formerly Revista Geologica de Chile
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Ophiolite Emplacement and the Effects of the Subduction of the Active Chile Ridge System: Heterogeneous Paleostress Regimes Recorded in the Taitao Ophiolite (Southern Chile)
Emplazamiento de ofiolitas y los efectos de la subducción de la dorsal activa de Chile: Regímenes heterogéneos de paleostress registrados en la Oflolita Taitao (Sur de Chile)
Eugenio E. Veloso1,2, Ryo Anma2, Atsushi Yamaji3
1 Facultad de Ingenieria y Ciencias Geologicas, Departamento de Ciencias Geologicas, Universidad Catolica del Norte, Av. Angarrios 0610, Antofagasta, Chile. eveloso@ucn.cl
2 Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japan. anma@arsia.geo.tsukuba.ac.jp
3 Division of Earth and Planetary Sciences, Graduate School of Sciences, Kyoto University, Kyoto 606-8502, Japan. yamaji@kueps.kyoto-u.ac.jp
ABSTRACT. The repeated north and southward migration of the Chile Triple junction, offshore the Peninsula de Taitao, is expected to have imposed contrasting stress fields in the forearc for the last 6 Ma because of changes in convergence direction and rate of subducting plates. NNW-SSE to E-W and minor NE-SW striking brittle faults developed in the plutonic units of the Mio-Pliocene Taitao Ophiolite, whereas NNE-SSW and minor NW-SE trending faults developed in its eastern border (Bahia Barrientos fault-zone). These brittle faults are studied to elucidate the style of ophiolite emplacement and the tectonic effects resulting from the alternated migration of the Chile Triple junction in the area. Analyses of heterogeneous fault-slip data on both areas suggest that faults were activated by different stress fields. Two different compressional stress fields were identified in the plutomc units (A and B), whereas three different stress fields, ranging from compressional to strike-slip, were identified in the Bahia Barrientos fault-zone (C, D and E). Calculated directions of Oj axes for A, C, D and E solutions are mostly E-W trending, roughly similar to the convergence direction of subducting plates, whereas that for B solution is counterclockwise rotated ca. 60° with respect to the previous E-W trend. Brittle structures related to solution B were attributed to an early deformation of the ophiolite, most probably developed shortly after its emplacement (ca. 6 Ma). These structures were further counterclockwise rotated, while new structures (related to solution A) developed in the plutomc units in order to absorb the continuous deformation. In the eastern margin of the ophiolite, the stress field divided inte compressional and strike-slip components. During periods of relatively strong compression (fast subduction of the Nazca plate), the fault-zone experienced well defined compressional and strike-slip movements (solutions C and D). In contrast, during periods of relatively weak compression (slow subduction of the Antarctic plate), the fault-zone experienced a complex mixture of thrust and strike-slip movements (solution E). Thus, the wide range of calculated stress ratios for all solutions could be attributed to the alternated change in convergent velocity of the subducting plates beneath the Taitao area.
Keywords: Brittle deformation, Paleostress, Heterogeneous stress field, Taitao Ophiolite, Chile Triple junction.
