TECTONICS, VOL. 24, TC4019, doi: /2004tc001771, 2005

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1 TECTONICS, VOL. 24,, doi: /2004tc001771, 2005 Paleomagnetism of the Gran Sasso range salient (central Apennines, Italy): Pattern of orogenic rotations due to translation of a massive carbonate indenter Sara Satolli Dipartimento di Scienze della Terra, Università G. D Annunzio, Chieti Scalo, Italy Fabio Speranza Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Fernando Calamita Dipartimento di Scienze della Terra, Università G. D Annunzio, Chieti Scalo, Italy Received 29 November 2004; revised 20 April 2005; accepted 17 May 2005; published 30 August [1] We report on the paleomagnetism (39 new sites) of Gran Sasso, an indenter-controlled salient of the external central Apennines formed by two orthogonal limbs. We find that Gran Sasso is a complex salient, paleomagnetically corresponding to neither a nonrotating nor an oroclinal end-member. Data from the core of the arc show that the indenter itself did not undergo any rotation. Conversely, rotations of variable magnitude and sign are observed along the curveshaped thrust fronts. Rotations are virtually absent at both end points of the arc. Moving toward the apex, progressively greater counterclockwise (CCW) and clockwise (CW) rotations occur along the E-W and N-S fronts, respectively. The rotations increase continuously and more than linearly, reaching their maximum values (80 CCW and 50 CW) around the apex. Here, the more strongly CCW and CW rotated fronts are separated by a domain characterized by local block rotations. The inequality between the maximum CCW and CW rotation values is likely a consequence of the asymmetry of the indenter displacement direction (N70 E) with respect to the preorogenic trends of its margins (E-W and N-S). From an oroclinal point of view, the fronts close to the end points virtually define a nonrotating arc, while the pattern around the apex is similar to that of an orocline. We conclude that close to end points, nonrotational thrusting normal to the indenter margins occurred, while in the vicinity of the apex, the peritidal carbonates acted as an ice breaker, pushing apart (and strongly rotating) the weaker multilayer located ahead. Citation: Satolli, S., F. Speranza, and F. Calamita (2005), Paleomagnetism of the Gran Sasso range salient (central Apennines, Italy): Pattern of orogenic rotations due to translation of a massive carbonate indenter, Tectonics, 24,, doi: /2004tc Copyright 2005 by the American Geophysical Union /05/2004TC001771$ Introduction [2] Curve-shaped segments of orogenic belts have received great paleomagnetic attention in the past. Significant vertical axis rotations have been revealed at the limbs of arcs, and several models have been proposed to explain the rotational-structural patterns documented. It is desirable to find clear correspondence between different tectonic settings and their characteristic rotational patterns. Such unequivocal evidence would make paleomagnetism a powerful proxy to unravel the tectonics and geodynamics of curved orogenic systems of controversial origin. [3] The size of a natural bend seems to be an important factor for discriminating its geodynamics. In fact, it has been proposed that the evolution of regional-scale (on the order of hundreds or thousands of kilometers) orogenic bends is mainly controlled by lithospheric-scale phenomena [Beck, 1998; Platt et al., 2003; Hall et al., 2004; Jolivet and Faccenna, 2000]. Conversely, frictional processes occurring at shallow crustal levels are believed to dictate the kinematics of small-scale arcs developed in orogenic wedges [Davis et al., 1983]. Here, arcs arise from several factors and processes occurring in the thrust belt-foreland basin system, such as the impingement of the wedge with foreland obstacles, the translation of a rigid hinterland indenter within weaker external successions, the lateral variation in the thickness of a predeformational basin fill, and the lateral variation in the strength of a detachment surface [Marshak, 1988; Macedo and Marshak, 1999]. Considerable paleomagnetic evidence has been gathered in the past of the rotational pattern of small-scale arcs (mainly salients) formed after collision of the orogenic wedge with foreland buttresses. Conversely, the arcs formed in response to other tectonic settings have not been paleomagnetically studied, and their rotational behavior was solely inferred relying upon the results of sandbox analogue experiments [Zweigel, 1998; Macedo and Marshak, 1999; Lickorish et al., 2002; Costa and Speranza, 2003]. [4] In this paper we report on the results of a detailed paleomagnetic sampling (33 new reliable sites) carried out 1of22

2 in the Gran Sasso range, a small-scale (20 km of radius) narrow hinge salient (with an interlimb angle of 90 ) located in the external central Apennines. Gran Sasso is a highrelief salient of carbonate strata in the hanging wall of a propagating thrust system, entering into a less competent basinal multilayer. Therefore our results may serve as the first evidence of the paleomagnetic rotation pattern occurring when a kilometer-scale hinterland indenter pushes into relatively weaker foreland successions, giving rise to a range salient. 2. Previous Paleomagnetic Evidence From Curve-Shaped Orogens [5] Most of the regional-scale arcs located above slabs of subducting oceanic lithosphere, showed oroclinal-type rotations [e.g., Eldredge et al., 1985; Marshak, 1988], when paleomagnetically investigated. Yet, a variety of different geodynamic explanations was used to explain the observed bending process. In the Andean margin of South America, a clear rotation change occurs corresponding with the Arica deflection, marking also a sharp change of the orogenic trends. The paleomagnetic structural evidence from the Andean reentrant was explained in several different ways, and related to different shortening rates, local block rotations in response to shear-driven subcrustal ductile flow, and subduction of a spreading ridge or an island arc [e.g., Beck, 1998, 2004; Taylor et al., 1999]. In the Mediterranean domain, some large-scale oroclines (such as the southern Apennines, Sicily, and the Aegean arc [e.g., Kissel and Laj, 1988; Gattacceca and Speranza, 2002]) were interpreted as due to the passive sinking of narrow oceanic lithosphere slabs inducing back arc spreading and synchronous belt migration and bending [Faccenna et al., 2001]. Conversely, the oroclinal pattern of the Betic-Rif arc (encircling the Alboran sea, another Mediterranean back arc basin) was related to the interaction between the northward translation of the Alboran domain and spreading of the basin itself [e.g., Platt et al., 2003]. In New Zealand, the great magnitude clockwise (CW) rotations measured in the south Island orocline have been recently related to the rotation of elongate blocks in response to continuous deformation in the underlying lithospheric mantle [Hall et al., 2004]. [6] Other great bends arising from continental collision do not show a clear oroclinal rotational pattern, thus significantly differing from arcs located at the continentocean boundary. The western Alps arc is characterized by ubiquitous counterclockwise (CCW) rotations [Thomas et al., 1999; Collombet et al., 2002], but its oroclinal character [see Eldredge et al., 1985] has not been quantitatively evaluated. Moreover, the relative importance of the factors possibly inducing rotations (i.e., Adriatic hinterland rotations, shear zones, gravitational spreading body forces, back arc opening of the Ligurian Sea) has not been clarified yet. Insignificant rotations were documented in the Jura arc, an intracontinental thrust belt from northern Europe, revealing that it can be considered a primary (nonrotational) arc [Hindle and Burkhard, 1999]. Several paleomagnetic data sets gathered along the great Himalayan salient show predominant CW rotations, and a pattern incompatible with an oroclinal bending model [Appel et al., 1991; Schill et al., 2001, 2003]. [7] At a smaller scale, several arcs (mainly salients) formed after collision of the orogenic wedge with foreland obstacles have been paleomagnetically investigated. As a rule, such salients reveal an oroclinal-type rotation pattern, as shown for the Wyoming-Idaho belt [Schwartz and Van der Voo, 1984] and the southern Pyrenees [Sussman et al., 2004]. The rotational nature of the northern Apennine arc remains controversial, as some work basically supports a primary nature [Muttoni et al., 1998], while Speranza et al. [1997] have suggested the oroclinal bending of the outer part of the arc. An oroclinal pattern has also been documented for the Cantabria-Asturias arc, a puzzling horseshoe-shaped reentrant from northern Spain with an interlimb angle of 180 [Weil et al., 2000]. The only salient formed in between foreland obstacles which paleomagnetically correspond to a nonrotational arc is the Gela Nappe of southern Sicily. Here, a uniform regional-scale CW rotation was observed, without any significant difference in the salient interiors and along its limbs [e.g., Channell et al., 1990; Scheepers and Langereis, 1993; Costa and Speranza, 2003; Speranza et al., 2003b]. 3. The Apennines: The Final Outcome of a Mediterranean Arc Back Arc Eastward Migrating System [8] The Apennine belt developed in Tertiary times as a consequence of the eastward roll-back of an Adriatic Ionian lithospheric slab passively sinking in the mantle [Malinverno and Ryan, 1986]. The passive subduction process gave rise to the synchronous spreading of back arc basins (Liguro- Provencal and Tyrrhenian seas), and the eastward (on average) migration of the Apennine wedge (and intervening microplates, such as Corsica-Sardinia [Alvarez et al., 1974]). From late Oligocene to mid-miocene, spreading of the Liguro-Provencal sea was accompanied by the eastward drift and 50 CCW rotation of both the Corsica-Sardinia block and the adjacent Apennine belt [Gattacceca, 2001]. At circa Ma (late Burdigalian early Langhian), Corsica- Sardinia stopped drifting (and rotating), and the locus of back arc extension jumped eastward, within the incipiently spreading Tyrrhenian Sea. It is still a matter of debate whether this major event was due to mantle dynamics [Faccenna et al., 2001], or to the collision of the migrating wedge with a peritidal carbonate platform corridor [Gattacceca and Speranza, 2002]. During mid-late Miocene to late Pliocene, back arc spreading of the Tyrrhenian Sea occurred synchronously with northeastward shortening in the Apennines [Mascle et al., 1988; Patacca et al., 1990]. It has long been well documented [Elter et al., 1975] that during such time span the Apennines were characterized by coeval occurrence of normal and thrust faults along the western and eastern belt margins, respectively. As a consequence, both synrift sediments (mostly deposited within intermontane basins) and foredeep sequences (later incorporated into the frontal thrust structures) are exposed in the Apennine belt. 2of22

