Jun 10, 2000 - Francesco Italiano, 1 Mauro Martelli, 2 Giovanni Martmelli, 3 and P.Mario ...... Hooker, P.J., R. Bertrami, S. Lombardi, R.K. O'Nions, and E.R..
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B6, PAGES 13,569-13,578, JUNE 10, 2000
Geochemicalevidenceof melt intrusionsalong lithosphericfaults of the SouthernApennines,Italy' Geodynamic and seismogenicimplications FrancescoItaliano,1Mauro Martelli, 2Giovanni Martmelli, 3andP.Mario Nuccio •'2 Abstract. Severalgasemissions, distributedalong200 km of the Southern Apennineaxial zone,havebeeninvestigated. Thisportionof the sedimentary chain,whichconstitutes the accretionary prismof thewestwardsubduction of theAdriaticplate,is subjected to a tensile stressfield responsible of high-magnitude earthquakes. The studiedemissions aregenerally
CO2-dommated, have3He/4He ratios intherange of0.09-2.84 Ra,anddisplay both3Heand 4Heoutputs intherangeofthose reported fortheItalianactive volcanoes Phlegrean Fieldsand Vulcano.The heliumisotoperatios,togetherwith the amountof releasedgas,indicatethat a hugeamountof mantle-derived heliumis releasedoverall the investigated area.Our geochemical dataandthegeophysical peculiarities of theregion,suchasthehighheatflow and thelow resistivityof rocksbelowa depthof 15 km, coherentlyindicatethepresenceof melt intrudedintothe crustalonglithospheric faults.Magmaintrusions alongthe axial part of the Southern Apenninesedimentary chainarecertainlyunusual,sincevolcanismin centraland southern Italy is concentrated alongtheperi-Tyrrhenianmarginof thepeninsula.We suggest thatmeltis promotedby adiabaticupriseof the asthenosphere in themantlewedgebeneaththe Apenninesandit is subsequently intrudedalongtensilelithospheric faults,stronglyaffecting thethermalstateof thelowercrust.An interrelationship betweent•hegeodynamics, themelt intrusions, andthegenesisof thelargeearthquakes (uptoM= 7.1) in theregionis suggested. 1. Introduction
usedasa geochemical tracer.In fact,mantleheliumis easily
The Apenmnesrepresentthe accretionaryprism of the westwardsubduction of the Adriaticplate.The southernpart of this chainis oneof the mostactiveseismiczonesof Italy, andduringthe lastfew centuries it hasbeenaffectedby large earthquakes (e.g., 1694,M= 6.9; 1857,M= 7.0; 1915,M= 7.1; 1962, M = 6.1; 1980, M- 6.7, [Boschiet al., 1995; Nostro et al., 1998]). In particular, the 1980 Irpinia earthquakewas the most destructiveeventoccurringin the region for over a century,killing more than 3000 people [WestawayandJackson,1984]. Several gas manifestationsoccur all over the Italian Apennines.As they are mostly characterizedby CO2dominantgas, differenthypotheses abouttheir origin have been proposed: carbonate hydrolysis, carbonate thermometamorphism, mantle outgassing, and mixing betweenmantleandcrustalCO2 (seeDoglioni et al. [1996] for a review).Thereforea conclusive answeraboutthe origin of the emittedgaseshasnotyet beengiven. In the studyof thepossiblesources of the gasesrisingfrom theEarth'sinterior,theheliumisotopicratiohasbeenwidely
3 ordersof magnitudecomparedto the radiogenichelium producedin continentalcrust.Continentalloss of mantle helium is closely associated with tectonic activity, particularly in thoseregionsundergoing extension [O Wions and Oxburgh, 1988]. Crustal fluids of the European continentoftencontainmantle-derivedgases,suchas those from the Pannonian basin (Hungary), Vienna basin [Sherwood Lollar et al., 1994], and Slovakia[Polyaket al.,
distinguishable fromcrustal helium, being enriched in3Heby
1985], displaying 3He/nile isotope ratios from0.2to2.9times theatmospheric ratio(Ra= 1.39x 10-6). Heliumisotope ratiosin Italy havebeeninvestigated by variousresearchers.
