doi: 10.1111/j.1365-3121.2004.00582.x Growth, interaction and seismogenic potential of coupled active normal faults ( Basin, central-southern Italy)

Daniela Di Bucci,1 Giuseppe Naso,1 Sveva Corrado2 and Igor M. Villa3 1Dipartimento della protezione civile, Servizio Sismico Nazionale, Via Vitorchiano n. 4, 00189 Rome; 2Dipartimento di Scienze Geologiche, Universita` degli Studi ‘Roma tre’, L.go S. L. Murialdo n. 1, 00146 Rome; 3Institut fu¨r Geologie, Universita¨t Bern, 3012 Bern, Switzerland; Dipartimento di Scienze Geologiche e Geotecnologie, Universita` di Milano Bicocca, 20126 Milan, Italy

ABSTRACT

The inception and growth of the active Carpino-Le Piane Basin than 253 ± 22 ka BP, and very probably as recently as <28 ka BP. Fault System (CLPBFS; central-southern Apennines, Italy) was These ages, combined with structural data (geometry and analysed with respect to the neighbouring Isernia and Surround- kinematics of the systems), indicate that the inception and ing (ISFS) and Boiano Basin (BBFS) extensional Fault Systems. development of the CLPBFS could be strictly related to the stress 39Ar–40Ar dating showed that the BBFS was already active changes caused by earthquakes occurring on the BBFS. 649 ± 21 ka BP and that the ISFS was active at least 476 ± 10 ka BP, whereas the activity of the CLPBFS started certainly later Terra Nova, 17, 44–55, 2005

Boiano Basin Fault System (BBFS; since Middle Pleistocene (Patacca Introduction Fig. 2). These are distinct fault seg- et al., 1992; Corrado et al., 1997; Fer- The boundary area between the cen- ments both in terms of geometry and ranti, 1997; Porfido et al., 2002). All tral and southern Apennines thrust geology (McCalpin, 1996, and refer- along the axis of the chain, this belt of the Italian peninsula includes ences therein); moreover, according to extension is still active today and the surroundings of Isernia, an some authors (e.g. Porfido et al., responsible for strong earthquakes important town of the Molize region 2002), they host the seismogenic faults (Gruppo di Lavoro CPTI, 1999; Mon- (Fig. 1). Here, a recent study identi- responsible for the 1805 Boiano earth- tone et al., 1999; Galadini and Galli, fied two local intramontane basins as quake mainshock (I0 ¼ X MCS; Gru- 2000; D’Addezio et al., 2001; Roberts half-grabens formed by a 10-km long, ppo di Lavoro CPTI, 1999) and for et al., 2004; Roberts and Michetti, N330 striking and NE dipping set of one of the related strong aftershocks. 2004; Fig. 1). normal faults (Di Bucci et al., 2002; This work aims at showing how the It is widely accepted in literature Figs 2 and 3). The late Holocene growth and interaction of two import- that extension on the BBFS is acting activity of these faults [called Carpi- ant adjacent extensional systems in the since Middle Pleistocene (for instance, no-Le Piane Basins Fault System Apennines, the ISFS and the BBFS, see Corrado et al., 1997; Galli and (CLPBFS); Fig. 4] was constrained led to the formation of new late Galadini, 2003, and references there- for the last 4000 years by analysing UpperPleistocene to Holocene faults in). This age is compatible with the 65 wells drilled on modern fluvio- and associated sub-basins within one tectonic evolution of the Mas- lacustrine deposits that fill the intra- of the said structures (the CLPBFS sif (Calabro` et al., 2003) and derives montane basins. within the ISFS). This is not well from Middle–Upper Pleistocene te- Field data suggest that the CLPBFS understood and can have significant phra deposits filling the Boiano Basin, formed during the Upper Pleistocene– impact on the knowledge of ongoing faulting of Middle Pleistocene tephra Holocene within the northernmost extensional processes and fault link- deposits and Pleistocene slope-scree, part of a regional tectonic depression age, as well as on the associated and offset of Pleistocene remnant N300 oriented, which developed landscape evolution and seismicity in surfaces. However, no 39Ar–40Ar iso- since Middle Pleistocene from the the Apennines. This topic was ad- topic ages were available before the Isernia surroundings to the dressed by applying structural analysis present work to quantitatively con- Plain, for a length of about 40 km and 39Ar–40Ar technique, which pro- strain the age of this extension. (Corrado et al., 1997; Figs 2 and 3). vided new data and allowed a totally The BBFS is a well-known active We refer to northernmost part of this novel interpretation based on the extensional structure (Cucci et al., depression as the Isernia and Sur- integration of geological data and 1996; Naso et al., 1998; Basili et al., roundings Fault System (ISFS), and physical models of interaction be- 1999; Guerrieri et al., 1999; Blumetti to the adjacent central part as the tween seismogenic sources. et al., 2000; Valensise and Pantosti, 2001; Porfido et al., 2002; Galli and Galadini, 2003; Figs 1 and 2), which Correspondence: Daniela Di Bucci, Structural data Dipartimento della protezione civile, develops with N300 strike for about Servizio Sismico Nazionale, Via Vitorchi- The Quaternary development of the 25–28 km, with a complex extensional ano n. 4, 00189 Rome, Italy. Tel.: +39 06 ISFS, BBFS and CLPBFS is an pattern. This consists of a NE-dipping 6820 4761; e-mail: daniela.dibucci@ expression of the SW–NE extension master fault, which develops down to protezionecivile.it that has been acting in the study area a depth of at least 9 km (Mazzoli

