Paleomagnetic evidence for a Neogene two-phase...

Tectonophysics 326 (2000) 241–253www.elsevier.com/locate/tecto

Paleomagnetic evidence for a Neogene two-phasecounterclockwise tectonic rotation in the

Northern Apennines (Italy)

G. Muttoni a,*, L. Lanci a, A. Argnani b, A.M. Hirt a, U. Cibin c,N. Abrahamsen d, W. Lowrie a

a Institute of Geophysics, ETH-Hoenggerberg, CH-8093 Zurich, Switzerlandb Istituto di Geologia Marina, via Gobetti 101, I-40129 Bologna, Italy

c Servizio Cartografico e Geologico, Regione Emilia-Romagna, viale Silvani 4/3, Bologna, Italyd Department of Earth Sciences, Aarhus University, Aarhus, Denmark

Received 9 April 2000; accepted for publication 30 May 2000

Abstract

Paleomagnetic directions have been determined for a new collection of Early Oligocene and Late Miocene–PlioceneEpiligurian clastic sediments from the frontal portions of the northern Apennines. These results are combined withCenozoic data from the literature to evaluate whether rotations of units in this region are related to the Oligo-Miocene Corsica–Sardinia rotation and/or to younger phases of deformation of the Apennine chain. WhenCorsica/Sardinia moved counterclockwise off the coast of France, the Ligurian units located at its front werepresumably pushed eastward and rotated counterclockwise above a main boundary thrust onto the Adria/Africamargin. We propose that about 24 out of a total of 52° of rotation observed in the Epiligurian units can be relatedto the Oligo-Miocene motion of the Corsica–Sardinia block, in partial agreement with previous conclusions, and theremaining 28° to the Pliocene tectonic phase at the Apennine chain front, which may have (re)activated thrust planesin the Adria/Africa succession below the Ligurian wedge. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Apennines; Italy; Neogene; paleomagnetism

1. Introduction: paleomagnetism and Apennine Late Miocene and Pliocene. Tectonic rotations asso-ciated with thrusting have recently been docu-tectonicsmented paleomagnetically along the NW–SE-trending, rectilinear northern Apennines (MuttoniThe Apennines are a Neogene thrust-and-foldet al., 1998), as well as in other portions of thebelt that developed in conjunction with the Oligo-northern and central Apennines (Mattei et al., 1996;Miocene Corsica–Sardinia block rotation and theSperanza et al., 1997). We present new data fromsubsequent opening of the Tyrrhenian sea in thethe northern Apennines and review data from theliterature to evaluate whether the observed tectonicrotations are related to the Oligo-Miocene Corsica–* Corresponding author. Tel.: +41-1-633-2633;Sardinia rotation and/or to younger phases offax: +41-1-633-1065.

E-mail address: [email protected] (G. Muttoni) structuration of the Apennine chain.

