Contractional tectonics and magma paths in volcanoes

Journal of Volcanology and Geothermal Research 176 (2008) 291–301

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

Contractional tectonics and magma paths in volcanoes Alessandro Tibaldi ⁎ Dipartimento di Scienze Geologiche e Geotecnologie, University of Milan-Bicocca, Italy

A R T I C L E

I N F O

Article history: Received 22 November 2007 Accepted 7 April 2008 Available online 24 April 2008 Keywords: compressional tectonics reverse faults volcanism Trohunco Los Cardos–Centinela El Reventador

A B S T R A C T This study aims to contribute a possible explanation for magma migration within volcanoes located in contractional tectonic settings, based on field data and physically-scaled experiments. The data demonstrate the occurrence of large stratovolcanoes in areas of coeval reverse faulting, in spite of the widely accepted idea that volcanism can develop only in extensional/transcurrent tectonic settings. The experiments simulate the propagation of deformation from substrate reverse faults with different attitudes and locations into volcanoes. The substrate fault splits into two main shear zones within the volcano: A shallow-dipping one, with reverse motion, propagates towards the lower volcano flank, and a steeper-dipping one, with normal motion, propagates upwards. In plan view, the reverse fault zone is arcuate and convex outwards, whereas the normal fault zone is rectilinear. Structural field surveys at volcanoes located in contractional settings show similar features: The Plio–Quaternary Trohunco and Los Cardos–Centinela volcanic complexes (Argentina) lie above Plio–Quaternary reverse faults. The Late Pleistocene–Holocene El Reventador volcano (Ecuador) is also located in a coeval contractional tectonic belt. These volcanoes show curvilinear reverse faults along one flank and rectilinear extensional fracture zones across the crater area, consistent with the experiments. These data consistently suggest that magma migrates along the substrate reverse fault and is channelled along the normal fault zone across the volcano. © 2008 Elsevier B.V. All rights reserved.

1. Introduction For decades, volcanism and regional extensional tectonics have been thought to be tightly linked, as this stress state favours magma upwelling along vertical fractures perpendicular to the regional horizontal least principal stress (σ3) (Anderson, 1951; Cas and Wright, 1987; Watanabe et al., 1999). For arc volcanism occurring at convergent margins, Nakamura (1977) states that the overall tectonics of the arcs should be strike–slip (with σ3 and greatest principal stress, σ1, both horizontal) instead of compressional reverse (σ3 vertical). This would allow magma to ascend through vertical dykes parallel to the direction of σ1 (Nakamura and Uyeda, 1980). This idea is consistent with the models of Hill (1977) and Shaw (1980) that suggest composite systems of tensional and shear fractures for dyke propagation as well as with field data (e.g. Tibaldi and Romero-Leon, 2000; Pasquarè and Tibaldi, 2003; Lara et al., 2006) and geophysical data (Roman et al., 2004). In other strike–slip settings, volcanism has been associated with local dilation occurring at releasing bends and pullapart basins (Pasquarè et al., 1988; Petrinovic et al., 2006; Busby and Bassett, 2007). By contrast, a true contractional tectonic environment, with reverse or transpressional faulting, is usually considered a highly unfavourable setting for volcanism (Glazner, 1991; Hamilton, 1995; ⁎ Complete address: Dipartimento di Scienze Geologiche e Geotecnologie, University of Milan-Bicocca, Piazza della Scienza 4, 20126 Milano, Italy. Fax: +39 02 64484273. E-mail address: [email protected]. 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.04.008

Watanabe et al., 1999) where only intrusive emplacement is expected (Cas and Wright, 1987). More recently, based on field mapping of plutonic rocks, it has been suggested that transpressional tectonics can be an efficient mechanism for moving magma through the lithosphere (Saint Blanquat et al., 1998), although Marcotte et al. (2005) suggest that transpression can result in the movement of only a small volume of magma to the surface. To investigate how magma rises through the brittle upper crust in the context of contractional tectonics, Galland et al. (2003) have performed experiments on scaled physical models. These authors suggest that magma in orogenic belts can rise along thrust faults, but horizontal compression favours thick flat-lying intrusions, and prevents magma from reaching the surface. However, in a more recent paper Galland et al. (2007a) suggest that magma can reach the surface along thrust faults. Field geological and structural data do show correlation between volcanism and reverse and strike–slip faults in the Mojave Desert area (USA, Glazner and Bartley, 1994), whereas field and geophysical data demonstrate that the entire history of development of the El Reventador volcano (Ecuador) occurred within a contractional tectonic regime with reverse or reverse-oblique faulting (Tibaldi, 2005). In recent years it has also been recognized that some other volcanoes lie close to major thrust faults, such as Guagua Pichincha in Ecuador (Legrand et al., 2002), Tromen in Argentina (Marques and Cobbold, 2002; Galland et al., in press), and some volcanic edifices in the Calchaquì valley of Argentina (Guzman et al., 2006) as well as in northern Japan (Yoshida, 2001). A true regional contractional tectonic environment must be distinguished from local compressional deformation resulting from

