Agosta Kirschner 2003

Fluid conduits in carbonate-hosted seismogenic normal faults of

central Italy

F. Agosta1 and D. L. KirschnerDepartment of Earth and Atmospheric Sciences, Saint Louis University, Saint Louis, Missouri, USA

Received 5 June 2002; revised 13 November 2002; accepted 4 February 2003; published 29 April 2003.

[1] We studied the structures and stable isotope geochemistry of carbonate fault rocks infour normal faults of central Italy. The faults juxtapose Meso-Cenozoic carbonates ofthe footwalls against continental basins of the hanging walls. Footwall rocks exposedalong fault scarps have been exhumed from depths of �1 km. The fault rocks aresystematically arranged in each fault and can be separated into five distinct domains.Farthest from the main fault contact are undeformed host rock and fractured host rock(domain 1). Progressively closer to the fault contact, in the core of the fault, are gouge(domain 2), cataclasite (domain 3), and cement-dominated horizons with planar slipsurfaces (domain 4). Thin horizons of brecciated hanging wall sediments (domain 5) areadjacent, and locally accreted, to the footwall. Structural and stable isotope data areconsistent with compartmentalization of fluid in the faults during exhumation. The dataare most consistent with these fluids being predominantly evolved meteoric water ratherthan fluids from the mantle, crustal magmas, and/or devolatilizing carbonate rocks.Meteoric water infiltrated domains 4 and 5 of the faults, either from hanging wallsediments or directly at the land surface. The gouge and cataclasites of domains 2 and 3were impermeable barriers to movement of meteoric water into domain 1 andundamaged host rocks of the footwall. The results of this study do not support modelsof earthquake nucleation and rupture that envision large volumes of deep-seated fluidspassing upward through shallow portions of these seismogenic faults. INDEX TERMS:

8010 Structural Geology: Fractures and faults; 8045 Structural Geology: Role of fluids; 1040 Geochemistry:

Isotopic composition/chemistry; 1045 Geochemistry: Low-temperature geochemistry; 7299 Seismology:

General or miscellaneous; KEYWORDS: stable isotope, normal fault, fluid, architecture, earthquake, Italy

Citation: Agosta, F., and D. L. Kirschner, Fluid conduits in carbonate-hosted seismogenic normal faults of central Italy,

J. Geophys. Res., 108(B4), 2221, doi:10.1029/2002JB002013, 2003.

1. Introduction

[2] Many faults in the upper crust have a core of highlydeformed rocks surrounded by lesser deformed to unde-formed rocks [Sibson, 1977; Chester and Logan, 1986;Byerlee, 1993; Scholz and Anders, 1994; Caine et al.,1996]. The fault core is a narrow, highly deformed zonethat forms around a fault surface(s) and is the site ofcomminution, dissolution/precipitation, mineral reactions,and other fault-related processes. A damage zone surroundsthe fault core and contains numerous fractures, slip surfaces,and recognizable fragments of host rock. The core anddamage zones are surrounded by relatively undamaged hostrock.[3] Development of this zonation, associated mesoscale

to microscale structures, and the mechanics of seismic andaseismic deformation might influence the movement offluids in the shallow crust [McCaig, 1988; Knipe et al.,

1991; Antonellini and Aydin, 1994, 1995; Olivier, 1986;Evans et al., 1997; Jones et al., 1998; Hanemberg et al.,1999; Cello et al., 2001]. Several models have beenproposed to relate subsurface fluid flow to the structure offaults [Scholz, 1990; Bruhn et al., 1994; Mozley and Good-win, 1995;Matthai et al., 1998; Ferrill et al., 1999; Rawlinget al., 2001]. In low-porosity rocks, such as the carbonatesof this study, these models assume that fault cores have alower long-term permeability than the surrounding damagedzones. This is broadly consistent with the results of numer-ical 3-D fluid flow models demonstrating that fractures arethe primary feature controlling fluid flow in low-porosityrocks [Caine et al., 1999].[4] Fluid flow in faults depends not only on the structural

arrangement of fault rocks but also on the dynamic inter-actions imposed by deformation on the fault zone-fluid flowsystem. The role of tectonic loading on fluid flow in faultswas examined by Scholz et al. [1973] and later modeled bySibson et al. [1975], Sibson [1990, 1992], and Byerlee[1993]. They proposed cyclic, heterogeneous fluid flow infaults during the seismic cycle due to variations in devia-toric stress. Individual faults can be both conduits andbarriers for fluid movement during one seismic cycle. For

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B4, 2221, doi:10.1029/2002JB002013, 2003

1Now at Department of Geological and Environmental Sciences,Stanford University, Stanford, California, USA.

Copyright 2003 by the American Geophysical Union.0148-0227/03/2002JB002013$09.00

ETG 18 - 1

a normal fault, numerous studies envision an increase inpermeability during the interseismic period that increases toa maximum value just after failure [e.g., Brown and Bruhn,1996]. Healing and sealing of fractures following migrationof fluid into a normal fault [Cello, 2000] may result in anincrease in pore fluid pressure if the healing/sealing rate ishigh enough [cf. Marone, 1998]. When tectonic loading andpore fluid pressure are sufficiently high, earthquake faultingmay occur, resulting in the discharge of fluids from the faultinto the surrounding host rock [Muir-Wood and King,1993]. Clearly, fluid flow varies spatially and temporallyin faults.[5] In this framework, we investigate the structures and

stable isotope geochemistry of four normal faults in centralItaly to understand fluid-fault interactions. The faults, whichare hosted in carbonate rocks, are well exposed along themargins of large intramontane basins and were chosen forstudy because they extend 10 km into the crust [Ghisetti andVezzani, 1999], are the loci of numerous earthquakes[Boschi et al., 1997], and exhibited the largest isotopicvariations between fault rocks and host rocks in an earlierstructural/geochemical study of the same region [Ghisetti etal., 2001]. This study documented contrasting paleofluidcirculations between contraction- and extension-relatedstructures of the orogen. Semiclosed to closed fluid systemconditions occurred during the earlier shortening of thethrust system, while semiopen to open system conditionsprevailed during later uplift, exhumation, and normal fault-ing. We now focus on the development of these fluidconduits along basin-bounding normal faults to betteraddress the fluid migration through faults that are structur-ally zoned.