0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0040-1951 ( 00 ) 00140-2

242 G. Muttoni et al. / Tectonophysics 326 (2000) 241–253

1.1. Oligocene–Miocene Corsica–Sardinia rotation 1.2. Late Miocene–Pliocene Tyrrhenian opening

The deformation of the Apennines continuedThe Apennines are composed of, among others,after the end of the rotational motion of thesediments and ophiolites from the MesozoicCorsica–Sardinia block. With the opening of theLigurian ocean. The Ligurian ocean was formedTyrrhenian sea since the Late Miocene, compres-as a consequence of the breakup of Pangea andsional deformation occurred at the front of theseparation of Africa from Europe in the Jurassicnorthern-central Apennines, while internal to theand was located between Corsica–Sardinia, whichfront, along the Tyrrhenian margin, the continentalat that time was attached to the southern Europeancrust has been subject to tension with the forma-margin of France, and the Africa margin of west-tion of NW–SE-trending grabens. Deformationern Adria. Later, during the Africa/Europe con-peaked in the Early Pliocene when a major episodevergence since Late Cretaceous times, the Ligurianof thrusting took place at the front of the northernoceanic units underwent deformation due toApennines (Boccaletti et al., 1990). Paleomagneticthe subduction of the Ligurian ocean below thedata from Messinian clastics outcropping in theEuropean margin of Corsica–Sardinia. In theMarche-Romagna foredeep at the front of theOligo-Miocene, Corsica–Sardinia rifted off Europenorthern-central Apennine belt (Speranza et al.,and moved away, rotating counterclockwise to1997) indicate a significant counterclockwise rota-attain its present-day position in the middle of thetion, whereas Messinian to Pliocene sediments andeventual western Mediterranean basin (e.g.intrusives from the Tyrrhenian margin of TuscanyAlvarez, 1972; Vigliotti and Langenheim, 1995).show no rotation of paleomagnetic vectors (LowrieAs a consequence of the motion of Corsica–and Alvarez, 1979; Sagnotti et al., 1994; MatteiSardinia, the Ligurian accretionary wedge waset al., 1996). Therefore, at the time the Apenninethrust onto the Cenozoic foredeep sediments,front was subject to compression and rotation,which were progressively deposited at the front ofTuscany was experiencing non-rotational exten-the Ligurian thrust sheets as these were migratingsion (cf. Speranza et al., 1997).

to the northeast onto the Adria passive continental To confirm the hypothesis that the paleomag-margin (e.g. Boccaletti et al., 1990). netic rotations observed by Muttoni et al. (1998)

Recently, Muttoni et al. (1998) carried out a in the Epiligurian units were associated with thepaleomagnetic study on clastic sediments deposited motion of the Corsica–Sardinia block, the precisein relatively undeformed basins located on top of timing of the Corsica–Sardinia block motionthe advancing Ligurian thrust sheets. These clas- would need to be constrained. This can be accom-tics, referred to in the literature as Epiligurian plished by finding the end of the northernunits, yielded a component of magnetization of Apennines rotation in Late Miocene or youngerLate Oligocene–Middle Miocene age. The derived Epiligurian sediments, thus allowing separation ofpaleomagnetic pole shows a large and significant the effects of the ‘Corsica–Sardinia’ from thecounterclockwise rotation with respect to the ‘Tyrrhenian’ tectonic phases. This paper presentscoeval (10–30Ma) African reference paleopoles new paleomagnetic data from Early Oligocene(Besse and Courtillot, 1991). The Late Oligocene– and Late Miocene–Pliocene sediments of theEarly Miocene paleopole of Corsica–Sardinia Epiligurian units, and discusses again the conclu-shows a similar, yet smaller, amount of rotation sions attained by Muttoni et al. (1998) in the lightwith respect to European reference paleopoles. of these new results and data from the literature.Therefore, Muttoni et al. (1998) suggested thatthe thrusting of the Ligurian units onto the Adriamargin was accompanied by the counterclockwise 2. Age and location of the sampling sitesrotations of the thrust sheets, and that these rota-tions may have been associated, although not This study focuses primarily on the youngestnecessarily rigidly coupled, with the counterclock- portion of the Epiligurian clastic succession, which

crops out in the Bologna area (Fig. 1a). Steeplywise motion of the Corsica–Sardinia block.

243G. Muttoni et al. / Tectonophysics 326 (2000) 241–253

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dipping clay-rich sediments of the Late Miocenesuccession were collected at seven sites, and theoverlying Intra-Apennine Pliocene was sampled atthree additional sites located a few kilometers tothe south in almost flat-lying sediments. The LateMiocene succession consists of Tortonian pre-evaporitic clays (Termina Formation), sampled atsites 31–34, followed by the Messinian evaporitesrelated to the Mediterranean salinity crisis and,finally, by post-evaporitic clays with resedimentedgypsum derived from the Colombacci Formationof Messinian age, sampled at sites 35–37. TheIntra-Apennine Pliocene consists of over 1000 mof shallow marine to continental clastics. Wesampled the marine clays of the Argille AzzurreFormation at sites 38 and 39, and some harderconcretions within looser Pliocene marine sand-stone beds at site 40. Further sampling was donein the flat-lying, older portion of the Epiliguriansuccession to confirm and expand the previousresults of Muttoni et al. (1998), and namely, wesampled in the Early Oligocene siliciclastic turbid-ites of the Ranzano Sandstones at sites 41 and 42,47–49 and 53 (Fig. 1a). Site 48 is a volcanoclasticlevel located within the Ranzano Sandstones.

