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The role of transfer structures on volcanic activity at Campi Flegrei (Southern Italy)
Journal of Volcanology and Geothermal Research 91 Ž1999. 123–139 www.elsevier.comrlocaterjvolgeores
The role of transfer structures on volcanic activity at Campi Flegrei žSouthern Italy / V. Acocella ) , F. Salvini, R. Funiciello, C. Faccenna Dipartimento Scienze Geologiche, UniÕersita´ degli Studi ‘‘Roma Tre’’, Largo San Leonardo Murialdo, 1, 00146 Rome, Italy
Abstract The Tyrrhenian margin of Central Italy underwent extension during Pliocene and Quaternary. Extension occurred mainly through NW–SE normal faults, bordering a sequence of Plio-Quaternary basins. These basins are offset by coeval NE–SW faults, which show strike–slip and normal motions and have been interpreted as transfer faults. Plio-Quaternary volcanic activity along the margin occurred along a NW–SE belt, systematically in correspondence with NE–SW transverse systems. The Campi Flegrei Volcanic District ŽCFVD., on the Southern Tyrrhenian margin, consists of an active NE–SW volcanic ridge developed along NE–SW fractures. We performed a structural field analysis with analogue and mechanical models to investigate the role of transverse structures upon volcanism at Campi Flegrei. Field analysis at Campi Flegrei recognized NE–SW and, to a lesser extent, coeval NW–SE active fractures. Analogue experiments have simulated the development of transfer fault systems in brittle extensional domains. The experiments show that subvertical transfer faults connect offset adjacent normal faults dipping 608. The mechanical model is based on the stress equations in uniaxial lithostatic conditions and absence of regional stresses. It shows how pre-existing subvertical fractures require the smallest magmatic pressures to be penetrated. For a given magmatic pressure, subvertical fractures might be penetrated more deeply, tapping more easily primitive magmas. These results suggest that the CFVD is located along a NE–SW transfer zone connecting NW–SE regional normal faults. Volcanic activity along such NE–SW trend would be induced by the subvertical dip of the transfer faults. The subvertical dip of transfer faults also suggests an explanation for the emission of the more primitive products along NE–SW systems at Campi Flegrei. These considerations find a wider application on the remaining volcanic districts of the margin, located in the same overall structural setting. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Campi Flegrei; transfer faults; sand-box modelling; permeability
1. Introduction A major phase of extension has been affecting the Tyrrhenian margin of Central Italy, between the Apenninic belt and the Tyrrhenian Sea, mainly from Upper Pliocene to Quaternary times. Extensional tectonics shifted over time, towards the Apenninic )
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belt and the Southern Tyrrhenian Sea. Volcanic activity, which shows a similar SE-migrating evolution pattern, has been following extension on the Tyrrhenian margin since upper Pliocene. Extension occurred mainly along NW–SE normal faults and, to a lesser degree, along coeval NE–SW transverse systems ŽFig. 1; Locardi et al., 1976; Wise et al., 1985; Mariani and Prato, 1988; Faccenna et al., 1994.. NW–SE faults predominantly show a normal throw and are responsible for the largest
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Fig. 1. Structural sketch of the Tyrrhenian margin of the Central Italy Peninsula, displaying the Plio-Quaternary volcanic districts.
amount of extension on the margin. These systems border a sequence of NW–SE Plio-Quaternary basins, defining the overall structural setting of the Tyrrhenian margin.
NE–SW fracture systems are mainly located on the western side of the volcanoes ŽFig. 1., interrupting the continuity of adjacent NW–SE extensional basins. The distribution of NE–SW transverse sys-
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tems indicates their secondary role in the extensional processes along the Tyrrhenian margin. NE–SW faults usually display a complex kinematics, often related to the superimposition of normal motions on previous oblique and strike–slip motions; when the normal motion is predominant, NE–SW faults usually border Plio-Quaternary transverse basins ŽFig. 1.. The coeval activity of NW–SE and NE–SW fractures has been recognized in several sedimentary and volcano-tectonic basins on the northern, central and southern Tyrrhenian margin ŽMariani and Prato, 1988; Liotta, 1991; Faccenna et al., 1994; De Rita and Giordano, 1996; Orsi et al., 1996; Acocella and Funiciello, 1999.. NE–SW fractures, due to their kinematics Žnormal to strike–slip component of motion. and age Žcoeval to NW–SE systems., have been interpreted as transfer systems of NW–SE normal faults under the same stress field ŽLiotta, 1991; Faccenna et al., 1994.. Transfer systems link rift segments characterized by different styles of extension ŽLister et al., 1986 and references therein.. Plio-Quaternary volcanic activity on the margin has occurred along a NW–SE belt. However, on a more detailed scale, volcanic activity systematically focuses along single NE–SW fractures, forming NE–SW eruptive fissures and dikes, or multiple NE–SW fractures, forming main volcanic edifices. This relationship is evident in Fig. 1, which shows that the volcanic districts on the margin are quite regularly spaced between 30 and 50 km and concentrated along NE–SW fractures. The occurrence of volcanic activity along the Tyrrhenian margin in coincidence with transverse fractures suggests a systematic relationship between transfer structures and volcanism. The Campi Flegrei Volcanic District ŽCFVD. consists of a 50-km long NE–SW active volcanic ridge ŽFig. 1. and displays the largest, most recent and best exposed NE–SW fractures. Thus, Campi Flegrei is the preferred site for studying the relationship between active transverse structures and volcanism. The aim of this paper is to try to investigate how volcanic activity develops at Campi Flegrei in relation to transverse fracture systems. With this purpose, previously performed structural analyses have been coupled with analogue and mechanical modeling. The results suggest that volcanic activity along
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the NE–SW trend at the CFVD is induced by the favourable, subvertical dip of the transfer faults. These considerations, relative to Campi Flegrei, also have a possible application on the remaining volcanic districts along the Tyrrhenian margin, located in the same overall structural setting. 2. The Campi Flegrei Volcanic District The CFVD is located on the Southern part of the Campania Plain, which is a NW–SE Plio-Quaternary graben-like structure bordered by Mesozoic carbonate platforms. The CFVD includes the Campi Flegrei Caldera ŽCFC. and the volcanic islands of Ischia and Procida ŽFig. 2., which constitute the surface feature of a transverse, buried, horst-like structure within the Campania Plain. The horst has a NE–SW trend and has been identified through seismic, gravimetric and magnetic data ŽCarrara et al., 1973; Finetti and Morelli, 1974; Fedi and Rapolla, 1987; Rapolla et al., 1989.. The CFVD is responsible for the emission of several ignimbrites, such as the Green Tuff Žabout 55 ka, Gillot et al., 1982., the Campanian Ignimbrite, with a DRE volume of approximately 150 km3 Žabout 37 ka; Barberi et al., 1978; Orsi et al., 1996; Rosi et al., 1996; Civetta et al., 1997. and the Neapolitan Yellow Tuff Žabout 12 ka; Orsi et al., 1992, 1995, 1996; Scarpati et al., 1993; Wohletz et al., 1995.. The last eruption, responsible for the build-up of a new volcanic edifice ŽMonte Nuovo., occurred in 1538. The compositional spectrum of the volcanic rocks of the CFVD ranges from trachytes Žlargely predominant. and alkali trachytes to trachybasalts and latites, with a subcrustal contamination ŽArmienti et al., 1983; Di Girolamo et al., 1984; Rosi and Sbrana, 1987; Vezzoli, 1988; Turi et al., 1991.. The CFC is the largest structural feature of the ridge constituting the CFVD ŽFig. 2.. The CFC consists ŽFig. 2. of a complex structure ŽRosi et al., 1983; Barberi et al., 1991., formed by two nested calderas, resulting from the interaction between regional and local tectonics ŽOrsi et al., 1996; Civetta et al., 1996a.. The formation of the larger, outer caldera has been related to the Campanian Ignimbrite eruption, while the inner caldera formed during the eruption of the Neapolitan Yellow Tuff ŽOrsi et al., 1996..