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gravitational spreading of substrata below the volcano load, such as at Socompa in Chile (van Wyk de Vries et al., 2001). In some cases the temporal coincidence between contractional tectonics and volcanism has not been unequivocably determined (e.g. Taapaca in Chile, Clavero et al., 2004), whereas in other instances it has been shown that the volcanic loading can induce a strain partitioning involving deflection and flattening of regional compressive structures (Branquet and van Wyk de Vries, 2001). Understanding whether magma can reach the surface in the presence of regional contractional tectonics is not only a “leading edge” matter of scientific debate, but also has major implications in terms of natural hazards mitigation and natural resource exploitation. The evaluation of volcanic and seismic hazards involves the reconstruction of the structural architecture and the stress state of the volcano and the surrounding basement. Moreover, when conducting hydrogeologic and geothermical studies in volcanic areas, it is crucial to be able to correctly identify the tectonic framework. This paper presents some field examples of clear correlations between major volcanism and coeval regional contractional deformation. Furthermore, results of experiments on scaled physical models, aimed at investigating how magma moves at shallower crustal depths across volcanic cones in a contractional strain field, are presented. These new experiments complete the work started by Vidal and Merle (2000) and Merle et al. (2001), who simulated deformation in volcanoes above vertical faults. The data presented here lead to a proposed mechanical model that explains: i) how volcanoes deform above moving reverse faults, ii) how magma can ascend across volcanoes under a regional compressional stress state, and iii) why classical morphometric analyses of volcano–tectonic relationships can be misleading in this tectonic environment. These observations cast some doubt on models of extensional magmatism that claim only crustal extension facilitates the transport of magma to the surface. Moreover, these data suggest the presence of volcanic edifices or extensional structures on volcanoes in a given area does not necessarily indicate regional extension.

2. Field examples Three examples are described here using a combination of new observations and published data. These are the El Reventador stratovolcano (Ecuador), toghether with the Trohunco and Los Cardos– Centinela volcanic complexes (Argentina). 2.1. El Reventador El Reventador is a late Quaternary active stratovolcano located in the Ecuadorian jungle of the sub-Andean zone near the Colombian border (Fig. 1) (Hantke and Parodi, 1966; Pichler et al., 1976; Hall, 1977). It has an average diameter of 14 km, a height of 3562 m a.s.l., and its substrate crops out at an altitude of 1800 m. Its lavas are basaltic andesites, andesites, dacites and rhyolites, which belong to a medium to high-K calcalkaline suite with an adakitic affinity (Barragan and Baby, 1999). Its oldest rocks have been dated to 0.34 Ma (INECEL, 1988). The volcano has a series of satellite volcanic domes and has produced two sector collapses towards the east (Fig. 2) (INECEL, 1988; Tibaldi, 2005). Its substrate is represented by a NNE– SSW-trending deformed belt made of early Palaeozoic–early Mesozoic crystalline rocks that were thrust onto the Sub-Andean zone in the Tertiary (Pasquarè et al., 1990). Around El Reventador, a series of N–S to NNE-striking faults, mostly west-dipping, have been active in Plio– Quaternary times with reverse and right-lateral reverse motion (Tibaldi, 2005). The largest of these faults crops out around the western part of the volcano and also affects the edifice (Fig. 2). Neotectonic field data and seismic data indicate that contractional tectonics have been active here during the Holocene and remain active at present, highlighting the coexistence of reverse and reverse-oblique faulting and volcanism (Tibaldi, 2005). Based on new field observations, the volcano succession has here been divided into three main units: the Late Pleistocene deposits (Coca Synthem), the Middle–late Pleistocene deposits (Malo Synthem), and

Fig. 1. Location of the studied field examples and their structural settings. The structures shown are the main faults of Quaternary age, based on Tibaldi (2005), Folguera et al. (2006) and Miranda et al. (2006). The different shading represent the various geological–physiographic domains, passing from the high Andean range to the West, to the Amazonian foreland to the East. Note the presence of other large Plio–Quaternary volcanoes aligned along the Andean frontal thrust zone both in Ecuador and in Argentina. CAF = Cayambe–Afiladores Fault.

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Fig. 2. Geological map of El Reventador volcano and the surrounding substrate based on field studies carried out in the 1980's for the Coca-Codo Sinclair Project in collaboration with others (INECEL, 1988) and updated by the author. The deposits of the Malo Synthem are affected by mainly N–S-striking faults (1) with downthrow to the east; their escarpments guided the successive emplacement of the lavas of the Coca Synthem. Note the presence of a system of arcuate faults (2) on the eastern cone flank. Compare structures (1) and (2) with the experimental results of Figs. 6 and 7 and model in Fig. 8.

the Holocene deposits (Fig. 2). These units are separated by major unconformities that are related to the development of the two sector collapses and resulting debris avalanches. The deposition of the Coca Synthem has been controlled by two main morphological depressions: the previous sector collapse amphitheatre that channelled subsequent lava flows eastwards, and a series of N–S-striking scarps, facing eastward (1 in Fig. 2), that channelled lava flows northwards and southwards. These N–S scarps correspond to high-angle faults with cumulative offsets of tens of metres, and dip–slip motion with relative downthrow to the east. The volcano is also affected by other faults: a series of short sub-vertical to vertical faults striking from N–S to NNE–SSW, are mostly present on the western flank of the volcano. On the northeastern flank there are some NW–SE-striking faults, whereas the southeastern flank is affected by NE–SW-striking faults. These faults seem to be linked by a series of lineaments that have been recognized from aerial photographs and airborne radar images (2 in Fig. 2). Although these structures have been partially hidden by the sector collapses and by the Holocene volcanic deposits, the overall pattern of structures found on the eastern volcano flank suggests the presence of arcuate faults that are concave to the east. It is also important to note that the bottom of the sector collapse depression displays a distinctive morphology where it intersects this arcuate fault zone. Here the floor of the sector collapse depression exhibits an abrupt increase in slope angle. Along this break in slope, there is a flat zone where Holocene lavas have ponded. This marked

