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TECTONICS, VOL. 28, TC4015, doi:10.1029/2008TC002385, 2009

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Imprint of foreland structure on the deformation of a thrust sheet: The Plio-Pleistocene Gela Nappe (southern Sicily, Italy)

Francesca C. Ghisetti,1,2 Andrew R. Gorman,3 Mario Grasso,4,5 and Livio Vezzani6

Received 28 August 2008; revised 20 March 2009; accepted 9 June 2009; published 22 August 2009.

[1] In Sicily, the progressive imbrication of the Apenninic thrust belt above the Pelagian-African Foreland is traced by the southward migration of marine basins that were progressively shortened during the late Miocene-Pleistocene. The outermost and youngest thrust sheet (Gela Nappe) displays a peculiar shortening, with Messinian to early Pliocene E-W folds refolded in the late Pliocene ­ early Pleistocene by approximately N-S folds (subparallel to the transport direction of the thrust sheets). This structural interference is documented in south Sicily within localized belts of refolding spaced $5­8 km apart. The significance of this fold interference pattern is highlighted by our analysis of the offshore seismic reflection line M23A (CROP Mare Project) that intersects the Gela Nappe along a trace suborthogonal to the thrust transport direction. Migration and depth conversion of the line reveal multiple imbrications and draping of the allochthonous units above structural highs of the foreland, delimited by inherited N-S faults. The largest faults bound mid-late Miocene extensional basins but were reactivated in compression during the late Pliocene­early Pleistocene, causing (1) superposed folding along discordant N-S structural trends, (2) compressional extrusion of the whole wedge of the Gela Nappe, and (3) offset of its sole thrust. The reactivation of faults subparallel to the transport direction accommodates differential flexure of the rigid foreland beneath the Apenninic wedge, and these late stage deformations in the foreland are responsible for the superposition of E-W finite shortening onto N-S shortening. Citation: Ghisetti, F. C., A. R. Gorman, M. Grasso,

and L. Vezzani (2009), Imprint of foreland structure on the deformation of a thrust sheet: The Plio-Pleistocene Gela Nappe (southern Sicily, Italy), Tectonics, 28, TC4015, doi:10.1029/ 2008TC002385.

TerraGeologica, Christchurch, New Zealand. Also at Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand. 3 Department of Geology, University of Otago, Dunedin, New Zealand. 4 Dipartimento di Scienze Geologiche, Universita degli Studi, Catania, ` Italy. 5 Deceased June 2007. 6 Dipartimento di Scienze della Terra, Universita degli Studi, Torino, ` Italy.

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1. Introduction

[2] For many fold-thrust belts worldwide, modes and styles of progressive shortening are successfully modeled by the dynamics of critical Coulomb wedges [cf. Chapple, 1978; Davis et al., 1983; Dahlen, 1984] and analog sandbox experiments [Malavieille, 1984; Storti et al., 1997]. However, complex kinematic histories that depart from simple plane strain modes of imbrication arise from syntectonic rotations, detachments along discontinuous weak horizons, and gravity sliding of unconsolidated units, resulting in irregular trajectories of fault propagation and disharmonic folding [e.g., Lujan et al., 2003; Comeau et al., 2004; Sherkati and Letouzey, 2004]. More importantly, the preexisting architecture of the foreland margin, with structural highs and sedimentary basins separated by inherited faults, often interferes with the transport direction of the overlying thrust sheets, and controls the location of transfer zones, the deflection of structural trends, and the superposition of discordant tectonic fabrics, as observed in many arcuate thrust fronts [e.g., Ghisetti and Vezzani, 1997; Macedo and Marshak, 1999; Lacombe et al., 2003; Tull and Holm, 2005]. [3] In this paper we present new field data on the Gela Nappe, the outermost and youngest thrust sheet of the Apennines in Sicily. Our field data collected from the onshore outcropping portions of the Gela Nappe emphasize an intriguing fold interference pattern of the allochthonous units, with a set of first generation (synpost Messinian to early Pliocene) E-W folds, refolded by a set of second generation (post middle late Pliocene to post early Pleistocene), N-S and NNE-SSW folds. This interference is not observed in the older and inner thrust sheets of the Sicily thrust belt and cannot be ascribed to soft-sediment gravitational deformation, to a constrictional strain field, or to a 90° switch in the trajectories of maximum horizontal compression during the advancement of the nappe. In fact, the northerly trends are subparallel to the regional transport direction of the thrust sheets. [4] We integrate our field data with offshore subsurface information along seismic reflection line M23A (CROP Mare Project [see Finetti, 2005]). This line crosses the Gela Nappe along a direction suborthogonal to the transport direction, and is therefore well positioned to reveal the geometry of the fold interference pattern. The original time domain stack acquired from the CROP Mare Project has been migrated and depth converted, for a better definition of the geometry of the deformed units. Our new interpretation of this line shows that late compressional reactivation of inherited N-S and NNE-SSW normal faults in the foreland

Copyright 2009 by the American Geophysical Union. 0278-7407/09/2008TC002385$12.00

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is the likely cause for the superposed deformation and the compressional extrusion of the units of the Gela Nappe. [5] The deformation history of the Gela Nappe demonstrates that the progressive incorporation of the foreland into the thrust belt interacts with, and modifies, the geometry of the thrusted units, and provides a favorable study case made simpler by (1) the 90° interference between the compressive structures within the translated wedge and the inherited extensional faults in the foreland; (2) the good competence contrast between the relatively thin package of poorly consolidated sediments deformed in the Gela Nappe and the underlying rigid substratum; and (3) the chronologic relationships recorded by the syntectonic stratigraphy in the thrust wedge.

