Pleistocene changes in the central Apennine fault kinematics: A key to decipher active tectonics in central Italy

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1 TECTONICS, VOL. 18, NO. 5, PAGES , OCTOBER 1999 Pleistocene changes in the central Apennine fault kinematics: A key to decipher active tectonics in central Italy Fabrizio Galadini Istituto di Ricerca sulla Tettonica Recente, CNR, Rome Abstract. Integrated structural, geomorphological, and stratigraphic data have permitted the definition of the Quaternary structural evolution of some tectonically generated basins in the central Apennines. Available data indicate that some N 130 ø- 140 ø trending Quaternary normal faults have not been active since the middle Pleistocene, while along some other faults with the same trend, left-lateral strike-slip or oblique-slip movements are superimposed on normal movements. Pure normal movements were found to affect only N 125 ø to E-W trending faults. This structural evolution indicates that fault kinematics in the central Apennines changed markedly between the early and middle Pleistocene. Current models infer that this change may have been a response to the bending which caused the arcuate shape of the Apennine orogen. The deformation related to the bending added to the effects of the regional stress field. As a result, post early Pleistocene fault kinematics were due to an extension that locally was not perfectly perpendicular to the inherited normal faults and that is consistent with models of oblique-slip tectonics. Processes of arc formation and related "incompatibility" between inherited Quaternary faults and post Early Pleistocen extension have significant implications regarding research on active tectonics. In fact, some nonactive faults show misleading features typical of recent activity while some active faults are poorly visible owing to the recent age of the present regime. This kinematic evolution therefore represents one of the major factors (others being the low slip rates and the fragmented structural setting) which hinders the definition of reliable active faulting schemes for the Apennine chain. 1. Introduction A complex succession of diverse deformational phases has characterized the building of the Apennine chain since the Miocene, with older compressional and more recent extensional structures being evident along the entire chain axis [CNR-PFG, 1983]. The present extensional regime has been defined through data on active tectonics e.g., [CNR-PFG, 1987], the characteristics of instrumentally recorded earthquakes, and in situ stress measurements [Montone et al., 1998]. Knowledge regarding the structural evolution of the Quaternary extensional phase is not very detailed, however, because some geological factors (such as structural complexity and low slip rates) hinder its definition and impede the identification of active faults. New data, however, highlight a major change in the Copyright 1999 by the American Geophysical Union. Paper number 1999TC /99/1999TC central Apennine fault kinematics during the Pleistocene which has significant implications for the definition of models of active tectonics for the region. As a result of this change, older faults show the typical geomorphological features of active faults, while the later structuresometimes do not show significant evidence of recent movements. This paper addresses Quaternary tectonics, focusing primarily on the factors which influenced the structural evolution and the implications for the active tectonic setting of a region in the central Apennines; this region is characterized by several 10, to 20- km-long Quaternary faults that have different kinematics and that were responsible for some large historical earthquakes. The Quaternary tectonic history of the studied sectors is described to highlight the changing strain characteristics during the Pleistocene, followed by a specific section on possible causes of this evolution and an analysis of the_ implications for research on active tectonics. 2. Central Apennine Structural Setting The formation of the Apennine chain is mainly related to compressive tectonic events that occurred along mainly ENE or NE verging thrusts (Figure 1). According to current views [e.g., Boccaletti et al., 1982; Malinverno and Ryan, 1986; Royden et al., 1987; Patacca et al., 1990; Doglioni, 1991; Doglioni et al., 1994], the Apennine kinematic evolution resulted from the opening of the Tyrrhenian basin, the counterclockwise rotation of the chain, and the eastward migration of the arc-foredeep system since the early Miocene. The progressive NE migration of the foredeep and active thrust front (Figure 1) was driven by the sinking of the lithosphere and the related flexural retreat of the lithospheric plate [e.g., Malinverno and Ryan, 1986]. Rifting activity in the Tyrrhenian area was contemporaneous with thrust front migration. Kinematic evolution related to this NE migration affected the entire Apennine structural system until the early Pleistocene, when the kinematic behavior changed primarily because of changes in the flexural retreat of the southern Apennine lithosphere [Cinque et al., 1993; Doglioni et al., 1994]. According to Patacca et al. [1990] and Cinque et al. [1993], two different Apennine arcs formed during the Quaternary, with the northern arc being characterized by active thrusting while the southern one is quiescent north of the Calabrian arc (Figure 2). The two arcs are separated by a main lithosphere tear (the so-called Ortona-Roccamonfina line (ORL), in Figure 2; see Di Bucci and Tozzi [1991] for a discussion of this structure). Arc evolution influenced fault kinematics along the chain axis through a complex deformation succession during the Pleistocene, as shown for the southern Apennines by Cinqu et al. and Hyppolitet al. [ 1994]. 877

2 878 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES I % GENOVA BOLOGNA,&' % -,?-'... Main volcanic... - complex [ ---'--- -:- J ; :J basin - ' >r.c Main intermontane ' Normal and \, strike-slip fault X Thrust fault ' deposits in the foredeep 0 90kin 10øE 12øE 14øE Figure 1. Structural scheme of the northern Apennine arc (modified from CNR-PFG [1983]); the rectangle highlights the study area. The active, or most recent, thrust front of the Apennines is now located in the Adriatic Sea (along the peninsula) and below the Po Plain (in northern Italy) [e.g., Boccaletti et al., 1982]. The Apennine topographic axis, however, is located behind the thrust front system (i.e., westward) in sectors which have been affected by extensional tectonics since the Pliocene (Figure 1) [e.g., CNR- PFG, 1987]. The main Plio-Pleistocene normal faults are parallel to the thrusts and their geometry is often controlled by the thrust geometry [Faccenna et al., 1995]. Present Apennine tectonics occurs along numerous faults that have displaced late Pleistocene-Holocene deposits; paleoseismological studies of some of these structures [e.g., Pantosti et al., 1996] as well as seismic data compiled in various catalogues [e.g., Boschi et al., 1997; Carnassi and Stucchi, 1997] show that earthquakes of M" _6 are concentrated along the Apennine axis. The present structural setting of the central Apennines is characterized by a complex pattern of thrust systems (the largest of which placed Meso-Cenozoic carbonates in contact with Miocene flysch) that are cut or reused by more recent normal faults [e.g., Bosi et al., 1994; Bigi et al., 1995a]. Extensional tectonics were responsible for the numerous intermontane basins of the central Apennines (Figure 3), and the typical NW-SE valleys of this sector (whose bottoms and flanks are carved into Miocene flysch and Meso-Cenozoic carbonates, respectively)usually show a normal fault along the eastern flank. Although normal faults are the most evident structural features related to Quaternary tectonics, evidence of Plio-Quaternary strike-slip movements is also visible in the central Apennines (Figure 3). 3. Quaternary Chronological Framework The continental environment (mainly lacustrine and fluvial) which existed during the Quaternary throughout peninsular Italy greatly hinders tectonic reconstructions for two main reasons: (1) the lateral and vertical discontinuity of sedimentary successions and (2) the difficulties in obtaining dates for continental sediments older than the chronological limits of the geochemical dating methods. As for the latter point, key paleontological data in the study area are represented by Elephas meridionalis vestinus (early Pleistocene) [e.g., Azzaroli, 1977] and Elephas Antiquus

