2010). The Messinian syndepositional activity of the deep-seated transpres-sional faults is suggested by (1) the intra-Messinian unconformities and abrupt lateral variation of thickness (Fig. 10) displayed at the basin scale between the Messinian turbidites and the overlying Messinian evaporites (Fig. 9c–e); (2) the stratal pattern that sug-gests a local and roughly north- to northwestward tilting of the

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basin margin, mainly along the San Leonardo Fault (Figs 9a and 10); (3) some transpressive structures (Site 1 in Fig. 9f) that deformed Messinian evaporite strata and are sealed by the Lower Pliocene Trubi limestones (Fig. 9d).

Other transpressive structures show that this tectonic phase con-tinued during the Early Pliocene. Avellone et al. (2011) described the syndepositional activity of transpressional structures during the Early Pliocene in the Lascari Basin, interpreting the Lower Pliocene succession as originally deposited within a narrow and relatively deep topographic trough (palaeostrait), confined by steep and unstable slopes, corresponding to the flanks of a growing large-scale syncline. The palaeo-bathymetry for the Trubi chalk as inferred by means of macroforaminifera content ranges between 500 and 1200 m according to Sprovieri (1977, 1979, 1981). Structural and stratigraphic evidence (e.g. growth geometry; Fig. 7c) suggests that the development and evolution of the Early Pliocene Lascari Basin was controlled by active, mostly south-dip-ping transpressional structures. These structures, consisting of both NE–SW-trending, left-lateral and NW–SE-trending, right-lateral transpressive or transcurrent faults, are superimposed on previ-ously formed shallow-seated structures (e.g. Sicilide thrust sheets), developed during the earlier compressional Event I. Transpression also produced the uplift of the lowermost structural level (carbon-ate platform units), along double-verging pop-up structures at pre-sent exposed at the surface (Cozzo S. Biagio; Fig. 7). The stress field orientation, as calculated by inverting the striated fault planes,

is characterized by a horizontal, roughly north–south-oriented, maximum compression axis, compatible with that calculated for the structures developed during the deep-seated Event II (Avellone et al. 2011).

In spite of the local uplift of faulted blocks, bounded by transpressional faults, the evolution of the studied sedimentary successions, from upper Tortonian continental–shallow-marine clastic deposits to bathyal lower Pliocene chalks, indicates a gen-eral deepening of the wedge-top depozone, suggesting a regional subsidence.

Evolution of the Sicilian foreland-thrust belt and the timing of tectonic events

After the deformation and the closure of the Numidian Flysch basin a new foreland basin system developed from Serravallian time in response to the migration of the fold flexural wave. Based on our data, we infer that between the late Serravallian and the early Tortonian the foreland basin accommodated the sedimentation of the Marne di San Cipirello in the external sectors, away from the thrust belt front (foredeep depozone and foreland ramp), and the Sicula Formation above a growing orogenic wedge (Stage 1 in Figs 12 and 13). The homogeneous stratigraphic and sedimentological characters observed in the field suggest also that the Sicula Formation was probably deposited filling wide and laterally con-nected wedge-top basins. The occurrence of the Marne di San

Stage 1Serravallian to lower Tortonian

Stage 2latest Tortonian

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onset of deep-seated transpression

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Fig. 12. Three-stage evolution of the Sicily fold-and-thrust belt with associated wedge-top deposits between the Serravallian and the Early Pliocene schematized by using simplified cross-sections obtained by integrating field and subsurface data. The sections are traced along an approximately north–south direction, parallel to the approximate mean tectonic transport, avoiding the lateral component of movement.