decrease southward to 50° and 20° in the lower Pleistocene sediments of the outermost thrust sheets [Channell et al., 1990; Oldow et al., 1990; Speranza et al., 1999; Mattei et al., 2007]. Overall, the mechanisms of right-lateral shear appear to have accommodated the southward increase in extension of the Tyrrhenian basin behind the subducting Ionian slab. 2.2. Gela Nappe [9] South of the Mount Kumeta ­ Alcantara fault zone (Figure 1) the Apenninic thrust sheets are largely buried beneath 6 km of upper Miocene ­ upper Pliocene clastic sediments, deposited in a system of amalgamated thrust-top basins (Caltanissetta Basin). These basins record the history of subsidence, infilling and uplift of the southward migrating frontal foredeep of the Apennines that was progressively shortened and incorporated into the thrust belt. In outcrop, the dominant structures are NE-SW to E-W fault propagation folds above north dipping, blind thrust faults. [10] The outermost thrust sheet (Gela Nappe [Ogniben, 1969]) is interpreted by many authors as a complex, imbricate wedge, involving the sequences of a foredeep basin originally located NW of the Hyblean Plateau. The Gela Nappe is well exposed along the south coast of Sicily between Agrigento and Gela (Figure 2) and is imaged in the adjacent offshore by seismic reflection lines [Trincardi and Argnani, 1990; Argnani, 1993; Catalano et al., 1996, 2000]. A number of commercial hydrocarbon wells (Figure 2) have crossed a 2 ­ 3 km thick body of repeatedly imbricated Messinian-Pliocene units, thrust above the middle to upper Miocene, lower Pliocene and lower Pleistocene sediments that stratigraphically overlie the Pelagian carbonates (e.g., wells Leone 1, Prezioso 1, Patty 1, Rabbito, Settefarine, and Ursitto [cf. Lickorish et al., 1999]). The basal thrust of the Gela Nappe has an overall displacement !10 km [Lickorish et al., 1999], defining a frontal south facing arc in the south Sicily offshore (Figure 2). South of this frontal arc (Figure 1) the underlying Pelagian Block is deformed by the PlioPleistocene and active rifting processes in the Sicily Channel [e.g., Corti et al., 2006]. 2.3. Stratigraphy of the Gela Nappe [11] Within the Gela Nappe (Figure 3a), the deepest units exposed in the cores of tight, detached anticlines are upper Oligocene calcareous mudstones and lower Miocene (Burdigalian) variegated clays. The onset of siliciclastic sedimentation in the inner flexural foredeep is recorded by the unconformable contact of the overlying upper Miocene (Tortonian) clays and sands of the Terravecchia Formation [Grasso and Butler, 1991], deposited in a prograding submarine delta, fed by the Miocene protothrust belt of north Sicily. The Terravecchia Formation is well exposed in the Caltanissetta Basin, but to the south it interfingers with distal pelagic and hemipelagic clays (Licata Formation). In the study area the Licata Formation ranges in age from early Langhian to late Tortonian [Grasso et al., 1995] and increases in thickness from $400 m inland (Licata area) to <3000 m in the offshore [Argnani, 1993]. Both the Terravecchia and Licata formations grade upward to dia-

2. Regional Geological Setting

[6] The Apennines are the result of Neogene convergence and subduction of oceanic and continental crust at the collisional boundary of the Calabrian microplate against the African margin [e.g., Malinverno and Ryan, 1986]. A residual narrow tongue of Ionian oceanic crust is still subducting beneath the Calabrian Arc (Figure 1) [e.g., Frepoli et al., 1996], but whether the processes of lithospheric convergence are still active is under debate [Mattei et al., 2007]. Weakly deformed portions of the African Foreland are submerged offshore south of Sicily (Pelagian block), where they are truncated (Figure 1) by the WNWESE normal faults of the Plio-Quaternary Sicily Channel rift [Corti et al., 2006]. Uplifted, fault-bounded ridges of the Pelagian block emerge in the Hyblean and Sciacca areas, on the islands of Malta (Figure 1), and farther to the south, on the island of Lampedusa. 2.1. Apenninic Thrust Belt in Northern Sicily [7] In the Sicily thrust belt, the highest and innermost units of the Apennines involve the Calabride crystalline basement (Figure 1) and an upper Jurassic ­ lower Miocene basinal sequence with Tethyan ocean affinity (Sicilide units). The external thrust sheets deform Mesozoic-Eocene basin-to-platform carbonate sequences and the overlying Oligocene ­ lower Miocene turbidites, originally deposited on African continental crust (see Finetti et al. [2005] for a recent review and references). [8] The Apenninic tectonic trends of Sicily are dominantly E-W, at high angle to the NE-SW trends of the Calabrian arc, and to the N-S trends of a post-Miocene submerged thrust belt (Adventure Bank) displaying the transpressive reactivation of inherited Mesozoic normal faults in the Pelagian Foreland [see Antonelli et al., 1988; Torelli et al., 1991; Argnani, 1993]. In northern Sicily, E-W trends (Figure 1) are associated with a regional belt of Miocene to middle Pliocene transpressive right-lateral faults, along the Mount Kumeta ­ Alcantara Fault System [Ghisetti and Vezzani, 1984]. Miocene-Pliocene thrusting and strike-slip faulting were accompanied by clockwise rotations of the thrusted units relative to the stable African Foreland [Channell et al., 1990]. Rotations up to 140° in the northernmost and oldest units (close to the Mount Kumeta ­ Alcantara Fault System)

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Figure 1. Regional geological setting of Sicily and the Calabrian Arc, showing the upper crustal tectonic elements of the Apenninic thrust belt in relation to the Ionian subducting slab (slab contours from Frepoli et al. [1996]). The Gela Nappe is the outermost and youngest thrust sheet, transported above the African-Pelagian Foreland along a south facing, arcuate front. Note the discordant trend between the E-W structures of the Sicily thrust belt and the N-S to NNE-SSW structures of the Pelagian-African Foreland.