3. Rock-magnetic properties

The magnetic mineralogy of selected samplesthat gave reliable paleomagnetic directions fromsites 31 and 33 (Tortonian Termina Formationclays), 35 (Messinian Colombacci Formationclays), 40 (Pliocene sandstones) and 48 (EarlyOligocene Ranzano Sandstones), that we considerrepresentative of the main lithologies encountered,was determined using standard rock-magneticexperiments. An incremental isothermal remanent

Fig. 2. Rock-magnetic properties of selected Epiligurian clasticssamples deduced from the acquisition curves of isothermalremanent magnetization (IRM) (a), the thermal unblockingcharacteristics of orthogonal-axes IRMs (Lowrie, 1990)induced at fields of 1, 0.6 and 0.15 T (b), and, finally, the ratiosof the hysteresis parameters coercivity of remanence (Hcr),coercivity (Hc), saturation remanence (Jrs) and saturation mag-netization (Js) plot on a Day et al. (1977) diagram (c). See textfor discussion.


magnetization (IRM) was imparted at 25–50 mT 35 from the Colombacci Formation clays ofMessinian age have lower Hcr/Hc and higher Jrs/Jssteps along the sample z-axis, and remanence was

measured after each magnetization step up to a ratios compared to Early Oligocene samples fromthe Ranzano Sandstones, suggesting a differentmaximum magnetizing field of 1 T. The IRM was

then AF-demagnetized to determine the value of magnetic grain size. The Hc and Hcr values arecomprised between 12 and 23 and between 28 andthe median destructive field (MDF). Finally, a

composite IRM at 1, 0.6 and 0.15 T fields was 33 mT, respectively, and are compatible with thoseexpected for magnetite.induced along sample orthogonal axes and ther-

mally demagnetized up to 680°C in 20–100°Csteps, measuring remanence after each demagneti-zation step (Lowrie, 1990). 4. Magnetic fabric

All samples have very similar IRM acquisitioncurves that saturate at fields between 0.25 and The anisotropy of magnetic susceptibility (AMS)

has been measured to determine whether the sedi-0.3 T (Fig. 2a). The median destructive fields areof 22±9 and 16.9±15 mT for the Mio-Pliocene ments have undergone any significant deformation

(Sagnotti et al., 1998). The AMS was measured onclays and Early Oligocene sandstones, respectively.The thermal unblocking characteristics of ortho- an AGICO KLY-2 susceptibility bridge where the

anisotropy can be expressed as a second-ordergonal-axes IRM show the presence of a dominantlow coercivity (<0.15 T ) magnetic phase with tensor with principal axes k1>k2>k3. An average

ellipsoid was defined using a modified version ofmaximum unblocking temperatures of 570°C(Fig. 2b). These observations suggest that magne- the principal component analysis of Jelinek (1978).

The AMS was measured at 77 K, using the sametite is the main carrier of the magnetic remanence.Hysteresis loops and backfield demagnetization of approach as Hirt and Gehring (1993), for one

sample per site to investigate the minerals responsi-selected samples were measured using a Micromagalternating gradient field magnetometer. Most of ble for the observed magnetic fabric.

