ELSEVIER Tectonophysics 299 (1998) 143–157

Neogene–Quaternary evolution of the centralApennine orogenic system (Italy):

a structural and palaeomagnetic approach in the Molise region

Fabio Speranza b, Massimo Mattei a,Ł, Giuseppe Naso a, Daniela Di Bucci a, Sveva Corrado a

a Dipartimento di Scienze Geologiche, Universita degli Studi ‘Roma Tre’ Largo S. Leonardo Murialdo, 1, 00146 Roma, Italyb Istituto Nazionale di Geofisica, Via di Vigna Murata 605, 00143 Roma, Italy

Received 17 October 1997; accepted 7 March 1998

Abstract

We report on new palaeomagnetic and magnetic fabric analyses of mainly Upper Miocene sedimentary sequences fromthe external central Apennine fold and thrust belt (Molise area), where the principal compressive structures are clearlynon-coaxial. The sampling was carried out on the E–W-oriented Matese–Frosolone thrust sheet, that for its geographicalposition and structural setting (superposition of thrusting, strike-slip and extensional tectonics since Late Miocene topresent-day) represents a key structure for the comprehension of the Neogene–Quaternary evolution of the entire Molisearea. Palaeomagnetic results suggest that the Matese–Frosolone thrust sheet counterclockwise rotated at least 35º afterMessinian times. These data confirm that the present-day trend variability observed in the main compressional structuresin the Apennine chain can be related to rotations about vertical axes rather than to changes in the stress field orientation,at least since Late Miocene times. Magnetic fabric analyses indicate that the studied sediments were subjected to verymild deformation, suggesting that the surface emergence of the thrust front of the Matese–Frosolone unit is located farthernorth, far from the studied area. Well-defined magnetic lineations of tectonic origin were only observed in sites close tolocalised belts of strike-slip deformation. 1998 Elsevier Science B.V. All rights reserved.

Keywords: Apennines; palaeomagnetism; anisotropy of magnetic susceptibility; structural geology; Neogene

1. Introduction

A large number of palaeomagnetic studies werecarried out in different regions of the Italian penin-sula since the seventies. In the first works, Creta-ceous northwestward palaeodeclinations obtained inthe Umbria–Marche Apennines, were interpreted asdue to a general counterclockwise rotation of the en-

Ł Corresponding author. Tel.: C39 (6) 54888027; Fax:C39 (6) 54888201; E-mail: mattei@uniroma3.it

tire Italian peninsula, partially related to the Neogeneopening of the Tyrrhenian Sea (Lowrie and Alvarez,1974, 1975). Later on, other authors suggested that,on the contrary, palaeomagnetic rotations (with re-spect to the adjacent major plates) observed in theApennines may be related to Neogene thrust activ-ity (Channell et al., 1978). In the last few years alarge number of studies tried to better define thecontribution of local tectonic movements to the mea-sured palaeomagnetic rotations all along the Italianpeninsula and Sicily (Channell et al., 1990; Sag-

0040-1951/98/$ – see front matter 1998 Elsevier Science B.V. All rights reserved.PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 2 0 2 - 9

144 F. Speranza et al. / Tectonophysics 299 (1998) 143–157

Fig. 1. Schematic map of the central-northern Apennines. Arrows represent tilt-corrected palaeomagnetic declinations (all transferred tonormal polarity) from mainly Upper Miocene sediments cropping out along the compressional structures of the Apennine chain. Data arefrom: Dela Pierre et al. (1992); Lanci and Wezel (1995); Mattei et al. (1995); Speranza et al. (1997).

notti, 1992; Scheepers et al., 1993). In particular, inthe northern and central Apennines, a large amountof palaeomagnetic data from the Miocene–LowerPliocene sediments cropping out within the arcuatecompressional structures is now available (Fig. 1).These data demonstrate that thrusting and strike-slipdeformation were quite everywhere accompanied bysignificant clockwise and counterclockwise rotationsabout vertical axes, which progressively turned anoriginally parallel belt to the present-day arcuate

shape (Mattei et al., 1995; Speranza et al., 1997). Onthe contrary, neo-autochthonous Messinian to Pleis-tocene sedimentary sequences, cropping out withinthe syn-rift basins from the northern Tyrrhenian mar-gin showed an irrotational behaviour (Sagnotti et al.,1994a; Mattei et al., 1996a,b). This large datasetdemonstrates that in the northern and central Apen-nines the compressional tectonic episodes were ac-companied by large rotations, while the contempo-raneous development of back-arc extensional basins

F. Speranza et al. / Tectonophysics 299 (1998) 143–157 145

in the Apennine hinterland and, at larger scale thenorthern Tyrrhenian Sea opening, were irrotationalprocesses.