44 2005 Blackwell Publishing Ltd Terra Nova, Vol 17, No. 1, 44–55 D. D. Bucci et al. • Growth and interaction of active normal faults ......

12°30'E 13°30'E 14°30'E 15°30'E 0 20 Bolsena A Lake km G ran PESCARA Sa sso Ra nge 42°20'N L'AQUILA N T iyb e M r a V i a e Adriatic Sea l l l le a

M

t SULMONA . Bracciano S Lake im Fucino 42°00'N b ru Basin in i M ts M a rs ROME ica Al to Mo lise Gargano Promontory 41°40'N Figs 2 and 3 ISERNIA Tyrrhenian Sea Vo lsc i M ts Matese Mts

Sa nn io

Marine and continental clastic deposits Carbonate platform Synorogenic hemipelagic and (Pliocene-Quaternary) (Trias-Miocene) turbiditic sequences (Tortonian-Pliocene) Volcanic deposits Slope and by-pass margin deposits (Pleistocene) (Lias-Miocene) Thrusts Clayey-carbonate and arenaceous turbidites -Sannio pelagic basin (Cretaceous-Eocene) (Oligocene-Miocene) Faults

Pescara B Viterbo Rieti

L'Aquila N 0 km 50 Central Apennines Rome

Isernia 1805 Campobasso Southern Magnitude scale Apennines >7.0 7.0–6.8 6.8–66.4 6.4–66.0 6.0–65.6 5.6–65.2 Benevento

Fig. 1 (A) Geological map of the central-southern Apennines (after Di Bucci et al., 2002, modified). (B) Largest earthquakes of the central-southern Apennines (data after Boschi et al., 1997, and Valensise and Pantosti, 2001). The size of the square symbols is proportional to an ‘equivalent magnitude’ (Me) derived from intensity data. et al., 2000; Butler et al., 2004), and of An example comes from S. Giorgio site gave us the possibility to quanti- synthetic and antithetic Quaternary area (Fig. 5), where normal faults tatively constrain the Middle Pleisto- normal faults linked by shallow, pre- affect a succession of Pleistocene la- cene tectonic activity of the BBFS. Quaternary, mainly E–W-oriented custrine and pyroclastic sediments This fault system strictly coincides high-angle faults, reactivated by the (samples San2 and San3) forming a with that revealed by drainage net- present-day extensional stress field terrace along the main slope of the work studies (Cucci et al., 1996) and (Fig. 3, plots G–M). BBFS (Brancaccio et al., 1979). This with the seismogenic fault inferred

2005 Blackwell Publishing Ltd 45 Growth and interaction of active normal faults • D. D. Bucci et al. Terra Nova, Vol 17, No. 1, 44–55 ......

C 41°40'00"N L P Fig. 4 B F S Montagnola di Isernia San30 IS San9b Venafro FS Mts Campobasso

.

R i l R o io n r

e

f

i

B

41°30'00"N San31 S. Massimo

R Roccamandolfi Boiano o Venafro n Matese Mts r u lt o V. San2 . Sannio T o Letino Fig. 5 n San3 i Saddle Mts B ir u B Q FS Sepino Plain Matese Lake

N

0 km 10 Plain 14°07'08''E 14°27'08''E 41°20'00"N

Calcareous and Siliciclastic deposits Calcareous and Siliciclastic deposits dolomitic (Upper Tortonian- dolomitic (Upper Messinian) Main Buried successions Lower Messinian) successions thrust thrust Apennines carbonate Apulia carbonate platform, by-pass platform and slope Strike-slip fault margin and slope (Meso-Cenozoic) (Meso-Cenozoic) Second order tectonic elements Marly-calcareous Marly-clayey Siliciclastic Sedimentary and successions successions deposits volcanic Molise-Sannio Molise-Sannio (Tortonian) continental deposits Extensional systems slope pelagic basin (Quaternary) discussed in the text (Meso-Cenozoic) (Oligocene-Miocene)