Hooker etal.[1985]observed thatthedistribution of•I--Ie/4He in Italy failsto reveala simplerelationship with the surface geologyandthatthepresence of mantleheliumis not always obviouslyassociated with surfacevolcanicactivity.Sano et al. [1989] suggested that the distributionof helium isotope ratios in southernItaly is compatiblewith both the subduction of a platealongthe Sicilian-Calabrian forearcor the upwellingof the mantlebeneaththe TyrrhenianBathyal Plain.
In this paper,we reportthe resultsof a studycarriedout on gaseous emissions occurringalongthe nearlyaxial zone of the Southern Apennines(Figure1). As the Apenninesare Xlstituto di Geochimica dei Fluidi,Consiglio Nazionaledelle an exampleof the few outcropping beltsassociated with west Ricerche,Palermo,Italy zones(mostof themarebelowsealevel), 2Dipartimento di Chimicae FisicadellaTerraed Applicazioni directedsubduction they may provide significant information on similar (C:F.T.A.),Universityof Palermo,Palermo,Italy 3ServizioSistemi Informativi Geografici,Regione Emilia inaccessible geodynamic settingswhich are commonin the Romagna,Bologne,Italy westernPacific.Our geochemical investigations, including gaschemistry,gas fluxes,and helium and carbonisotopes, Copyright2000 by the AmericanGeophysical Union. are mainly aimed at better def'mmgthe crust-mantle Papernumber2000JB900047. 0148-0227/00/2000
JB900047 $09.00
interactionsin this region and their possiblerelationswith seisrnogenic processes. 13,569
13,570
ITALIANO ET AL.' MELT INTRUSIONS ALONG SOUTHERN APENNINES
subduction • TYRR.HENIAN SEA
0 I
50 km I
I
'•
OUTER APULIA •,• PLATFORM
FLYSH
APENNINE
PLATFORM
VOLCANIC AREAS
Figure1. Geological scheme of theinvestigated area(modifiedfromSellaet al. [ 1988])andlocationof the sampled sites:1,Termedi Agrippa;2, BagnideMasi;3, Telese; 4, Mefitinielle; 5, Mefited'Ansanto; 6, San Sisto;7, Contursi; 8, Tramutola; 9, SanCataldo;ORL, Ortona-Roccamonfina Line [afterPataccaet al., 1993].
seismicity ratewith a highermagnitude (M- 7) [Coccoet al., 1993]. Presently,the SouthernApenninechain is The Italian Apennines are composed of Neogeneand experiencing a NE-SW orientedstretching, whichcauses a Quaternarysedimentary thrust sheets,movingover the widespread NW-SE normalfaulting[AmatoandMontone, Adriaticplate.Thelatter,according to bothseismicity and 1997; Ciaranfi et al., 1983]. As a result, severalPliotomography, hasbeensubducting westwardat leastsincethe Pleistocenegrabenscrosscutthe Apennine thrust pile earlyMiocene, generating boththeaccretionary wedgeof the [Hippolyteet al., 1994;Doglioniet al., 1996].Largeseismic
2. GeodynamicSetting
Apenninesand the internal Tyrrhenianback arc basin eventsoccuralongnormalfault segmentswhich extendfor [Doglioniet al., 1994,andreferences therein]. 40-50 km in theNW-SE direction, parallelto theApennine The whole Apenninesubductionsystemshows an belt [Chiarabbaand Amato, 1997; Nostro et al., 1998]. eastward rollback motion,at a rateof 1-7 cm yr'] as a Crustalthicknessdisplaysa maximumvalueof •--30-35 km consequence of eastward asthenospheric flow pushingthe beneaththe axis of the SouthernApennine chain and subducting Adriaticslab[Doglioni,1991'Doglioniet al., decreases to 10-15km in theTyrrhenian area[Ciaranfiet al.,