This paper deals with the structural evolution ofthe Matese–Frosolone region (Figs. 1 and 2), whichis characterized by a strong lack of coaxiality of the

Fig. 2. Schematic structural map of the Molise area and location of the sampling sites selected for palaeomagnetic and anisotropy ofmagnetic susceptibility analyses.

compressive structures. This feature, coupled withthe complex geological evolution of the region northof the Matese–Frosolone zone, turns out this lastarea to be a key zone to better understand, throughpalaeomagnetic methods, the origin of non-coaxialstructures in the central Apennines.

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2. Geological setting

Five palaeogeographic domains, which can beobserved in the study area (Fig. 2), are describedbelow from the shallowest to the deepest:

(1) The Sannio Unit, interpreted in literature asa regional rootless thrust sheet, tectonically over-lies the Molise and the Apennine domains (Fig. 3).From bottom to top, its representative stratigraphicsuccession consists of multicoloured pelagic clays,hemipelagic and pelagic limestones and turbiditiccalcarenites and sandstones; the age of the succes-sion ranges from Late Oligocene to Late Miocene.

This unit has been interpreted as located, beforethrusting, toward the hinterland with respect to theApennine domain (Patacca et al., 1992b).

(2) The Apennine domain, consisting of a widecarbonate platform with associated slope facies, isdeformed in a series of structural units that cropout in the Marsica region (Fig. 1) with a NNW–SSE structural trend and ENE-vergence, and in theMatese massif (Figs. 1 and 2) with a predominantE–W trend and N-vergence.

In its southern and central part, the Matese struc-tural unit shows a typical Upper Triassic–UpperCretaceous subsiding carbonate platform succession,unconformably overlain by Middle Miocene rampcalcarenites (Cusano Fm.). Moving to the north,slope deposits become prevalent, replacing the car-bonate platform succession. Both these sequencesevolve into mainly terrigenous Tortonian–Messiniansediments as the hemipelagic marls of the LonganoFm., followed by the turbidites of the Molise flysch.

The structural setting of the Matese massif is dom-inated by an E–W structural trend in which compres-

Fig. 3. Schematic geological cross-section across the study area. See Fig. 2 for location.

sive structures (mainly N-verging fault-propagationfolds) are subordinate if compared to the kilometrichigh-angle mainly left-lateral transpressional faults(Scrocca et al., 1995; Corrado et al., 1997) that cross-cut the entire unit (Fig. 2). These structures are gen-erally reactivated later on by extensional kinematics(Scrocca et al., 1995). To the southwest, the Matesemassif is limited by NW–SE-oriented SW-dippingnormal faults (Servizio Geologico d’Italia, 1961; Fer-ranti, 1994). It must be underlined that the position ofthe thrust front of the Matese structure has been fora long time object of debate. For a long period it hasbeen located in correspondence with an E–W-orientedN-verging fold detectable at the macro-scale, that in-volves the whole Matese structure (Patacca et al.,1992a). However, recent papers (see Scrocca et al.,1995) also referring to works of the French school inthe seventies (i.e. Pironon, 1980) suggest that this foldhas not a primary importance, and that the main thrustfront should be found moving to the north, in a moreadvanced position (Figs. 2 and 3).

The tectonic setting of the Marsica thrust sheets(Fig. 1) strongly differs from that just described forthe Matese, as the main structures strike NNW–SSE.The representative stratigraphic succession is madeup of Upper Triassic to Upper Cretaceous platformcarbonates, which are disconformably overlain byLower–Middle Miocene limestones and marls thatgrade upward into Upper Miocene siliciclastic flyschdeposits.

(3) The Greco-Genzana Unit (Fig. 2) is repre-sented by an ENE-verging thrust sheet placed be-tween the overlying Latium–Abruzzi and the un-derlying Apulia realms in the Marsica region. Itsstratigraphic succession characterises a pelagic envi-

F. Speranza et al. / Tectonophysics 299 (1998) 143–157 147

ronment and is made up of well-bedded, calcareous–marly–siliceous lithotypes that range in age fromLate Jurassic to Middle Miocene. This successionoverlies Late Triassic shelf carbonates and is coveredby Late Miocene siliciclastic flysch deposits.