3 Figure 1. permission. Structural map of the central Apennines, modified from Calamita et al. [2004] with [9] The central Apennines are the result of this latest (late Tortonian late Pliocene) evolutionary stage of the Tertiary Mediterranean arc-back arc system (Figure 1). Here late Tortonian to mid-pliocene foredeep turbidites (progressively younger moving eastward [e.g., Cipollari et al., 1999]) are exposed in the footwalls of several imbricate thrust sheets, mainly formed by Mesozoic carbonates. Similarly, latest Messinian to late Pliocene synrift sequences are spread within the belt, and mostly crop out within intermontane continental basins. Extension (and related deposits) continued also during Pleistocene-Holocene (and to date), likely induced by the final postorogenic collapse of the belt. [10] Pn [Mele et al., 1996] and shear wave attenuation [Mele et al., 1997] beneath the central northern Apennines 3of22

4 suggest the presence of asthenospheric material (rising obliquely from the Tyrrhenian back arc region) up to km beneath the belt. Relying on this evidence, it has been suggested that the extensional-compressive pair of the central Apennines has been driven by an eastward mantle flow [Carminati et al., 2004], or by an asthenospheric corner flow above the Adriatic subducting slab in retrograde motion [Cavinato and De Celles, 1999]. Seismic reflection and borehole data from the Adriatic coast of central Italy show that compression stopped in late Pliocene times along the central Apennine front [Bally et al., 1988; Bolis et al., 2003]. This chronological datum, together with the evidence of an upper mantle window within the subducted Adriatic slab as documented by seismic tomography data [Lucente et al., 1999], indicates that subduction at the central Apennine trench has stopped a few million years ago. 4. Geological Setting of the Gran Sasso Salient [11] Gran Sasso is one of the most external salients of the central Apennines. It displays a striking curved shape, characterized by two roughly orthogonal E-W and N-S limbs (Figure 2). The two limbs are virtually rectilinear, and are separated by a narrow arc apex. Inside the arc, predominantly Mesozoic-Tertiary marine carbonates are exposed [Servizio Geologico d Italia, 1963], at an average altitude of m above sea level (Corno Grande, located at midpoint of the E-W salient front, is the highest top of the Apennines (2912 m)). Conversely, Messinian lower Pliocene siliciclastic turbidites crop out at lower altitudes north and east of the carbonate salient. [12] Paleogeographically, the Mesozoic to Tertiary carbonates of Gran Sasso belong to both a persisting peritidal carbonate platform domain (Latium-Abruzzi platform) and to the slope connecting such shelf to a basin deepening northward (Umbria-Marche basin) and eastward (Figure 2). The slope and the basin formed in lower-mid-liassic (early Jurassic) times, as a consequence of a regional extensional tectonic event, which partially drowned a regional-scale peritidal carbonate platform, previously thriving in the whole central northern Apennines domain [Cantelli et al., 1978]. Though the sedimentation of shallow water carbonates during late Triassic early Liassic times was virtually ubiquitous, local small-scale basins have been exceptionally documented, and one of them is exposed at Monte Camicia [Adamoli et al., 1990] (Figure 2). [13] The sedimentary successions of the slope domain at Gran Sasso display highly variable characteristics and thicknesses, and are formed by both thin-bedded pelagic strata and massive carbonate-turbidite bodies slid from the southern Latium-Abruzzi platform. Slumps and resedimented beds are common, as well as preorogenic seamounts bounded by Liassic normal faults, topped and surrounded by condensed successions displaying frequent hiatuses. Though strictly speaking Gran Sasso is not entirely formed by peritidal shelf carbonates, its overall character is that of a thick (4 km) and rigid carbonate indenter entering into an outer less competent basinal domain, consisting of a calcareous, clayey, and siliceous turbidite multilayer. Field data, in fact [Adamoli, 1992; Calamita et al., 2002, 2003b; Speranza, 2003], show that the salient thrust fronts roughly follow the trends of the preorogenic normal Liassic faults, which bounded the shelf and the shallower slope, and separated them from the outer deep basin. A paleogeographic (preorogenic) greater relief of the Gran Sasso indenter with respect to the surrounding basins is confirmed also by the distribution and thicknesses of the Messinian turbidites and Pliocene terrigenous deposits related to foreland basins [Servizio Geologico d Italia, 1963; Calamita et al., 2003a]. These deposits are ubiquitous and very thick (3 4 km) around Gran Sasso (Figure 2), but are not found within the salient (not even within tectonic grabens, where they should have been preserved from erosion). Lower Messinian turbidites crop out again just southwest of the salient, near L Aquila (Figure 2), but their overall thickness does not exceed a few hundreds of meters. The shelf carbonate salient is not completely massive, as it is cut by several normal faults, active since the early Pleistocene. However, thrust faults are visible only SW of the salient and do not cut its core (Figure 2), so that its behavior during the Messinian-Pliocene shortening episodes may be, at first order, compared to that of a rigid body entering into less competent outer basinal sediments. [14] The exposed thrust sheets forming the Gran Sasso salient fronts arise from tectonic stacking of slope domains, paleogeographically interposed between the carbonate indenter and the outer deep basin. At present, there is not an overall consensus about the number of thrust faults forming the Gran Sasso fronts, and on the geometry of the imbricate stack. Along the E-W limb, there is clear evidence for a lower thrust fault located at the range toe (never cropping out but crossed by a highway tunnel, Figure 3b), and for an upper thrust, spectacularly exposed along steep northward facing slopes and walls between Corno Grande and Monte Camicia. Along the E-W Gran Sasso thrust front, the amount of shortening increases eastward. In fact, both the lower and the upper thrust faults evolve, west of Monte Corvo, into a frontal fold of the range (Figures 2 and 3a). The upper thrust, where exposed, shows a 1 2 km displacement [Adamoli, 1992; Ghisetti and Vezzani, 1986a, 1986b, 1990, 1991; Calamita et al., 2002, 2003b]. The lower thrust, located at the range toe, accommodates an uncertain amount of shortening, though along the highway tunnel a minimum shortening of about 8 km may be calculated [Costruzioni Generali Spa Milano, 1979] (Figure 3b). [15] At Monte Camicia (Figure 3c), the upper thrust undergoes an abrupt along-strike eastward plunge of 1 km, joining downward with the lower thrust. The N-S Gran Sasso limb comprises some carbonate slices, bounded by splay thrusts, which extend further east of the main exposed front. These slices are exposed (Colle Madonna, Monte La Queglia, Figures 1 and 3d) or buried beneath the Messinian-Pliocene turbidites [Bigi et al., 1995a, 1995b]. [16] South of the upper thrust, there are other south dipping faults, juxtaposing younger-on-older carbonates. Ghisetti [1987] and Ghisetti and Vezzani [1986a, 1986b, 1991, 1997] interpret these faults as out-of-sequence thrust 4of22