3 GALADINI: FAULT KINEMATICS THE CENTRAL APENNINES 879 NORTHERN 'Yrrheni n $e Lithosphere the foreland dip in direction Orogenic transport Passively sinking lithosphere dip Boundary of the foreland- *ß foredeep system in the ß southern Apennines Lithosphere flexural zone < Major tear faults bounding sectors with different rates of flexural retreat Figure 2. Kinematic sketch of the Apennine chain, showing the northern and southern Apennine arcs. The former is characterized by active flexural retreat of the lithospheric plate and NE to N migration of the compressive front, while flexural retreat is not active in the latter north of the Calabrian arc. The Ortona-Roccamonfina line (ORL) separates the two Apennine arcs (modified from Patacca et al. [ 1990]). (middle Pleistocene) [Maini, 1956], which were found close to buted these breccias to the later part of the early Pleistocene on the town of L'Aquila and in the middle Aterno valley, respecti- the basis of their stratigraphic position, while recent paleomagnevely (Figure 4). tic analysis of the breccia matrix has specifically related these de- One indirect dating method is related to the presence of volca- posits to the Matuyama interval [D;4gostino et al., 1997], connic layers in continental depositsince the middle Pleistocene, firming an age older than 0.78 Ma. owing to the beginning of the main volcanic activity at the Previous stratigraphic works conducted on the different inter- Tyrrhenian margin about Ma [Fornaseri, 1985] (Figure montane basins of the study area [e.g., Bertini and Bosi, 1993; 4). An important stratigraphic unit that is ubiquitous throughout Galadini and Messina, 1993; Bosi et al., 1995] were based on lithe central Apennines is represented by a characteristic slope-de- thostratigraphy and geomorphology, thus permitting the definirived carbonate breccia, whose deposition was related to debris tion of "unconformity-bounded stratigraphic units" [Salvador, flows which mainly occurred along the SW and south slopes of 1987]. Their dating is based on the few chronological elements the mountain ranges (Figure 4). Bosi and Messina [1991] attri- which are available, correlations between the different basins, and

4 880 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES Figure 3. Map of Quaternary faults and Schmidt diagrams of available Quaternary structural data.' Acronyms highlighthe faults mentioned in the text: MMF, Mount Marine fault; MPF, Mount Pettino fault; LF, Lucoli fault; CFF, Campo Felice fault; SVF, $alto valley fault; PF, Ovindoli-Pezza fault; VF, Velino fault; TMF, Tre Monti fault; MHF, Marsicana highway fault; PRSF, Pescina Railway Station fault; SBGF, San Benedetto dei Marsi-Gioia dei Marsi fault; TF, Trasacco fault; LVF, Liri valley fault; VLF, Vallelonga fault; USFZ, Upper Sangro fault zone; PF, Pescasseroli fault. Schmidt diagrams 1 and 5 show the contouring of divergence axes of striated pebbles (Schrader[ 1988]), diagram 4 shows poles of extensional fractures affecting the pebbles of early Pleistocene gravels, and diagrams 2, 3, 6, 7, 8, and 9 show striations measured on the main fault planes reported in the map. Diagrams 1 and 5 are from Giuliani and Galadini [1998] and Galadini and Giuliani [1995], respectively; data for diagrams 7 and 9 were provided by E. Vittori and V. Bosi, respectively, and reported by Serafini and Vittori [1995] and Bosi et al. [ 1994]. a consideration of the main erosional and depositional events that are ubiquitous in the study area [Bosi and Messina, 1991]. The definition of the Quaternary kinematic evolution reported below is based on available stratigraphic data for the different sectors of the study area (Figure 4). 4. Quaternary Kinematic Evolution 4.1. Liri River Valley The flanks of the NW-SE trending Liri river valley (Figure 3) are carved into carbonate bedrock, while its bottom is cut into the Miocene clayey-arenaceous fiysch. Along the western flank an important thrust places the Meso-Cenozoic succession over the Miocene flysch [Accordi et al., 1969]. In contrast, the eastern flank of the valley is characterized by the 15-km-long Liri valley fault (LVF in Figure 3) whose kinematic history appears to be quite complex since both strike-slip and normal movements have been recorded along it [Accordi et al., 1969; Ciotoli et al., 1993; Serafini and Vittori, 1995]. According to Serafini and Vittori [1995], the extensional activity represents the most recent kinematic behavior of the Liri valley fault (Figure 3, diagram 7). This fault is responsible for the displacement of early Pleistocene alluvial fan sediments that are fed from the eastern border of the valley [Carrara et al., 1995a, b]. As a result of fault activity,

5 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES 881 Salto valley L'Aquila] Middle Fucino basin basin [Aterno valley Sangro valley Liri valley Middle Pleistocene Middle Pleistocene (lower part) I l I ø _øl Colle di Sassa Petrara Early Pleistocene (upper part) Aquilente Mt. Marine Anzano Pliocene Boulders and large carbonate blocks : in silty matrix (debris flows, rockfalls) Clay and silt (lacustrine environment) Sand with tephra levels(fiuvial alluvial Carbonate fans) breccia (talus, scree, j v.:... lacustrine environment) Gravel (fluvial environment) Partially preserved top surface "... of the succession environments) Sand (fluvial and lacustrine Paleolandscape (erosional) Elephas antiquus Eleph vestinus rneridionalis Figure 4. Schematicorrelation of the continental sedimentary successions in the area of Figure 3. Circled names refer to the formation reported in the literature or to the place where the succession was studied. This scheme was originally proposed by Bosi and Messina [1991] and subsequently supported by more recent works (Bertini and Bosi [1993]; Galadini and Messina [1993]; Bosi et al. [1995]). Data for the Liri valley are taken from Carrata et al. [1995a]. the top surface of an alluvial fan is affected by a 50-m-high fault scarp near Balsorano (Figure 5a). The displacement which affected the fan deposits was related to normal movements, with no horizontal components. Although geomorphological data (such as the presence of bedrock fault scarps along the fault) led Bosi [1975] to consider the Lift valley fault as one of the "probably active" faults of the central Apennines, more recent research has indicated that the activity of this fault terminated in the later part of the early Pleisto- cene or at the beginning of the middle Pleistocene with the sealing of the fault with middle Pleistocene alluvial fan deposits [Carrata et al., 1995b]. Presumed geomorphological evidence of more recent activity along this fault (mainly bedrock fault scarps) is probably related to exhumation phenomena driven by the different erodibility of the carbonate bedrock and the Miocene flysch. The freshness of the scarp affecting the early Pleistocene deposits (Figure 5a) is also misleading as it is likely due to strong cementation and not to a recent scarp formation.