RESUMEN. Es esperable que la repetida migracion norte-sur del Punto Triple de Chile cercana a la costa de la Peninsula de Taitao haya impuesto campos de esfuerzos distintos en el antearco por los ultimos 6 Ma debido a cambios en la direccion de convergencia y tasa de subduccion. Fallas con rumbos NNO-SSE y E-O y otras menores con rumbos NE-SO se desarrollaron en las unidades plutonicas de la Oflolita Miocena-Pliocena de Taitao, mientras que fallas con rumbos NNE-SSO y otras menores con rumbos NO-SE se desarrollaron en el borde Este de esta (Zona de falla de Bahia Barrientes). Estas estructuras fragiles son estudiadas para dilucidar el estilo de emplazamiento de la oflolita y los efectos tectonicos resultantes de la migracion alternada del Punto Triple de Chile en al area. Analisis de datos heterogeneos de fallas en ambas areas sugieren que fueron activadas por campos de esfuerzos diferentes. Dos campos de stress compresionales distintos fueron identificados en las unidades plutonicas (A y B), mientras que 3 campos de stress diferentes, desde compresionales a transcurrentes, lo fueron en la zona de falla de Bahia Barrientes (C, D y E). Las direcciones de los ejes a1 calculadas para las soluciones A, C, D y E son principalmente este-oeste, similares a la direccion de convergencia de las placas subductantes, mientras que aquella para la solucion B esta rotada antihorario ca. 60° con respecto del patron anterior este-oeste. Las estructuras fragiles relacionadas con la solucion B fueron atribuidas a una deformacion temprana de la ofiolita, muy probablemente desarrolladas despues de su emplazamiento (ca. 6 Ma). Estas estructuras fueron posteriormente rotadas antihorario, mientras nuevas estructuras (relacionadas con la solucion A) se desarrollaron en las unidades plutonicas para absorber la deformacion continua. En el margen este de la ofiolita, el campo de esfuerzo se dividio en componentes compresionales y transcurrentes. Durante periodos de compresion relativamente alta (subduccion rapida de la placa de Nazca) la zona de falla experimento movimientos compresionales y transcurrentes bien definidos (soluciones C y D). En contraste, durante periodos de relativa baja compresion (lenta subduccion de la placa Antartica) la zona de falla experimento una compleja mezcla de movimientos inversos y transcurrentes (solucion E). Asi, el amplio rango de radios de stress calculados para todas las soluciones puede ser atribuido al cambio alternado en la velocidad de convergencia de las placas subductantes bajo el area de Taitao.
Palabras claves: Deformaciones fragiles, Paleostress, Campos de stress heterogeneos, Ofiolita Taitao, Punto Triple de Chile.
1. Introduction
The processes of emplacement and the tectonic evolution of the Late-Miocene Taitao Ophiolite (southern Chile) has long been a matter of debate, although is one of the best natural examples of ophiolite emplacement and ridge-trench interaction (e.g., Lagabrielle et al, 2000). The Taitao Ophiolite is located approximately at 75°30'W and 46°40'S, at the westernmost point of the southern coast of Chile (Fig. 1a). It consists of a complete sequence of oceanic lithosphere including ultramafic rocks, gabbros. dike complexes and volcaniclastic rocks (Fig. 1b). All these rock units are northward tilted (Nelson et al., 1993;Lagabrielle etal, 2000; Veloso etal, 2005) and surrounded by young plutonic intrusions (Mpodozis et al., 1985; Lagabrielle et al., 1994; Guivel et al., 1996,1999; Herve etal., 2003).
Several authors have pointed to a ciose spatial and temporal relation between the processes of 6 Ma ophiolite emplacement (Anma et al., 2006) and the evolution of the active Chile Ridge system that separate the Nazca (to the north) and Antarctic (to the south) oceanic plates (e.g., Cande and Leslie. 1986; Forsythe and Nelson, 1985; Nelson et al., 1993; Keading et al., 1990; Bourgois et al, 1996; Lagabrielle et al, 1994, 2000; Veloso et al, 2005). According to global piate reconstructions and numerical models (e.g., NUVEL1-A) the Nazca piate is presently subducting at a rate of about 8.5 cm/yr in a N80°E direction whereas the Antarctic piate subducts at a rate of ca. 2 cm/yr in a N90°E direction (Fig. la) (e.g., Cande and Leslie, 1986; DeMets et al, 1994; Somoza, 1998). The Nazca and Antarctic oceanic plates are separated by the Chile Ridge System, which is presently subducting beneath the South American piate ca. 40 km north of the Taitao area and defining a triple junction named 'Chile Triple Junction'. The obliquity of convergent directions and orientation of the active Chile Ridge system resulted in an alternated north and southward migration of the Chile Triple junction (Cande and Leslie, 1986). Because of this migration pattern. three different segments of the Chile Ridge system collided withthe westernborderof the South American piate offshore the Peninsula de Taitao at ca. 6 Ma, ca. 3 Ma and 0.3 Ma to Present (Cande and Leslie, 1986). The particular configuration of the active Chile Ridge system, the alternated migration pattern and the change in convergent directions and velocities are expected to induce contrasting stress conditions inthe forearc (e.g., Forsythe and Nelson 1985; Yanez and Cembrano, 2000, 2004) and the entire belt (Lagabrielle et al., 2004).