5 Figure 2. Geological map of the Gran Sasso range. Previous paleomagnetic data: (1) paleomagnetic declinations from Dela Pierre et al. [1992] and Sagnotti et al. [2000] and (2) paleomagnetic rotations at Corno Grande from Speranza et al. [2003a] (site names are omitted). Sampled sites are (a) Eocene(?) mafic sill, (b) lower Pliocene clays, (c) Messinian clays, (d) Paleocene-Oligocene Scaglia limestones, (e) middle-upper Cretaceous Scaglia limestones, (f) upper Lias-Dogger limestones, and (g) upper Triassic dolomites. The age of sites GS16, GS21, GS24, and GS25 was extrapolated according to the available geological maps from Servizio Geologico d Italia [1963], Vezzani and Ghisetti [1993], and Bigi et al. [1995b]. The age of the remaining 35 sites was detailed by new biostratigraphic determinations provided by M. Chiocchini. 5of22

6 Figure 3. Geological sections across the Gran Sasso thrust system, modified from Calamita et al. [2002, 2003b] with permission, for (a) Monte Corvo; (b) Monte Aquila (highway tunnel); (c) Monte Camicia; and (d) Colle Madonna. See Figure 2 for location. 6of22

7 surfaces, separating eight different thrust sheets. Conversely, Adamoli et al. [1990], D Agostino et al. [1998], Calamita et al. [2002, 2003b], Speranza et al. [2003a], and this work suggest an extensional nature for these tectonic contacts. [17] Though at first approximation, both the E-W and N-S limbs of the Gran Sasso arc seem to be characterized by predominantly dip-slip thrust fronts [Ghisetti, 1987; Ghisetti and Vezzani, 1986a, 1986b, 1990, 1991; Bigi et al., 1995a, 1995b; Calamita et al., 2002, 2003b], a thorough description of the slip direction all along the frontal thrust faults of the salient is lacking at present. Therefore a direct comparison with the displacement vectors observed in analogue models [Zweigel, 1998; Lickorish et al., 2002] for indentercontrolled salients (which could help understanding of the indenter s kinematics) is not possible. [18] In any case, the preorogenic indenter s boundaries are roughly parallel to the salient s fronts, i.e., E-W and N-S (in present-day coordinates). Conversely, the evaluation of the translation direction of the indenter is not straightforward, as the kinematic trajectories inside and outside the salient describe strongly irregular paths and sudden changes from northward to eastward shortening directions [e.g., Ghisetti and Vezzani, 1997]. However, Gran Sasso is one of the most external carbonatic structures of the central Apennines, and the exposed and buried fronts located north and east (displaying synchronous or slightly younger deformation), show regular subparallel directions, and follow a rather constant N20W trend [Bally et al., 1988; Consiglio Nazionale delle Ricerche, 1991; Bolis et al., 2003]. Moreover, the E-W Gran Sasso limb is stacked over N-S to N20 W trending thrust fronts (Monte dei Fiori, Montagnone, Figure 1), which did not rotate [Speranza et al., 1997] and underwent N70 E shortening during the latest Messinian early Pliocene [Mattei, 1987; Averbuch et al., 1995; Calamita et al., 1998]. Therefore we infer that the average direction of displacement of the indenter may be approximated to N70 E, which is normal to the N20 W thrust fronts located north and east of the salient. Relying on this assumption, Gran Sasso may be characterized as a salient formed after a slightly oblique or diagonal (to follow the terminology of Zweigel [1998] or Lickorish et al. [2002], respectively) indenter displacement. The indentation angle, i.e., the angle between the advance direction (approximately N70 E) and the normal to the frontal face of the indenter (the N-S Gran Sasso limb), is around 20. Zweigel [1998] suggests that similar small values of the indentation angle translate into a small radius of salient curvature, in agreement with the very narrow hinge observed at Gran Sasso (Figure 2). [19] The Messinian lower Pliocene age of both the turbidites exposed in the footwall of the salient and some thrust top deposits exposed close to the arc apex (Figure 2) indicates that thrusting occurred at Gran Sasso from upper Messinian to middle Pliocene times. Moreover, the N-S to N20 W thrust sheets plunging below Gran Sasso (Monte dei Fiori Montagnone, Figure 1), were internally imbricated during the latest Messinian early Pliocene [Mattei, 1987; Calamita et al., 1998]. Therefore N70 E directed shortening in the Monte dei Fiori Montagnone domain was synchronous with the earlier thrust activity at Gran Sasso, which eventually truncated and overthrust the adjacent approximately N-S belt in mid-pliocene times [Ghisetti and Vezzani, 1988]. [20] From early Pleistocene onward, the orogenic stack was fragmented by several normal faults (partially reactivating older thrust faults), which gave rise to some intermontane basins filled by continental deposits [D Agostino et al., 1998]. Extensionally offset moraines (deposited during the Last Glacial Maximum), and the historical seismicity recorded in the area, are both proof that the postorogenic collapse of Gran Sasso is ongoing. 5. Previous Paleomagnetic Data From Gran Sasso [21] The distinctive arcuate shape of the Gran Sasso range has stimulated previous paleomagnetic interest. Several works [Dela Pierre et al., 1992; Speranza et al., 1997; Sagnotti et al., 2000] have documented that the Messinian- Pleistocene siliceous turbidites and clays external to the salient have not rotated, implying that the rotations found within the salient itself have to be attributed to indenter s and limbs rotations synchronous with thrusting over the outer Neogene sediments. [22] Within the Gran Sasso range, Dela Pierre et al. [1992] hand-sampled eight sites from upper Cretaceous- Oligocene slope sediments ( Scaglia facies, Figure 2). They showed northward paleomagnetic declinations at three sites in the western part of the arc (west and SW of Corno Grande), and westward declinations at five sites spread in the eastern sector of the salient (Figure 2). As a consequence, they concluded that Gran Sasso had rotated homogeneously CCW by 90 around a pole located in the westernmost part of the arc, in agreement with the increase of the N-S shortening observed moving eastward along the E-W front. This model required an almost pure right-lateral shear (i.e., a quasi-absence of thrusting) along the N-S front. [23] Speranza et al. [2003a] analyzed in detail Lias to Oligocene strata and sedimentary dykes at 11 sites from Corno Grande. Consistent results from seven sites indicated the absence of rotation with respect to the Adriatic/African foreland in that point of the E-W front (Figure 2). Relying on the previous paleomagnetic evidence from Dela Pierre et al. [1992], they interpreted the Gran Sasso arc as a composite structure, made up of an unrotated domain (at and west of Corno Grande) and a 90 CCW rotated eastern salient. The great semirigid rotation of the eastern part of the chain was considered as a northward lateral escape due to the collision of two different carbonate platforms during the northeastward migration of the Apennine wedge. 6. Paleomagnetic Sampling and Analyses [24] Given the well-constrained evidence of null orogenic rotation documented at Corno Grande [Speranza et al., 2003a], we carried out a careful paleomagnetic investigation on the eastern belt salient, where the puzzling 90 CCW 7of22

