Mem. Soc. Ceol. Il., 52 (1998), 469-478, 4 ff. PALEOMAGNETIC CONSTRAINTS TO THE TECTONIC EVOLUTION OF THE NORTHERN ITALIAN PENINSULA MASSIMO MATTEI ("), FABIO SPERANZA ('d,), LEONARDO SAGNOTTI ('d'), RENATO Fu ICIELLO (',) & CLAUDIO FACCEN A (i,) ABSTRACT Several paleomagnetic studies were performed in the northern Italian peninsula since the mid 1970s, when various research groups investigated the MesoCenozoic pelagic carbonatic sequences of the Apenninic chain. The first paleomagnetic results generally showed western declinations which were interpreted as due to a general counterclockwise (CCW) rotation of the whole Italian peninsula, a concept that had a broad influence on nearly all the geodynamic models proposed for the centraI Mediterranean. At the same time the paleomagnetic results obtained along the arc showed larger CCW rotations in the northern part of the arc than in its southern part, initiating a long-standing scientific debate about the oroclinal origin of the Umbria-Marche Apennines. In the last few years our group faced these two topics by means of new paleomagnetic investigations. We sampled 131 sites (about 1300 samples) in marine clay units deposited in the Neogene foredeeps of the Apenninic chain and in «neo-autochthonous» sequences along the TuscanLatium extensional Tyrrhenian margino A characteristic remanent magnetization (ChRM) was isolated in 101 sites which were used for the tectonic interpretation. Magnetic mineralogy analyses, positive fold and reversal tests demonstrated the primary ori gin of the ChRM. Paleomagnetic results show that the tectonic evolution of the Umbria-Marche-Romagna and Latium-Abruzzi regions was accompained, after the Messinian, by large differential rotations about vertical axes. Extensional tectonics developed at the same time along the Tuscan-Latium Tyrrhenian margin without any rotation. The central-northern Apenninic chain, originally straight and oriented about N320°, assumed its present arcuate shape throughout differential rotations connected to thrust sheets emplacement and strike-slip activity. The null rotation of the Tuscan-Latium «neo-autuchthonous» basins demonstrates that the opening of the northern Tyrrhenian Sea was not accompained by rotations of its eastern margin, thus implying a different tectonic evolution with respect to the southern Tyrrhenian Sea and the Liguro-Provençal basino (*) Dipartimento di Scienze Geologiche, Università ROMA TRE, Largo S. Leonardo Murialdo 1,00146 Roma. ('''') Istituto Nazionale di Geofisica, Via di Vigna Murata 605 - 00143, Roma. KEy WORDS: Paleomagnetism, Apenninic Tyrrhenian Sea, chain, Neogene. RIASSUNTO L'Appennino settentrionale è stato oggetto, a partire dagli anni settanta, di numerosi studi paleomagnetici che hanno analizzato le successioni carbonatiche meso-cenozoiche diffusamente affioranti nell'area. I primi risultati ottenuti nell'area umbro-marchigiana mostrarono declinazioni paleomagnetiche nord-occidentali, di maggiore entità nei settori settentrionali rispetto a quelli meridionali dell'arco appenninico. Questi dati vennero largamente utilizzati a conforto della postulata rotazione antioraria della penisola italiana e furono l'oggetto di un'ampia discussione sull'origine oroclinalica dell'arco umbro-marchigiano. Negli ultimi anni il nostro gruppo di ricerca si è occupato dello studio di queste rotazioni pianificando una nuova, estesa campagna paleomagnetica. Sono stati campionati 131 si ti nelle unità argillose neogeniche affioranti nei settori esterni della catena e nelle unità post-orogene del margine estensionale tosco-laziale. In 101 siti è stata isolata una magnetizzazione rimanente caratteristica di origine primaria. I risultati ottenuti mostrano che rotazioni di segno ed ampiezza differenti hanno condizionato l'evoluzione tettonica post-messiniana dell'arco umbro-marchigiano-romagnolo e della piattaforma laziale-abbruzzese, mentre il margine tirrenico tosco-Iaziale non ha subito alcuna rotazione a partire almeno dal Messiniano. In particolare la catena appenninica centro-settentrionale si è evoluta a partire da una struttura rettilinea ad andamento circa N320°. La progressiva rotazione delle strutture fino alloro assetto attuale è da imputarsi all'attività di fronti di accavallamento e, nell'Appennino centrale, ad importanti discontinuità trascorrenti. Durante lo stesso lasso di tempo il margine tirrenico, interessato esclusivamente da tettonica estensionale, non è stato soggetto ad alcuna