2. Regional Setting

[6] Oligocene to Pliocene continental collision and A-type subduction of the Adriatic-Apulian plate below theEuropean margin resulted in the development of an accre-tionary wedge in which Meso-Cenozoic sediments weredeformed [Royden et al., 1987; Patacca et al., 1990;Doglioni, 1991]. The result of this deformation is theApennine fold-and-thrust belt, a narrow, NW-SE elongatedbelt of tectonic units arranged in arcs of different size andcurvature [cf. Locardi, 1988]. Concurrent shortening inthe foreland and downfaulting and crustal thinning in thehinterland characterize much of the deformation in theApennines. Models taking into account coeval contractionin the foreland and extension in the hinterland includerollback of the subducting Adriatic slab [Malinverno andRyan, 1986; Jolivet et al., 1998], north-south Africa-Eurasiaconvergence [Mantovani et al., 1996], and upwelling ofasthenospheric magma [Locardi, 1988].[7] The contraction and extension has resulted in the

development of four distinct structural domains in thecentral Apennines, from west to east (Figure 1a). Farthestwest is the Tyrrhenian Sea back-arc region, with a 10-km-thick crust in the central portion of the basin and waterdepths of 2–3 km. To the east is the thinned and down-faulted peri-Tyrrhenian inner belt, which is composed of a25- to 30-km-thick crust dissected by NW-SE trendingnormal faults and associated basins [Bosi et al., 1995].Farther east is the strongly shortened peri-Adriatic outer

belt, which is composed of a 35-km-thick crust that wasactively shortening until late Pliocene-early Pleistocene[Ghisetti and Vezzani, 1999]. The peri-Adriatic outer beltis currently undergoing extension and collapse by activenormal faulting. The Adriatic-Apulia foredeep, which is theeasternmost structural domain, is actively shortening.[8] The four normal faults investigated in this study are

located in the peri-Adriatic outer belt of central Apennines(Figure 1b). The tectonostratigraphic units of this regionare composed of Mesozoic-Tertiary shallow water andtransitional platform-to-basin carbonates of the Adriatic-Apulian margin [Vezzani and Ghisetti, 1998]. This regionhas been uplifted at rates of 1.4–2.5 mm/yr since 3.5 and1.6 Ma, respectively [Ghisetti and Vezzani, 1999]. It hasbeen dissected by late Pliocene to Pleistocene high-anglenormal faults that have either reactivated preexisting thrustfaults [D’Agostino et al., 1998] and/or formed along theback limb of large folds [Ghisetti et al., 1994]. Manyearthquakes in central Italy are due to slip along thesenormal faults. These faults are oriented approximatelyNW-SE and lie in a 30- to 50-km-wide, 800-km-longseismogenic zone that extends through the central andsouthern Apennines with an overall direction of extensiontoward the NNE [Piccardi et al., 1999, and referencestherein].

3. Geology of the Normal Faults

[9] The four normal faults investigated in this study arethe (1) Venere-Gioia dei Marsi, (2) Campo Imperatore, (3)Monte Capo di Serre, and (4) Roccacasale faults. Thesefaults are some of the larger-displacement normal faults inthe region with throws of 1–2 km. The footwalls of theseemergent faults expose fault rocks exhumed from �1 kmdepth or less. The faults occur in a seismically active regionand two of the faults are known to have been seismogenic.We assume the other two faults have also been seismicallyactive during part or all of their history because of theirsimilarity and proximity to historically documented seismo-genic faults.[10] We describe in detail the structures and fault rocks of

the four fault zones (Figures 2 and 3). The structures andfault rocks are similar among the faults and are distributedsystematically in the faults. In the discussion, we provide ageneralized model for the fault zones that includes fivestructural domains.

3.1. Venere-Gioia dei Marsi Fault

[11] The Venere-Gioia dei Marsi fault is part of a NWtrending, SW dipping fault zone along the eastern margin ofthe Fucino Plain. Mesozoic and younger carbonate rockscompose the footwall of the fault. The fault’s hanging wallis composed of late Pliocene to late Pleistocene fluvio-lacustrine sediments of the Fucino basin [Galadini andMessina, 1994]. The basin is a half-graben structure thatformed by movement of the Venere-Gioia dei Marsi andother normal faults along the eastern margin of the basin[Mostardini and Merlini, 1988].[12] The Venere-Gioia dei Marsi fault ruptured during the

Ms = 7.0 Avezzano earthquake of 1915, resulting in morethan 30,000 casualties and structural damage in a 500 km2

area [Boschi et al., 1997]. The 1915 earthquake epicenter

ETG 18 - 2 AGOSTA AND KIRSCHNER: FLUID CONDUITS IN SEISMOGENIC NORMAL FAULTS

was near the town of Gioia dei Marsi [Galadini et al.,1995], �1 km north of our study section. Focal mechanismsolutions place the T axis in the E-W [Gasparini et al.,1985] to NNE-SSE directions [Basili and Valensise, 1991],consistent with oblique extension, while trench investiga-tions documented pure dip-slip motion of this fault duringthe 1915 rupture [Micchetti et al., 1996]. Normal, right-lateral slip of this fault has been documented by localgeomorphological analyses [Piccardi et al., 1999]. Thisfault and others in the Fucino area have been responsiblefor numerous earthquakes in the past, including thosebetween 885 and 1349 A.D., 550 and 885 A.D., 6000 and3600 B.C., and 10,800 and 5600 B.C. [Micchetti et al.,1996; Galadini and Galli, 1999]. The fault has accommo-

dated an average displacement rate of 0.4–1.0 mm/yr, withdown-dip motion toward 229 [Micchetti et al., 1996;Piccardi et al., 1999].[13] This fault is made of three NW striking segments.