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WEDGE-TOP BASINS AND ThRuST BELTC. 13

Cipirello between the tectonic wedge made up of imbricated thin deep-water units and the carbonate platform unit (shown in Fig. 6) suggests that the shallow-seated floor thrust migrated until the late Tortonian, when it overrode the thicker external Trapanese carbon-ate platform (Stage 2 in Figs 12 and 13). The angular unconformity that marks the base of the Sicula Formation suggests no deposition or erosion in the wedge-top depozone owing to the underlying thrust wedge growth during the early Serravallian. In the study region the forward migration of the shallow-seated structures (compressional Event I) ended during the late Tortonian, when the underlying thicker carbonate platform unit was reached by defor-mation that developed at deeper levels (about 6–8 km on average). Since that time, both the depositional and structural features of the foreland-thrust belt system changed: a large portion of the wedge-top depozone was uplifted and partially emerged, accommodating coarse-grained sediments in a transition to continental settings, as suggested by the Terravecchia Formation sedimentary facies. The angular unconformity observed at the base of the Terravecchia Formation marks the first episode of emergence of some portion of the thrust belt and was forced by the ‘thickening’ of the orogenic wedge owing to the involvement of the thicker carbonate platform unit. An interaction with sea-level changes in the Mediterranean area is believed to have enhanced the effect of relative sea-level lowering (haq 1991; Esteban 1996; Gugliotta et al. 2013).

Following recent models (e.g. De Celles & Giles 1996), the regional slope associated with the wedge-top depozone dips toward the foredeep depozone, parallel to the main tectonic transport of the thrust sheets. In our case study both the stratigraphic and deforma-tional differences observed longitudinally (from the present-day ENE to WSW) across the study region account for the reconstruc-tion of a regional palaeoslope dipping towards the present-day WSW connecting the inner wedge-top depozone with the outer wedge-top depozone during the Sicula Formation and Terravecchia Formation deposition. This inferred palaeoslope dip is consistent with the mean southwestward tectonic transport calculated for the tectonic units emplaced during Event I in this sector of Sicily (Avellone et al. 2010), rather than with the present-day southward tectonic transport of the Sicilian fold-and-thrust belt. We believe that this difference is due to the vertical-axis rotation of the thrust sheets during their progressive growth.

The occurrence of syndepositional transpressive structures already active during the latest Tortonian in the inner wedge-top basins (Scillato and Ciminna–Bosco basins) suggests that, during this time interval, the structural setting of the wedge-top depozone was influenced by both inherited compressional structures of Event I and newly formed deep-seated transpressional structures of Event II. Thus, our data indicate that in this portion of Sicily the onset of deep-seated structures occurred in the latest Tortonian, during the Terravecchia Formation deposition.

From the Messinian and into the Early Pliocene (Stage 3 in Figs 12 and 13) the influence of the transpressional structures strongly increases. The structural highs generated by transpressional faults progressively dismembered the already existing sedimentary basins, which became progressively narrower and discontinuous. Sedimentation in these basins appears to be controlled by transpres-sional structures (Ciminna–Bosco and Lascari basins). Many of these transpressional faults dip towards the south, generating large north-verging (i.e. back-verging) structures.

Field data suggest that transpressive faults developed after the involvement in the deformation of the Trapanese carbonate plat-form unit. As documented by other researchers (Di Stefano et al. 2002; Mallarino et al. 2002; Martire & Bertok 2002; Basilone et al. 2010) Jurassic–Cretaceous extensional and/or transtensional faults, with a present-day WNW–ESE to NW–SE trend, related to the

Fig. 13. Scheme summarizing the timing of the main tectonosedimentary stages in the study area.

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GuGLIOTTA Et AL.14

Mesozoic rifting affected the Trapanese carbonate platform. When the compressional deformation reached this thick rock body the occurrence of the inherited Mesozoic extensional structures oblique to the compressional front could have forced the development of diffuse transpression and back-thrusting as demonstrated by reacti-vated neptunian dykes, and striated planes with multiple slicken-lines whose cross-cutting relationships indicate a later transpressional reactivation of the Mesozoic extensional faults (Avellone et al. 2010).

The late Tortonian–Early Pliocene sedimentary evolution of the Sicilian wedge-top depozone recorded a regional subsidence increasing with time that probably began in response to an increased orogenic load, owing to the emplacement of the Calabrian–Peloritani basement units above the Sicilian units (according to Sartori 1989; Agate et al. 1993; Butler & Grasso 1993), occurring within the framework of a continental lithosphere subduction pro-cess (Catalano et al. 2013).

