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Recent tectonic and morphostructural evolution of Byers Peninsula (Antarctica): insight into the development of the South Shetland islands and bransfield basin/Evolucion tectonica y morfoestructural reciente de la Peninsula Byers (Antartida): evidencias sobre el desarrollo de las Islas Shetland del sur y la Cuenca de Bransfield.

1. Introduction

Byers Peninsula, located in the westernmost part of Livingston Island (South Shetland Islands), is the largest ice free area in the South Shetland Archipelago and constitutes an interesting place to study the recent tectonic evolution of the South Shetland Block. This continental fragment of the southern branch of the Scotia Arc is bordered by active deformation zones (Fig 1): the South Shetland Trench to the north, the Bransfield back-arc basin to the south, and extends eastwards along the South Scotia Ridge. Westwards it is connected to the Antarctic Plate by a broad deformation zone located at the southern prolongation of the Hero Fracture Zone.

The region has had a complex geodynamic evolution because it has been influenced by the extension related to the opening of the Bransfield Basin (Barker et al., 1991), the left-lateral displacement of the Scotia and Antarctic plates along the transtensional fault zone that extends along the South Scotia Ridge (Galindo-Zaldivar et al., 1996, 2004), and the subduction of the Phoenix Plate under the Antarctic Plate that progressively ends north-eastwards along the Pacific margin of the Antarctic Peninsula (Dalziel, 1983) (Fig. 1). Therefore, rocks of Byers Peninsula have been subject to different regional geodynamic processes.

Several geological studies have been carried out in Byers Peninsula, mainly focused on stratigraphy and paleontology (e.g. Smellie et al., 1980, Crame et al., 1993), petrology (e.g. Hobbs, 1968, Smellie et al., 1984; Demant et al., 2004) and geomorphology (e.g. Lopez-Martinez et al., 1996a). There are not specific studies on palaeostress analysis in the study area. This paper is focused on the integration of morphostructural data and brittle mesostructural analysis. We made a paleostress analysis showing stress tensors that were related to the recent tectonic evolution that produced brittle structures. Results are compared with others obtained in Livingston Island, different areas of the South Shetland Islands and the Antarctic Peninsula. Finally, the local stress regime estimated for Byers Peninsula is integrated in the regional geodynamic models proposed for the region.

2. Geological context

2.1. Regional tectonic setting

Byers Peninsula (Livingston Island) is included in the South Shetland Block, a Jurassic-Quaternary magmatic forearc generated by Mesozoic and Cenozoic subduction processes along the South Shetland Trench (Smellie et al., 1984). More specifically, Byers Peninsula is located along the segment between the Former Phoenix Plate and the Antarctic Plate, bounded by the Shackleton and Hero Fracture zones (Fig. 1). This segment, bounded to the north by the South Shetland Trench, is presently characterized by active subduction, according to most authors (Gamboa and Maldonado, 1990; Larter and Barker, 1991, Maldonado et al. , 1994). Nevertheless, other studies indicate that convergence of the oceanic Phoenix and Antarctic plates concluded about 4Ma ago (Barker et al., 1991).

In this geodynamic context, the southern border of Livingston Island is characterized by longitudinal E-NE trending faults, which are responsible for the relative uplift of the South Shetland Block. High-angle normal faults constitute the southern boundary of the tectonic horst (Gonzalez-Casado et al., 1999, Galindo-Zaldivar et al. , 2004). Uplift has been attributed to different causes such as the emplacement of Tertiary plutonic intrusions (Ashcroft, 1972), or to passive subduction of the former Phoenix Plate and rollback of the South Shetland Trench (e.g. Smellie et al., 1984; Maldonado et al., 1994; Lawver et al., 1995, 1996). Another mechanism proposed was the sinistral trans-tensional movement between the Antarctic and Scotia plates causing oblique extension along the Antarctic Peninsula continental margin generating the Bransfield Basin and defining the South Shetland tectonic block (e.g. Rey et al., 1995; Klepeis and Lawver, 1996; Lawver et al., 1996; Gonzalez Casado et al., 2000; Galindo-Zaldivar et al., 2004; Maestro et al., 2007; Solari et al., 2008). The opening rate of Bransfield Basin seems to have accelerated from 1.1 mm/yr during Oligocene-Miocene (Sell et al., 2004) to 2.5-7.5 mm/yr for the last 2 Ma (Gonzalez-Ferran, 1991).