1994].Furthermore, theslopeoftheslabisenhanced bothby 1983;Scarasciaet al., 1994]. the weightof the slab itself and by the asthenospheric horizontal pushes [Doglioniet al., 1994]. Methods As a resultof thepost-Tortonian evolution, theApennine chainis currently splitintotwo majorarcuatefeatures' the We collectedthe gaseoussamplesinto glasscontainers northern and the southern Apenninearcs.Theymergein with twovacuumvalves.Gaseshavebeenanalyzedfor their centralItaly along a marked feature, called the Ortona- chemical composition andcarbonandheliumisotopic ratios. Roccamonfina Line (ORL, Figure 1), which has been Chemicalanalysesof He, N2, CH4, and CO2 were carriedout interpreted as a deepdiscontinuity separating lithosphericwith a PerkinElmer8500 gaschromatograph equipped with sectors characterized by differentfeatures[Mantovaniet al., a 4-m Carbosieve 5A columnand doubledetector(Flame 1993' Patacca et al., 1993]. One of the main differences Ionisation Detectorwith methanizer andHot Wire Detector). betweenthesetwo arcs is their seismic character:the former The detection limitsare2 ppmvfor He andN2 and 1 ppmv is characterized by a diffusehigh seismicity rate,rarely for CH4 and CO2.Analyticalerrorsare + 5% for He and + exceeding magnitude5, and the latter displaysa lower 3% for the otherspecies.Isotopicanalysesof CO2 carbon
ITALIANO ET AL.: MELT INTRUSIONS ALONG SOUTHE•
APENNINES
13,571
Table 1. ChemicalComposition of the SampledGasEmissions. Site
Date
He
N2
CH4
CO2
Bagni de Masi
June26, 1998
254
24.3
1.9
73.8
Termedi Agrippa
Oct. 28, 1998
22
1.9
0.02
98.1
Telese
May 23, 1998
13
1.5
0.1
98.4
Telese
June 30, 1998
12
3.6
0.1
96.3
Telese
July30, 1998
13
1.5
0.1
98.4
July 1, 1996
9
2.4
0.4
97.2
Mefite d'Ansanto
Mefitinielle
June29, 1996
8
1.3
0.2
98.5
Mefite d'Ansanto
Sept.29, 1996
9
1.4
0.2
98.4
San Sisto
June30, 1996
22
2.0
0.3
97.7
Contursi
May 23, 1998
5
0.5
0.1
99.5
Tramutola
April 8, 1996
108
46.5
50.8
2.7
Tramutola
April 15, 1996
259
16.9
82.0
1.1
Tramutola
May 18, 1996
186
18.2
80.7
1.1
SanCataldo
May 6, 1996
178
77.3
21.0
1.7
Values are correctedfor air contaminationby eliminatingoxygenalong with the corresponding amountof nitrogenfoundin the atmosphere. N2, CH4, and CO2 are expressed in vol %. Helium is in ppm by volume
origin (02 < 0.1%). Gasesfrom wereperformedusinga FinniganDelta-Smassspectrometer. (23%), of nonatmospheric Theresultingisotopicvaluesareexpressed in g %0versusPee Tramutolaand San Cataldo are methaneand nitrogenDee Belemrote,and the uncertaintyof each measure is dominated,respectively, with a minor CO2 content(N1%). +0.1%o. In order tomeasure 3He/4He ratio,eachgassample Helium concentrationusually ranges between 5 and 30 was introducedinto a metallic vacuum line and purified ppmv,while the highestconcentrations (200-300ppm) have using two charcoaltraps held at 77 K and two getter been measuredin samplesfrom Bagni de Masi and
adsorbers, onehotandtheothercold.The4He/2øNe ratiowas
Tramutola.