6 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES *:i %. : :.,....-,?, :,......:.

7 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES NW SE WSW ENE :::::::::::::::::::::::::::::::::::::;:::: I:':::::::::::::::' "'t /% 600!. :.. 1 a.s.l. Palco-land surfaces:... 1 '"': "':":' 32 _ 5 6 (Early (Middle Pleistocene-Midd Pleistocene) [ 1 3 (Pliocen½-Early Pleistocene) 7 c... Portion of paleo-land surface observed in the field Figure 6. Schematic drawing of the paleo-land surfaces along a section perpendicular to the Salto valley fault (SVF in Figure 3). More recent paleo-land surfaces, related o the middle Pleistocene, are not displaced by the fault (modified from Chiarini et al. [1997]). In short, the Liri valley fault was active during the early Plei- no appreciable displacement has occurred along the Salto valley stocene and was characterized by normal kinematics; its activity fault, and therefore it has to be considered inactive. terminate during the later part of the early Pleistocene or at the beginning of the middle Pleistocene. Therefore this fault must 4.3. Vallelonga Area presently be considered as inactive. The Vallelonga basin formed during the Pleistocene owing to 4.2 Salto River Valley the activity of the Vallelonga fault (VLF in Figure 3), which is located along the eastern flank of the basin. Geophysical data Similar to the Liri valley, the Salto valley trends NW-SE and show the presence of very thick Pleistocene alluvial sediments is carved into carbonate bedrock and Miocene flysch. Plio-Plei- whose deposition is related to the tectonic deepening of the basin stocene alluvial and lacustrine sediments are deposited in the de- [Galadini and Messina, 1994]. pression, while debris deposits are visible along the NE border of The Vallelonga fault is about 20 km long, consists of two main the valley. This border is characterized by a >20-km-long fault en echelon branches, and is perfectly in-line with the Trasacco (Salto Valley Fault (SVF) in Figure 3), which is exposed along fault in the Fucino Plain (active during the Holocene; TF in carbonate bedrock scarps (Figure 5b). Available slip data show Figure 3) and with the above mentioned Salto valley fault. The the fault to have mainly normal kinematics (Figure 3, diagram 9). Vallelonga fault is sealed at some places, however, by late Pleis- According to Bosi et al. [1994], the normal fault reutilizes old tocene fluvial sediments. This is visible at the northern end of the compressive structures (ramps) as the result of Plio-Pleistocene Vallelonga depression where it is buried by gravels of a large, inversion tectonics. Moreover, episodes of gravity tectonics late Pleistocene alluvial fan[giraudi, 1988]. [Nijman, 1971] also affected the Salto valley fault [Mariotti and Therefore, despite evidence of Pleistocene tectonic activity, Capotorti, 1988]. The Pleistocene activity of this fault is demon- the Vallelonga fault has to be considered inactive. Future research strated by the displacement of Pliocene and early Pleistocene de- will be aimed at collecting chronological data about the presumed posits and landforms [e.g., Bertini et al., 1986]. end of its activity. A definition of activity chronology has been attempted. through the study of Quaternary paleo-land surfaces, which con Fucino Plain sisted mainly of s rath terraces carved into carbonate bedrock, Miocene flysch, and Quaternary continental deposits [Chiarini et The Fucino Plain, the largest intermontane basin of the central al., 1997]. The sections reported in Figure 6 show that the oldest Apennines, is filled by more than 1 km of lacustrine and fluvial paleo-land surfaces were displaced across the Salto valley fault, Plio-Quaternary deposits and is surrounded by active NW-SE and while the youngest ones were not affected by the fault. The ages NE-SW faults (Figure 2). Its tectonic evolution is related to the of the continental units into which the paleo-land surfaces are complex superposition of two half grabens since the Pliocene carved enabled Chiarini et al. [1997] to infer that this fault had [Galadini and Messina, 1994] (Figure 7). The first graben formed significant activity until the middle Pleistocene. Since this time, up to the late Pliocene, owing to the activity of the NE-SW Tre