The present geodynamic setting of the region can be investigated by seismotectonic and geodetic studies. Stress orientations deduced from earthquake focal mechanisms and fault analysis is related to a sinistral movement between the Antarctic and Scotia plates (Pelayo and Wiens, 1989; Galindo-Zaldivar et al., 1996; Gonzalez-Casado et al., 2000). Geodetic data show that Bransfield Basin is opening at 5-20 mm/yr in a NW-SE direction (Dietrich et al. , 1996). In addition, the South Shetland Block moves 17 mm/yr in a N020E direction with respect to the Antarctic Plate (Dietrich et al., 2001).

[FIGURE 1 OMITTED]

2.2. Geology of Byers Peninsula

Hobbs (1968) carried out the first reconnaissance work in 1957-58. Several papers contain geological maps of all parts of Byers Peninsula (Hobbs, 1968; Valenzuela and Herve, 1972; Pankhurst et al., 1979; Smellie et al., 1980). Early studies covered the paleontology and general stratigraphy of the area (Araya and Herve, 1966; Gonzalez Ferran et al., 1970; Tavera, 1970; Hernandez and Azcarate, 1971; Valenzuela and Herve, 1972; Covacevich, 1976). Smellie et al. (1980, 1984) proposed a stratigraphic subdivision of the Mesozoic rocks of Byers Peninsula. This was revised and updated by Crame et al. (1993) and by Hathway and Lomas (1998).

The Byers Peninsula includes a succession of Upper Jurassic-Lower Cretaceous sedimentary deposits assigned to the Byers Group (Smellie et al., 1984; Crame et al. , 1993) (Fig. 2). This is a thick sedimentary sequence characterized by over 1 km of marine clastic rocks, unconformably overlain by 1.4 km of non-marine volcanoclastic strata (Smellie et al., 1984; Crame et al., 1993; Hathway and Lomas, 1998). According to Hathway and Lomas (1998), the sedimentary sequence cropping out on Byers Peninsula (also in Rugged Island and President Head on Snow Island) is composed, from bottom to top, of the Anchorage Formation (Kimmeridgian-Tithonian), the President Beaches Formation and the Start Hill Formation (Berriasian), the Chester Cone Formation (Upper Berriasian to Valanginian) and the Cerro Negro Formation (early Aptian). Hathway and Lomas (1998) indicated an equivalence of these Formations with those previously proposed by Smellie et al. (1980) and Crame et al. (1993). Penecontemporaneous intrusive igneous rocks (mainly sills, dykes and plugs of basalt-basaltic andesite composition) are present in much of the succession (Smellie et al., 1980), especially in the marine strata (Fig. 2).

In a regional study, Smellie et al. (1980) carried out the first tectonic study of Byers Peninsula. They described several faults and folds, but focused on the dyke trends and their relationship with faults. In general, Smellie et al. (1984) assumed that most of the faults and dykes are not much younger than their host rocks. The dykes of Byers Peninsula have a wide range of orientations concentrated toward the SE to ESE (Lopez-Martinez et al., 1996a).

Quaternary deposits and landforms are extensively exposed on Byers Peninsula. The general morphology of the Peninsula is dominated by a series of raised marine platforms and beaches at different altitudes, as well as, a well developed drainage network including temporary streams and many lakes and ponds (Lopez-Martinez et al., 1996b).

3. Methods

Palaeostress analysis was conducted on fault planes, measured at 16 sites in Mesozoic rocks of Byers Peninsula (Fig. 2). Most sites are in intrusive rocks but four sites are located in sedimentary rocks. A total of 359 faults have been measured and analyzed. The slip on the normal fault planes varies between centimeters and a few meters, whereas the slip measured on reverse faults was only a few centimeters (Fig. 3). Slickenlines and chatter marks on the fault surfaces are scarce due to rock properties.

We used a method of stress inversion described by Galindo-Zaldivar and Gonzalez-Lodeiro (1988) for situations in which some slip-sense indicators are lacking. This is named Search Grid Inversion Paleostress Determination method, and provides data on the main axis orientation and the axial ratios of the stress ellipsoids. This method tries to justify as many as possible of the measured faults with the minimum number of overprinted stress ellipsoids, using a systematic search on a grid and involves striae from both known and unknown fault regimes. At the sites were there is lack of fault regime determinations in all of the measures, the method provides two alternative stress ellipsoids for each faulting stage that corresponds to the two possible opposite fault regimes of each fault.

Photointerpretation of vertical aerial photographs obtained in December 1956 and February 1957 (Falkland Islands and Dependencies Aerial Survey expedition) shows several linear elements on the Byers Peninsula. Some of these are reflected in the topography (CGE-UAM-BAS, 1992) by relief features (e.g. contour patterns or coastline). Other linear elements are faults or dykes, but most are composite features, including structurally controlled streams, lake lineaments, scarps and cliffs (Lopez-Martinez et al., 1995).

