1 Journal of Geodynamics 37 (2004) Early Middle Pleistocene eastward migration of the Abruzzi Apennine (central Italy) extensional domain Fabrizio Galadini*, Paolo Messina CNR, Istituto di Geologia Ambientale e Geoingegneria, Rome, Italy Received 24 March 2003; received in revised form 30 September 2003; accepted 8 October 2003 Abstract The evolution of the Apennine arc is related to the flexural retreat of the Adriatic lithosphere plate, the NE migration of the compressive front and the contemporaneous rifting of the Tyrrhenian basin. Evidence of the NE migration of the orogen can also be inferred from the analysis of the intra-apennine Quaternary extensional tectonics. Stratigraphic and structural data collected along NE SW transects in the Abruzzi Apennines (central Italy), indeed, indicate that the westernmost normal faults (activated during the Pliocene) have to be considered inactive since the Early Pleistocene (Turano valley fault) or the beginning of the Middle Pleistocene (Salto valley and Liri valley faults). In contrast, the easternmost faults (Campo Imperatore and Mt. Morrone faults) were activated during the Early Pleistocene and are still active, as demonstrated by the displacement of Late Pleistocene Holocene deposits and landforms. Within the innermost portion of the extensional domain, the Fucino and upper Aterno valley fault systems show persistent activity since the Pliocene. Besides the evidence of a progressively NE-shifted intra-apennine extension, available data suggest that a period of co-existing activity of the presently inactive and newly activated faults (e.g. Liri valley and Mt. Morrone faults, respectively) occurred during the Early Pleistocene. This indicates that for a few hundred thousand years active faults in the central Apennines were more numerous than in the period of the present tectonic regime. This may have resulted in a reduced activity per fault (lower slip rates) or an increased intra-apennine extension rate. Quaternary faults in the Abruzzi Apennines generally coincide with pre-existing discontinuities such as old thrust ramps or syn-orogenic normal faults. Based on a simple geometric criteria, i.e. direction, dip, length of the pre-existing discontinuities located east of the investigated area, and assuming a still active NE-migration of the extensional domain, it is possible to hypothesize that the old and presently inactive normal faults of the Maiella and Montagna dei Fiori anticlines may accomodate active extension of the future. The NE-ward shifting of the extensional domain would be, instead, locked if the process of lithospheric flexural retreat which drove the Neogene-Quaternary p.p. structural evolution of the Apennine chain has ended during the Early Pleistocene. # 2003 Elsevier Ltd. All rights reserved. * Corresponding author. Fax: address: (F. Galadini) /$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi: /j.jog
2 58 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Introduction As known, young orogens usually permit the study of problems related to the structural evolution better than the old and inactive ones. The exogenic forces, indeed, could not erase significant traces of the different tectonic episodes, and the recent deposits and landforms usually represent a mosaic whose understanding gives the possibility of checking the progress of the deformation with a periodicity of a few hundred thousand years. On the other hand, the structural evolution of an orogen within a close time window is more complex than that inferred from reconstructions based on evolution stages of some million years. In fact, processes such as the abandonment of shear zones in favour of the activation of others (e.g. Galadini, 1999), fault kinematic changes resulting from the evolution of the deformation (e.g. Scotti et al., 1991) or changes in the stress field orientation (e.g. Bellier and Zoback, 1995; Bonini et al., 1997), pulsating tectonic activity (Wallace, 1984, 1987; Bertini and Bosi, 1993), and geometrical modifications during the growth of a structure (e.g. Stewart and Hancock, 1988; Jackson and Leeder, 1994) are quite common in active orogens. The Apennine chain appears to be one of the most suited orogens to address this kind of problems, in fact: (1) Plio-Quaternary tectonics has been documented for the entire chain (e.g. CNR PFG, 1987); (2) a reconstruction of the structural evolution since the Miocene by means of the stratigraphy related to marine sediments is available (e.g. Patacca et al., 1990); (3) the increasing amount of stratigraphic data related to the Plio-Quaternary continental deposits permits to define chronologically constrained evolutive patterns; (4) some kinematic changes affected the Quaternary tectonic history (Cinque et al., 1993; Hyppolite et al., 1994; Calamita et al., 1995; Galadini, 1999), and the characteristics of the present tectonic regime are not completely clear. As for point 2, available reconstructions are related to the evolution of the NEmigrating compressive structures, while few data on the general evolution of the intra-apennine extensional tectonics have been published (e.g. Martini and Sagri, 1994; Bartole, 1995; Cavinato and De Celles, 1999) after the publication of the Neotectonic Map of Italy (CNR-PFG, 1987). As for point 3, most data on the Apennine continental stratigraphy are available for the Abruzzi sector of the central Apennines, where works have been published since the 70s aimed at creating a database which may be used for the reconstruction of the Plio-Quaternary tectonics (Bertini and Bosi, 1976, 1993; Bertini et al., 1986; Giraudi, 1988; Bosi and Messina, 1991; Galadini and Messina, 1993; 1994; Carrara et al., 1995a; Cavinato and Miccadei, 1995; Messina, 1996; Chiarini et al., 1997). The present paper addresses the problems of the early Quaternary structural evolution of the intra-apennine extensional belt in the central Apennines (Abruzzi sector). The main purpose is the reconstruction of the early Quaternary displacement history of the recent faults through integrated geomorphological, stratigraphic and structural field investigations. The data gathered in this paper illustrate a complex evolutive pattern with kinematic changes occurring in a few hundred thousand years. After a section dedicated to the current views on the building processes of the Apennines, some sections will deal with the available chronologically constrained data on the structural evolution. Finally, a discussion about the Quaternary kinematic history will be proposed.