126 V. Acocella et al.r Journal of Volcanology and Geothermal Research 91 (1999) 123–139 Fig. 2. Structural map of the CFVD, which includes the CFC, the islands of Ischia and Procida. The reported structural setting of the Neapolitan area is based on data from Orsi et al. Ž1996. and Acocella and Funiciello Ž1999. and references therein. Insets report structural data from previous investigations: Ža. orientation and frequency Žproportional to the length of the bars. of the main fractures inside the CFC Žmodified after Cosentino et al., 1984.; Žb. simplified structural sketch of Ischia Žmodified after Acocella and Funiciello, 1999..
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Vertical ground movements have accompanied volcanic activity both inside and outside the CFC ŽGillot et al., 1982; Cinque et al., 1985; Rosi and Sbrana, 1987.. The most recent Ž1969–1984. ground movement consisted of a 3.5-m uplift and a subsequent Ž1985–1994., partial recovery at Pozzuoli, inside the CFC ŽCorrado et al., 1976; Barberi et al., 1984; Berrino et al., 1984; Civetta et al., 1996b.. The 1969–1984 uplift has been related to caldera-resurgence ŽLuongo et al., 1991; Civetta et al., 1995., which involves both brittle and ductile deformation ŽOrsi et al., 1996..
3. Structural analysis at the CFVD Two structural analyses were performed at the CFVD, inside the CFC and on the island of Ischia. Fig. 2 Žinset. shows the main fracture systems within the CFC, evaluated from field analysis; the data has been previously collected and is reported in
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Cosentino et al. Ž1984.. Based on this data, the regional structures, as well as the local deformations related to caldera activity, are reported in Fig. 2. The data shows that NE–SW trending fractures are the most widespread active regional systems ŽFig. 2., while a narrow zone of NW–SE trending fractures suggests the presence of a discontinuity cross-cutting the north-eastern part of the caldera. N–S fracture systems are moderately present and could be due to local tectonics induced by the development of the western sector of the caldera. The overall collected data is consistent with other published data, which shows that the main fracture systems in the area have a NE–SW, NW–SE and, secondarily, a N–S trend ŽRosi and Sbrana, 1987; Bellucci, 1994; Orsi et al., 1996.. The recent activity of NE–SW transverse systems inside the CFC is shown in Figs. 3 and 4. Fig. 3 displays a NE–SW fracture at the Solfatara, near Pozzuoli, in the centre of the caldera formed during the 2-m uplift which occurred in the 1982–1984
Fig. 3. The Solfatara area, near Pozzuoli, in 1984. The arrow indicates a NE–SW trending fracture formed during the 1982–1984 bradyseism.
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Fig. 4. N608E trending fracture Žsee arrows. inside the Flavio Roman Amphitheatre at Pozzuoli. The fracture, as visible in the picture, also affects the recently restored portions of the building.
bradyseism. He isotopic analyses at the Solfatara suggest a subcrustal contribution ŽAllard et al., 1991.. N608E fractures, possibly related to the 1969–1984 bradyseisms, are also present in the recently restored portions of the Flavio Roman Amphitheatre, at Pozzuoli ŽFig. 4.. A structural analysis was also performed, by means of remote sensing, field analysis and soil gas prospecting, at Ischia island, on the SW portion of the CFVD. The results ŽFig. 2, inset., reported in Acocella et al. Ž1997. and Acocella and Funiciello Ž1999., permitted to propose the structural model schematically shown in Fig. 2 Žinset., indicating the overall same deformation pattern as in the CFC. The active regional fracture systems, developed in the last 130 ka ŽVezzoli, 1988., show in fact a NE–SW and NW–SE direction, while secondary N–S fractures are related to local tectonics induced by a resurgent dome. NE–SW fractures control most of the volcanic activity at Ischia and Procida, forming a
sequence of aligned monogenetic vents ŽFig. 2.. On Ischia, at a broad scale, NE–SW fractures usually offset NW–SE fractures ŽFig. 2 inset.: in particular, the main NW–SE fractures on the central part of the island are crosscut by the NE–SW fractures located along the south-eastern part of the Island. However, the geometrical relationships between coeval NW–SE and NE–SW fractures are often locally masked by their superimposition over a wide area.
4. Analogue modeling 4.1. Experimental procedure and scaling Analogue modeling helps one understand the deformation mechanisms that might be responsible for the studied natural example. Such modeling consists of the scaled simulation of natural deformative processes. Analogue models were performed to investi-
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gate the relationship between NW–SE and NE–SW fractures in the Neapolitan area and along the Tyrrhenian margin, testing the hypothesis that NE– SW structures might be transfer systems of the NW– SE normal faults ŽLiotta, 1991; Faccenna et al., 1994.. In our experiments, performed at the Roma Tre Laboratories of Experimental Geology, we tried to reproduce the formation of transfer systems in brittle extensional domains. Extensional tectonics has been widely simulated by means of analogue modeling Žsee Benes and Davy, 1996; Keep and McClay, 1997 and references therein.. Previous experiments reproduced the formation of transfer zones and transform faults ŽCourtillot et al., 1974; Elmohandes, 1981; Serra and Nelson, 1988., using clay as analogue material. More recently, transform fault geometry in oceanic domains has been reproduced by means of brittle–ductile materials such as sand and silicone putty ŽMauduit and Dauteuil, 1996.. Models are geometrically, kinematically and dynamically scaled, following principles discussed by Hubbert Ž1937. and Ramberg Ž1981.. We chose a ratio length of 10y5 Ž1 cm of the model corresponds to 1 km in nature.. The densities of natural rocks Ž2.0–2.7 g cmy3 . and of the experimental materials commercially available Ž0.9–1.8 g cmy3 . require a density ratio of approximately 0.5. Since the models were performed at 1 g, the gravity ratio is 1. These ratios impose the stress ratio between model and nature to be approximately 5 = 10y5 . We assumed a Mohr–Coulomb failure criterion for the rocks in the
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brittle crust, with an angle of internal friction of approximately 308 and a mean cohesion of approximately 10 7 Pa. Cohesion, having the dimensions of stresses, must be scaled at approximately 5 = 10y5 in the experiments; this requires the use of an analogue material with a cohesion of approximately 500 Pa. For this purpose, we used well sorted, rounded grain, dry quartzose sand with an angle of an internal friction of approximately 358 and a cohesion in the order of few hundreds of Pascal. Dry quartzose sand is commonly used in analogue modelling. Its rheological properties and use as analogue material are discussed in Mandl et al. Ž1977. and Krantz Ž1991.. We placed a 7-cm thick pack of sand in the experimental apparatus to simulate the Mohr– Coulomb behaviour of the brittle crust ŽFig. 5.. The experimental apparatus, similar to the one used by Mauduit and Dauteuil Ž1996., consisted of a mobile basal plate sliding along an underlying fixed basal plate; two vertical walls were attached to the opposite sides of the fixed and mobile plates ŽFig. 5.. A screwing jack piston, connected to a computer, moved the upper plate outward, inducing a velocity discontinuity between the two plates, at the base of the model. The velocity discontinuity propagated the deformation in the sand-pack, inducing the formation of faults. The velocity discontinuity was, thus, responsible for the localized formation of two enechelon extensional segments at deeper crustal levels. Extension was driven at a constant velocity of 4 cm hy1 . The model run was stopped at about 2 cm
Fig. 5. The experimental apparatus used in analogue modeling to simulate the formation of transfer systems in brittle extensional domains. The migrating velocity discontinuity between the two basal plates is responsible for the propagation of the extension in the overlying, undeformed sand-pack.