8 homogeneous rotations had been previously reported by Dela Pierre et al. [1992]. Since shelf and slope-to-basin carbonates exposed at Gran Sasso have a very weak (or absent) magnetic remanence, we performed a preliminary sampling campaign to select lithologies and outcrops characterized by measurable intensity of magnetization and stable paleomagnetic directions. We hand collected samples in Triassic to lower Pliocene sediments from 142 different localities spread all over the eastern Gran Sasso salient. These samples were paleomagnetically analyzed in the shielded room of the paleomagnetic laboratory at the Istituto Nazionale di Geofisica e Vulcanologia (INGV, Rome, Italy). The natural remanent magnetization (NRM) of the specimens was measured with a 2G Enterprises DC-SQUID cryogenic magnetometer, and its stability was checked by alternating field (AF) cleaning. [25] As a result of such preliminary sampling, 39 sites were selected and sampled in detail (Figure 2 and Table 1). Most of the sites were sampled in the thin-bedded Scaglia pelagic limestones, ranging in age from midupper Cretaceous (28 sites) to Eocene-Oligocene (five sites). Furthermore, one site (GS34) was collected in upper Triassic dolomites, one (GS38) in upper Lias-Dogger pelagic limestones ( Verde Ammonitico Formation), one (GS16) in a (likely) Eocene mafic sill, and the last three in the clayey levels of the siliciclastic turbidites exposed east of the N-S carbonate front: two (GS21 and GS24) in the Messinian Laga Formation and one (GS25) in the lower Pliocene Cellino Formation. We detailed the age of all the Mesozoic to Oligocene carbonate sites relying on the microfossil assemblage analyses performed for us by M. Chiocchini on thin sections made on rock samples from the individual sites (Table 1). The age of the remaining (clayey and igneous) four sites was extrapolated according to the available geological maps from Servizio Geologico d Italia [1963], Bigi et al. [1995b] and Vezzani and Ghisetti [1993]. [26] In the selected 39 sites, we sampled 415 cylindrical samples, 25 mm in diameter, using a petrol-powered portable drill. We collected 8 14 cores (10 11 on average) from each site, spaced in different beds of the outcrop, and oriented in situ with a magnetic compass (except the igneous site GS16, oriented by both a sun and a magnetic compass). The magnetic orientation values of the cores (as well as the bedding attitude values) were corrected to account for the present-day magnetic declination at Gran Sasso (+2 in the summer 2003, according to Istituto Nazionale di Geofisica e Vulcanologia [2001]). [27] The cores were cut into standard cylindrical specimens, and their NRM was measured by the cryogenic magnetometer in the shielded room of INGV. All samples (except those from site GS16) were stepwise demagnetized by thermal heating in steps. The samples from site GS16 were magnetically cleaned by AF demagnetization in 13 steps. Demagnetization data were plotted on both orthogonal demagnetization diagrams [Zijderveld, 1967] and on equal-area projections, and the magnetization components were isolated by principal component analysis [Kirschvink, 1980]. Site-mean directions were computed using either Fisher s [1953] statistics, where only characteristic components of magnetization (ChRM) were isolated for all samples from a site, or the McFadden and McElhinny [1988] method, combining direct observations with remagnetization circles. In the clayey site GS25, where layering was not apparent in the field, we used the site-mean magnetic foliation (as observed from anisotropy of magnetic susceptibility analyses performed with a KLY-3 bridge) as proxy for the bedding plane. 7. Evaluation of the Site-Mean Paleomagnetic Directions and Field Tests [28] In the AF-cleaned site GS16, a viscous component was eliminated at 10 mt, and a ChRM was isolated in the mt interval (Figure 4a). For most of the carbonate samples, a viscous component was eliminated at 200 C (Figure 4b) or 300 C (Figure 4c), and a well-defined ChRM was isolated between C and 580 C. The samples from sites GS07 and GS08, sampled in Oligocene Scaglia limestone beds, show markedly different low-temperature (LT) and high-temperature (HT) components, after the elimination of the viscous component at 200 C (Figures 4d and 4e). In tilt-corrected coordinates, the LT component (defined between 200 C and 460 C) is upward and southeastward directed, while the HT component (observed between 460 C and 580 C) trends downward and northward. The LT component is far from the geocentric axial dipole (GAD) field direction in in situ coordinates, so that it cannot arise from a recent overprint. For some samples from site GS08 we could not isolate the HT component, and we calculated the plane (projected as a remagnetization circle) containing both (LT and HT) components. Finally, in all the clayey samples, a ChRM could be defined in the C temperature interval (Figures 4f and 4g). The unblocking temperature spectra observed during thermal cleaning suggest that in the carbonates the remanence is carried by almost pure magnetite, while iron sulphides (greigite?) likely dominate the magnetic mineralogy of the Neogene clays. [29] The site-mean paleomagnetic directions are well defined for 33 (out of 39) sites (Table 1). Three sites (GS09, GS37, GS39) have an intensity of magnetization that is too low to be measured, and three sites (GS06, GS27, GS31) were discarded because they are characterized by a 95 values > Both the in situ and tilt-corrected site-mean directions are far from the GAD field direction expected at Gran Sasso (D = 0 ; I = 61.2 ), thus excluding the possibility of a recent magnetic overprint (Figure 5). After tilt correction, both normal and reverse polarity directions are observed, though the normally magnetized sites are predominant. When considered all in the normal polarity state, the paleodeclinations are spread from WSW to NE. Given such great declinational scatter, we could not perform the fold and the reversal tests on the whole set of sitemean directions. Consequently, the fold test (according to McFadden [1990]) was solely evaluated for the individual ChRMs from the upper Cretaceous sites GS13 and GS19 (both sampled in the limbs of metric-scale folds). The fold 8of22

9 Table 1. New Paleomagnetic Results From Gran Sasso a In Situ Tilt Corrected Site Latitude N Longitude E Age Age, Ma Bedding n/n D, deg I, deg k a95, deg D, deg I, deg k a95, deg R, deg F, deg GS upper Turonian-Coniacian Variable 9/ ± ± 6.0 GS Santonian-Campanian r 4/ ± ± 11.5 GS middle Eocene r 4/ ± ± 7.5 GS Santonian-Campanian / ± ± 15.6 GS middle Campanian / ± ± 10.5 GS06 b Maastrichtian / ± ± 16.4 GS Oligocene / ± ± 10.3 GS Oligocene / ± ± 14.9 GS upper Cretaceous-Oligocene /11 / / / / / / / / / / GS upper Turonian / ± ± 5.0 GS upper Turonian-lower Coniacian / ± ± 4.4 GS Campanian / ± ± 8.0 GS Coniacian-Santonian Variable 13/ ± ± 5.8 GS Turonian Variable 10/ ± ± 11.2 GS upper Turonian-Coniacian Variable 6/ ± ± 7.4 GS16 c Eocene? r 12/ ± ± 9.1 GS upper Turonian-Coniacian / ± ± 6.8 GS lower Coniacian / ± ± 8.0 GS upper Turonian Variable 13/ ± ± 4.9 GS middle Eocene / ± ± 5.2 GS Messinian / ± ± 4.3 GS lower Turonian r 8/ ± ± 14.7 GS Turonian Variable 10/ ± ± 11.4 GS Messinian Variable 10/ ± ± 5.0 GS lower Pliocene / ± ± 5.6 GS middle-upper Eocene Variable 10/ ± ± 7.4 GS27 b upper Albian-lower Cenomanian / ± ± 18.8 GS upper Campanian / ± ± 8.7 lower Maastrichtian GS Turonian-lower Coniacian / ± ± 6.0 GS upper Campanian / ± ± 8.4 GS31 b upper Cretaceous / / / GS Turonian-lower Coniacian / ± ± 8.1 GS upper Aptian-lower Albian / ± ± 9.3 GS Norian-Rhaetian / ± ± 10.1 GS upper Aptian / ± ± 5.0 GS Cenomanian / ± ± 11.6 GS upper Cretaceous-Oligocene /12 / / / / / / / / / / GS upper Lias-Dogger Variable 5/ ± ± 13.8 GS lower-mid- Cretaceous /14 / / / / / / / / / / a Epoch and stage ages (in Ma) for the Mesozoic and Tertiary are from timescales of Gradstein et al. [1994] and Berggren et al. [1995], respectively. Bedding is expressed in dip azimuth and dip values (r indicates overturned strata); n/n is number of reliable samples/total number of studied samples at a site; D and I are site-mean declination and inclination; k and a 95 are statistical parameters after Fisher [1953]; and R and F are the site-mean rotation and flattening (according to Demarest [1983]) relative to coeval D and I African values expected at Gran Sasso (latitude N, longitude E). The reference African poles used are from Besse and Courtillot [2002] for Jurassic-Tertiary sites and from Muttoni et al. [2001] for the upper Triassic site GS34. b Discarded sites (see text). c Site sampled in a mafic sill. 9of22