3 2. Geological framework F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) The evolution of the Apennines is part of the complex geodynamic history of the Mediterranean area. In particular, the present structure results from the opening of the Tyrrhenian sea (the westernmost structural domain) and the forelandward migration of the Apennine compressive front since the Late Cenozoic (CNR-PFG, 1983; Boccaletti et al., 1986; Malinverno and Ryan, 1986; Vai, 1987; Royden et al., 1987; Patacca et al., 1990; Doglioni, 1995; Patacca and Scandone, 2001; Mantovani et al., 2002). These kinematic processes are due to the eastward and northeastward flexural retreat of the Adria (foreland) lithosphere plate boundary (Fig. 1) (e.g. Patacca et al., 1990; Doglioni, 1995). The Tyrrhenian sea is considered as a back-arc basin resulting from a velocity of flexural retreat of the lithosphere boundary largely exceeding the velocity of plate convergence between Africa and Europe (Meletti et al., 2000). Presently, the youngest compressive front is located in the Adriatic sea along the Italian peninsula (e.g. Patacca et al., 1990; Doglioni, 1991) and is buried below the Po Plain in the northern Apennine sector (Fig. 1) (Patacca et al., 1990). From the structural point of view, this evolution resulted in thrust sequences affecting the entire Apennine chain (CNR-PFG, 1983). The NE migration of the orogenic process has been Fig. 1. Schematic kinematic setting of the Northern Apennine arc and location of the investigated area. See text for a detailed kinematic explanation.
4 60 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) documented through the observation of progressively younger thrusts affecting the Italian peninsula towards NE (e.g Patacca et al., 1990; Cipollari et al., 1999). After the orogenic phase, since the Pliocene, the Apennines experienced normal faulting and uplift (e.g. CNR-PFG, 1987; Bartolini, 1999; Coltorti and Pieruccini, 2000; D Agostino et al., 2001). The former process has been responsible for the formation of the main extensional intermontane basins. In the Abruzzi sector of the central Apennines (Fig. 2), some normal faultbounded intermontane basins are characterized by continental deposition (lacustrine facies) since the Pliocene (Fig. 3) (e.g. Bertini and Bosi, 1976; Bosi et al., 1995). Normal fault activity is persistent through most of the Quaternary (Fig. 2); Late Pleistocene Holocene activity has been proved by means of paleoseismological investigations for some central Apennine faults (e.g. Pantosti et al., 1996; Galadini and Galli, 1999; D Addezio et al., 2001). The persistent fault activity is also demonstrated by the strong seismicity which affects the chain, with events characterized by magnitude up to 7 (e.g Working Group CPTI, 1999). Investigations on the extensional tectonics of the Northern Apennines have suggested that also extension has been characterized by a progressive northeastward migration (Martini and Sagri, 1994; Bartole, 1995). A similar process has been defined for the Umbria-Marche sector of the central Apennines (Calamita et al., 1999) and hypotheses about progressively NE-shifted extensional domains are also available for the Abruzzi sector of the chain (e.g. Lavecchia et al., 1994; Cavinato and De Celles, 1999). However, the lack of a detailed Plio-Quaternary continental stratigraphic framework for the latter sector has so far prevented the definition of a reliable structural evolution related to the extensional tectonics. More recently, a summary of the available data on the continental stratigraphy of the Abruzzi Apennines has been produced (Bosi et al., 2003). In Fig. 3 we report only part of this stratigraphic framework, i.e. that related to the area discussed in the present paper. Methodological aspects and the data on which the chronological attributions are based have been discussed in detail in the paper by Bosi et al. (2003). In short, the chronology of the different units is based on the available paleontological findings, on paleomagnetic data (giving an age older than the limit Bruhnes Matuyama, i.e. older than 0.78 Ma), and on the occurrence of deposits with a high content of volcanic minerals (indicating an age of deposition subsequent to the onset of the most recent central Italy volcanic phase, i.e. 0.6 Ma). The recognition of similar successions of erosional and depositional events in the different basins has permitted a correlation of the different units across the basins. Chronological constraints are only lacking for the first depositional event attributed to the Pliocene (-Early Pleistocene?) by Bosi et al. (2003). This attribution is not based on quantitative data, but on the consideration that the deposits related to the first event are subsequent to sediments attributed to the Messinian-Lower Pliocene (Bosi and Messina, 1990; Cipollari et al., 1999) and are much older than those related to the Early Pleistocene (since the physiography of the first event-related basins is completely different from that of the Quaternary basins; see Bosi et al., 2003, for details). The stratigraphic framework of Fig. 3 represents the basis for the discussion of the chronology of the fault activity reported in the next sections. However, the stratigraphic successions of the Sulmona plain and the Liri valley are not reported in the mentioned figure, since they have not been discussed in the work by Bosi et al. (2003). Therefore, we will refer to the work by Carrara et al. (1995a) to define the chronology of the Liri valley fault activity and to the papers by Cavinato and Miccadei (1995) and Miccadei et al. (1998) to define the chronology of the Mt. Morrone fault activity.