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Fig. 6. Main geometrical and kinematical features of the performed set of sand-box experiments. The initial configuration of the upper plate and thus the configuration of the velocity discontinuity, varies in each model. The corresponding final configuration of the extended sand-pack is also schematically reported.
of extension Ž3% of total length of the model.. Subsequently, the model was wet and carefully cut
into cross-sections orthogonal to the direction of the main faults.
Fig. 7. Evolution of the transfer zone in model 06, characterized by S 4 L. Ža. Initial stage Žmap view., no extension. Žb. Intermediate stage, 6 mm extension. Žc. Final stage, 15 mm extension. Arcuate normal faults border the transfer zone. Inside the transfer zone strike–slip X faults are present. Žd. The vertical section X–X shows normal faults dipping approximately 608 bounding one of the basins. Že. Section X Y–Y shows a normal fault bounding the basin Žleft. and a strike–slip transfer fault Ždipping approximately 808. inside Žright..
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Fig. 7 Žcontinued..
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4.2. Results and interpretation of the experiments A different geometry of the velocity discontinuity imposed in each experiment reproduced various initial configurations for the interacting rift segments, thus simulating different geometries for transfer zones ŽFig. 6.. Particularly, the overlap Ž L. and overstep Ž S . between en-echelon rift segments were varied; consequently, also angle f Žbetween the direction perpendicular to the extensional direction and the strike of the velocity discontinuity in the transfer zone. varied ŽFig. 6.. The initial configuration of models 02, 03 and 07 displays the same overstep Ž10 cm., but a different overlap Žand thus a different angle f .. Conversely, the initial configuration of models 03, 05 and 07 displays a constant angle f Ž908., but a variable offset Ž10, 7 and 20 cm, respectively.. The results of the experiments are synthesised and shown in model 06 ŽFig. 7., which displays the closest geometrical similarities to the structures present on the Tyrrhenian margin. The initial configuration of model 06 is characterized by S 4 L, being L s 0 cm and S s 20 cm ŽFig. 7a.. At 3 mm of extension, two en-echelon basins form on the top of the sand-pack, along the underlying velocity discontinuity. Both basins are bordered by master faults Žnamed A and AX in Fig. 7. and antithetic normal faults ŽB and BX .. At 6 mm of extension, strike–slip transfer faults, subparallel to the extension direction, develop in the transfer zone between the en-echelon basins. Transfer faults join normal faults B and AX , while the two en-echelon basins deepen ŽFig. 7b.. Clock-wise rotations around vertical axes, connected to the strike–slip faults, occur inside the transfer zone. At 15 mm of extension, normal faults A and BX curve progressively towards normal faults AX and B, respectively, trending, towards their tips, subparallel to the extension direction; both faults decrease the extensional component towards the tips ŽFig. 7c.. By the end of the experiment, strike–slip faults had developed inside the transfer zone, whereas arcuate normal faults were present at its borders, showing strain partitioning. Fig. 7d displays section X–XX , which is orthogonal to one of the basins. The basin is bordered by normal faults A and B, whose dip is about 608. Fig. 7e displays a section orthogonal to the axis of the
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transfer zone ŽY–YX . of model 06. It displays both normal fault BX Žleft. dipping about 608 and a strike–slip fault Žright. inside the transfer zone, dipping about 808. The final deformative configurations of the remaining experiments Ž02, 03, 05 and 07. are schematically shown in Fig. 6. All the transfer zones display an overall similar architecture, characterized by normal faults bordering the transfer zone and strike–slip faults inside the transfer zone; normal faults have a dip between 588 and 648, whereas strike–slip transfer faults have a dip between 788 and 878. Strike–slip faults, however, do not form in model 02 Žwhere f s 458 and the two rifts underlap., which is characterized only by oblique-normal faults bordering the basin. This is consistent with the oblique rifting experiments of Tron and Brun Ž1991., where strain partitioning between normal and strike–slip faults also occurs for f ) 458. Curved normal faults systematically border the transfer zone. We interpret that the curved normal faults bordering the transfer zone as the result of a variation of the stress field induced by the shear stresses in the transfer zone ŽPollard and Aydin, 1984.. The overall geometry of the transfer zones in the experiments shows some differences from the classical models of transfer faults ŽGibbs, 1984.. Particularly, our transfer zones display arcuate normal faults propagating from the en-echelon depressions; this feature is, however, seen in several natural examples from narrow rifts worldwide ŽNelson et al., 1992 and references therein.. On the other hand, the transfer zone in experiment 06 shows geometric and kinematic similarities to classical models, with transfer faults nearly orthogonal to the normal faults under the same extensional stress field. The results of experiment 06 are similar to the results of transform faults on oceanic domains of Mauduit and Dauteuil Ž1996., which show subvertical strike–slip systems parallel to the extension direction. The analogue experiments confirm that NE–SW transverse fractures at Campi Flegrei and on the Tyrrhenian margin constitute transfer fault zones between offset adjacent NW–SE normal faults. The experiments also show that transfer faults are steeper Žapproximately 208–258. than the adjacent normal faults. Moreover, in the central Apennines NW–SE
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Fig. 8. Ža. The pressure P necessary to penetrate a pre-existing fracture is a function of the angle u Žcomplementary to the dip of the fracture, between the fracture and the direction of the vertical lithostatic stress. and the depth z, being r , g and n constant. Žb. Graph displaying the values of the pressure P as a function of the angle u for various depths Žshown by different curves.. The arrows on the top of the graph show the dip of transfer and normal faults as measured from the cross-sections of the analogue models.
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normal faults have often reactivated pre-existing low-angle thrust faults ŽFaccenna et al., 1995; D’Agostino et al., 1998., thus displaying a lower dip Žabout 308. than the one Žabout 608. shown in the experiments.
5. Mechanical model The analogue models suggest that a possible explanation for the occurrence of volcanic activity at Campi Flegrei is connected to the subvertical dip of NE–SW transfer fractures. A simplified mechanical model is proposed here in order to try to Ž1. investigate the relationships between the dip and depth of the fractures in the transfer zone and their permeability to magma and Ž2. define the role of transverse structures upon volcanic activity. The purpose of the mechanical model is to investigate the relative influence of the dip and depth of a pre-existing large fracture on magma permeability. With regards to this, we did not consider Ža. tectonic stresses, Žb. pore pressure, Žc. stress modifications nearby pre-existing fractures and Žd. physical and chemical properties of magma, even if these parameters may alter the absolute stress values ŽPollard, 1973; Rubin, 1993a,b; Takada, 1994.. From the stress equations in uniaxial lithostatic conditions, the pressure P necessary to intrude a pre-existing fracture is proportional to ŽJaeger and Cook, 1976; Gudmundsson, 1984; Delaney et al., 1986.: P s Ž sv q s H . r2 y cos2 u Ž sv y s H . r2
Ž 1.