1 2 San2 Sampling site

Fig. 2 Schematic map of the Isernia and Boiano areas. Box 1, surface projection of the fault model proposed by Cucci et al. (1996) based on dislocation modelling of selected landscape features. The fault dips to the NE and projects to the surface along the NE edge of the Matese Mts. Box 2, surface projection of the source of the 1805 earthquake, as proposed by Gasperini et al. (1999) based on modelling of intensity data. Sampling sites are indicated. ISFS, Isernia and surroundings fault system; BBFS, Boiano Basin Fault System; CLPBFS, Carpino-Le Piane Basin Fault System.

from the damage field of the 1805 Middle Pleistocene ‘Riempimento syn-sedimentary faults of Fig. 6; sam- earthquake (Io ¼ X MCS; Gasperini principale’ Fm., which constitutes the ple San9b was collected in this site) et al., 1999; Gruppo di Lavoro CPTI, upper part of a Lower–Middle Pleis- and by E–W oriented faults. The latter 1999). Thus, it is widely accepted that tocene continental succession (Corra- pre-existed in the Meso-Cenozoic bur- the BBFS is a source of severe seis- do et al., 1997 and references therein; ied units and propagated in the over- micity. Brancaccio et al., 2000). Integrated lying Quaternary succession with an Moving towards the town of Iser- data (geomorphology, field survey, oblique component of motion, be- nia, mesostructural analysis provided stratigraphic correlations) indicate cause of the reactivation under the two different pieces of evidence which that this continental succession did extensional regime (Fig. 7). Therefore, allowed to better discriminate the not overcome the Middle–Upper fault orientation and kinematics of CLPBFS from the ISFS. Pleistocene boundary (Coltorti, this Middle Pleistocene pattern are The first mesoscale evidence refers 1983). At the outcrop scale, a fault comparable with those of the BBFS to the ISFS, that is characterized at network can be observed (Fig. 3, plots (Fig. 3, plots H–M). As shown in the landscape scale by the Pesche fault A–E), which is composed of newly Fig. 3, this fault network is pervasive (Figs 2 and 3). Mesostructural data formed WNW–ESE to NW–SE-ori- and widespread all over the study come from some outcrops of the ented normal faults (among these the area.

46 2005 Blackwell Publishing Ltd Terra Nova, Vol 17, No. 1, 44–55 D. D. Bucci et al. • Growth and interaction of active normal faults ......

13˚59'54''E' Mt. Totila 14˚30'08''E '

0 Duronia 0

' Continental clastic 0 0 5 10 Km deposits 4 . (Quaternary)

1 Miranda

4˚ N r Sessano d n Travertine aT F (Quaternary) V a Foredeep and thrust-top-basin siliciclastic A Fornelli deposits Pesche N (Miocene) Colli Isernia Pelagic basin a Volturno B Frosolone successions E (Meso-Cenozoic) C Carbonate re R. Pettoranello platform and Cavalie slope successions A G S. Elena Sannita (Meso-Cenozoic) Macchia S. Angelo in Grotte Montaquila d'Isernia H D Mt Bedding . Pa B tal il Serrone S. Agapito ecc hia Cantalupo Faults (dashed when buried or inferred). Arrows indicate dip of nel Sannio fault plane; dots indicate the sense Roccavindola . of hangingwall motion. Thicker lines

R indicate the extensional systems

il R discussed in the text. J io T. o I La Montagnola n r

. e f Thrusts R K i o B rn Venafro u S. Massimo Reverse and lt Roccamandolfi o Boiano Vinchiaturo transpressive C V L M faults Capriati 70 a Volturno o S. GiulianoStrike-slip faults in r del Sannio Gallo S Polomatese i u QT. Letino Mt. Miletto Normal faults T Campochiaro a m m a Rocca ro R Pipirozzi Prata . Sannita M.a D tes L Plot location e L Mt. Cesima Sepino

N N

Presenzano Ailano S. Gregorio Mt. Moschiaturo S. Angelo d'Alife Matese T. a '

' m

0 m 0 ' E F G H I J K L M a

0 r o 2 Cusano Mutri Pietraroia Morcone R 1

4˚13˚59'54''E N 14˚27'08''E

Fig. 3 Detailed structural map of the Isernia (ISFS and CLPBFS) and Boiano (BBFS) areas. Mesostructural data on Schmidt net (lower hemisphere. Faults, solid line; bedding, dashed line; r1, five-point star; r2, four-point star; r3, three-point star). Original field data concerning the extensional systems are plotted along with data from SGN (1970, 1971), Ferranti (1993) and De Corso et al. (1998).