5 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Fig. 2. (A) Map of the main Quaternary faults in the Abruzzi Apennines; continuous lines define active faults (thick if discussed in the present paper) while dashed and dotted lines identify inactive faults: TVF, Turano valley fault; SVF, Salto valley fault; DMF, Duchessa Mts. fault; LVF, Liri valley fault; MMF, Magnola Mts. fault; MHF, Marsicana Highway fault; SBGF, San Benedetto dei Marsi-Gioia dei Marsi fault; MMOF, Mt. Morrone fault; CIF, Campo Imperatore fault. For the nomenclature related to the upper Aterno valley (see Fig. 5). Some of the faults showing evidence of Late Pleistocene-Holocene activity have been responsible for historical earthquakes with magnitude M>6 (Galadini and Galli, 2000). Faults whose activity ended during the Early Pleistocene or at the beginning of the Middle Pleistocene are sealed by deposits and landforms of this age; (B, C) topographic profiles across the Abruzzi Apennines and location of the main Quaternary faults.
6 62 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Fig. 3. Stratigraphic setting of the investigated area, modified after Bosi et al. (2003).
7 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) The following sections are dedicated to the description of two different transects across the central Apennines, roughly perpendicular to the direction of the main extensional faults (Fig. 2). 3. The NW transect 3.1. Turano valley fault The southern portion of the Turano valley has been affected by lacustrine deposition during the Pliocene, as indicated by the numerous outcrops of layered clayey sandy deposits related to the Santa Croce unit (Fig. 3). These deposits dip 20 towards NE as a result of the tilting due to the activity of the NW SE trending normal fault located along the right valley flank (Fig. 4). The defined structural relationship between the lacustrine deposits and the fault indicates that tectonic activity affected the investigated area during and/or after the deposition of the Pliocene sediments. The second depositional event recorded in the Turano valley is represented by extensively outcropping slope-derived breccias related to the Early Pleistocene ( Le Grotte breccias in Fig. 3). The breccias are layered, dip towards SW and evidently seal the fault (Fig. 4), thus indicating the lack of displacement after the deposition. Tectonic data on the Turano valley indicate, therefore, that this area has experienced the effects of fault activity before the deposition of the Early Pleistocene breccias. Since this depositional event the fault has no longer been active Salto valley fault system Similarly to the Turano valley, also the Salto valley has been affected by lacustrine deposition during the Pliocene ( Piagge unit in Fig. 3). The first unit is, indeed, made of layered sands and silty sands fed by the Miocene flysch in the lower part. The upper part is made of coarser deposits fed by the carbonate bedrock. The total thickness of the outcropping deposits related to the Pliocene unit is 300 m (Bosi et al., 1989). After the deposition of the Piagge unit, the basin was drained and the subsequent deposition has been characterised by fluvial and colluvial facies. The Salto valley is bordered by two normal fault segments: the northern one has been reported as Salto valley fault by Galadini and Messina (2001), while the southern one is reported as Duchessa Mts. fault by the same authors. Both segments are NW SE striking, although minor portions are characterised by different directions. The plane of the Salto valley fault is exposed along a bedrock scarp in the Fiamignano area (Bosi, 1975). In this sector, the fault activity has been responsible for the displacement of the Pliocene deposits which are clearly tilted and dip up to 50 towards N and NE. By contrast, Quaternary deposits of alluvial facies are generally characterised by a sub-horizontal bedding (Bosi et al., 1989). The Salto valley fault system has been considered as active by Morewood and Roberts (2000), mainly because the fault was responsible for the formation of the bedrock scarp in the area of the Fiamignano village. However, Galadini and Messina (2001) indicated that the formation of the scarp is only due to the activation of a deep seated gravitational movement (whose effects are also represented by the tilting of more than 40 of Early Pleistocene slope-derived breccias). Since no
8 64 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Fig. 4. Turano valley fault (see Fig. 2 for location): (A) geological sketch of the investigated area; the normal fault exposed between the villages of Ascrea and Paganico is sealed by the Early Pleistocene breccias of the Le Grotte unit; (B) exposed plane of the Turano valley fault; (C) panoramic view of the Le Grotte breccias sealing the Turano fault; (D) layered sands and silts of the Pliocene Santa Croce lacustrine unit; the deposits dip towards NE. other evidence of Late Pleistocene Holocene has been detected along the fault, the mentioned authors considered that the gravitational movement has not a tectonic origin. Moreover, geomorphological data indicate that the activity of the Duchessa Mts. fault ended at the beginning of the Middle Pleistocene. The fault, as reported by Chiarini et al. (1997), is sealed