8 884 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES Pliocene Hiocene-E. Pleistocene M. Pleistocene-Holocene transcurrent faults Figure 7. Structural evolution of the Fucino basin during the Pliocene and the Quaternary. of to the evolution of the half grabens Monti fault (TMF in Figure 3), followed by the formation of the second half graben along NW-SE faults of the eastern border (the San Benedetto dei Marsi-Gioia dei Marsi (SBGF) and Marsicana Highway faults (MHF) in Figure 3). These faults, responsible for the most recent structural evolution, are right-hand en echelon normal faults and about 10 km long each. Minor faults affecting the area east of the Fucino basin have been considered active by Piccardi [1995]. The author infers the occurrence of a kinematic change, based on the crosscutting relationship of fault plane striations, but does not propose any chronological constraint for it. Holocene slope deposits and is highlighted by a carbonate scarp (Figure 5d). The slip rate evaluated for the last 20,000 years is mm yr -1[Galadini and Galli, 1999]. Toward the north the Fucino basin is bordered by the Mount Velino fault (VF in Figure 3; Figure 5e), a N 110 ø normal fault whose Quaternary activity is shown by the displacement of early and middle Pleistocene slope-derived breccias and late Pleistocene till deposits [Galadini and Messina, 1994]. Kinematic indicators define a purely normal movement along the Mount Velino fault (Figure 3, diagram 8), and offset observed in early Pleisto- cene breccias yields an estimated slip rate of 0.8 mm yr-1. The present activity of the N130 ø trending San Benedetto dei Although the evolution of the Fucino area since the late Plio- Marsi-Gioia dei Marsi fault is demonstrated by the displacement cene was driven by the fault system along the eastern border of Holocene deposits and landforms (Figure 8), as illustrated in [Galadini and Messina, 1994] (Figure 7), some kinemati changes numerous paleoseismological works [Giraudi, 1988; Michetti et in this system have been detected. East of the Marsicana al., 1996; Galadini and Galli, 1999] and by the displacement Highway fault the N135 ø trending Pescina Railway Station fault which occurred during the 1915 earthquake [Oddone, 1915; (PRSF in Figure 3) was responsible for the displacement of Plio- Serva et al., 1986; Galadini and Galli, 1999] (Figure 9). In the cene - early Pleistocene lacustrine deposits and part of the middle southeastern sector of the Fucino basin the fault was responsible Pleistocene succession (Figure 8). The fault, however, is sealed for the displacement of early Pleistocene and late Pleistocene by more recent middle Pleistocene deposits (Casoli Formation in slope deposits and is highlighted by an impressive carbonate Figure 8b) and has to be considered inactive since this age scarp (Figure 5c). The minimum slip rate evaluated for the last [Messina, 1996]. The N 120 ø trending Marsicana Highway fault is 12,000 years is about 0.3 mm yr -1[Galadini and Galli, 1999]. characterized by a scarp affecting lacustrine and fluvial deposits; The Plio-Quaternary activity of the N120 ø trending Marsicana paleo-land surfaces and terraces are suspended m above Highway fault is demonstrated by the displacements, which in- the Fucino Plain in the footwall of this structure (Figures 8 and crease with age, of Pliocene and Pleistocene formations (Figure 10). These suspended terraces are no older than the middle Plei- 8) [see Galadini and Messina, 1994; Messina, 1996]. The present stocene, thus marking a scarp formation since this age along the activity of this fault is shown by the displacement of late Pleisto- Marsicana Highway fault. Finally, the largest vertical slip rate in cene-holocene sediments (figure 8), as highlighted by paleosei- the region (0.8 mm yr -1 for the period upper part of the early smological studies [Galadini and Galli, 1999] and surface faul- Pleistocene present) was calculated along the N110 ø trending ting related to the 1915 earthquake [Serva et al., 1986; Galadini Mount Velino fault. and Galli, 1999]. Toward the SW a minor fault in line with the In short, all of these points infer that a kinemati change af- Marsicana Highway fault is responsible for the displacement of fected the eastern faults of the Fucino basin at the beginning of

9 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES 885 SW S. Benedetto-Venere wave-cut terrace- 1 Pescina-S. Benedetto soft-rock pediment I 670 NE S.Benedetto dei M.-Gioia -- / i 640 rn dei M. fault 0 km '"'"':' Lacustrine deposits (historical period) a.s.1. (Late Pleistocene) SW Pescina Railway Station fault NE Pescina-Collarr ele palco-land su 1000 rn a s l Marsicana Hwy fault :._. rfa ' ß ':'-:->:-:i:!:i!!:.,,,,,,-,,-,,-,.".,_'- - -'-- 4-;...::::.:.-. :. ::..--:...::..-.'.: :,,,,... Carbonate bearock800] "... ß " deposits Lacustrine (Late and Pleistocene) colluvial I ",'l Gravel (Casoli formation, Middle Pleistocene) 'Gravel and sand (Pervole formation, 'Middle Pleistocene) ::...:... ß..- Middle Gravel (Pescina Pleistocene) formation, Gravel and silt (Aielli and Cupoli complexes, Pliocene-Early Pleistocene) 0 km 0.5 I Figure 8. Geological and geomorphological sections through (a) the San Benedetto dei Marsi-Gioia dei Marsi fault (SBGF in Figure 3; modified from Giraudi [1988] and (b) the Marsicana Highway fault (MHF in Figure 3; modified from Messinn [ 1996]). In the latter the Pescina Railway Station fault (PRSF in Figure 3) is reported. the middle Pleistocene, resulting in extensional movements that controlled basin. The first depositional phase resulted in the thiare more consistent with the N 110 ø- 120 ø trending faults than with ckest gravel deposits close to the eastern flank of the valley the N 135 ø trending Pescina Railway Station fault. (along the NW-SE trending main fault; Figure 1 la). The depositional geometries, however, define a progressive migration of the 4.5. Upper Sangro River Valley depocenter toward the Pescasseroli basin (Figure l lb), and the The southern prolongation of the Fucino faults is represented second depositional phase shows the greatest gravel thickness by the Upper Sangro fault zone (USFZ in Figures 3 and 11). Du- close to Pescasseroli and along an E-W trending portion of the ring the later part of the early Pleistocene the Sangro valley was Pescasseroli fault (PF in figures 3 and 11). This geological evocharacterized by fluvial deposition in a NW-SE normal fault lution is related to the "new" role of the Pescasseroli fault as the... - ".,:'.:.:.. : :....:-' _ , y..,... : ;... ß ,..?::...:r... :-?.,...;:;...:... ' ;&, ß.:..:..., " :::L...:...::h.:':...., ß '.m- :.:::: "...%:.:,.::..::,.)....., t-,. Fibre 9. San Benedetto dei Marsi-Gioia dei Marsi fault. A ve gentle sca (indicated by a color change) marks the emergence of the fault and represents the remnant of the sca which fo ed during the 1915 ea,hquake (Ms=7.0, according to Margottini et al. [ 1993]).

