Genesis and evolution of a curved mountain front: paleomagnetic and geological evidence from the Gran Sasso range (central Apennines, Italy)

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1 Tectonophysics 362 (2003) Genesis and evolution of a curved mountain front: paleomagnetic and geological evidence from the Gran Sasso range (central Apennines, Italy) Fabio Speranza a, *, Leo Adamoli b, Rosanna Maniscalco c, Fabio Florindo a a Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, Italy b Via Cona 114, Teramo, Italy c Istituto di Geologia e Geofisica, Università di Catania, Catania, Italy Received 8 December 2000; received in revised form 30 April 2001; accepted 10 May 2002 Abstract The Gran Sasso range is a striking salient formed by two roughly rectilinear E W and N S limbs. In the past f 90j counterclockwise (CCW) rotations from the eastern Gran Sasso were reported [Tectonophysics 215 (1992) 335], suggesting west east increase of rotation-related northward shortening along the E W limb. In this paper, we report on paleomagnetic data from Meso-Cenozoic sedimentary dykes and strata cropping out at Corno Grande (central part of the E W Gran Sasso limb), the highest summit of the Apennine belt. Predominant northwestward paleomagnetic declinations (in the normal polarity state) from both sedimentary dykes and strata are observed. When compared to the expected declination values for the Adriatic foreland, our data document no thrusting-related rotation at Corno Grande. The overall paleomagnetic data set coupled with the available geological information shows that the Gran Sasso arc is in fact a composite structure, formed by an unrotated-low shortening western (E W trending) limb and a strongly CCW rotated eastern salient. Late Messinian and post-early Pliocene shortening episodes documented along the Gran Sasso front indicate that belt building and arc formation occurred during two distinct episodes. We suggest that the southern part of a late Messinian N S front was reactivated during early middle Pliocene time, forming a tight range salient due to CCW rotations and differential along-front shortening rates. The formation of a northward displacing bulge in an overall NW SE chain is likely a consequence of the collision between the Latium-Abruzzi and Apulian carbonate platforms during northeastward propagation of the Apennine wedge, inducing lateral northward extrusion of Latium-Abruzzi carbonates towards ductile basinal sediment areas. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Gran Sasso range; Mountain front; Central Apennines 1. Introduction For at least two decades, it has been observed that paleomagnetic data are useful to constrain mountain * Corresponding author. address: speranza@ingv.it (F. Speranza). belt kinematics and to understand curved range formation (e.g. Marshak, 1988; Van der Voo, 1993). This is the main target of several paleomagnetic investigations carried out in the Mediterranean region, where the Alpine belt is largely exposed. However, the various Alpine segments have been paleomagnetically investigated so far by sampling over large /02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi: /s (02)

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3 F. Speranza et al. / Tectonophysics 362 (2003) regions, and averaging the paleomagnetic datum over many different mountain fronts and nappes (e.g. Scheepers, 1994; Speranza et al., 1997; Duermeijer et al., 2000). On the other hand, smaller-scale investigations documenting rotations from different localities within the same curved thrust front are needed to understand the tectonic setting and reconstruct the deformation history of range salients. In Italy, previous paleomagnetic data from different regions have shown that the orogenic process induced large rotations and disruption of the original geometry of the compressive structures (Channell et al., 1990; Ghisetti and Vezzani, 1997). One of the most amazing arc-shaped structures of the Apennine belt is the Gran Sasso range (Figs. 1 and 2), which is formed by two almost orthogonal E W and N S limbs where high-relief carbonates contrast with terrigenous deposits exposed at the range toe. The E W limb faces N S thrust sheets located farther north, which seem to plunge beneath the Gran Sasso carbonates. Reliable seismic reflection data from the Gran Sasso (as well as from most of the Apenninic ranges) are lacking. Consequently, the deep structural style is unknown. It has been proposed both that the Gran Sasso sediments are simply folded and contain small second-order detachments (Servizio Geologico d Italia, 1963), and that this range is a far-travelled (with tens of kilometres of displacement) thrust sheet complex (Ghisetti and Vezzani, 1991; Bigi et al., 1995a). Paleomagnetism may help to discriminate among these different hypotheses, because large rotations suggest large shortening values. Previous paleomagnetic data obtained at Gran Sasso (Dela Pierre et al., 1992) showed large counterclockwise rotations at five sites in the eastern belt (Fig. 2), and no significant rotation in the westernmost part of the range. Such rotational pattern left unclear the transition between the rotated and unrotated part of the belt, and the possible tectonic boundaries decoupling them. In this paper, we report on new paleomagnetic data from the frontal range structures at Corno Grande, the highest top of the Apennine belt (2912 m), located at about the centre of the E W Gran Sasso limb. Our data, integrated with geological observations, suggest different tectonic styles and kinematic evolution in the western and eastern Gran Sasso belt sectors. 