(4) The Molise Unit (Fig. 2) has highly vari-able structural trends, which range from E–W in thesouth to N–S in the north. It is made up of Juras-sic to Miocene slope and pelagic basin carbonates(Frosolone sub-Unit), which become progressivelymore distal moving away from the Matese massifto the north, where exposure of this unit is exten-sive (Agnone sub-Unit; Fig. 2). Towards the top,the Molise domain is followed by Lower Messinianflysch deposits (Patacca et al., 1992a).

(5) The Apulia Unit (Figs. 1 and 2) crops outwith a predominant NNW–SSE trend in the Mt.Rotella, Mt. Pizzalto and Mt. Maiella areas, deepen-ing towards the east, beneath the Molise Unit, at anaverage depth of 2500–3000 m below ground level.It mainly consists of Upper Triassic–Cretaceous car-bonate platform deposits that locally grade upwardinto Lower Cretaceous, slope and pelagic basin fa-cies. They are followed by a carbonate ramp sedi-ment that was deposited from the Langhian to theTortonian. From the SW to the NE, the ApuliaUnit succession is overlain by progressively youngersiliciclastic flysch deposits that range from lateMessinian to Early Pliocene (Globorotalia puncticu-lata Biozone).

Concerning the tectonic regimes that acted in thisregion during the Neogene–Quaternary formation ofthe Apennine Chain, three deformational momentsoccurred:

(1) The earliest deformation was thrusting, thatacted between Early Messinian and Late Pliocenep.p. In this sector of the Apennines deformationresulted in the tectonic stacking of the five mainpalaeogeographic domains (Patacca et al., 1992a).

(2) The thrust structures were then overprintedby Late Pliocene–Early Pleistocene strike-slip tec-tonics (Figs. 2 and 3), which caused the dissec-tion of pre-existing compressive structures by meansof mainly high-angle strike-slip faults (Corrado etal., 1997). To the east (Matese Mts., Molise Unit),these elements show a pattern predominantly char-acterised by N–S right-lateral strike-slip faults, and070º–080º left-lateral strike-slip faults. These sys-

tems cut through the buried Apulia Unit alongnarrow localised shear zones which propagate atshallower structural levels into wider deformationalbelts having ‘flower’ geometries. Strike-slip tecton-ics contributed to the shallow deformation of theMolise realm, presently deformed by a series ofN- and W-directed thrust sheets that evolved as aconsequence of motion along strike-slip fault dis-continuities. To the west (Marsica Range), mainlyNNW–SSE- and N–S-trending, right- and left-lat-eral, strike-slip faults cut the outcropping Apennine,Genzana and Apulia Units blinding the pre-exist-ing thrust geometries; this is observed especially incorrespondence with the frontal ramps of the mainthrust sheets (Miccadei, 1993; Corrado et al., 1995).

(3) Finally, since Middle Pleistocene times exten-sional tectonics has been resulting primarily fromthe collapse of the orogenic wedge; this is recordedin the outcropping areas of the Apennine, Gen-zana and Apulia domains through the developmentof high-angle NW–SE normal faults (Servizio Ge-ologico d’Italia, 1961, 1968; Ferranti, 1994). In theMolise area (Matese massif and surrounding Quater-nary basins, such as the Isernia basin), the last de-formational event also caused the reactivation of pre-existing high-angle mainly strike-slip faults (Corradoet al., 1998).

3. Palaeomagnetic sampling

Palaeomagnetic analyses in the Molise–Frosoloneregion were proven to be particularly difficult becauseof the few available outcrops and the low magneticquality of sediments. In order to select favourablesites for palaeomagnetic investigation, we performedpreliminary magnetic analyses on about 50 hand-col-lected samples, mainly Late Miocene in age. On thebasis of preliminary results, palaeomagnetic samplingwas carried out in thirteen new palaeomagnetic sites(a site refers to an outcrop several metres wide andhigh): ten sites in the Tortonian–Messinian clays ofthe Longano Formation and Molise flysch, two sitesin the Eocene marly limestones of the Scaglia RossaFormation, and one site in Middle Pleistocene clays inthe Isernia basin (Fig. 2). Sites MT01, MT02, MT04,MT09 were sampled along the roughly E–W-orientednorthern part of the Matese structure. Sites MT03 and