The Tortonian clays of the Termina Formationthe samples show a large paramagnetic componentof the induced magnetization. The values of the from sites 31, 33 and 34 near Bologna at the

Apennine front possess a lineation of under 1%coercivity of remanence (Hcr), coercivity (Hc),saturation remanence (Jrs) and saturation magne- and a weak compaction of less than 1.5% (Fig. 1,

Table 1). Site 33 exhibits an inverse fabric in whichtization (Js), when corrected for the paramagneticsignal, plot on a Day diagram (Day et al., 1977) the k1 axes are sub-parallel to the bedding pole,

and the k2 and k3 axes are distributed in thein the pseudo-single domain (PSD) range (Fig. 2c).Samples from sites 31 and 33 from the Termina bedding plane (bedding attitudes are reported on

the AMS stereoplots and in the caption to Fig. 1).Formation clays of Tortonian age and from site

Table 1Normalized magnitude and in-situ orientation of the principal axes of the ansiotropy of magnetic susceptibility k1>k2>ka

Site k1 Declination Inclination k2 Declination Inclination k3 Declination Inclination Lineation Foliation

31 1.007 279.0 24.1 1.000 47.1 55.2 0.993 278.9 24.0 1.008 1.00633 1.007 178.6 19.6 0.997 278.9 26.7 0.996 56.8 55.9 1.010 1.00134 1.005 35.9 61.0 1.004 271.7 17.3 0.991 174.3 22.5 1.001 1.01341 1.017 38.4 4.9 0.992 133.1 43.4 0.992 303.3 46.2 1.025 1.00147 1.013 85.9 8.3 1.004 354.1 11.8 0.984 210.3 75.5 1.009 1.02148 1.063 270.7 5.4 1.024 1.4 7.21 0.919 143.9 81.0 1.038 1.11449 1.023 252.0 2.5 1.014 161.5 9.5 0.964 356.7 80.2 1.009 1.05253 1.079 110.9 37.8 1.037 206.5 7.2 0.895 305.5 51.3 1.041 1.158

a The lineation is defined as k1/k2 and the foliation as k2/k3.


Fig. 3. Zijderveld demagnetograms of representative Epiligurian units samples from the Tortonian clays oucropping at the northernApennines front at Bologna (a) and from the Early Oligocene flat-lying sandstones located at sites internal to the chain front (b).Closed symbols are projections onto the horizontal plane, and open symbols are projections onto the vertical plane. All diagramsare in geographic coordinates.

In the flat-lying older portion of the Epiligurian are distributed as expected for the superpositionof a horizontal compaction on the bedding com-succession internal to the Apennine front, we

studied the AMS fabric in the Early Oligocene paction. The magnetic fabric at site 53 is the bestdeveloped with an average flattening of 16% in thesiliciclastic turbidites of the Ranzano Sandstones

at sites 41, 53 and 49–47 (Fig. 1, from east to bedding plane. Although the average lineation is4%, the k1 axes are distributed in the beddingwest; Table 1). Site 41 has a lineation of 2.5% and

a prolate AMS where the k1 axes are sub-parallel plane. The magnetic fabric at site 47 is dominatedby the bedding compaction, although the fabric isto the structural fold trend and the k2 and k3 axes


Fig. 4. Equal-area projections before and after bedding-tilt correction of the characteristic component directions from Epiligurianclastics at Tortonian site 31 and site 32–33 combined from the northern Apennine chain front (a), Tortonian and Pliocene sites 34and 40 (b), Messinian and Pliocene sites 35–39 (c), and from Early Oligocene site 47–49 combined and site 53 (d), and site 41–42combined from flat-lying sandstones located internal to the chain front (e). Solid symbols refer to the lower hemisphere.

weakly developed. Sites 48 and 49 display bedding In conclusion, the magnetic fabric of theTortonian clays at the Apennine front is generallycompaction but also a well-developed lineation

that is similar to the structural trends in the region. weak, whereas that of the Ranzano Sandstonesinternal to the front is dominated by beddingThe orientation of the principal axes of the

AMS ellipsoids did not change at 77 K, but all compaction.sites except 48 and 53 showed an increase by afactor of 3.0–3.8 in the intensity of the principalsusceptibilities at 77 K. This is in broad agreement 5. Paleomagnetic resultswith the Curie–Weiss Law for a purely paramag-netic material, and therefore, we conclude that Paleomagnetic samples were collected with a

water-cooled rock drill and oriented with a magneticparamagnetic minerals dominate the magnetic fab-rics at these sites. Sites 48 and 53 showed an compass. The natural remanent magnetization