Theoretically, the internal structures of any accreted ophiolite can potentially provide infor-mation related to the successive stress fields that this ophiolite experienced from its generation to its emplacement. Thus, it is necessary to separate the structures generated during spreading ridge processes, as for example those present in the Troodos Ophiolite (Moores and Vine, 1971; Varga and Moores, 1985; Dilek et al, 1990), from those related to the emplacement process. Nelson et al. (1993) pointed that most of the brittle deformation observed in the Taitao Ophiolite is a direct result of its emplacement process. Henee, brittle structures present in the central part and in the easternmost margin of the ophiolite and of the nearby forearc domain may become a key to understand the changing stress field induced by the subduction of the Chile Ridge system.
In this study we report the results of paleostress analyses of structural data collected from the plutonic section of the Taitao Ophiolite and from the major lineament that bounds the ophiolite in the East (the Bahia Barrientes fault-zone). Calculated patterns of stress field were studied in order to reveal the different stress fields that were involved during the emplacement process, their relation to previously reported rotations and to the effeets of the migration of the Chile Triple junction in the area.
2. The Brittle Deformation of the Taitao Ophiolite
Previous works onthe Peninsula de Taitao (e.g., Forsythe and Nelson, 1985; Nelson et al, 1993; Guivel et al, 1996; Lagabrielle et al, 2000; Veloso et al., 2005) showed that the Taitao Ophiolite is in fault contact with surrounding Pre-Jurassic basement and young (<4 Ma) Cabo Raper pluton in the south (Forsythe and Nelson, 1985; Guivel et al, 1996; Nelson et al, 1993; Herve et al, 2003; Veloso etal., 2005). Internal lithologicalboundaries of the ophiolite (Fig. 1b) were inferred mostly to be fault contaets, based on our own field observations and interpretation of aerial photographs.
Forsythe and Nelson (1985) reported thrust, normal and strikeslip brittle faults in the easternmost margin of the Peninsula de Taitao. In this study, reported fault data was mainly collected from the plutonic units of the ophiolite (gabbros and ultramafic rocks) and from the coastal outerops in the northern and central parts of the Bahia Barrientes faul-tzone developed mostly in volcaniclastic deposits of the Chile Margin Unit (e.g., Bourgois et al, 1993; Nelson et al, 1993) (Fig. 1b) in the eastern part of the ophiolite. These rock units were chosen because they recorded the deformation that affected the interior and the border of the ophiolite, respectively; providing contrasting and complementary information about the emplacement and accommodation processes.
Ultramafic rocks show different geochemical affinities and commonly present strong serpentinization (e.g., Nelson et al, 1993), whereas gabbros commonly present compositional layering and well preserved primary structures (such as folds and compositional grading). The Chile Margin Unit is composed of thick deposits of highly vesicular pillow lavas and pillow breccias (Bourgois et al, 1993; Nelson et al, 1993; Lagabrielle et al, 2000) interbedded with conglomerates and sandstones. Accordingto Guivel et al. (1996,1999) this unit was emplaced ciose to its actual location, and perhaps as a result of the counterclockwise rotation of the ophiolite package (Veloso et al, 2005).
Little information about the internal brittle structures of the ophiolite was available until now. Brittle structures developed in the inner parts of plutonic units (gabbros and ultramafic rocks) of the ophiolite are not only restricted to fault contaets between different lithologies. We found and documented abundant meter-scale fault planes with dip-slip and/or strike-slip movements in the surrounding areas of the contact between ultramafic and gabbroic rocks (Fig. 2) during two expeditions (summers of 2000-2001 and 2003-2004). Two sets of brittle faults were recognized, one with mainly NNW-SSE to E-W trends and other with NE-SW trends (Fig. 2a). Both sets are characterized by low angle fault planes that dip towards north and south. Major structures can be followed several meters until they are covered by the dense vegetation or submerged inte Bahia Barrientes waters. No gouge was observed, yet fault planes present kinematic indicators such as those described by Petit (1987) and Doblas (1998). These indicators suggest thrust movements with small strike-slip components. Orientations of fault planes and of their kinematic indicators together with inferred senses of move-ment are plotted on figure 2a.