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Figure 2. Extent of the Gela Nappe (light gray) in south Sicily. See inset and Figure 1 for regional location. The north boundary of the nappe is not marked by any surface thrust fault and is hypothetically located, based on the extent of superposed folding. The south facing, arcuate frontal thrust is reconstructed from subsurface data in the foredeep and in the offshore (redrawn after Bigi et al. [1983], Grasso et al. [1995], Grasso [1997], and Lickorish et al. [1999]). Note the lateral deflection of the frontal thrust against rigid carbonate salients (Sciacca structural high to the west and Hyblean Plateau to the east). Seismic line M23A (CROP Mare Project) crosses the Gela Nappe along a trajectory suborthogonal to the nappe transport direction. The portion of the line that has been migrated, depth converted, and interpreted (Figure 6b) is marked by a solid black line. tomites and bituminous clays (<50 m thick) that herald the onset of sediment starvation, restricted water circulation, and marine lowstand during the Messinian ``salinity crisis.'' In the study area, the Messinian evaporites comprise brecciated, bituminous limestones ( 100 m thick) and the overlying gypsum, gypsiferous marls and gypsarenites ( 100 m thick), with an intra-Messinian unconformity. The evaporitic depocenters are localized in synclinal cores and locally host thick salt accumulations (mainly KCl and NaCl). The syndepositional physiography within the evaporitic sequence and the intra-Messinian unconformity suggest that the basin's morphology was primarily controlled

Figure 3. (a) Simplified stratigraphy of the external zones of southern Sicily and (b) geological-structural map of the Gela Nappe inland of Gela, Licata, and Punta Bianca (based on work by Servizio Geologico d'Italia [1955, 1959], Roda [1967], and Grasso et al. [1997] and unpublished geological maps surveyed by F. C. Ghisetti and M. Grasso). See Figure 2 for regional location of the map. Note that thicknesses in Figure 3a are approximate and based on the sequence exposed in the Licata-Gela area [Grasso et al., 1998]; in fact, significant variations occur from north to south and from onshore to offshore. The whole upper Oligocene to Pleistocene sequence is detached and deformed within the allochthonous Gela Nappe and transported above the Pelagian Foreland and its Plio-Pleistocene Foredeep (see Figures 5 and 6). The map shows the dominant shortening by folding, with deflection of a set of synpost-Messinian to early Pliocene E-W folds (first generation) by sets of superposed middle to late Pliocene to early Pleistocene, N-S and NNE-SSW folds (second generation). The latest second generation folds deform the early Pleistocene sediments in open anticlines and synclines with axial traces scattered from N-S to NE-SW orientation. Note that the axial traces of the second generation synclines and of the youngest second generation anticlines are omitted to simplify the map pattern.

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Figure 3

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by thrusting and fold growth in the actively deforming foredeep [Butler et al., 1995]. [12] The evaporitic sequence (with topmost brackish water units) is unconformably overlain [Ogniben, 1969; Rio et al., 1991; Butler et al., 1992] by $200 m of rhythmically bedded lower Pliocene chalks (Trubi Formation, Figure 3a), deposited during reestablished highstand marine conditions (water depths 300 m). Both the Messinian evaporites and the Trubi Formation are repeatedly interrupted by interposed gravity mass flows (Argille Brecciate, not shown in Figure 3a) that testify to the instability of the basin's slopes, controlled by synsedimentary fold growth. A prominent unconformity occurs at the top of the Trubi Formation [Rio et al., 1991; Patacca and Scandone, 2004], with a hiatus of $800,000 years in the top Zanclean ­ lower Piacenzian stratotypes (lower to middle Pliocene). The overlying deposits are middle to upper Pliocene bluish claystones, mudstones and sandy claystones ( 400 m thick), grading upward into sands and calcarenites. Starting with the latest Pliocene ­ early Pleistocene the deposition of a regressive arenaceous sequence discordant onto the underlying folded units records the progressive uplift, southward tilting and erosion of the foredeep.

3. Deformation of the Gela Nappe From Outcrop Data

3.1. Geometry of the Deformed Units [13] The array of surface structures mapped in the inland exposure of the Gela Nappe along the south coast of Sicily (from Punta Bianca to Gela) is shown in Figure 3b. [14] In outcrop, concentric folds with wavelength of $1 km, amplitude of several hundred meters and circular, box and chevron hinge zones are well traced for several kilometers by the competent units of the Messinian evaporitic limestones. Folding of the rigid beds has been assisted by the development of sets of bed-perpendicular, radial fractures. The cores of the tightest anticlines and synclines show complex accommodation geometry, with flow of the incompetent layers, and development of second-order, tight disharmonic folds. Almost all of the formation boundaries in the stratigraphy are detachment horizons, with the weakest layers located within the Messinian evaporites (diatomites and gypsiferous units, see Figure 3a). The basal detachment of the folded sequence transported above the Pelagian carbonates is nowhere exposed in outcrop. [15] In the mapped area, the internal structure of the Gela Nappe is dominated by a ``type I'' fold interference pattern [Ramsay, 1967], with sets of early approximately E-W folds (first generation folds), refolded by upright N-S to NNESSW folds, (second generation folds). This geometry is well revealed by the large-scale undulations of the double

plunging axial traces of the first generation folds, with north facing recesses and south facing salients (Figure 3b). This interference pattern often results in four-way plunging fold cores, with ``dome and basin'' geometry. The second generation folds are spaced at wavelengths of nearly 5­ 8 km, i.e., much larger than the 1 km wavelengths of the first generation folds (Figure 3b). The earlier E-W folds vary from upright to recumbent, whereas the later N-S and NNE-SSW folds maintain upright axial surfaces and plunge mainly to the north. The latest stages of shortening are documented by open concentric folds with axial traces trending from N-S to NE-SW that gently bend the base of the lower Pleistocene sandstones (Figure 3b) and detach within the upper Pliocene ­ lower Pleistocene beds. [16] A good example of the fold interference geometry at the eastern margin of the Gela Nappe (Figure 4) is illustrated in some detail by the contours of the base of the Messinian evaporites that depict the deflection of an early E-W syncline about N-S, north plunging anticlines. [17] The largest component of shortening in the shallow cover sequence is by folding, but sets of discontinuous faults accommodate the differential shortening and rotation associated with detachment, folding, and superposed deformation. Many faults dip at high angle, maintain an orientation subparallel to the easterly and northerly structural trends, and possess reverse and normal displacements of 10­ 100 m. Subsurface thrusting within the Gela Nappe is well documented, but outcropping thrust faults are generally discontinuous and/or of limited extent. Notably, there is no surface evidence for an inner thrust fault that bounds the hanging wall sequence of the Gela Nappe, probably as a consequence of dominant mechanisms of folding in the lowcompetence successions, eventually detached above blind thrusts. The north boundary of the nappe has been tentatively located in Figure 2, based on the inland extent of refolded first generation folds. In the whole mapped area, the only significant thrust fault (Figure 3b) is located a few kilometers inland from the Gela coast (Settefarine Thrust Fault [Grasso et al., 1995]). This E-W fault dips $30° north at the surface and overthrusts the refolded MessinianPliocene sediments above lower Pleistocene clays in the footwall. Olistoliths of Messinian evaporites and Trubi chalks derived from the folded hanging wall are embedded at different horizons within the footwall sequence, with the youngest being inserted within the upper part of the lower Pleistocene succession [Grasso et al., 1995]. 3.2. Age of Deformation [18] Field data consistently show that the middle to late Miocene claystones and mudstones (Licata Formation, see Figure 3a) and the overlying Messinian evaporites were already deformed by first generation E-W folds prior to the