9 by Middle Pleistocene deposits and paleo-landsurfaces (basically strath terraces carved into the Meso-Cenozoic substratum and Quaternary deposits), while it has been responsible for the displacement of older deposits and landforms. In conclusion, available data on the Salto valley fault system indicate that the activity of this structure began during the Pliocene and ended at the beginning of the Middle Pleistocene Upper Aterno valley fault system The upper Aterno valley fault system is made of four minor segments, each one bordering a specific basin which experienced Quaternary continental deposition. From the south, we defined: the Mt. Pettino fault, bordering the l Aquila basin; the Mt. Marine fault, bordering the Arischia basin; the San Giovanni and Capitignano faults to which the Montereale basin is related (Fig. 5). The length of each segment does not exceed 10 km. The kinematics is normal or normal-oblique with slight left-lateral motion (Galadini, 1999). The presence of clayey sandy deposits of lacustrine origin attributed to the Pliocene (Pianola unit in Fig. 3) in the L Aquila area indicates that the L Aquila basin originated at least during pre-quaternary times. This defines a Pliocene age for the beginning of the extension in the upper Aterno valley. Fault activity is persistent during the Quaternary, as indicated by the displacement of Early Pleistocene slope-derived breccias (related to the Madonna della Strada-San Marco unit of Figs. 3; Fig. 5) along the Mt. Pettino and San Giovanni faults (Fig. 5; Galadini and Messina, 2001), or by the displacement of alluvial gravels (related to the above mentioned unit) in the Arischia basin (Fig. 5). Moreover, available data indicate that the entire fault system has to be considered as having been active during the Late Pleistocene Holocene. The Mt. Pettino and Arischia faults have been responsible, indeed, for the displacement of slope deposits which have been radiocarbon dated at BP and BP, respectively (Galadini and Galli, 2000). Finally, paleoseismological data collected along the Mt. Marine fault showed that it has been responsible for the displacement of colluvial units containing pottery shards of historical age (Moro et al., 2002). Available literature (e.g. Blumetti, 1995; Galadini and Galli, 2000) suggests that the upper Aterno fault system was responsible for the February 2, 1703 earthquake (Me 6.67, according to Working Group CPTI, 1999), on the basis of the damage distribution and of the historical reports on the coseismic geological effects. Data on the chronology of the activity indicate, therefore, that the upper Aterno valley fault system has to be considered as active during the Quaternary (probably since the Pliocene) until the Present Campo Imperatore fault F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) NE of the Aterno valley the topography rises up to the Gran Sasso chain, where the highest peaks of the Abruzzi Apennines are located (Fig. 2). The chain is about E W trending and bordered to the south by a N normal fault system (about 30 km long) whose recent activity is demonstrated by the displacement of Late Pleistocene glacial till, colluvial, alluvial and slope deposits and Holocene palustrine deposits and paleosols (Fig. 6) (Carraro and Giardino, 1992; Giraudi and Frezzotti, 1995; Galli et al., 2002). One of the segments of this fault system was also responsible for the displacement of a glacial cirque related to the last glacial maximum (Fig. 6).
10 66 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Fig. 5. Upper Aterno fault system (see Fig. 2 for location): (A) Photomosaic of aerial photographs (by courtesy of the Italian Istituto Geografico Militare, survey 1954); small triangles indicate the segments of the fault system and the related bedrock fault scarps: CF, Capitignano fault; SGF, San Giovanni fault; MMAF, Mt. Marine fault; MPF, Mt. Pettino fault; (B) displaced paleosol in the Arischia area; the paleosol is contained in a succession of fine gravels and sands and can be correlated to the Late Pleistocene (about 30,000 BP) paleosols identified in the area by Blumetti (1995) and Moro et al. (2002); (C) eastern wall of the paleoseismological trench excavated in the Pizzoli area by Moro et al. (2002); the fault plane places Late Pleistocene alluvial deposits (30, BP, radiocarbon age from Moro et al., 2002) in contact with colluvial deposits containing pottery shards; (D) displaced Early Pleistocene alluvial gravels outcropping in a quarry in the southern portion of the Arischia basin; (E) panoramic view of the early Pleistocene breccias outcropping in the area of the San Marco village; the breccias are affected by a fault scarp representing the NW termination of the Mt. Pettino fault; (F) displaced Late Pleistocene slope deposits (see Galadini and Galli, 2000, for the age of these deposits) along the Mt. Pettino bedrock fault scarp.
11 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Fig. 6. Campo Imperatore fault (see Fig. 2 for location): (A) Photomosaic of aerial photographs (by courtesy of the Italian Istituto Geografico Militare, survey 1954) related to the central portion of the Campo Imperatore fault; small triangles indicate the fault scarps (both in the bedrock and in unconsolidated sediments); in the Mt. Paradiso area, minor faults were responsible for the displacement of an alluvial fan which formed mainly after the Last Glacial Maximum (about 22,600 BP in the investigated area, according to Frezzotti and Giraudi, 1992); in the Mt. Brancastello area, the fault was responsible for the displacement of a glacial cirque which fed a glacial tongue during the LGM; (B) panoramic view of the Mt. Paradiso alluvial fan, affected by fault scarps; (C) view of one of the fault scarps affecting the alluvial fan and of the paleoseismological trench site which revealed the displacement of a paleosol dated at / BP (14C calibrated age, Galli et al., 2002).