10 .: GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES - :-::i m -:--": d....?' "' : :z ::- : : : :½?... ": :- :. : : [½[: :,.:. : 4: ; ; :.-I bottom of fie historical lak,: :... " ::" -- %.L " : :,: : :: ::-- :.: :::...: ' : %... lm [ -. : :?:... ::, - % :: : : :--;;.. : '-% "" : ' -:4:':='- X' ;:.- :x: : ½.;?..,: : : :';x,:.:...:.. :, :... -" ; M :ddle Pleistocene terrace ;.:.":'. :55:-E;: :.N z [:.... ':: 4-: 47 : :,:... : :..:- : ;: "½ '"'"'"' -'"'"""": ': ' : %, '"'... ::;t?:-.:.: -...,.,,.. :, ::..,... :....:. :-.-..:.-...x'.-.:? : :,::... :. x:: :::::: :.:: : - ::.-::: :-: : -...."% ':' :... :..: ; :; :::::'"" ::} :: : ::.::. : ;'::'::'......' -: : :: :v "::. : : ".....<::g. ' ']: ;:2'::...:.... :::.:... "..:': :. '. :. '... :...:.." : " ff :"'....:,. m:-. :" ' : :5: '.?::: :'* r'."?... :: : ':.':.... '.:.:... r".' " - ½:::::: : ;. :'.::... :. :::.::::...:-:'...- ::.'.½...'... :.'-. : ;::. : _..&.-... :;... -:? z. - :... -: ' -- :' : : :. ::.....:.:.:......, , ß :' :'?g: :½t' :> '. ".' :':"".'? : :: ' :9'":":?' : f' :,,'-::.':.:.:: ::.'.:..: '... '-<': :"'... ",::. :,... :...-::.. :.. :... ß..... ':: ' -: -::.-.'. -.,..:... '"' '" :.,..,- : : -. ß....:....<..:.::.,..... :'"". ';-::"-.-'-.: :..- w?:'":.':.::..} r;:::... :... "" v.... : ::......, ' "... '"":... ::.:: : ' ' ; Figure 10. Panoramic view (from north) of the suspended middle Pleistocene terraces along the eastern border of the Fucino basin. ^ 70 to 80-m-high fault scarp separates the terraced area on the left (east) from the present plain. fault responsible for the evolution of the related basin during the later part of the early Pleistocene [Galadini and Messina, 1993]. The recent activity of the Pescasseroli fault is shown by the formation of a ve 3' sharp, several tens' of meters high scarp affecting the gravels (Figure 12) and by the displacement of slope deposits younger than 27,000 years [Galadini et al., 1999]. As for the Upper Sangro fault zone, the main NW-SE normal fault of the eastern flank appears to have been inactive since the later part of the early Pleistocene, as it is sealed by the upper portion of the early Pleistocene gravel formation (Figure 1 lb). Other NW-SE 2" Y'.? Carbonate (M½so-Cenozoic) succession [- Sand (Late Pleistocene- gravel a flysch (Miocene) Landslide deposits (Campo Rotondo fm., Pliocene) Grav Early el Pleistocene (Pe scas scroli fm., Holocene) Thrust -,r.b Left-lateral strike- slip fault ' " -' Left-lateral oblique fault Normal fault: a) active, b) not active Fault with uncertain kinematics Palcodrainage Figure 11. Geological and structural evolution of the Pescasseroli area, Sangro valley: (a) the beginning of early Pleistocene gravel deposition (Pescasseroli Formation in Figure 4) influenced by a typical NW-SE Apennine valley characterized by a main normal fault on the eastern flank, (b) the subsequent river drainage and related deposition controlled by the E-W portion of the Pescasseroli fault (PF), owing to a new left-lateral regime affecting the NW-SE faults; the main fault of the Upper Sangro fault zone cut the older and inactive normal fault; (c) the present setting showing that the Pescasseroli basin structure is strongly related to the activity of the PF.

11 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES 887 '...-{:'"'; :?... displ'aced terrace ' "' '* '" :e- -? :, :..2: :?: ::..... : -'*:"'?,:. :.:.-; :j}... Figure 12. View (from north) of the fault scarp affecting the early Pleistocene gravels along the Pescasseroli fault, ESE of Pescasseroli (see Figure 11 for location). The fault also affects the incised surface which roughly represents the top of the gravel succession (see also Figure 4 for the stratigraphic setting). faults subsequently became active, affecting the entire early Pleistocene gravel formation with left-lateral strike-slip kinematics [Galadini and Messina, 1993]. The main strike-slip fault of the Upper Sangro fault zone affects the eastern flank of the Pescasseroli basin, cutting the previously active normal fault (Figure 11). Along the faults of this fault zone, Galadini and Giuliani [ 1995] analyzed the deformation of pebbles in the above mentioned fluvial formation and identified NW-SE to E-W trending divergence axes (Schrader[1988]; Figure 3, diagram 5). These deformations indicate a local NW-SE to E-W shortening axis, which is consistent with the left-lateral strike-slip component of the Upper Sangro fault zone. In such a deformational regime the extensional activity of the E-W portion of the Pescasseroli fault is the result of normal movements along a releasing bend, which is consistent with the leftlateral activity of the NW-SE faults. In short, the geological and structural evolution described above is related to a change in the kinematic behavior which occurred since the later part of the early Pleistocene, when the normal fault flanking the eastern side of the Sangro valley ceased its activity. The extensional activity migrated along the E-W sector of the Pescasseroli fault, conditioned by the left-lateral strike-slip movements along "new" NW-SE faults Altopiano Delle Rocche- Campo Felice North of the Fucino Plain the recent activity of the Ovindoli- Pezza fault (OPF in Figure 3) is shown by the displacement of till related to the last glacial maximum and of more recent fluvial deposits and landforms [Biasini, 1966; Giraudi, 1989; Pantosti et a!., 1996]. Along the N160 ø portion of the fault a left-lateral component which is slightly larger than the vertical one (7-8 m vertical and 8-10 m horizontal, according to Giraudi [1995]) is highlighted by the displacement of a Holocene stream channel (Figure 13). Similar to the Sangro Valley, normal movements also affect the œ-w portion of the Ovindoli-Pezza Fault (fig. 3), which is also characterized by Holocene activity [Pantosti eta!., 1996] and is responsible for the formation of the Piano di Pezza intermontane basin. The present activity of the Campo Felice fault (CFF in Figure 3; Figure 5f) has been hypothesized by Bosi et al. [1993] on the basis of geomorphological data (e.g., the presence of a continuous bedrock fault scarp and a suspended valley at the footwall of the fault). Giraudi [1995] reported the formation of fault scarps, related to minor branches associated with the Campo Felice fault, which were subsequent to the last glacial maximum. The Pleistocene activity of this normal fault resulted in the formation of the Campo Felice intermontane basin, which is mainly filled by lacustrine and glacial deposits. The northern prolongation of the Campo Felice fault, the Lucoli fault (LF in Figure 3), shows no direct evidence of Pleistocene activity due to the lack of Quaternary deposits along the Lucoli valley. Fluvial pebbles deposited in the Scoppito area (at the northern end of the Lucoli fault) during the later part of the early Pleistocene [Bosi, 1989] (Figure 4) are, however, affected by deformations similar to those observed in the Sangro valley area and consistent with left-lateral strike-slip movements (Figure 3, diagram 1). These data are coherent with the kinematics of the Lucoli fault inferred by carbonate bedrock striations and the displacement of the carbonate succession [Bigi et al., 1995b]. Despite the abundant data indicating the present activity of the faults affecting this area, the beginning of the presentectonic regime is not easy to infer because of the scarcity of local Pleistocene formations and the consequent lack of stratigraphic data necessary to date the tectonic activity. The Scoppito Formation, deformed by the Lucoli fault, is related to the later part of the early Pleistocene, but it does not chronologically constrain the beginning of the last deformational phase. Stratigraphic data therefore only indicate that the mentioned faults and their kinematics have been active since at least the middle Pleistocene. Some chronological indications may, however, be derived from geomorphological data. During the analysis of paleo-land surface successions around the Fucino Plain, Bosi et al. [1996] distinguished variou sectors which are characterized by homogeneous geomorphological characteristics, are separated by major faults, and appear to have been stable during the Quaternary. One of the apparently stable sectors defined in this work is the Alto-