rotazione. Ciò implica che il meccanismo di apertura del bacino tirrenico settentrionale è profondamente differente da quello del TilTeno meridionale e del bacino ligure-provenza le. TERMINI CHIAVE: Paleomagnetismo, no, Appennino, Neogene. Mar Tirre- M. MATTEI ET ALli 470 INTRODUCTION These data were first interpreted as the result of an oroclinal evolution of the northern Apenninic arc (CHAN ELL et alii, 1978; ELDREDGEet alii, 1985) and then attributed to a complex deformative mechanism that involved pre-folding rotations of the sedimentary units (HIRT & LOWRIE,1988). In any case the paleomagnetic data demonstrated that the northern sector of the arc rotated CCW more than the southern part, even if they did not «strictly» (i.e. by applying rigorous statistical tests) prove definitively the oroclinal origin of the arco In order to better constrain the tectonic evolution of the northern Italian peninsula we carried out, in the last few years, new paleomagnetic studies. We sampled sedimentary sequences coeval to the main tectonic events (opening of the Tyrrhenian Sea and building of the Apenninic chain), and distributed over different structural provinces. The aim was to distinguish the rotations related to the opening of the Tyrrhenian Sea from those due to the deformation of the Apenninic chain. 131 sites (more than 1300 samples) were collected in Neogene and Quaternary sedimentary sequences located throughout the TuscanLatium Tyrrhenian margin and the foredeep basins in northern and centraI Apennines. In this paper we discuss the tectonic evolution of the northern Italian peninsula, integrating our paleomagnetic results with structural, stratigraphical and geophysical data. A comparison between the tectonic evolution of the northern Tyrrhenian Sea wi th respect to the Liguro- Provençal basin will be also discussed. The northern Italian peninsula was intensively paleomagnetically investigated since the mid 1970s, when different group of reserchears studied the Meso-Cenozoic sediments cropping out in the Umbria-Marche Apennines. The first paleomagnetic results were published by LOWRIE& ALVAREZ(1974), concerning the upper Cretaceous Scaglia Formation in the Umbria-Marche region. The western paleomagnetic declinations reported in this first study were later also recognized both on the Apulian foreland (CHANNELL,1977; MARTO & VELJOVIC, 1983; VANDENBERG, 1983) and elsewhere in the Apenninic chain (LOWRIE& ALVAREZ,1975; CHANNELL& TARLI G, 1975; CHAN ELL et alii, 1978; VANDENBERGet alii, 1978; HII~.T& LOWRIE,1986). The results from Umbria, were originally interpreted by LOWRIE & ALVAREZ(1974, 1975), who assumed the studied sequences as «autochthonous» and representative of the whole Italian peninsula, in terrns of a 30°-40° Cretaceous and 25° post-Eocene counterclockwise (CCW) rotation with respect to Europe, and then by VANDENBERG et alii (1978) as the result of a post-Early Eocene 25° CCW rotation of the peninsula with respect to Africa. On the other hand, CHANNELL& TARLING(1975) and CHAN ELL et alii (1978) interpreted the paleomagnetic results from Umbria-Marche region as due to rotations of the sedimentary cover, linked with the emplacement of the Apenninic thrusts. More recently CHANNELL (1992) showed that the whole set of paleomagnetic data does not imply a Tertiary CCW rotation of the whole Italian peninsula. In spite of this debate about the geodyGEOLOGICAL SETTING AND PALEOMAGNETIC SAMPLING namic significance of the paleomagnetic data, the idea of the CCW rotation of the Italian The northern Italian peninsula is formed peninsula became a common assumption in the tectonic reconstructions of the centraI by several geodynamic provinces with distincMediterranean region, and was considered tive geological, tectonic, and geophysical characters. The Apenninic chain is mainly comthe cause for the opening of the Tyrrhenian Sea, as the CCW rotation of the Corsica-Sarposed of sedimentary sequences arranged in arcuate thrust sheets, verging toward the Adridinia block was for the Ligure-Provençal basin (CAREY,1938; BOCCALETTI et alii, 1974; atic foreland (fig. 1). The regional metamorSARTORI,1990; DOGLIONI,1991; CASTELLARIN phic basement crops out only in the western et alii, 1992). part of the chain, whereas the sedimentary cover, which includes carbonatic platform to Paleomagnetic results from the UmbriaMarche region showed variations of the paleo- pelagic Meso-Cenozoic sedimentary sequenmagnetic declinations along the arc, the more ces, is widespread in the axial sector of the westerly declination being reported from its chain. Syn-orogenic siliciclastic units, deposited in foredeep basins, constitute the last epinorthern part. 471 PALEOMAGNETIC CONSTRAINTS TO THE TECTONIC EVOLUTION 45"- o 100 km I i I 44"- ,() 43'L f++ : ~+ +.1 Volcllnic and p/utonie unirs Extensional sedimentar)) basi"s lhe Tyrrhenian margin _ Plioce[le./ored!!ep ApelllllfllC cltaln l:-:-:-:·:j .... Messinian Apelltliflic 6G basins foredeep ellain ~:-,;.:-;l "Epi-Liguriall" Carbonatic basins or . oi file oJ lITe units platforms Umbria-Marche Uflit.