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MT08 were sampled in the Messinian flysch units tothe north of the Matese structure. Sites MT05, MT06and MT07 were collected in the flysch units crop-ping out around the Montagnola di Frosolone, wherethe MT12 site in the Longano Formation, and theMT11 and MT13 sites of the Scaglia Rossa Forma-tion were also collected. The age of the sites has beenreferred to detailed geological maps and stratigraphicstudies published for the investigated area (ServizioGeologico d’Italia, 1961; Patacca et al., 1990, 1992a).

We collected 117 cylindrical core samples, 25mm in diameter. Nine to ten cores were taken fromevery site, each of them giving two to three speci-mens. Cores were drilled using an ASC 280E petrol-powered portable drill, and oriented in situ with amagnetic compass.

4. Laboratory procedures and vector analysis

All magnetic remanence measurements were car-ried out in the magnetically shielded room of thepalaeomagnetic laboratory at the Istituto Nazionaledi Geofisica (Rome, Italy). Measurements of re-manence directions and intensities were performedusing a 2G three-axis cryogenic magnetometer.

Rock magnetic experiments were performed forall sites on at least one specimen for each site.We investigated magnetic mineralogy by means ofisothermal remanent magnetization (IRM) acquisi-tion curves with a maximum field of 0.9 T, removalof the IRM by progressive application of an increas-ing back-field and estimation of the coercivity ofremanence Hcr (Fig. 4). These analyses showed that,except for the small fraction of a higher coercivitymineral, probably hematite, observed in the ScagliaRossa sites MT11 and MT13, in all the samples theremanence is hold by low-coercivity magnetic min-erals. Thermal demagnetization curves show that themaximum unblocking temperature for these samplesranges between 450ºC and 550ºC, suggesting titano-magnetite as the magnetic carrier of the measuredremanence.

Natural remanent magnetization (NRM) valuesrange from 3:0ð 10�5 A=m to 3:6ð 10�1 A=m.

Two pilot specimens for each site were stepwisedemagnetized, one by AF and the other by thermaltreatment. Results from each couple of pilots are con-

Fig. 4. Rock-magnetism analyses for some representative spec-imens. (A) Acquisition of isothermal remanent magnetization(IRM). (B) Back-field demagnetization curve.

sistent (Fig. 5), and indicate that for most of thesites the AF treatment gives a more efficient magneticcleaning and a better defined demagnetization path.

On the basis of the pilots behaviour, one specimenfor each core was stepwise demagnetized with a 2GAF demagnetizer up to a maximum field of 0.1 T.AF treatment was stopped when NRM decreased toabout one tenth of the initial value or when artificialchanges of palaeomagnetic directions appeared dur-ing AF demagnetization. Demagnetization data wereplotted on orthogonal demagnetization diagrams (Zi-jderveld, 1967) and evaluated using the standardprincipal component analyses (Kirschvink, 1980). In

F. Speranza et al. / Tectonophysics 299 (1998) 143–157 149

Fig. 5. Vector diagrams of typical thermal and AF demagnetization data, tilt corrected. Open and solid symbols represent projections onthe vertical and horizontal planes, respectively.

most cases, apart from a small viscous componentremoved below 20 mT, a characteristic component ofmagnetization (ChRM) was isolated in the orthogo-nal demagnetization diagrams. In the samples show-ing two components of magnetization with overlap-ping coercivity spectra, remagnetization circles werefitted to the demagnetization paths. Mean directionsfor each site were obtained using the Fisher (1953)

statistic (where only ChRMs were isolated), or theMcFadden and McElhinny (1988) method.