Figure 4. (a) Structural contour map of the base of the Messinian limestones in the Disueri area (see Figure 3a for location). The contour lines depict the refolding of the early set of E-W (first generation) folds by later N-S (second generation) folds. For simplicity, only the axial trace of the first generation synclines and of the second generation anticlines are marked. (b) Poles to bedding (and density distribution) in the refolded Disueri synclines, showing the ``type I'' fold interference pattern [Ramsay, 1967].

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Figure 4

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Figure 5. Synthesis of the structural setting of the Gela Nappe along an ideal N-S transect, subparallel to the transport direction (redrawn and modified after Catalano et al. [1996], Grasso et al. [1998], and Lickorish et al. [1999]). The section portrays (1) the folding and imbrications of the thrust sheet along E-W structures, (2) the mid-Pliocene to early Pleistocene insertion of the Gela Nappe into the foredeep of the Pelagian-African Foreland and the crosscutting of the base of the Pleistocene sequence at the frontal tip of the nappe, (3) the onlap of early to mid-Pleistocene strata above the imbricated units in the offshore, and the ``out-of sequence'' crosscutting of the lower Pleistocene sequence by the Settefarine Thrust Fault inland, and (4) the compressional reactivation of the normal faults of the foreland substratum that were progressively incorporated in the thrust belt. unconformable deposition of the Trubi Formation in the early Pliocene. The Trubi Formation onlaps the Messinian folds, but the onlapping strata are in their turn shortened by folds that maintain the same axial trace of the underlying Messinian folds. The apex of this deformation is marked by an unconformity and a hiatus at the top of the Trubi Formation. During these deformation stages, sediments were weakly consolidated, as indicated by the abundant occurrence of slumping and gravity sliding in the Messinian and lower Pliocene units. [19] Continuous internal deformation of the allochthonous Gela Nappe is testified by the second generation N-S to NNESSW folds that refold the first generation E-W folds and deform the middle to upper Pliocene and the lower Pleistocene sediments that discordantly overlie the Trubi Formation. Syndepositional shortening is indicated by internal unconformities and growth strata geometries within the middle to upper Pliocene and the lower Pleistocene succession [cf. Patacca and Scandone, 2004]. The youngest stages of deformation recorded in the mapped area (Figure 3b) are associated with the late sets of N-S to NE-SW second generation folds (post early Pleistocene), and with the crosscutting of the E-W Settefarine Thrust Fault, that truncates the refolded units and was active during sedimentation of the uppermost lower Pleistocene section.

4. Gela Nappe in the South Sicily Offshore

[20] A great deal of information on the Gela Nappe comes from a number of industry seismic lines, hydrocarbon wells and from deep seismic reflection lines (11 s records) of the CROP Mare Project in the Sicily Channel [Finetti, 2005; Finetti et al., 2005]. In fact, the geometry of the arcuate offshore thrust front (Figure 2), as well as the definition of thickness, amount of displacement and age of thrusting of the Gela Nappe, are all based on subsurface data [Argnani et al., 1989; Argnani, 1993; Trincardi and Argnani, 1990; Butler et al., 1992; Catalano et al., 1993, 1996, 2000; Grasso et al., 1998; Lickorish et al., 1999] (see also Figure 5). However, most interpretations rely on transects parallel to the transport direction (i.e., along N-S to NE-SW trends), unable to account for the fold interference pattern within the nappe. Also, the poor seismic resolution of the older commercial seismic lines does not provide a reliable image of the internal deformation of the nappe. The resolution of the CROP Mare Project seismic lines is much higher, but

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most authors have focused on the regional crustal-scale setting [e.g., Catalano et al., 2000], with minor emphasis on the structural detail in the topmost 5 km. [21] Figure 5 shows a synopsis of the geometry of the Gela Nappe, modified from the cross sections published by Catalano et al. [1996], Grasso et al. [1998], and Lickorish et al. [1999]. The most relevant features are (1) the insertion of the frontal tip of the Gela Nappe into the mid-Pliocene flexural foredeep of the Pelagian Foreland. The frontal tip of the Gela Nappe basal thrust truncates the lower Pleistocene sequence and is onlapped by mid-Pleistocene strata [cf. Lickorish et al., 1999]; (2) the frontal displacement of the nappe >10­ 15 km for the early Pliocene cutoff and 2 ­ 3 km for the early Pleistocene cutoff; (3) the undulation of the basal overthrust above faults of the Pelagian Foreland that maintain normal offsets in the outer foredeep (to the south) but were reactivated with reverse mechanisms in the inner domains (to the north); and (4) the onlap of the lower Pleistocene sequence onto the imbricate wedge transported by the basal thrust of the nappe. However, the same lower Pleistocene horizons are truncated in the footwall of the more internal Settefarine Thrust Fault (Figures 3b and 5). These later deformations have been interpreted [Grasso et al., 1995] in terms of ``out of sequence'' deformation of the Gela Nappe, coeval with the final stages of frontal advancement into the external foredeep during the early Pleistocene. [22] In order to better understand the regional versus local significance of the geometry of folding observed in outcrop (Figure 3b) we have chosen to reprocess and reinterpret the western segment of seismic line M23A (CROP Mare Project), located in the immediate offshore of the south Sicily coast (Figure 2). The line crosses the Gela Nappe for a length of nearly 70 km along a trend suborthogonal to the transport direction, i.e., along an ideal trajectory for revealing the refolding of the E-W folds. Interpretations of this line have already been published [Catalano et al., 2000; Finetti et al., 2005], but they are based on the nondepthconverted stack, and do not provide a robust image of the geometry, thickness and structural relationships within the deformed units of the Gela Nappe. 4.1. Migration and Depth Conversion of the CROP Mare Line M23A [23] The migration and depth conversion of line M23A required the construction of a detailed velocity model. No information on stacking velocities was available for the M23A data set, so an interval velocity model was developed based on a stratigraphic interpretation of the M23A stacked