~' EEEIJ Cervaro/a units E3 Fig. TUSCQII units ~ Ligurian units Il Il Il Il Il Basemell! unirs • ~ Sampling sites Main trh"slFonlS 1 - Schematic map of the central-northern Italian peninsula and location of the sampling sites. extensional regime developed along the Tuscan-Latium Tyrrhenian margin after the Late Miocene (ELTER et alii, 1975). This extensional phase was accompanied by the formation of large sedimentary basins, bounded by normal faults, whose orientation changes from N-S in the eastern sector to NW-SE in the 472 M. MATTE! ET ALII western sector. The sedimentary extensional basins are filled by late Miocene-Pleistocene continental, brackish and marine sequences of the «neo-autochthonous» sedimentary cycle. These sediments unconformably cover the compressional Apenninic structures. The extensional tectonics was also accompanied by diffuse magmatic activity. Crustal stretching migrated progressively eastward, following the Apenninic compressional episodes, from the Tyrrhenian coast to the axis of the Apennines chain, where it is active today. In the centraI Apennines we sampled 39 sites in Middle Miocene to Lower Pliocene marls and clays, mainly deposited in the migrating Apennines foredeep, distributed over the Sabina, Latium-Abruzzi and Marsica regions (MATTEIet alii, 1995). In the northern Apennines we sampled 32 sites mainly in Messinian sediments at the external front of the Umbria-Marche-Romagna arc, from the Gran Sasso chain to the Sillaro line (SPERANZAet alii, 1997). In the Tuscan-Latium Tyrrhenian margin we sampled 60 sites in Messinian-Pleistocene clay-rich sediments from the Volterra, Val di Fine, Orcia, Siena-Radicofani, Mignone, Tiber and Ardea basins (SAGNOTTIet alii, 1994; MATTEI et alii, 1996a; MATTEI et alii, 1996b). In the following sections we will briefly overwiew the paleomagnetic results from the Apenninic chain and from the Tyrrhenian margin and discuss their implications for the tectonic evolution of northern Italy. PALEOMAGNETIC RESULTS APENNINICCHAIN In 45 out of 71 sites a Characteristic Remanent Magnetization (ChRM) was isolated. This component is interpreted of primary origin, as suggested by positive fold and reversal tests. Detailed description of laboratory experiments, magnetic mineralogy and paleomagnetic data can be found in MATTEIet alii (1995) and SPERANZAet alii (1997). Most of the data show a paleomagnetic inclination significantly shallower than the present-day one. Such inclination flattening is not correlated to the geographic or stratigraphic distribution of the sampling sites and clearly reflects compaction-related effects on the studied sediments. We exclude that the inclination scatter results from tectonic processes, and we discuss paleomagnetic results only in terms of paleodeclination values. Because of the absence of a reference pole for the Adriatic foreland and the Italian peninsula since the Late Miocene, we interpret paleomagnetic declinations in terms of vertical axes local tectonic rotations. In fig. 2 we show the paleomagnetic directions obtained in the Apenninic chain. From a generaI point of view it is clear that the northern Apennines underwent differential clockwise (CW) and CCW rotations which depict a complex deformative history. Distinct rotational domains can be observed, which are separated by major tectonic elements or, such as in the Umbria-Marche region, represent different segments of arcuate structures. In the centraI Apennines three different domains were recognized (MATTEIet alii, 1995): the Sabina region, that underwent a post middle Miocene CW rotation of about 16°; the Latium-Abruzzi platform with a post Messinian CW rotation of about 28° and the western border of the Latium-Abruzzi platform (Eastern Marsica) with post Messinian CW rotations of variable amplitude. Along the Umbria-Marche-Romagna arc different paleomagnetic