We investigated the central segment, south of Venere, whereseveral quarries crosscut the fault’s contact and footwall.Here the fault strikes between 110 and 140, and dipsbetween 55� and 65� SW (Figure 2a). The fault surface ispolished and striated with striations and slickenfibers con-sistent with normal dip-slip motion and associated minorleft-lateral movement. A noticable scarp demarcates thefault trace. Inversely graded, cemented slope scree drapesover, and is partly offset by, the fault in some places. Thefault’s footwall is composed of dark white to gray, centi-

Figure 1. (a) Schematic cross section across central Italy [after Ghisetti et al., 2001]. Four distinctstructural domains are present: (1) Tyrrhenian Sea back-arc region, (2) thinned and downfaulted peri-Tyrrhenian (p-T) inner belt, (3) strongly shortened peri-Adriatic (p-A) outer belt, and (4) Adriatic-Apuliaforedeep. Study area is in the peri-Adriatic outer portion of the belt, in the Abruzzo region of central Italy.This region has been uplifted and dissected by late Pliocene to Pleistocene high-angle normal faults. (b)Simplified tectonic map of central Apennines [after Ghisetti et al., 1994] and the four faults studied indetail: A, Venere-Gioia dei Marsi; B, Campo Imperatore; C, Serra Capo di Monte; and D, Roccacasalefaults. Faults juxtapose deformed carbonates of footwalls against continental sediments of hanging walls.

AGOSTA AND KIRSCHNER: FLUID CONDUITS IN SEISMOGENIC NORMAL FAULTS ETG 18 - 3

Figure

2.

Geologicalmapsandorientationdataofminorstructuresof(a)Venere-GioiadeiMarsiandCam

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faultsand(b)MonteCapodiSerre

andRoccacasalefaultzones.Structuraldataareplotted

inlower

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isphere,equal-area

stereonetprojections.Rosediagramsdepicttrendsofminorstructuresconsistentw

iththeright-handruleforstrikeanddip.N

isnumber

ofdatadisplayed;M

isthepercentofdatain

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Figure 3. Representative outcrops of faults and fault rocks. (a) Campo Imperatore normal fault zoneviewed from SW. Fault separates core and damage zones of footwall from Quaternary sediments (Q.A.)of hanging wall. (b) Venere-Gioia dei Marsi normal fault viewed from SW. Fault separates core anddamage zones of footwall from Quaternary sediments (Q.A.) of hanging wall Subsidiary normal faults infootwall damage zone are oriented orthogonal to the main fault. (c) Hand sample from Campo Imperatorefault. Fault surface is planar and coated with carbonate cements and thin bands of fine-grained cataclasite.Within cement there are microscopic, rounded survivor clasts, while those in cataclasite are commonlyangular. Cemented horizons are commonly sheared. (d) Hand sample from Roccacasale fault. Fault rockadjacent to fault surface contains matrix and clast-supported cataclasites with centimeter-thick horizons ofred and pink cements. Youngest slip surface is between striated alluvium (Q.A.) and relativelyundamaged alluvium (Q.A. at top of sample). (e) Photomicrograph of Campo Imperatore hand sample.Survivor clasts and thin calcite veins are in cement-rich carbonate horizon. Sizes of survivor clastsincrease away from fault surface (f.s.). (f) Photomicrograph of Roccacasale hand sample. Fault surface(f.s.) juxtaposes sheared alluvium (Q.A.) against matrix-supported cataclasite of footwall.


meter- to meter-layered, lower Cretaceous limestones [Vez-zani and Ghisetti, 1998].[14] Two high-angle, E-W oriented left-lateral faults off-

set the fault in the western portion of the study area. Thesefaults are coated with pink, centimeter-thick cements; stria-tions on internal slip surfaces are consistent with bothstrike- and dip-slip motions. Low-angle stylolites that strikeparallel to the main fault and dip 10�–15� NE are alsopresent in this area. They are associated with millimeter-thick veins of white carbonate, which display a radiatingpattern about a central point.[15] The fault zone is 70–100 m thick. In the footwall,

the fault core contains several distinct fault rocks. A 5- to20-cm-thick cement-supported cataclasite with centimeter-to millimeter-diameter survivor clasts is adjacent to the faultsurface. Next to this horizon is a meter-thick, clast- andmatrix-supported cataclasite with centimeter- to millimeter-diameter survivor clasts. Adjacent to the cataclasite is ameter-thick gouge horizon with clasts that are arranged incentimeter- to decimeter-thick layers of uniform clast size.The clasts are more rounded closer to the fault. Three sets offractures and one set of minor faults are present in the faultcore (Figure 2a).[16] Numerous fractures, slip surfaces, and some veins

crosscut the host rock in the damage zone. This zone, whichis several decameters thick, is separated from the relativelyundeformed host rock by a subsidiary, dip-slip fault thatforms a distinctive morphologic scarp subparallel to the mainfault (Figure 3b). There are two primary sets of fractures, oneset of minor faults, and two sets of veins in the damage zone(Figure 2a). The centimeter-thick veins are filled with whitecarbonate. On the basis of a few crosscutting relations, theSE striking veins formed before the east striking veins.