data. Six horizons beneath sea level (seafloor, base of Pleistocene clays, top of lower Pliocene Trubi Formation, top of Messinian evaporites, top of the middle to upper Miocene units in the foreland and of the Langhian-Tortonian Licata Formation and, top of carbonate substratum see Figures 3a, 6b and 6c) were interpreted on the original stacked data. Using the velocity analysis capabilities of GLOBE Claritas [Ravens, 2001], an interval velocity model was constructed by (1) manually smoothing the interpretations so that no layers were repeated vertically (i.e., removing the effect of stacked layers within thrust packages); (2) assigning initial velocities to the layers based on geological interpretations and on previous analyses of seismic lines in the Apennines [Bally et al., 1988] (Table 1); (3) lateral (25 CDPs) and vertical (25 ms) smoothing of the velocities to remove discrete changes in velocity which can cause artifacts during migration; and (4) picking a finely spaced grid of interval velocities through the entire data set (every 25 CDPs horizontally and 25 ms vertically). [24] The stacked data were migrated (Figure 6a) using a finite difference downward continuation time migration algorithm that made use of the velocity model described above. The downward continuation time slice interval was set to 25 ms, the same as for the gridded velocity model. Depth conversion was accomplished using the same gridded interval velocity model to appropriately stretch each time-migrated trace. Traces were resampled to a 5 m interval and truncated at 6000 m. Unfortunately, there is no tying well along the trace of the line. However, the formation depths interpreted on the depth-converted migration have been checked for consistency in relation to the logs of the closest, nearby wells (Leone 1, Prezioso 1 and Patty 1, see Figure 2 and Lickorish et al. [1999]) There is good agreement, though the geometry of the imbricates may change substantially over relatively short distances, especially along the frontal wedge. 4.2. Interpretation of the Migrated and Depth-Converted Line M23A [25] The portion of seismic line M23A (from shotpoint 9200 to shotpoint 7000) that has been interpreted is shown in Figures 6a and 6b, together with the seismic facies of the key stratigraphic units (Figure 6c). The resolution of the seismic horizons is generally good down to 5 ­ 6 km, except beneath the allochthonous Gela Nappe, where the internal chaotic geometry is a barrier to energy propagation. [26] The top of the Hyblean and Sciacca carbonate substratum is marked by high-energy reflections. Within

Figure 6. (a) Migrated and depth-converted portion of line M23A (CROP Mare Project). See text for details on procedure, Table 1 for velocities within the stratigraphic intervals, and Figure 3a for the stratigraphy. The trace of the line (with shotpoints (SP) annotated) is shown in Figure 3b. Nominal SP spacing is 50 m. Note that vertical exaggeration (VE) is 3. (b) Interpretation of the depth-converted line, with definition of top of Pelagian carbonate substratum, top of middle to upper Miocene claystones and mudstones and of the Langhian-Tortonian Licata Formation, top of Messinian evaporites, top of Trubi Formation, top of the upper Pliocene ­ lower Pleistocene sequence, and top of the Pleistocene units, seabed. Only the topmost horizons of the Gela Nappe (Trubi Formation and Messinian evaporites) can be resolved on the seismic line, whereas the internal structure of the nappe remains undefined. (c) Seismic facies of the units recognized for the interpretation.

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Figure 6

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Table 1. Velocities Prior to Smoothing and Resampling Used for Construction of the Interval Velocity Modela

Layer Ocean Seafloor to base of Pleistocene clays Base of Pleistocene clays to top of Trubi Formation Top of Trubi Formation to top of Messinian evaporites Top of Messinian evaporites to top of upper to middle Miocene claystones and mudstones and Licata Formation Top of upper to middle Miocene claystones and mudstones and Licata Formation to top of Pelagian carbonate substratum Top of Pelagian carbonate substratum to base of section (10 s)

a

Velocity at Top (m/s) 1500 2000 2600 2800 3200 3400 3500

Velocity at Bottom (m/s) 1500 2600 2800 3200 3400 3500 6000

See text for more details.

the carbonate sequence it is possible to distinguish some marker horizons, but there has been no attempt at a stratigraphic subdivision within the substratum (see Finetti et al. [2005] for an interpretation). [27] The overlying package of upper Oligocene to midMiocene sediments displays discontinuous reflections (especially well-marked at the base), but they become irresolvable in the strongly deformed portions of the Gela Nappe. [28] A good seismic contrast occurs at the base of the Messinian evaporites, identified by a couple of high energy reflections. This seismic facies probably belongs to the evaporitic limestones, with lack of gypsiferous and saline depocenters along this transect, as also confirmed by hydrocarbon exploration wells in the area [Lickorish et al., 1999]. [29] The Trubi Formation (early Pliocene) is seismically transparent; above lie well bedded upper Pliocene to lower Pleistocene sediments, with sharp, continuous reflections from the sandstones intercalations. Bed disruption within these units defines bodies of interposed, shallow landslides. The topmost horizon of well-bedded, undeformed units immediately beneath the seafloor is attributed to the upper Quaternary deposits. [30] The notable structural features shown by the cross section of Figure 6b are as follows: [31] 1. The WNW-ESE section is suborthogonal to the transport direction. However, it reveals a closely spaced set of folds in the shallow cover, truncated by reverse faults. The Pelagian carbonate substratum is strongly dissected by a number of faults that propagate through the Miocene ­ lower Pliocene units. The orientation of these structures cannot be defined from a single seismic line, but at the west end of the cross section the splay of steep faults with flowerlike geometry is relatable to the N-S transpressive sinistral system that bounds the Sciacca structural high (Figure 2). Between shotpoint (SP) 8600 and SP 8500 normal faults dipping 60° east control the depocentral thickening of the middle to upper Miocene sediments in their hanging walls; elsewhere the substratum is dissected by high-angle reverse faults dipping 40° ­ 60° west, with low displacement (except for the fault at SP 7400). [32] 2. The Gela Nappe is intersected between SP 8600 and 7300. The section shows the opposite vergence (to the west and east) at the tips of the arcuate basal thrust (Figure 2),