The second piece of evidence is of the succession; these facies are (2000). Results are shown in Fig. 8 given by the CLPBFS that, instead, interfingered and have a total thick- and Table 1. is concentrated along a N330 trend- ness that can exceed 100 m. All samples show some degree of ing narrow zone (Di Bucci et al., 2002; The remaining two samples came chemical heterogeneity, highlighted by Fig. 4). This trend is significantly dif- from the succession of lacustrine and variations in the Cl:K and Ca:K ratios ferent (30) with respect to the general pyroclastic sediments that form a (for a discussion about these chemical direction of the BBFS and ISFS tec- terrace along the main slope of the indicators, see Villa, 2001, and refer- tonic depression (N300; Fig. 3). Its BBFS (Brancaccio et al., 1979; ences therein). Basing on Ar isotope activity post-dates the ‘Riempimento Fig. 5). These deposits are deformed systematics, the analysed samples can principale’ Fm., whose deposits con- by syn-sedimentary faults belonging be divided into two groups. stitute the bedrock for the fluvial- to the BBFS. The thickening of tens of The first group, formed by samples lacustrine sediments directly related to metres of these lacustrine sediments San2, San3 and San9b, shows modest the CLPBFS (Di Bucci et al., 2002). towards faults belonging to the BBFS chemical variations, probably indica- is derived from field survey and sug- ting an almost monogenetic origin. gests synsedimentary tectonic activity For the second group (samples San30 Geochronological data (Corrado et al., 2000). Moreover, and San31), a heterogeneous proven- Five samples of pyroclastic rocks these sediments are presently faulted, ance, i.e. a higher degree of reworking, were analysed. Three (San9b, San30 suggesting a more recent activity of is suggested by the concomitant pres- and San31) came from the surround- the same faults. ence of large variations in the chem- ings of Isernia (concerning both The sedimentary environments des- ical indicator ratios and of a younger ISFS and CLPBFS), and two (San2 cribed imply that all the samples are component mixed to one or more and San3) came from the BBFS area affected by some reworking. However, older components. A reliable age can (Fig. 2). the large size and angular shape of the be calculated by only considering The former three samples were col- pumices rule out significant transport ‘isochemical’ steps, i.e. those steps lected in the ‘Riempimento principale’ after their deposition. Sanidines were whose uniform chemical indicators Fm. Conglomerates, gravels, sands separated for 39Ar–40Ar dating. Ana- imply the smallest chemical (and gen- and pyroclastic deposits form this part lytical protocols follow Villa et al. etic) heterogeneity (Table 1).

2005 Blackwell Publishing Ltd 47 Growth and interaction of active normal faults • D. D. Bucci et al. Terra Nova, Vol 17, No. 1, 44–55 ......

80 concordantly assign <28 ka to the Alto Molise hills 0 m 14˚14'10''E 0 1 41˚37'50''N km inception of the filling of the Carpino m 600 and Le Piane basins, and therefore to 5 00 1m 622 1 m 1 0 0 0 0 the inception of the CLPBFS activity.

0

m 7 0

0 A significant lapse of time (more than

Pratocicala m Montagnola 90 kyr) separates the inception of 614 tr di Frosolone Le Piane 937 Carpino and Le Piane basins filling basin from the end of ‘Riempimento princi- 900 m R 458 605 a v a pale’ Fm. deposition (0.125 Ma). R r i e v iv e o R 23 r rd So N 60 Pesche 931

5 0 0

m 47 Discussion and conclusions a 16

454 10 10 The BBFS and the ISFS are oriented 17 1 617 according to the extensional stress 2 b 603 3 39 field which is acting along the core of 4 471 600 m the Apennines (Montone et al., 1999), Isernia c while the CLPBFS is not aligned. Thus, a local variation of the stress 10 623 field orientation could be invoked for tr d 508 31 the evolution and present-day activity m 0 0 Carpino 6 basin of this fault system.

e i It is well known that the occurrence of earthquakes perturbs the stress 5 00 m state in the surroundings of the source f j fault (Stein et al., 1992; King and Cocco, 2001, and references therein).