where sv is the vertical lithostatic stress, s H the horizontal lithostatic stress, and u the angle Žcomplementary to the dip of the fracture, Fig. 8a. between the fracture and the direction of the vertical lithostatic stress. r gz is the vertical lithostatic stress, r being the rock density, g the gravity acceleration Ž9.8 mrs 2 . and z the depth. The horizontal component of the lithostatic pressure is:
s H s sv nr Ž 1 y n . n being the Poisson modulus Ž n s 0.25; Turcotte and Schubert, 1982.. Fig. 8b displays a series of curves, based on Eq. Ž1., reporting the pressure necessary to intrude a
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pre-existing fracture as a function of the angle u and the depth z in km. Vertical fractures, characterized by angle u s 08, require the lowermost values of magmatic pressure P to be penetrated. The pressure required to penetrate fractures increases with u , becoming larger for normal faults, reverse faults and largest of all for horizontal fractures Ži.e., sills.. This difference increases significantly with the depth of intrusion, where the pressure variation as a function of u is larger. Arrows on the top of the graph in Fig. 8b show the dip for the transfer faults Žtransfer fault domain. and normal faults Žnormal fault domain. as measured from the cross-sections of the analogue models. These results suggest that subvertical transfer fractures, since they require the lowermost values of pressure to be penetrated, constitute a preferential pathway for magma rise. Moreover, Fig. 8b shows that, in lithostatic conditions, for a same depth reached by the fractures and for a given magmatic pressure, subvertical fractures might be penetrated more deeply. Thus, subvertical fractures might tap the potentially more basic magmas, enhancing the emission of the primitive components. As stated, the effects of pore pressure and tectonic stresses have not been taken quantitatively into account. From a general point of view, the presence of pore pressure decreases the magmatic pressure P computed from Eq. Ž1., necessary to intrude a preexisting discontinuity. Tectonic stresses vary the pressure required to dilate a pre-existing discontinuity. Extensional domains are commonly characterized by a tensile, horizontal tectonic contribution which adds to the lithostatic stress ŽTurcotte and Schubert, 1982. and the result is a lowering of the normal stress on fractures. Thus, from a comparative point of view, the magmatic pressure Pe necessary to penetrate a fracture in extensional domains, because of the lower horizontal component, will be lower than the pressure computed from Eq. Ž1.. The opposite occurs in compressional domains, where the magmatic pressure Pc necessary to intrude will be larger than P. 6. Discussion The extension that occurred during Pliocene and Quaternary times on the Tyrrhenian margin of Cen-
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tral Italy is responsible for the formation of NW–SE normal faults and coeval NE–SW transfer fractures. Plio-Quaternary volcanic activity on the margin is consistently associated with transfer fractures; such relation is best shown at the active CFVD. Structural analysis performed at Campi Flegrei has shown the presence of NE–SW systems usually interrupting the continuity of NE–SW fractures at a regional scale. Analogue experiments were performed to investigate the geometrical and kinematical relationship between coeval, orthogonal structures in extensional domains. The experiments show en-echelon depressions bordered by normal faults dipping 608 and connected by vertical transfer faults subparallel to the extension direction. These results confirm that NE–SW systems at the CFVD, as well as on the margin, represent subvertical transfer fractures of the NW–SE normal faults. The simplified mechanical model considered the permeability of fractures as a function of their dip and depth. It shows how vertical fractures require the lowermost magmatic pressures to be penetrated and how the required pressure increases while decreasing the dip of the fracture.
These results suggest that the NE–SW volcanic ridge constituting the CFVD is located along a transfer zone connecting adjacent, offset NW–SE regional faults parallel to the Tyrrhenian margin. In such framework, the rise of magma in the upper crust would be enhanced along subvertical transfer systems, inducing volcanic activity along the preferred NE–SW trend at the CFVD. Furthermore, Fig. 8b has shown that, considering a constant magmatic pressure, subvertical faults are able to be penetrated more deeply, tapping the potentially more primitive magmas. This is consistent with the evidence that the most primitive products at the CFVD, both inside the caldera and outside Žislands of Ischia and Procida., have been erupted along NE–SW fractures ŽDi Girolamo and Rolandi, 1975; Beccaluva et al., 1985; Poli et al., 1987; Vezzoli, 1988; Allard et al., 1991; Civetta et al., 1991; D’Antonio et al., 1999-this volume.. Although the model here proposed has been applied to Campi Flegrei, it also suggests an explanation for the structural setting of the remaining volcanic districts on the Tyrrhenian margin, located in correspondence to transverse fractures. In fact, as
Fig. 9. Ža. Transfer fault zones and related volcanic activity along the western branch of the African Rift System. Inset shows the structure of the Kivu-Rusizi transfer fault zone and the relative volcanic products Žmodified after Ebinger, 1989b.. Žb. The main transfer fault zone of the Rio Grande Rift and the related Jemez Volcanic Field.
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can be seen from Fig. 1, major NE–SW fractures, eruptive fissures and dikes cluster at the volcanic districts of Amiata, Vulsini, Sabatini, Vico, Colli Albani, Roccamonfina and Vesuvio. Narrow rifts worldwide also display volcanic activity at transfer fault zones. Volcanism on the western branch of the African Rift System occurs, with a constant spacing, at transfer fault zones ŽFig. 9a; Ebinger, 1989a; Nelson et al., 1992 and references therein., where strike–slip faults have been observed ŽEbinger, 1989b.. The Rio Grande Rift also displays volcanic activity in correspondence with strike–slip faults, at the main transfer fault zone ŽFig. 9b; Aldrich, 1986.. These evidences suggest a wide-scale control of subvertical transfer fractures on volcanic activity during the extension of the Tyrrhenian margin as well as in other continental extensional frameworks worldwide ŽWestern African Rift System, Rio Grande Rift.. 7. Conclusions The CFVD shows both active tectonics and volcanism along a predominant NE–SW trending, constituting a preferred site to study the role of transverse fractures upon volcanic activity along the Tyrrhenian margin. Such a role has been investigated here by means of analogue and numerical modeling, coupled with previous structural analyses ŽCosentino et al., 1984; Acocella et al., 1997; Acocella and Funiciello, 1999.. The results suggest that the NE–SW volcanic ridge constituting the CFVD is characterized by subvertical transfer faults connecting adjacent, offset NW–SE regional faults parallel to the Tyrrhenian margin. In such context, volcanic activity along the preferred NE–SW direction at the CFVD would be induced by the subvertical dip of the transfer fractures. Transfer systems might also be responsible for the emission of the most primitive products, as consistent with the petro-chemical data collected at Campi Flegrei. Such a model, though best applied to Campi Flegrei, also allows us to explain the selective location of the remaining volcanic districts on the Tyrrhenian margin, systematically in correspondence to NE–SW fracture systems.
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Acknowledgements The authors wish to thank F. Rossetti for his help during the setting up of experiments, G. Giordano and the reviewers K.E.C. Krogh and G. Pasquare´ for their helpful comments that substantially improved the paper. The authors thank L. Civetta, G. Orsi and G.A. Valentine for their encouragement. This work was financed with Protezione Civile ŽOsservatorio Vesuviano. funds Žcode 21027r16..