rotations were found (fig. 2). In this region we took into account also the paleomagnetic results obtained by DELAPIERRE et alii (1992) and LANCI & WEZEL(1995). North of the Gran Sasso thrust front paleomagnetic declinations are about NS, suggesting no rotation of the Acquasanta and Montagna dei Fiori structures since the Messinian. Toward the north, CW rotations of about 10°-20° were found in the Messinian Laga Formation. To the north of the Cingoli anticline different amounts of CCW rotations were recorded mainly in the Upper Messinian Colombacci Formation. The sites between the Cingoli anticline and the Val Marecchia unit show about 20° CCW rotation, whereas the sites to the north of the Val Marecchia unit show CCW rotations up to 60°. CCW rotations of about 20° were measured further north up to the Sillaro tectonic line. The whole set of data from the Central-Northern Apennines shows that the paleomagnetic declinations and the structural directions (fold axes and thrust fronts) are significantly correlated, both at regional and at local scale. In particular, the structures oriented N-S to NNW-SSE (such as those in the Sabina, Marsica and at the southern Sibillini front) are generally associated to CW rotations, whereas for the PALEOMAGNETIC CONSTRAINTS TO THE TECTONIC EVOLUTION 473 Paleomagnetic declinations from the Tuscan-Latium Tyrrhenian margin 45~ Paleomagnetic declinatiol1s from the Apenninic chain 44~ {) 43~ ,+. :: :1 Va/cmlic and plutOllic UflilS Il L-....J Extellsio1lal sedimemary c!le Tyrrheniall margin _ Pliocefle.fored.eep ApenllllJlC challl I:-:':-:-:~Mess;,~;a.'l fore.deep . . .. Apelllll1l1C challl B W B Umbria·Marche ~ Liguria" unils Basemellt wlits ~:-;':-j.l "Epi·Ligurian" EJ vv vv Carbollatic Cervaro/a basins 0/ basifls hasills of file of che units platforms un;/s ullits TusCQIl uflits ~ Maill trllUslfrollls Fig. 2 - Ti!t corrected pa!eomagnetic declinations (ali transferred Apennines and Tuscan-Latium Tyrrhenian margino structures oriented NW-SE (such as LatiumAbruzzi platform and northern Umbria-Marche-Romagna arc) the paleomagnetic data indicate consistent CCW rotations. Local structures also support this generaI pattern. The Roveto Valley, for example, is formed by NW-SE oriented structures bounded by thrust to norma! polarity) along the central-northern and left-Iateral strike-slip faults (MONTONE& SALVINI,1993). This trend is almost constant along the valley, except nearby the Canistro structure, where the fold axes turn toward a N-S orientation. Paleomagnetic data show a generaI post-Messinian CCW rotation along the Roveto Valley except in the Canistro struc- M. MATTEl ET ALll 474 80 60 Y=7.3+1.2X R=O.814 40CI) Z 20 O •..• 0~ b -20~ -40 -60-80 -60 20 40 60 STRUCTURAL DIRECTIONS Fig. 3 - Declination deviations relative to fold axis deviations. The Y axis represents the measured paleomagnetic declination, the X axis represents the deviation of the fold axis respect to a reference direction 315°. ture, where CW rotations were observed, demonstrating that the change of structural directions is due to differential rotations. This behavior was also observed in the Romagna region, where the geometry of minor arcuate structures, separated by major tectonic boundaries (e.g. Forlì line) appears related to differential vertical axis rotations. In order to test statisticalIy the relationships between the measured paleomagnetic rotations and the structural trends of the compressional structures, we performed a regression analysis between the paleomagnetic declinations and the fold axis orientations. Fig. 3 shows that, except for the Acquasanta-Montagna dei Fiori structures, the data are positively correlated (correlation coefficient R=0,814), thus implying that the present-day orientation of the compressional structures is totalIy due to differential rotations after Late Miocene-Early Pliocene. The statistical significance of the inferred correlation was proved (at a 99% significance level) performing a t test (MATTEIet alii, 1995; SPERANZAet alii, 1997). TUSCAN-LATIUM TYRRHENIANMARGIN In the Tuscan-Latium Tyrrhenian margin 39 out of 60 sites provided reliable paleomag- netic results (fig. 2). Positive fold and reversal tests indicate the primary origin of the isolated ChRM. Detailed description of laboratory experiments, magnetic mineralogy and paleomagnetic data can be found in SAGNOTTI et alii (1994); MATTEIet ali i (1996a); MATTEI et alii (1996b). The paleomagnetic results are consistent throughout the whole region and within the different sampled basins. In the Val di Fine Basin the mean direction calculated for the Messinian sites is D=I.4°; 1=54.9°; k=93; a9s=9.6°. In the Volterra Basin 13 sites in Messinian-Lower Pliocene sequences give a mean direction defined by: D=350.2°; 1=58.0°; k=58.4; a9s=5.5°. In the Siena-Radicofani Basin 12 sites in Lower Pliocene sediments define a mean paleomagnetic direction D=357.3°; 1=56.9°; k=76.7; a9s=3. 