12 888 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES Colluvial (Late Pleistocene- deposits ', :... ] Glacial (Late till Present) Pleistocene) Alluvial fan deposits e..,>,f Carbonate bedrock (Holocene) and dayey-arenaceous flysch Alluvial (Late Pleistocene) fan deposits Erosion scarp Fault scarp Figure 13. Geological sketch map of the Ovindoli-Pezza fault (OPF in Figure 3), ESE of the Pezza Plain (see Figures 3 and 7 for location). Modified from Giraudi [1989]. piano delle Rocche, which, in contrast, is affected by the Ovindoli-Pezza fault. As such the authors concluded that the beginning of the Ovindoli-Pezza fault activity, and of the faults affecting this area, occurred too recently (i.e., a few hundred thousand years) to have left any macroscopic evidence on the landscape (such as fault-bounded mountain ridges, clear displacements of paleo-land surface successions, etc.). Moreover, the formation of the small intermontane basin of Piano di Pezza, along the E-W trending portion of the Ovindoli-Pezza fault, is related to the last important basin formation phase in the central Apennines. This phase of basin formation affected paleo-land scapes that were already strongly modeled by the continental geological evolution (C. Bosi, personal communication, 1998) and was dated at the boundary between the early and middle Pleistocene in surrounding areas (e.g. the area between the Gran Sasso Range and the Aterno Valley) [Galadini and Giuliani, 1991]. It seems reasonable, therefore, to date the beginning of the present tectonic regime in this area at the earlier part of the middle Pleistocene. This regime is characterized by left-lateral oblique-slip movements on N160 ø to N130 ø trending faults and purely extensional kinematics on N130 ø to E-W trending faults The L'Aquila Area Some active faults in the northernmost part of the investigated area, close to L'Aquila, are characterized by impressive carbonate bedrock fault scarps. For example, the Mount Pettino fault (MPF in Figure 3, N 110 ø striking) is responsible for the displacement of late Pleistocene slope deposits (Figure 14a). Moreover, the presence of some fault scarps affecting loose debris deposits along the southern slope of Mount Pettino testifies to the activity of this fault during the last few centuries. Late Pleistocene slope deposits are also displaced by the Mount Marine fault (MMF in Figure 3, N130 ø striking), the vertical offset being about 10 m throughout the entire fault zone. As a result of this offset, the slope deposits appear to be suspended over the present piedmont area along the entire slope (Figure 14b). According to Blumetti [1995], the activity of this fault is younger than 29,690_ years B.P. (radiocarbon age obtained from a displaced paleosol). The activity of the Mount Pettino-Mount Marine fault system is also highlighted by associated microseismicity [Bagnaia et al., 1996] and high-magnitudearthquakes, such as the February 2,

13 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES 889,,... Figure 14. (a) Panoramic view of the Mount Pettino fault (MPF in Figure 3) and a view (inset) of the displacements affecting late Pleistocene slope deposits along the fault scarp, (b) Panoramic view of the Mount Marine fault (MMF in Figure 3) and a view (inset) of the suspended late Pleistocene slope deposits outcropping the footwall of the fault; the location of the same deposits in the hanging wall defined a vertical offset of about 10 m. Small white arrows indicate the bedrock fault scarps while large arrows show the locations of the outcrop along the scarps; small black arrows indicate some fault planes affecting the late Pleistocene slope deposits along the Mount Pettino fault. 1703, earthquake (Ms=6.2, according to Carnassi and Stucchi [1997]; Me=6.6 MCS, according to Boschi et al. [1997]) which was responsible for strong damage in L'Aquila. Coseismic deformations along the Mount Marine fault are reported by Blumetti on the basis of historical sources. The Mount Pettino-Mount Marine fault system was responsible for the formation of the L'Aquila basin. The surficial structural setting (e.g., the tilting of the Quaternary deposits toward the NE), which is similar to that of other central Apennine intermontane basins, is typical of a half graben. However, the kinematic indicators sampled along the two fault branches (Figure 3, diagrams 2, 3, and 4) indicate that a major vertical component and related extension affected the Mount Pettino fault, while small left-lateral oblique components affected the Mount Marine fault. This kind of kinematics is reflected by the general morphology of the L'AqUila basin, which is very large along the Mount Pettino sector and narrow along the Mount Marine fault (Figure 3). This kinematic setting is quite similar to that observed in the pre- viously described areas, being characterized by an almost Purely normal co,..ponent of movement along approximately l -W tren- ding faults and left-lateral components along NW-SE to NNW- SSE faults. In short, while available data are conduciv to active kinematics, the chronological data on the evolution 0f the kinematic cha- racteristics are poor, thus making it impossible to identify and constrain these changes. Old lacustrine deposits in the L'Aquila area may be Pliocene in age, but the structural features of this Pliocene basin are completely unknown, and therefore a reconstruction of the structural evolution, similar to that proposed for the Fucino area, is presently impossible. 5. Discussion 5.1. Kinematic Data and Tectonic Implications The strain regime of the study area changed at the beginning of the middle Pleistocene, as some NW-SE normal faults (namely the Liri valley and Salto valley faults) ceased their activity, and quite different kinematics began to affect the other faults in the area. Available data highlight the new kinematic characteristics of the investigated sector of the central Apenninesince the middle Pleistocene. NW-SE to NNW-SSE faults are characterized by purely left-lateral to oblique left-lateral movements. The horizontal component of movement decreases from south to north, as the Upper Sangro fault zone is characterized by almost purely strike-slip kinematics and the Mount Marine fault only shows minor left-lateral activity. Between these two structures the Ovindoli-Pezza fault shows a left-lateral oblique behavior with 1:1 ratio between the normal and horizontal components. Purely