2. Tectonics of the central Apennines in the frame of the central Mediterranean geodynamics Gran Sasso is one of the most external structures of the central Apennines belt (Fig. 1), resulting from Messinian to middle Pliocene thrusting episodes. The Mesozoic lower Tertiary carbonates exposed at Gran Sasso were deposited in the core of the Adriatic foreland area. Here, synsedimentary extensional activity determined the distribution of domains characterised by persisting carbonate shelf or pelagic sedimentation, respectively (Consiglio Nazionale delle Ricerche, 1988). Since late Cretaceous, compressive deformation related to the Africa Europe convergence begun at the northern western margins of the Adriatic domain (Marroni et al., 1992), and the central western Mediterranean Alpine chain started to rise. During late Cretaceous middle Oligocene times, the Africa Europe collision induced the formation of an accretionary prism formed by tectonically stacked oceanic and internal paleogeographic domains (Lonergan and White, 1997). Conversely, since late Oligocene, tectonic stacking in the Alpine Apennine chain was synchronous with back-arc spreading of the Liguro Provencal (late Oligocene middle Miocene) and Tyrrhenian (late Miocene Pleistocene) seas (Elter et al., 1975). This process was likely the consequence of passive subduction and eastward roll-back of the Adriatic Ionian lithosphere beneath the Alpine Apennine chain (Malinverno and Ryan, 1986). Therefore, the Apenninic thrusting events were coeval to extensional tectonics at the rear of the accretionary wedge, and Fig. 1. Structural map of the central Apennines, redrawn from Cipollari et al. (1999). Legend: (1) Meso-Cenozoic slope-basin sediments, (2) Meso-Cenozoic Latium-Abruzzi shelf carbonates, (3) Meso-Cenozoic Apulian shelf carbonates, (4) upper Miocene-lower Pliocene foredeep siliceous turbidites, (5) Messinian lower Pliocene thrust-top sediments, (6) buried Pliocene marine sediments, (7) middle upper Pleistocene volcanics, (8) Plio-Pleistocene and Holocene marine and continental deposits, (9) thrusts, (10) undifferentiated faults, (11) isobaths (in metres) of the base of the Pliocene sediments in the external belt.

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5 F. Speranza et al. / Tectonophysics 362 (2003) spreading of the Tyrrhenian Sea. The compressiveextensional pair of the Apennine Tyrrhenian system migrated eastward, towards the Adriatic foreland area, since late Miocene times (Patacca et al., 1990). In the central Apennines (Fig. 1), thrusting affected the western calcareous ridges (Lepini Ausoni Aurunci Mts.) during late Tortonian early Messinian times, the Simbruini Mts. and Marsica area during Messinian early Pliocene, and finally the Maiella Mt. and Adriatic foothills during middle late Pliocene (Cipollari and Cosentino, 1995; Cavinato and De Celles, 1999). The Gran Sasso arc underwent both late Messinian and early middle Pliocene shortening episodes (Ghisetti and Vezzani, 1991). On the other hand, extension affected first the western ridges during late Messinian early Pliocene times, subsequently migrated eastward, and is still active today in the central eastern belt sector (e.g. D Agostino et al., 1998). 3. Tectonic setting of the Gran Sasso arc and previous paleomagnetic data The Gran Sasso arc is predominantly made up of Mesozoic lower Tertiary carbonates (Fig. 2) cropping out at about 2000 m mean altitude, whereas to the north and to the east Messinian lower Pliocene siliceous turbidite deposits are exposed at m altitude. The N S arc limb is formed by west-dipping thrust planes with both dip slip and right-lateral strike-slip shear components (Bigi et al., 1995b). Conversely, folds and thrusts exposed along the E W limb show pure N S compression without any strike-slip component (Ghisetti, 1987). The eastern E W limb is made of two units tectonically superimposed by a clearly visible subhorizontal décollement (Upper Thrust, Fig. 2). The Upper Thrust shows progressively reduced displacement westward and disappears a few km west of Corno Grande. South of the Upper Thrust, there are several southward-dipping faults systematically juxtaposing younger on older strata. Ghisetti and Vezzani (1991, 1997) argue for the out-of-sequence compressive nature of these faults, whereas Adamoli et al. (1990) and D Agostino et al. (1998) interpret them as Pleistocene Holocene extensional faults, which became active after the late Messinian middle Pliocene compressive episodes. Other unambiguous Pleistocene Holocene extensional faults can be observed further south, separating the Gran Sasso range from the L Aquila intermontane plain (Fig. 2). In the western E W limb, two km-long fault escarpments dipping south by 50 70j (Tre Selle and Assergi faults) show 600 and 1500 m offset, respectively (Ghisetti and Vezzani, 1990; D Agostino et al., 1998). In the eastern E W limb, a south-dipping normal fault separates the chain front from the Campo Imperatore plain, filled by Pleistocene Holocene alluvial and glacial deposits. Further south, prevalently NE-dipping low- and high-angle normal faults separate