1°. In the Latium Tyrrhenian margin 12 sites sampled in Upper Pliocene-Lower Pleistocene sequences, provide a mean paleomagnetic direction: D=358.00; 1=55.2°; k=115.l; a9s=4.l°. The overall mean direction calculated far alI the studi ed Tuscan-Latium extensional basins is D=357.5"; 1=56.5"; k=84.3; a9s=2.5°, which indicates that the northern Tyrrhenian margin as a whole did not rotate since the Messinian. DISCUSSION The new paleomagnetic data from the northern Italian peninsula concur with other geological and geophysical data to outline distinct geodynamic provinces, which underwent differential rotations during Neogene and Quaternary times. During such time interval the Apenninic chain underwent large and complex rotations while, at the same time, the northern Tyrrhenian margin did not rotate. The bearings of such data far the geological and tectonic reconstructions of the northern Italian peninsula and the Tyrrhenian Sea will be discussed in the next sections. NEOGENETECTONICEVOLUTIONOF THE CENTRAL-NORTHERN APENNINICCHAIN The paleomagnetic data demonstrate that rotation about vertical axis is a main deformative mechanism characterizing the PliocenePleistocene thrust sheets emplacement and strike-slip activity in the central-northern PALEOMAGNETIC CONSTRAINTS Apennines. If we rotate back the present-day fold axes for the measured paleomagnetic rotations they became constantly oriented around N320°, indicating that during the Messinian, the chain-foredeep system was almost rectilinear (fig. 4). Progressive rotations during the thrusts propagation produced the complex geometry of the compressional structures and the present-day arcuate shape of the Apenninic chain. Most of the rotations should have occurred during the emplacement of the major out-of-sequence thrust sheets, such as the Gran Sasso front and the Olevano-Antrodoco line. Rotations associated to thrust activity along these structures resulted in the present-day non coaxiality of the chain-foredeep system and allowed single thrust units to overlie foredeep basins of different ages (CIPOLLARI& COSENTINO,1992; MATTEI et alii, 1995, Bg. 14). TO THE TECTONIC 475 EVOLUTION The detailed knowledge of the sense and amount of rotations in the Apennines allows to define precisely the thrust sheets trajectory and can strongly support a correct balancing of cross-sections. In the northern Apenninic chain, since BALLY et alii (1986), a large number of balanced cross-sections has been proposed, mainly considering surface geology and interpretation of seismic data. The thrust sheets trajectories, reconstructed restoring balanced cross section (e.g. BALLY et alii, 1986; ME ICHETTI,1996), generally show larger CCW rotations in the southern part of the Umbria-Marche arc, which are opposi te to the evidences from the paleomagnetic data (see also CHANNELLet alii, 1978). It seems clear that an integrated approach to the balancing of cross-sections using together structural, seismic and paleomagnetic data allows to define better the thrust sheets geometry at Messinian (-6.5- .. :-'l: :- :- . ".'::::.;----r::; l:. :..:...:...:.....1 Rifted area filled by "'neoautochthonous' continental deposits. []j]]]]J Cervarola and Tuscan nappes, internai units. = Internai Umbria-Marche-Romagna domain. 1= = 1 ~ External Umbria-Marche-Romagna ~ Lazio-Abruzzi platform. marine and domain. Flysch deposits ~ Faults: Fig. 4 - Schematic model of the tectonic evolution of the northern External frent of the Apenninic chain ~ thrust I italian peninsula. strike-slip \ norma I 476 M. MATTEI depth and also contributes to the estimatation of the exact amounts of shortening along the different sectors of the chain. OPE l G OF THE TYRRHENIANSEA Different mechanisms have been proposed to explain the origin of the Tyrrhenian Sea basin (see SERRIet alii, 1991 for a review). In a recent paper FACCE A et alii (1996) explored, by means of analogue lithospheric model, the possible contributions of three of these different proposed mechanisms: the