3.2. Campo Imperatore Fault

[17] The Campo Imperatore fault delimits the northernmargin of the Campo Imperatore Plain, which is an intra-montane Quaternary basin. This basin is �17 km long, up to4 km wide, and filled with middle to middle upper Pleis-tocene gravels and glacial sediments, and upper Pleistocenefluvial, lacustrine, and alluvial fan sediments [Giraudi,1994]. The fault has been active since the middle Pleisto-cene. It offsets the middle Pleistocene sediments by severaltens of meters, the middle upper Pleistocene sediments by�10 m, and the upper Pleistocene alluvial fans and terracesby �1 m [Giraudi, 1994].[18] The study area is on the western margin of the

Campo Imperatore Plain. The fault cuts through an earlyJurassic dolomitic host rock of the Vado di Corno tectonicunit [Vezzani and Ghisetti, 1998], and juxtaposes thesedecimeter-thick, cement-supported carbonates against pink,very cohesive, fluvial and lacustrine Quaternary sedimentsof the hanging wall. Slope debris locally drapes the faultsurface. The fault contains several segments that strikebetween 105 and 130, and dip 50�–60� SW (Figure 2a).The fault surface is polished, striated, and coated with newcarbonate minerals. The striations are consistent with obli-que, right-lateral slip along the fault segments striking 105–110, and pure dip-slip along the 120–130 segments.[19] In the study area, the deformed footwall of the fault is

up to 130 m thick. The fault core with an average thicknessof 15–20 m contains decimeter- to meter-thick clast-sup-

ported cataclasites, and decameter-thick fault gouge. Thewhite, clast-supported cataclasites close to the fault consistof centimeter-diameter, angular to subrounded survivorclasts, which are occasionally embedded in a fine-grainedmatrix and white carbonate cements. In the footwall, fartherfrom the fault, the core zone is composed primarily of gouge,which contains decimeter- and centimeter-diameter, angularfragments arranged in thin layers of uniform clast size. Thegouge is thicker in areas where the strike of the fault changessignificantly or where subsidiary faults merge into the mainfault. There are three sets of high-angle fractures, three setsof minor faults, and three sets of veins in the fault’s core(Figure 2a). On the basis of crosscutting relations, the NWstriking, NE dipping reverse faults were the first structures toform in the core zone. The east striking, south dipping veinsare filled with white and pink carbonates and display twophases of growth. The SE and NE striking veins are filledwith white carbonate minerals; the NE striking veins exhibitevidence for several episodes of growth. The damage zone,which is very distinctive in the footwall, contains three setsof fractures (Figure 2a). Minor faults subparallel to the mainfault separate the damage zone from the relatively undam-aged host rock.[20] Major element analyses of samples collected along

the fault are consistent with the host rocks and cataclasite’sfine-grained matrix being a mixture of dolomite and low-Mg calcite (Table 1). Cements collected on the fault surfaceare composed primarily of low-Mg calcite.

3.3. Monte Capo di Serre Fault

[21] The Monte Capo di Serre fault borders the westernflank of the Le Riparate Mountain, a few kilometers south-east of the Campo Imperatore Plain. The fault is a metric-scale undulated surface, both parallel and perpendicular tothe strike. It juxtaposes Cretaceous limestones of the foot-wall against Jurassic dolomites of the hanging wall [Vezzaniand Ghisetti, 1998]. Slope debris with angular to sub-rounded clasts embedded in yellow to pink carbonatecements covers the fault surface, and is locally crosscutby numerous minor fractures. Centimeter-thick portions of

Table 1. Relative Abundance of Principal Elements in Carbonate

Host Rocks and Cemented Cataclasites of Domain 4 in Three

Faultsa

Rock TypeCa,wt %

Mg,wt %

Fe,wt %

Sr,ppm

Si,wt %

LOI,wt %

Campo Imperatore FaultHost rock 23.9 10.8 0.03 96 0.05 65.1Yellow cement 32.8 4.23 0.16 36 0.83 61.5White cement 25.8 9.66 0.01 56 0.06 64.4Yellow cement 33.2 4.72 0.05 87 0.15 61.8

Monte Capo di SerreHost rock 39.2 0.37 0.02 162 0.07 60.3Pink cement 39.4 0.20 0.04 77 0.17 60.0White cement 39.5 0.11 b.d. 26 0.03 60.3Pink cement 39.1 0.17 0.03 70 0.13 60.4

RoccacasaleHost rock 39.2 0.35 0.06 178 0.29 59.9Red cement 34.3 3.22 0.10 45 0.13 62.1Yellow cement 28.7 7.40 0.23 52 0.44 64.0Pink cement 26.6 8.67 0.10 55 0.35 64.8

aData obtained by XRF analysis.


slope debris and talus have been locally sheared andaccreted onto the fault’s footwall.[22] In the study area, the Monte Capo di Serre fault

strikes between 095 and 150 and dips 50�–70� W (Figure2b). A fault scarp, up to 9 m high, characterizes the faulttrace. The fault surface is polished and striated with kine-matic indicators consistent with normal oblique faulting.Uncommon 5- to 10-cm-long ring cracks (i.e., cracks withan arcuate shape [see Scholz, 1990, Figure 2.12]) formed onthe fault surface in the western portion of the study area.Cemented cataclasites adjacent to the fault surface containmillimeter- and centimeter-diameter, angular to subroundedsurvivor clasts embedded in carbonate cement. Locally, upto three different millimeter- and centimeter-thick red hori-zons are present within these rocks, each of which containvery small survivor clasts embedded in carbonate cements.These red horizons are characterized by internal wavysurfaces on the footwall side and polished, striated, planarsurfaces on the hanging wall side.[23] The fault zone is not well developed along the Capo

di Serre fault; in the footwall, its width ranges from a fewmeters up to �35 m and consists of a discontinuous faultcore flanked by a narrow damage zone. The fault corecontains decimeter-thick, clast-supported cataclasites, anddiscontinuous meter-thick fault gouge. The cataclasites aremade of centimeter-diameter, subangular survivor clastswith rare matrix and white and light pink cements. Thefault gouge is present only along the eastern portion of thefault in the study area. It consists of centimeter- anddecimeter-diameter, angular to subrounded fragmentedclasts, commonly embedded in a fine-grained matrix.Numerous millimeter- to centimeter-thick white veins arepresent locally along the fault contact. They occur where thefault zone is poorly developed and parallel the NW strikingand gently SW dipping bedding of the limestone host rock.There are two sets of fractures and one set of minor faults inthe fault’s core (Figure 2b).[24] In the damaged host rock of the footwall, the original

sedimentary fabric is well preserved. There are two sets offractures in the damage zones (Figure 2b). The fractures areparallel and orthogonal to the fault surface, similar to thosein the fault core. The few minor faults crosscutting thedamage zone are parallel and oblique to the main fault, andare associated with poorly cemented breccia.[25] The Monte Capo di Serre fault zone host rocks and

cements are primarily low-Mg calcite (Table 1). East of thestudy area, centimeter-thick flame structures cut across theentire fault core. These structures contain centimeter- anddecimeter-thick, angular clasts embedded in pink carbonatecements. They may be evidence of high-pressure, coseismicfluid flow through the fault.