narrow WNW ESE calcareous ridges from adjacent sigmoidal-shaped grabens (narrow ridges high plateau (NRHP), D Agostino et al., 1994, Fig. 2). The frontal part of the Gran Sasso E W limb is formed by an anticline syncline couplet overturned towards the north which shows an (apparently) continuous sedimentary sequence from Meso-Cenozoic limestones to the Messinian lower Pliocene turbidites exposed at the range toe. Therefore, the question arises as to whether the frontal overturned anticline overlies a basal décollement (Lower Thrust, Fig. 2) far displacing northward the Gran Sasso carbonates over the Messinian lower Pliocene turbidites, or the calcareous and turbidite strata are simply folded implying small horizontal shortening. Counterclockwise (CCW) rotations synchronous with thrusting are expected in a northward-displacing E W mountain front showing a west to east increase of shortening (as it is observed along the Upper Thrust). Fig. 2. Geological map of the Gran Sasso arc and previous paleomagnetic data. Legend: (1) Pleistocene Holocene continental deposits filling extensional intermontane basins, (2) lower Pliocene foredeep siliceous turbidites, (3) Messinian foredeep siliceous turbidites, (4) Messinian and lower Pliocene thrust-top sediments, (5) middle Lias middle Miocene slope-basin limestones and marls between the Lower and Upper Thrust, (6) upper Trias lower Lias shelf carbonates and middle Lias middle Miocene slope-basin limestones and marls above the Upper Thrust, (7) upper Trias middle Cretaceous shelf carbonates of the Latium-Abruzzi platform, (8) anticline axis, (9) syncline axis, (10) thrust fault, (11) extensional fault, (12) inferred location of the footwall ramp of the Gran Sasso range, (13) inferred location of left-lateral decoupling boundary between the western and eastern Gran Sasso sectors, (14) paleomagnetic declinations from Dela Pierre et al. (1992), (15) locations of geological cross-sections shown in Fig. 7.

6 188 F. Speranza et al. / Tectonophysics 362 (2003) Paleomagnetic investigations in the Gran Sasso range (Dela Pierre et al., 1992) showed j CCW rotations at five sites from the eastern belt (Fig. 2). Further west, small clockwise (CW) and CCW rotations were observed at three sites. The large CCW rotations are clearly related to the Gran Sasso thrusting, as no rotation was measured in the Messinian lower Pliocene turbidites exposed to the north (Dela Pierre et al., 1992; Speranza et al., 1997). The documented CCW rotations seemed to confirm the variable amount of shortening along the E W Gran Sasso front, but the along-strike rotational evolution was still largely obscure, and the tectonic boundary between rotated and unrotated sectors unclear. Moreover, most of the sites of Dela Pierre et al. (1992) are not located on the frontal compressive structures, but on the extensionally downfaulted area south of it (Fig. 2). Here extensionrelated block tilting may have caused small additional rotations and formed plunging-axis structures, where spurious rotations are calculated because of the erroneous tilt-correction applied. 4. Geology of Corno Grande and paleomagnetic sampling The Gran Sasso range culminates at the 2912 m of Corno Grande (Fig. 2), which hosts a small circ glacier (Ghiacciaio del Calderone), still representing the southernmost glacier of Europe. Corno Grande is formed by 600 m thick massive lower Lias shelf limestones ( Calcare Massiccio ) on top of upper Triassic dolomites (Alessandri et al., 1968, Fig. 3). North of Corno Grande the Calcare Massiccio is overlain by middle Lias pelagic cherty limestones and marls ( Corniola ). The time-dependent evolution from reef (Calcare Massiccio) to basinal (Corniola) environment is tectonically driven, as extensional activity drowning the Calcare Massiccio carbonate platform is widely documented elsewhere in the central northern Apennines (Consiglio Nazionale delle Ricerche, 1988). Such strong early Lias extensional phase is likely related to the opening of the Atlantic Ocean and Pangea breakup (e.g. Muttoni et al., 1996). Clear evidence of early Lias extensional activity is also directly observed NW of Corno Grande, where steep Corniola strata onlap the Calcare Massiccio along an E W subvertical early Lias normal fault (Speranza, in press) (Figs. 3 and 4). After the early Lias, Corno Grande remained emerged or at shallow depth as a seamount, while adjacent areas were drowned and subsequently characterized by basinal sedimentation. The articulated paleobathimetry during the Mesozoic is testified by the Corniola thickness, varying in the Gran Sasso range from 50 to 600 m. North of Corno Grande, the Corniola strata are conformably overlain by upper Lias lower Malm green reddish nodular limestones and marls ( Verde Ammonitico ), followed by upper Jurassic lower Cretaceous calcareous turbidites and pelagic limestones. The whole upper Triassic lower Cretaceous succession forms a N-verging anticline tectonically overlying (along the Upper Thrust) an overturned syncline exposing middle Lias to Messinian pelagic and slope sediments (Fig. 3). The Upper Thrust displacement progressively decreases westward, and vanishes at Pizzo Intermesoli, few km west of Corno Grande (Fig. 2), where the anticline and syncline form a continuous fold. The early Lias extensional tectonics at Corno Grande is also testified to by a swarm of spectacular f E W