increase of slope at the intersection with the arcuate fault zone could be the expression of a hidden reverse fault. 2.2. Trohunco Trohunco is a Plio–Pleistocene volcanic complex located in the Eastern Neuquén Andes of Argentina (Fig. 1). The data presented here are mainly based on the work of Folguera et al. (2006) and Miranda et al. (2006) integrated with new observations based on high resolution (1 m) satellite images. The volcano has an average diameter of 15 km, a height of 2700 m a.s.l., and its substrate crops out at an altitude of 1500 m. Volcanic deposits made of andesitic lavas and breccias dip radially outwards from a 15-km-wide semicircular depression interpreted as a caldera (Fig. 3, Folguera et al., 2006). Inside the caldera, Miocene volcanic breccias and tuffs outcrop on the hanging wall blocks of two west-dipping reverse faults (1 and 2 in Fig. 3). These faults are confined within the Miocene rocks and are interrupted by the Pliocene deposits of the Trohunco volcano, suggesting that the two faults moved prior to the emplacement of the volcano. In the middle of the caldera, Quaternary volcanic centres are aligned approximately N–S with two semi-circular scarps that probably originated from landslides. Note that the western parts of these scarps are aligned and strike N–S. A N–S-striking rectilinear fault offsets the Pliocene–Quaternary succession within the central and

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Fig. 3. Geological map of the Trohunco caldera complex and surrounding substrate based on interpretation of high-resolution satellite images and data from Folguera et al. (2006) and Miranda et al. (2006). In the center of the volcanic complex, note the presence of a Plio–Quaternary vertical fault with downthrow of the eastern block (3) and reverse faults on the eastern cone flank. (1) and (2) are pre-volcano faults. Compare structures (3) and reverse faults on the eastern flank with the experimental results of Figs. 6 and 7 and model of Fig. 8.

northern parts of the caldera floor (3 in Fig. 3). This structure has already been regarded as a reverse fault by Folguera et al. (2006), but here I point out its rectilinear trace in plan view across a rugged topography, indicating a vertical dip. The block east of this fault is relatively downthrown. On the eastern caldera floor, Miocene volcanic breccias are thrust onto Miocene sedimentary rocks along a series of west-dipping fault planes. Some of these planes are confined within the Miocene rocks, but others also cut the Pliocene or the Plio– Quaternary deposits indicating faulting coeval to the emplacement of the Trohunco volcanic complex. The orientation, kinematics and location of this fault set are consistent with the regional trace of the Quaternary Antiñir–Copahue fault zone (Figs. 1 and 3) (Miranda et al., 2006). 2.3. Los Cardos–Centinela Los Cardos–Centinela volcanic complex is also located in the Eastern Neuquén Andes of Argentina, about 15 km northeast of the Trohunco caldera (Fig.1). The volcano has an average diameter of 19 km, a height of 2841 m a.s.l., and its substrate crops out at an altitude of 1500 m. The rocks are represented by lava flows and breccias of basaltic composition and pyroclastic deposits (Miranda et al., 2006). The older rocks are composed of basal breccias and lava flows dated 3.2–26 Ma (Rovere, 1993). Above these there is a sequence of volcanoclastic deposits and lava flows of Quaternary age with some eruptive centres still recognizable, mantled by lavas erupted from other small centres (Fig. 4). Analysis of morphometric parameters of coeval centres such as crater and cone base elongation, and multiple cone alignment (Tibaldi, 1995), indicates the control exerted by NW-striking fractures on their emplacement in the northeastern part of the complex (C3 zone in Fig. 4), and by NNW- to N–S-striking fractures in the central part of the complex (C2 zone in

Fig. 4). As the trace of the fractures is rectilinear in plan view across a rugged topography, their dip should be vertical. The age of these eruptive centres has been assigned to the late Pliocene or Pleistocene by Folguera et al. (2006) and, at least in part, younger than 30 ka by Miranda et al. (2006). These authors also indicate the presence of a kink fold affecting the eastern slope of the Los Cardos–Centinela volcanic complex. Here the Pliocene lavas have a primary dip of 15 , but a kink increases the average dip to 25°. Folguera et al. (2006) suggest that this kink fold is associated with a west-dipping thrust fault exhuming the core and substrate of the volcanic complex. Farther east, the main frontal thrust of the Plio–Quaternary Guanacos fold-and-thrust belt, known as the Nahueve Fault, is located (Fig. 1 and F3 in Fig. 4). This fault strikes NW–SE and dips to the SW. A NNW-striking reverse fault (F1 in Fig. 4) affects the western flank of the volcanic complex, confined within the Miocene rocks, whereas a series of Plio–Quaternary folds with NNWtrending hinge lines are present farther west. Another fault (F2 in Fig. 4) dips WSW to W and offsets Plio–Quaternary deposits, suggesting it might belong to the Quaternary Antiñir–Copahue fault zone (Fig. 1) (Miranda et al., 2006). NNE- to ESE-striking dykes are present along the scarp of a huge landslide (not reported in Fig. 4) but their orientation might be related to instability or unbuttressing along this volcano flank, similar to many other volcanoes where flank failure has taken place (e.g. Tibaldi, 1996; Tibaldi et al., 2008a,b and references therein). 3. Analogue modelling 3.1. Scaling procedure The experimental apparatus (Fig. 5) has a fixed rigid substratum and a moveable block that is displaced by a regulatable engine and a