indentation of the African plate, the consumption of the Adria-Ionian plate and the collapse of a thickened Alpine crust. Other mechanisms, such as astenospheric (horizontal or vertical) mantle flow, have not been experimentally modelled. Despite the over-semplification of our models, they simulate the extensional process in the Tyrrhenian Sea with the combination of far field-stress, generated by plate interactions and/or subduction, with near field-stress, mainly linked to body forces (FLEITOUT& FROIDEVAUX,1982; MOLNAR& LYON-CAEN,1988) stored in the crust previously affected by the Alpine-Apenninic thickening evento Our paleomagnetic data show that, at least since the Messinian, the northern Tyrrhenian Sea opened without any rotation of the Tuscan-Latium margin, indicating that the change in the structural trend within the extensional northern Tyrrhenian basin (from N-S in the Corsica basin to NW-SE in Tuscany) does not result from passive rotation of the already formed extensional structures (fig. 4). On the base of our analogue models and numerical simulation (see for a review BASSI, 1995), we suggest that at regional scale location, geometry and mechanics of extensional tectonics is connected to the crustal heritage of a thickened Alpine-Apenninic chain. At smaller scale, a similar process can be envisaged in many sectors of the Appenninic chain, where normal faults follow the trend of the main compressional structures and its arcuate geometry under the influence of the pre-existing topography and crustal weakness zone (i.e. Gran Sasso Range, D'AGOSTINOet alii, 1997). We underline that a non rotating opening mechanism is not common to all the extensional basins within the Mediterranean Sea. As a matter of fact, the adjacent Liguro-Pro- ET ALIl vençal Basin opened during the CCW rotation of the Corsica-Sardinia block (VIGLIOTTl& KENT, 1990). This difference, which implies a distinct geometry and evolution of the sedimentary deposits (FUNICIELLOet alii, 1997), can be due to a difference in the pre-rift rheology of the area. In the Corsica-Sardinia block rifting acted on a Hercynian «norma!» crust, whereas in the Tyrrhenian Sea the extensional tectonics affected a thickened and thermally relaxed unstable Alpine crust. ROTATlO OF THE ITALIA PE I SULA The hypothesis of a CCW rotation of the Italian peninsula was originally proposed by LOWRIE& ALVAREZ(1974) on the basis of the first paleomagnetic results obtained from the Upper Cretaceous-Lower Tertiary sedi me ntary sequences of the Umbria-Marche region, considered as autochthonous and representative of the entire Italian peninsula. Even if other paleomagnetic studies (e.g. CHANNELL et alii, 1978; CHANNELL,1992) demonstrated the existence of different rotations throughout the Italian peninsula, this mechanism is stili largely used to explain the tectonic evolution of the Centrai Mediterranean region. The new paleomagnetic data demonstrate that, at least for the Neogene and Quaternary periods, the concept of an «Eulerian» CCW rotation of the Italian peninsula, considered as a unique, rigid block, is properly a non sense. We show that not only the major tectonic domains, but also the minor adjacent structures, underwent different sense and amount of rotations during their tectonic evolution. Therefore reconstructions which derive the originai orientation of Meso-Cenozoic faults or paleogeographic domains on the base of a generai «CCW rotation of the Italian peninsula» (e.g. TAVARNELLI, 1996) should be avoided, or, in case, should use precise and detailed paleomagnetic data obtained in the examined structures. Moreover, it should be reminded that the paleomagnetic data from Mesozoic sequences may represent the summation of different tectonic rotations (e.g. movements of «microplates» in the Mediterranean and localized deformations associated with the orogenic building of the Apenninic chain). Even if only the Neogene and Quaternary sequences are concerned, we showed that the paleomagnetic data point out a complex deformation pattern that prevents the extrapolation of 477 PALEOMAGNETIC CONSTRAINTS TO THE TECTONIC EVOLUTION the vertical axis rotations found in the Apenninic chain or in the northern Tyrrhenian region to the adjacent geologic provinces (Le. the Adriatic foreland). Paleomagnetic and structural evidences clearly show a differrent behaviour of the Adriatic foreland with respect to the Apenninic chain. Otherwise VAN DER Voo (1993) and CHANNELL(1996) recently