subvertical sedimentary dykes up to 10 m thick cutting gently dipping Calcare Massiccio strata and filled by Corniola-equivalent doloarenites (Scarsella, 1958; Adamoli, 1992; Speranza, in press; Fig. 4). Persisting extensional activity during Mesozoic early Tertiary time is documented by other sets of more recent sedimentary dykes, filling fractures related to the collapse of the isolated Corno Grande seamount. Verde Ammonitico dykes are exposed east of the early Lias normal fault, whereas N S dykes filled by Eo- Oligocene Scaglia reddish limestones crop out at and around the Corno Grande summit (Scarsella, 1955; Adamoli, 1992; Speranza, in press; Fig. 3). We paleomagnetically investigated the Corno Grande area by hand-sampling calcareous blocks at 11 localities (Fig. 3). We overcome the problem of extremely weak remanence in the Calcare Massiccio shelf limestones by sampling the sedimentary dykes filled by pelagic sediments. Three sites (Co05bis, Co09, and Co19) were sampled in Verde Ammonitico dykes, and four (Co06, Co07, Co11, and Co11bis) in the reddish Scaglia dykes around the Corno Grande top (Fig. 3 and Table 1). Furthermore, two sites (Co18 and Co32) were sampled in the Corniola strata

7 F. Speranza et al. / Tectonophysics 362 (2003) Fig. 3. Geological map of Corno Grande and thrusting-related paleomagnetic rotations. Geological features are superposed over a topographic background from an Istituto Geografico Militare (Italy) 1:25,000 scale map. Legend: (1) Pleistocene Holocene cemented breccia and debris, (2) middle Lias Messinian slope-basin sediments below the Upper Thrust, (3) upper Jurassic lower Cretaceous calcareous turbidites and pelagic limestones, (4) Verde Ammonitico nodular limestones and marls (lower Malm upper Lias) and Corniola cherty limestones and marls (middle Lias), (5) Calcare Massiccio shelf limestones (lower Lias) and upper Trias dolomites. Sampling sites: (6) Eocene Oligocene Scaglia dykes, (7) Santonian Coniacian Scaglia, (8) Verde Ammonitico dykes, (9) Verde Ammonitico, (10) Corniola, (11) vertical-axis rotations with respect to foreland (Table 1), (12) and (13) bedding (and dip angle) in normal polarity and overturned strata, respectively. north of Corno Grande, one (Co21) in the Verde Ammonitico just above the Upper Thrust, and one (Co22) in the upper Cretaceous Scaglia limestones just beneath the Upper Thrust (Fig. 3 and Table 1). The sediment ages (as reported in Table 1) were inferred by Adamoli (1992), although the micropaleontological content of Verde Ammonitico dykes and strata and of upper Cretaceous Scaglia strata was also determined. The Verde Ammonitico samples are systematically sterile, whereas the Scaglia limestones (site Co22) contain the planktonic foraminifers Dicarinella sp., Marginotruncana sp., Heterohelix sp., and Hedbergella sp., and the dinoflagellate cyst of the genus Calcisphaerula sp., thus being referable to the early Senonian (Coniacian Santonian). Each hand sample was drilled in the laboratory, yielding

8 190 F. Speranza et al. / Tectonophysics 362 (2003) Fig. 4. Onlap of Corniola (Co) over Calcare Massiccio (CM) strata along the E W early Lias normal fault (close to site Co18). The bold arrow in the right-side of the photo indicates two persons sit on top of an f 8-m-thick Corniola sedimentary dyke filling E W fractures parallel to the early Lias fault oriented standard paleomagnetic cores per sampling locality. 5. Paleomagnetic results The magnetic measurements were done in the shielded room of the paleomagnetic laboratory of the Istituto Nazionale di Geofisica e Vulcanologia (Roma). All the specimens were demagnetized in steps and the natural remanent magnetization (NRM) was measured after each step with a 4.5-cm access pass-through 2G cryogenic magnetometer. Alternating field (AF) cleaning was systematically used, except for the Verde Ammonitico strata samples (site Co21), which contained high-coercivity magnetic Table 1 Paleomagnetic results from Corno Grande Site Formation Age Age (Ma) N D b (deg) I b (deg) D a (deg) I a (deg) k a 95 (deg) R (deg) Co07 Dyke SC Eocene Oligocene F 8 Co11bis Dyke SC Eocene Oligocene F 18 Co22 SC Coniacian Santonian F 11 Co05bis Dyke VA upper Lias lower Malm F 5 Co09 Dyke VA upper Lias lower Malm F 7 Co19 Dyke VA upper Lias lower Malm F 7 Co21 VA upper Lias lower Malm F 7 Co18 Corniola middle Lias F 10 Formations: VA, Verde Ammonitico; SC, Scaglia. Epoch and stage ages in mega annum are from time scales of Gradstein et al. (1994) and Berggren et al. (1995). N, number of studied samples at a site. D and I are site mean declinations and inclinations calculated before (D b and I b ) and after (D a and I a ) tilt correction. Value k and a 95 are statistical parameters after Fisher (1953). The value R is the thrusting-related vertical-axis rotation calculated (according to Demarest, 1983) by comparing the obtained paleodeclinations to the coeval reference declinations of the Adriatic foreland domain. Reference Adriatic paleomagnetic poles used for the Mesozoic sites (from Channell, 1992) are: LatN = 62.96j/ LongE = j/A95 = 12.3j (for the Coniacian Santonian site), LatN = 44.03j/LongE = j/A95 = 3.0j (for upper Lias lower Malm sites), LatN = 64.71j/LongE = j/A95 = 4.5j (for the middle Lias site). Conversely, Eocene Oligocene rotations are calculated by comparing to the declinations to the local north (see text for explanation).