associated with multiple imbrications, and the convex geometry of the allochthonous wedge, $2 km thick in the middle (SP 8100 ­ 7700), and few hundred meters thick at the frontal edge. The top of the wedge culminates in a central Messiniancored anticline at $500 m below sea level. The internal geometry of the nappe remains undefined, except for the uppermost layers, with short-wavelength folds in the Messinian horizons passing upward to open folds in the Trubi Formation. The base of the allochthonous body has been picked at the boundary between nonreflective and reflective units. This distinction is quite arbitrary for the central, thicker part of the wedge (SP 8200 ­7800), where the base of the nappe has been positioned by maintaining a maximum thickness of $2 km and a depth consistent with the logs of the Leone 1, Prezioso 1 and Patti 1 wells (see Figure 2 and Lickorish et al. [1999]). If the overall interpretation is correct, then the basal thrust of the Gela Nappe is offset by the upsection propagation of reverse faults that truncate the upper Pliocene ­ lower Pleistocene beds (e.g., SP 8000, 7300). [33] 3. The sole thrust of the Gela Nappe cuts up section from the top (or near top) of the Pelagian carbonate substratum into the Plio-Pleistocene sequence of the foredeep, with lateral onlap of the Pleistocene horizons above the allochthonous wedge. [34] 4. The topmost Pleistocene sequence onlaps the nappe but is itself folded in a gentle dome (culminating between SP 8200 and SP 7800), with lateral detachment of two shallow landslides (SP 8900 ­ 8200 and SP 7800­ 7000) along the west and east flanks of the nappe. This gentle arching is consistent with continuous shortening of the nappe during the Pleistocene [Trincardi and Argnani, 1990] and with the deformation associated with the youngest second generation folds observed onshore (Figure 3b).

5. Discussion

[35] The Gela Nappe is the youngest and outermost thrust sheet of the Sicilian Apennines, but it displays a sequence of interfering deformations that are not recognized for older thrust sheets in more internal positions. The blind basal thrust of the nappe delineates a large south facing arc (Figure 2), laterally constrained against emergent salients of the Pelagian-African platform (Sciacca high to the west

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and Hyblean Plateau to the east), and frontally advanced above the Plio-Pleistocene flexural foredeep. The internal structure of the nappe is exposed along the south coast of Sicily, where it shows prevailing shortening by folding. A first generation of upright to overturned, doubly plunging, E-W folds deform the Messinian evaporitic sequence and the overlying lower Pliocene Trubi Formation. The unconformable contact of the Trubi Formation onto Messinian cored folds, and the later folding of the Trubi beds in structures that maintain E-W trends are consistent with multiple episodes of progressive shortening. These deformations are recorded by the intra-Messinian unconformity, by the unconformity at the base of the Trubi Formation, and by the unconformity and hiatus at the top of the Trubi Formation (Figure 3a). [36] The first generation E-W folds are refolded by a second generation of upright N-S to NNE-SSW, north plunging folds, that also deform the middle to late Pliocene and the early Pleistocene sequence. We do not have a precise dating for this second episode of shortening, but internal unconformities repeatedly occur within the middle to late Pliocene and early Pleistocene sequence, consistent with deformation of their substratum during deposition [cf. Patacca and Scandone, 2004]. The wavelength of the second generation folds ($5 ­ 8 km) controls the position of localized bands of ``basin and dome'' interference extending $20 km inland (Figure 3). Gentle folding affects also the topmost (post early Pleistocene) sediments in outcrop, but these folds show a more scattered orientation of their axial traces (from N-S to NE-SW). [37] The ``dome and basin'' interference pattern designed by the first and second generation folds was already mapped by Servizio Geologico d'Italia [1955, 1959] and Roda [1967] and quoted in previous analyses of the Gela Nappe [e.g., Lickorish et al., 1999], but it has never been incorporated into a consistent structural model. [38] It could be argued that the superposed N-S and E-W shortening within the Gela Nappe arises from multiple detachments in the shallow, poorly consolidated cover sequence. This may be the case for the youngest, shallow folds of second generation, that display a large scatter in the orientation of their axial traces, probably related to surficial rotations and gravity sliding. These mechanisms are in fact detected in the topmost horizons of the Gela Nappe offshore (see e.g., Figure 6b). [39] However, (1) the maintenance of a regular pattern of fold interference over tens of kilometers, (2) the regular spacing of the belts of localized deformation, and (3) the chronology of superposed folding tied to major unconformities in the sedimentary record (Figure 3) are all elements consistent with a tectonic rather than gravitational origin for the first and second generation fold interference patterns. [40] The accumulation of increasing shortening by approximately E-W and N-S to NNE-SSW folding is synchronous with the progressive southward translation of the Gela Nappe, culminated in the overthrusting of the frontal tip of the nappe above the lower Pleistocene sediments of the foredeep (Figure 5). The imbricate stack of the Gela Nappe is sutured by onlapping early Pleistocene strata offshore, but