5. Palaeomagnetic results

Sites MT03, MT06, MT09 and MT12 were dis-carded because of the very low NRM values or be-

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Table 1Summary of palaeomagnetic results

Site Treatment N n c DBTC IBTC DATC IATC k Þ95 Age

MT01 Af 9 9 0 342.5 �25.0 344.9 37.5 36.6 8.6 Late MioceneMT02 a Af 9 9 0 7.5 60.4 7.5 60.4 92.7 5.4 Late MioceneMT04 Af 7 6 1 115.1 �41.1 106.3 �67.5 53.9 8.3 Late MioceneMT05 Af 7 7 0 202.4 51.6 191.4 55.7 15.9 14.2 Late MioceneMT07 Af 10 10 0 147.5 �28.4 153.2 �57.9 40.6 7.7 Late MioceneMT08 Af 7 7 0 337.8 56.9 311.2 54.5 61.8 7.7 Late MioceneMT10 a Af 7 7 0 3.6 60.8 3.6 60.8 38.5 9.8 PleistoceneMT11 Af 7 5 2 277.0 33.3 270.4 36.5 17.2 15.3 EoceneMT13 a Af 9 8 1 189.9 �65.2 250.8 �77.9 68.7 6.3 EocenePescorosito b 24 304.5 36.1 313.1 56.6 172 4.2 Late Miocene

Late Miocene regional mean (sites MT01, MT04, MT07, MT08, Pescorosito)BTC (5 sites, 55 samples) 321.5 30.1 5.4 36.1ATC (5 sites, 55 samples) 321.9 56.4 25 � D 4:6 � D 24

N , number of studied samples; n, number of stable directions; c, number of great circles; BTC and ATC, values before and after tiltcorrection; k and Þ95, statistical parameters; �; � , minor and major axes of the 95% confidence ellipse.a Discarded sites.b Site from Iorio et al., 1996.

cause of instable behaviour during demagnetization.In all the other sites a well defined palaeomagneticmean direction was calculated, the Þ95 value beinggenerally lower than 10º out of two sites (MT05and MT11) with quite larger within-site directionalscatter (Table 1).

Since no reliable Tertiary APWP (Apparent PolarWander Path) is available for the Adriatic promon-tory, the observed palaeomagnetic directions cannotbe compared to those expected for the Adriatic fore-land. However, on the basis of the correspondencebetween the Adriatic promontory and African AP-WPs during Mesozoic times, it is not believed thatAdria has significantly rotated or latitudinally driftedduring the Neogene (Van der Voo, 1993). In fact theEocene to Neogene expected magnetic declinationsfor stable Africa are systematically smaller than 1º(Besse and Courtillot, 1991). For these reasons theobserved palaeomagnetic directions are comparedwith the present-day geocentric axial dipole (GAD)field direction (in both normal and reverse polaritystates).

Eight Eocene–Late Miocene and one Pleistocenesite-mean palaeomagnetic directions were obtainedby the procedure discussed above (Table 1).

After bedding correction four out of eightEocene–Late Miocene sites have normal polarity.All sites but MT02 and MT13 are far from the

GAD field direction in geographic coordinates. Thenfor such sites the possibility of a recent remag-netization process could be ruled out. The in-situMT02 and MT13 site-mean directions coincide withthe GAD field direction. Moreover, after tectoniccorrection, the Eocene MT13 site-mean direction ischaracterized by an anomalous high inclination value(I D �77:9º) (Table 1). Recent remagnetization pro-cesses likely occurred for these latter sites, whichhave been discarded from further analyses.

When reported to normal polarity, four LateMiocene sites (MT01, MT04, MT07, MT08) andone Eocene site (MT11) show variable amount ofwestward declinations, whereas site MT05 is char-acterized by an eastward declination. Site MT05was not further considered for the regional interpre-tation because of the presence of local strike-slipfault activity, and of the overturned bedding attitude.Moreover, we added the Pescorosito direction (Iorioet al., 1996) from the northern part of the Matesestructure (Fig. 6) to the four newly obtained Miocenesite-mean directions.

The data distribution from these five latter sites,according to the Mu=Me test (Tauxe et al., 1991),is Fisherian in geographic coordinates but non-Fish-erian after bedding correction. A bootstrap statistichas been applied for palaeomagnetic directions afterbedding correction. We calculate a Late Miocene

F. Speranza et al. / Tectonophysics 299 (1998) 143–157 151

Fig. 6. Equal-area projection of the site-mean directions fromthe Molise region. Open (solid) circles represent projection ontoupper (lower) hemisphere. Triangle indicates the result from thePescorosito site (Iorio et al., 1996).

mean palaeomagnetic direction for the Matese–Frosolone structure characterized by: D D 325:6º,I D 50:7º, � D 4:55, � D 23:99. A bootstrap foldtest (Tauxe and Watson, 1994) indicates that the dataare mostly tightly grouped at about 100% unfold-ing, indicating between 40 to 160% unfolding as theappropriate coordinate system at the 95% level ofconfidence.