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Fig. 4. Geological map of the Los Cardos–Centinela volcanic complex and surrounding substrate based on the interpretation of high-resolution satellite images and data from Folguera et al. (2006) and Miranda et al. (2006). Fault (1) is a pre-volcano structure. In the middle of the volcanic complex the N–S alignment of volcanic centers (C2) can be linked to the reverse fault (F2), similar to the experimental results of Figs. 6 and 7 and model of Fig. 8. The same goes for structures C3 and F3.

pulley system. A constant sliding rate is maintained enabling measurement of displacements with a 1-mm precision. During the 80 experiments performed, geometric values were maintained as close as possible to field conditions, whereas several factors were varied to investigate their influence on the deformation processes. These included: a) the dip of the fault plane, b) the material between the table

and the modelled volcano (plastic film, dry sand, wet sand), c) the material composing the cone, d) the position of the cone with respect to the fault trace, and e) the fault offsets. During each experiment, a scaled volcanic cone was built with a funnel above the substratum where a fault dipping to the west with respect to the reference system has an offset = 0 at time t = 0. At each tn

Fig. 5. Sketch of the experimental device used during analogue tests. High precision (1 mm = 40 m in nature) vertical offset is monitored by movements of the pulley-wire system. The volcano has been constructed with a funnel in different positions with respect to the surface fault trace. The hanging wall and footwall blocks have been changed after each set of experiments in order to simulate faults with different dips.

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two fixed digital cameras captured in plan view and in section view the image of the cone deformation for a given offset increment Δn. Fault location, strike, dip, length and offset were accurately measured at each incremental step for each experiment. In section view, with the aid of piercing points, the deformation paths were measured by superimposing the digital images of the various incremental deformations. At the end of each experiment, when the scaled maximum value of offset was reached, the deformed volcano was cut vertically in slices that enabled verification of the geometry, kinematics and amount of deformation inside and across the cone. In some cases, the model was sliced at lower incremental offsets to illustrate earlier, inner deformation. The theory of scale modelling requires that to be a valid representation of a natural prototype, a model must be scaled both kinematically and dynamically (Hubbert, 1937; Ramberg, 1981). A well-developed scaling methodology has been established for tectonic (e.g. Corti et al., 2002; Lohrmann, 2003) and volcano (e.g. Tibaldi, 1995; Merle and Borgia, 1996; Van de Wyk de Vries and Merle, 1996; Vidal and Merle, 2000; Lagmay et al., 2000; Acocella, 2005; Tibaldi et al., 2006) deformation. To achieve dynamic similarity, the ratio of gravitational to cohesive forces (R = ρgL/C, where ρ = density, g = gravity acceleration, L = characteristic length, C = cohesive strength) must be the same in the model and in nature (R prototype / R model = 1; Hubbert, 1937; Ramberg, 1981; Weijermars and Schmeling, 1986). This condition is achieved in natural gravity (g prototype = g model) by choosing the right linear scale factor (L prototype / L model) as a function of prototype and model material parameters (ρ and C). The results of scaling are shown in Table 1 as values of the parameters of the prototype versus those of the model. Under brittle conditions, deformation is not time-dependent provided that inertial forces are negligible (Hubbert, 1937). Three different analogue materials have been selected (SQS sand, Q100 sand and flour) that simulate the main types of volcanic materials directly observed at El Reventador volcano, and which should also be representative of the other cones (properties are listed in Table 2). The apical angle of the cone is similar in the model and in nature, in agreement with the principle that angular quantities, being dimensionless, should be equal in the prototype and in the model (Hubbert, 1937). Thus, the scaled variables are: density, cohesion, height and width of the volcano, and fault offset. The increments of the substrate fault displacement were reproduced at 1-mm steps, corresponding to 40 m in nature, reaching a total reverse-slip offset of 3.4 cm (1.3 km in nature). This amount corresponds to the measured slip rate of 4.3 ± 2.2 mm/yr in Holocene reverse faults located near El Reventador volcano (Tibaldi et al., 2007b) and to the volcano age of 0.34 Ma (INECEL, 1988). I also modelled lower offsets corresponding to deformation in parts of the cone made of younger deposits or lower slip rates of 1 mm/y, such as those found on oblique reverse faults north of El Reventador (Tibaldi and Romero-Leon, 2000). In this case, 1 cm was the maximum offset reached in a series of experiments, corresponding to 340 m in nature. This range of slip rates and total fault offset amounts is also quite reasonable for the other studied volcanoes. The analogue cone height and diameter have been scaled for the volcano dimensions (El Reventador av. Ø = 14 km, h above the

Table 2 Physical properties of analogue materials used in the experiments and volcanic rock masses at the studied volcanoes

Model

Prototype

Material

Density (t/m3)

Cohesion (Pa)

Refs.