measured with a quadrupole massspectrometer. Helium was Thefi•3Cco2 values of theinvestigated emissions range completelyseparated fromneonby a cryogeniccharcoaltrap between+0.3 and-3.6%0versusPDB, partiallyoverlapping held at 40 K and transferredinto a high-precisionmass thatreported fortheApulianBasingases(-3 to -4 %o),which spectrometer (modifiedVG5400, VG Isotopes).Ion beamsof were interpretedas the result of metamorphicreactionsof 3He+and4He + werecontemporarily detected by a double volcanicCO: with limestone[Ricchiutoand Schoell, 1988]. collectorsystemwhich keeps the error on isotopicratios The only exceptionconcernsSan Cataldo gas, displaying within 0.5%. Purified atmospherichelium was used as a g•3Cco: =-14.1%o, where uponconsidering boththelowgas runningstandard. flux and the CO2 content(N 1%), an organicorigin can be Gas outputat the San Sistodry emissionwas calculated implied. consideringthe emissionsurfaceand by measuringthe In Table 2 the heliumisotopecomposition, togetherwith velocityof the escapinggasby meansof a cupanemometer. the4He?øNe ratio,is listed.Thegenerally low valuesof On the other emissions, output measurementswere 4He/2øNe testifyto the low atmospheric contamination of performedon bubblinggasby usinga stainlesssteelfunnel samples. to dram the gas into a connectedoverturnablebottle of Althoughthe investigated gasemissionsare locatedalong knownvolumeand measuringthe time requiredto displace the sedimentaryApennine chain, the measuredhelium waterout (uncertainty+ 10 %). isotoperatios(Table2), rangingbetween0.09 and 2.84 Ra, are abovethe valuestypicallyexpectedfrom the continental crust(-• 0.02 Ra, [O •Vionsand Oxburgh, 1983]). We have
found thehighest 3He/4He ratiosoftheinvestigated region at 4. Results and Discussion
4.1. Chemicaland IsotopicComposition The chemicalcompositionof the sampled gases is reportedin Table 1. Usually,gasesare CO:-dominated,with minoramountof N2, CH4, H2, CO, and He. Bagni de Masi gas is also CO2-dominated but displaysa high N2 content
the Mefite d'Ansantodegassingarea (Figure 1), both in gas vents (2.60-2.84 Ra) and in the drilled well Pozzo Mefite (2.79 Ra). Hence regardlessof their gas composition,the studiedemissionsare affectedby a contributionof mantlederivedhelium. Accordingto O •Vionsand Oxburgh [1988, p. 345], mantlehelium"is thebesttracerfor mantlevolatiles currentlyavailable and its distributionprovidesa crude
13,572
ITALIANO
ET AL.: MELT INTRUSIONS
ALONG
SOUTHERN
APENNINES
Table2. 4He/2øNe Ratioand3He/4He Isotope Composition oftheExamined GasEmissions. Study site
Date
4He/aøNe (R/Ra){3
Bagni de Masi
June26, 1998
270
0.86 + 0.01
Termedi Agrippa
Oct. 28, 1998
12
0.69 + 0.01
Telese
May 23, 1998
6
1.73 + 0.02
Telese
June 30, 1998
38
1.73 + 0.02
Telese
July30, 1998
56
1.76 + 0.04
Mefitinielle
July 1, 1996
46
2.35 + 0.02
Mefite d'Ansanto
June29, 1996
20
2.77 + 0.02
Mefite d'Ansanto
Sept.29, 1996
6
2.84 + 0.02
San Sisto
April 17, 1996
41
1.37 + 0.01
San Sisto
June30, 1996
5
1.33 + 0.02
San Sisto
June27, 1998
98
1.21 + 0.03
Contursi
May 23, 1998
28
1.35 + 0.05
Contursi
June 27, 1998
12
1.43 + 0.04
Tramutola
April 8, 1996
7
1.19 + 0.02
Tramutola
April 15, 1996
110
1.11 + 0.02
Tramutola
May 18, 1996
123
1.17 + 0.02
SanCataldo
May 6, 1996
12
0.09 + 0.01
SanCataldo
April 1, 1999
19
0.07 + 0.01
April 1982
0.6 - 16 a
0.95 - 1.77
813Cc02 %0versus PDB 0.3
-1.9
0
-3.6
-14.1
Active volcanic areas:
Napoli(geothermal well)
2.2- 2.7 b
Vesuvius(fluidsinclusions)
Vesuvius (fumaroles)
1987-1991
Phlegrean Fields
1983-1993
IschiaIsland
1985-1993
2.2- 2.7b 330a
2- 3.2b.a,e 3.56 e
0c -1.5 -2.5 to -5 f
The3He/4He isotope ratiois expressed asR/Ra= (3He/4He)ob•/ (3He/4He)•n andis corrected from atmosphericcontamination.He isotopevalues at Mefite d'Ansanto are slightly higher than those
previouslyreported(2.57+2.74Ra [Hookeret al., 1985; Balderer and Martinelli, 1995]). Data for emissions of the Vesuvius,Ischia,andPhlegrean Fieldsvolcanicareasarealsoreportedfor comparison. •Hookeret al. [1985].
