From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy)

GR-01329; No of Pages 28 Gondwana Research xxx (2014) xxx–xxx

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From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy) Giovanni Luca Cardello a,b, Carlo Doglioni c a b c

ISTO — Institut des Sciences de la Terre d'Orléans, Université d'Orléans, 1A Rue de la Ferollerie, F-45071 Orléans Cedex 2, France Department of Earth Sciences, Swiss Federal Institute of Technology (ETH), ETH Zentrum, CH-8092 Zürich, Switzerland Department of Earth Sciences, University Sapienza, Piazzale Aldo Moro 5, I-00185 Roma, Italy

a r t i c l e

i n f o

Article history: Received 14 August 2014 Received in revised form 24 September 2014 Accepted 25 September 2014 Available online xxxx Handling Editor: M. Santosh Keywords: W-directed subduction zones Apennine accretionary prism Gran Sasso salient Passive margin inversion Fold back limb tilt Active normal faultingt

a b s t r a c t The Apennines are a low-temperature accretionary prism generated by the west-directed subduction of the Adriatic–Ionian plate, whose structural origin is still to be fully understood. The highest and best-exposed segment of the Apennines, the Gran Sasso range is here documented to unravel the tectonic history of the northern tip of Gondwana. It is located along a NE-verging salient of thrust sheets decoupling the sedimentary cover of the subducting Adriatic lithosphere. Field mapping and structural analysis along the E–W trending left-lateral transpressive segment of the salient highlight the interplay of the inherited Mesozoic passive margin stratigraphic and tectonic framework with the Neogene contraction. The rheological differences between the massive carbonate platform and the well-bedded turbiditic and pelagic limestones determined along-strike undulations of the thrusts geometries and fold styles during shortening. Heterogeneities are due to inherited syn- and postrift Mesozoic tectonics. The Gran Sasso overturned anticline shows a backlimb anomalously tilted toward the foreland and we infer this dip as being related to a deeper back-thrust of a triangle zone. The pinching out of the foredeep sequence on the growth anticline forelimb dates the contractional phases of the region to the late Messinian. From the late Pliocene to Present, the area has been uplifted and extended about 2 km by oblique normal faults cross-cutting the accretionary prism. Some of them are seismically active, as shown by the 2009 Mw 6.3 L'Aquila earthquake. © 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geological setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stratigraphic setting and Mesozoic–Cenozoic rifting . . . . . . . . . . . . . . . . . . Neogene compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Reverse faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Veins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Quaternary extension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Structural interplay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1. Late orogenic back-tilt . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2. Reactivation vs. non-reactivation . . . . . . . . . . . . . . . . . . . 6.1.3. Interpretation of the cross-sections . . . . . . . . . . . . . . . . . . 6.1.4. Retrodeformation of Neogene structures . . . . . . . . . . . . . . . . 6.2. Structural events recording northern Gondwana dismembering and later accretion 7. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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http://dx.doi.org/10.1016/j.gr.2014.09.009 1342-937X/© 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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1. Introduction The Apennines are an accretionary prism formed in the hangingwall of a W-directed subduction zone. The tectonic evolution of belts formed within these subduction zones is poorly studied, particularly because they are generally low elevated and located below sea-level (e.g., Aleutians, Barbados, Nankai, South Sandwich arc, Tonga, von Huene, 1986; Moore et al., 1990; Yamamoto et al., 2009; Safonova and Santosh, 2014). Among W-directed subduction zones, the Apennines are the highest belt in the world because of their deeper décollement at the base of the accretionary prism with respect to subduction zones having the same polarity. In fact W-directed subduction zones are worldwide characterized by shallow (upper crustal) décollement layers (Doglioni et al., 2007). Within this class of accretionary prisms, the deep (N6 km) décollement of the Apennines involves larger volumes of the lower Adriatic plate, affecting both the Mesozoic–Cenozoic passive margin and the Neogene active margin sequences and determining a relatively higher topography (Bigi et al., 2003; Lenci and Doglioni, 2007). The involvement of the underlying basement, if any, is constrained to the shallow layers due to the shallow positioning of the basal décollement of the accretionary prism (e.g., Scrocca et al., 2005). This is also supported by analysis of the prism with the area balancing technique (Lenci et al., 2004). These accretionary prisms share a single eastward vergence and low topography and are generally associated to subduction hinges migrating away from the upper plate (Harabaglia and Doglioni, 1998; Doglioni et al., 2007). Moreover, W-directed subduction zones are characterized by deep foredeeps and backarc basins having fast subsidence rates on the order of N 1 mm/yr and N 0.6 mm/yr, respectively (Doglioni et al., 1999a,b). The Apennines (Fig. 1) are a ca. 1500 km long Cenozoic to Present accretionary prism (Boccaletti and Guazzone, 1974; Treves, 1984; Bally et al., 1986; Malinverno and Ryan, 1986; Royden et al., 1987; Doglioni, 1991, 1994; Buiter et al., 1998; Doglioni et al., 1999a,b; Jolivet and Faccenna, 2000; Rosenbaum and Lister, 2004; Panza et al., 2007; Brandmayr et al., 2011; Roure, 2014). Unlike the opposite E- or NE-directed Andes or Alpine–Himalayan end-member orogens (both oceanic and continental–collisional stages), the accretionary prisms of W-directed subduction zones form without collision and are mostly composed of shallow layers scraped-off from the retreating lower plate; moreover they show few or absent outcropping metamorphic rocks, apart from the inherited slices of pre-existing dismembered boudinaged belts (Doglioni et al., 1999a,b). With regard to the other end-member, the Alps rather have the subduction hinge of the European lower plate converging relative to the Adriatic upper plate (Carminati and Doglioni, 2012). The geological setting of the Alps is defined by the presence of ultra-high pressure rocks and vast areas of the metamorphic basement outcrop (i.e., Beltrando et al., 2010), which are missing in the Apennines. Moreover, there is no backarc basin, the belt is doubly verging, the two foredeeps have low subsidence rates (0.1–0.2 mm/yr) and are relatively thinner. The Alpine belt is at least two times more elevated than the

Apennines and, in fact, in the Alps, thrust planes deeply involve the entire crust of both the upper and lower plates (Carminati and Doglioni, 2012). This paper addresses the evolution of a segment of the central Apennines (Vezzani et al., 2010), which, due to its well-exposed outcrops (Fig. 2), may represent an example of the stratigraphic (Fig. 3) and tectonic history of the whole belt and similar geodynamic settings worldwide. The Gran Sasso range is the highest of the Apennines, having a mean altitude of about 2200 m and peaks that reach 2914 m. Due to its elevation, the study area is one of the least vegetated of the Apennines, representing a geological window to analyze the structure and evolution of the belt. Moreover, glacial erosion has contributed to the exposure of relatively deep portions of the structure, which are not commonly observable elsewhere in the outer lower sectors of the orogen. Therefore this research also aims to contribute to a better definition of the regional structural evolution, focusing on the interplay between pre- and syn-orogenic structures. In particular, we document the Mesozoic–Cenozoic syn-sedimentary rifting evolution of the region in order to define its control on the Neogene contractional evolution of the Gran Sasso. Moreover, the study area may represent an example of the geometry and kinematic evolution of the whole Apennines, which in turn can help to unravel the tectonics of similar belts associated with W-directed subduction zones. Our study is based on detailed mapping of the central Gran Sasso area on a scale of 1:10,000 (Cardello, 2008; Fig. 4). New geological cross-sections (Fig. 5) and lithostratigraphic columns (Fig. 6) are presented in order to document stratigraphic variations and geometries in the study area. Analysis of the outcrops allows a detail reconstruction of the paleotectonic evolution of an important segment of the southern continental margin of the Tethys. For the lithostratigraphy (Fig. 3) we use here a combination of traditional names (e.g. Passeri et al., 2008) and newer, formally defined units for the Jurassic; for the Cretaceous and the Paleogene we have adopted names and subdivision of van Konijnenburg et al. (1998, 1999); and for the Neogene units the Sheet “Gran Sasso d'Italia” of the Geological Map of Italy, 1:50,000 (Calamita et al., 2010). 2. Geological setting The Apennine accretionary prism formed at the expense of the western passive continental margin of the Adriatic plate (Bernoulli et al., 1979; Bernoulli, 2001; Scrocca et al., 2007; Vezzani et al., 2010; Calamita et al., 2011) or the Ionian oceanic embayment (Catalano et al., 2001), all elements of the Alpine Tethys at the northern tip of Gondwana (i.e., Nance et al., 2014). The present active prism is located to the “east” of the belt buried beneath the Po Basin, the western Adriatic and the Ionian Sea. The Apennine chain represents an exhumed earlier stage of the accretionary prism, now uplifted and cross-cut by Quaternary normal faults (Bigi et al., 1990; Galli et al., 2002; Roberts and Michetti, 2004; Papanikolaou et al., 2005; Cardello and Tesei, 2013).

Fig. 1. Sketch of the geodynamic setting. The Gran Sasso massif is located in the hangingwall of the “westerly” directed and “eastward” retreating Apennine subduction zone, where the slab is the western side of the thinned Mesozoic passive continental margin of the Adriatic plate. Note the magnification in morpho-bathymetry with respect to the deep structure.

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 2. Location of the study area in the E–W limb of the Gran Sasso salient, and its tectonic and Mesozoic paleogeographic setting (modified after Bernoulli, 2001). The red and orange patches along the Gran Sasso thrust are the syn-orogenic Monte Coppe Conglomerate and Rigopiano Conglomerate respectively, deposited and deformed during the Early Messinian– Early Pliocene, and Early Pliocene. Thrust trajectories close to the Adriatic coast are from Scrocca (2006).