References Acocella, V., Funiciello, R., 1999. The interaction between regional and local tectonics during resurgent doming: the case of the island of Ischia. Italy. J. Volcanol. Geotherm. Res. 88, 109–123. Acocella, V., Funiciello, R., Lombardi, S., 1997. Active tectonics and resurgence at Ischia island ŽSouthern Italy.. II Quaternario 10 Ž2., 427–432. Aldrich, M.J., 1986. Tectonics of the Jemez Lineament in the Jemez Mountains and Rio Grande Rift. J. Geophys. Res. 91, 1753–1762. Allard, P., Maiorani, A., Tedesco, D., Cortecci, G., Turi, B., 1991. Isotopic study of the origin of sulfur and carbon in Solfatara fumaroles, Campi Flegrei Caldera. J. Volcanol. Geotherm. Res. 48, 139–159. Armienti, P., Barberi, F., Bizouard, H., Clocchiatti, R., Innocenti, F., Metrich, N., Rosi, M., Sbrana, A., 1983. The Phlegraean Fields: magma evolution within a shallow chamber. J. Volcanol. Geotherm. Res. 17, 289–311. Barberi, F., Innocenti, F., Lirer, L., Munno, R., Pescatore, P., 1978. The Campanian Ignimbrite: a major prehistoric eruption in the Neapolitan area ŽItaly.. Bull. Volcanol. 41, 1–22. Barberi, F., Corrado, G., Innocenti, F., Luongo, G., 1984. Phlegraean Fields 1982–1984: brief chronicle of a volcano emergency in a densely populated area. Bull. Volcanol. 47 Ž2., 175–185. Barberi, F., Cassano, E., La Torre, P., Sbrana, A., 1991. Structural evolution of Campi Flegrei caldera in light of volcanological and geophysical data. J. Volcanol. Geotherm. Res. 48, 33–49. Beccaluva, L., Di Girolamo, P., Serri, G., 1985. High-k calc-alkaline, shoshonitic and leucitic volcanism of Campania ŽRoman Province, southern Italy.: trace elements constraints on the genesis of an orogenic volcanism in a post-collisional, extensional setting. IAVCEI Sci. Ass. Potassic Volcanism, Etna Volcano, Giardini Naxos, Italy, 46 pp. Bellucci, F., 1994. Nuove conoscenze stratigrafiche sui depositi vulcanici del sottosuolo del settore meridionale della Piana Campana. Boll. Soc. Geol. Ital. 113, 395–420. Benes, V., Davy, P., 1996. Modes of continental lithospheric extension: experimental verification of strain localization processes. Tectonophysics 254, 69–87.
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The role of transfer structures on volcanic activity at Campi Flegrei žSouthern Italy / V. Acocella ) , F. Salvini, R. Funiciello, C. Faccenna Dipartimento Scienze Geologiche, UniÕersita´ degli Studi ‘‘Roma Tre’’, Largo San Leonardo Murialdo, 1, 00146 Rome, Italy
Abstract The Tyrrhenian margin of Central Italy underwent extension during Pliocene and Quaternary. Extension occurred mainly through NW–SE normal faults, bordering a sequence of Plio-Quaternary basins. These basins are offset by coeval NE–SW faults, which show strike–slip and normal motions and have been interpreted as transfer faults. Plio-Quaternary volcanic activity along the margin occurred along a NW–SE belt, systematically in correspondence with NE–SW transverse systems. The Campi Flegrei Volcanic District ŽCFVD., on the Southern Tyrrhenian margin, consists of an active NE–SW volcanic ridge developed along NE–SW fractures. We performed a structural field analysis with analogue and mechanical models to investigate the role of transverse structures upon volcanism at Campi Flegrei. Field analysis at Campi Flegrei recognized NE–SW and, to a lesser extent, coeval NW–SE active fractures. Analogue experiments have simulated the development of transfer fault systems in brittle extensional domains. The experiments show that subvertical transfer faults connect offset adjacent normal faults dipping 608. The mechanical model is based on the stress equations in uniaxial lithostatic conditions and absence of regional stresses. It shows how pre-existing subvertical fractures require the smallest magmatic pressures to be penetrated. For a given magmatic pressure, subvertical fractures might be penetrated more deeply, tapping more easily primitive magmas. These results suggest that the CFVD is located along a NE–SW transfer zone connecting NW–SE regional normal faults. Volcanic activity along such NE–SW trend would be induced by the subvertical dip of the transfer faults. The subvertical dip of transfer faults also suggests an explanation for the emission of the more primitive products along NE–SW systems at Campi Flegrei. These considerations find a wider application on the remaining volcanic districts of the margin, located in the same overall structural setting. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Campi Flegrei; transfer faults; sand-box modelling; permeability
1. Introduction A major phase of extension has been affecting the Tyrrhenian margin of Central Italy, between the Apenninic belt and the Tyrrhenian Sea, mainly from Upper Pliocene to Quaternary times. Extensional tectonics shifted over time, towards the Apenninic )
Corresponding author. Tel.: q00-39-6-548-88082; fax: q0039-6-548-88201; E-mail: [email protected]
belt and the Southern Tyrrhenian Sea. Volcanic activity, which shows a similar SE-migrating evolution pattern, has been following extension on the Tyrrhenian margin since upper Pliocene. Extension occurred mainly along NW–SE normal faults and, to a lesser degree, along coeval NE–SW transverse systems ŽFig. 1; Locardi et al., 1976; Wise et al., 1985; Mariani and Prato, 1988; Faccenna et al., 1994.. NW–SE faults predominantly show a normal throw and are responsible for the largest
0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 3 2 - 3
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Fig. 1. Structural sketch of the Tyrrhenian margin of the Central Italy Peninsula, displaying the Plio-Quaternary volcanic districts.
amount of extension on the margin. These systems border a sequence of NW–SE Plio-Quaternary basins, defining the overall structural setting of the Tyrrhenian margin.
NE–SW fracture systems are mainly located on the western side of the volcanoes ŽFig. 1., interrupting the continuity of adjacent NW–SE extensional basins. The distribution of NE–SW transverse sys-
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tems indicates their secondary role in the extensional processes along the Tyrrhenian margin. NE–SW faults usually display a complex kinematics, often related to the superimposition of normal motions on previous oblique and strike–slip motions; when the normal motion is predominant, NE–SW faults usually border Plio-Quaternary transverse basins ŽFig. 1.. The coeval activity of NW–SE and NE–SW fractures has been recognized in several sedimentary and volcano-tectonic basins on the northern, central and southern Tyrrhenian margin ŽMariani and Prato, 1988; Liotta, 1991; Faccenna et al., 1994; De Rita and Giordano, 1996; Orsi et al., 1996; Acocella and Funiciello, 1999.. NE–SW fractures, due to their kinematics Žnormal to strike–slip component of motion. and age Žcoeval to NW–SE systems., have been interpreted as transfer systems of NW–SE normal faults under the same stress field ŽLiotta, 1991; Faccenna et al., 1994.. Transfer systems link rift segments characterized by different styles of extension ŽLister et al., 1986 and references therein.. Plio-Quaternary volcanic activity on the margin has occurred along a NW–SE belt. However, on a more detailed scale, volcanic activity systematically focuses along single NE–SW fractures, forming NE–SW eruptive fissures and dikes, or multiple NE–SW fractures, forming main volcanic edifices. This relationship is evident in Fig. 1, which shows that the volcanic districts on the margin are quite regularly spaced between 30 and 50 km and concentrated along NE–SW fractures. The occurrence of volcanic activity along the Tyrrhenian margin in coincidence with transverse fractures suggests a systematic relationship between transfer structures and volcanism. The Campi Flegrei Volcanic District ŽCFVD. consists of a 50-km long NE–SW active volcanic ridge ŽFig. 1. and displays the largest, most recent and best exposed NE–SW fractures. Thus, Campi Flegrei is the preferred site for studying the relationship between active transverse structures and volcanism. The aim of this paper is to try to investigate how volcanic activity develops at Campi Flegrei in relation to transverse fracture systems. With this purpose, previously performed structural analyses have been coupled with analogue and mechanical modeling. The results suggest that volcanic activity along
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the NE–SW trend at the CFVD is induced by the favourable, subvertical dip of the transfer faults. These considerations, relative to Campi Flegrei, also have a possible application on the remaining volcanic districts along the Tyrrhenian margin, located in the same overall structural setting. 2. The Campi Flegrei Volcanic District The CFVD is located on the Southern part of the Campania Plain, which is a NW–SE Plio-Quaternary graben-like structure bordered by Mesozoic carbonate platforms. The CFVD includes the Campi Flegrei Caldera ŽCFC. and the volcanic islands of Ischia and Procida ŽFig. 2., which constitute the surface feature of a transverse, buried, horst-like structure within the Campania Plain. The horst has a NE–SW trend and has been identified through seismic, gravimetric and magnetic data ŽCarrara et al., 1973; Finetti and Morelli, 1974; Fedi and Rapolla, 1987; Rapolla et al., 1989.. The CFVD is responsible for the emission of several ignimbrites, such as the Green Tuff Žabout 55 ka, Gillot et al., 1982., the Campanian Ignimbrite, with a DRE volume of approximately 150 km3 Žabout 37 ka; Barberi et al., 1978; Orsi et al., 1996; Rosi et al., 1996; Civetta et al., 1997. and the Neapolitan Yellow Tuff Žabout 12 ka; Orsi et al., 1992, 1995, 1996; Scarpati et al., 1993; Wohletz et al., 1995.. The last eruption, responsible for the build-up of a new volcanic edifice ŽMonte Nuovo., occurred in 1538. The compositional spectrum of the volcanic rocks of the CFVD ranges from trachytes Žlargely predominant. and alkali trachytes to trachybasalts and latites, with a subcrustal contamination ŽArmienti et al., 1983; Di Girolamo et al., 1984; Rosi and Sbrana, 1987; Vezzoli, 1988; Turi et al., 1991.. The CFC is the largest structural feature of the ridge constituting the CFVD ŽFig. 2.. The CFC consists ŽFig. 2. of a complex structure ŽRosi et al., 1983; Barberi et al., 1991., formed by two nested calderas, resulting from the interaction between regional and local tectonics ŽOrsi et al., 1996; Civetta et al., 1996a.. The formation of the larger, outer caldera has been related to the Campanian Ignimbrite eruption, while the inner caldera formed during the eruption of the Neapolitan Yellow Tuff ŽOrsi et al., 1996..