bGraham etal. [1993]. CAllard et al. [1997].
aSano etal.,[1989]. •this work.
f!nguaggiato andPecoraino [1998].
ITALIANO
ET AL.' MELT INTRUSIONS
ALONG SOUTHERN APENNINES
13,573
Table1. TotalGas,3He,and4HeOutputs FromtheMainExamined Emissions andFromTwo Italian Active Volcanic Areas.
Site
TotalGasOutput, m3s-1 Heppmv 3He/nile 4HeOutput, atoms s-13HeOutput, atoms s'•
Mefite d'Ansanto
5
8
3.95E-06
9.9E+20
3.9E+ 15
0.35
22
1.90E-06
1.9E+20
3.6E+ 14
Telese
0.3
15
2.45E-06
1.1E+20
2.7E+14
Tramutola
0.2
250
1.65E-06
1.2E+21
2.0E+ 15
PhlegreanFields
a
a
3.50E-06
5.6E+19
1.9E+14
Vulcano
a
a
8.40E-06
1.5E+21
1.3E+ 16
San Sisto
aHeliumoutputestimations for Phlegrean Fieldsand Vulcanoare performedby scalingheliumisotoperatios(this work) with steamoutput[Italiano et al, 1984a,b] and heliumconcentration [Bolognesiet al., 1986;Badalamentiet al., 1991]. Read3.95E-06 as 3.95 x 10-6.
mappingof regionswhere basalt melts are enteringthe order of magnitudeof those from Mefite d'Ansanto.As continental lithosphere." shown in Table3, both3Heand4Heoutputs fromalmost all Fluids emitted from the close active volcanoes or included
in recentlavasare characterized by heliumisotopicratiosup
the investigated SouthernApenninegas emissionsrange between those from active volcanic areas of the Island of
to3.5Ra(Table2) and•3Cco2 values of0 to- 1.5%o versusVulcanoandthePhlegreanFields. PDB[Allardetal., 1997],except forIschia gases (313Cco2 =The studiedgas emissionsare renownsincea very long 2.5 to-5%0 [Inguaggiato and Pecoraino, 1998]). These time ago (someof them sinceRomanage), and historical values,considerably far from typicalmid oceanridgebasalt manuscripts report short but strongincreasesof the gas signatures (R/Ra= 8 [Lupton, 1983];313Cco2 = - 4 to- 8%0 output[Sinno,1969].Duringthe studyperiod(~ 4 years)the [davoy and Pineau, 1991]), have been interpretedas a measured fluxesdid not changesignificantly with the time. consequence of the possiblecontaminationof the magmatic Theseevidencesshowthat the observedgas fluxes are not source[Inguaggiatoand Pecoraino, 1998, and references relatedto quick dischargeof gas accumulated in shallow therein].As shown,the investigatedgas emissionsdisplay reservoirs; on the contrary,they are specificand permanent both helium and carbonisotopecompositions in the same properties of theregion. range as those shown by the adjacentactive volcanoes, 4.3. Source of the Collected Gases possiblysuggesting a similaroriginof gases. Thefew volcanicrocksfoundin the studyregionarethose Thetotalheliumflux fromthe investigated emissions is ~ of Tertiaryage crossedby drillings[MostardiniandMerlini, 7 ordersof magnitudelargerthanthe averageHe flux from 1986], and the probablyallochthonous diabaseof Frigento continentalareas[O'Nions and Oxburgh, 1983]. Hencethe [Sinno, 1969]. Hence the only potential sign of a recent heliumdegassing from the continentalcrustcontributes only magmaticactivityis the detectionby aeromagnetic surveyof a very small extentto the observedfluxes.In addition,the someburiedbodies[Cassanoet al., 1986]; however,herewe measured •rte/nHe isotope ratios in sampled gases indicate an are not consideringthem as evidenceof recentintrusions, appreciable mantle-Hecontribution.We want to stressthat theiragebeingunreliable. the average mantle-He flux from continentalcrust is 4.2.