As such, the belt is undergoing backarc extension in the hangingwall of the easterly retreating lower plate and therefore the accretionary wedge is less elevated to the west. As testified by the migration of the accretionary prism front and of the foredeep depocenters (Ricci Lucchi, 1986), the eastward migration of the backarc extension and the related magmatism (Jolivet et al., 1998; Lustrino and Wilson, 2007; Carminati et al., 2012), the paired contraction–extension tectonic settings have moved from “west to east” since the Late Eocene or Oligocene. The exposed units in the axis and in the external parts of the belt evolved in very low-temperature conditions (b100 °C), and exhumation has been no more than 3–4 km (Rusciadelli et al., 2005 and references therein). The eastward retreat of the Apenninic slab is responsible for the eastward migration of the paired tectonic regimes: compression to the east and extension to the west (Cuffaro et al., 2010 and references therein). In fact, the Apennines evolved as a single “easterly” verging accretionary wedge, followed to the west by an extensional wave, as shown in Fig. 1. The lower plate is presently moving away from the upper plate, but subduction and related accretion are still operating because the subduction hinge moves faster to the east than the lower plate relative to the upper plate at a rate N2 mm/yr (Devoti et al., 2008).

The speed of the slab retreat decreases moving along the chain from southeast to northwest, and appears to be on the order of a few to 10 mm/yr (Doglioni, 1991; Devoti et al., 2008). Since the subduction hinge is migrating away relative to the upper plate, the Apennines are a subduction zone without a converging upper plate (Doglioni et al., 2007). The curvature of the subduction hinge determines the dip of the foreland regional monocline. East of the Gran Sasso area, along the front of the accretionary prism below the Adriatic Sea, the regional foreland monocline dips 2–3° to the west, becoming steeper (6–10°) beneath the prism (Mariotti and Doglioni, 2000). The dip of the monocline may deeply influence the evolution and spacing of the thrust planes (e.g., Doglioni et al., 1999a,b; Koyi and Vendeville, 2003). The ongoing subduction of this part of the Apennines is inferred by compressive seismicity in the accretionary prism along the Adriatic coast, and extensional earthquakes to the west along the Apennine belt. The scarce deep slab-related seismicity in the central Apennines (Selvaggi and Amato, 1992; Chiarabba et al., 2005) can be explained by the continental lithosphere rheology (Carminati et al., 2002, 2005) and slab tears (Doglioni et al., 1994). The study area did not experience significant burial as testified by the absence of metamorphism and the low temperature recorded (Rusciadelli et al., 2005; Aldega et al., 2007). The depth of

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 3. Stratigraphic column of the study area. Formational thicknesses can be found in Fig. 8.

the décollement flooring the accretionary prism and the amount of subduction determine the volume of the prism (Bigi et al., 2002, 2003). The stratigraphic horizons where the basal décollement is located can be recognized from seismic reflection profiles (Bally et al., 1986; Billi et al., 2006; Scrocca, 2006). The highest elevation of the Gran Sasso range along the belt can be correlated with the deeper location (8–12 km depth) of the basal décollement in the Triassic evaporites located in the lower layers of the passive margin sequence (4.5–6 km thick) and the active margin sequence (flysch and molasse) of the foredeep basin (3–5 km at the depocenter). Therefore, larger volumes of the upper crust (mostly sedimentary cover) of the lower plate have been accreted in the fold and thrust belt of the central-northern Apennines with respect to other segments of the Apennines where the subduction and related shortening have been larger (e.g., Calabria) because the décollement layer is shallower and located within the Messinian evaporites (e.g., 2–3 km; Lenci et al., 2004). The stratigraphic

units of the external Apennines, detached along Late Triassic anhydrites (Burano Fm; Martinis and Pieri, 1964) up to the Plio-Quaternary layers form thrust sheets piled upon each other, often displaying a frontal ramp anticline (e.g. Sibillini, Gran Sasso, Maiella). The central Apennine fold and thrust belt is constituted by both passive and active margin stratigraphic sequences, respectively composed of carbonate and siliciclastic rocks (Fig. 2). These successions are affected by syn-sedimentary faults and folds of different age, recording the various steps of the tectonic history, that starts with continental rifting during the Late Triassic–Early Jurassic and end with a contrasted evolution of the different sectors of the Adriatic microplate (Fig. 3). Early Jurassic syn-sedimentary faults were considered to be the most relevant for the determination of the stratigraphic setting and the later orogenic structure (Adamoli et al., 1978). During Jurassic–Cretaceous times, the area of the future central Apennines was part of a large, Bahamiantype carbonate platform–basin system, whereby the area of the western and central Gran Sasso range was situated between the carbonate platform of Latium and the Abruzzi in the west and the deeper basinal area of Umbria–Marche to the east and north (Adamoli et al., 1978; Accordi and Carbone, 1986). However, further Late Jurassic to Late Cretaceous syn-sedimentary normal faults are characterized by remarkable displacement occurring throughout different domains of the Apennines (Montanari, 1988; Marchegiani et al., 1999; Graziano, 2000; Casabianca et al., 2002; Santantonio et al., 2012). The passive margin stratigraphic sequence is overlaid by a Neogene transitional sequence of foreland to foredeep sediments of the active margin. The geometry of faults and the amount of their reactivation during the Neogene can, indeed, provide information to quantify shortening in the shallower part of the accretionary prism, and the later ongoing stretching. Reactivation of pre-existing faults (Williams et al., 1989) is supposed to have taken place in the area (Adamoli, 1992; D'Agostino et al., 1998; Speranza, 2003; Calamita et al., 2011) but without much agreement about their mode and importance. Although the influence of the Early Jurassic faults during Alpine orogeny has been recognized (e.g. Dela Pierre et al., 1992; Speranza et al., 2003; Satolli et al., 2005), the kinematics of thrusting have been interpreted in different ways during recent years (Ghisetti and Vezzani, 1991; Dela Pierre et al., 1992; Speranza et al., 2003; Satolli et al., 2005; Billi et al., 2006; Patacca et al., 2008; Di Luzio et al., 2009; Vezzani et al., 2010; Calamita et al., 2011; Pace et al., 2014). The Gran Sasso range is the salient with the highest morphologic and structural elevation in the Apennines and has a NE-vergence (Vezzani et al., 2010). At its western termination it disappears into a recess where the depocenter of the Messinian Laga Flysch is located (Bigi et al., 1999). The recess merges into the Sibillini salient to the north (Fig. 2). In order to detect block rotations at the thrust fronts that determined the salient geometries, Dela Pierre et al. (1992) conducted paleomagnetic investigations in the Marche–Abruzzi region, reporting a remarkably homogenous Counter Clock Wise (CCW) rotation (~ 90°) of the Gran Sasso thrust sheet with respect to the Messinian–Pliocene Laga Flysch in the footwall. Satolli et al. (2005) studied the paleomagnetic rotations of the Gran Sasso Unit, recognizing a progressive increase of the CCW rotation at the northern Gran Sasso thrust front, where the Corno Grande rocks rotated only 7.9° ± 4.3° CCW. During the Late Miocene, the external Apennines were involved in thrusting and syn-orogenic conglomerates date the timing of compression. Indeed, some kilometers to the east (Fig. 2) along the thrust front, the Monte Coppe Conglomerate (Late Messinian–Early Pliocene) unconformably overlies the previously deformed structures, sealing the first compressive structures. The conglomerates are composed of extrabasinal clasts of various origins, derived from eroded Ligurian units (Calamita et al., 2004). Subsequently, the area was involved in a second event of NE-vergent thrusting during which the Monte Coppe Conglomerate (MCC in Fig. 2) and the Early Pliocene Rigopiano Conglomerate (RC in Fig. 2) were folded soon after their deposition. Both crop out in the easternmost part of the Gran Sasso salient. In

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 4. Geological map of the central Gran Sasso massif, modified after Cardello (2008). Panels a), b), c) and d) are the traces of the cross sections of Figs. 5 and 11. The formational symbols in the legend are followed by their ages (e.g., LJ, Late Jurassic; MJ, Middle Jurassic; EK; Early Cretaceous; LM, Late Miocene).

contrast to the Monte Coppe Conglomerate, the younger Rigopiano Conglomerate is exclusively composed of limestone clasts (Calamita et al., 2004) derived from the eroded Gran Sasso Unit, suggesting that in a later stage of compression the present day structure was already set

up before being uplifted to the present day altitude and faulted by the high-angle normal faults affecting the whole Apennines. The recent L'Aquila earthquake (Mw 6.3) testifies to the ongoing activity of the extensional faulting in the area (Chiarabba et al., 2009).

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 6. Stratigraphic correlations and lateral thickness variations of the study area. Note the occurrence of paleo-highs and lows controlling thinner and thicker sequences respectively. Paleo-highs also remained structural highs within the accretionary prism.

3. Stratigraphic setting and Mesozoic–Cenozoic rifting In this section we discuss the Mesozoic and Cenozoic Gran Sasso paleogeographic and syn-sedimentary structural evolution. A stratigraphic sequence spanning from Late Triassic to Late Miocene and Quaternary outcrops in the study area (Figs. 3, 4 and 6). Following a Triassic tidal flat (Dolomia Principale), the area was characterized by a widespread carbonate platform during the Earliest Jurassic (Calcare Massiccio). During the late Early Jurassic, the platform was dismounted by faulting and marine ingression and mostly drowned and covered by pelagic and turbiditic calcareous sediments (Corniola and Verde Ammonitico, Fig. 7). Other common lithologies are chert interbedded with marls, graded, mainly bioclastic calcarenites, and pebbly lime mudstones. Pelagic marly limestone intercalated by mass flow platform-derived resediments characterize the later Middle–Late Jurassic and Cretaceous times (Figs. 8 and 9). Moreover, thickness and lateral facies variations show that paleo-highs developed (Figs. 7 and 10) along Mesozoic normal faults. The most prominent of this is in the Corno Grande

(Scarsella, 1955, 1958) to the east (Fig. 10), and a second one formed in the western side (Acqua San Franco) with an intervening basin (west of Campo Pericoli, Fig. 11) where Early Jurassic pelagic deposits and platform-derived resediments are characterized by thicker successions, lying between the paleo-highs. This setting was determined by the Early Jurassic rifting, and thinner and discontinuous pelagic patches occur on the paleo-high tops (Calcare Massiccio B, Corniola, Verde Ammonitico, ‘Calcari a Posidonia’, Fig. 10). The Corno Grande paleohigh coincides with the highest structural and morphologic elevation in the Apennine fold–thrust belt. Thickness variations allow inferring the occurrence of Mesozoic syn-sedimentary faults. In the paleo-highs (Corno Grande, Acqua San Franco), carbonate platform sedimentation persisted longer than in the inherited structural lows (Pizzo d'Intermesoli, M. Corvo) into which carbonate detritus from the Lazio– Abruzzi platform was channeled (Fig. 12). However, by the middle Early Jurassic to the latest Jurassic, all platform relics had turned into paleo-highs (Fig. 7). The Corno Grande paleo-high persisted as a submarine relief throughout the Mesozoic (Fig. 10), whereas other tectonic

Fig. 5. Geological cross sections of the study area. The uncolored parts represents the deep interpretation. For cross-section location see Fig. 4. Full colored lines are formational boundaries; dashed colored lines are interpreted formational boundaries; full black lines faults, dotted red lines interpreted thrusts, full violet lines Mesozoic faults. Shortening and later extension decrease from section a) to section c), that is, from east to west. Moreover, thrusts have a left-lateral transpressional component, whereas normal faults have a right-lateral transtensional component, which prevents 2D accurate balancing restoration.