Crosscutting relationships between identified sets are complex; NE-SW trending fault planes commonly cut and displace NNW-SSE to E-W trending ones, yet opposite relationships were also observed at some places. This complex crosscutting relationship suggests that activity of both sets was most probably synchronous.
Faults with scattered orientations were observed in the dike complex and volcaniclastic units of the ophiolite, in where the relative displacement of bedding markers indicate a normal movement. This suggests a NE-SW extension in the northern part of the ophiolite. Veloso et al. (2005) suggested that such extension was a direct result of a counter-clockwise rotation of the ophiolite sometime after 6 Ma due to its emplacement and accommodation into the forearc. No kinematic indicators -such as those described by Petit (1987) and Doblas (1998)- were observed on these faults, and therefore they were excluded from the further analysis.
The Bahia Barrientes fault-zone corresponds to a major lineament in the Peninsula de Taitao (Bour-gois et al, 1993; Nelson et al, 1993), extending from the southernmosttip of the Cabo Raperpluton in the south towards NNE until it submerges under the water at Seno Hoppner bay (Fig. 1b). Two set of fault planes, one striking NNW-SSE and dipping moderately to the west and other striking NE-SW and dipping to the east, are present in ultramafic rocks and gabbros in the southern part along the coast of Bahia Barrientes as well as in several volcaniclastic outcrops in the eastern border of the peninsula (Chile Margin Unit; Figs. 1b and 2b) (Forsythe and Nelson, 1985; Nelson et ai, 1993; Veloso et ai, 2005).
In the southern porcion of the Bahia Barrientes fault-zone strike-slip and/orthrust movements were inferred from the orientations of kinematic indicators as well as from the relative displacements of bedding markers (Fig. 2). On the contrary, in the northern portion of the fault-zone normal and/or strike-slip movements were inferred. The orientations of individual fault planes and of inferred senses of movement suggest mainly dextral strike-slip movement for the fault-zone with minor vertical components (either normal or thrust), perhaps similar to a scissors fault. Orientations of fault planes and of their kinematic indicators together with inferred senses of movement are plotted on figure 2b.
3. The Multiple Inverse Method and Fault Data Sampling
Stress tensor inversion is a method for determining stress fields from fault-slip data obtained from outcrops, borehole cores or active seismic clusters (Angelier, 1994; Ramsay and Lisie, 2000). Among several computer routines and numerical techniques the 'multiple inverse method' is designed to separate stress fields from heterogeneous fault-slip data which recorded polyphase stress histories (Yamaji, 2000; Otsubo and Yamaji, 2006). The graphic interface of the method allows an easy and fast separation of different stress fields recorded on the fault-slip data. The method is an adaptation of the generalized Hough Transform (Ballard, 1981; Yamaji, 2000). It assumes that a sub-population (containing a number of k-elements) of the het-erogeneous fault-slip data responds to the Wal-lace-Bott hypothesis (e.g., Angelier, 1994), thus allowing determination of the principal stress axes associated to it. A cluster analysis of the obtained solutions for each one of all possible sub-sets allows the identification of homogeneous stress fields recorded within a heterogeneous population of fault-slip data (see Yamaji et al, 2000 and Ot-subo and Yamaji, 2006, for details). To apply this method fault-slip data must include the orientation of the fault plane and of the striae (or some other kinematic indicator) plus the sense of movement. In this study the version 5.31 of the 'multiple in-verse method' (Otsubo and Yamaji, 2006) was used to analyze collected brittle fault-slip data.