12 68 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) The area between the Gran Sasso chain and the middle Aterno valley is affected by a large number of minor faults (not longer than 6 km) which were responsible for the formation of small and narrow NW SE trending intermontane basins (Figs. 2 and 7). The basins are grabens and half-grabens and the deformation distributed over the entire area has been modelled by D Agostino et al. (1998). The authors defined a direct relationship between the low-angle geometry of the Gran Sasso normal fault and the deformation distributed along the secondary faults located in the hangingwall. According to Galadini and Giuliani (1991), the formation of the small depressions along the secondary faults affected a paleolandscape gently dipping towards the Aterno Valley. Alluvial fan sediments ( Valle Valiano layered carbonate breccias, incorporated in the box Valle Valiano-San Nicandro of Fig. 3) deposited on the paleolandscape and in the marginal area of the Aterno paleovalley, where these sediments are interfingered with lacustrine deposits ( San Nicandro unit, incorporated in the box Valle Valiano-San Nicandro of Fig. 3) of Early Pleistocene age (Bosi and Messina, 1991; Bertini and Bosi, 1993). The formation of the above mentioned small depressions along the secondary faults of the Gran Sasso chain began after the deposition of the alluvial fan sediments, since in some cases the narrow ridges separating the fault-bounded depressions are entirely made of the Early Pleistocene alluvial fan breccias (Fig. 7) and the tectonic activity disrupted the continuity of the breccias paleolandscape. The flanks of the small depressions have been affected by the deposition of slope-derived breccias (Fig. 7; Fonte Vedice unit of Fig. 3) which are also related to the Early Pleistocene (Galadini and Giuliani, 1991) and are common to many slopes of the Abruzzi Apennines (Bosi and Messina, 1991). Paleomagnetic analyses made on the matrix of these breccias indicated that the deposition occurred during the reverse Matuyama Chron and therefore their age is older than 0.78 Ma (D Agostino et al., 1997). Based on these data it is possible to conclude that the nucleation of the small depressions between the Gran Sasso chain and the Aterno river began during the Early Pleistocene. Since a direct relationship exists between the activation of these minor faults and the main Campo Imperatore fault (D Agostino et al., 1998), it is possible to infer that the beginning of the activity of the latter dates back to the Early Pleistocene. Based on the above reported data, the chronology of the activity of the Campo Imperatore fault is Early Pleistocene Present. 4. The SEtransect 4.1. Liri Valley fault The 40-km-long Liri valley fault (Fig. 2) places the carbonate bedrock in contact with the Miocene flysch and is characterised by bedrock fault scarps (Carrara et al., 1995a). The kinematic history of the fault is quite complex and both strike-slip and normal movements have been detected (Accordi et al., 1969; Ciotoli et al., 1993; Serafini and Vittori, 1995). According to Serafini and Vittori (1995), however, kinematic indicators collected along the fault show that the youngest deformative regime is characterised by normal movements. The fault was responsible for the displacement of alluvial fan deposits related to the Early Pleistocene (Carrara et al., 1995a) and is sealed by slope-derived breccias which deposited during
13 Fig. 7. (A) Map of the area between the Middle Aterno Valley and the Gran Sasso Chain (see Fig. 2 for location); Quaternary deformation in this area is distributed over a number of a few-kilometre-long faults responsible for the formation of bedrock fault scarps; (B) panoramic view of one (Valle Force fault) of the numerous bedrock fault scarps affecting the investigated sector. F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004)
14 70 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) the earlier part of the Middle Pleistocene ( Balsorano breccias ; Carrara et al. 1995a,b). As for the beginning of the activity, based on geological and geomorphological data, Carrara et al. (1995b) observed that the extensional tectonics in the Liri valley and surrounding areas began during the Late Pliocene. In short, extensional activity along the Liri valley fault may be documented between the Late Pliocene and the earlier part of the Middle Pleistocene. Since this age the fault has to be considered as inactive Fucino fault system Located east of the Liri valley, the Fucino Plain is the largest intermontane basin of the Apennines (Fig. 2); it is bounded to the east and north by a normal fault system which has been responsible for the formation of a half-graben since the Pliocene (Galadini and Messina, 1994). Three segments