(NRM) was measured on a 2G D.C.-squid cryogenicincrease in the susceptibility at 77 K by a factorof 2, which suggests that ferromagnetic minerals magnetometer at ETH Zurich. After pilot thermal

and alternating field (AF) demagnetization studies,are also responsible for the observed fabric atthese sites. all the standard 11.4 cm3 specimens were subjected

248 G. Muttoni et al. / Tectonophysics 326 (2000) 241–253T

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to progressive AF demagnetization. A least-squares coherence during Ligurian thrust sheets emplace-ment. In the Umbria-Marche Adriatic units under-analysis was applied to determine the component

directions of NRM (Kirschvink, 1980), chosen by lying the Ligurian allochthon (Fig. 1c), the thrustfronts are also mainly linear and trending parallelinspection of vector end-point demagnetization dia-

grams (Zijderveld et al., 1967). Site mean directions to the Apennine front, as suggested by the spatialdistribution of the Miocene foredeep depocentres.were determined using standard Fisher statistics.

The Tortonian clays at site 31 and site 32–33 It is only in the Pliocene that local deviations fromlinearity of the front occurred, as testified by the(i.e. sites 32 and 33 combined) are characterized by

an average NRM intensity of 0.87 mA/m. After formation of small-scale arc-shaped thrust frontsburied under the Po Plain sediments. Therefore,removal of an occasional present-day field compo-

nent in the 0–5 to 10 mT range, a bipolar character-istic remanent magnetization (ChRM) oriented NEand up or SW and down in geographic coordinateswas isolated up to a maximum of 70 mT (Fig. 3a).Upon application of the bedding tilt, the ChRMdirections become SE and up (NW and down)(Fig. 4a, Table 2). Other clay samples of Tortonian,Messinian and Pliocene age from sites 34, 35–37combined and 38–39 combined, respectively, as wellas the Pliocene sandstones at site 40, with NRMintensities normally comprised between 0.71 and66 mA/m, were discarded because the paleomag-netic components were either remagnetized in thepresent-day field direction (Fig. 4b) or were unsta-ble or scattered (Fig. 4c). The older Epiligurian site47–49 combined and site 53, comprising theRanzano Sandstones of Early Oligocene age, arecharacterized by an average NRM intensity of250 mA/m. AF demagnetization revealed the occur-rence of a characteristic magnetization componentoriented E or SE and up in geographic and tilt-corrected coordinates (Figs. 3b and 4d; Table 2).Finally, site 41–42 combined, sampled in the EarlyOligocene Ranzano Sandstones at Rioveggio(Fig. 1), with an average NRM intensity of520 mA/m, yielded a characteristic component ofmagnetization-oriented NE and down (i.e. rotatedclockwise) in both geographic and tilt-correctedcoordinates (Fig. 4e, Table 2).

6. Paleomagnetic regional mean directions and Fig. 5. Equal-area projections before and after bedding correc-tion of the characteristic component site-mean directions frompaleopolesthe Tortonian–Messinian sites located at the Apennine chainfront (a), and the Early Oligocene–Middle Miocene EpiligurianThe litologies sampled come from the frontalflat-lying clastics located at sites internal to the chain front (b).

portions of thenorthern Apennines, which are Numbers refer to the sites from this study. Solid symbols refercharacterized by a generally linear NW–SE thrust- to the lower hemisphere. (c) Comparison of the overall mean

directions from (a) and (b).fold trend (Fig. 1a), suggesting substantial tectonic


Fig. 6. Late Miocene Apennine Chain Front-paleomagnetic pole (solid cross) and the Early Oligocene–Middle Miocene paleopolefrom the Epiligurian flat-lying units internal to the front (solid square, position ‘A’) are compared with the apparent polar wanderpaths of Africa (open circles, Besse and Courtillot, 1991), and Corsica–Sardinia (open squares, Muttoni et al., 1998). The EarlyOligocene–Middle Miocene Internal to Front-paleopole, when restored for the 28° Pliocene chain front rotation (solid square, position‘B’), falls close to the Late Oligocene–Early Miocene Corsica–Sardinia paleopole. See text for discussion.