reviewed the available paleomagnetic data from Adria, suggesting no relative rotation of this region respect to Africa during Cenozoic. Infact they demonstrated that the Cretaceous paleomagnetic poles obtained from Istria, Gargano Apulia, Istria and Southern Alps, are not significantly different from those expected from the coeval African reference poles, whereas paleomagnetic directions from northern Umbria (Bosso/Sentino/Burano river valleys) are rotated CCW with respect to Africa. basins. In our opinion geometry and orientation of extensional tectonics is influenced by the preexisting rheology and topography and, as a result, normal faults passively follow the arcuate shape of the Apenninic chain. Manoscritto pervenuto il 28 gennaio 1997. Testo approvato per la stampa il 2 luglio 1997. Ultime bozze restituite 1'11 novembre 1997. REFERENCES BALLY A.W., BURBI L., COOPER C. & GHELARDO I R. (1986) - Balanced sections and seismic reflection profìles across the Cel1tral Apennines. Mem. Soc. Geol. Ita!., 35, 257-310. BASSI G. (1995) - Relative importance of strain rate and rheology for the mode of col1tinental extension. Geoph. Journ. Intern., 122, 195-210. BOCCALETTI M., GUAZZONE G. & MA NETTI P. (1974) Evoluzione paleogeografìca e geodinamica del Mediterraneo: bacini marginali. Mem. Soc. Geo!. Ita!., 13(2), 18-21. CAREY S. W. (1938) - Tectonic evolution of New Guinea and Melanesia, PHD thesis, University or Sidney. CASTELLARlN A., COLACICCHI R. & PRATURLO A. (1978) - Fasi distensive trascorrenze e sovrascorrimenti lungo la «linea Ancona-Anzio» dal Lias medio al Pliocene. Geologica Romana, 17, 161-189. CHAN ELL J.E.T. (1977) - Paleomagnetism of limestones from the Gargano peninsula (ltaly), and the implications or these data. Geophys. J.R. Astron. Soc., 51,605-616. CHANNELL, J.E.T., LOWRIE W., MEDIZZA F. & ALVAREZ W. (1978) - Paleomagnetism and tectonics in Umbria, 1taly. Earth Piane t. Sci. Lett., 39, 199-210. CIPOLLARl P. & COSENTINO D. (1992) - Analisi biostratigrafìca dei depositi terrigeni a ridosso della linea Olevano-Antrodoco. Studi Geologici Camerti, 199112,143-150. i TRE UPPER NEOGENE TECTONIC EVOLUTION OF TRE NORTRERN-CENTRAL ITALIAN PENINSULA In fig. 4 we present a tentati ve scheme of the tectonic evolution of the northern Italian peninsula as deduced by paleomagnetic, structuraI and strathigraphic data. In this mode!, the internaI or external position of the different structural units is based on the age of the foredeep deposits, which are assumed progressively younger toward the Adriatic Sea (PATACCAet alii, 1990; CIPOLLARI& COSENTI o, 1992), whereas the average orientation of the external front of the chain during the Messinian is obtained restoring the different structures by means of the measured pale 0magneti c rotations. The age of the tectonic rotations is assumed to be the same of the main tectonic structures which separate the different rotational domains (Gran Sasso front, Olevano-Antrodoco line; Sabina fault, Sibillini thrust). During the Pliocene the chain progressively evolved from a rectilinear bel t toward the present-day arcuate shape. Rotations are connected to thrust activity, mainly to out-of sequence thrust, and enhanced by strike-slip faulting. During the rotations of the Apenninic structures, the extensional Tyrrhenian margin did no rotate. The changing in orientation of the Neogene extensional basin does not result from passive rotations which affected the already formed sedimentary D'AGOSTI o ., CHAMOT ROOKE ., FUNIClELLO R., JOLIVET L & SPERA ZA F.. The role or pre-existing thrust raulis and topography on the styles or extension in the Gran Sasso range (Centrai Italy). Submitted. DELA PIERRE F., GHISETTI F., LANZA R. & VEZZANI L. (1992) - Paleomagnetic and structural evidence or Neogene tectonic rotation or the Gran Sasso range (Centrai Apennines, Italy). Tectonophysics, 215, 335-348. DOGLIONI C. (1991) - A proposal ror the kinematic modelling or W-dipping subduction. Possible applications to the Tyrrhenian-Apennines system. Terra Nova, 3, 423-434. ELDREDGE S., BACHTADSE V. & VA DER VOO R. (1985) - Paleomagnetism and the orocline hypothesis. Tectonophysics, 119, 153-179. ELTER P., GIGLlA G., TONGJORGI M. & TREVI SAN L. (1975) - Tensional and compressional areas in recent (Tortonian to Present) evolution or north Apennines. Boll. Geofis. Teor. Appl., 17, 3-18. 