126 V. Acocella et al.r Journal of Volcanology and Geothermal Research 91 (1999) 123–139 Fig. 2. Structural map of the CFVD, which includes the CFC, the islands of Ischia and Procida. The reported structural setting of the Neapolitan area is based on data from Orsi et al. Ž1996. and Acocella and Funiciello Ž1999. and references therein. Insets report structural data from previous investigations: Ža. orientation and frequency Žproportional to the length of the bars. of the main fractures inside the CFC Žmodified after Cosentino et al., 1984.; Žb. simplified structural sketch of Ischia Žmodified after Acocella and Funiciello, 1999..
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Vertical ground movements have accompanied volcanic activity both inside and outside the CFC ŽGillot et al., 1982; Cinque et al., 1985; Rosi and Sbrana, 1987.. The most recent Ž1969–1984. ground movement consisted of a 3.5-m uplift and a subsequent Ž1985–1994., partial recovery at Pozzuoli, inside the CFC ŽCorrado et al., 1976; Barberi et al., 1984; Berrino et al., 1984; Civetta et al., 1996b.. The 1969–1984 uplift has been related to caldera-resurgence ŽLuongo et al., 1991; Civetta et al., 1995., which involves both brittle and ductile deformation ŽOrsi et al., 1996..
3. Structural analysis at the CFVD Two structural analyses were performed at the CFVD, inside the CFC and on the island of Ischia. Fig. 2 Žinset. shows the main fracture systems within the CFC, evaluated from field analysis; the data has been previously collected and is reported in
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Cosentino et al. Ž1984.. Based on this data, the regional structures, as well as the local deformations related to caldera activity, are reported in Fig. 2. The data shows that NE–SW trending fractures are the most widespread active regional systems ŽFig. 2., while a narrow zone of NW–SE trending fractures suggests the presence of a discontinuity cross-cutting the north-eastern part of the caldera. N–S fracture systems are moderately present and could be due to local tectonics induced by the development of the western sector of the caldera. The overall collected data is consistent with other published data, which shows that the main fracture systems in the area have a NE–SW, NW–SE and, secondarily, a N–S trend ŽRosi and Sbrana, 1987; Bellucci, 1994; Orsi et al., 1996.. The recent activity of NE–SW transverse systems inside the CFC is shown in Figs. 3 and 4. Fig. 3 displays a NE–SW fracture at the Solfatara, near Pozzuoli, in the centre of the caldera formed during the 2-m uplift which occurred in the 1982–1984
Fig. 3. The Solfatara area, near Pozzuoli, in 1984. The arrow indicates a NE–SW trending fracture formed during the 1982–1984 bradyseism.
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Fig. 4. N608E trending fracture Žsee arrows. inside the Flavio Roman Amphitheatre at Pozzuoli. The fracture, as visible in the picture, also affects the recently restored portions of the building.
bradyseism. He isotopic analyses at the Solfatara suggest a subcrustal contribution ŽAllard et al., 1991.. N608E fractures, possibly related to the 1969–1984 bradyseisms, are also present in the recently restored portions of the Flavio Roman Amphitheatre, at Pozzuoli ŽFig. 4.. A structural analysis was also performed, by means of remote sensing, field analysis and soil gas prospecting, at Ischia island, on the SW portion of the CFVD. The results ŽFig. 2, inset., reported in Acocella et al. Ž1997. and Acocella and Funiciello Ž1999., permitted to propose the structural model schematically shown in Fig. 2 Žinset., indicating the overall same deformation pattern as in the CFC. The active regional fracture systems, developed in the last 130 ka ŽVezzoli, 1988., show in fact a NE–SW and NW–SE direction, while secondary N–S fractures are related to local tectonics induced by a resurgent dome. NE–SW fractures control most of the volcanic activity at Ischia and Procida, forming a
sequence of aligned monogenetic vents ŽFig. 2.. On Ischia, at a broad scale, NE–SW fractures usually offset NW–SE fractures ŽFig. 2 inset.: in particular, the main NW–SE fractures on the central part of the island are crosscut by the NE–SW fractures located along the south-eastern part of the Island. However, the geometrical relationships between coeval NW–SE and NE–SW fractures are often locally masked by their superimposition over a wide area.
4. Analogue modeling 4.1. Experimental procedure and scaling Analogue modeling helps one understand the deformation mechanisms that might be responsible for the studied natural example. Such modeling consists of the scaled simulation of natural deformative processes. Analogue models were performed to investi-
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gate the relationship between NW–SE and NE–SW fractures in the Neapolitan area and along the Tyrrhenian margin, testing the hypothesis that NE– SW structures might be transfer systems of the NW– SE normal faults ŽLiotta, 1991; Faccenna et al., 1994.. In our experiments, performed at the Roma Tre Laboratories of Experimental Geology, we tried to reproduce the formation of transfer systems in brittle extensional domains. Extensional tectonics has been widely simulated by means of analogue modeling Žsee Benes and Davy, 1996; Keep and McClay, 1997 and references therein.. Previous experiments reproduced the formation of transfer zones and transform faults ŽCourtillot et al., 1974; Elmohandes, 1981; Serra and Nelson, 1988., using clay as analogue material. More recently, transform fault geometry in oceanic domains has been reproduced by means of brittle–ductile materials such as sand and silicone putty ŽMauduit and Dauteuil, 1996.. Models are geometrically, kinematically and dynamically scaled, following principles discussed by Hubbert Ž1937. and Ramberg Ž1981.. We chose a ratio length of 10y5 Ž1 cm of the model corresponds to 1 km in nature.. The densities of natural rocks Ž2.0–2.7 g cmy3 . and of the experimental materials commercially available Ž0.9–1.8 g cmy3 . require a density ratio of approximately 0.5. Since the models were performed at 1 g, the gravity ratio is 1. These ratios impose the stress ratio between model and nature to be approximately 5 = 10y5 . We assumed a Mohr–Coulomb failure criterion for the rocks in the
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brittle crust, with an angle of internal friction of approximately 308 and a mean cohesion of approximately 10 7 Pa. Cohesion, having the dimensions of stresses, must be scaled at approximately 5 = 10y5 in the experiments; this requires the use of an analogue material with a cohesion of approximately 500 Pa. For this purpose, we used well sorted, rounded grain, dry quartzose sand with an angle of an internal friction of approximately 358 and a cohesion in the order of few hundreds of Pascal. Dry quartzose sand is commonly used in analogue modelling. Its rheological properties and use as analogue material are discussed in Mandl et al. Ž1977. and Krantz Ž1991.. We placed a 7-cm thick pack of sand in the experimental apparatus to simulate the Mohr– Coulomb behaviour of the brittle crust ŽFig. 5.. The experimental apparatus, similar to the one used by Mauduit and Dauteuil Ž1996., consisted of a mobile basal plate sliding along an underlying fixed basal plate; two vertical walls were attached to the opposite sides of the fixed and mobile plates ŽFig. 5.. A screwing jack piston, connected to a computer, moved the upper plate outward, inducing a velocity discontinuity between the two plates, at the base of the model. The velocity discontinuity propagated the deformation in the sand-pack, inducing the formation of faults. The velocity discontinuity was, thus, responsible for the localized formation of two enechelon extensional segments at deeper crustal levels. Extension was driven at a constant velocity of 4 cm hy1 . The model run was stopped at about 2 cm
Fig. 5. The experimental apparatus used in analogue modeling to simulate the formation of transfer systems in brittle extensional domains. The migrating velocity discontinuity between the two basal plates is responsible for the propagation of the extension in the overlying, undeformed sand-pack.