the deformation that affected the Epiligurian sedi- stratigraphic relationships. This allows us to calcu-late overall paleomagnetic mean directions fromments during the emplacement of the Ligurian

allochthon did not generally disrupt the original site-mean directions from this study and the litera-

Fig. 7. Schematic reconstruction of the western Mediterranean area for the Oligocene–Miocene Corsica–Sardinia rotation phase (a)and the Late Miocene–Pliocene Tyrrhenian-opening phase (b). The positions of Africa and Europe relative to fixed North Americaare based on Euler poles derived from Central Atlantic marine magnetic anomalies 6 and 5, respectively (Dewey et al., 1989).


ture that we consider representative of the regional 7. Discussion and conclusionsthrust-fold trend of the frontal portions of thenorthern Apennines. We exclude from calculation The Early Oligocene–Middle Miocene Internalsite 41–42 combined and site 53. These sites were to Front-paleopole falls close to, yet displacedlikely affected by local deformation related to counterclockwise with respect to, the Latetectonic structures oriented transversal to the Oligocene–Early Miocene Corsica–Sardinia paleo-regional NW–SE Apennine trend, as reported pole of Muttoni et al. (1998) (Fig. 6). We proposealong the Setta river close to site 41–42 (Panini, that this offset can be associated with the Pliocene1994; Bertolini and Pizziolo, 1997; Camporesi and tectonics at the chain front. By subtracting thePizziolo, 1997) and in the Vetto syncline area at approximately 28° counterclockwise rotation ofsite 53 (Papani et al., 1987; Denardo et al., 1992), the Tortonian–Messinian Chain Front-paleopolewhere the bedding strike is also nearly perpendicu- to the Early Oligocene–Middle Miocene Internallar to the regional structural trend. to the Front-paleopole, we find a good match

Paleomagnetic data from the Tortonian clays between this latter paleopole and the Corsica–of site 31 and site 32–33 combined from this study Sardinia paleopole (Fig. 6, Internal to Front-indicate a counterclockwise rotation similar to paleopole in position B). This observation hasdata from 17 sites from Messinian age clastics important geological implications, namely the pos-outcropping south of the study area in the Marche- sibility that the Pliocene frontal tectonics may haveRomagna foredeep at the front of the Apennine affected also the Ligurian units at least as far intobelt (Speranza et al., 1997) (Fig. 1b, area 2). These the chain, as inferred from the geographical distri-19 Tortonian–Messinian sites are therefore com- bution of the sampled Epiligurian units (i.e.bined (Fig. 5a), and an overall site-mean direction Fig. 1b, area 1).in tilt corrected coordinates of Dec.=334.6°, A tectonic model can now be proposed for theInc.=51.8° is calculated (Table 2). The Tortonian– Cenozoic evolution of the northern Apennines.Messinian overall paleomagnetic direction, posi-