10 Figure 4. Vector diagrams of alternating field (Figure 4a) and thermal (Figures 4b 4g) demagnetization data, tilt-corrected coordinates. Open and solid symbols represent projection onto the vertical and horizontal planes, respectively. Demagnetization step values are expressed in mt (Figure 4a) and C (Figures 4b 4g). Data are from (a) an Eocene (?) mafic sill, (b) upper Turonian Scaglia limestones, (c) upper Campanian lower Maastrichtian Scaglia limestones, (d e) Oligocene Scaglia limestones (complete demagnetization diagram and detail of the high-temperature component, respectively), (f) Messinian clays, and (g) lower Pliocene clays. test is indeterminate for the site GS13 (though maximum k and minimum x values are observed at 87% and 100% of complete unfolding, respectively), and positive at the 99% significance level for the site GS19. These results indicate a prefolding (i.e., pre-messinian) age of magnetization acquisition. The reversal test (according to McFadden and McElhinny [1990]) was performed on the ChRMs from sites GS20 and GS38, showing dual-polarity paleomagnetic directions. The result is positive (of class C) for the site GS20, and indeterminate for the site GS38. [30] The magnetic polarities gathered from the 33 reliable sites and their (mostly biostratigraphically determined) ages were superposed to the Cenozoic [Berggren et al., 1995] and Mesozoic [Gradstein et al., 1994] geomagnetic polarity timescales (Figure 6). The age intervals estimated for the Tertiary and Jurassic-Triassic sites encompass several polarity chrons, so that the individual site polarities could not be framed within the available timescales. Conversely, the ages of 16 Scaglia sites ranging from Santonian to Aptian are entirely comprised within the long normal 10 of 22

11 Figure 5. Equal-area projections of the site mean paleomagnetic directions from Gran Sasso. Solid (open) circles represent projection onto the lower (upper) hemisphere. The star represents the normal polarity geocentric axial dipole (GAD) field direction for the study area. Open ellipses are the projections of the a 95 cones about the mean directions. Cretaceous superchron. Consistently, these sites all show a normal polarity (except site GS32), supporting the primary nature of their ChRMs. The reverse polarity observed in the site GS32 may suggest that the age of this site is, in fact, Campanian (or younger), and that the Turonian lower Coniacian foraminifera observed in thin sections are reworked. 8. Comparison With the African Apparent Polar Wander Path and Flattening Values [31] In order to evaluate the orogenic rotations related to thrust sheet emplacement, the tilt-corrected paleomagnetic directions from Gran Sasso (Figure 5 and Table 1) were compared to the coeval directions expected for the Adriatic foreland. An apparent polar wander path (APWP) from the backbone of Adria is not available to date, but several paleomagnetic data from Italy have convincingly shown that Adria has paleomagnetically mirrored the African drift since at least Permian times [e.g., Channell, 1992; Van der Voo, 1993; Muttoni et al., 2001]. Therefore we used the updated versions of the African APWP, to evaluate the orogenic rotations occurring at Gran Sasso with respect to the foreland. We used the African paleopoles from Besse and Courtillot [2002] for the Jurassic-Tertiary sites, and a late Triassic African pole from Muttoni et al. [2001] for the site GS34. Absolute ages of the sampled sites (to compare with strictly coeval African poles) were inferred relying upon timescales from Berggren et al. [1995] and Gradstein et al. [1994] for the Tertiary and Mesozoic sites, respectively. The rotation and flattening values with respect to the foreland (Table 1) were computed according to Demarest [1983]. [32] In Figure 7 we show the inclination and flattening values evaluated for the 33 reliable sites. The flattening values range between 11.8 to 30.9 for the three clayey Messinian lower Pliocene sites, while they are negligible or slightly positive for the other carbonate sites (except sites GS33 and GS34, showing 43.1 and 31.9 values, respectively). The flattening values in the clays are easily explained considering the effects of diagenesis and compaction [e.g., Deamer and Kodama, 1990] and are frequently observed in similar sediments elsewhere in the Apennines [Speranza et al., 1997]. We speculate that the very large flattening observed for the site GS33 may arise from an incorrect bedding correction, as massive limestones at the paleogeographic boundary between the peritidal shelf and the slope were sampled at this site (Figure 2). Conversely, we have no solid explanation for the high flattening value documented for the Triassic site GS34. [33] The rough agreement between observed and expected inclinations from most of the Mesozoic-Tertiary carbonates is a proof for both the first-order validity of the Besse and Courtillot [2002] African poles, and the primary nature of the ChRMs gathered from the whole preorogenic sedimentary succession. In fact, the ages of the sampled sediments are generally much older than age of folding, so that the evidence of the prefolding magnetization acquisition documented by us does not exclude the possibility of magnetic overprints occurring before folding. Yet, the expected African inclination values at Gran Sasso (using the Besse and Courtillot [2002] poles) increase rather continuously from the early Cretaceous onward, due to the well-known northward African drift occurring along this time span. Therefore prefolding magnetic overprints would translate into negative flattening values, which, as a rule, were not observed in our sites. We note, however, that site GS32, that we suspected on magnetostratigraphic grounds to be younger than its biostratigraphically deter- 11 of 22

12 Figure 6. Comparison between the global polarity timescale of the Cenozoic and Mesozoic, and the magnetic polarity of our biostratigraphically dated sites from Gran Sasso. Numerical ages of both polarity chrons and geological stage boundaries are from Berggren et al. [1995] and Gradstein et al. [1994]. Solid, open, and half solid circles represent normal, reverse, and dual polarity sites, respectively. 12 of 22

13 Figure 7. Paleomagnetic inclination and flattening values (with respect to the coeval African poles from Besse and Courtillot [2002]) versus absolute age of the studied sites. Circles, squares, and triangles represent clays, carbonates, and a mafic sill, respectively. Open and solid symbols represent inclination and flattening values, respectively. Error bar half length for inclination and flattening is a 95 and according to the Demarest [1983] method, respectively. mined age due to sediment reworking, displays a negative ( 8.9 ±8.1 ) flattening. This may be additional evidence for its younger age. [34] As a summary, there are four lines of evidence concurring to support the primary nature of the ChRMs isolated at Gran Sasso: (1) both the in situ and tilt-corrected paleomagnetic directions are far from the GAD field direction; (2) fold tests indicate a prefolding magnetization acquisition; (3) 15 sites with biostratigraphic ages entirely falling within the long normal Cretaceous superchron boundaries show (consistently) a normal polarity; and (4) the observed inclinations are generally similar to those expected from the Besse and Courtillot [2002] African poles. From these considerations, we conclude that our paleomagnetic data from Gran Sasso can be safely used to evaluate the orogenic rotations and the kinematics of the salient. 9. Pattern of Orogenic Rotations at Gran Sasso [35] The orogenic rotations calculated with respect to the Adriatic/African foreland for the 33 reliable sites are shown in Figure 8 and listed in Table 1. The rotational pattern is clearly inhomogeneous, and characterized by rotations of different sign and greatly variable magnitudes. Null rotations are observed within the salient interiors, while the fronts are almost invariably rotated. As a rule, CCW rotations characterize the E-W and CW rotations the N-S limb. Excluding the sites located close to Monte Fiore and Monte La Queglia (in the N-S front, Figure 8), the rotations are generally similar (in sign and magnitude) for nearby sites located over the same front segment, suggesting that entire thrust sheets (instead of small fault-bounded blocks) rotated. [36] The sites from Monte Fiore and Monte La Queglia show great scatters of the rotation values, suggesting local block rotations. At Monte Fiore our site GS17 is rotated CCW by 64.9, and two Scaglia sites (SC12 and SC13) by Dela Pierre et al. [1992] show westward declinations (in the normal polarity state), suggesting similar CCW rotation values (these sites are undated, so that the precise rotation value cannot be evaluated). Conversely, our sites GS18 and GS19, sampled adjacent to SC12, show no rotation, implying that Monte Fiore is fragmented in small rotating blocks. A similar tectonic pattern is inferred to characterize Monte La Queglia, where our site GS16 is rotated CCW by 48, while Messinian clays sites FL9 and FL13 from Dela Pierre et al. [1992] show no rotation and 120 CCW rotation, respectively (Figure 8). At both Monte Fiore and Monte La Queglia several E-W to WSW-ENE strike-slip transfer faults locally cut the N-S front [e.g., Bigi et al., 1995b]. Thus we infer that these faults induced vertical axis rotations in the intervening small-scale blocks (the distance between differently rotated sites implies block widths not exceeding 2 3 km). We note that in an overall CW rotating N-S Gran Sasso limb, the block-rotating sites display either no rotation or (up to 120 ) CCW rotation, implying that all these sites greatly rotated CCW with respect to the rest of the N-S limb. Therefore, by considering the block rotation model by Sonder et al. [1994], we infer that the roughly E-W to WSW-ENE strike-slip faults separating the rotating blocks were characterized by leftlateral kinematics. [37] Excluding the Monte Fiore and Monte La Queglia block rotation domains, we have averaged the rotations observed in neighbor sites (Figure 9), in order to provide a simpler picture of the gross rotations characterizing the different sectors of the salient. We have also reevaluated and reaveraged the rotations at Corno Grande (Figures 8 and 9 and Table 2), by using the original paleodeclination data as 13 of 22