14 890 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES normal kinematics affect N125 ø to E-W faults throughout the study area. This kinematic framework may be interpreted two different ways: (1) strike-slip components may be related to a regional stress field with a horizontal C l, and therefore extensional movements along the N125 ø to E-W faults may be the result of activity along releasing bends, or (2) the described kinematics may be due to a regional stress field with a vertical C l, and thus the strike-slip components on NW-SE to NNW-SSE faults are the result of a local extension which is not perfectly perpendicular to these faults. As such, the crucial variable is the orientation of the regional main compressional axis. The Active Stress Map in Italy [Montone et al., 1998], which reports data from borehole breakouts, as well as centroid moment tensor (CMT) solutions for earthquakes that occurred during (magnitude Mw ranging between 4.2 and 7) and seismic sequences that occurred during (magnitude Md ranging between 2.8 and 4.8), shows that the minimum horizontal stress (( 3) is perpendicular to the chain axis. The focal mechanisms of the main earthquakes (M>5) which have affected the Apennine chain in the last 20 years indicate that these earthquakes are mainly related to normal faulting [Montone et al., 1998]. This is consistent with the vertical orientation of (I 1 obtained from dynamic modeling of stress accumulation in central Italy (A.M. Negredo et al., Dynamic modelling of stress accumulation in central Italy, submitted to Geophysical Research Letters, 1999) and with models which try to relate Apennine fault geometries and kinematics to seismicity [e.g., Lavecchia et al., 1994]. Finally, kinematics related to a vertical (I 1 in the Apennines are modeled by showing the complex interaction between the opening of the Tyrrhenian basin and the NE migration of the thrust front [e.g., Meletti et al., 1995; Faccenna et al., 1997]. Therefore, although some hypotheses have been recently proposed on the major role played by strike-slip tectonics in the central Apennines [e.g., Cello et al., 1997], available data indicate that the present tectonic regime is consistent with a vertical ( 1. On this basis, strike-slip components are necessary to accommodate an extension (c 3) that locally (i.e., in the investigated portion of the Apennines) is not perfectly perpendicular to the inherited structural framework, following current models of oblique-slip tectonics [e.g., Bonini et al., 1997]. The structural evolution data reported in sections show that "incompatibility" between inherited structures and extension has affected the investigated Apennine sector since the middle Pleistocene. As such, discovering the cause of local distortion of the regional stress field may be an important point for understanding the structural evolution of the Apennine chain Possible Causes of the Kinematic Change The identification of a recent change in kinematicharacteristics is generally difficult because few traces (structural, geological, or geomorphological) related to the new regime usually affect the inherited geological framework. These difficulties increase if the strain change re-uses preexisting discontinuities withouthe formation of new structures. In the case of reutilization the best evidence of strain change is the crosscutting relationship of differently aged striations a fault plane. Interpretation problems may, however, also exist in this case, such as when interpreting a deformational framework which resulted from the coexistence of different strain characteristics on parallel faults (strain partition) or from the superimposition of differently aged stress regimes. Examples of areas affected by a Pleistocene kinematichange include (1) the deformational succession of the Walker Lane Zone (Basin and Range), for which substantially different hypotheses, ranging from coexistence to superimposition. of different deformations, are available [Wright, 1976; Wesnousky and Jones, 1994; Bellier and Zoback, 1995]; (2) the recent tectonics of the Malawi Rift, which is also interpreted substantially different ways [Ring et al., 1992; Chorowicz and Sorlien, 1992]; and (3) the tectonics of the Ethiopian Rift [Bonini et al., 1997; Boccaletti et al., 1998]. In regards to the study area the change in the strain characteristics shows features which are similar to the above mentioned cases. There is no clear reorientation of faults, such that the main structural trends remain basically unchanged. Only a minor deviation (about 20 ø ) is observed in the trend of the main faults responsible for major normal movements and the formation of new normal-oblique to strike-slip faults (the horizontal component always being left-lateral, with trends parallel to those of the previous normal faults). In terms of stress distribution the present local situation may be represented by a main extensional axis (c 3) which divergeslightly (about 20 ø ) from the early Pleistocene trend. Defining the cause of the strain change is not straightforward since numerous processes may be invoked, such as block rotation or the eastward migration of the central Apenninextensional belt. Post-Neogene block-rotations in central Italy were hypothesized by Mattei et al. [1995] on the basis of paleomagnetic data. Rotation may be responsible for the locking or change in kinematics of faults whose attitude and activity is no longer favorable to a certain stress field [Ron et al., 1990; Scotti et al., 1991]; in this case the stress field remains constant during time, and the structures change their orientation. In the study area, however, there is no significant difference in the trends of the active and inactive faults. Moreover, no data regarding Pleistocene strikeslip motion are available for the possibly rotated and presently inactive faults, i.e., the Liri valley and Salto valley faults. As reported in section 2, the eastward migration of the back arc extension is one of the most strikin geodynamic characteristics of the Apennine chain. The migration process may imply the progressive shift of the most uplifted area, which is characterized by extensional activity and the progressive locking of the faults present on the western side of the active belt. This kind of evolution, however, does not explain the "new" left-lateral kinematics which affect some of the faults mentioned in the previousec- tions Arc Formation Related Kinematics One of the most striking features of the Apennine chain is its arc-shaped structure. Doglioni [1991] showed the complex deformational pattern of an arc due to west dipping subduction of the lithosphere. The deformations were mainly represented by compression at the easternmost boundary of the arc and back arc extension, as well as by strike-slip movements along the two "arms" of the arc. The formation of minor arcs inside the major one is due to heterogeneities in the subducting lithosphere (Figure 5). As mentioned in section 2, the evolution of the Apennine chain resulted in two separate arcs being formed (a northern and a