About 24 out of a total of 52° of counterclockwisetive in the fold test according to the McFaddenrotation observed in the Epiligurian units internal(1990) criteria, is considered representative of sitesto the Apennine front can be related to the Oligo-affected by the Pliocene deformation at theMiocene counterclockwise motion of the Corsica–Apennine front.Sardinia block, in partial agreement with theThe overall site-mean direction from the flat-conclusions of Muttoni et al. (1998). When Corsica–lying Early Oligocene–Middle Miocene EpiligurianSardinia moved off the coast of France, the Ligurianclastics outcropping internal to the front (Fig. 1b,units were presumably pushed eastward and rotatedarea 1), and therefore not directly affected by theabove a main boundary thrust onto the Adria/AfricaPliocene frontal deformation, is upgraded frommargin (Fig. 7a). The remaining 28° of the totalMuttoni et al. (1998) with sites 47–49 combinedrotation can be related to the Pliocene frontal defor-from this study (Table 2).mation of the Tyrrhenian-tectonic phase (Fig. 7b)The overall mean directions and the associatedwhich might have (re)activated thrust planes in thepaleomagnetic poles in tilt-corrected coordinatesUmbria-Marche succession below the Ligurianof the Tortonian–Messinian sites at the Apenninewedge (Fig. 1c). The Pliocene rotational compres-front and the Early Oligocene–Middle Miocenesion along the Apennine front was accompaniedsites internal to the front are statistically differentalong the Tyrrhenian margin by non-rotational(Fig. 5c, Table 2). The Tortonian–Messinianextension associated with the Tyrrhenian sea openingpaleopole is rotated counterclockwise with respect(Speranza et al., 1997 and references therein)to the Africa reference paleopoles (Besse and(Fig. 1b, area 5; Fig. 7b). The rotations associatedCourtillot, 1991) by about 28(±9.5)° (Fig. 6,with the Tyrrhenian-tectonic phase were explainedChain Front-paleopole). The Early Oligocene–by Argnani and Frugoni (1997) as being due toMiddle Miocene paleopole is rotated with respectoblique collision of thrust fronts against an obstacleto coeval Africa poles by about 52(±7)° (Fig. 6,

Internal to the Front-paleopole in position A). located in the Adriatic foreland. Alternatively, the


Hirt, A.M., Gehring, A.U., 1993. Thermal alteration of mag-differential rotations of thrust sheets in the northernnetic ferruginous rocks. Journal of Geophysical ResearchApennines were recently interpreted as the result of96, 9947–9953.

the lateral bending of a lithospheric slab subducting Kirschvink, J.L., 1980. The least-squares line and plane and theunder the Apennine front (Lucente and Speranza, analysis of palaeomagnetic data. Geophysical Journal of the

Royal Astronomical Society 62, 699–718.1998).Jelinek, V., 1978. Statistical processing of magnetic susceptibil-

ity on groups of specimens. Stud. Geophys. Geod. 22,50–62.

Acknowledgements Lowrie, W., Alvarez, W., 1979. Paleomagnetism and RockMagnetism of the Pliocene Rhyolite at San Vincenzo Tus-cany Italy. Journal of Geophysics 45, 417–432.Massimo Mattei, Charon Duermeijer and an

Lowrie, W., 1990. Identification of ferromagnetic minerals in aanonymous reviewer substantially improved therock by coercivity and unblocking temperature properties.manuscript. Geophysical Research Letters 17, 159–162.

Lucente, F.P., Speranza, F., 1998. Belt bending as a conse-quence of lateral bending of subducting lithospheric slab:geophysical evidences from the northern Apennines (Italy),ReferencesEuropean Geophysical Society, Nice. Annales Geophisicae1, Supplement, 14.

Alvarez, W., 1972. Rotation of the Corsica–Sardinia micro- McFadden, P.L., 1990. A new fold test for palaeomagneticplate. Nature Physical Science 235, 103–105. studies. Geophysical Journal International 103, 163–169.

Argnani, A., Frugoni, F., 1997. Foreland deformation in the Mattei, M., Kissel, C., Funiciello, R., 1996. No tectonic rotationCentral Adriatic and its bearing on the evolution of the of the Tuscan Tyrrhenian margin (Italy) since late Messin-Northern Apennine. Annali di Geofisica 15, 771–780. ian. Journal of Geophysical Research 101, B2, 2835–2845.

Bertolini, G., Pizziolo, M., 1997. Carta geologica dell’Appen- Muttoni, G., Argnani, A., Kent, D.V., Abrahamsen, N., Cibin,nino Emiliano–Romagnolo in scala 1:10.000 ‘Marzabotto’ U., 1998. Paleomagnetic evidence for Neogene tectonic rota-sezione 237070 Regione Emilia–Romagna Universita degli tions in the northern Apennines, Italy. Earth and PlanetaryStudi di Modena. Archivio Cartografico, Bologna. Science Letters 154, 25–40.