14 Figure 8. Geological map of Gran Sasso and paleomagnetic rotations with respect to the coeval African poles from Besse and Courtillot [2002]. Paleomagnetic data are (1) rotations at Corno Grande (reevaluated from original paleodeclination data from Speranza et al. [2003a]), and paleodeclinations from Dela Pierre et al. [1992] and Sagnotti et al. [2000], (2) rotations from our sedimentary sites, and (3) rotation from our igneous site GS of 22

15 reported by Speranza et al. [2003a] and the new African poles from Besse and Courtillot [2002] (where as Speranza et al. [2003a] used African/Adriatic paleopoles from Channell [1992] to evaluate the rotations). Conversely, we did not reevaluate the site-mean rotations from original data from Dela Pierre et al. [1992], because they did not specify the age of all their sites. The average rotational value of the domain Colle Madonna Monte Cappucciata (N-S limb) was calculated excluding the site GS24, rotated by 7.2 CCW and surrounded by five sites rotated consistently CW by 51.6 ±6.6. The rotation value of site GS24 is different by about nine standard deviation values from the average rotation value of the domain, so that we infer a local block rotation for this site as well. [38] In Figure 9, we first note the absence of rotation in the internal part of the indenter (eight sites with mean value of 1.7 ±8.7 CCW). This datum shows that Gran Sasso is a salient arising from a purely nonrotating indenter displacement. The null rotation documented for the whole salient interiors (i.e., also for its eastern sector) is at odds with the suggestions from both Dela Pierre et al. [1992] and Speranza et al. [2003a]. [39] Along the E-W salient limb, systematic mean CCW rotations are observed for the three front sectors averaged. The mean rotation values increase from 7.9 ±4.3 at Corno Grande, to 80.0 ± 18.2 close to the apex. A similar increase of the CW mean rotation values is observed along the N-S limb, moving northward from the end point to the apex. The southern part of the N-S limb rotates CW by 10.5 ± 9.4, while at Monte Cappucciata Colle Madonna the average CW rotation reaches the 51.6 ±6.6 value. The two strongly CCW and CW rotated domains are finally separated, just south of the apex, by the Monte Fiore block rotation domain. [40] It may be argued that the different rotation magnitudes are related to the site locations in the distinct thrust sheets, i.e., that the exposed thrust faults induced the observed rotations. This mechanism would yield the greatest rotation values on top of the nappe pile, and stepwise decreasing values descending along the underlying thrust sheets (as observed in the Maghrebian belt of Sicily, Oldow et al. [1990]). Instead, we note that there is no systematic relation between the rotation magnitude and the position of the sites within the nappe pile. Along the E-W limb there are three domains characterized by an increasing amount of average CCW rotation values moving eastward (7.9, 24.4, and 80.0, Figure 9). Yet the 24.4 rotated domain is located below the upper thrust, while the other two (showing almost null and very large rotations, respectively) are placed above it. Therefore it appears that the upper thrust fault of the E-W limb did not introduce any rotation. Similarly, in the N-S limb, the 10.5 CW rotated domain is located on top of the nappe pile, while the sites CW rotated by 50 are spread along more external thrust sheets. We conclude that, in each limb point, the rotations occurred along the lowermost thrust fault of the salient, which passively rotated all the nappe pile located above. Such rotation pattern supports a normal in-sequence propagation timing of the thrust fault activity at Gran Sasso. [41] The 51.6 ±6.6 CW rotation value along the N-S limb was obtained averaging out five individual values from sites spread over different, juxtaposed, thrust sheets (Figure 9). Moreover, two (out of the five) sites are Messinian lower Pliocene in age, confirming that the Gran Sasso salient formed (and rotations occurred) during such time span. The similar rotation values observed in the outer Neogene sites and along the (more internal) main carbonate fronts, suggest that in fact, the clayey sites overlie buried N-S thrust sheets genetically linked with the Gran Sasso salient, and that splay faults are connected at depth with the main N-S carbonate front. This is in agreement with both the field evidence of narrow N-S carbonate ridges (Colle Madonna, Monte La Queglia) exposed east of the main salient front, and with seismic reflection data from the area [Bigi et al., 1999]. According to Zweigel [1998], the ratio between the widths of the frontal and lateral parts of salients increases linearly with decreasing indentation angles. Given the inferred displacement direction of the indenter (N70 E) at Gran Sasso, a larger N-S (than E-W) front is expected in this frame. Accordingly, the CW rotations observed over several (exposed and buried) N-S fronts define a wider N-S (than E-W) limb. 10. Comparison With Previous Paleomagnetic Data [42] As evident from Figure 8, our new data, documenting a complex pattern of rotations at Gran Sasso, are not fully consistent with previous results from Dela Pierre et al. [1992], which were suggestive of an uniform 90 CCW rotation in the whole eastern Gran Sasso. Though Dela Pierre et al. [1992] did not compare the paleodeclinations to the African poles, their westward declination values from five (partly undated) Scaglia sites are so large ( 87 on average) that the inconsistency would persist even when evaluating the rotations. In fact, the Scaglia Formation ranges in age between upper Cretaceous to Oligocene, so that a 87 mean paleodeclination translates (after comparison with the Besse and Courtillot [2002] African poles) to orogenic rotation values comprised between 85 and 65 CCW. [43] South of Corno Grande, the two sites SC9 and SC10 from Dela Pierre et al. [1992] yielded 97 and 46 declination values, respectively, while five neighbor sites from us display a null rotation (Figure 8). This inconsistency might be explained by considering the demagnetization behavior (Figures 4d and 4e) of the two Oligocene sites (GS07, GS08) sampled by us adjacent to the two strongly rotated sites from Dela Pierre et al. [1992]. The samples from sites GS07 GS08 showed (in tilt-corrected coordinates) a reverse-polarity southeastward directed LT component from 200 C to 460 C (Figure 4d), and a normal-polarity northward directed HT component (that we considered as characteristic) from 460 C to 580 C (Figure 4e). Our hypothesis is that Dela Pierre et al. [1992] considered as characteristic a similar LT component, thus finding the westward directed paleodeclinations. 15 of 22