may be distinguished as part of the fault system, i.e. (from south) the San Benedetto dei Marsi-Gioia dei Marsi fault, Marsicana Highway fault and Magnola Mts. fault for a total length of 33 km (Figs. 2 and 8). The first segment was responsible for the displacement of Early and Middle Pleistocene slope-derived breccias (Cupoli unit in Fig. 3 and Tre Monti Formation of Galadini and Messina, 1994, the latter being synchronous with the Pescina unit of Fig. 3) and Late Pleistocene Holocene lacustrine and colluvial deposits (Giraudi, 1988). The fault, together with the Marsicana Highway and perhaps the Magnola Mts. segments, activated during the 1915 earthquake (Ms=7.0, Working Group CPTI, 1999) as testified by a number of works on paleoseismology and historical seismology (Serva et al., 1986; Michetti et al., 1996; Galadini et al., 1998; Galadini and Galli, 1999). As for the kinematics, data from paleoseismological investigations indicated movements with a prevalent normal component. The Marsicana Highway segment (Fig. 8) was responsible for the displacement of Pliocene (Aielli unit of Fig. 3) to Holocene lacustrine, alluvial and colluvial deposits (the entire succession reported in Fig. 3 for the Fucino basin plus the younger formations reported in Giraudi, 1988). The Plio-Pleistocene units are tilted towards east (in the hangingwall of the fault). The decreasing tilt which affects the progressively younger deposits indicates that the activity of the fault is persistent with a mainly normal kinematics since the Pliocene (Galadini and Messina, 1994). The Magnola Mts. segment is characterised by a continuous bedrock fault scarp which affects the southern slope of the Mt. Velino-Magnola Mts. branch. The fault plane is exposed along the scarp and kinematic indicators show prevalently normal movements (Galadini, 1999). The fault places the carbonate bedrock in contact with Early and Middle Pleistocene slope-derived breccias and Late Pleistocene slope deposits outcropping in the hangingwall. Early Pleistocene breccias (incorporated in the Cupoli box of Fig. 3) have also been found in the footwall of the fault (Fig. 8b) and Galadini and Messina (1994) evaluated a vertical offset of about 700 m. Towards west, Frezzotti and Giraudi (1992) reported the displacement of a till of the Last Glacial Maximum (about 22,600 BP in the central Apennines, according to Giraudi and Frezzotti, 1997) along the Magnola Mts. segment. In conclusion, available data on the Fucino area indicate a continuous fault activity between the Pliocene and the Present.
15 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Fig. 8. Fucino fault system (see Fig. 2 for location): (A) structural sketch of the Fucino Plain area; the basin evolution has been conditioned by the activity of the fault system bordering the depression to the east and to the north: MMF, Magnola Mts. fault; MHF, Marsicana highway fault; SBGF, San Benedetto dei Marsi-Gioia dei Marsi fault; (B) panoramic view of Early Pleistocene layered breccias, outcropping in the footwall of the Magnola Mts. fault; the correlative deposits located in the hangingwall define a vertical displacement of about 700 m; (C) Marsicana highway fault: the fault plane (uncovered during an excavation for the laying of a gas-pipeline) places Pleistocene lacustrine deposits in contact with Late Pleistocene Holocene colluvial deposits (Galadini and Galli, 1999); (D) panoramic view of the Mt. Parasano bedrock fault scarp along the Marsicana highway fault; (E) panoramic view of the Serrone bedrock fault scarp along the San Benedetto dei Marsi-Gioia dei Marsi fault; (F) detail of the contact between the layered slope deposits and the Serrone bedrock fault plane; the layers (right) abruptly end against thick and unconsolidated friction material (left).
16 72 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Mt. Morrone fault The Mt. Morrone fault is 20 km long, NW SE trending and characterised by normal kinematics (Figs. 2 and 9). As in the case of other faults affecting the central Apennines, it can be easily identified in the field since it was responsible for the formation of an impressive carbonate Fig. 9. Mt. Morrone fault (see Fig. 2 for location): (A) Photomosaic of aerial photographs (by courtesy of the Italian Istituto Geografico Militare, survey 1954) related to the NW portion of the Mt. Morrone fault; triangles indicate the bedrock fault scarps; similarly to the Magnola Mts. fault of Fig. 8, also in this case Early Pleistocene breccias are preserved in the footwall and are suspended over the present plain bottom due to the fault activity; triangles indicate the fault scarps; (B) detail of the contact between the layered slope deposits and the bedrock fault plane; the layers (left) abruptly end against the fault plane (right).