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Fig. 6. Main geometrical and kinematical features of the performed set of sand-box experiments. The initial configuration of the upper plate and thus the configuration of the velocity discontinuity, varies in each model. The corresponding final configuration of the extended sand-pack is also schematically reported.
of extension Ž3% of total length of the model.. Subsequently, the model was wet and carefully cut
into cross-sections orthogonal to the direction of the main faults.
Fig. 7. Evolution of the transfer zone in model 06, characterized by S 4 L. Ža. Initial stage Žmap view., no extension. Žb. Intermediate stage, 6 mm extension. Žc. Final stage, 15 mm extension. Arcuate normal faults border the transfer zone. Inside the transfer zone strike–slip X faults are present. Žd. The vertical section X–X shows normal faults dipping approximately 608 bounding one of the basins. Že. Section X Y–Y shows a normal fault bounding the basin Žleft. and a strike–slip transfer fault Ždipping approximately 808. inside Žright..
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Fig. 7 Žcontinued..
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4.2. Results and interpretation of the experiments A different geometry of the velocity discontinuity imposed in each experiment reproduced various initial configurations for the interacting rift segments, thus simulating different geometries for transfer zones ŽFig. 6.. Particularly, the overlap Ž L. and overstep Ž S . between en-echelon rift segments were varied; consequently, also angle f Žbetween the direction perpendicular to the extensional direction and the strike of the velocity discontinuity in the transfer zone. varied ŽFig. 6.. The initial configuration of models 02, 03 and 07 displays the same overstep Ž10 cm., but a different overlap Žand thus a different angle f .. Conversely, the initial configuration of models 03, 05 and 07 displays a constant angle f Ž908., but a variable offset Ž10, 7 and 20 cm, respectively.. The results of the experiments are synthesised and shown in model 06 ŽFig. 7., which displays the closest geometrical similarities to the structures present on the Tyrrhenian margin. The initial configuration of model 06 is characterized by S 4 L, being L s 0 cm and S s 20 cm ŽFig. 7a.. At 3 mm of extension, two en-echelon basins form on the top of the sand-pack, along the underlying velocity discontinuity. Both basins are bordered by master faults Žnamed A and AX in Fig. 7. and antithetic normal faults ŽB and BX .. At 6 mm of extension, strike–slip transfer faults, subparallel to the extension direction, develop in the transfer zone between the en-echelon basins. Transfer faults join normal faults B and AX , while the two en-echelon basins deepen ŽFig. 7b.. Clock-wise rotations around vertical axes, connected to the strike–slip faults, occur inside the transfer zone. At 15 mm of extension, normal faults A and BX curve progressively towards normal faults AX and B, respectively, trending, towards their tips, subparallel to the extension direction; both faults decrease the extensional component towards the tips ŽFig. 7c.. By the end of the experiment, strike–slip faults had developed inside the transfer zone, whereas arcuate normal faults were present at its borders, showing strain partitioning. Fig. 7d displays section X–XX , which is orthogonal to one of the basins. The basin is bordered by normal faults A and B, whose dip is about 608. Fig. 7e displays a section orthogonal to the axis of the
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transfer zone ŽY–YX . of model 06. It displays both normal fault BX Žleft. dipping about 608 and a strike–slip fault Žright. inside the transfer zone, dipping about 808. The final deformative configurations of the remaining experiments Ž02, 03, 05 and 07. are schematically shown in Fig. 6. All the transfer zones display an overall similar architecture, characterized by normal faults bordering the transfer zone and strike–slip faults inside the transfer zone; normal faults have a dip between 588 and 648, whereas strike–slip transfer faults have a dip between 788 and 878. Strike–slip faults, however, do not form in model 02 Žwhere f s 458 and the two rifts underlap., which is characterized only by oblique-normal faults bordering the basin. This is consistent with the oblique rifting experiments of Tron and Brun Ž1991., where strain partitioning between normal and strike–slip faults also occurs for f ) 458. Curved normal faults systematically border the transfer zone. We interpret that the curved normal faults bordering the transfer zone as the result of a variation of the stress field induced by the shear stresses in the transfer zone ŽPollard and Aydin, 1984.. The overall geometry of the transfer zones in the experiments shows some differences from the classical models of transfer faults ŽGibbs, 1984.. Particularly, our transfer zones display arcuate normal faults propagating from the en-echelon depressions; this feature is, however, seen in several natural examples from narrow rifts worldwide ŽNelson et al., 1992 and references therein.. On the other hand, the transfer zone in experiment 06 shows geometric and kinematic similarities to classical models, with transfer faults nearly orthogonal to the normal faults under the same extensional stress field. The results of experiment 06 are similar to the results of transform faults on oceanic domains of Mauduit and Dauteuil Ž1996., which show subvertical strike–slip systems parallel to the extension direction. The analogue experiments confirm that NE–SW transverse fractures at Campi Flegrei and on the Tyrrhenian margin constitute transfer fault zones between offset adjacent NW–SE normal faults. The experiments also show that transfer faults are steeper Žapproximately 208–258. than the adjacent normal faults. Moreover, in the central Apennines NW–SE
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Fig. 8. Ža. The pressure P necessary to penetrate a pre-existing fracture is a function of the angle u Žcomplementary to the dip of the fracture, between the fracture and the direction of the vertical lithostatic stress. and the depth z, being r , g and n constant. Žb. Graph displaying the values of the pressure P as a function of the angle u for various depths Žshown by different curves.. The arrows on the top of the graph show the dip of transfer and normal faults as measured from the cross-sections of the analogue models.
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normal faults have often reactivated pre-existing low-angle thrust faults ŽFaccenna et al., 1995; D’Agostino et al., 1998., thus displaying a lower dip Žabout 308. than the one Žabout 608. shown in the experiments.