15 600 m Not only can this promote and/or 50 Pettoranello 7 g k 0 0 m 0

m 725 delay subsequent ruptures, but it can Matese Mts also influence the attitude of adjacent h l seismogenic sources, which could be 14˚14'50''E 35 41˚34'10''N rotated with respect to the considered source fault (depending on the ampli- Fig. 4 Geological map of the CLPBFS (after Di Bucci et al., 2002, modified; see Fig. tude of coseismic stress changes with 2 for the location). (a) bedding; (b) faults from: 1, well log analysis; 2, well log analysis respect to the regional stress; King and aerial photograph interpretation; 3, aerial photograph interpretation; 4, et al., 1994). In particular, the effects geological survey; (c) entrenched rivers; (d) trench location after Giraudi et al., of a normal faulting earthquake have 1999; (e) Late Quaternary fluvial and lacustrine deposits (Giraudi et al., 1999); (f) been analysed by Nostro et al. (2001). Late Quaternary slope talus and associated alluvial fans (Giraudi et al., 1999); (g) Middle Pleistocene terraced slope talus and alluvial fans (Coltorti and Cremaschi, For a seismogenic normal fault com- 1982); (h) palaeolandslide; (i) Middle Pleistocene conglomerates, gravels, sands and parable with the BBFS (Fig. 9), they volcanoclastic deposits (‘Riempimento principale’ Auct.; Delitala et al., 1983; obtain a stress increase in well-defined Corrado et al., 2000); (j) Early–Middle Pleistocene travertine and concretional sands lobes at the tips of the source fault; (AA, 1983); (k) Early–Middle Pleistocene lacustrine clays (Esu, 1983); (l) Meso- one of these coincides with the loca- Cenozoic marine successions – mainly carbonate and terrigenous deposits (SGN, tion of the CLPBFS. Moreover, they 1971). calculate the expected focal mechan- ism for an earthquake generated in such zone, obtaining nodal planes In summary, inception of BBFS the ‘Riempimento principale’ Fm., it comparable with the attitude of the predates 649 ± 21 ka, as the deposits is younger than sample San30 from CLPBFS. containing samples San2 and San3 the upper part of this formation, This suggests a possible evolution show syn-sedimentary deformation which is 253 ± 22 ka. Thus the for the analysed active fault systems. related to this fault system. CLPBFS inception occurred well later The study area underwent an exten- Some faults of the pervasive and with respect to the ISFS activity. As a sional tectonic regime since Middle widespread Middle Pleistocene ISFS matter of fact, the fluvio-lacustrine Pleistocene. This stress field caused acted during the deposition of the sediments filling Carpino and Le Pi- the progressive development of a sediments containing sample San9b. ane basins are generally attributed to N300 oriented tectonic depression, Therefore, the ISFS was active at least the Upper Pleistocene–Holocene (Di given by a pattern of new NW–SE- 476 ± 10 ka (Fig. 6); it can be com- Bucci et al., 2002 and references there- oriented faults and of reactivated pre- pared with the BBFS inasmuch it has in). In detail, they can be more pre- existing E–W-oriented faults, which coincident orientation, kinematics and cisely attributed to the final part of the encompasses the ISFS and BBFS. The age. Upper Pleistocene and Holocene, BBFS was growing for at least 620 ka; On the contrary, as the CLPBFS based on the lines of evidence des- by c.28kaBP, it should have been activity post-dates the deposition of cribed in Table 2. These arguments sufficiently developed to cause earth-

48 2005 Blackwell Publishing Ltd Terra Nova, Vol 17, No. 1, 44–55 D. D. Bucci et al. • Growth and interaction of active normal faults ......

Fig. 5 Geological setting of the Serra San Giorgio area, within the BBFS (see Fig. 2 for the location). During the Middle Pleistocene, the interplay of NW–SE normal faults and SW–NE pre-existing faults controlled the creation of a small lake (schematically shown in the block diagram) and the subsequent dissection of the associated lacustrine sediments (San2 and San3) under an extensional stress field. Note the growth of the lacustrine sediments toward the faults in the geological cross-section A–A’.