17 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) fault scarp. Along the scarp the fault plane is exposed and places the carbonate bedrock in contact with slope deposits (Fig. 9). The Mt. Morrone fault bounds the Sulmona basin. This intermontane depression experienced lacustrine deposition during the Quaternary (Cavinato and Miccadei, 1995; Miccadei et al., 1998). The fault is considered as active (e.g. Vittori et al., 1995; Galadini and Galli, 2000), since it has been responsible for the displacement of Late Pleistocene (related to the Last Glacial Maximum) alluvial fan and slope deposits. Slope derived breccias (related to the Early Pleistocene, Miccadei et al., 1998) outcrop in the footwall and are suspended m above the bottom of the present depression (Fig. 9). The subhorizontal attitude of the breccias indicates that they deposited close to the local base level. The difference of elevation between the outcrops of breccias and the present bottom of the basin indicates significant displacement due to the Mt. Morrone fault activity. This means that the fault has been active for most of the Quaternary. As for the beginning of the activity, a key-aspect concerns the age of formation of the Sulmona basin. Pre-Quaternary deposits have never been found in this basin (Miccadei et al., 1998). The latter authors, indeed, reported only Quaternary deposits also in the geological section crossing the basin. This may indicate a Quaternary age of basin nucleation. Geological surveys in the Tirino valley (see Fig. 2 for location), a large basin which formed along the northern prolongation of the Mt. Morrone fault, did not identify pre-quaternary continental deposits too (Fig. 3; Giuliani and Sposato, 1995). Therefore, available data indicate the lack of pre-quaternary deposits in a very large basinal area (Tirino valley plus Sulmona basin). This is anomalous in the framework of the continental stratigraphy of the Abruzzi Apennines and corroborates the hypothesis that the basin formation began during the Quaternary. On this basis we consider that the evolution of the Sulmona basin (and therefore the recent activity of the Mt. Morrone fault) started during the Early Pleistocene (age of the first depositional episode which affected the area). The activity is persistent during the Late Pleistocene (after the Last Glacial Maximum) and therefore the fault has to be considered as active. 5. Discussion 5.1. Chronological constraints and implications Fig. 10 shows the chronological constraints available for the activity of the previously discussed faults. The NE migration of the extensional tectonics along the NW transect can be inferred from: 1. the end of the activity along the Turano valley fault before the deposition of the Early Pleistocene breccias; 2. the end of the activity along the Salto valley fault before the formation of lower Middle Pleistocene landforms; 3. the beginning of the activity along the Campo Imperatore fault in the Early Pleistocene. The latter fault and those affecting the upper Aterno valley have to be considered still active. The migration of the extensional tectonics along the SE transect can be inferred from: (1) the end of the activity along the Liri valley fault during the lower Middle Pleistocene; (2) the beginning of the activity along the Mt. Morrone fault in the Early Pleistocene.
18 74 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Fig. 10. Chronology of the normal fault activity along the transects reported in Fig. 2. Both sections define an eastward migration of the extensional domain. The NE migration of the active extensional belt in the Apennines is probably related to the process of migration of the structural domains resulting from the lithosphere flexural retreat (e.g. Patacca et al., 1990; Ghisetti and Vezzani, 1999; Cavinato and De Celles, 1999). However, the evidence of a migration of the activity during the Quaternary from a structural framework whose westernmost normal fault is located in the Liri or the Salto valleys to a new structural framework which has the faults of the Fucino area or those of the upper Aterno valley as the western boundary has not been documented yet. A crucial point is the time requested for the migration of the activity. As already mentioned, the activation of the Sulmona fault (part of the easternmost fault set) occurred during the Early Pleistocene, while the activity of the Liri valley fault system ended at the beginning of the Middle Pleistocene. This fault, however, was active during the Early Pleistocene since it displaced Ma-year-old alluvial fan deposits (Carrara et al., 1995a,b). Therefore it is probable that the migration of the activity took some hundred thousand years and during the later part of the Early Pleistocene all the faults reported in Fig. 2 were active. Available data do not permit to understand if during this phase a reduced activity per fault occurred to maintain the intra-apennine extension constant or if the slip rates per fault remained constant for a long period causing an extension larger than the post-early Pleistocene one. This aspect leads to the problem of the long-term stationarity of the fault activity. The reconstructions of the Quaternary geological evolution of intermontane basins generally indicate the occurrence of periods of reduced tectonic activity. As an example, Bertini and Bosi (1993) defined two episodes of reduced tectonic activity during the Middle Pleistocene and hypothesized a pulsating tectonics with periodicity of 3 5 hundred thousand years in the middle Aterno valley. The first episode of reduced activity occurred in the earlier part of the Middle Pleistocene and may well be related to the above mentioned tectonic processes. A reduced slip rate per fault, with a larger number of active fault sets, may define an approximately constant extensional rate throughout the Apennines during the Quaternary. The inconstant fault behaviour may therefore be the effect