Conclusions

Our results illustrate a case-history where the onset and evolu-tion of a set of wedge-top basins are mainly affected by transpres-sional tectonics, driven by late, deep-seated faults. The Sicilian orogen evolved, like other thrust belts, through two subsequent main tectonic events (shallow-seated Event I and deep-seated Event II), progressively involving more external areas and deeper structural levels. The Sicilian thrust belt represents an unconventional and interesting case study because thrust sheets underwent large clockwise rotations during their forward migra-tion, in both of the compressional tectonic events. This process produced (1) the rotation of the structures (folds and thrusts) relative to different tectonic events, which were superimposed on each other non-coaxially, producing a wide range of interfer-ence patterns (e.g. dome and basin pattern of interference and folded thrusts), and (2) the rotation of the ‘apparent’ tectonic transport through the time.

Intraformational unconformities, growth structures, sudden changes in sediment transport pathways and rates, and spatial dis-tribution of erosion and sediment deposition indicate that the upper Serravallian–Lower Pliocene wedge-top basin deposits have recorded the progressive growth of the main structures.

By means of a detailed structural analysis as well as stratigraphic and sedimentological studies, we highlight that the onset of deep-seated transpression (Event II) in the NW Sicily thrust belt occurred in the latest Tortonian (Figs 12 and 13), involving thick carbonate platform units in the thrust belt. The transition from shallow-seated to deep-seated structures can be summarized in the following steps.

(1) Between the late Serravallian and the early Tortonian the foreland basin system was characterized by relatively wide, marine wedge-top basins (Sicula Formation sedimentation), connected with a foredeep (Marne di San Cipirello sedimentation). The loca-tion, geometry and sedimentary evolution of the wedge-top basins during this time-span was influenced by shallow-seated structures developed during the compressional Event I.

(2) During the late Tortonian the deep-water Imerese units over-rode the thicker Trapanese carbonate platform unit and the latter began to be involved in the thrust belt, forcing the onset of the deep-seated structures from latest Tortonian time. Some regions of the thrust belt were strongly uplifted and experienced subaerial exposure for the first time. Deep-seated transpressional faults (dou-ble-verging pop-up structures) induced localized deformation into the inner wedge-top basins, where continental to transitional sedi-mentation occurred in narrow to wider basins that were poorly con-nected laterally.

(3) Between the Messinian and the Early Pliocene an extensive deep-seated transpression affected a large portion of the wedge-top depozone. The Messinian–Lower Pliocene syntectonic successions were accommodated as basin fill in progressively narrower, local-ized and laterally discontinuous wedge-top basins bounded by transpressional structures (mostly back-verging).

The synkinematic rotation of the thrust sheets is clearly recorded by the wedge-top successions. The correlation of the late Serravallian–early Messinian wedge-top basin deposits (Sicula Formation and Terravecchia Formation) across the accretionary wedge suggests the occurrence of an ‘inherited’ regional palae-oslope with a present-day WSW dip. This orientation is different from the present regional slope (roughly south-dipping). In this sector of Sicily the trend of the palaeoslope is consistent with the southwestward tectonic transport recognized for the compressional Event I. In our opinion, synkinematic rotations that occurred during each of the above-described steps progressively involved the wedge-top successions, thus explaining the angular discrepancy between the present-day regional slope (south-dipping) and the Late Miocene palaeoslope (WSW-dipping).

This work was supported by research grants from CARG and SI.RI.PRO projects (R. Catalano coordinator). The Editor E. Bozkurt, D. van hinsbergen and an anonymous reviewer are warmly acknowledged for their helpful reviews and constructive comments. The paper has also ben-efited from useful suggestions and stimulating discussion with A. Sulli and F. Speranza.

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Received 12 February 2013; revised typescript accepted 7 October 2013.Scientific Editing by Erdin Bozkurt.

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