6. Low-field anisotropy of magnetic susceptibility(AMS) results

The study of the anisotropy of magnetic suscep-tibility (AMS) is getting an increasing role in theanalysis of the strain patterns in weakly deformedsedimentary sequences. Relationships between mag-netic fabric and structural setting have been widelydemonstrated all around the world. In sedimentaryrocks with low ferromagnetic (sensu lato) contentthe AMS provides an accurate image of the petro-fabric and reflects the average preferred orientationof the rock matrix (i.e. Rochette, 1987). In the pastyears several studies reported successful attempts ofdetecting subtle strain-related features in sedimentfabrics by means of extensive AMS analyses (seerecent reviews by Tarling and Hrouda, 1993, andKodama, 1995). In the Italian peninsula, in particu-lar, the AMS studies focused on different clay andpelagic limestone sequences distributed through theApennine chain and the extensional Tyrrhenian mar-gin (Lowrie and Hirt, 1987; Sagnotti and Speranza,1993; Sagnotti et al., 1994b; Scheepers and Lan-gereis, 1994; Winkler and Sagnotti, 1994; Averbuchet al., 1995; Mattei et al., 1995). Results alwaysindicated a direct relationship between the magneticfabric and the structural setting, showing that theAMS pattern in the studied sequences is the resultof the earliest stages of tectonic modification of anoriginal compacted sedimentary fabric.

We measured AMS for all the samples usedfor palaeomagnetic analyses. The low-field mag-netic susceptibility of one specimen for each corewas measured with a KLY-2 bridge, and the AMSat both the specimen and the site levels was cal-culated using the Jelinek statistics, (Jelinek, 1977,1978). The studied sediments are characterized bylarge differences in the bulk susceptibility values(Kmean). The Longano and Scaglia Formations showthe lower Kmean values, ranging between 10 and110ð 10�6 SI, whereas the highest values have beenmeasured in the Molise flysch site MT02, whereKmean D 3880 ð 10�6 SI (Table 2). This differencein the bulk susceptibility values is mainly due tothe high carbonatic content of the Scaglia Rossa andLongano Formation sediments. Anyway, apart thespecimens from site MT02, all the samples showKmean values lower than 311 ð 10�6 SI. These low

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Table 2List of anisotropy parameters

Site Lithology N km L F Pj Tj D, I (Kmax) E1–2

MT01 (Longano Formation) Marly intervals 10 111.2 (2.8) 1.006 (0.003) 1.064 (0.004) 1.079 (0.004) 0.820 (0.076) 235.3 9.6Late MioceneMT02 (Molisan Flysch) Marly intervals 9 3880.2 (489.7) 1.006 (0.004) 1.036 (0.016) 1.046 (0.015) 0.616 (0.391) 220.2 30.9Late MioceneMT03 (Longano Formation) Marly intervals 9 70.2 (4.1) 1.006 (0.002) 1.023 (0.015) 1.032 (0.017) 0.462 (0.361) 258.17 35.6Late MioceneMT04 (Molisan flysch) Marly intervals 9 223.5 (29.4) 1.003 (0.002) 1.108 (0.016) 1.129 (0.020) 0.937 (0.032) 175.18 27.4Late MioceneMT05 (Molisan flysch) Marly intervals 9 301.4 (18.5) 1.024 (0.006) 1.081 (0.003) 1.112 (0.006) 0.532 (0.083) 2.9 5.0Late MioceneMT06 (Molisan flysch) Marly intervals 10 83.5 (3.8) 1.002 (0.001) 1.045 (0.008) 1.053 (0.009) 0.928 (0.033) 193.18 42.5Late MioceneMT07 (Molisan flysch) Marly intervals 10 204.9 (13.1) 1.004 (0.002) 1.090 (0.009) 1.107 (0.011) 0.909 (0.039) 134.30 17.2Late MioceneMT08 (Molisan flysch) Marly intervals 9 165.2 (3.9) 1.002 (0.001) 1.051 (0.016) 1.060 (0.010) 0.930 (0.043) 263.16 56.5Late MioceneMT09 (Molisan flysch) Marly intervals 10 140.2 (23.0) 1.006 (0.005) 1.033(0.010) 1.043 (0.012) 0.714 (0.200) 90.2 8.6Late MioceneMT11 (Scaglia Formation) Limestone 10 10.5 (10.4) 1.020 (0.017) 1.027(0.014) 1.044 (0.025) 0.215 (0.476) 259.4 13.1EoceneMT012 (Longano Formation) Marly intervals 9 22. (5.2) 1.004 (0.003) 1.017 (0.006) 1.022 (0.008) 0.627 (0.201) 206.24 38.1Late MioceneMT13 (Scaglia Formation) Limestone 9 145.0 (56.4) 1.003 (0.002) 1.023(0.008) 1.028 (0.009) 0.739 (0.214) 140.20 31.7Eocene