478 M. MATTEI ET ALII FACCENNA c., DAVV P., BRUN J.P., FUNICIELLO R., GIARDINI D., MATTEI M. & NALPAs T. (1996) - The dynamics approach of back-arc extension: a laboratory to the opening of the Tyrrhenian Sea. Geoph. Journ. Int., 126, 781-795. FUNICIELLO R., MATTEI M., SPERANZA F. & FACCENNA C. (1997) - La geodinamica del sistema TirrenoAppennino. Le Scienze, 343, 44-53. FLETOUT L. & FROIDEVAUX C. (1982) - Tectonics and topography for a lithosphere containg density heteregeneties. Tectonics, 1, 21-56. HIRT A. & LOWRIE W. (1988) - Paleomagnetism of the Umbrian-Marches orogenic belt. Tectonophysics, 146,91-103. JOUVET L., DUBOTS R, FOURNIER M., GOFFE B., MICHARD A. & JOURDAN C. (1990) - Ductile extension in Alpine Corsica., Geology, 18, 1007-1010. LANCI L. & WEZEL C.F. (1995) - Rotazioni tettoniche di età Messiniana nell'Appennino marchigiano (abs.), in Tettonica Attiva e Geodinamica del Sistema Tirreno-Appennino, LOWRlE W. & LOWRIE W. & 330-332, Camerino. ALVAREZ W. (1974) - Rotation peninsula. Nature, 251, 285-288. ALVAREZ W. (1975) dence for rotation Geophys. MARTON E. Res., & of the of ltalian (1983) results from the Istria peninsula tonophysics, 91, 73-87. MATTEl M., FUNlClELLO R. - Paleomagnetic (Yugoslavia). Tec- & KrSSEL C. (1995) - Paleo- magnetic and structural evidence for Neogene block rotations in the Centrai Apennines (Italy). J. Geophys. Res., 100, 17.863-17.883. MATTEI M., FUNICIELLO R. & & SALVINI F. (1991) - Evidences of NW-SE Late Miocene strike-slip tectonics in Centrai Apennines near Tagliacozzo, Abruzzi. Boll. Soc. Geol. MONTONE P. It., 110,617-619. R & SCANDONE P. (1990) - Tyrrhenian basin and apenninic arcs: Kinematic relations since late Tortonian times. Mem. Soc. Geol. PATACCA, E., SARTORI Ital., 45, 425-451. & SCANDONE P. (1987) - Segmentation and configuration of subducted lithosphere in ltaly: An important control on thrust-belt and foredeep-basin evolution. Geology, 15,714-717. SAGNOTTI L., MATTEl M., FUNICIELLO R. & FACCENNA C. (1994) - Paleomagnetic evidence for no tectonic rotation of the Centrai !taly Tyrrhenian ma l'gin since upper Pliocene. Geophys. Res. Lett., 21, 481-484. SARTORI R. (1990) - The main results of ODP leg 107 in the frame of Neogene to Recent geology of Perityrrhenian areas. Proc. Ocean Drill. Program Sci. ROYDEN L., PATACCA E. Results, - Paleomagnetic eviItalian peninsula. J. 80, 1579-1592. VELJOVIC D. tal deformation, Clark S.P., Burchfield B.C. and Suppe J. eds., Geol. Soc. Am., special papers, 218, 179-208. KrSSEL C. (1995) - No tectonic rotation of the Tuscan Tyn-henian margin (Italy) since Late Messinian. Journ. Geoph. Res., 101, 2835-2845. MATTEI M., KTSSEL c., SAGNOTTT L., FUNICTELLO R & FACCENNA C. (1996) - Lack of Upper Miocene to Present rotation in the Northern Tyrrhenian margin (!taly): a constraint on geodynamic evolution. In Morris A. & Tarling D. (eds.) Paleomagnetism and 107,715-730. SERRI G., INNOCENTI F., MANETTI P., TONARINl S. & FERRARA G. (1991) - Il magmatismo neogenico- quaternario dell'area tosco-Iaziale-umbra: Implicazioni sui modelli di evoluzione geodinamica dell'Appennino settentrionale. Studi Geol. Camerti, 1991(1),429-463. & MATTE I M. (1997) - Tectonics of the Umbria-Marche-Romagna Arc (centrai northem Apennines, ltaly): New paleomagnetic constraints. J. Geophys. Res., 102, 3153-3166. TAVERNELU E. (1996) - Tethyan heritage in the development of the Neogene Umbria-Marche fold-andthrust belt, !taly: a 3D approach. Terra Nova, 8, SPERANZA F., SAGNOTTI L. 470-478. VANDENBERG J., KLOOTWlJK C.T. & WONDERS A.A.H. (1978) - Late Mesozoic and Cenozoic movements of the ltalian peninsula: Further paleomagnetic data from the Umbrian sequence. Geol. Soc. Am. Bull., 89, 133-150. - Reappraisal of paleomagnetic data from Gargano (South ltaly). Tectonophysics, Tectonics of the Mediterranean Region, Geological Society Special Pubblication, 105, 141-146. MENlCHETTl M. (1996) - Analisi strutturale ed evoluzi- VANDENBERG J. (1983) one cinematica dell'Appennino umbro-marchigiano attraverso la costruzione di sezioni geologiche bilanciate. In «Presentazione dei risultati del profilo VAN DER VOO R. (1993) sismico VTGUOTTI L CROP 03» Roma 7-9 Novembre, MOLNAR P. & LVON CAEN H. (1988) P. 67. - Some simple physical aspects of the support, structure and evolution of mountain belts. In: Process of continen- 98, 29-41. - Paleomagnetism of the Atlantic, Tethys and lapetus Oceans. Cambridge Univer- sity Press, Cambridge, & KENT UK. D.Y. (1990) - Paleomagnetic results of Tertiary sediments from Corsica; evidence of Post-Eocene rotation. Phys. Earth and PIan. Int., 62, 97-108.