9 F. Speranza et al. / Tectonophysics 362 (2003) fraction and were thermally demagnetized. Efficiency of AF cleaning and the low to intermediate coercivity values (below mt) observed imply that magnetite is likely to represent the main magnetic carrier. Demagnetization data were evaluated using principal component analysis (Kirschvink, 1980). In the Corniola site Co18 (the remaining one Co32 was not magnetic) and in all the Verde Ammonitico dyke sites (Co05bis, Co09, Co19), a single component of magnetization was observed in the mt interval (Fig. 5a). The Scaglia dyke sites (Co06, Co07, Co11, Co11bis) proved to be strongly magnetized and to carry a stable magnetic signal (Fig. 5b), but yielded randomly distributed magnetization components. This behaviour is attributed to the effect of lightning striking the sharp Corno Grande summit. In two out of four Scaglia dyke sites (Co07, Co11bis), a characteristic component of magnetization (ChRM) was isolated by thermal demagnetization in the jc interval, after having eliminated the lightningrelated component by applying the peak (160 mt) AF field available. In the Scaglia site Co22, a viscous Fig. 5. Vector diagrams of typical alternating magnetic field (a c) and thermal (d) demagnetization data, in situ coordinates. Demagnetization step values are in mt (a c) and jc (d), respectively. (a) Verde Ammonitico dyke sample, (b) Eocene Oligocene Scaglia dyke sample, (c) Santonian Coniacian Scaglia sample, (d) Verde Ammonitico sample. The diagram (d) is an enlargement of the vector diagram after thermal demagnetization at 400 jc (see text for explanation).

10 192 F. Speranza et al. / Tectonophysics 362 (2003) component was eliminated at 10 mt, a low-coercivity component at 40 mt, and finally a ChRM was isolated in the mt interval (Fig. 5c). In the Verde Ammonitico site Co21, an intermediate temperature component was eliminated below 500 jc, and a ChRM was observed in the jc interval (Fig. 5d). Site-mean paleomagnetic directions are well defined, the a 95 confidence angles being generally well below 10j (Fig. 6 and Table 1). The in situ directions are far from the normal (and reverse) geocentric axial dipole field at Corno Grande (Fig. 6), excluding the possibility of a recent overprint. The fold test (according to McFadden, 1990) is not significant at the 95% level due to similar bedding attitudes in most of the sites (n in situ = 3.179, n unfolded = 0.355, and n 95% = 3.298). However, maximum k and minimum n values are observed at 100 and 99% of complete unfolding, respectively, supporting the pre-folding magnetization acquisition. After tilt correction, all directions (but the one from the Scaglia site Co22) have reverse polarity (Fig. 6). The normal polarity at site Co22 is in agreement with the Coniacian Santonian age determined by the microfossil content (the top Santonian corresponds to the upper bound of the Cretaceous normal superchron, e.g. Gradstein et al., 1994). The Corno Grande area underwent a polyphasic tectonic evolution involving Liassic (and more recent) foreland-plate extension, and late Messinian middle Pliocene shortening. Therefore, the question arises as to whether a simple tilt correction can restore beds in their original attitude. We note that all the sites but Co22 are located on a unique north-verging anticline, where the fold-axis plunging may be easily evaluated. At Corno Grande, bedding in the Calcare Massiccio strata shows that the anticline axis plunges westward by about 20j (Figs. 3 and 4). Such plunging value, coupled with the weak ( < 30j) bedding dip value, implies that the paleomagnetic declination error arising from an incorrect tilt correction is < 5j (e.g. Tarling, 1983). We conclude that the possible declination error introduced in our data is small, and thus negligible, compared to the large ( f 90j) rotations documented elsewhere at Gran Sasso in the past (Dela Pierre et al., 1992). The tilt-corrected paleodeclinations were compared to the coeval declinations expected for the Adriatic foreland area in order to calculate (according to Demarest, 1983) the thrusting-related rotations at Fig. 6. Equal-area projection of the site-mean paleomagnetic directions from Corno Grande. Open (solid) symbols represent projection onto upper (lower) hemisphere. Site-mean directions: B Eocene Oligocene Scaglia dyke (sites Co07, Co11bis); n Coniacian Santonian Scaglia (Co22); o Verde Ammonitico dykes (Co05bis, Co09, Co19); D Verde Ammonitico (Co21); w Corniola (Co18). Site symbols are the same as in Fig. 3. The asterisk represents the normal polarity GAD field direction for the study area (latitude 42.5jN, longitude 13.5jE). Open ellipses are the projections of the a 95 cones about the mean directions.