N D number of specimens.km D .kmax C kR C kmin/=3 (mean susceptibility, in 10�6 SI units).L D kmax=kR .F D kR =kmin.Pj D exp

ý2ð.�1 � �/2 C .�2 � �/2 C .�3 � �/2

Ł1=2(corrected anisotropy degree).

Tj D 2.�2 � �3/=.�1 � �3/� 1 (shape factor).�1 D ln kmaxI �2 D ln kR I �3 D ln kminI � D .�1 C �2 C �3/=3.E1–2: semi-angle of the 95% confidence ellipse around the mean Kmax axis in the Kmax–KR plane.For each locality the line shows the arithmetic means of the individual site-mean values (standard deviation in parentheses).

F. Speranza et al. / Tectonophysics 299 (1998) 143–157 153

values of the bulk susceptibility indicate that themain magnetic contribution is due to the param-agnetic fraction (Tarling and Hrouda, 1993) andthat ferromagnetic minerals (probably magnetites)slightly influence the magnetic fabric in all sites butMT02.

The magnetic fabric detected by AMS measure-ments is described by a second-order tensor, whichmay be visualized as an ellipsoid characterized bythree principal axes .Kmax > Kint > Kmin/ cor-responding to the eigenvectors of the tensor. Themagnetic fabric ranges from oblate ellipsoids of puresedimentary origin (Kmax D Kint with Kmin perpen-dicular to the bedding) to intermediate ellipsoids(tectonic and sedimentary) with a well defined mag-netic lineation and Kmin still perpendicular to thebedding (Fig. 7).

For each site, the degree of anisotropy and theshape of the susceptibility ellipsoids were evaluatedusing the P 0, T (as defined by Jelinek, 1981), Fand L parameters. The AMS parameters show val-ues typical of weakly deformed sediments, with lowP 0 values (less than 1.13) and with oblate shape ofthe ellipsoids .T > 0/ (Fig. 8). The P 0 values areless than 1.022 in the Longano and Scaglia RossaFormations, and show slightly higher values (up to1.129) in the Molise flysch sediments, which arealso characterized by strong oblate ellipsoids, withT values up to 0.937. The magnetic foliation is al-ways well defined and parallel to the bedding plane,suggesting a sedimentary origin due to compactionprocesses. Magnetic lineation is well defined in sitesMT01, MT05, MT09 and MT11, where L is greaterthan 1.005, the E1–2 values (the semi-angle of the95% confidence ellipse in the Kmax–Kint plane) areless than 15º, and a triaxial shape of the anisotropyellipsoid is observed. On the contrary, sites MT04,MT06, MT07, MT08 and MT12 are characterizedby pure oblate ellipsoids, with less defined lineation.L < 1:005/.

The sites MT01, and MT09, located in the north-ern sector of the Matese, and the site MT11, locatedin the southern part of the Montagnola di Frosolonestructure, are characterized by a magnetic lineationoriented ENE–WSW to E–W, which is sub-parallelto the local structural axis. Site MT05 is charac-terized by a well defined magnetic lineation, N–S-oriented. This orientation is in good agreement

Fig. 7. Examples illustrating the magnetic fabric in the studiedsediments. Schmidt equal-area projections, lower hemisphere;ellipses indicate the 95% confidence regions around the principalsusceptibility axes. Both sites are characterized by triaxial-oblatesusceptibility ellipsoids with a magnetic lineation of tectonicorigin.

with the presence of a N–S-oriented right-lateralfault which defines the western border of the Mon-tagnola di Frosolone structure.