SQS sand (0.150–0.210 mm) Q 100 sand (0.100 mm) Flour Pyroclastic deposits Andesitic/basaltic fractured lavas Andesitic/basaltic poorly fractured lavas

1.50 1.20 1.20 2.40 2.65 2.65

25 69 330 1.E+06 5.E+06 2.E+07

a, c a, c a b, d b, d b, d

(a) Krantz (1991), (b) Saotome et al. (2002), (c) Lohrmann et al. (2003), (d) Apuani et al. (2005).

substrate = 1.7 km; Trohunco av. Ø = 15 km, h = 1.2 km; Los Cardos– Centinela av. Ø = 19 km, h = 1.3 km, see also Table 1). The cone was composed of alternating layers of different materials (SQS, Q100 sand and flour) to simulate internal lithologic variations with different cohesion. Colored flour has been used to emphasize the strata geometry in cone slices. 3.2. Results of analogue experiments In all the experiments the substrate reverse fault has the same dip direction to the west, but its position has varied by 2, 4 and 6 cm to the west and to the east with respect to the cone top. This has been done in order to verify the various possible deformation modes of a volcano with respect to the location of the substrate fault. The substrate fault dip was 25°, 30°, 35° or 40° in different experiments, in order to check if different dips might enhance deformation in the volcano. Six selected main types of tests are described here (see also Table 3): The first three have a substrate fault dip of 25°; the fault is displaced 6 cm to the west with respect to the cone top, located below the cone top,

Table 3 Main characteristics of the experiment types and results Type Figure Substrate Fault dip Fault Position of fault type direction dip volcano 1

7A

Reverse

West

25°

2

7B, 6

Reverse

West

25°

3

7C

Reverse

West

25°

4

7D

Reverse

West

40°

5

7E

Reverse

West

40°

Table 1 Results of scaling expressed as values of model vs. prototype parameters

Model El Reventador Model Trohunco Model Los Cardos–Centinela

Cone height

Cone ray

Density (g/cm3)

Gravity (m/s2)

Cohesion (Pa)

4.3 cm 1.7 km 3.0 cm 1.2 km 3.3 cm 1.3 km

17.5 cm 7 km 18.7 cm 7.5 km 23.7 cm 9.5 km

1.35 2.6 1.35 2.6 1.35 2.6

9.8 9.8 9.8 9.8 9.8 9.8

80–100 107 80–100 107 80–100 107

6

7F

Reverse

West

40°

Footwall block

Type of structures in the cone

Reverse faults Normal faults Central Reverse faults Normal faults Extension fissures Hangingwall Reverse block faults Normal faults Extension fissures Footwall Reverse block faults Central Reverse faults Normal faults Extension fissures Hangingwall Reverse block faults Normal faults Extension fissures

Position of structures in the cone North flank South flank Summit

East flank Summit East flank

West flank

East flank Summit East flank

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and displaced 6 cm to the east with respect to the cone top. Another three tests are similar but with a substrate fault dip of 40°. 3.2.1. Test type 1 The substrate fault dips 25° to the west (i.e. to the left in Figs. 6 and 7). The analogue cone is placed with its vertical axis (i.e. summit crater) 6 cm towards the substrate footwall block (i.e. 6 cm east of the fault). After an initial series of incremental deformation steps, at 0.5 cm dip– slip displacement two structures are clearly visible: 1) An extensional fissure (Ef) striking N–S, rectilinear in plan view, and visible in the northern and southern lower flanks of the cone, and 2) a curvilinear zone of deformation (F2), with an outward convex side, developed from the substrate fault passing close to the cone apex. The successive increments of substrate fault offset cause an increase in cone deformation (Fig. 7A) with a slip increase at F2. The Ef turns into a shear zone F1 with relative downthrow of the eastern block. The complete development of F1 and F2 isolates the block W1 (e.g. Fig. 7C, E and F). 3.2.2. Test type 2 The analogue cone is placed with its vertical axis (i.e. summit crater) above the substrate fault. At 0.5 cm of dip–slip displacement two structures are presented: 1) An extensional fissure (Ef) cutting across the upper part of the cone with a rectilinear trace in plan view, and 2) a curvilinear zone of deformation F2, with an outward convex side, that develops from the substrate fault towards the middle of the eastern cone flank. At 1 cm of substrate fault offset, there is a slip increase at F2, and Ef turns into shear zone F1 with a relative downthrow of the eastern block. At 2 cm of substrate fault offset (Fig. 7B), two sets of normal faults with converging dips produce a N–S-trending graben passing through the cone summit area. The new fractures show an earlier development towards the west and a successive one towards the east (i.e. within W1). NNW-striking fractures and faults develop in the northern lower cone flank between F1 and F2; NNE-striking structures develop in the southern lower flank. Along the surface trace of F2 some landslides appear. The slicing of the cone at the end of each experiment (e.g. Fig. 6B, C and D for Type 2) enables verification and completion of the observations made in plan view (note that in Fig. 6C and D, due to the perspective of the photographs, the substrate fault is hidden by the hangingwall block and is shown by a dashed line). During the first incremental steps of deformation, vertical to sub-vertical tension fissures start to develop (Ef in Fig. 6B). F2 starts from the substrate fault with the same dip but immediately rotates, gradually acquiring a lower dip. With major offset of the substrate fault, Ef turns into fault F1, dipping at high angle (80–90°) above the substrate fault and showing normal offset (i.e. downthrow of the eastern block). A wedgeshaped block (W1) is formed between F1 and F2, showing, in section view, a general tendency to movements of the topographic surface upwards and outwards. The W1 block is subject to internal drape-like deformation around the propagating tip of F2. At about half the distance between the substrate fault and the volcano topographic surface, the F2 slip plane parallels the attitude of the volcanic strata. As a whole, F2 has an upward-convex shape in section. At the largest substrate offsets, F1 is composed of a series of normal faults with converging dips. 3.2.3. Test type 3 The analogue cone is placed with its vertical axis 6 cm towards the substrate hanging wall block, i.e. 6 cm west of the substrate fault. At 0.5 cm dip–slip displacement two structures are recognizable: 1) An extensional, highly rectilinear fissure (Ef) cutting across the middle eastern flank of the cone, and 2) a curvilinear, convex outward, zone of deformation (F2) that develops from the substrate fault towards the lower eastern cone flank. At larger substrate fault offsets (Fig. 7C), there is a slip increase at F2 and Ef turns into shear zone F1 with a relative downthrow of the eastern block. With the largest offsets, F1 is