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 7. (a) Simplified stratigraphic architecture of the southern sector (Venacquaro–Campo Pericoli). The Jurassic succession reflects the submarine topography resulting from Early Jurassic extensional faulting. Along the N–S trending (half?-)graben; pelagic and mass-flow deposits of the Corniola Fm are onlapping onto the Passo del Cannone Fault separating the evolving Corno Grande paleo-high from the adjacent basin. The differences in formational thickness of the Corno Piccolo Fm between Val Maone and Corno Piccolo suggest that the submarine relief was not completely leveled in the Late Jurassic. Persisting submarine topography and differential compaction of the basinal deposits (Corniola) may have allowed for increased accommodation space in the west. The difference in accommodation space between the paleo-high and the basin to the west is about 2 km. The red line indicates the location of the future upper thrust that obliquely ramps up across the older geometry. (b) Scaled experimental models showing how the presence of a viscous layer, such as salt, facilitates the development of ‘forced’ extensional folds above active normal faults, from Withjack and Callaway (2000; model 12, p. 639). The orange dashed box indicates the area with a similar geometry as in (a).

elements were buried earlier and temporarily invaded by the mass-flow deposits. On the top of the Acqua San Franco paleo-high (Fig. 8), the stratigraphic relationships between the Calcare Massiccio A and B and the younger deeper marine sequence, already observed by Adamoli et al. (1978), clearly argue for a paleotectonic, pre-Alpine structure, i.e. a drowned submarine high overlain by a deep-water cover. Despite that, Adamoli et al. (1978, their Fig. 2) already considered this submarine high to represent a down-faulted block. During the Early Sinemurian (earliest Jurassic), the elevation of the paleo-highs reached shallow water depths (e.g., Acqua San Franco, Fig. 8) or even subaerial exposure (e.g., Corno Grande, Fig. 6). The water depth attributed to the pelagic Early Jurassic Corniola Fm is considered to be less than 1 km. In similar contexts but in more distal areas, a paleodepth of a few hundreds of meters was recognized for the pelagic carbonatic paleo-highs (Gill et al., 2004). After the Early Jurassic rift, the faults bounding the paleo-highs appear to have been inactive or only active to a limited extent. Along the margins of the carbonate platforms, pelagic limestones interfingered with platform-derived mass-flow deposits were laid down at the base of submarine escarpments and non-depositional slopes (e.g. Bernoulli et al., 1979; Eberli et al., 2004). We have tried to summarize new and old observations in order to reconstruct the original arrangement of the fault blocks and estimate the amount of Early Jurassic extension. A sketch illustrating the evolution of the normal faults and the paleohighs of the area is shown in Fig. 13. During thermal subsidence of the margin, the structural basins were invaded by carbonate mass-flows (shed from the adjacent carbonate platform) channeled around and between the isolated paleo-highs. The long-lived submarine topography, inherited from the Early Lias rifting, persisted during the rest of the Mesozoic and into the early Cenozoic, as suggested by the pronounced facies and thickness differences between the basinal base-of-slope deposits and the pelagic limestone caps of the ‘paleo-highs’ whose condensed sequences were punctuated by submarine hiatuses. Variations of formational thicknesses are less pronounced up-section from the Cherty Detrital Limestone (Calcari

Diasprigni Detritici), suggesting that the submarine relief was, with the exception of the Corno Grande high, gradually buried by the sediments in the Late Jurassic. At Corno Grande, Late Cretaceous–Paleogene sediments occur only as breccias filling sedimentary dykes in the Calcare Massiccio, and might reflect minor extensional faulting or, simply, gravitational instabilities. Additionally, the occurrence of platformderived shallow-water biota in these sediments indicates that parts of the paleo-highs were invaded by redeposited shallow-water sediment. A prominent and new feature is the increase in sedimentary thickness from the Corno Grande paleo-high and Campo Pericoli plateau to the west, which together occur parallel to a general westward tilt of the Corno Grande paleo-high (Fig. 10). The stratigraphic relationships between the extremely reduced and discontinuous succession of Campo Pericoli in the east (Fig. 11), and the stratigraphically complete, thick and partly redeposited succession in the west (Pizzo d'Intermesoli– Monte Corvo) are illustrated in Fig. 13. In the southern sector, there is no evidence of reactivation of preexisting Early Jurassic faults. Rather small syn-sedimentary faults could be assigned to the Middle–Late Jurassic, being the Cherty Detrital Limestone affected by an abrupt change of thickness in the order of tens of meters across some faults. These structures do not represent the main structure but correspond to the beginning of the differential subsidence between the eastern and western sectors. The tilt and the sudden change in formation thickness could reflect the evolution of a flexure above the deeper-seated forced normal faults similar to the model of Withjack and Callaway (2000; their model 12, on p. 639) and also to Bergerat et al. (1990) and Childs et al. (1993). Therefore deep seated and possibly buried normal faults are likely to have been active during thermal subsidence and possibly affected the basement and the (Burano) anhydrites that should occur below the Late Triassic Dolomia Principale (Fig. 7). In the Early to Late Cretaceous successions of the Apennine examples of post-break-up normal faults are common but their relevance has not yet been addressed in any study. In the Marchean basin, besides deep scars eroding the carbonate platform and filled by pelagic sediments

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Fig. 8. a) and b) Panoramic view of Pizzo Cefalone from the south. The Early Jurassic Acqua San Franco paleo-high is separated from the coeval, more basinal deposits by a steep Early Jurassic fault that is sealed by middle (Corniola) and late Early Jurassic (Verde Ammonitico) pelagic deposits. Paleo-high and basinal units were subsequently buried by pelagic and mass-flow deposits derived from the Lazio–Abruzzi carbonate platform (Corno Piccolo Fm., Maiolica Fm., etc.). c) and d) The same outcrop viewed from the west. The basinal Corniola Fm onlaps onto the eroded flank of the paleo-high. The entire structure was tilted to the north during Neogene.

(Corniola), syn-sedimentary normal growth-faults affecting the Scaglia Fm were recognized by Montanari (1988) and Marchegiani et al. (1999). Marginal sectors of the Lazio–Abruzzi platform were affected as well by Cretaceous normal faults (Praturlon and Sirna, 1976). Moreover just north of Rome (M. Soratte, Fig. 1), sedimentary dykes of Scaglia Fm occur in the Earliest Jurassic Calcare Massiccio, suggesting extension or local collapse during Late Cretaceous (late Turonian–Coniacian; S. Arragoni and M. Caporaletti, personal communication). In the central Apulia platform, Cenomanian to Santonian syn-sedimentary faults have been documented in seismic profiles offshore and inland, indicating active extension during the Late Cretaceous (Santantonio and Carminati, 2011; Santantonio et al., 2012). Significant extension has also been documented in northern Apulia, where Albian listric growth faults and related escarpments have been inferred (Graziano, 2000) with a displacement of about 850 m on the largest fault. At Maiella (Fig. 2), normal faults of similar age have a maximum displacement of 200 m and affected facies and thickness distribution (Casabianca et al., 2002). In the Tuscan domain of the Northern Apennines, pre-syn and post-break up syn-sedimentary normal faults have also been observed (Molli and Meccheri, 2012 and references therein). In the Helvetic Alps (and elsewhere in the Alps) post-breakup significant syn-sedimentary normal faults occur in the Middle–Late Jurassic and in the Cretaceous (Cardello, 2013 and references therein; Cardello and Mancktelow, 2014). During the deposition of the Oligocene to middle Miocene marly limestones and calcarenites, extensional movements resumed along the western segment of the Tre Selle Fault. In the middle Miocene, growing ridge structures accompanying the formation of major syn-

sedimentary normal faults have been recognized in different domains of the external Apennines (Centamore et al., 2009; Di Francesco et al., 2010, and reference therein). Moreover, normal faults affected the sedimentary sequence prior to thrusting during collision (Bigi and Costa Pisani, 2005; Calamita et al., 2009; Pace et al., 2014). In general, the middle Miocene movements have been interpreted by Calamita et al. (2002) as related to deformation in the foreland, which could already have started in the Oligocene, as suggested, and by the formation or the reopening of neptunian dykes during the Paleogene. The Miocene succession begins at the base with calcarenites with chert and glauconite, followed by the calcarenites and pelagic marls of the Cerrogna Marl. 4. Neogene compression In this section the structural data are exposed in detail in order to constrain the time and mode of shortening. The youngest preorogenic formation of the study area is the Orbulina Marl (Tortonian pp.–early Messinian pp.), consisting of 10 to 15 m bioturbated hemipelagic silty marls rich in Orbulina, a planktonic foraminifera. The overlying Laga Flysch (or Laga Fm, Fig. 5) is a 2 to 2.5 km thick succession of turbiditic sandstones and shales wedging out against the frontal fold of the Gran Sasso Unit (Milli et al., 2007). The pinching out of the Messinian Laga Flysch onto the forelimb of the Gran Sasso anticline constrains the age of the Gran Sasso thrust and related fold (Fig. 5). The result of the contractional deformation is concentrated at the frontal thrust zone (Figs. 14, 15 and 16), although minor thrusts occur north of Pizzo Cefalone and in the southern part (Fig. 5). The undulated, W- to

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 9. Panoramic view of Campo Pericoli, Pizzo Cefalone and Pizzo d'Intermesoli. The normal western Tre Selle Fault cuts across the late Jurassic to early Cenozoic base-of-slope succession. The Corno Piccolo Fm consists of coarse mass-flow breccias. Note large glides and slumps at the base of the Cefalone Fm (green). Dotted brick pattern: breccias and calcarenites; brick pattern: pelagic limestones.