3.4. Roccacassale Fault

[26] The Roccacasale fault is part of a hundreds of meterswide fault zone crosscutting the western flank of MorroneMountain and bordering the eastern part of the Sulmonabasin [Vezzani and Ghisetti, 1998]. This basin is a NWelongated trough that developed after the late Pliocene andis the easternmost intramontane basin of central Apennines.It is filled with alluvial and lacustrine Pleistocene sediments,upper Pleistocene-Holocene fluvial sediments, and veryrecent colluvium and alluvial fans [Miccadei et al., 1998].

According to historical records [Boschi et al., 1997], therewere no significant earthquakes in this area until 1706.Since then, there have been earthquakes of intensity V(Mercalli scale) in 1789 and 1933 A.D.[27] Our investigation focused on a prominent fault scarp

above the Roccacasale village. The fault separates upperCretaceous limestones and Jurassic dolostones of the foot-wall from Quaternary fluvial, lacustrine, and alluvial depos-its of the hanging wall [Vezzani and Ghisetti, 1998]. Thestudy portion of the Roccacasale fault comprises two mainsegments, which strike 105–110 and 120–130, respectively(Figure 2b). The fault surface is polished and striated, andforms a distinctive fault scarp. The surface formed on acentimeter-thick, yellow to pink, cemented horizon that isoccasionally associated with centimeter-thick fault breccia.These cements consist of millimeter-diameter, angular tosubrounded survivor clasts embedded in light coloredcarbonate cements. Two thin horizons containing milli-meter-diameter clasts embedded in red carbonate cements,plus a few millimeter-thick veins filled with white carbo-nates, are present within this rock.[28] The fault is covered locally by cohesive, pinkish

slope scree that drapes the fault surface. Kinematic indica-tors along the fault surface are consistent with oblique slip(normal with minor right-lateral movement) on the 105–110segment, and pure dip slip on the 120–130 segment. East ofthe study area, the fault is buried beneath Quaternarycontinental deposits of the hanging wall. In the footwall,the fault zone consists of a narrow, discontinuous fault coreand a very wide damage zone. The fault core containsdecimeter-thick, clast- and matrix-supported cataclasites,and discontinuous meter-thick gouge. The cataclasites con-tain centimeter-diameter, subangular clasts in a white to lightcolored matrix. The fault gouge is made of centimeter- anddecimeter-diameter, angular to subrounded fragmentedclasts embedded in fine-grained matrix. There is one primaryset of fractures and three sets of minor faults in the core ofthe fault (Figure 2b). The damage zone is hundreds of metersthick and has two sets of fractures and two sets of minorfaults (Figure 2b). There are many fractures near these minorfaults, resulting in a very incohesive rock in outcrop.[29] The cemented horizons in the fault core are com-

posed of dolomite survivor clasts embedded in a low-Mgcalcite cement. Both dolomite and low-Mg calcite arepresent in the host rock of the footwall.

4. Stable Isotope Analyses

[30] We use data from stable isotope geochemistry toreconstruct the occurrence and origin of fluid(s) within thefault, and the movement of fluid through the fault zone.

4.1. Methodology

[31] Hand samples were selected from the outcrops in thestructural context provided by our detail mapping andstructural analysis. Most samples were cut perpendicular tothe orientation of the main fault at each locality and parallelto the inferred direction of slip. Powder samples wereobtained by microdrilling on the cut surface or by wholerock powdering. More than two hundred powder samplesobtained from 139 hand samples were analyzed by stableisotope geochemistry to determine the isotopic composition


of (1) survivor clasts, (2) uncemented matrix surrounding thesurvivor clasts, (3) nonmineralized slickensides, (4) slick-enfibers, (5) veins, (6) mineralized slickensides of slope screeand Quaternary fluvio-lacustrine deposits, and (7) cementsurrounding survivor clasts. The survivor clasts, uncementedmatrix, and nonmineralized slickensides (groups 1–3) do notcontain mesoscopically observable cement or veins, or anyother evidence for significant fluid-rock interaction. Slick-enfibers, veins, mineralized slickensides, and cements sur-rounding survivor clasts (groups 4 – 7) formed byprecipitation from a fluid. The isotopic values obtained fromthe latter set of powder samples should more closely correlatewith the isotopic value(s) of the fluid(s) in the faults.[32] The stable isotope compositions of the powders were

obtained by two different techniques, both of whichinvolved phosphoric acid digestion of the carbonates[McCrea, 1950]. The first technique involved the reactionof 10–20 mg aliquots of sample powder in a conventionalvacuum-extraction line (details are given by Kirschner et al.[2000]). The extractions were done at 50�C for 5–6 hours.At least one in-house standard was analyzed with each

series of nine samples. The extracted gas was analyzed onan isotope ratio, gas source mass spectrometer at Washing-ton University (St. Louis). The second technique involvedthe reaction of �0.5 mg of sample powder in an automatedcarbonate extraction device connected to a continuous flow,isotope ratio, gas source mass spectrometer at Saint LouisUniversity. These reactions were done at 90�C for a mini-mum of 4 hours. Approximately one standard was analyzedfor every five samples. Values of d13C and d18O for the in-house standard were calibrated relative to NBS-19. Accep-ted NBS-19 values are d13C VPDB = 1.95% and d18OVSMOW = 28.64% [Copeland et al., 1983]. For conven-tional extractions, the standard deviation is 0.05% for d13Cand 0.19% for d18O for 21 in-house standard analyses. Forautomated extractions, the standard deviation is 0.03% ford13C and 0.23% for d18O for 21 in-house standard analyses.

4.2. Results

[33] Isotopic data of each fault zone define similar arcuatetrends in d18O - d13C space with d18O values ranging from22 to 34% and d13C values from �8 to +4% (Figure 4).