11 F. Speranza et al. / Tectonophysics 362 (2003) Gran Sasso. We used reference Adriatic paleomagnetic poles for Mesozoic times from Channell (1992), who studied pelagic sediments from the northern Apennines equivalent (in age and facies) to those sampled by us (Table 1). Conversely, the Eocene Oligocene declinations from the Scaglia dyke sites were compared to the local north, because the Adriatic foreland (coupled to Africa) is thought to have undergone no significant rotation since lower middle Tertiary (e.g. Channell, 1992, 1996; Besse and Courtillot, 1991). All the rotation values are < A10jA (and generally < A5jA) except one from the site Co19 (Verde Ammonitico dyke), where a 57 F 7j CW rotation is calculated (Fig. 3 and Table 1). The reason for this large-angle CW rotation is not understood, because it is completely different (in sign and amplitude) from those observed by us at Corno Grande and by Dela Pierre et al. (1992) in other Gran Sasso localities. The general coherence of rotations inferred from sedimentary dykes and strata implies that in all these rocks the secular variation of the geomagnetic field is averaged, contrary to what is observed in igneous dykes. This suggests that the magnetic grains were free to reorient parallel to the ambient magnetic field after the dyke filling episode, and that the lock-in time for the remanence of the dykes was of the same order of magnitude of the hosting sediments. Therefore, our data show that sedimentary dykes are in principle good candidates for paleomagnetic investigations. 6. Discussion 6.1. Rotational difference between the western and eastern Gran Sasso sectors Our paleomagnetic results indicate that the Gran Sasso arc is in fact a composite belt, made up of a western non-rotated segment and a strongly CCW rotated eastern sector (Fig. 2). The null rotation at Corno Grande and further west (and the disappearance of the frontal thrusts) suggests that the northward horizontal displacement is minimal, and only the frontal structures are tectonically duplicated by blind thrust surfaces. We infer that here the footwall ramp is close to the frontal structures and is inverted by the Assergi Fault (Figs. 2 and 7) showing 1500 m of extensional offset (D Agostino et al., 1998). Paleomagnetism also documents that the western E W Gran Sasso front is an original feature and does not arise from rotation of an originally NW SE belt (i.e. the average Apenninic structural trend). The E W trend of the early Lias extensional fault at Corno Grande (Figs. 3 and 4) suggests that the local deviation from the mean Apenninic shortening direction arises from inheritance of pre-existing discontinuities interacting with the orogenic wedge propagation. The two strongly CCW rotated sites south of Corno Grande and an unrotated site SW of the Assergi Valley (Dela Pierre et al., 1992, Fig. 2) help to trace the boundary between the rotating and non-rotating part of the Gran Sasso range. This boundary represents the left-lateral shear zone decoupling the western and eastern Gran Sasso ranges. Along this zone, the limestones are affected by pervasive cataclasis and were easily eroded by the Raiale River, connecting the Assergi Valley with L Aquila Plain (Fig. 2) Implications for the shortening occurring along the range-front décollements The strong rotation of the eastern Gran Sasso sector implies a large (and eastward increasing) northward displacement along the basal detachments. A semi-rigid 90j CCW rotation of the eastern Gran Sasso structures pivoting about a pole located SE of Corno Grande requires about 20 km of minimum horizontal shortening for the easternmost Gran Sasso range (Fig. 7). Given the presence of two major décollements at the eastern Gran Sasso front (the exposed Upper Thrust and the inferred Lower Thrust), the question arises on which of these two detachment surfaces gives rise to the CCW rotations. We note that at M. Camicia (Fig. 2) the Upper Thrust produces about 2 km of horizontal displacement (D Agostino et al., 1998), i.e. an order of magnitude smaller than the 20 km required by a semi-rigid 90j rotation of the eastern Gran Sasso salient. This suggests that the CCW rotation is predominantly related to shear along the Lower Thrust. The large displacement inferred for the Lower Thrust requires the duplication of the sedimentary cover beneath the NRHP (Fig. 7), as proposed by D Agostino et al. (1998). In this frame, the closely spaced NRHP normal faults formed to accommodate