WNW-trending, Gran Sasso ridge consists of a NE-verging faultpropagation fold related to the upper thrust (Fig. 15). The fold-andthrust structure can be distinguished with respect to a fault-bend fold or a detachment fold because the anticline is highly asymmetric, the forelimb is overturned, and its connection to the footwall syncline is truncated by the thrust that propagated upward. In the footwall of the Gran Sasso thrust, the siliciclastic turbidites of the Laga Flysch represent the Late Miocene infill of the Neogene foredeep. The western part of the Gran Sasso Unit is defined by an asymmetric, sub-cylindric, overturned frontal anticline, whose fold axis is generally striking east–west and plunging 5–10° to the west (Figs. 4 and 5). The frontal anticline is continuous over 8 km (Fig. 4). To the northwest of Monte Corvo it plunges below the sediments of the Laga Flysch (Fig. 15), whereas east of Corno Grande the frontal fold is juxtaposed on the Montagnone structure and eroded (Fig. 16). The forelimb of the

fold structure is either vertical or overturned (e.g. Pizzo d'Intermesoli, Fig. 14) supporting a fault-propagation folding mechanism, in which the upper thrust propagated along the core hinge of the anticline (anticlinal breakthrough of Suppe and Medwedeff, 1990, their Fig. 11d), and the lower thrust with a low-angle breakthrough. Especially in the western sector, the forelimb is partly covered by the late Messinian foredeep deposits of the Laga Flysch (Fig. 5) which suggests a growing anticline: The siliciclastic Laga Flysch onlaps the previously tilted structures (i.e., Gran Sasso and Montagnone), that were later involved in the further Pliocene deformation (Milli et al., 2007; Bigi et al., 2009), while the foredeep depocenter was shifted eastward and gradually filled by syn-orogenic and extrabasinal sandy or pelitic deposits (Bigi et al., 2011). Along the front of the growing Gran Sasso anticline, however, no elements from the fold-like limestone olistoliths, have been found in the purely siliciclastic sequence. This indicates that there was

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Fig. 10. Panoramic view of the western flank of the Corno Grande paleo-high. The Corno Grande paleo-high is capped by irregularly distributed and spatially limited occurrences of different pelagic formations. Occasionally, the paleo-high has been reached by mass flows with shallow-water material. The Early Jurassic Passo del Cannone Fault has, in its upper reaches, been shaped by submarine erosion and gravitational collapse and turned into a submarine escarpment onto which the syn-rift Corniola Fm is onlapping. Sedimentary dykes in the Calcare Massiccio Fm, filled by Corniola sediments are related to tectonic fracturing and/or gravitational movements. The Passo del Cannone Fault has been tilted to the north during folding and faulting during the Neogene. Lighter lines are faults of undefined age.

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 11. Campo Pericoli: (a) Detail from the geological map in Fig. 4. The white circle shows the outcrop on hill 2244 m. (b) A very thin and discontinuous succession of Late Jurassic to Oligocene deposits crops out at this place. The original stratigraphic relationships are only weakly disturbed by later normal faults. The Jurassic succession dips to the west (inset); only the younger Maiolica Fm and Venacquaro Fm dip toward the north. (c) Detail of Venaquaro sedimentary breccia with sub-rounded clasts in a floatstone matrix rich in larger benthic foraminifera. (d) Geological profile (section d on the geological map in Fig. 4). From WNW to SE decrease the formational thicknesses. Some of the thickness changes occur across faults (e.g., in the Late Jurassic Cherty Detrital Limestone) suggesting their syn-sedimentary nature.

no submarine erosion of the forelimb of the Gran Sasso during the Laga Flysch deposition. Possibly that could be related to stronger regional subsidence than local uplift of the anticline as observed elsewhere by Doglioni and Prosser (1997). The axis of the frontal fold plunges to the west in the direction of the deepening of the Mesozoic–Cenozoic basin. In its depocenter, the earliest Jurassic to Paleogene basinal succession above the Calcare Massiccio is about 3.5 km thick, whereas, in the east, it is around 1 km thick in Campo Pericoli and, at Corno Grande, the same succession was originally (before erosion) not thicker than 0.5 km (Fig. 6). West of Rio Arno (Pizzo Intermesoli, Monte Corvo, Figs. 4, 5 and 15), the frontal fold is well developed; the entire Late Cretaceous–Cenozoic

sequence is folded around the upper thrust, which has a blind-thrust ramp geometry and accommodates a few hundred meters up to zero displacement at the fault tip. On the east side of Rio Arno, the frontal anticline is more open, as it could be defined ‘embryonic’ and the fold axis turns to strike roughly ENE. In this sector, the upper thrust accommodates a displacement of about 0.6 km (calculated from the Calcare Massiccio cutoff) involving the more competent limestones of the Calcare Massicio and Corno Piccolo Fms (Fig. 16). West of Rio Arno, in the more basinal deposits, the thrust obviously ramps into higher stratigraphic levels (Figs. 4 and 5) since the ramp cuts obliquely across the former fault(s) limiting the Corno Grande paleo-high. In contrast the lower thrust, north of Pizzo d'Intermesoli, brings a tight syncline with

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Fig. 12. Panoramic view of the south-western slope of Monte Corvo. The thickness of successions varies among the walls of the Quaternary Tre Selle Fault. This may be related to its earlier syn-sedimentary nature and/or to its strike slip component, which juxtaposed originally separated paleogeographic domains.

a Miocene core into contact with the Laga Flysch unit (Fig. 15). The overturned succession that defines the intermediate unit is involved in the frontal folding and thrusting of the western Gran Sasso. The plot of the poles to beds belonging to the Intermediate Unit (Fig. 17g), shows a cluster in the south suggesting top-to-the-north shear. The geometry of the frontal anticline, compared to the poles to beds of the Gran Sasso Unit and of the Intermediate Unit (Fig. 17g, h and f) indicate top-to-the-north emplacement of the upper units onto the lower ones (i.e. the Laga Flysch). Moving along the strike of the Gran Sasso anticline and related thrusts, the deformation may locally change. Consistent with the en-échelon pattern of minor folds and thrusts along the thrust front and the trend of striations on the bed-to-bed flexural slip, the E–W trend of the northern limb of the Gran Sasso salient is interpreted as a left-lateral transpressional segment. Mesoscale and regional scale structural data (Fig. 18) indicate a mean NE-directed compression (Fig. 5). At Rio Arno and Prati di Tivo, the C-planes dip gently to the south or the SSW whereas the S-planes generally dip to the WSW or to the SW (Fig. 18). The lineation marked on the S planes by the elongation and cataclasis of rigid grains and the slicken-lines on the small-scale faults indicate transport consistently directed toward the NE or locally toward the ENE. The foliation in the shear zones is often refolded into open to tight asymmetric folds generally characterized by fold axes plunging mainly 45° to the SE and by axial planes striking NW–SE. The displacement along the Gran Sasso thrust front decreases from east to west, along the left-lateral oblique and transpressive ramp. Along the geological section of Fig. 5a, the minimum displacement in the shallow structure is on the order of about 7 km, summing upper and lower thrusts plus folding (Gran Sasso Anticline and related thrust

planes). The core of the frontal fold is composed of highly competent platform limestones (Calcare Massiccio) and dolomites (Dolomia Principale), which are thrust onto the overturned, stratigraphically complete, and relatively thick base-of-slope succession of the footwall. Still more to the east (Fig. 16), at Valle dell'Inferno, the direction of transport, deduced from the structural relationship of the Dolomia Principale with the Corniola, is top to the ENE, as already documented by Adamoli (1992) and Speranza (2003). In the northern part of the map, the orientation of the folds in the Laga Unit follows a WSW trend, which is different from the WNWstriking one in the Gran Sasso Unit. Provided that all these folds formed at the same time, they can be compared for relative block rotation. Since the difference in orientation between the fold axes belonging to the different units of the frontal zone is rather small, the potential relative rotation was 15–35°. A CCW block-rotation is therefore consistent with a left-lateral transpressive component of the “E–W” trending segment of the Gran Sasso range. 4.1. Reverse faults The reverse faults strike WNW and the slicken-lines are always rather oblique (Fig. 17). The slicken-fibers on the reverse fault planes also show two different mean directions, which generally indicate slip topto-the NE and to the NW. The overprinting relationships of the slicken-lines in the outcrops of Rio Arno indicate that the NE-directed movement occurred before the one to the NW (Fig. 18). Unfortunately, at Prati di Tivo the database is not sufficient to prove a correlation. Instead, in the Corno Grande Unit (Fig. 16), the cross-cutting relationships

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

Fig. 13. Tectono-sedimentary evolution from the early to the late Early Jurassic. a–b: Situation prior to (a) and during early extensional faulting (b): location of the major faults active during early rifting. In c) subtidal platform limestones (Massiccio B facies) are deposited whereas pelagic and mass-flow deposits of the Corniola Fm onlap onto the flanks of the structural highs. In d) on the Acqua San Franco Paleo-high and on Corno Grande, a reduced sequence of pelagic limestones of Corniola and Verde Ammonitico without mass-flow deposits overlies Massiccio facies B. In e) from the late Early Jurassic onwards, pelagic and redeposited limestones overlie the drowned remaining portions of the former platform: only the Corno Grande paleohigh and some minor plateaus with reduced sedimentation (Campo Pericoli) persist as submarine topographic highs. In f) the beginning of the syn-sedimentary tilt toward the west is marked by the change of thickness of the Maiolica pelagic layers.