Besse, J., Courtillot, V., 1991. Revised and synthetic apparent Panini, F., 1994. Carta geologica dell’Appennino Emiliano–Ro-polar wander paths of the African, Eurasian, North Ameri- magnolo in scala 1:10.000 ‘Rioveggio’ sezione 237110. Regi-can and Indian Plates, and true polar wander since 200 Ma. one Emilia–Romagna Universita degli Studi di Modena.Journal of Geophysical Research 96, 4029–4050. Archivio Cartografico, Bologna.

Boccaletti, M., Calamita, F., Deiana, G., Gelati, R., Massari, Papani, G., Tellini, C., Torelli, L., Vernia, L., Iaccarino, S., 1987.F., Moratti, G., Ricci hucchi, F., 1990. Migrating foredeep- Nuovi dati stratigrafici e strutturali sulla Formazione di Bis-thrust belt system in the northern Apennines and southern mantova nella Sinclinale di Vetto-Carpineti (AppenninoAlps. Palaeogeography, Palaeoclimatology, Palaeoecology Reggiano–Parmense). Mem. Soc. Geol. It. 39, 245–275.77, 3–14. Sagnotti, L., Mattei, M., Faccenna, C., Funiciello, R., 1994.

Camporesi, D., Pizziolo, M., 1997. Carta geologica dell’Appen- Paleomagnetic evidence for no tectonic rotation of thenino Emiliano-Romagnolo in scala 1:10.000 ‘Vado’ sezione Central Italy Tyrrhenian margin since Upper Pliocene. Geo-237080 Regione Emilia–Romagna Universita degli Studi di physical Research Letters 21 (6), 481–484.Bologna. Archivio Cartografico, Bologna. Sagnotti, L., Speranza, F., Winkler, A., Mattei, M., Funiciello,

Day, R., Fuller, M., Schmidt, V.A., 1977. Hysteresis properties R., 1998. Magnetic fabric of clay sediments from the exter-of titanomagnetites: grain-size and compositional depen- nal northern Apennines (Italy). Physics of the Earth anddence. Physics of the Earth and Planetary Interiors 13, Planetary Interiors 105, 73–93.260–267. Speranza, F., Sagnotti, L., Mattei, M., 1997. Tectonics of the

Denardo, M.T., Iaccarino, S., Martelli, L., Papani, G., Tellini, Umbria–Marche–Romagna arc (central-northern Apen-C., Torelli, L., Vernia, L., 1992. Osservazioni sull’evoluzione nines, Italy): new paleomagnetic constraints. Journal ofdel bacino satellite Vetto–Carpineti–Canossa (Appennino Geophysical Research 102, 3153–3166.Settentrionale). Mem. Descr. Carta Geol. d’Italia 46, Thio, H.K., 1988. Magnetotectonics in the Piemont Tertiary209–220. Basin. Physics of the Earth and Planetary Interiors 52,

Dewey, J.F., Helman, M.L., Turco, E., Hutton, D.H.W., Knott, 308–319.S.D., 1989. Kinematics of the western Mediterranean. In: Vigliotti, L., Langenheim, V.E., 1995. When did Sardinia stopCoward, M.P., Dietrich, D., Park, R.G. (Eds.), Alpine Tec- rotating? New palaeomagnetic results. Terra Nova 7,tonics. Geological Society Special Publication, pp. 265–283. 424–435.

Funiciello, R., Parotto, M., Praturlon, A., 1981. Carta Tetton- Zijderveld, J.D.A., 1967. A.C. demagnetization of rocks —ica d’Italia Scala 1:500.000. Consiglio Nazionale delle Ric- analysis of results. In: Collinson, D.W., Creer, K.M., Runc-erche, Progetto Finalizzato Geodinamica, Unita Operativa orn, S.K. (Eds.), Methods in Paleomagnetism. Elsevier,

New York, pp. 254–286.5.2.1/76, Pubblicazione n. 269.

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