12 194 F. Speranza et al. / Tectonophysics 362 (2003) Fig. 7. Geological cross sections from the western (AAV) and eastern (BBV) Gran Sasso range (see Fig. 2 for location). The null rotation of the Lower Thrust footwall (BBVsection) is inferred by paleomagnetic data obtained north of Gran Sasso by Dela Pierre et al. (1992) and Speranza et al. (1997). Bold (open) arrows indicate compressive (extensional) displacements. Double tip grey arrows indicate displacements along inverted thrust ramps. AF and TSF are the traces of the Assergi and Tre Selle fault, respectively. the mechanical adjustment of the hanging-wall over a major basal detachment underlying the whole eastern Gran Sasso belt. The mechanically weak layer inferred by D Agostino et al. (1998) may be represented by either middle late Miocene marls or Messinian turbidites (containing evaporite beds) below the Lower Thrust (Fig. 7) Age and kinematics of tectonic deformation The Gran Sasso range tectonically covers Messinian and lower Pliocene turbidites along the E W and N S belt fronts, respectively (Fig. 2). Moreover, both Messinian ( Conglomerati di M. Coppe ) and lower Pliocene ( Conglomerati di Rigopiano ) thrust-top deposits unconformably cover the Meso-Cenozoic multilayer at the arc apex (Ghisetti and Vezzani, 1986, 1991, Fig. 2). This is suggestive of a two-fold compressive evolution, determining the subsequent thrusting episodes over the Messinian and the lower Pliocene foredeep turbidites located at the front of the migrating Apennine wedge. The early middle Pliocene tectonic phase led to the disappearance of the Messinian foredeep area below the Gran Sasso carbonates. This along-time geological evolution may be combined to the rotation pattern to reconstruct the formation of the Gran Sasso arc (Fig. 8). We infer that during the late Messinian, eastward thrusting (causing the deposition of thrust-top sediments) occurred along a roughly N S front. This late Messinian N S front is still visible NE of Corno Grande (Fig. 2) and continues further north for some tens of kilometres connecting to the N S Montagna dei Fiori thrust sheet (Consiglio Nazionale delle Ricerche, 1991). Its original N S direction is testified by paleomagnetic data from the Messinian turbidites reported by Dela Pierre et al. (1992) and Speranza et al. (1997).

13 F. Speranza et al. / Tectonophysics 362 (2003) Fig. 8. Synthetic cartoon showing the inferred late Messinian to early middle Pliocene evolution of the Gran Sasso range. Legend: (1) Messinian lower Pliocene thrust-top deposits, (2) Latium-Abruzzi platform carbonates, (3) inferred location of the thrust systems footwall ramps, (4) trace of the future range front location. We suggest that the Messinian N S chain was subsequently disrupted by the early middle Pliocene tectonics. The southern N S front was extruded northeastward, determining large-scale duplication of the Meso-Cenozoic cover associated with a 90j CCW rotation. The extruded block assumed a curved shape, overrode the lower Pliocene foredeep sediments, and induced deposition of thrust-top coarse sediments at the far-travelled arc apex (Fig. 8). Likely, the northward displacing salient induced further west the positive inversion of Liassic inherited E W extensional faults, and the building of the western Gran Sasso E W front. Corno Grande is located at the conjunction between rotated and unrotated Gran Sasso sectors, and in correspondence of the intersection with the Messinian N S thrust front to the north (Fig. 2). This location may indicate that the high elevation of Corno Grande is due to interference between N S and E W fronts, each producing duplication of the sedimentary sequences. The superb evidence of pre-orogenic extension at Corno Grande shows that it was an isolated seamount during Mesozoic, completely bounded by normal faults. This suggests that both the N S and E W compressive fronts reactivated inherited mechanical weakness surfaces Possible evolution of the Gran Sasso arc in the frame of the Apennine tectonics The CCW rotated eastern Gran Sasso arc contains a backbone of persisting shelf carbonates surrounded by sediments deposited in basinal and slope-to-basin environments (Fig. 2). The latter dominate the western unrotated belt, suggesting a relationship between rotations and reef deposits at Gran Sasso. We suggest (as also proposed by Ghisetti and Vezzani, 1991; Dela Pierre et al., 1992) that the thick and rigid Latium- Abruzzi platform carbonates were laterally extruded due to the interference of the Apennine wedge with the Apulian shelf carbonates, located about 30 km southeast of Gran Sasso (Fig. 1). Although the Apulian carbonates are not observed adjacent to the Gran Sasso arc, they crop out on the southern part of the N S Maiella structure. The buried northern part of the Maiella front is located just east of Gran Sasso, where it causes a c 3000-m vertical offset of the base of Pliocene sediments (Fig. 1). After the early Pliocene, the forward eastward migration of the Apennines was inhibited by the presence of the rigid Apulian carbonates and the pre-existing N S Maiella thrust front. This likely caused the out-of-sequence lateral escape of the Latium-Abruzzi carbonates towards the north, over