inland subsequent episodes of deformation are coeval with sedimentation of the uppermost lower Pleistocene succession, and require renewed internal shortening of the nappe during propagation of the E-W Settefarine Thrust Fault [Grasso et al., 1995]. [41] The superposition of structures indicative of approximately E-W and N-S shortening during deformation of the Gela Nappe is intriguing. In fact, the N-S to NNE-SSW trends of the second generation folds lie subparallel to the transport direction of the Sicily thrust sheets, and cannot be explained by a sudden 90° rotation of the maximum horizontal compression (SHmax) in the external areas since the middle Pliocene. Borehole breakouts in deep oil wells indicate consistent N-S orientation of SHmax in the area of the Gela Nappe [Grasso et al., 1995], concordant with the SHmax trajectories in the more internal domains of the thrust belt [Ghisetti and Vezzani, 1980, 1984]. [42] Nor can syntectonic clockwise rotations during translation of the thrust sheets explain the interference, because the N-S folds are the second generation folds formed in a stress field that has maintained N-S trajectories of maximum horizontal compression. Clockwise rotations of the lower Pliocene units sampled along the Agrigento coastline are of the order of 25­ 48°, and appear to be associated with the lateral bending of the frontal thrust against the Sciacca structural high [Speranza et al., 1999; Mattei et al., 2007]. [43] The emplacement of the Gela Nappe since the early Pliocene was constrained by the two lateral ramps of the Sciacca structural high to the west and the Hyblean Plateau to the east, but the further frontal advancement of its toe to the south (Figure 7b) was probably facilitated by a faultbounded basinal depression in the foreland, inherited from the Mesozoic paleotectonic setting [cf. Lickorish et al., 1999]. The arcuate deflection of the front of the Gela Nappe against two bounding rigid blocks (Figure 2) suggests that components of constrictional E-W shortening of the nappe occurred during its progressive southward advancement. While E-W constrictional shortening can indeed be responsible for the earliest components of N-S refolding, it cannot satisfactorily explain (1) the localized, spaced belts of superposed N-S folding in the whole body of the Gela Nappe and (2) the formation of the N-S to NNE-SSW second generation folds and the refolding of the whole body of the Gela Nappe during the middle to late Pliocene to post-early Pleistocene, i.e., when the frontal thrust of the Gela Nappe had already reached its outermost position. [44] Our interpretation of the migrated and depthconverted line M23A (Figure 6b) shows that the whole nappe is shortenend along a section perpendicular to the transport direction, and that the sole thrust of the Gela Nappe is offset by the late propagation of crosscutting faults. The orientation of these faults cannot be constrained by one line only, but N-S and NNE-SSW trends are consistent with the orientation of the bounding faults in the Sciacca structural high at the western end of the line (Figure 2). N-S and NNESSW faults also control the structure of the Pelagian Foreland in the Hyblean substratum (e.g., Perla oil field, see Figure 2) and are well exposed onshore [Ghisetti and Vezzani, 1980].

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Figure 7

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Subsurface data show that some of these faults are inherited Mesozoic normal faults that underwent compressional reactivation during the Pliocene and early Pleistocene [e.g., Argnani et al., 1989; Antonelli et al., 1988; Argnani, 1993]. [45] Along line M23A (Figure 6b) some faults maintain normal displacement (with thickening of the middle to upper Miocene units in their hanging wall, e.g., SP 8600, 8500), but other faults with similar thickening of the Miocene sequence in their hanging wall (e.g., SP 7400) have small components of reverse displacement that contrasts with 4 ­ 5 km of downdip penetration of the faults. These elements suggest that faulting in the external areas was still controlled by an extensional stress field during the late Miocene, when the units of the Gela Nappe were in a more internal position and already shortened by the first generation folds (Figure 7a). The progressive incorporation of the Pelagian carbonate substratum into the southward advancing thrust belt is testified by the post middle to late Pliocene­ early Pleistocene compressional and transpressive reactivation of the N-S to NNE-SSW inherited normal faults, resulting in (1) approximately E-W shortening of the shallow cover sequence within the translated units, with refolding of the first generation E-W folds along the deformation belt of the reactivated normal faults; (2) compressional extrusion of the allochthonous wedge with convex doming of the nappe and lateral sliding and slumping of the topmost lower Pleistocene unconsolidated cover; and (3) offset of the basal thrust of the Gela Nappe (Figure 7c). [46] Superposed folding and deformation in the Gela Nappe occurred in a regional stress field with N-S trajectories of maximum horizontal compression. In fact, newly formed thrust faults (e.g., the Settefarine Thrust Fault, Figure 3b) propagated with E-W orientation and at $30° to SHmax. The late, ``out of sequence'' crosscutting of the Settefarine Thrust Fault is coeval with the advancement of the basal thrust of the Gela Nappe in the outer foredeep, and is possibly indicative of thickening of a subcritically tapered wedge [Grasso et al., 1995; Lickorish et al., 1999]. Thus, E-W shortening imposed by deformations in the substratum was synchronous with renewed N-S shortening in the shallower portions of the wedge. However, the Settefarine Thrust Fault is the only

surface structure postdating the second generation folds. We do not have data to elucidate its subsurface geometry, and its eventual deformation above N-S structural highs of the substratum, as inferred from the seismic line M23A for the base of the Gela Nappe at $3 km of depth. [47] In our interpretation, finite deformation in the outer domains of the thrust belt (Figure 7d) is related to the compressional reactivation of N-S to NNE-SSW inherited normal faults in the Pelagian substratum during and after the late stages of emplacement of the Gela Nappe (middle to late Pliocene to post early Pleistocene). The consistent northward plunge of the second generation folds within the belts of superposed folding (Figure 3b) suggests that the compressionally reactivated faults may be traced for at least 20 km inland from the coastline (Figure 2). This is comparable with the length of the fault systems exposed in the Hyblean plateau. Geophysical data [Bianchi et al., 1989; Ben Avraham and Grasso, 1991] indicate northward continuity of the African-Pelagian units as far as the north slope of Mount Etna. The extension of the flexed Pelagian platform to the north is also hinted by geoelectric resistivity data [Sestini and Flores, 1986] that depict salients (<3 km deep) and depressions (>7 km deep) of the top of the carbonates in the substratum (Figure 7d). However, these ridges are not necessarily bounded by normal faults and they are mapped in the subsurface with NE-SW trends, i.e., at an angle to the N-S to NNE-SSW trends of the faults in the external Pelagian platform. [48] Thus, for the inner domains of the thrust belt we have no data to define location and orientation of Mesozoic faults in the Pelagian substratum, nor can we infer that buried, inherited faults underwent compressional inversion during the development of the Apenninic thrust belt and interfered with the geometry of the allochthonous thrust sheets. On the contrary, the presence of N-S to NNE-SSW inherited normal faults is well documented in the more external areas [Ghisetti and Vezzani, 1980; Argnani, 1993], where the allochthonous cover is thinner, and blocks of the Pelagian substratum are emergent or buried at shallow depths. [49] Mechanical decoupling and reactivation of older faults suborthogonal to the collisional front is partly the