Fig. 6. Experimental results from tests with different amounts of substrate fault offset, with a substrate fault trace located below the volcano top. A. shows the volcano prior to slicing. Piercing points enable measurement of the deformation path of the cone topographic surface (arrows). In the left column there are photos of the original sliced walls, whereas in the right column B., C. and D. show the relative interpretation, with increasing amounts of substrate fault offset passing from B. to D. Note the different deformations in the cone with increasing substrate fault offset, with the transition from extensional fissures (Ef) to normal faults (F1) and the offset increase of F2. In C. and D. the development of F1 and F2 creates a wedge-shaped block, labelled W1. In Fig. 6C and D, due to the perspective of the photographs, the substrate fault is hidden by the hangingwall block and is shown by a dashed line.

characterized by two parallel faults with converging dips giving rise to a very narrow N–S-trending graben. 3.2.4. Test type 4 The substrate fault dips 40° to the west. The analogue cone is placed with its vertical axis 6 cm towards the substrate footwall block (Fig. 7D). All the deformation history is characterized by the development of a rectilinear N–S-trending zone of shear with upthrow of the western block. This fault zone is located in the middle of the western cone flank and, given its surface trace, it probably represents the development of a high-angle reverse fault. At 2 cm of substrate fault offset, the traces of two reverse faults become visible. 3.2.5. Test type 5 The analogue cone is placed with its vertical axis above the substrate fault, dipping 40° to the west. At 0.5 cm of dip–slip displacement it is possible to observe: 1) An extensional fissure (Ef) cutting across the upper part of the cone with a slightly curved trace in

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Fig. 7. Experimental results from tests with the same amounts of substrate fault offset but different fault dips and locations. The dip direction is always to the left (i.e. to the west in a chosen system of reference). The column to the left illustrates experiments with a substrate fault dip of 25°, the column to the right shows experiments with a fault dip of 40°. In A. and D. the volcano top is placed above the substrate fault hanging wall (i.e. west of the fault); in B. and E. the substrate fault intersects the substrate-volcano interface below the cone top; in C. and F. the volcano top is above the footwall block (i.e. east of the fault).

plan view (convex to the west), and 2) a more curvilinear, convex east, zone of deformation (F2) that develops from the substrate fault towards the middle of the eastern cone flank. At 1 cm of substrate fault offset, there is a slip increase at F2 and Ef turns into shear zone F1 with dominant downthrow of the eastern block. Approaching 1.5–2 cm of substrate fault offset (Fig. 7E), a rectilinear normal fault, dipping west, develops to the east of F1, producing a N–S-trending asymmetric graben passing through the cone summit area. Very rare NNWstriking and NNE-striking fractures/faults develop in the northern and southern, respectively, lower cone flanks between F1 and F2. Along the surface trace of F2 some landslides appear. Slicing of the cone enables confirmation of the data as seen in plan view and unveils a general geometry similar to the one described in Test type 2. 3.2.6. Test type 6 The analogue cone is placed with its vertical axis 6 cm towards the substrate hanging wall block (Fig. 7F). With incremental steps of the substrate fault displacement and at the maximum fault offset, the results are very similar to Test type 3. 4. Discussion and conclusions 4.1. Deformation of the studied volcanoes The data presented here indicate that contractional deformation within regional compressional tectonic settings developed during the emplacement of the El Reventador, Trohunco and Los Cardos– Centinela volcanic complexes. These volcanoes grew above reverse faults and are offset by reverse faults, thus they actually developed under the direct influence of a substrate compressional state of stress. This also demonstrates that in these conditions magma can rise to the surface to build large edifices and feed volcanic activity for a long time, as these volcanoes are from 0.35 Ma (El Reventador) to about 3 Ma old

(Trohunco and Los Cardos–Centinela). This is more remarkable than other examples of volcanoes developed in regional contractional tectonic settings, which are situated entirely on the hanging wall block of reverse faults: In these latter cases the volcanoes were piggyback transported along reverse faults without being directly affected by the faults, such as at Guagua Pichincha in Ecuador (Legrand et al., 2002) and Tromen in Argentina (Marques and Cobbold, 2002; Galland et al., in press). Common to the three example volcanoes here reported is the presence of a larger number of faults than in their substrates, of course excluding pre-volcano substrate faults. This suggests that some of the faults coeval to the volcano might represent the splitting of the substrate fault into secondary planes or readjustment of the cone in the framework of the general deformation field. It is important to point out that most of the outcropping faults and fractures have a general trend that mimics the substrate faults. For example, inside the Trohunco caldera the faults have a N–S strike, which is parallel to the faults affecting the substrate. The folds found in the area indicate an E– W-directed shortening (Folguera et al., 2006) consistent with dip–slip reverse motion along the N–S-striking faults. In plan view, however, these faults show some differences: The faults located along the western caldera floor and those located immediately east of the caldera show sinuous traces that suggest a gentle dip. By contrast, the fault passing through the summit of the intracaldera volcano has a clear rectilinear trace in plan view, compatible with a vertical dip. In the southwestern flank of the Los Cardos–Centinela volcano, faults strike NNW, just like the fault swarm located in the substrate farther SW, whereas immediately east of this volcanic complex, the main frontal thrust (Nahueve Fault) of the Plio–Quaternary Guanacos foldand-thrust belt can be recognized (Miranda et al., 2006). The N–S faults across the eastern part of the Trohunco caldera, belonging to the Antinir–Copahue fault zone, and the NW–SE faults east of the Los Cardos–Centinela volcanic complex are connected