The output of the method is shown in a pair of stereograms containing the orientations of the principal maximum (σ1) and minimum (σ3) stress axes (Fig. 3). Each sub-solution is represented by a tadpole indicating the orientation of one of the principal axis with an attached 'tail' pointing towards the orientation of the complementary principal axis (i.e., σ1's tadpole points toward the orientation of σ3 and vice versa). The tadpole is, additionally, color coded according to the calculated stress ratio obtained through the relation(σ2-σ3)/(σ1-σ3). Thus, groups of tadpoles with similar colors and with similar orientations of their tails indicate a single stress field solution (e.g., Yamaji, 2000; Otsubo and Yamaji, 2006). Resolution and visualization of obtained solutions can be enhanced using a couple of numerical parameters; 'k' which defines the number of data contained on each subset, and 'e'which enhances the resolution, allowing to thin out erroneous solutions and enhance correct ones (see Otsubo and Yamaji, 2006 for details).
4. Paleostress Results
Numerical parameters used for this study were k=5 and e=l 1, which were found to be optimal for the current data set. The output of the method revealed different stress fields on both studied areas (plutonic units and Bahia Barrientes fault-zone) (Fig. 3), suggesting a complex polyphase brittle deformation history for the emplacement process of the ophiolite.
Two different stress field solutions were obtained from fault-slip data collected from the plutonic units of the ophiolite (ultramafic rocks and gabbros); these werelabeledsolutions 'A'and 'B'(Fig. 3a). Solutions have their σ1 axes oriented ciose to the horizontal plane, but with different azimuths and stress ratios values. Onthe contrary, they have a cormnon vertical orientation of their o3 axes (Fig. 3a). Henee, these solutions represent compressional stress fields. Calculated range of stress ratios are similar; the one for solution 'A' ranges between ca. 0.2 and ca. 0.9 while the one for solution 'B' ranges between ca. 0.4 and 0.7 (Fig. 4).
Orientations of σ1 axes of solution 'A are wide-spread, varying between NW-ward and SSW-ward azimuths (Fig. 3). A statistical analysis of these orientations indicates that they follow an elliptical distribution (e.g., Kent, 1992), with a westward mean orientation (Table 1). However, this mean orientation cannot be considered as fully representative of an imposed stress field since it only considers the orientation of the axes and not their associated stress ratio values. Thus, the nearly westward oriented compressional axis of solution 'A is considered here as a first-order approximation
In contrast to solution 'A', azimuths of σ1 axes of solution 'B' are mainly SW-ward oriented. Following the same procedure, the first approximation of the mean orientation of the compressional axis for solution 'B' trends nearly SW-ward (Table 1).
For fault-slip data collected from the southern and central parts of the Bahia Barrientes fault-zone (mostly in volcaniclastic deposits of the Main Volcante Unit), three different stress field solutions were obtained. These were labeled solutions 'C, 'D' and 'E' (Fig. 3b). Similar to solutions 'A and 'B', orientations of σ1 axes for these solutions are oriented ciose to the horizontal plane indicating a general E-W to SWW-NEE compression. Excep-tions are the orientations of some axes of solution 'E', which have steepest plunge angles related to SW-ward to southward azimuths.
Orientations of σ3 axes for these solutions vary from nearly vertical (solution 'C') to nearly horizontal (solution 'D'). Namely, calculated solutions represent a wide range of stress fields between nearly compressional (solution 'C') and nearly strike-slip (solution 'D') regimes. Calculated stress ratios also vary; for solution C it ranges between ca. 0.1 and ca. 0.7, for solution 'D' between ca. 0.1 and 0.8 and for solution 'E' between ca. 0.4 and ca. 0.9 (Figs. 3b and 4).
Orientations of σ1 axes for solution C are well-clustered in subhorizontal SWW-ward plung-ingpoint maximum. The orientations of these axes follow an elliptical distribution with a mean azimuth of ca. 253° (Table 1). Similarly, the orientations of related σ3 axes are well-clustered, following also an elliptical distribution with a mean northward azimuth and a plunge angle of ca. 75°.
Orientations of σ1 axes of solution 'D' plot on a subhorizontal eastward plunging point maximum, similar to those of solution C. The orientations of these axes also follow an elliptical distribution, with nearly eastward azimuths oriented ciose to the horizontal plane. In contrast to solution C, σ3 axes of solution 'D' are oriented near to the horizontal plane with a mean northward azimuth (Table 1). Differences in the orientations among o3 axes of solutions C and 'D', as well as the difference intheir stress ratios, suggest that these solutions represent different stress fields.