quakes which are able to induce sig- lineaments have been recognized (they fault systems comparable with the nificant stress changes at its tips. This have been described within the ISFS) BBFS, nor destructive historical or in turn caused the inception and the and are not oriented like the CLPBFS. instrumental earthquakes) is presently development of the CLPBFS, which is Furthermore, all over the Apennines known at the north-western tip of the in fact younger, less developed the extensional tectonics overprints CLPBFS. (10 km), differently oriented (N330) previous compressional structures, A third hypothesis considers the and located in the right position (at which do not condition the direction CLPBFS simply as a set of late, small the western tip of the BBFS). of the active normal faults. In addi- faults in the subsiding hangingwall of Contrasting interpretations are less tion, this hypothesis does not explain the Pesche fault, i.e. as part of the likely. A first alternative hypothesis on why inception of the CLPBFS incep- ISFS. This hypothesis is compatible the geometrical relationship between tion is younger than that of the other with the role of seismogenic fault BBFS and CLPBFS active fault sys- systems. system attributed by some authors to tems is that there is in fact no rela- A second hypothesis considers the the ISFS (Porfido et al., 2002) and tionship at all: this part of the CLPBFS as a transfer fault (Gaw- with the relatively younger age of Apennine fold-and-thrust belt has a thorpe and Hurst, 1993) of the BBFS the CLPBFS. However, we consider complex structural setting (Patacca towards the NW. In this way, the the inception and growth of the et al., 1992; Speranza et al., 1998) development of the CLPBFS would be CLPBFS as mainly induced by the and the orientation of the CLPBFS strictly connected to the evolution of BBFS development for the following could be controlled by local pre-exist- the BBFS, also explaining why the reasons. ing lineaments. In this way, the former is younger. However, the 1 The orientation of the CLPBFS is CLPBFS would evolve independently transfer system appears dead-ended, a relevant datum. The CLPBFS is from the BBFS. However, pre-existing as no seismic activity (neither active

2005 Blackwell Publishing Ltd 49 Growth and interaction of active normal faults • D. D. Bucci et al. Terra Nova, Vol 17, No. 1, 44–55 ......

N S

Conglomerates and sands

Paleosoil with pyroclastics San9b A B

Fig. 6 Site to the South of Isernia (San9b; for the location, see C in Fig. 3). (A) Syn-sedimentary faults cutting the Middle Pleistocene deposits; (B) Schmidt diagrams (lower hemisphere) of the strata – dashed lines – and one fracture (top), and of the fault planes (bottom).

E W

A B

Fig. 7 Normal faults cutting the fluvio-lacustrine stratigraphic succession in the Isernia plain (Locality: il Mondo; for the location, see (A) in Fig. 3). (A) Meso-scale synthetic and antithetic normal faults. Outcrop orientation: N215; (B) Schmidt diagrams (lower hemisphere) of the strata – dashed lines – and fractures (top), and of the fault planes (bottom). The three, four and five-pronged stars represent r3, r2 and r1 respectively.

not inherited, and the different (Fig. 3), caused by inherited 2 The CLPBFS strike (as well as its fault trend is not comparable with (mainly E–W oriented) faults reac- location at the tip of the BBFS) is the variable fault orientation char- tivated by the present-day stress not random, but it coincides with acterizing the ISFS and BBFS field. the location and orientation expec-

50 2005 Blackwell Publishing Ltd Terra Nova, Vol 17, No. 1, 44–55 D. D. Bucci et al. • Growth and interaction of active normal faults ......

San2 San3

1.2

0.75 1.0

0.8 Age (Ma)

Age (Ma) 0.65

0.6

0.55 0.4 0 20 40 60 80 100 0 20 40 60 80 100 39 % Ar % 39Ar San9b San30

0.5 1.2

1.0 0.4

0.8 0.3 0.6

Age (Ma) 0.2 Age (Ma) 0.4 0.1 0.2

0.0 0.0 0 20 40 60 80 100 0 20 40 60 80 100 % 39Ar % 39Ar San31

San2 - San3: Long. 14°24'34"E 0.6 Lat. 41°28'40"N

0.5 San9b: Long. 14°13'38"E 0.4 Lat. 41°34'54"N

Age (Ma) 0.3 San30: Long. 14°10'27"E Lat. 41°35'38"N 0.2

0.1 San31: Long. 14°08'04"N Lat. 41°31'00"E 0.0 0 20 40 60 80 100 % 39Ar

Fig. 8 Age spectra of sanidines from Boiano (San2 and San3) and Isernia (San9b, San30 and San31).

2005 Blackwell Publishing Ltd 51 Growth and interaction of active normal faults • D. D. Bucci et al. Terra Nova, Vol 17, No. 1, 44–55 ......