7. Discussion

Palaeomagnetic data show that the Matese–Frosolone structure underwent an about 35º coun-terclockwise rotation after the Late Miocene, in

154 F. Speranza et al. / Tectonophysics 299 (1998) 143–157

Fig. 8. Magnetic anisotropy plot of the analysed specimens. P 0 anisotropy degree, T shape factor (according to Jelinek, 1981).

agreement with the result obtained from one palaeo-magnetic locality by Iorio et al. (1996). Consideringthe age of the sampled formations, such rotation oc-curred after Messinian times and ended before themiddle Pleistocene, as the null rotation observed atthe site MT10 testifies (Table 1).

The new palaeomagnetic data confirm that thepresent-day orientation of compressional structuresin the Apennine chain is mainly due to differen-tial rotations about vertical axes of thrust units,as already documented for other thrust sheets inthe central and northern Apennines (Dela Pierreet al., 1992; Mattei et al., 1995; Speranza et al.,1997). In particular, the present-day E–W orien-tation of the Matese–Frosolone unit reflects thepost-Messinian 35ºCCW rotation, which is oppo-site to the post-Messinian 30ºCW rotation measuredin the NNW-oriented structures of the Meta andGreco-Genzana units (Mattei et al., 1995) (Fig. 9).

The small number of reliable data does not al-low to fully explain the existence of small-scalestructures with different orientation, nor to preciselyconstrain the role played by strike-slip tectonics ac-tivity for the occurrence of the rotations. Except forsite MT05, located nearby an important N–S right-lateral strike-slip fault, and showing a CW rotationof 22º, we have no data to relate the measured rota-tions to the strike-slip faults activity. In fact, even if

some additional rotations should be expected, thereis not a clear relationship between the amount ofthe measured rotations and the occurrence of majorleft-lateral strike-slip faults.

When considered at a regional scale, the overalldataset available for the Abruzzi–Molise area mayjustify the variability of structural trends as a resultof differential rotations about vertical axes.

In this context, an important theme of debateis the maximum depth at which the rotations ofthe Matese–Frosolone unit can be extrapolated. Wesuggest that the measured palaeomagnetic rotations,related to the thrust tectonic activity, involved afirst-order block that reaches at least the main de-collement level for the Apennine carbonate platformunit, which corresponds to the Upper Triassic evap-orites (Bally et al., 1986; Mostardini and Merlini,1986; Casero et al., 1988). On the other side, if thepalaeomagnetic rotations are also induced by tran-scurrent belts, which cut through at least two thrustsheets units (Corrado et al., 1997), they probablyinvolve also deeper structural units.

AMS results are generally typical of weakly de-formed sediments. These results confirm that theMatese massif and the Montagnola di Frosolone arequite far from the emergence of the thrust fault lo-cated at the base of the Matese–Frosolone regionalthrust sheet, and that no important thrust faults di-

F. Speranza et al. / Tectonophysics 299 (1998) 143–157 155

Fig. 9. Schematic model showing the tectonic evolution of theeastern portion of the central Apennines during the Neogene.

vide the two main carbonate structures. Surface andsubsurface data suggest that the emergence of thisthrust fault is located a few kilometres to the northof the Montagnola di Frosolone and to the east of theMatese massif (Fig. 2).

Local highly concentrated deformation due tostrike-slip kinematics is, instead, the probable causeof the N–S lineation developed along the northwest-ern margin of the Montagnola di Frosolone carbon-ates (site MT05).

8. Conclusions

Palaeomagnetic, AMS, and structural analysisdata obtained in the Matese–Frosolone structuresdemonstrate that differential rotations about verticalaxes played an important role in the tectonic processleading to the present-day complex geometry of thissector of the Apennine chain. In particular, the pre-sent-day E–W trend of the Matese–Frosolone unitsis explained by large CCW rotations which occurred

after Messinian times. Middle Pleistocene sedimentsdid not undergo any rotation, suggesting that nor-mal fault activity had no influence in the rotationalprocesses of the area. Structural and palaeomagneticdata concur to indicate that, as already showed fornorthern and central Apennines, non-coaxiality ofstructures is generally associated to differential tec-tonic rotations. A main unresolved problem remainsthe relative contribution of thrust and strike-slip tec-tonics to the documented rotations. On the otherhand, the overall dataset available for the northernand central Apennines demonstrates that palaeomag-netic rotations observed in the allochthonous unitscannot be used to extrapolate rigid rotations aboutfar located poles.

Acknowledgements

We wish to thank G. Muttoni and an anonymousreviewer for their careful review of the manuscript.

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