14 196 F. Speranza et al. / Tectonophysics 362 (2003) the ductile basinal Umbria Marche sediments and the overlying Messinian turbidites. Such mechanism justifies the formation of an E W front in an overall NW SE belt and the reactivation of pre-orogenic E W extensional discontinuities as thrust ramps. 7. Conclusions Paleomagnetic data from sedimentary dykes and strata from Corno Grande (central western Gran Sasso belt) reveal that no rotation occurred synchronous with shortening. When compared to the 90j CCW rotation previously reported from the eastern belt, our data show that Gran Sasso is in fact a composite arc. The null rotation at Corno Grande suggests little shortening in the western Gran Sasso E W front, in agreement with other geological evidence. East of Corno Grande, the 90j rotation is related to shear occurring along the Lower Thrust, producing a minimum 20 km northward horizontal shortening in the vicinity of the arc apex. Geological and paleomagnetic data highlight a two-fold sequence of compressive episodes leading to the formation of the Gran Sasso arc. We suggest that the southern part of a late Messinian N S belt system was reactivated during early middle Pliocene producing large-scale shortening associated with 90j CCW rotation. The rotated belt contains a shelf carbonate core, which was northward laterally extruded towards basinal sediments because of the collision between the rigid Latium-Abruzzi and Apulian carbonates during the northeastward propagation of the Apennine wedge. The northward displacement of shelf carbonates possibly induced the reactivation of Liassic E W extensional faults as thrust ramps, building up the western Gran Sasso E W front. Our data highlight the importance of detailed paleomagnetic investigations to unravel the structure and evolution of small-scale curved fronts. Acknowledgements N. D Agostino, L. Alfonsi, J. Dinarès, and F. D Ajello are greatly thanked for their help and discussions in the field. We are grateful to the referees G. Muttoni and M.E. Beck for providing valuable comments on the manuscript. FS wishes to dedicate this paper to his daughter Claudia, who came into this world last June References Adamoli, L., Evidenze di tettonica d inversione nell area Corno Grande Corno Piccolo (Gran Sasso d Italia). Boll. Soc. Geol. Ital. 111, Adamoli, L., Bigozzi, A., Ciarapica, G., Cirilli, S., Passeri, L., Romano, A., Duranti, F., Venturi, F., Upper Triassic bituminous facies and Hettangian pelagic facies in the Gran Sasso range. Boll. Soc. Geol. Ital. 109, Alessandri, D., Scandone, P., Scarsella, F., Il Trias della parete orientale del Corno Grande (Gran Sasso d Italia). Boll. Soc. Nat. Napoli 77, Berggren, W.A., Kent, D.V., Swisher III, C.C., Aubry, M.-P., A revised Cenozoic geochronology and chronostratigraphy: geochronology time scales and global stratigraphic correlation. Spec. Publ.-SEPM 54, Besse, J., Courtillot, V., Revised and synthetic apparent polar wander paths of the African, Eurasian, north American and Indian plates, and true polar wander since 200 Ma. J. Geophys. Res. 96, Bigi, S., Calamita, F., Paltrinieri, W., 1995a. Modi e tempi della strutturazione della catena centroappenninica abruzzese dal Gran Sasso alla costa adriatica. Studi Geologici Camerti, Special Volume 1995/2, pp Bigi, S., Calamita, F., Centamore, E., 1995b. Caratteristiche geologico-strutturali dell area abruzzese ad oriente del Gran Sasso. Studi Geologici Camerti, Special Volume 1995/2, pp Cavinato, G.P., De Celles, P.G., Extensional basins in the tectonically bimodal central Apennines fold-thrust belt, Italy: response to corner flow above a subducting slab in retrograde motion. Geology 27, Channell, J.E.T., Paleomagnetic data from Umbria (Italy): implications for the rotation of Adria and Mesozoic apparent polar wander paths. Tectonophysics 216, Channell, J.E.T., Palaeomagnetism and palaeogeography of Adria. In: Morris, A., Tarling, D.H. (Eds.), Palaeomagnetism and Tectonics of the Mediterranean Region. Geol. Soc. Spec. Publ. No. 105, pp Channell, J.E.T., Oldow, J.S., Catalano, R., D Argenio, B., Paleomagnetically determined rotations in the western Sicilian fold and thrust belt. Tectonics 9 (4), Cipollari, P., Cosentino, D., Miocene unconformities in the Central Apennines: geodynamic significance and sedimentary basin evolution. Tectonophysics 252, Cipollari, P., Cosentino, D., Esu, D., Girotti, O., Gliozzi, E., Praturlon, A., Thrust-top lacustrine lagoonal basin development in accretionary wedges: late Messinian (Lago Mare) episode in the central Apennines (Italy). Palaeogeogr. Palaeoclimatol. Palaeoecol. 151, Consiglio Nazionale delle Ricerche, Progetto finalizzato geodinamica. Note illustrative alla carta delle litofacies del

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