Table 1 Radiometric age of the analysed samples Sample Constrained Considered steps no. fault system (refer Fig. 8) Average age San2 BBFS Four adjacent steps having Cl/K <0.00012 and containing 52% of the sample Ar 621 ± 6 ka San3 BBFS Four adjacent steps having Ca/K <0.021 and containing 70% of the sample 649 ± 21 ka Ar. Note that the ages of San2 and San3 are statistically indistinguishable, and indistinguishable from the age of the sanidine-bearing basal eruption of the nearby Roccamonfina volcanic complex, 630 ± 2 ka (Ballini et al., 1991). San9b ISFS Five adjacent steps having Ca/K <0.030 and containing 63% of the sample Ar 476 ± 10 ka San30 CLPBFS An anticorrelation trend (Fig. 8) is defined by the fifth step (age 647 ± 24 ka, 253 ± 22 ka (weighted average age Ca/K ¼ 0.0261 ± 0.0009) and by three steps (3, 4, 8) having the highest Ca/K of the younger component) ratios (uniform at 0.0299 ± 0.0004) and the lowest ages. The age of the older component is identical to that of samples San2 and San3; this could represent erosion of a primary 0.63 Ma tuff exposed on the hillslope above the basin San31 ISFS/CLPBFS At least three components can be recognized 504 ± 12 ka (mean age of the three steps with the lowest Ca:K ratio: 5, 6, 9)

Table 2 Age of sedimentation of the Carpino and Le Piane basins’ fill Reference Constraint Additional constraint Age of sedimentation

Giraudi et al. (1999) The development and areal distribution of the In Central Italy this occurred at 18 000 years <18 000 years BP sediments filling Le Piane basin requires a climatic BP (Frezzotti and Giraudi, 1989). phase characterized by relevant stream load D’Addezio et al. (2001) In the region, small intramontane basins similar to the <21–18 ka Carpino and Le Piane basins (e.g. Aremogna, Cinque Miglia) are filled by sediments deposited during the deglaciation following the last glacial peak (21–18 ka) and by modern alluvial fans Di Bucci et al. (2002) The stratigraphy of the infill of the Carpino and De Rita and Giordano (1996) proposed to <295 ± 2 ka Le Piane basins is totally different from the derive these pyroclastic sediments from the ‘Riempimento principale’ succession. In particular, activity of the nearby Roccamonfina volcanic the thick strata of pyroclastic deposits characterized complex, which ended its explosive phase by centimetric pumices, occurring in the 295 ± 2 ka (Ballini et al., 1991). Therefore, ‘Riempimento principale’ Fm., are totally lacking the basins’ filling post-dates this age in the basins’ filling Giraudi et al. (1999), The sedimentation rate for the youngest part of An indirect attempt to identify the substratum <20 ka, >14 ka, <28 ka ) Di Bucci et al. (2002) sediments filling Le Piane basin is of 0.75 mm a 1 depth by applying the Nakamura technique (value derived from the trench analysed by Giraudi (D’Amico et al., 2004) returned a maximum et al., 1999). This corresponds to a deposition rate of depth of about 50 m. In this case, the age 1.8 mm a)1 in the basin depocentre. The for the inception of sedimentation in homogeneity of sediments shown by the wells Le Piane basin would be 28 ka analysed by Di Bucci et al. (2002) for both Carpino and Le Piane basins suggests that this sedimentation rate also prevailed in the older part. For the Carpino basin, the filling thickness at the depocentre is about 35 m; therefore we obtained a maximum age of 20 ka for the sedimentation inception. For Le Piane basin, the minimum depth explored by the wells is of 25 m (Di Bucci et al., 2002); this depth corresponds to 14 ka

ted according to the changes of the set of minor antithetic faults with Jurassic–Cretaceous boundary (SGN, Coulomb stress described before. respect to the Pesche fault. If so, the 1971), and activity in the final part of 3 As the CLPBFS is responsible for filling of the basins should be the Middle Pleistocene (Di Bucci the development of the most rele- thicker towards the latter, and this et al., 2002, and references therein). vant late Upper Pleistocene–Holo- is not the case. Its present-day activity is hypothes- cene features of the area (i.e. the ized by some authors (Michetti et al., The Pesche fault is a well-developed asymmetric filling of the Carpino 2000; Porfido et al., 2002; Galli and normal fault. It shows a displacement and Le Piane basins), this fault Galadini, 2003) based on the inter- of hundred of metres measured on the system cannot be interpreted as a pretation of historical records related

52 2005 Blackwell Publishing Ltd Terra Nova, Vol 17, No. 1, 44–55 D. D. Bucci et al. • Growth and interaction of active normal faults ......

41°40'00"N

C struck this area in the past (e.g. in AD P Isernia L B F S 41°30'00"N 1456; Gruppo di Lavoro CPTI, 1999) Boiano -0.03 BB FS are still unknown. Further studies,

0 km 10 14°07'08''E 14°27'08''E 41°20'00"N such as a long-term analysis of instru- mental data or a deeper revision of the -0.10 historical information, are needed to substantiate the CLPBFS as a seism- 0.00 0.10 ogenic fault system. 1.00 -1.00 0.03 Acknowledgements 0.00 Thanks are due to A. Trigari and I. 0.00 0.10 Leschiutta for their help in field survey, to F. Marra for his help in the early stages of -1.00 this work, to R. Nappi and D. Karner for 0.03 the mineral separation, and to B. Narcisi N and C. Nostro for useful discussions. Reviews by A.M. Michetti and two anony- mous referees are gratefully acknowledged.