Figure 7. Schematic reconstruction (a) of the progressive history of emplacement of the Gela Nappe since the late Miocene and of its subsequent deformation during (b) the early Pliocene and (c) middle to late Pliocene to post early Pleistocene. Transect in Figure 7c is a simplified version of the cross section in Figure 6b. Note that the WNW-ESE transects in Figures 7a, 7b, and 7c are suborthogonal to the nappe transport direction, with movement of the nappe toward the observer. Retrodeformation of the cross sections from transect in Figure 7c to transect in Figure 7a has been performed with the software LithotectTM (Geo-Logic Systems), but retrodeformation along this transect does not account for the first generation set of E-W folds and faults. Progressive deformation of the Gela Nappe by N-S structures is ascribed to compressional reactivation of inherited Mesozoic normal faults of the Pelagian-African Foreland, that maintained extensional mechanisms till the late Miocene. (d) Interpretation of the geometry of the frontal thrust of the Gela Nappe offset by the late compressional reactivation of N-S to NNE-SSW inherited normal faults in the Pelagian-African Foreland. Fault position is extrapolated from the interpretation of seismic line M23A (Figure 6b), but orientation cannot be constrained from the line. The faults are likely to be subparallel to the faults of the Sciacca ridge and Hyblean Plateau. The northward extension of the Pelagian Foreland is supported by subsurface data which depict the top of the buried carbonate substratum at depths of 2 ­7 km beneath the allochthonous units of the Sicily thrust belt (contour lines from Sestini and Flores [1986]). Note, however, that in the northern regions the buried ridges are oriented NE-SW (and not necessarily bounded by normal faults), whereas in the southern areas the structural highs in the foreland substratum are trending N-S to NNE-SSW.

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consequence of the flexural inflection of the foreland plate beneath the thrust belt [Cogan et al., 1989; Grasso et al., 1995], but the variety of reactivation mechanisms, with development of transpressive and transtensional transfer zones (with contrasting dextral and sinistral components, see Figure 2) is probably controlled by differential buoyancy of adjacent blocks with strong inherited variations in crustal thickness [Reuther et al., 1993].

6. Conclusions

[50] The Miocene to Pleistocene sequence of superposed deformation within the Gela Nappe of Sicily is intriguing, because it involves the most external and youngest thrust sheet of the Apenninic thrust belt, and is not observed for older thrust sheets in more internal positions. Field data integrated with analysis of the migrated and depth-converted seismic reflection line M23A (CROP Mare Project) show that the translated units shortened by synpost-Messinian to early Pliocene sets of approximately E-W folds are refolded by middle to late Pliocene to post early Pleistocene N-S and NNE-SSW folds that impart a distinct ``dome and basin'' interference, associated with the final approximately E-W shortening of the nappe. [51] This setting does not result from gravitational detachments or syntectonic rotations, nor can it be explained by a 90° switch in the orientation of the stress field in the outer zones. A constrictional E-W shortening exerted by the Sciacca and Hyblean bounding rigid blocks during the southward emplacement of the nappe can explain the earliest components of superposed N-S folding but is not compatible with the refolding of the whole nappe after its final emplacement in the outer foredeep.

[52] Our interpretation is that the late stage E-W shortening of the Gela Nappe was imposed by the compressional reactivation of inherited, Mesozoic N-S and NNE-SSW normal faults in the carbonate substratum, during the progressive incorporation of the Pelagian Foreland in the thrust belt. These faults are oblique to subparallel to the regional trajectories of maximum horizontal shortening, and their reactivation (probably with transpressive components) appears to accommodate the differential buoyancy and the flexure of the rigid carbonate substratum beneath the thrust belt. [53] The deformation of the Gela Nappe demonstrates that the thrust belt-foreland interaction results in a mixed style of thin- and thick-skinned tectonics, in high-angle structural interference, and in components of finite shortening at 90° from the regional transport direction. [54] Thus, even in the outermost and youngest thrust sheets of a foreland fold-and-thrust belt, the assumptions of plane strain kinematics and a planar basal detachment above the undeformed foreland may result in the incomplete balancing of interpretive cross sections and lead to a poor definition of the progressive deformation history.

[55] Acknowledgments. The research project synthesized in this paper was initiated together with our colleague and friend Mario Grasso. Many preliminary ideas drafted in former manuscripts were discussed and modified when Mario was still alive. We dedicate this paper to his memory. The scientific committee of the CROP Project provided the original data of line M23A (time stack and navigation data). Seismic processing and analysis of line M23A made use of the University of Otago's academic license for GLOBE Claritas. We thank R. Maniscalco for providing some of the original drafts of M. Grasso; A. Argnani for sending logs of the wells Leone 1, Prezioso 1, and Perla 1; and Bob Ratliff (Geo-Logic Systems) for help in using the LithoTectTM software. Critical reviews by the Editor, R. Catalano, and an anonymous reviewer helped improve the manuscript.

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À F. À À À À C. Ghisetti, ÀÀÀ

TerraGeologica, 55 Mansfield Avenue, Christchurch 8014, New Zealand. (francesca. [email protected]) A. R. Gorman, Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand. ([email protected]) L. Vezzani, Dipartimento di Scienze della Terra, Universita degli Studi, Via Valperga Caluso 35, I-10125 ` Torino, Italy. ([email protected])

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