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can be locally established but their interpretation has to consider that the older terrains were fractured and faulted during pre-orogenic extension. Some of the walls of the Early Jurassic sedimentary dykes, in fact, show in fact later polyphase reactivation. Therefore, numbers 1 and 2 of the chronological succession of the kinematic indicators in the stereoplots of the faults have to be considered as representing individual outcrops. Faults and fractures were therefore formed in a different structural context. A reactivation of the pre-Alpine fractures and faults during the propagation of the thrust front is well possible. Unfortunately, straightforward time correlations of age relationships recorded on fault planes do not provide univocal interpretation. 4.2. Veins Consistent cross-cutting relationships between two different vein systems have been detected at Rio Arno in the Intermediate Unit (Fig. 18a, b) and at Sella dei due Corni in the Gran Sasso Unit (Fig. 18c). The first set of mode-1 veins strikes N20°E. The second set is defined by mode-1 veins striking N35°W. In both sets, shear fractures associated to the tensional fractures are also observed. Tension gashes occur at 20–30° to the mode-1 veins that formed perpendicular to σ3 and therefore to the stretching directions. Provided the vein settlement was related to sub-horizontal compression, two different stages can be distinguished. Thus, a CCW rotation of almost 55° from the shortening direction with time can be recognized. Since the veins show similar cross-cutting relationships in the intermediate unit as well as in the Gran Sasso Unit, the two units did not experience any additional differential block rotation after veining. Though these data are rare, they are consistent and together with the other structural evidences, contribute to the definition of the structural evolution of the Gran Sasso thrust front; they cannot, therefore be easily disregarded. 5. Quaternary extension The entire area is cross-cut by Pleistocene to post-last-glacial normal faults of different magnitudes and lengths. They appear to overlap in time with the deposition of early Quaternary cemented breccias outcropping near Pietracamela on the northern side of the study area. These are calcirudites overlying an erosional surface, which mainly crops out at the northern slope of the Gran Sasso range. The breccias also seal the front of the Gran Sasso anticline and the thrust front in the western Gran Sasso range (Fig. 16), indicating that thrusting occurred before the early Quaternary and constraining its activity to the Late Messinian–Pliocene pp. There is a younger erosional surface on top of the breccias. Both basal and top erosional surfaces have been related to erosion during the glacial period (Nisio, 1997). The Quaternary breccias are fractured and moraines of the last glacial maximum are displaced by the Tre Selle Fault at Venacquaro and in Val Maone (Galadini and Galli, 2000). The Quaternary faults are concentrated between the Tre Selle Fault and the Assergi–Valle Fredda Fault, especially in the south-eastern sector, where numerous faults with modest displacement (30–40 m) were mapped. Four main sets of normal faults are distinguished: (1) WNW-striking, reactivated Cenozoic faults (e.g. the western segment of the Tre Selle Fault); (2) ENE-striking faults (e.g. the eastern segment of the Tre Selle Fault); (3) NW-striking faults usually with a normal displacement; and (4) NE-striking faults, which are characterized by a strike-slip component. More to the east, the Campo Imperatore Fault is a complex system of faults defining a series of south-dipping fault blocks and with a cumulative normal displacement which is around 1.8 km (Figs. 4 and 5). The Assergi–Valle Fredda Fault is characterized by an impressive topographic relief of about 1.5 km, reflecting a displacement of about the same amount in the area SW of Pizzo Cefalone (Fig. 5c) and 0.5 km east of Fonte Cerreto (Fig. 4a) (cf. Calamita et al., 2010).

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South of Corno Grande, the displacement along the eastern Tre Selle Fault is approximately 1 km (Fig. 19). South of Pizzo d'Intermesoli, the apparent displacement along its western segment, calculated on the cross-section (Fig. 5), is also around the same amount. Considering the original dip of the strata toward the west (Figs. 4 and 5) and the strike-slip component, the normal displacement is about 800 m. The fault pitch measured between lineation and fault strike for the western Tre Selle Fault at Venacquaro and Campo Pericoli is around 75°. This suggests a dextral transcurrent component of displacement of only 120 m. South of Monte Corvo, the total vertical displacement along the Tre Selle Fault is estimated to be ca. 850 m, whereas the pre-orogenic displacement accumulated during the Oligocene–middle Miocene, was around 270 m. The total displacement of the Miocene formations is about 630 m, reflecting the minimum Quaternary displacement of the western Tre Selle Fault. The conservative horizontal stretching (Fig. 20) is about 1.7–2 km. 6. Discussion 6.1. Structural interplay What is the evidence for the interplay between the stratigraphic and structural inheritance on the geometry and kinematic evolution of the area? The Meso-Cenozoic rifting was followed by the Neogene shortening and, finally, cut by the Quaternary extension. The first answer is the continuous along-strike structural undulation of the contractional structures, the systematic transfer of shortening from one geometry to the other (e.g. from oblique flexural slip and folding to oblique flexural shear and thrusting). The Neogene–Quaternary evolution shows few main tectonic features, such as: 1) the formation of the whole Gran Sasso salient in correspondence of the carbonatic paleo-high. 2) The westward plunge of the Gran Sasso anticline axial plane, corresponding to the passage from the Mesozoic paleo-high to the basin. The preexisting rifting-related heterogeneous geometry including steep synsedimentary extensional faults, combined with a general pre-orogenic and syn-sedimentary tilt of the structure to the west, must have influenced the tectonic style in the two sectors. 3) The change in geometry of the Gran Sasso anticline moving across the walls of the inherited Mesozoic normal fault. One further peculiar issue in the area is the diffused northward dipping monocline of the E–W limb of the Gran Sasso Salient, the origin of which is still unclear. Mesoscopic buckling is confined to the formations dominated by thinner, i.e. pelagic strata that crop out mainly in the west, where a thick basinal succession is present. In the east, cohesive platform limestones and dolomites dominate. In this area, thrusting prevails over folding and shortening is accommodated by brittle faulting and fracturing; whereas in the more shaly-marly units, the outcrop-scale structural features are represented by asymmetric folds, S/C structures (Fig. 17), small-scale faults (e.g. few cm displacement), and veins parallel to shear planes or by mode-1 veins (Fig. 18). The local variations of the shear and thrusting directions are intimately related to the heterogeneities of the stratigraphic pile, i.e. a heterogeneous mechanical multilayer. 6.1.1. Late orogenic back-tilt As shown in Figs. 4 and 5, most of the study area regionally dips toward the foreland. The Cenozoic flysch in the footwall of the northern thrust also tilts northward for about 5–10 km (Fig. 20). The internal limb of the fault-propagation fold, unlike what is predicted from the typical geometry of this type of structures, is dipping north in the direction of vergence (i.e., toward the foreland) rather than toward the hinterland (Fig. 14). This indicates that the entire structure was tilted northward after its emplacement. Indeed, along the ~30 km long E–W trending northern branch of the Gran Sasso salient, both the footwall and the hangingwall show a dip that indicates a later regional tilt. This attitude may be explained in different ways, including an underlying

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 15. Panoramic view of the frontal anticline–syncline system north of Monte Corvo. The anticline is highly asymmetric and the forelimb is overturned. These characteristics are typical of a fault-propagation-fold. However, unlike the geometric rule of this kind of structure, the inner flank of the anticline is tilted toward the foreland to the right. The thinning and pinching-out of the Laga Flysch on the forelimb indicates a Late Messinian age of the fold. The Laga siliciclastic deposits are also tilted toward the foreland, suggesting the occurrence of a later, deepseated, SSW-verging backthrust, i.e., a triangle zone, active during (and after) deposition of the Laga Flysch.

triangle zone, the footwall uplift of the Quaternary normal faults, or the construction of a duplex antiformal stack (Fig. 21), e.g., Couzens-Schultz et al. (2003) and references therein. We interpret this generalized dip to be generated by a deeper south-verging backthrust. This would be the case in a triangle zone located deeper down (Figs. 5 and 20), which formed after a first stage of northward propagation of the northdirected thrusts. Then the in-sequence forward and downward propagation of thrusts would have been accompanied by the formation of a triangle zone, which is very frequent at the front of many prisms and occurs in several sectors both along the Apenninic axis and front (Doglioni et al., 1999a,b; Patacca et al., 2008). As mentioned the flat of the upper thrusts dips toward the north, suggesting that a late tilt occurred prior to the Quaternary normal

faulting. As evident from the fault-to-bed cut-off relationships (Fig. 20), normal faults indeed cut across a previously inclined structure. Bedding in the fault block between the Tre Selle and the Assergi–Valle Fredda Fault is also too steep for preserving the usual cut-off angle of a normal fault. Assuming an originally horizontal or weakly southdipping flat of the thrust, we may assume a later tilt of the Gran Sasso–Laga area of about 15–20° to the NNW. 6.1.2. Reactivation vs. non-reactivation The Gran Sasso range provides examples of well-preserved Mesozoic structures but also of fault reactivation and new generations of faults formed during different tectonic phases. Three examples of Jurassic faults that have not been reactivated in the study area. Along the

Fig. 14. Panoramic view of the Pizzo d'Intermesoli group. The thick base-of-slope succession, rich in massive resediments (Corno Piccolo Fm), is characterized by internal angular unconformities (Maiolica and Cefalone Fm). On the right, the frontal Gran Sasso anticline is connected with a blind thrust along which the competent Corno Piccolo Fm is thrust onto the overturned limb of the fold. The internal (southern) limb of the fold dips toward the north, indicating that the entire structure was tilted after folding. On the left, the Quaternary active Tre Selle normal fault displaces the entire succession. Numbers refer to the geodynamic evolution recorded in the outcrop exemplified in the sketch at the bottom: 1) carbonatic slope deposits and Jurassic normal faulting in the passive continental margin of the Adriatic plate; 2) involvement in the Miocene–Pliocene Apennines accretionary prism by fault-propagation fold; and 3) Quaternary ongoing dissection by normal faults.