16 Figure 9. Geological map of Gran Sasso, rotational domains, and corresponding average rotation value. Paleomagnetic data are (1) rotations at Corno Grande (reevaluated from original paleodeclination data from Speranza et al. [2003a]), and paleodeclinations from Dela Pierre et al. [1992] and Sagnotti et al. [2000], (2) rotations from our sites, (3) rotational domains and corresponding average rotation value (mean and standard deviation of the individual site-mean rotations); and (4) block rotation domains. The paleodeclinations from Dela Pierre et al. [1992] were not used to evaluate the mean rotation values, as some of their sites are undated, so that the rotations could not be evaluated. The average rotation value of the domain Colle Madonna Monte Cappucciata (51.6 ±6.6 ) was calculated by excluding the site GS24 (see text). 16 of 22

17 Table 2. Updated Rotation Values at Corno Grande Calculated From Original Paleodeclination Data From Speranza et al. [2003a] a In Situ Tilt Corrected Site Age Age, Ma N D, deg I, deg D, deg I, deg k a 95, deg R, deg F, deg Co07 Eocene-Oligocene ± ± 6.9 Co11bis Eocene-Oligocene ± ± 12.8 Co22 Coniacian-Santonian ± ± 9.4 Co05bis upper Lias-lower Malm ± ± 9.9 Co09 upper Lias-lower Malm ± ± 8.5 Co21 upper Lias-lower Malm ± ± 7.9 Co18 middle Lias ± ± 9.5 a All data are from Speranza et al. [2003a], except the rotation (R) and flattening (F) values. Epoch and stage ages (in Ma) for the Mesozoic and Tertiary are from timescales of Gradstein et al. [1994] and Berggren et al. [1995], respectively. N is the number of studied samples at a site; D and I are site-mean declination and inclination; k and a 95 are statistical parameters after Fisher [1953]; and R and F are the updated site-mean rotation and flattening (according to Demarest [1983]) relative to coeval D and I African values expected at Gran Sasso. The reference African poles used are from Besse and Courtillot [2002], while Speranza et al. [2003a] used Mesozoic poles from Channell [1992] and compared the Tertiary directions to the local north. Results from the site Co19 (discarded by Speranza et al. [2003a]) are omitted. [44] The sites SC11 and SC13 from Dela Pierre et al. [1992], sampled in the apical region of Gran Sasso, display a westward declination and a normal polarity, in good agreement with directional data from our respective sites GS36 and GS17, visibly sampled in the same outcrops (Figure 8 and Table 1). The site SC12 sampled by Dela Pierre et al. [1992] north of Monte Fiore, displays a westward paleodeclination (in the normal polarity state), while our close sites GS18 and GS19 show north-nnw directed declinations. Clearly, SC12 and GS18 GS19 were sampled within two decoupled blocks of the Monte Fiore domain, and underwent different rotations. 11. Oroclinal Versus Nonrotational Arc Behavior of the Gran Sasso Limbs and Comparison With Results From Sandbox Models [45] In Figure 10 we show the changes in rotation moving along the arc limbs from Corno Grande to the southeastern end point. Data are from our sites (predominantly), and from Speranza et al. [2003a] (seven sites at Corno Grande). Data from the Monte Fiore and Monte La Queglia block rotation domains are omitted. The rotation pattern of Figure 10 is unusual, as it does not fit either oroclinal or nonrotational arc behavior (or all intermediate models between these end-members). A nonrotational arc would be characterized by ubiquitous null rotations, while in a perfect orocline the rotations should reach maximum (and opposite sign) values at the end points, and then progressively decrease toward the apex. [46] Conversely, we find quasi-null rotations at the end points (two sites from Dela Pierre et al. [1992] at the western endpoint show on average a northward declination, Figure 8). Then the rotations rise (rather progressively but more than linearly) moving toward the apex. Here, there are not intermediately rotated sites between the 80 CCW and 50 CW rotated fronts, but rather the small blocks of Monte Fiore undergoing individual rotations. The absence of rotations along the limbs far from the apex, and the rough dip-slip thrusting documented here [Ghisetti, 1987; Ghisetti and Vezzani, 1986a, 1986b, 1990, 1997; Calamita et al., 2002, 2003b], implies a radial pattern of slip directions occurring normal to the preexisting indenter s margins. Conversely, in the vicinity of the apex, large-magnitude CCW and CW rotations occur, suggesting that here the foreland successions were pushed apart and rotated by the indenter s apex, resembling a sort of ice breaker mechanism. The Monte Fiore block rotation domain, just south of the apex, likely corresponds to the region of frontal accretion of the indenter, cut by several strike-slip transfer faults inducing local rotations. [47] The CCW and CW rotations at the apex s proximity reach 80 and 50 values, respectively, raising the question as to why there is such magnitude of asymmetry of the rotational pattern. We note that the average displacement direction of the indenter (approximately N70 E) is not intermediate between the trends of the limbs (E-W and N-S). Thus we speculate that a quasi-frontal collision along the N-S front caused smaller CW rotations, while left-lateral shear was dominant along the E-W indenter s margin, inducing the large (80 ) CCW rotations documented by us. [48] The nonrotational versus oroclinal character of an orogenic bend can be quantitatively evaluated by analyzing the pattern of rotation deviations versus structural trend deviations (as introduced by Schwartz and Van der Voo [1983]). Horizontal and unit slope best fit lines are expected to define a nonrotational arc and a perfect orocline, respectively. In Figure 11a, the rotation deviations are plotted versus the corresponding bed strike (assumed as a proxy of the structural direction) deviations. Clearly, no linear trend is evident. A more regular pattern appears when fold axes as observed on the available geological maps (instead of bed strikes) are used as structural trend indicators (Figure 11b). The greater scatter of data from Figure 11a rather than from Figure 11b is likely a consequence of the strong axial plunge of folds and thrusts observed in some sectors of the front (namely, at Monte Camicia). Here, the individual beds strikes may strongly deviate from the fold axis direction. 17 of 22

18 Figure 10. Changes of paleomagnetic rotations versus horizontal distance from Corno Grande to the southern end point of the Gran Sasso arc. Distances are calculated along the limbs of the salient, starting from the western end point. Circles represent rotations reevaluated (considering updated African poles from Besse and Courtillot [2002]) after original paleodeclination data from Speranza et al. [2003a]. Rhombs represent rotations from this work. Data from the Monte Fiore and Monte La Queglia domains, and from site GS24, characterized by local block rotations, are omitted. Error bars for rotation data are according to the Demarest [1983] method. The shaded area indicates the rough rotation evolution moving from both end points toward the apex. The rotations seem to increase continuously and more than linearly. [49] Figure 11b shows that the sites on the limbs close to end points (open circles and rhombs) are roughly aligned along a horizontal line, thus approaching a nonrotational arc behavior. Conversely, the sites in the proximity of the apex (solid symbols) can be fitted by a line different from a zeroslope line at the 99% significance level, following the statistical t test. The t test of the slope of the regression line compared to zero slope gives t = This is greater than the critical t value at the 99% significance level (for number of data N = 9), t 99 = This evidence implies that the limbs close to the apex did follow an oroclinal evolution (Figure 12). However, the slope value of the regression line (m = 2.2) is about double the (unit) value expected for a perfect orocline, meaning that limbs at present forming a salient were arranged in a sort of reentrant shape, before tectonic deformation. The intersection between the best fit line and the abscissa is expected to define the unrotated structural trend (333 ) of the orocline. This value is similar to the orientation of the thrust fronts located north and east of Gran Sasso, strengthening our hypothesis that the salient underwent a N70 E direction of orogenic translation. [50] The first approximation dip-slip behavior documented for the Gran Sasso curved fronts [Ghisetti, 1987; Ghisetti and Vezzani, 1986a, 1986b, 1990; Calamita et al., 2002, 2003b] is not unexpected, and in agreement with indenter-controlled sandbox analogue experiments, clearly showing that the younger thrusts on the outer arc accommodate dip-slip displacement [Lickorish et al., 2002]. Yet, in such models, the intensity of strike-parallel shear increases toward the inner zones of the sand wedges, so that strike-slip faults separating the nonrotational N70 E displacing indenter from the (roughly dip-slip stacking and rotating) salient fronts are kinematically needed at Gran Sasso (Figure 12). An E-W left-lateral fault should be more evident and accommodate larger displacement than a N-S right-lateral fault, due to the more pronounced frontal collision along the N-S limb of the N70 E displacing indenter. However, at present, no clear field evidence of such strike-slip faults has been reported in the northern sector of Gran Sasso (where such faults are kinematically expected), though an E-W sinistral shear is roughly consistent with en échelon pattern of WNW-ESE trending thrusts and related folds documented by Calamita et al. [2002] close to the E-W limb end point, between Monte San Franco and Monte Corvo (Figure 2). The E-W to WSW-ENE strike-slip faults of Monte Fiore and Monte La Queglia, which we have inferred on rotational grounds to be left lateral, are rather local transfer faults cutting the N-S front, and do not propagate further west into the salient interiors. 18 of 22