5. Mechanical model The analogue models suggest that a possible explanation for the occurrence of volcanic activity at Campi Flegrei is connected to the subvertical dip of NE–SW transfer fractures. A simplified mechanical model is proposed here in order to try to Ž1. investigate the relationships between the dip and depth of the fractures in the transfer zone and their permeability to magma and Ž2. define the role of transverse structures upon volcanic activity. The purpose of the mechanical model is to investigate the relative influence of the dip and depth of a pre-existing large fracture on magma permeability. With regards to this, we did not consider Ža. tectonic stresses, Žb. pore pressure, Žc. stress modifications nearby pre-existing fractures and Žd. physical and chemical properties of magma, even if these parameters may alter the absolute stress values ŽPollard, 1973; Rubin, 1993a,b; Takada, 1994.. From the stress equations in uniaxial lithostatic conditions, the pressure P necessary to intrude a pre-existing fracture is proportional to ŽJaeger and Cook, 1976; Gudmundsson, 1984; Delaney et al., 1986.: P s Ž sv q s H . r2 y cos2 u Ž sv y s H . r2
Ž 1.
where sv is the vertical lithostatic stress, s H the horizontal lithostatic stress, and u the angle Žcomplementary to the dip of the fracture, Fig. 8a. between the fracture and the direction of the vertical lithostatic stress. r gz is the vertical lithostatic stress, r being the rock density, g the gravity acceleration Ž9.8 mrs 2 . and z the depth. The horizontal component of the lithostatic pressure is:
s H s sv nr Ž 1 y n . n being the Poisson modulus Ž n s 0.25; Turcotte and Schubert, 1982.. Fig. 8b displays a series of curves, based on Eq. Ž1., reporting the pressure necessary to intrude a
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pre-existing fracture as a function of the angle u and the depth z in km. Vertical fractures, characterized by angle u s 08, require the lowermost values of magmatic pressure P to be penetrated. The pressure required to penetrate fractures increases with u , becoming larger for normal faults, reverse faults and largest of all for horizontal fractures Ži.e., sills.. This difference increases significantly with the depth of intrusion, where the pressure variation as a function of u is larger. Arrows on the top of the graph in Fig. 8b show the dip for the transfer faults Žtransfer fault domain. and normal faults Žnormal fault domain. as measured from the cross-sections of the analogue models. These results suggest that subvertical transfer fractures, since they require the lowermost values of pressure to be penetrated, constitute a preferential pathway for magma rise. Moreover, Fig. 8b shows that, in lithostatic conditions, for a same depth reached by the fractures and for a given magmatic pressure, subvertical fractures might be penetrated more deeply. Thus, subvertical fractures might tap the potentially more basic magmas, enhancing the emission of the primitive components. As stated, the effects of pore pressure and tectonic stresses have not been taken quantitatively into account. From a general point of view, the presence of pore pressure decreases the magmatic pressure P computed from Eq. Ž1., necessary to intrude a preexisting discontinuity. Tectonic stresses vary the pressure required to dilate a pre-existing discontinuity. Extensional domains are commonly characterized by a tensile, horizontal tectonic contribution which adds to the lithostatic stress ŽTurcotte and Schubert, 1982. and the result is a lowering of the normal stress on fractures. Thus, from a comparative point of view, the magmatic pressure Pe necessary to penetrate a fracture in extensional domains, because of the lower horizontal component, will be lower than the pressure computed from Eq. Ž1.. The opposite occurs in compressional domains, where the magmatic pressure Pc necessary to intrude will be larger than P. 6. Discussion The extension that occurred during Pliocene and Quaternary times on the Tyrrhenian margin of Cen-
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tral Italy is responsible for the formation of NW–SE normal faults and coeval NE–SW transfer fractures. Plio-Quaternary volcanic activity on the margin is consistently associated with transfer fractures; such relation is best shown at the active CFVD. Structural analysis performed at Campi Flegrei has shown the presence of NE–SW systems usually interrupting the continuity of NE–SW fractures at a regional scale. Analogue experiments were performed to investigate the geometrical and kinematical relationship between coeval, orthogonal structures in extensional domains. The experiments show en-echelon depressions bordered by normal faults dipping 608 and connected by vertical transfer faults subparallel to the extension direction. These results confirm that NE–SW systems at the CFVD, as well as on the margin, represent subvertical transfer fractures of the NW–SE normal faults. The simplified mechanical model considered the permeability of fractures as a function of their dip and depth. It shows how vertical fractures require the lowermost magmatic pressures to be penetrated and how the required pressure increases while decreasing the dip of the fracture.
These results suggest that the NE–SW volcanic ridge constituting the CFVD is located along a transfer zone connecting adjacent, offset NW–SE regional faults parallel to the Tyrrhenian margin. In such framework, the rise of magma in the upper crust would be enhanced along subvertical transfer systems, inducing volcanic activity along the preferred NE–SW trend at the CFVD. Furthermore, Fig. 8b has shown that, considering a constant magmatic pressure, subvertical faults are able to be penetrated more deeply, tapping the potentially more primitive magmas. This is consistent with the evidence that the most primitive products at the CFVD, both inside the caldera and outside Žislands of Ischia and Procida., have been erupted along NE–SW fractures ŽDi Girolamo and Rolandi, 1975; Beccaluva et al., 1985; Poli et al., 1987; Vezzoli, 1988; Allard et al., 1991; Civetta et al., 1991; D’Antonio et al., 1999-this volume.. Although the model here proposed has been applied to Campi Flegrei, it also suggests an explanation for the structural setting of the remaining volcanic districts on the Tyrrhenian margin, located in correspondence to transverse fractures. In fact, as
Fig. 9. Ža. Transfer fault zones and related volcanic activity along the western branch of the African Rift System. Inset shows the structure of the Kivu-Rusizi transfer fault zone and the relative volcanic products Žmodified after Ebinger, 1989b.. Žb. The main transfer fault zone of the Rio Grande Rift and the related Jemez Volcanic Field.
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can be seen from Fig. 1, major NE–SW fractures, eruptive fissures and dikes cluster at the volcanic districts of Amiata, Vulsini, Sabatini, Vico, Colli Albani, Roccamonfina and Vesuvio. Narrow rifts worldwide also display volcanic activity at transfer fault zones. Volcanism on the western branch of the African Rift System occurs, with a constant spacing, at transfer fault zones ŽFig. 9a; Ebinger, 1989a; Nelson et al., 1992 and references therein., where strike–slip faults have been observed ŽEbinger, 1989b.. The Rio Grande Rift also displays volcanic activity in correspondence with strike–slip faults, at the main transfer fault zone ŽFig. 9b; Aldrich, 1986.. These evidences suggest a wide-scale control of subvertical transfer fractures on volcanic activity during the extension of the Tyrrhenian margin as well as in other continental extensional frameworks worldwide ŽWestern African Rift System, Rio Grande Rift.. 7. Conclusions The CFVD shows both active tectonics and volcanism along a predominant NE–SW trending, constituting a preferred site to study the role of transverse fractures upon volcanic activity along the Tyrrhenian margin. Such a role has been investigated here by means of analogue and numerical modeling, coupled with previous structural analyses ŽCosentino et al., 1984; Acocella et al., 1997; Acocella and Funiciello, 1999.. The results suggest that the NE–SW volcanic ridge constituting the CFVD is characterized by subvertical transfer faults connecting adjacent, offset NW–SE regional faults parallel to the Tyrrhenian margin. In such context, volcanic activity along the preferred NE–SW direction at the CFVD would be induced by the subvertical dip of the transfer fractures. Transfer systems might also be responsible for the emission of the most primitive products, as consistent with the petro-chemical data collected at Campi Flegrei. Such a model, though best applied to Campi Flegrei, also allows us to explain the selective location of the remaining volcanic districts on the Tyrrhenian margin, systematically in correspondence to NE–SW fracture systems.
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Acknowledgements The authors wish to thank F. Rossetti for his help during the setting up of experiments, G. Giordano and the reviewers K.E.C. Krogh and G. Pasquare´ for their helpful comments that substantially improved the paper. The authors thank L. Civetta, G. Orsi and G.A. Valentine for their encouragement. This work was financed with Protezione Civile ŽOsservatorio Vesuviano. funds Žcode 21027r16..
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