The orientations of σ1 axes of solution 'E' range between southward and NWW-ward azimuths; mainly clustered in a NNW-ward direction (Fig. 3b). This wide range in the orientations of σ1 axes is similar to that observed for solution 'A'. Orientations of the σ1 axes of solution 'E' are slightly overlapped with those of solution C. This makes difficult separation between these two solutions. yet differences in their stress ratios help to separate them (Fig. 5). Orientations of the σ3 axes of solution 'E' are clustered about a mean orientation of ca. 36751° (azimuth/plunge) (Table 1), and few of them overlap with solutions ' C and 'D'. Again, differences in stress ratio help to distinguish between these solutions, as well as the orientations of their related tadpoles (Fig. 3).
5. Discussion
Analysis of heterogeneous fault-slip data re-veal a complex brittle deformation pattern for the plutonic units of the Taitao Ophiolite and for the southern and central parts of the Bahia Barrientes fault-zone. In the plutonic units of the ophiolite we determined two different compressional stress fields whereas inthe Bahia Barrientes fault-zone we determined three different ones (ranging between compressional and strike-slip) (Fig. 3). Orientations of calculated σ1 axes (for all obtained solutions) agree with those previously reported by Forsythe and Nelson (1985); i.e., a main E-W compression most probably resulted from the subduction of the Chile Ridge system and the two oceanic plates (Nazca and Antarctic) offshore the Taitao area. However, there are some important differences among obtained solutions.
The mean orientation of the σ1 axis of solution Ti' is counterclockwise rotated ca. 60° with respect to the orientation of the σ1 axis of solution 'A. This suggests, preliminarily, that two different compressional events affected the ophiolite; a westward oriented compressional event and a SW-ward one. However, stress field imposed onthe area for the last 6 Ma has been mainly due to the eastward subduction of the Chile Ridge system and the two oceanic plates (Yanez and Cembrano, 2000; Yanez et al, 2002).
Veloso et al. (2005) suggested that the plutonio units of the ophiolite experienced a large coun-terclockwise rotation after 6 Ma based on paleo-magnetic analyses. A consistent explanation for the different orientations of σ1 axes of solutions 'A' and 'B' is that solution 'B' represents a coun-terclockwise rotated portion of solution 'A. This is supported by the similarities among the stress ratios of both solutions, both roughly following a normal distribution with similar mean and standard deviation values (Figs. 4 and 5).
The complex crosscutting relationships among the different sets of brittle structures developed in the plutonio units and the similar stress ratio values between solutions 'A and 'B' suggest that the compression imposed on the ophiolite was similar before and after the rotation. The wide range of orientations of the σ1 axes of solution 'A could thenbe attributed to fault activities (or development) synchronous to rotation (Fig. 3a). Early generated brittle structures were most probably the result of compression imposed by the subduction of the Chile Ridge system, and then rotated while new brittle reverse slip structures developed in response to the continuous compression.
The σ1 axes of solutions C, 'D' and 'E' are similarly oriented in E-W directions; compatible with the compressional direction expected from the subduction of the Chile Ridge system and the two oceanic plates offshore the Taitao area. The main difference between these three solutions is the orientation of their minimum principal stress (σ3) axes (Fig. 3b). These are clearly clustered in three different orientations arranged in a N-S great circle. probably representing a rotation of the σ3 axis about an E-W horizontal axis.
In particular, orientations of principal stress axes of solution C are similar to orientations of principal stress axes of solution 'A', both representing compressional regimes. On the contrary, orientation of the σ3 axis of solution 'D' suggests a strike-slip regime, which is consistent with field observations and with the general sense of movement of the Bahia Barrientes fault-zone (Fig. 1). Here, orientation of the σ3 axis of solution 'E' suggests that it represents an intermediate (or connecting) stress state between solutions C and 'D' (Figs. 3 and 5).