19 [51] It might be suggested that the expected E-W leftlateral fault was later overprinted by the southward dipping normal faults (showing several hundreds of meters of displacement), which separate at present the E-W front from the indenter itself [e.g., D Agostino et al., 1998, Figure 2]. Conversely, the N-S right-lateral fault could follow the eastern boundary between the peritidal shelf and the slope carbonates. Here, some minor northeastward dipping thrust faults documented by D Agostino et al. [1994] (just west of the sites GS10 GS14) could be reinterpreted as restraining bands kinematically related to the N-S right-lateral fault. Pronounced cataclasite belts are exposed along both the northern and eastern margins of the indenter, so that the original kinematic indicators relative to the expected strikeslip activity could have been obliterated. [52] Apart for the apex zone of the arc, where the outer slope sediments were bulldozed and strongly rotated by the indenter s corner, the (virtually unrotated) fronts close to end points define a radial shortening pattern, suggesting that preexisting discontinuities within the foreland sequences constitute a primary control on the local shortening directions. Moreover, the E-W Gran Sasso limb is stacked over approximately N-S fronts (Monte dei Fiori, Montagnone), which did not undergo any orogenic rotation [Speranza et al., 1997]. Yet N70 E shortening in these approximately N- S fronts is partly synchronous with northward displacement in the eastern sector of the E-W Gran Sasso limb. Therefore the overall deformation pattern in this sector of the external Apennines seems to be dominated by the irregular path of the preorogenic normal faults. During orogenesis, the thrust fronts formed parallel to the preexisting normal faults, sharing their azimuth scatter within a regional direction of tectonic transport which probably remained constant at N70 E. [53] Indenter-controlled salients observed in sandbox models are considered to arise from critical-tapered orogenic wedge factors, so that their rotation pattern should compare with our results. Still, these models have revealed so far rotational scenarios completely different from those reported by us. Marshak [1988] originally suggested that deformation along irregular continental margins or indenter margins leads to the genesis of purely nonrotational arcs. More recently, Macedo and Marshak [1999] have basically confirmed the findings from Marshak [1988], though they suggest that the thrust fronts may subsequently undergo slight (oroclinal-type) rotations during progressive convergence. Zweigel [1998] and Lickorish et al. [2002] did not discuss the rotations for their models, but the displacement vectors (for materials points of nonrotational indentergenerated salients) seem roughly to follow nonrotational trajectories. Costa and Speranza [2003], by paleomagnetically studying indenter-generated sandbox salients, find a nonrotational outer front and small, oroclinal-type, rotations in the more internal thrusts. We conclude that the 19 of 22 Figure 11. Paleomagnetic rotation deviations at the limbs of the Gran Sasso arc relative to (a) strike of beds and (b) fold axis deviations [e.g., Schwartz and Van der Voo, 1983]. Data in Figure 11a are uniquely from 20 sites from us, data in Figure 11b are from 20 sites from us and seven sites from Speranza et al. [2003a]. Data from the Monte Fiore and Monte La Queglia domains and from site GS24, characterized by local block rotations, are omitted. R is the observed rotation at a site, R0 is the reference rotation (0 ). S is the observed bed strike at a site, and F is the fold axis direction at a site determined from the geological map from Vezzani and Ghisetti [1993]. The reference bedding strike (S0) and fold axis (F0) direction is 315, i.e., a perfectly NW-SE trend. Error bars for rotation data are according to the Demarest [1983] method. The best fit line of Figure 11b is calculated by linear regression analysis to the sole data from the limbs around the salient apex (solid symbols).

20 Figure 12. Schematic cartoon (not in scale) showing the modes of salient formation and the paleomagnetic rotations at Gran Sasso after indentation of a nonrotating convex hinterland corner. The slope sediments ahead of the hinterland corner are pushed apart and rotated by the indenter s translation. A wider wedge but smaller-magnitude rotations are observed along the N-S limb, accommodating a more frontal collision than the E-W limb. Strike-slip faults are kinematically required to separate the displacing indenter from the (roughly dip slipping) thrust fronts. The E-W Gran Sasso limb is tectonically juxtaposed over unrotated approximately N-S stacks (Monte dei Fiori Montagnone). The approximately N-S fronts underwent N70 E directed shortening synchronous with the earlier shortening episodes at Gran Sasso (Messinian early Pliocene), prior to being overthrust by the Gran Sasso salient in mid-pliocene times. Thus the geometry of the preorogenic faults seems to be the main factor controlling both the orientations of the thrust fronts and the local kinematic paths in the framework of an overall N70 E direction of orogenic transport. new paleomagnetic evidence from Gran Sasso is in conflict with the results from sandbox models available so far. 12. Conclusions [54] The detailed paleomagnetic study of the Gran Sasso arc has first offered the opportunity to document how rotations change along the limbs of a kilometric-scale indenter-controlled range salient. The rotational behavior is complex, and does not compare with either nonrotational arc or oroclinal models (or with all intermediate patterns). It also differs significantly from results from indentercontrolled sandbox analogue models. New data from the hinterland backbone of the range indicate that Gran Sasso is a salient generated by a nonrotating indenter translation. [55] Along the fronts, the rotations do not depend upon the structural position of the sites within the nappe pile, meaning that they occurred synchronously with shear along the outer (lowermost) thrust fault of the system. Along the limbs, the rotations are virtually absent close to the end points and increase more than linearly (with opposite sign) moving toward the arc apex. Around the apex, a 80 CCW and 50 CW rotated sites (separated by a block rotation domain) are observed along the E-W and N-S limbs, respectively. The difference between the maximum value of the CCW and CW rotations documented by us is likely the consequence of the asymmetry between the preorogenic orientation of the indenter margins (E-W and N-S) and the inferred direction of orogenic translation of the indenter itself (approximately N70 E). Also, similar value CW rotations observed in several stacked thrust sheets along the N-S limb are a proof for a wider N-S (than E-W) wedge, in agreement with previous evidence from sandbox models. [56] Our tectonic model of an indenter bulldozing the outer weaker sedimentary successions requires two strikeslip faults, subparallel to the limbs, able to decouple the rigid indentation of the core of the arc from the dip-slip outer thrust fronts. Yet there is no clear evidence of these expected first-order strike-slip faults, so that the paleomagnetic and geologic evidence are somehow in conflict. Our data from Gran Sasso show that when the oblique indentation of a convex hinterland corner occurs, completely different kinematics are observed close to end points and around the apex. The salient limbs close to end points arise from nonrotational thrust stacking along directions circa normal to the local indenter margins. Conversely, close to the apex, the outer foreland sediments are strongly rotated and pushed apart (with an ice breaker like mechanism) by the orogenic translation of the hinterland corner. The large amount of shortening around the apex is also probably related to the occurrence of small blocks undergoing independent rotations (Monte Fiore domain). Such local block rotation is due to shear along strike-slip transfer faults cutting the salient s hanging wall. [57] Relying on our evidence from Gran Sasso, we suggest that paleomagnetic rotations from the limbs of a salient may represent a valuable proxy to infer the direction of orogenic translation of a hinterland indenter. In fact, rotations are expected to rise quasi-symmetrically toward the point of frontal accretion, and greater magnitude rotations should be observed along the lateral limb of the displacing hinterland corner. [58] Acknowledgments. The work was supported by COFIN 2003 and MUIR grants to F. Calamita. We are indebted to J. Gattacceca for his help in the field and to M. Chiocchini for biostratigraphically analyzing and dating most of 20 of 22