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 16. Panoramic view of the Gran Sasso thrust front at the eastern slope of Corno Grande (‘il Paretone’). The view is slightly oblique to the strike of the thrust zone. The upper thrust appears as very irregular because of lateral and frontal ramps. Formational boundaries in black, faults in red, Early Jurassic faults in violet and Corno Piccolo Fm in light-blue. On the right, Quaternary stratified breccias seal the thrusts.

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 17. Prati di Tivo upper thrust. (a) S/C structures define the local sense of movement. (b) Sigmoidal clast of limestone with chert nodule involved in the shearing of the basinal limestones of the Fonte Gelata Formation (42°29.125′N; 013°33.579′E), demonstrates a top to the ENE sense of shear. (c–d–e) Great circles of foliation and shear direction. (f–g–h) Poles to beds of the major units of the area; Gaussian contour lines mark their statistical occurrence. The Gran Sasso Unit (h) is offset by 15–25°with respect to the Laga (f) and Intermediate units (g). (i) Great circles of axial planes and plot of lineations of their fold axes that plunge to the SE. (j) Great circles of reverse faults and their slicken-fibers.

Corno Grande paleo-high, the WSW-striking Passo del Cannone Fault correspond to the early Jurassic fault that was bordering the paleohigh to the north and was passively involved in the fold-and-thrust structure. Other early Jurassic faults striking WSW are preserved at

Acqua San Franco and at Campo Pericoli where a major NNE-striking Jurassic fault is suspected to be present at depth. After the initial stage of rifting (early Jurassic), which generated ENE-striking normal faults, the area was again involved in extensional faulting during the Oligocene

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 18. Cross-cutting relationships of veins (a–d). In the Intermediate Unit (a–b), vein 1 is cross cut by vein 2, Fonte Gelata Fm (42°29.105′N; 013°32.554E). (a) Red arrows indicate the direction of shortening whereas the white arrows indicate the direction of stretching perpendicular to the mode-1 veins. (b) The occurrence of antithetic en-echelon veins, and mode-1 veins better defines the orientation of the shortening direction (orange arrows). The shear direction is marked by violet half-arrows; (c) similar cross-cutting relationships are observed in the Gran Sasso Unit (42°28.278′N; 013°33.765′E). (d) A conceptual sketch of the succession of deformational events suggests a sharp CCW rotation of the main shortening direction. The oldest direction of compression (1) is NNE–SSW, whereas the second (2) strikes NNW–SSE; it is defined by well-developed en-echelon arrays. (e) Great circles of fault planes and their slicken-lines in the Intermediate Unit (outcrops at Rio Arno and Prati di Tivo) and in the Gran Sasso Unit (Corno Grande). 1 and 2 indicate overprint of slicken-fibers on the older ones. This cross-cutting relationship is consistent at the outcrop scale.

Fig. 19. Normal–dextral transtensional active Tre Selle Fault outcropping between Val Maone and Campo Pericoli (a–b). A similar ribbon is present also in the adjacent valley to the west of Pizzo d'Intermesoli. (b) The surface rupture can be more than 2 m high (detail in the picture to the right) and indicates that the fault could have generated earthquakes with Mw N 7.

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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and Middle Miocene. During this time, the spacing between the synsedimentary Cenozoic faults is in the order of 2–5 km, suggesting that the basal décollement is once more located in the Triassic evaporites. This event probably reactivated the earlier levels of décollement active during the early Jurassic. South of Val Maone, the set of normal faults clearly crosscuts the pre-existing Jurassic faults; it was active contemporaneously with the deposition of the Oligocene and middle Miocene sediments. There, the western Tre Selle Fault loses displacement, a great part of which was transferred to the eastern segment (Fig. 19). A possible reactivation of the eastern segment during the Cenozoic is thus suggested. Not all the Mesozoic faults may have been reactivated during Cenozoic because of an orientation of the fault plane unsuitable for reactivation. Later on, the Tre Selle Fault and other Early Jurassic faults were passively transported with the thrust sheets without any remarkable reactivation (Fig. 20). It is possible that some of the preexisting syn-sedimentary faults were reactivated as minor reverse faults during folding; however, the dip of most of these faults was not suitable for reactivation because of over-steepening. During Pliocene uplift and subsequent subaerial erosion, the area was again under extension. Late Pliocene(?)–Quaternary normal faults cut across the older structures along three major fault zones: the Tre Selle Fault (Ghisetti and Vezzani, 1990, 1991); the Campo Imperatore Fault; and the Assergi–Valle Fredda Fault (D'Agostino et al., 1998), possibly reactivating pre-existing normal faults (Tavarnelli, 1999; Calamita et al., 2002; Pace et al., 2014) or the lower thrust (D'Agostino et al., 1998) during the uplift and the back-thrusting as geometrically explainable by the restoration of our geological cross-sections. However, the youngest extensional faults likely began to form at the end of the late Pliocene–early Quaternary when the Central Apennines were already uplifted (Centamore and Nisio, 2003). The faults are sub-parallel to the Paganica Fault to the southwest that generated the April 6th 2009 L'Aquila earthquake (Chiarabba et al., 2009). During this event, the Tre Selle Fault and the Assergi Fault where only marginally affected by the seismic activity probably because: 1) their strike direction (WNW) diverges ~25° from that of the Paganica Fault (NW); and 2) their kinematics have a right-lateral transtensive component. We interpret the western segment of the Tre Selle Fault as a reactivated Cenozoic fault; however, our data do not allow inferring whether the Tre Selle Fault is a branch of the Assergi–Valle Fredda Fault or independently ‘roots’ at depth. Since the pre-Apenninic faults lost their original roots during late Miocene thrusting, new active faults must have merged at the surface with the old ones. We cannot exclude that the active Tre Selle Fault is connected at depth via the ancient thrust ramp with the more southerly Assergi–Valle Fredda Fault. This alternative interpretation suggests listric geometry for the Quaternary Tre Selle Fault, leading to a rotation of the hanging-wall block. In any case, the Assergi–Valle Fredda Fault that shows a larger displacement than the Tre Selle Fault and dips still 50° at a depth of 8–10 km (Chiarabba et al., 2009). Moraines related to the last glacial maximum (12,500 years BP), cut by recent faults and topographic steps (in the order of 1–2 m), testify to their post-glacial activity, which could be generated in more than one seismic event. Considering its total displacement and if the Tre Selle and Assergi–Valle Fredda Faults are explained as a cumulative coseismic slip, then according to Wells and Coppersmith (1994), more than 1000 earthquakes with magnitude Mw 7 are needed to produce the observed cumulative offset during the last million year. Moreover, no aseismic fault creep has been detected in this area, if there is any it is below the present resolution of GPS data. 6.1.3. Interpretation of the cross-sections The geologic cross sections (Figs. 5, 11, and 19) intersect each other and allow us to postulate a 3D model of the structural evolution resulting from the different phases of Mesozoic extension, Neogene shortening and Quaternary extension. While the Mesozoic extension direction is rather uncertain (possibly about E–W in the present coordinates), the main Neogene–Quaternary shortening and extension

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directions in the area are coaxial and trending roughly NE–SW. Therefore, the E–W trending axis of the study area is oblique to the regional stress directions (Neogene left-lateral transpression and Quaternary right-lateral transtension), preventing an accurate balancing technique and 2D retrodeformation. In fact the thrusts have an oblique-lateral ramp geometry. However, in the sections of Fig. 5 it is evident that both the amount of shortening and the extension in the study area decrease from east to west. In the cross-section of Fig. 5a, the interpretation of the structure is also based on data obtained during construction of the highway tunnel below Campo Imperatore (Catalano et al., 1986). In this cross-section, stratigraphic columns are plotted in order to fix guide lines. At Valle della Portella–Vena Rossa (Fig. 4), an important NNW-dipping highangle fault, which juxtaposes Corniola Fm with Cefalone Fm, is interpreted as a steep back-thrust. The Corno Grande paleo-high is bounded by Jurassic faults (Figs. 10 and 13). Along the northern slope of the paleo-high, a thin pelagic succession onlaps its flank. The frontal structure is a fault-propagation-fold bounded by two main thrust surfaces, the upper one, which occurs in the hanging wall of the overturned limb of the frontal anticline, and the lower thrust, which is characterized by a ramp–flat–ramp geometry. The inferred N–S striking fault bounding to the west the Mesozoic paleo-high (horst) of the Corno Grande probably acted as transfer fault along the Maone Valley during the development of the thrust-related fold, which is better developed with the preserved overturned forelimb only along the western side of the valley. The Laga Flysch in the footwall of the lower thrust is represented by the Montagnone Structure, a NNE–SSW striking anticline, which plunges to the SSW. The thrusting direction affecting the Montagnone Structure is toward the east (Bigi et al., 2011). As shown in Fig. 5a, such independent thrust planes are located in the footwall of the Gran Sasso anticline and moving out of the section. In the western part of the map, the Laga unit dips to the NNW (Figs. 4 and 5b). There, the geometries affecting the Laga Flysch are intimately related to the Gran Sasso frontal zone. The plot of the poles to beds documents NNW–SSE shortening and folding (Fig. 17) probably produced after thrusting of the Gran Sasso Unit on to the Laga Flysch. This is likely to have affected mainly the siliciclastic Laga deposits rather than the thick middle Miocene calcareous succession from which the siliciclastic deposits may be detached (Figs. 5 and 19). Immediately to the NE of Corno Grande, the thrust zone affecting the Montagnone Structure at its southern end, dips to the west doubling the Miocene succession (Calamita et al., 2010), suggesting an E-directed transport direction. The underthrusting of the Montagnone structure beneath the Gran Sasso Unit should have occurred prior to the sedimentation of the ‘evaporitic member’, whose occurrence is restricted to the west, i.e. during the middle Messinian. At that time, the region west of Montagnone was uplifted as its piggy-back basin. Only on the eastern side of the Laga basin, in the footwall of the Montagnone thrust, were siliciclastic sediments with reworked gypsum deposited (Centamore et al., 1993). Therefore, the westward plunge of the Gran Sasso frontal fold can be possibly related to this late stage of deformation within the Laga basin. In this reference frame, the CCW (~ 55°) rotation of the shortening direction can be related to local redistribution of the strain after the underthrusting of the Montagnone structure during the middle Messinian and the effect of tilt toward the foreland in the area of Pizzo d'Intermesoli–Monte Corvo. In the cross-section of Fig. 5b, along the footwall of the Gran Sasso Unit, the lower thrust dips gently to the north like the inner flank of the recumbent frontal fault-propagation fold. Here, the lower thrust cuts the upright flank of the return syncline, accommodating a displacement on the order of 1.5 km, considering the off-set of the Calcare Massiccio. In contrast, the Cretaceous and Cenozoic (basinal) succession is folded around the upper thrust whose displacement grows toward the south. The pre-existing Early Jurassic faults and the Oligocene–Miocene Tre Selle Fault were not reactivated but were rather cut and passively transported during thrusting. In this case, oblique normal faulting is