Figure 4. Isotope data of samples in d18O - d13C space. Structures are subdivided into different groups;those denoted by solid symbols are relicts of the host rock (groups 1–3 in text), while open symbolsrepresent samples precipitated from a fluid (groups 4–7). Most samples in groups 1–3 have isotopicvalues typical of Mesozoic marine carbonates. Many samples in groups 4–7 have lower values,consistent with these samples having exchanged with, or precipitated from, an externally derived fluid,probably of meteoric origin.


Survivor clasts, uncemented matrix, and nonmineralizedslickensides have the highest isotopic values, similar tothe isotopic values of most Mesozoic and younger marinelimestones and dolostones in central Italy [Corfield et al.,1991; Jenkyns et al., 1994; Bartolini et al., 1996; Ghisetti etal., 2001]. Slickenfibers, veins, mineralized slickensides,and cement surrounding survivor clasts have the lowestisotopic values, consistent with isotopic exchange and/orprecipitation from a fluid in isotopic disequilibrium with thehost rock carbonates.

5. Discussion

[34] The four normal faults of this study are similar intheir dimensions, displacements, isotopic values, and iso-topic trends in d18O - d13C space. In the following dis-cussion, we assume the processes responsible for the faults’development were similar in order to develop a generaldescription and model of the faults’ structure and evolution,and the spatial-temporal variation in fluid-rock interactions.We then consider briefly the possible dynamic fluid/faultinteractions occurring at depth in these seismogenic faults.

5.1. Fault Structure

[35] Most of the fault rocks and structures in the fournormal faults can be separated into five spatial domains(Figure 5a). Within the footwall, farthest from the fault is adomain (domain 1) of moderately to highly fractured rockthat is many tens of meters thick. Although fractured,bedding planes and stratigraphy are still recognizable. Thefractures strike parallel and perpendicular to the main faultsurface (Figure 5b). Small, centimeter-thick veins arepresent locally in this domain. Domain 1 is commonlyseparated from the relatively undeformed host rock by asubsidiary normal fault that subparallels the main fault anddelimits the outer margin of the fault’s damage zone.[36] Domains 2–5 all reside in the fault’s core. Domain 2

is composed of fault gouge that is commonly 1 m thick, butmay be locally thicker in areas where two segments of thenormal fault link, intersect, or overlap. Bedding planes andstratigraphy are very disrupted and generally unrecogniz-able. The gouge contains angular to subrounded survivorclasts that are graded according to clast size in horizons thatsubparallel the main fault. In general, the clasts decrease insize toward the fault.[37] Domain 3 is a centimeter- to decimeter-thick catacla-

site. The rock is made of millimeter- to centimeter-diameter,angular to rounded survivor clasts in a finer-grained, lightcolored friable matrix. Sparse, centimeter-thick veins locallycrosscut the cataclasite. The dominant set of fractures in thisdomain strikes parallel to the main fault; a second set offractures strikes perpendicular to the main fault (Figure 5b).[38] Domain 4 contains the primary slip surface(s) of the

normal fault. This surface is usually planar, polished, andstriated with slickensides and some slickenfibers. Sense-of-shear displacement from slickensides and slickenfibers isalmost always consistent with predominantly normal, dip-slip movement. Locally, there are centimeter-thick zones ofcement-rich horizons (classified above as mineralized slick-ensides), each of which has a slickensided planar surfacefacing the hanging wall and an irregular, slightly gradationalcontact with the underlying cataclasite. We interpret these

horizons to be, or have been, the primary slip surface duringpart of the fault’s history.[39] Domain 5 contains Quaternary sediments of the

hanging wall. These sediments were deformed by slip ofthe fault, and form up to a meter(s)-thick, poorly cementedbreccia along the fault. Locally, centimeter-thick slices ofhanging wall sediments have been accreted onto the foot-wall by outward migration of the primary fault surface. Thefault trace is commonly covered by, and locally offsets,slope scree that drapes over the fault.

5.2. Proposed Evolution of Faults

[40] We propose a model for the temporal development ofthe shallow portion (�1–2 km depth) of the generalized

Figure 5. (a) Schematic cross section of fault rocks incarbonate-hosted normal fault. Fault zone contains fivedomains from fractured host rock of footwall (domain 1) tobrecciated Quaternary alluvium (domain 5). (b) Rosediagrams of subsidiary faults and fractures in plan andcross-sectional orientations for core and damage zones. Forease of comparison, structures have been rotated so that themain fault at each locality parallels the Venere-Gioia deiMarsi fault. Most structures in core zone strike parallel tofaults’ contacts, while in damage zone, most structures areeither parallel or orthogonal to the faults’ contacts.


fault zone presented above based on the current distributionof the exhumed fault rocks and mesostructures, and fewcrosscutting relations. As a working hypothesis, we assumethat deformation began in a broad zone and then localized inthe core of the fault during continued slip. In the earlieststages of deformation in late Pliocene-early Pleistocene, therocks currently exposed in the fault’s footwall began frac-turing at depth (Figure 6a). Both the footwall and hanging

wall fractured in tens of meters thick zones, which estab-lished the approximate overall width of the fault zone in thefootwall. Fractures formed parallel and perpendicular to theevolving fault contact. Some calcite coatings on minorfaults, veins, and breccias formed at this time from rock-buffered fluids circulating through the fault rocks.[41] Deformation progressively localized in the incipient

core of the fault with continued fault slip. Fracturing andcementation/veining continued in both hanging wall andfootwall (Figure 6b). By this stage, core and damage zoneswould have been established and centered about the principalfault contact. New fractures subparallel to the main faultscontinued to form primarily in the fault core. Fault displace-ment controlled thedevelopment of small intramontanebasinson the hanging wall of these emergent faults. Eventually,fluvial and lacustrine sediments in the basins were juxtaposedagainst the fault rocks of the footwall. Calcite continued toprecipitate in the fault zone from rock-buffered fluids.[42] The latter stage of deformation was characterized by