15 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES 891 a) s'l'.- :. qrst -ORDF ARC. t'r kwi N <:./?::X..::."; ;.; ':. s :,:....'//' cl. se: :. ": '"'- : extemion - mpression ' ß '' Figure 15. (a) Simplified kinematic model related to the evolution of an arc due to subduction processes, (b) application of the arc evolution model to the northern Apennine arc (modified from Doglioni, [1991]). i southern arc). According to Patacca et al. [1990] and Cinque et al. [1993], the cause of this structural evolution was the end of the flexural retreat of the southern Apennine lithosphere north of the Calabrian arc, the related end of thrusting during the Pleistocene, and the continued evolution of the arc-foredeep system in the central northern Apennines. This kinemati change is not only recorded along the eastern compressive margin of the southern Apennines but also in the inner portion of this sector of the chain [Hyppolit et al., 1994]. In agreement with this evolution, paleomagnetic data collected along the northern sector of the Apennines show that the bending of the chain related to the NE migration of the compressive front is one of the most significant events in the Plio-Pleistocene structural evolution of the northern Apennines [Speranza et al., 1997]. ß...:.11 :.i:... L'Aquila bas, ' ' g "-".:-/;. LAquila basin ' 't a) ":-."... extensional asins ';:"'] -...: ß - basin ';: :..,.,, of the basin Figure 16. Hypothetical structural evolution of the study area related to a local bending phase of the active compressive margin of the central Apennines: (a) before (until the later part of the early Pleistocene) the beginning of the bending phase and (b) after (since the later part of the early Pleistocene or the earlier part of the middle Pleistocene) the beginning of the bending phase. Because of the latter phase, left-lateral strike-slip or oblique-slip movements affect faults with a trend which in the previous phase characterized normal faults; purely normal kinematics affect N125 ø to E-W trending faults. Note that arrow lengths are proportional to the strike-slip / normalslip offset rate.

16 892 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES The study area is located at the southern boundary of the northern Apennine arc (Figure 1), and therefore the observed strike-slip kinematics may be the result of arc-bending processes affecting either the entire northern Apennine arc or a smaller local structure, as shown in the general model and local evolution schemes given in Figures 15 and 16, respectively. This bending may be highlighted by the trend of the isobaths of the Pliocene base, which shows a curved shape at the southernmost part of the northern Apennine arc (Figure 1). This trend may have resulted from two different processes: (1) an eastward migration of the compressive front while its SE termination remained stationary or (2) an actual bending of the entire compressive front. In the latter case, slip vectors would be directed toward the SW at the SE termination of the front and early-middle Pleistocene shortening should affect the area between the extensional basins of Figure 16a and the southern portion of the compressive front in Figure 16b. This shortening should be highlighted by an out-of-sequence activation of compressive structures. However, evidence of compressive features of such a recent age is lacking in the investigated area, and therefore the hypothesis of arc formation reported in point 1 may be preferable. Because of the evolution of the arcuate Apennine orogen, the fault kinematics described in the previous sections resulted from the summation of the local stress field (related to the effects of arc bending) and the regional stress field (responsible for extension in the Apennine chain). In short, a change of the fault kinematics occurred in the central Apennines at the boundary between the early and middle Pleistocene, which was likely due to bending processes affecting the arcuate Apennine orogen. This bending may have been caused by the interaction between a NE migrating compressive front (the northern arc) and a stationary front (the southern arc north of the Calabrian region). According to this interpretation, the strike-slip components of movement along the N 130 ø- 160 ø trending faults only represent the kinematic effect of the bending in a system driven by extensional tectonics. I 13ø5' E Figure 17. Map of active faults in the L'Aquila-Avezzano area according to Bosi [ 1975]. More recent data (compare the map of Figure 3) highlighthat some of the presumed active faults are no longer considered active Implications for Active Tectonics active faults that Bosi [1975] produced on the basis of geomor- The most important factors which reduce the "visibility" of phological criteria (Figure 17). This document, which was very active faults in the central Apennines appear to be (1) the low slip innovative when published, reports some faults as being rates (generally lower than 1 mmyr']), (2) the complex structural "probably active" on the basis of geomorphological evidence; if setting which is strongly imprinted by previous tectonic regimes this is compared with the map in Figure 3, one observes that this and consists of a fragmented fault pattern (no more than 20-km- interpretation is no longer supported by geological data. long faults), and (3) the frequent exhumation of nonactive faults and the related formation of misleading fault scarps (Figure 5). 6. Conclusions The last factor is due to the location of the main faults along the carbonate mountain fronts, thereby generally placing carbonate The integration of structural, stratigraphic, and geomorrocks in contact with more erodible flysch. The exhumation pro- phological data has permitted a definition of the Quaternary cesses are thus responsible for the convergence of forms between structural evolution of a central sector of the Apennine chain. active and nonactive Pleistocene faults [Brancaccio et al., 1986; Available data highlighted a change in the strain characteristics at Bosi et al., 1993]. the end of the early Pleistocene or at the beginning of the middle Data reported in the previous sections, however, show that Pleistocene. During this period, some previously active N130 ø- another factor is responsible for the misleading "visibility"(see 140 ø oriented normal faults ceased their activity. At the same Figure 5) of the central Apennine active faults, i.e., the relatively time other similarly trending faults began a new active phase with young age of the pre ent tectonic regime. As a result of the men- strike-slip or oblique-slip left-lateral movements, while purely tioned factors, some nonactive Pleistocene faults present in the normal kinematics are observed along N125 ø to E-W trending central Apennineshow typical aspects of active faults (e.g., the faults. This change may have been caused by the bending proc- Liri valley and Salto valley faults), while some active faults are esses which led to the present arcuate shape of the northern Apless easily recognizable owing to the short duration of the most ennine arc, possibly through an eastward migration of the comrecentectonic phase. This is clearly highlighted by the map of pressive front while its SE portion remained stationary.

17 GALADINI: FAULT KINEMATICS IN THE CENTRAL APENNINES 893 The change in the strain characteristics has significant implications for research on active tectonics since it is one of the major factors influencing the poor "visibility" of active Apennine faults. As a result of the recent deformational regime, some inactive faults show misleading geomorphological features, mainly bedrock scarps, and the displacement of Pleistocene deposits (also typical of active faults). In contrast, some active faults have not left any clear trace of their activity on the present landscape, owing to the small amount of cumulative slip along faults related to the presently active tectonic regime. The reported structural evolution and the related implications for research on active tectonics render invalid the traditional view that extensional activity in the Italian Apennine simply followed compressive tectonics since the Pliocene. Before the identification of the change of the tectonic regime during the Pleistocene, each Quaternary fault affecting the Apennine chain and showing some geomorphological evidence of recent activity was considered to be active and possibly responsible for earthquakes. The present study presents a different interpretation and suggests the need for a detailed reconstruction of structural and geological evolution during the Quaternary in order to better define a framework of active tectonics in the Apennine chain. Acknowledgments. The work has been partly supported by CNR- GNDT, Italian National Group for the Defense Against Earthquakes. The author's activity inside GNDT has benefited from discussions with P. Scandone (University of Pisa). Discussions with C. Bosi and P. Messina (CNR, Istituto di Ricerca sulla Tettonica Recente, Rome) on the stratigraphic setting of the central Apennines were fundamental for the reconstruction of the structural evolution. 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