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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Fig. 21. The backlimb of the Gran Sasso anticline, visible along the Intermesoli cross-section, is dipping toward the foreland to the north. This geometry is inconsistent with fault-propagation fold kinematics (top panel). Three possible scenarios could explain this setting: 1) the entire Late Miocene structure has been tilted by a deeper in sequence backthrust associated with a triangle zone, as interpreted in Fig. 20; this structure also affects the syn-folding strata and it has been later cross-cut by the Quaternary normal fault. 2) Footwall uplift associated with Quaternary normal faults. 3) A duplex antiformal stack where the upper thrust is folded as well as the outcropping shallow anticline; a normal fault finally cross-cuts the structure. We hypothesize that scenario 1 is more consistent with the local and regional data, including the steep cut-off angle of the normal fault, suggesting a pre-normal fault tilt, and the dip of the syn-folding flysch deposits in the footwall of the earlier thrust. The flysch shows a constant dip toward the foreland, suggesting the occurrence of a backthrust at depth affecting the entire structure, later cross-cut by the normal fault.

explained by a local deviation of this fault from the main direction of extension (NE–SW) as is common in the Apennines. According to our reconstruction, the late Neogene compression was characterized by: 1) the NNE-verging fault-propagation folding (Gran Sasso anticline) associated with a minor back-thrust, followed by an inferred one; and 2) a deeper triangle zone, whose backthrust determined the northward tilting of the shallower anticline and related footwall syncline (Fig. 20). The whole structure shows no sign of sub-aerial erosion during the greatest part of Neogene thrusting and folding indicating that the regional subsidence was stronger than uplift by folding. Indeed, there is no record of carbonate resediments in the foredeep that would indicate syn-orogenic erosion during the first two stages. The latest compressive event recorded by the Rigopiano Conglomerates in the east of the Gran Sasso Salient could be coeval with the back-tilt of the western

Gran Sasso–Monti della Laga, and with the under-thrusting of the Montagnone structure. In the cross-section of Fig. 5c, the lower thrust terminates within the Paleogene formations, whereas the upper thrust does not show any relevant displacement. However during the Quaternary the Tre Selle Fault was reactivated. The Cenozoic part of the displacement can be easily recognized. Along the western Tre Selle Fault it grew from about 130 m in section b) to 270 m in section a) of Fig. 5. In this section, the Assergi–Valle Fredda Fault accommodates almost 1400 m of apparent displacement. 6.1.4. Retrodeformation of Neogene structures We propose a structural model based on field data and a subsurface interpretation down to a few kilometer depth based on the geometrical

Fig. 20. Kinematic retrodeformation of the geological section a) of Fig. 5. The balancing technique does not account for the horizontal 3D component due to the left-lateral transpression during shortening and right-lateral transtension during the final stretching. Starting from the bottom: in the Middle Miocene the area likely represented the Apennines foreland with the inherited rifting-related Mesozoic–Cenozoic normal faults, and the paleo-high of the Gran Sasso–Corno Grande area. During the Messinian the overturned Gran Sasso anticline developed associated with a décollement in the Triassic evaporites, ramping upward into the overlying Mesozoic–Cenozoic sequence. Messinian growth strata of the Laga Flysch constrain this dating. At a later stage (Middle–Late?) Pliocene, a backthrust associated with a triangle zone tilted the entire structure, determining a northward dip of both the backlimb of the Gran Sasso anticline and the Laga Flysch in the footwall of the earlier thrusts. During the Late Pliocene(?)–Quaternary normal and right-lateral transtensional faults cross-cut the whole pre-existing architecture. Legend as in Fig. 5. Heavy lines: faults active during evolution of the respective tectonic stage. The shallow fault-propagation Gran Sasso fold accounts for about 7 km shortening. The buried back-thrust may add a similar amount, but the footwall cut-off is unconstrained. The Plio(?)-Quaternary stretching of the section is about 1.7 km, being the Tre Selle Fault documented as active.

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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relationships at the surface. In the geological cross-section of Fig. 5a, borehole and tunnel excavation data from Catalano et al. (1986) have been integrated with our surface field data. Unfortunately, no seismic data which could help in the construction of the depth section are available. The section has been drawn along the mean direction of transport, based on the orientation of mesoscopic folds and outcrop-scale structures, such as S–C fabrics, slickensides, duplexes, and major and minor reverse faults. The resulting section (Fig. 20) has been restored by hand by performing the following procedures: (1) kinematic inversion of the movement along faults according to the amount of displacement; and (2) unfolding folds assuming flexural slip. Pin lines were located along zones of no inter-bed slip and the back-restoration has been performed by using the line length method. The sequence of restoration follows the three main stages of deformation, as discussed in the previous chapters. Each stage represents the restoration of the geometries prior to the next tectonic ‘phase’. These are, in the sequence of kinematic inversion from the youngest to the oldest: 1) Quaternary normal faulting; 2) tilting to the north of the Gran Sasso–Laga structure and uplift; and 3) folding and thrusting during deposition of the Laga Flysch. As can be observed in geological cross-section (Fig. 5b), the strikeslip component both in thrusting and in later normal faulting does not allow 2D geometric analysis; 2D restorations commonly assume that there is no movement of material into or out of the plane of the section. Nevertheless, a semi-quantitative trial is attempted in order to produce a first back-restoration model for the western Gran Sasso range (Fig. 20). Our retrodeformation gives a conservative value for shortening along the section of ~ 7.5 km (Fig. 20). In a first stage of thrusting, 1) the frontal fold started to form together with the upper thrust and the steep backthrust in the southern sector, a pop-up geometry. Later, 2) the thrusts related to the Montagnone Structure were active. Those thrusts are east vergent and doubled the Miocene succession in the northern part of the section. In a later stage 3), probably related to the younger vein set and to the folding event of the Rigopiano Conglomerate, the Gran Sasso lower thrust juxtaposed the Gran Sasso unit on the Montagnone structure and Laga Unit. However, the total shortening is unconstrained due to the scarcity of information about the buried thrusts and back-thrusts, whose displacement has to be as large or larger than the length of the ramp (on the order of about 10 km). Tilting of the unit toward the foreland during the early Pliocene caused uplift of the western Gran Sasso and initiated the sub-aerial erosion of the already formed upper structure during the middle–late Pliocene, as suggested by Centamore and Nisio (2003). The structure of Gran Sasso was uplifted and eroded during the Pliocene pp.–early Quaternary (Fig. 20). Most probably, 3.5–4.0 km of crust were eroded in less than 1 Myr, with a mean post-thrusting uplift rate of ~ 0.37 cm/year. The event of back-thrusting preceded the onset of Quaternary extension and probably contributed to the uplift of the structure (Fig. 21). We suggest three possible options to explain this setting: 1) i.e., the entire Late Miocene structure has been tilted by a deeper, in-sequence back-thrust associated with a triangle zone affecting also the syn-folding strata; the whole contractional structure has been later cross-cut by the Quaternary normal fault (option 1, Fig. 21); 2) it is the effect of the footwall uplift due to the Quaternary normal fault (option 2, Fig. 21); and 3) it is the surface expression of a deep antiformal stack eventually cross-cut by normal fault (option 3, Fig. 21). In this last option, the foreland dipping thrust is a folded and tilted fault, suggesting a “normal” displacement. Therefore, we hypothesize that option 1 is more consistent with the data (including steep cut-off angle of the normal fault), suggesting a pre-normal fault tilt, and the dip of the syn-folding flysch deposits that show a constant dip toward the foreland, suggesting the occurrence at depth of a backthrust affecting the entire structure, later cross-cut by the normal fault. Our retrodeformation gives ~ 2 km of extension between the early Quaternary–Today. The horizontal component of the right-lateral transtension is poorly constrained, but it is supported by the oblique (WNW) trend of the faults relative to the regional NE–SW