complete localization of slip at the hanging wall-footwallcontact, and the episodic outward migration of the principalfault surface (Figure 6c). Red, pink, and white low-Mgcalcite cements precipitated along the fault surface, and weresheared. The outer surface of these centimeter-thickcemented horizons (i.e., the surface facing the hanging wall)would have been the main slip surface of the fault zone forone or more slip events. Seismic release during this latefaulting was potentially responsible for the formation of thering cracks on these surfaces in that their arcuate shape isconsistent with high tensile stresses behind the trailing edgeof a contacting asperity [cf. Scholz, 1990]. Locally, themigrating fault surface crosscuts centimeter-thick slivers ofslope scree draping the fault, resulting in the accretion of thebrecciated scree onto the footwall. This occurred primarily inthe nonplanar regions of the fault surface. Two contrastingprocesses, smearing and segmentation of the fault surfaces,characterized the latter stages of deformation: smearing dueto shearing and accretion of slope scree breccia onto thefootwall, and segmentation of fault surface due to the activityof minor faults laterally offsetting the principal fault surface.

5.3. Source of Syntectonic Fluid

[43] The veins, slickenfibers, and cements attest to thepresence of fluids in the fault zone during deformation. Theprimary fluid was probably evolved meteoric water for fourreasons. First, the emergent, active normal faults have beenexhumed from shallow depth, and thus the fault rocks formedin close proximity to near-surface aquifers. Second, themineralized slickensides, slickenfibers, and veins have iso-topic values similar to the cements of the hanging wall sedi-ments, which clearly precipitated from a fluid of meteoricorigin.Third,meteoricwater isoneof themore common fluidsin the earth’s crust with sufficiently low d18O and d13C valuesto account for the low isotopic values of calcite precipitated atlow temperatures. Fourth, strontium concentrations of themineralized slickensides, slickenfibers, veins, and cementsare much lower than the carbonate host rocks (Table 1),consistent with their precipitation from a low-salinity fluid[Banner, 1995]. Although not definitive, the incursion ofdeeply sourced CO2-bearing fluids (e.g., from mantle, mag-mas, or metamorphic decarbonation of limestones) into theupper crustal levels of the normal faults is not supported by the

Figure 6. Cartoon showing evolution of normal faults. (a)Carbonate host rock fractured in tens of meters to hundred-meter-thick zone during initiation of faulting. Fracturesformed parallel and perpendicular to eventual principalfault. (b) Deformation localized around main fault duringprogressive displacement and exhumation, resulting information of core and damage zones. Small, continentalbasins developed on fault’s hanging wall. (c) Quaternarycontinental alluvium eventually juxtaposed against faultrocks of footwall, resulting in complete localization of slip.The principal slip surface migrated into hanging wall due toformation of cement-dominated horizons and accretion ofhanging wall slivers.


data. This interpretation is consistent with the analysesof Chiodini et al. [1999] for CO2 fluxes in regional aquifersand Quattrocchi et al. [2000] geochemical data of a thermalspring in the eastern part of central Apennines.[44] Assuming constant isotope composition of the mete-

oric water that entered the faults, then the relative positionof individual datum in the d18O - d13C arcuate trend is due tovariations in the partly evolved nature of the water and/orquantity of water that formed or exchanged with eachsample (Figure 4). Those samples with isotopic valuessimilar to the carbonate host rocks either interacted withsmall quantities of meteoric water or precipitated from ameteoric water that had previously equilibrated with thehost rock. Conversely, those samples with lower isotopicvalues either interacted with larger quantities of meteoricwater or precipitated from meteoric water that had partlyretained its original isotopic values. It is not possible withthe data to eliminate any of these possibilities.

5.4. Paleohydrogeology

[45] Fault rocks collected at different distance from theprincipal fault surfaces are characterized by different stableisotope values (Figure 7). In domains 1, 2, and 3, all samplesincluding cements and veins have d18O values between 26and 32% and d13C of �2 and +2%, consistent either with arock-dominated isotopic system or fluid-rock interactionwith fluid that had equilibrated with the host rocks prior toentering the investigated segments of the fault zones. Incontrast, most mineralized slickensides, slickenfibers, andveins from domains 4 and 5 have isotopic values that are up to6% lower, similar to values of the cements in the Quaternarysediments and consistent with a fluid-dominated isotopicsystem. The lower isotopic values of domains 4 and 5 relativeto domains 1–3 preclude large quantities of fluid havingpervasively entered the faults from the footwall side. The dataare most consistent with meteoric water having entered thefaults either from the hanging wall side and/or channeled intothe faults directly at the land surface [cf. Antonellini andAydin, 1995]. Both flow paths are compatible with the data.[46] The differences in isotopic values between domains

1–3 and domains 4 and 5 are consistent with compartmen-talization of meteoric water in the fault zone. Meteoric waterwas able to move along the fault surface (domains 4 and 5)and into/out of the hanging wall, but not move laterally intoadjacent footwall (domains 1–3). We propose that the faultgouge and cataclasites of domains 2 and 3 were the primarybarriers to fluid flow [cf. Zhang and Tullis, 1998] and thatthe fractured carbonates in domain 1 might have accom-modated fluid flow albeit in a closed, isotopically rock-buffered system. Such compartmentalization of fluidswould be a variant of the combined conduit barrier perme-ability structure discussed by Caine et al. [1996] and Caineand Forster [1999]. In contrast to the other three moremature faults, the Monte Capo di Serre fault does not havemuch gouge and cataclasites (domains 2 and 3) and thuseffectively forms only one fluid conduit, which would beclassified as a localized conduit permeability structure ofCaine et al. [1996] and Caine and Forster [1999].