-0.10 0.00 References 0.00 AA, VV.1983. Isernia La Pineta. Un accampamento piu` antico di 700.000 anni. -0.03 50 km Calderini Editions, Bologna, Italy, 127 pp. Ballini, A., Barberi, F., Laurenzi, M.A., Fig. 9 Comparison between changes in the static stress field induced by a normal fault Mezzetti, F., Oddone, M. and Villa, comparable with the BBFS (after Nostro et al., 2001, redrawn) and the geometry of I.M., 1991. Chrono-stratigraphy of the studied fault systems (inset, at the same scale of the main figure). The map shows Roccamonfina Volcanic Complex. the total stress changes (in bars) after 100 years from the occurrence of an earthquake Plinius, 4, 35–36. Basili, R., Galadini, F. and Messina, P., generated by a normal fault striking N310 and dipping 60NE. Changes in the static 1999. The application of palaeolandsur- stress field persist over a long time span; therefore, faults and fractures developing at face analysis to the study of recent tec- the tips of the BBFS must be coherent with these stress field variations. The CLPBFS tonics in central Italy. In: Uplift, Erosion satisfies this requirement. and Stability: Perspective on Long-term Landscape Development (B.J. Smith, W.B. Whalley and P.A. Warke, eds). to the 1805 Boiano earthquake. How- scenario would be one where the Geol. Soc. Lond. Spec. Publ., 162, 1–9. ever, no detailed and specific geologi- entire 40-km long extensional belt Blumetti, A.M., Caciagli, M., Di Bucci, D., Guerrieri, L., Michetti, A.M. and Naso, cal studies on the recent activity of the ruptures in a single Mw 7.0–7.2 G., 2000. Evidenze di fagliazione super- Pesche fault are available in the lit- earthquake. The discussion before ficiale olocenica nel bacino di Boiano erature. Summing up, data available suggests that at present this scenario (Molise). In: Atti 19 GNGTS/01.09, at the moment for the study area is perhaps unlikely and that this 9 p., CD-ROM. Prospero Editions, suggest that Holocene activity mainly extensional belt tends to rupture in Trieste, Italy. occurred on the CLPBFS; therefore, limited size episodes rather than all Boschi, E., Guidoboni, E., Ferrari, G., at depth this should prevail on the at once. However, the long-term Valensise, G. and Gasperini, P., 1997. Pesche fault. However, Pesche fault implication of the interpretation here Catalogo dei forti terremoti in Italia dal activity cannot be ruled out, and presented is that CLPBFS and BBFS 461 a. C. al 1990. Istituto Nazionale di Geofisica and S.G.A., Bologna, Italy, further geological studies are needed are going to link more and more 644 pp. for a complete definition of this struc- efficiently, and that in the geological Brancaccio, L., Sgrosso, I., Cinque, A., ture. future 40 km coseismic ruptures will Orsi, G., Pece, R. and Rolandi, G., 1979. The relationship of the CLPBFS be increasingly frequent. Lembi residui di sedimenti lacustri with the seismic activity of the BBFS This conclusion leaves the fate of pleistocenici sul versante settentrionale is in agreement with the hypothesis the CLPBFS fully open and in need del Matese, presso S. Massimo. Boll. Soc. of a seismogenic fault in the Isernia for further investigations. Up to now Natur. Napoli, 88, 275–286. area. Simple empirical relationships only a moderate instrumental seismic- Brancaccio, L., Di Crescenzo, G., Ros- (Wells and Coppersmith, 1994) sug- ity is known for the Isernia area skopf, C., Santangelo, N. and Scarciglia, gest that the BBFS has a potential (Valensise and Pantosti, 2001, and F., 2000. Carta geologica dei depositi quaternari e Carta geomorfologica del- for an Mw 6.6–6.8 earthquake. Based catalogues therein); neither palaeoseis- l’alta valle del fiume Volturno (Molise, on the same relationships, in case of mological data nor detailed seismolo- Italia meridionale). Note illustrative. Il. CLPBFS rupture for the entire gical information is available for this Quaternario, 13, 81–94. length mapped, this would yield c. structure. Moreover, the causative Butler, R.W.H., Mazzoli, S., Corrado, S., Mw 6.0 estimation. A rather different sources for historical earthquakes that De Donatis, M., Scrocca, D., Di Bucci,

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