extension, the pitch of the slicken-fibers and, in some cases, by the juxtaposition of stratigraphic sequences of variable thickness in the two walls of the faults. 6.2. Structural events recording northern Gondwana dismembering and later accretion The Gran Sasso massif represents a morphologic and structural high due to the presence of a Mesozoic paleo-high caused by syn-rift to postbreak-up tectonics (Fig. 20), the study of which allows to define the dismembering of northern Gondwana through the Mesozoic and later in the Cenozoic (Nance et al., 2014). Considering the structural location, the range in study is now at the apex of a NE-verging salient. Moving along the E–W limb of the salient where a left-lateral transpression has been documented (see above), the Miocene–Pliocene shortening increases from west to east. The opposite is true for the Quaternary right-lateral transtension, which increases to the west, implying different levels of décollement for the compressive and extensional structures. In our opinion, the result of the syn-sedimentary tectonic evolution determines the geometry of the ensuing fold-and-thrust structure. This is testified by the reduced and discontinuous sequences overlying the Corno Grande paleo-high, where a Paleogene conglomerate is sitting on top of Early Jurassic platform sediments (Scarsella, 1955; Adamoli et al., 2012), the Acqua San Franco paleo-high, and the Campo Pericoli plateau. Therefore, the more condensed sequences representing Mesozoic structural highs are located in the most structurally and morphologically elevated areas during the growth of the accretionary prism and that is due to the lateral ramp related to the preorogenic and syn-sedimentary westward tilt (Fig. 7). Our reconstruction of the geological history of the Gran Sasso range allows us to distinguish six phases in its tectonic and sedimentary evolution: (1) Late Triassic–early Jurassic rifting determined the shape of the ‘Umbrian paleo-highs’, which persisted into the early Jurassic (Acqua San Franco) or even into the late Cretaceous–early Paleogene. These paleo-highs were interspersed with deeper base-ofslope areas that were swamped by carbonate detritus from the adjacent Lazio–Abruzzi platform. The paleo-highs were bounded by active fault scarps that were partially eroded, overlapped by pelagic and turbiditic basinal sediments and partially obliterated by later tectonic events. (2) Local westward tilt of the post-break-up Jurassic to early Paleogene formations to the west of the Corno Grande Paleo-high. This tilt could indicate ongoing extensional tectonics and/or inherited submarine relief combined with differential compaction of the basinal sediments. (3) Oligocene and middle Miocene syn-sedimentary extension initiated a new generation of faults (western Tre Selle Fault) and possibly a partial reactivation of its eastern segment. (4) Messinian NNE-directed forward thrusting of the calcareous units onto the siliciclastic deposits with a growth anticline. In the east (Corno Grande–Campo Pericoli) the basal décollement appears to follow the Triassic evaporites. Moreover, balancing techniques, although limited by the left lateral transpressive component along the E–W trending segment of the Gran Sasso salient, indicate that the thrust ramps should flatten out at the Triassic level. Toward the west (Pizzo d'Intermesoli, Monte Corvo), the décollement must ramp up across the stratigraphic section in order to produce the frontal folds in the younger Jurassic–middle Miocene succession. Therefore, the pre-existing stratigraphic architecture and the related differences in competence between paleo-high and basinal successions controlled the Late Miocene–Pliocene thrusting and folding. The geometry of the frontal Gran Sasso anticline varies across the N–S trending

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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transfer zone occurring along the Mesozoic fault-related western margin of the Corno Grande Paleo-high. The thrust front is characterized by CCW block rotation, where the different mechanics of the more competent paleo-high successions in the east and of the less competent basinal successions in the west determined the geometries of folds and thrusts. (5) Earliest Pliocene tilt and back-thrusting toward the foreland to the north, which involved the western edge of the Gran Sasso–Laga front together with the inner flank of the faultpropagation fold. This structure is possibly related to the formation of a subsurface triangle zone and its uplift and with the second event of contraction affecting other regions of the external Apennines (i.e. Rigopiano, Fig. 1). (6) Post-shortening extension (Quaternary to present day), which may have locally intercepted pre-existing synsedimentary faults that had been passively transported within the thrust sheets. The NE–SW extension is expressed by orthogonal normal faults or WNW–ESE-trending right-lateral transtensional faults (e.g., Tre Selle and Campo Imperatore faults). Examples of inversion tectonics involving the basement are documented in the Taiwan forebelt (Camanni et al., 2014), in the northern China craton (Wang et al., 2013) and in Egypt (Fowler and Osman, 2013). Another classic example of inversion is recognized in the Permian trough of the Glarus Verrucano of the Alps (Pfiffner, 2014). However, despite previous interpretations, new field mapping in the study area did not show recognizable reactivation or inversion of the Mesozoic normal faults (e.g., Fig. 14), although we cannot exclude that this mechanism could have partly occurred. There is no outcropping evidence of inversion tectonics, i.e., a thicker basinal sequence uplifted in a structural high of the accretionary prism and passive margin-related normal faults reactivated by thrusting. On the contrary, the highest structural and morphological elevation of the Apennines along the Gran Sasso salient occurs along a Mesozoic paleo-high (Figs. 6, 13, 20), not along a paleo-basin. Moreover, the inherited normal or transtensional Mesozoic faults are passively cut and transported by thrust sheets. A discussion of the amount of basement involved during the inversion from the Mesozoic passive margin architecture to the Neogene– Present accretionary wedge and related back-arc extension evolution is beyond the scope of this research. However, similar to what was observed elsewhere (e.g., Booth et al., 2004; Kaneko et al., 2007), the basal décollement of the prism is located within the sedimentary cover atop the basement (Bigi et al., 2003). Since the Apennines are the result of a Wdirected subduction zone where the sole thrust is shallower than along the orogens related to the opposed E- or NEdirected subduction zones (e.g., Doglioni et al., 2007), the amount of basement involved can be estimated by the volumes accreted in the hangingwall of the subduction zone. The basement does not outcrop in the study area and seismic reflection profiles of the external sectors of the Apennines do not show basement involvement (e.g., Doglioni et al., 1999a, b; Patacca et al., 2008). Moreover, area balancing limits the involvement of the metamorphic basement (e.g., Bianchi et al., 2010), if any, only to its shallowest layer (Bigi et al., 2003; Lenci et al., 2004; Scrocca et al., 2005). 7. Concluding remarks The Gran Sasso range records the steps of dismembering of the Gondwana supercontinent and its later involvement into the Apennines accretionary prism. The Apennines formed in the hangingwall of a Wdirected subduction zone, where the slab hinge diverges relative to the upper plate (Devoti et al., 2008) and represent an archetype of an

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accretionary prism associated to the subduction zones having this geographic polarity. Both the accretionary prism and the following backarc extension to the west migrated E-ward. The passage of this tectonic wave is recorded in the Gran Sasso range (e.g., Fig. 14). Although the stratigraphy and the rates of extension and contraction change along the strike of the Apennines, the evolution of the study area can be taken as a reliable example of the tectonic evolution of the whole chain. The Gran Sasso salient formed due to the offscraping of the sedimentary cover from the westerly subducting Adriatic plate beneath the Apennines. The stratigraphic setting of the Adriatic cover outcropping in the study area was influenced by synsedimentary normal faults of different ages. The resultant heterogeneous non-layer caked setting influenced the later Neogene kinematics and geometries. The regional high elevation of the central Apennines can be explained by the deeper location of the basal décollement plane with respect to other segments of the Apennines (e.g., in Calabria, Lenci et al., 2004). Moreover the lower erodibility of the limestones may have also played a role. The Gran Sasso morphologic peak represents the sum of all these parameters. However, the short wavelength of this morphologic and structural high supports the dominant effect of the inherited Mesozoic horst. Quaternary extensional tectonics from the Gran Sasso Range continues to the west toward the Tyrrhenian backarc basin and it can be related to the retreat of the Adriatic slab relative to the European upper plate. Since the subduction hinge has been moving away relative to the upper plate, the upper plate was and still is not converging relative to the lower plate. In this setting the Apennine accretionary prism formed along the subduction hinge, that while retreating set a rate of subsidence that was faster than the uplift of the fold and thrust belt. This is the main reason why the prisms associated to W-directed subduction zones are mostly below sea-level and are accompanied by deep trenches or foredeeps (Doglioni, 1994) that may be filled or unfilled as a function of the sediment supply (Garzanti et al., 2007). Therefore, the Gran Sasso salient represents a case history not only for the Apennines, but also for similar subduction settings in which i) the subduction hinges diverge relative to the upper plate; ii) the accretionary prism forms at the expenses of the lower plate alone; and iii) the accretionary prism is eventually uplifted and cross-cut by the backarc extensional wave.

Acknowledgments Daniel Bernoulli gave us a fundamental contribution to unravel the stratigraphy of the Gran Sasso area. We also thank him for the repeated critical reading of the manuscript. Adrian Pfiffner, Giancarlo Molli, and François Roure and Glen Stockmal provided very helpful reviews and suggestions. Neil Mancktelow and Jan Pleuger are thanked for supporting a preliminary version of this manuscript. We are grateful to Alexandra Kushnir for improving the English. Danilo Seccia, Christian Montanaro, Daniele Girasoli, Jara Schnyder, Danilo Mangiacapra and Simone Arragoni are sincerely thanked for their support in the field. A special thank goes to Johannes Pignatti and Silvia Spezzaferri for fossil determinations and to the Forestry Guard of Pietracamela and administration of the Cable Car of Assergi for logistical help.

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Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009

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G.L. Cardello, C. Doglioni / Gondwana Research xxx (2014) xxx–xxx Giovanni Luca Cardello is a Postdoc Researcher at the Orléans University in France since 2013. He received his BSc (2006) and MSc (2008) from the Sapienza University of Rome, and PhD (2013) from the Swiss Federal Institute of Technology ETH-Zürich. A field geologist, his primary research interests focus on tectonics and structural geology of some Mediterranean chains, e.g. Apennines, Corsica, Cycladic Islands, Helvetic Alps, and their mechanisms of exhumation from different depths, where syn-sedimentary features constitute the heterogeneities onto which deformation localizes in further deformation events.

Carlo Doglioni is a Professor of geology at the Sapienza University of Rome. He is field geologist, seismic interpreter and modeler of geodynamic processes, and member of the Accademia dei Lincei and the Academy of Europe. He is also a recipient of the Spendiarov Prize of the Russian Academy of Sciences and Wegener Award of the EAGE.

Please cite this article as: Cardello, G.L., Doglioni, C., From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy), Gondwana Research (2014), http://dx.doi.org/10.1016/j.gr.2014.09.009