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southwards with the NNE-striking right-lateral strike–slip Liquine– Ofqui fault zone (Fig. 1) (Melnick et al., 2006; Miranda et al., 2006). The geometric relationships between these structures confirm at a regional scale the kinematics observed more locally. In fact, the rightlateral strike–slip motions of the Liquine–Ofqui fault zone should induce transpressional motion along the N–S faults at Trohunco caldera with a possible right-lateral component, and reverse dip–slip (pure contractional) motion on the NW–striking faults at Centinela volcano. At El Reventador, the presence of west-dipping reverse faults is consistent with the dominant regional architecture in which Quaternary west-dipping reverse and right-lateral reverse faults are present in the substrate, indicating that the regional state of deformation coeval with the volcano has been governed by a component of E–W contraction (Fig. 1) (Tibaldi, 2005). Close to the summit zone of El Reventador there is the N–S-striking vertical scarp with downthrow of the eastern block that channelled the emplacement of the younger deposits. This structure represents a high-angle fault linked to local extension. Local extension in a volcanic cone can be recognized also by the preferential orientation of dykes and eruptive fissures, because these propagate upwards as hydrofractures that result from magma overpressuring orthogonal to the local least principal stress (σ3, Gudmundsson, 1990, 1995). Where dykes or eruptive fractures are lacking, Dhont et al. (1998) and Adiyaman et al. (1998) propose, as suitable indicators, alignments of volcanic clusters. Similarly, Tibaldi (1995) recommends the use of morphometric parameters of monogenic cones. Although at El Reventador there is no clear alignment of monogenic cones that could suggest the orientation of the feeding system, this is possible at the other two studied volcanoes. At Los Cardos–Centinela, tens of monogenic cones have characteristics indicating a NNW–SSE to N–S orientation of the fractures feeding magma to the surface in the central part of the volcanic complex and a NW–SE orientation in its northeastern part. At the Trohunco caldera, the limited number of vents show a N–S alignment. 4.2. Comparing field data with analogue modelling All analogue experiments performed in this study resulted in the propagation of the substrate fault through the cone. In cases where the volcano summit was located on the footwall block, the substrate fault cut across the cone flank creating essentially one zone of deformation, which was characterised by reverse motion (e.g. Fig. 7A and D). This pattern was found to be the same for all the investigated fault dips, from 25° to 40°. This can be compared with the reverse fault that outcrops along the substrate and the western lower flank of El Reventador volcano (west of fault 1 in Fig. 2), which maintains a single fault plane and a low dip when crossing the cone. In the experiments where the cone was modelled with its top lying above the surface trace of the substrate fault (Fig. 7B and E), the substrate fault splays into a subvertical normal fault F1 (to the west) and a subhorizontal thrust F2 (to the east). This is similar to the findings of Vidal and Merle (2000) and Merle et al. (2001), although they refer to a substrate vertical fault and hence not to a contractional setting. In all the experiments conducted here, F1 has a rectilinear trace in plan view, is associated with parallel tension fissures, and passes through the cone summit zone. This outcome indicates that, in this case, a zone of local extension forms across the cone, whereas contractional deformation is concentrated along a shear zone located at lower elevations in the cone flank and with an arcuate trace in plan view (Fig. 8). All my field examples bear similarities to this case: At El Reventador the N–S rectilinear fault trace (1 in Fig. 2) might have the same significance as F1 in the experiments, whereas the arcuate zone of deformation affecting the eastern volcano flank (2 in Fig. 2) may represent the shallow-dipping thrust (F2 in the experiments). At Trohunco, the F1 structure of the experiments is represented by the N–S rectilinear fault crossing the recent active vent zone (3 in Fig. 3),

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Fig. 8. Plan and section view of a general model, based on the field data and analogue modelling here presented, that documents the splaying of a substrate reverse fault into two faults inside a volcanic cone and their relationship with magma paths. One fault in the cone (to the right, F2) has a shallower dip than the substrate fault and tends to acquire a horizontal geometry (thrust fault) and to prolong the contractional deformations towards the lower sector of the volcano flank (converging arrows). The other fault (to the left, F1) of the splay has a steeper dip than the substrate fault and is the site of local extensional deformation (diverging arrows). This favours upwelling of magma in the cone, that tends to migrate along the substrate reverse fault and then along the steeper-dipping fault zone, as suggested by the field data discussed in the main text.