Gas Fluxes
A diffuseandfrequentlyintensedegassing occursall over the investigated area,for • 200 km alongthe axial zoneof the Southern Apennines [Sinno, 1969; Balderer and Martinelli, 1995, and references therein],and particularlyin theIrpiniaregion,wherean anomalousexhalingareaseveral squarekilometerswide hasbeenidentified.The mostintense outgassing zoneis the gas-bubbling muddypondof Mefite
extremely low(oftheorder of 103atoms cm'2S'l) [O'Nions and Oxburgh,1983].Even thoughin the tensilefault region
ofthePannonian basinit reaches 105-106 atomscm-2s-1,this relativelyhighflux was interpreted as dueto the presence of magmaintrusionsat depth [Martel et al., 1989]. In fact, besidesthe mantleitself,mantle-Hecanbe suppliedonlyby magmaticbodies.Also in Japan,mantle-Hefluxes of the orderof 3 x 10lø atomscm-2s-1 from areasof ~50 x 30 m alignedalong a fault zone, were attributedto intracrustal
d'Ansanto, wherewemeasured a gasoutput of ~ 5 m3 s-I magmaticbodies[Wakitaet al., 1978]. The latterfluxesare from the 100 x 50 m wide area.The averagegas flux from
thisareaisthen4.1x 10-6molcm'2s'l, whiletheheliumflux
still 100 times lower than the fluxes we measured.
Darcy'slaw may accountfor the generallylow mantle-gas
is 1013 atoms cm'2s'l. Assuming a typicalMORBhelium fluxesobservedat the surface.Accordingto this law the isotoperatioof 8 Ra, themantle-Heflux at Mefite d'Ansanto magnitudeof fluid flow q is a functionof the intrinsic is 1012atomscm'2 s'l. The 3He and 4He fluxes from the San permeability k of the geologicmedium,the fluid viscosity/•, P and gravitationalenergygradientsacting Sisto,Telese,andTramutolagasemissions resultin the same andthe pressure
13,574
ITALIANO
ET AL.' MELT INTRUSIONS
ALONG SOUTHERN APENNINES
Figure 2. Heat flow map (modifiedfromMongelli et at, [1996] andBarbier et at, [1998]) andlocationof
thesampled sites. Isolines arein mWm-2.A andB aretheextreme points of theFigure3 profile. The CROP04Agropoli-Barletta profile(dashedX-Y line)is alsoshown.Numbersareasin Figure1.
on a unit volume of fluid. Thus
lithosphericfaults, since they have extensionalslip rates
ranging from~1 to ~3 mmyr4 [Chiarabba andAmato, 1997].
This suggestion is also supportedby somegeophysical anomaliespresent in the study region. Magnetotelluric investigations,carried out along the CROP04 (AgropoliBarletta)profile(Figure2), detectedextremelylow resistivity where p is the fluid density,g is the gravitational values(~5 • m) of the rocksbelow 15 km [Loddoet al., acceleration,z is the elevationabove a datum, and •P + 1995;Marsella et al., 1998].Thesewereinterpreted to be the pgz)/c3c is the energygradientfor fluid flow alongthe flow consequence of a risein theisotherms affectingdeepresistive pathx. Numericalexperiments on a wide rangeof geologic structures, whichnormallydisplaya resistivityof~ 10000• environments showthat permeabilityis usuallythe primary m [Loddoet al., 1995' Marsella et al., 1998]. It shouldbe control on fluid flux. This is because the measured intrinsic notedthat in the TibetanPlateau,wherethe presence of melt permeabilityof commongeologicmedia varies by many intrusions hasbeenproposed[Phamet al., 1986;Yokoyama ordersof magnitude, whilefluidviscosity andtypicaldriving et al., 1999], an analogousconductive(