The different obtained solutions and the orientations of their principal stress axes suggest that the stress field imposed by the subduction of the Chile Ridge system divided inte two components; one mainly compressional and other mainly transcurrent. Such division of the stress field (e.g., Fitch, 1972) has beenproposedfor other parts of the Andean orogen, like for example the Chiloe block north of the Peninsula de Taitao (e.g., Forsythe and Nelson, 1985;Nelson et al., 1994) and, in particular, for the dextral transpression documented in the are at the latitude of the Peninsula de Taitao associated in time and space to the subduction of the Chile Ridge system (Cembrano et al, 2002; Thomson. 2002).
According to Yanez and Cembrano (2000) and Yanez et al. (2002) the subduction of the Chile Ridge systemimposes different stress regimes inthe forearc; southof the Chile Triple junction the stress field is expected to be transtensional due to the slow subduction of the Antarctic piate (ca. 2 cm/year; Cande and Leslie, 1986; DeMets, 1994; Somoza, 1998), while north of it is expected to be transpressional due to the fast subduction of the Nazca piate (ca. 8.5 cm/year; Cande and Leslie, 1986; DeMets, 1994; Somoza, 1998). The change in stress regime, or velocity of subducting oceanic piate, might be thus reflected in the stress ratios of the different solutions. Solutions 'A, 'B' and 'E' correspond to mainly compressional to transpressional solutions (mean stress ratio larger than 0.5) while C and 'D' are mainly transtensional to transpressional solutions (mean stress ratios smaller than 0.5) (Fig. 4). Thus, from the different solutions it is possible to observe the stress variability imposed by the alternated north and southward passage of the Chile Ridge system in the area.
A closer analysis of the stress ratios variation and related orientations of their axes gives more information about the repeated passage of the Chile Triple junction in the Taitao area (Fig. 5). For low stress ratio values a clear division of the stress field into compressional (solutions 'A and 'B') and strike-slip (solution 'D') regimes can be observed. On the contrary, for high stress ratio values only compressional stress regimes can be observed. Intermediate stress ratio values (solution 'E') display a complex mixture of both strike-slip and compressional stress regimes. In particular, solution 'E' corresponds to a mixture between compressional and strike-slip regimes, but orientation of its axes is widespread and they seem to gradually merge with solution C (Fig. 5). This suggests that solution 'E' represents a linkage between compressional (solution 'C') and strike-slip (solution 'D') regimes, most probably reflecting the change in subducting piate (from Nazca to Antarctic and vice versa) off-shore the Taitao area.
Using available ages (e.g., Anma et al, 2006; Guivel et al, 1996; Mpodozis et al, 1985) and global piate tectonic reconstructions of the migration of the Chile Ridge system (Cande and Leslie, 1986) it is possible to establish a relative timing for the different solutions. Previously published fission track and U-Pb ages obtained from gabbros indicate that they were uplifted and cooled rapidly just after their generation about 5.7 My ago (Anma etal, 2006). Also, global piate reconstructions have shown that the Chile Ridge system subducted at about ca. 6Ma, ca. 3Maandca. 0.3-Present(Cande and Leslie, 1986) offshore the Peninsula de Taitao. The brittle deformation event must have oceurred during and/or after the ophiolite was incorporated onto the forearc, i.e., after ca. 5 Ma. Although there are no constraints to establish a youngest age boundary for the brittle deformation, identified stress fields account for the alternated subduction episodes of the Chile Ridge system.
Variation in the orientation and in the stress ratio values for the different solutions (Fig. 5) suggests that the style of brittle deformation was controlled by the external conditions induced by the subduction of the different oceanic plates (i.e., different convergence directions and velocities). The relative strong compression imposed by the subduction of the Nazca piate for the last ca. 6 Ma (e.g., Cande and Leslie, 1986) can be related to high stress ratio values -represented by transpressional stress ratios- which are clearly divided into strike-slip and compressional regimes in the Bahia Barrientes fault-zone and a mainly compressional regime in the innerpart of the ophiolite (Fig. 5).