whereas the curvilinear reverse faults of the Antiñin–Copahue fault zone, located along the lower eastern flank, represent F2 in the experiments. At Los Cardos–Centinela there are two main zones of magma emplacement: One in the middle of the volcanic complex (C2 in Fig. 4) and one farther NE (C3 in Fig. 4). The C2 zone is characterised by a N–S-striking fracture feeding magma to the surface, whereas the C3 zone is represented by NW-striking fractures that favoured multiple eruptions. Here I suggest that C2 is the zone of extension associated with the reverse fault F2; in fact both strike N–S and are compatible, in terms of position and kinematics, to the model in Fig. 8. C3, in turn, is the zone of extension associated with the reverse fault F3, having once again the same (NW–SE) strike and compatibility with the model. Finally, the kink fold reported by Folguera et al. (2006) in the eastern volcano flank might represent the deformation observed along the F2 shear zone of the analogue models. The analogue experiments simulate the cumulative displacement and do not consider erosion and deposition of erupted materials during development of the deformation. Thus, examples in nature cannot be identical to those in laboratory: Erosion and deposition along the volcano slopes can mask the trace of the low angle fault F2; moreover, this trace can be largely disrupted by huge lateral collapses such as has occurred at El Reventador. Also in the volcano summit zone, the emplacement of fresh lavas or the construction of monogenic cones can hinder the recognition of the extensional structures F1; this could be the case for the widespread summit activity at the Los Cardos–Centinela volcanic complex. 4.3. A general model The modelling presented here complements the work carried out by Galland et al. (2007a) on the movement of magma in the crust along thrust planes, demonstrating that magma can propagate to the surface in a tectonic regime characterised by contractional deformation. If the thrust in the substrate is arcuate in plan view, vertical tension fractures can form in the shallower portion of the hanging wall almost parallel to the imposed shortening (Galland et al., 2007a). These structures can be attributed to superposition of a local load, due to the uplifted area, on the regional stress field (Johnson, 1970). Although some volcanoes located in contractional settings have dykes parallel to the direction of σ1, as found at Iwate (NE-Japan) during the 1999 crisis (http://hakone.eri.u-tokyo.ac.jp/vrc/erup/iwate.html) and at Tromen (Galland et al., in press), the experiments presented here do not reveal fractures parallel to the contraction direction. In fact, in the

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cases investigated here, the reverse/thrust faults affecting the substrate are not arcuate in plan view, differing to those simulated in the experiments of Galland et al. (2007a). In my experiments, the propagation of the rectilinear (in plan view) reverse faults of the substrate across the volcanic cone results in a local extensional field with σ3 parallel to the general contraction direction. This is sketched in Fig. 8 where a general model summarizes the main features obtained by the field data and by analogue modelling. The model presents in plan and section view the splaying of a substrate reverse fault into two faults within a volcanic cone and their relationship to magma paths. One fault in the cone (to the right in Fig. 8, F2) has a shallower dip than the substrate fault and tends to acquire a horizontal geometry (thrust fault) and continues the contractional deformation towards the lower sector of the volcano flank (converging arrows). The other fault (to the left in Fig. 8, F1) of the splay has a steeper dip than the substrate fault and is the site of local extensional deformation (diverging arrows). I suggest that in a contractional tectonic setting, below a volcano, magma might migrate along the substrate reverse fault and then, within the volcano, along the steeper-dipping fault zone F1. This magma migration along the steeper fault is facilitated by the localised extension and is supported by the field occurrences of eruptive vents close or along the F1 zones. This case corresponds to the situation where the substrate reverse fault intersects the cone base below the volcano summit (Fig. 7B, E). In this case, in fact, the location of the substrate fault leads to a fault splay directly beneath and intersecting the volcano summit at the location of the actual vent. In the other conditions investigated, if the cone is located towards the footwall block (i.e. eastwards in the experiments), with a shallower-dipping substrate fault, the summit of the volcano goes into compression (Fig. 7A). Or, with a steeper substrate fault, contractional deformation concentrates west of the summit (Fig. 7D). These conditions are not favourable to magma migration into the cone and may favour more frequently sills and other subvolcanic intrusions. If the cone is located towards the hangingwall block (i.e. westwards in the experiments), the steeper normal fault splay intersects the eastern volcano flank, whatever the substrate fault dip (Fig. 7C and E). This might suggest that this condition leads to the migration of the vent towards the footwall block (i.e. eastwards in the models). Magma migration along faults is consistent with several cases found in different geodynamic settings and spanning from evidence at deep crustal level (e.g. Rosenberg, 2004), to near surface levels (e.g. Tibaldi et al., 2008a,b), and to emplacement of dykes and vents along pre-existing fractures (Tibaldi, 1995; Ziv and Rubin, 2000; Corazzato and Tibaldi, 2006). In most of these cases magma propagates and emplaces in the form of tabular sheets that have been classically regarded as forming perpendicular to σ3 (Anderson, 1951; Tsunakawa, 1983). However, this assumption is invalid when dyke injection occurs within strike–slip shear zones, including transtensional and transpressional regimes (e.g. Rossetti et al., 2000). In the case of the contractional regime discussed here, at Los Cardos–Centinela volcanic complex there are two main zones of aligned vents that could indicate dyking with a N–S to NW–SE strike. These alignments are normal to the regional direction of compression, here from E–W to NE–SW. If the rising magma acquires a dyke geometry with the same strike as the splay fault, then the application of the classical Nakamura (1977) criteria (that considers the direction of the regional σHmax as coincident with dyke or vent alignment) can be misleading in contractional settings. In fact, in all the field cases and analogue experiments illustrated here, the possible strike of the fracture feeding magma to the surface is perpendicular rather than parallel to the regional σHmax. Acknowledgements Constructive reviews of a previous version of the manuscript by Olivier Merle and Diana C. Roman are greatly appreciated. Derek Rust is acknowledged for having improved the English style. This is a

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