19 of the synchronuous activity of all the faults reported in Fig. 2, W and SW of the Gran Sasso chain. That is the activity per fault may have decreased between the later part of the Early Pleistocene and the earlier part of the Middle Pleistocene in order to maintain the long-term intra-apennine extension rate constant Geometric constraints Another aspect which permits to better characterize the Abruzzi Apennine extension is represented by the location of the normal faults within the structural framework of this Apennine sector. Normal faulting generally occur along pre-existing discontinuties, such as the ramps of older thrusts, as in the case of the Duchessa Mts. and Campo Imperatore faults (Faccenna et al., 1995; D Agostino et al., 1998). Some normal faults (e.g. Mt. Morrone, Salto valley, Liri valley faults) are located in the hangingwall of the main thrusts, close and parallel to the thrust surficial expressions. In such cases extension may have re-used pre-existing discontinuities related to the thrust emplacement similarly to the already documented cases of the Duchessa Mts. and Campo Imperatore faults. Alternatively extension may re-use pre-existing normal faults which formed in pre- or syn-orogenic times. Faults of this kind have already been documented in the investigated region (Calamita et al., 1998; Scisciani et al., 2002). According to the mentioned authors, they were the cause of major complexities in the pre-thrust paleogeography and influenced the location and development of the thrusts. Normal faults were subsequently involved in the passive transport during the forelandward thrust migration. However, the W-dipping normal faults did not experience significant geometrical variations What next? F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) Hypotheses about the development of a regional structural system can only be proposed after a general view of the recent history of the entire Apennines. In this light, the most debated point is represented by the current activity of the Apennine migration process which certainly driven the Neogene-Quaternary p.p. building of peninsular Italy (e.g. Patacca et al., 1990). This migration has been admitted for the central-northern Apennines through the reconstruction of the Neogene kinematic evolution (e.g. Patacca et al., 1990). However, the kinematic frameworks defined during the last twenty years were largely based on surficial and sub-surficial geological data (related to the thrust tectonics) which were not directly focused on the Quaternary kinematics. For example, reflection seismic surveys have been generally addressed to the reconstruction of the Neogene thrust evolution, while the interest for the more recent kinematics was definitely subordinate (e.g. Bally et al., 1986; Coward et al., 1999); only very recent papers try to cast light on the problem of the Apennine migration during the Quaternary (e.g. Di Bucci and Mazzoli, 2002). Migrationists define the occurrence of low-moderate magnitude compressive earthquakes as evidence of active thrusting in the Adriatic region (e.g. Meletti et al., 2000), i.e. as the evidence of an active NE-ward migration of the compressive domain. Moreover, stress data along the eastern sectors of the Apennine chain indicate active compression (Mariucci et al., 1999; Mariucci and Mu ller, 2003) and compressive seismogenic sources have been hypothesised along the Adriatic margin (Basili et al., 2002). This view perfectly agrees with the results of the seismic tomography based on the velocity anomalies of the P waves showing the presence of a lithospheric slab under
20 76 F. Galadini, P. Messina / Journal ofgeodynamics 37 (2004) most of the central and northern Apennines (Amato et al., 1998; Di Stefano et al., 1999; Lucente et al., 1999; Lucente and Speranza, 2001). Other geophysical and structural data, from studies made in the central-northern Apennines at different scales, seem to indicate that the flexural retreat of the lithosphere, which conditioned the migration of the structural domains towards E or NE during part of the Quaternary, may have ended (e.g. Argnani et al., 1997; Bertotti et al., 1997; van der Meulen et al., 1998; Di Stefano et al., 1999; Di Bucci and Mazzoli, 2002). This geodynamic change possibly occurred during the Early Pleistocene (Argnani et al., 1997; Di Bucci and Mazzoli, 2002). According to Lucente et al. (1999), the lack of oceanic crust in the central-northern Apennine sector may have caused a recent termination of the subduction process. Indeed, the available tomographic images indicate the complete involvement of continental lithosphere in the subduction (Di Stefano et al., 1999). Structural data related to the most recent deformation affecting the axial areas of the central Apennines indicate a change of extension direction during the Quaternary, from NE SW to NNE SSW (Calamita et al., 1995; Galadini, 1999). The latter author attributed this kinematic change to the later part of the Early Pleistocene or to the earlier Middle Pleistocene. The new NNE SSW extension may indicate that the NE-ward migration of the Adriatic compressive front along peninsular central Italy is no more active. However, this extension may be driven by a still active NNE- to N-ward migration of the orogen in the northern Apennines. The presence of an active compressive axis directed about N S in the northern Apennines has been documented on the basis of borehole breakout data and focal mechanisms (Montone and Mariucci, 1999). If this reflects the direction of the Apennine migration, an intra-apennine extension similar to that related to the post-early Pleistocene kinematics may be expected. In such a case thrusting activity along the Adriatic NW SE structures may be presently reduced or absent and horizontal (rightlateral) movements may occur along the previous thrust faults. In our opinion, available data cannot fully cast light on the fundamental issue of the present migration activity or inactivity in the central Apennines, since different data suggest alternative pictures. The inactive NE migration of the structural domains in the central Apennines (i.e. inactive flexural retreat of the lithospheric plate) implies that the migration of the extensional activity within the Apennine chain towards NE, documented for the Pliocene Early Pleistocene in the previous sections, has been inactive during the Late Quaternary. This hypothesis may be corroborated by the lack of activation of normal faults east of the Gran Sasso chain-mt. Morrone alignment after the Early Pleistocene. As for the hypothesis of active NE migration of the structural domains, we may hypothesize the location of the main normal faults of the future, if the migration process is still working. The previous section has indicated, indeed, that old normal faults and thrust ramps, characterised by a NW SE direction, represent the pre-existing discontinuities which are naturally predisposed to accomodate the recent extension which affects the Apennines. The present knowledge of the structural framework of the sectors located east of the investigated area identifies major discontinuities in the faults bordering to the west the Maiella and Montagna dei Fiori anticlines (see Fig. 2 for location; Calamita et al., 1998; Scisciani et al., 2002). These structures, old pre- and synorogenic normal faults according to the mentioned authors, are km long. Therefore they have the typical length of the presently active normal faults affecting the Abruzzi Apennines. According to the available literature (e.g. Barchi et al., 2000; Galadini and Galli, 2000) this class