Here, we propose a tectonic model (Fig. 6) in which all identified stress fields (and related brittle structures) are consistently explained as the result of the subduction of the Chile Ridge system and of the alternated north and southward migration of the Chile Triple junction offshore the Taitao area. In this model, early generated compressional structures developed in the plutonic units of the ophiolite (gabbros and ultramafic rocks) were progressively counterclockwise rotated together with the whole ophiolite (e.g., Veloso et al., 2005), while new structures were generated due to the continu-ous compression (solutions 'A' and 'B'). Rotated structures were then progressively abandoned due to their incapability to accommodate the imposed deformation. Such behavior and rotation of small faults has been already discussed for complex deformed terranes (e.g., Peacock et al., 1998). Meanwhile, in the eastern border of the ophiolite the mainly dextral transcurrent activity of the Bahia Barrientos fault-zone perhaps drove the counterclockwise rotation of the ophiolite. During periods of relatively strong compression (Le., subduction of the Nazca piate) the southern and central portion of the fault-zone experienced vertical and strike-slip movements (solutions 'C and 'D'); whereas during periods of relatively weak compression (Le., subduction of the Antarctic piate) the fault-zone experienced a complex mixture of strike-slip and/or vertical movements (solution 'E').
This tectonic setting agrees with the general structure of the toreare proposed by Lagabrielle et al. (2000). Deformation of the forearc was dominated by horizontal displacements and tilting of blocks controlled by a network of strike-slip faults generated by the subduction of different segments of the Chile Ridge system offshore the Peninsula de Taitao.
6. Conclusions
Brittle structures developed in the plutonic units of the Taitao Ophiolite and inthe southern and central parts of the Bahia Barrientes fault-zone (Fig. 1b) were analyzed by using the multiple inverse method (e.g., Otsubo andYamaji, 2006) inorderto revealthe complex tectonic history that affected the ophiolite during and after its emplacement inte the South American forearc. Five different stress fields were identified, two from the structures developed in the plutonic units of the ophiolite and three from those present along the Bahia Barrientos fault-zone(Fig. 3). Our conclusions are summarized as follows:
• The alternated change of the subducted oceanic piate (from Nazca to Antarctic and vice versa) beneath the Taitao area induced different stress regimes inte the forearc, with major variations inthe stress ratiobutwithoutsignificantchanges in the orientation of the maximum stress axes.
• Brittle deformation of the ophiolite started shortly after it was incorporated into the forearc. i.e., after ca. 5.7 Ma.
• The stress field induced by the subduction of the Chile Ridge system resulted in different styles of brittle deformation within and around the ophiolite. In the plutonic units early generated compressional structures were progressively counterclockwise rotated (about 60°) as a result of rotation of whole ophiolite package (represented by stress field 'B'). New compressional structures were then generated because of the continuous subduction (represented by stress field 'A). The development of these new structures was synchronous with rotation of the ophiolite.
• The subduction of two different oceanic piafes resulted in different style of brittle deformation in the eastern border of the ophiolite (soufh-ern and central parts of the Bahia Barrientos fault-zone). During periods of relatively strong compression (subduction of the Nazca piate) the fault-zone experienced dip- and strike-slip movements (representedby stress fields C and 'D'). In contrast, during periods of relatively weak compression (subduction of the Antarctic piate) the fault-zone experienced a mixture of thrust and strike-slip movements (mostly repre-sentedby stress field 'E').
Acknowledgements
This work was supported by the grant -in-aid of the Science Research Project N° 13373004 financed by the Ministry of Education, Sports, Science and Technology (MEXT) of Japan. Special thanks to the crew of the R/V Petrel for their professional work and cooperation. Field data collection was assisted by Drs. Y. Kaneko (Yokohama National University), M. Terabayashi (Kagawa University), T. Otha and T. Komiya (Tokyo Institute of Technology), S. Kagashima (Yamagata University), I. Katayama, (Hiroshima University), Miss C. Herrera (Universidad de Chile), Mrs. M. Schilling (Universidad de Chile), S. Yamamoto, T. Shibuya, Y. Kon (Tokyo Institute of Technology) and R. Endo (University of Tsukuba). We would like to thank Dr. J. Cembrano, Dr G. Yanez and an anonymous reviewer for their helpful comments which help to improve the manuscript
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Manuscript received: May 07, 2007; accepted: June 24, 2008
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