5.5. Dynamic Fluid/Faults Interactions

[47] Data from paleoseismic studies [Giraudi, 1994; Mic-chetti et al., 1996; Pantosti et al., 1996] and historical

seismicity are consistent with these faults having beenseismically active during exhumation [Armijo et al., 1985;Gasparini et al., 1985; Galadini et al., 1995; Basili andValensise, 1991]. These west dipping, high-angle normalfaults merge asymptotically with west dipping, low-angledetachment surfaces at depths of 10–15 km [Bally et al.,1988; Amato et al., 1998]. The zone of earthquake nucleationis localized at these depths, in the region where the Tyr-rhenian crust overrides the subducting Adriatic plate (focalvolume region of Figure 1a). There are potentially deep-seated fluids at these depths due to dehydration reactionsoccurringwithin Triassic evaporites and Paleozoic crystallinerocks of the Tyrrhenian basement [Quattrocchi, 1999]. Thefluids are thought to play an important role in the earthquakenucleation/rupture in central Italy [cf. Cello, 2000]. Becauseof the load-weakening character of normal faults after

Figure 7. The d18O and d13C data of fault rocks relative totheir position in fault zones (compare Figure 5). Domain‘‘Q’’ represents data from Quaternary sediments on hangingwalls. A clear isotopic jump occurs between domains 3 and4, consistent with a rock-buffered system in domains 1–3,and potentially fluid-buffered (open) system in domains 4and 5. Low isotopic values are consistent with meteoricfluid that entered faults either from hanging wall and/ordirectly at land surface. Symbols are identical to those inFigure 4.


coseismic slip, models predict dilation and vertical fracturingof faults just after failure, subsequent incursion of fluids intothe faults with episodes of fluid overpressure development,and further seismicity [e.g., Quattrocchi, 1999].[48] Occurrence of coseismic fluid flow is thought to take

place along the study faults, consistent with the centimeter-thick flame structures in the Monte Capo di Serre fault core.At least at shallow levels of 1–2 km, slip localized on thesefaults during their progressive exhumation, concurrent withthe flow of meteoric water along the main fault contacts.There is no isotopic evidence of deep-seated fluids havingpassed these faults at shallow levels. Consequently, thosemodels that envision deep-seated fluids passing up throughshallow levels of these seismogenic faults are not supportedby this study. If there are deep-seated fluids passing upthrough the faults, then the fluids are either being divertedfrom the fault or isotopically equilibrated with the Meso-Cenozoic carbonates before they reach 1–2 km depth.[49] The corrugated fault surfaces observed in this study

potentially form channels for fluid flow at shallow depths.These conduits are oriented both parallel and perpendicularto the faults’ strikes along the hanging wall-footwall con-tacts. The along-strike hydraulic communication is pre-dicted to occur at depth within normal faults [Sibson,2000]. The observation of multiple aftershock sequenceprogressively extending along strike after the 1997–1998Umbria-Marche sequence in central Italy [Amato et al.,1998], and experimental results obtained by Evans et al.[1997] also support this prediction.

6. Conclusions

[50] Internal structures of four carbonate-hosted normalfaults in central Italy are similar and can be separated intofive structural domains. Farthest from the main fault contactin the footwall is undamaged host rock and a zone offractured host rock (domain 1). The fractures in domain 1are both parallel and orthogonal to the main fault contact.Progressively nearer to the fault contact is gouge (domain 2),cataclasites (domain 3), and cement-dominated horizonswith polished, slickensided fault surfaces (domain 4). Anarrow breccia horizon (domain 5) of hanging wall sedi-ments is present locally next to domain 4. Small slivers ofthis breccia had been accreted onto the footwall, resulting inlaterally migration of the main fault contact toward thehanging wall. Fractures in the core of the fault (domains2–5) are primarily parallel to the main fault contact. Ingeneral, fracture density increases toward the main faultcontact.[51] Stable isotope data from the four faults form an

arcuate trend in d18O - d13C space, consistent with variablefluid-rock interaction and compartmentalization of fluid inthe fault domains. The relatively high d13C and d18O valuesof host rocks, fault rocks, veins, and cements from domains1 and 3 are consistent with little to no isotopic exchangewith a fluid in isotopic disequilibrium with the host rocks.In contrast, the relatively low d13C and d18O values of thecemented horizons in domain 4 and brecciated hanging wallof domain 5 are consistent with meteoric water havinginfiltrated and isotopically exchanged with these fault rocks.The meteoric water entered fault domains 4 and 5 eitherlaterally from the hanging wall sediments or longitudinally

down from the land surface, and were prevented frompassing laterally into the footwall carbonates by the gougeand cataclastic barriers of domains 2 and 3.[52] Two of the four study faults have produced large,

historic earthquakes; the other two are presumed to havebeen seismogenic due to their proximity and similaritieswith seismogenic faults. At least at shallow levels of 1–2km, displacement on these seismogenic faults has been verylocalized. Meteoric fluids have flowed along the main faultcontacts. There is no isotopic evidence of deep-seated fluidshaving passed through these faults. Consequently, anymodel of earthquake nucleation/rupture that envisions largequantities of deep-seated fluids passing up through shallowlevels of seismogenic faults is not supported by our data.

[53] Acknowledgments. We thank F. Ghisetti and L. Vezzani for theirhelp during the fieldwork and input during the writing of an earlier versionof the manuscript. We thank R. Criss and D. Kremser of WashingtonUniversity (St. Louis, Missouri) for providing us access to their labs.Fieldwork and laboratory expenses were partly supported by Sigma XI toAgosta and American Chemical Society (Petroleum Research Fund 31943-GB2) to Kirschner. We gratefully acknowledge an equipment grant fromNational Science Foundation (DUE 9952256) to Kirschner that supportedthe stable isotope laboratory at Saint Louis University. The reviews of R.Bruhn, an anonymous reviewer, Rudi Wenk (JGR Associate Editor), andAndreas Mulch are much appreciated.

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��F. Agosta, Department of Geological and Environmental Sciences,

Stanford University, 450 Serra Mall, Stanford, CA 94305-2115, USA.([email protected])D. L. Kirschner, Department of Earth and Atmospheric Sciences, Saint

Louis University, 3507 Laclede Ave., Saint Louis, MO 63103, USA.([email protected])


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Agosta Kirschner 2003