The high number of lineaments identified (1,259) made necessary to use an automatic exploration program for the determination of some of their characteristics. This program reads vectorial files (DXF in our case) and explores systematically first along the X-axis and then along the Y-axis. This program generates a file that provides, among other results, the length of each line and its orientation. From these data, different conventional statistical programs were used for the analysis of lineaments.

Lineament distribution is shown here as density fracture maps. These maps are built making a net of square cells and calculating the length of the lines contained within the individual cell limits. The result is divided by the cell area. From the data file containing the coordinates of the beginning and end of each fracture, the program calculates the number of fractures beginning or ending within each cell. Nevertheless, an automatic computation program is necessary to determine the length of lineaments or the length of segments of lineaments included within each cell (program LINDENS by Casas et al., 2000).

The elaboration of density maps begins with the determination of the most appropriate cell size. The critical size of the grid is conditioned by the average size of the lineaments and the distance between them. To determine these distances, the Delaunay triangulation method (Preparata and Shamos, 1985) was applied. Each fracture is represented by its middle point. The vertices of Delaunay's triangles are constituted by these middle points. The distance from each lineament (point) to its two nearest neighbours is then calculated. The average distance between three lineaments is considered as the arithmetic mean of the three sides of the triangle, and plotted at the centre of each triangle. This process is achieved by means of an automatic program (TRIANGLE, by J. Bernal, unpublished) which calculates the arithmetic mean of the three sides of each triangle. To appreciate the variations of this distance and the most representative distances from their distribution over the area, a contour map of average distances between lineaments was drawn.

[FIGURE 2 OMITTED]

4. Results of the analysis of lineaments and brittle mesostructures

4.1 Paleostress analysis from brittle mesostructures

We have analyzed faults at 16 sites in Byers Peninsula in Mesozoic rocks of the Byers Group. Most of the sites are located along the peninsula coast. At the north coast, site 3 is located at the north beach next to Rotch Dome, site 4 at Lair Point and site 14 at Punta Varadero, all of them in igneous intrusive rocks. Four sites (5, 6, 7 and 8) are located along the south coast, between the SW of Cerro Negro and Devils Point. They are located in igneous rocks except site 6 which

is located in volcanoclastic rocks of the Cerro Negro Formation. Along the western coast, between Devils Point and Start Point, sites 9 and 15 are located in mudstones of the President Beaches Formation, while sites 10, 11 and 16 are located in igneous rocks. At the center of the peninsula, sites 1 and 2 are located in igneous rocks of the Usnea Plug and Chester Cone respectively. Finally, sites 12 and 13 are located respectively along a stream which flows into the South Beaches, in sedimentary rocks of the Chester Cone Formation and in igneous rocks (Fig. 2).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The structures studied at outcrop scale are different types of faults with offsets from centimetric to as much as a few meters. The methodology for the quantitative study of orientation is based on the definition of different fracture sets and a determination of the dominant direction in each exposure (16 sites in total and 359 surface faults). The fracture orientation data are displayed by means of rose diagrams (Fig. 4). Overall, faults at the outcrop scale show a WNW-ESE orientation maxima. Although some of them show well developed striae, it is difficult to determine the fault regime in most of the cases owing to the poor exposure of these markers. Observed fault regimes are variable: some of them correspond to normal faults (20), reverse faults (9) and strike-slip faults (25). Joints, most of them with vertical planes, are not included in this study.

The results of the paleostress analysis are summarized in Table 1 and they have been represented in figure 5. Stress tensors are defined as orientations of the most probable main axes ([[sigma].sub.1], [[sigma].sub.2] and [[sigma].sub.3]) and the stress ratio (R = ([[sigma].sub.2] - [[sigma].sub.3])/([[ sigma].sub.1] - [[sigma].sub.3])). Orientations of [[sigma].sub.1] show two main axes trending NE-SW and NW-SE, whereas the [[sigma].sub.3] direction is NNW-SSE to NNE-SSW. At the outcrop scale, the relative chronology between extensional and compressional structures is not always clear.

The results obtained using this method indicate the existence of overprinted deformations on the Byers Peninsula because, in most of the measurement sites, two fault stages have been established. Most of the calculated faulting stages corresponds to stress ellipsoids characterized by a main axis (odd-axis) with a magnitude very different to the other two main axes. The odd axis corresponds to the maximum well defined compression in prolate ellipsoids or the well constrained extension in oblate ellipsoids. In stations 4, 6 and 16 the stress ellipsoids have axes with different magnitudes. The odd axes show very regular orientations in the different sectors of the region supporting the hypothesis that Byers Peninsula has undergone several well defined overprinted stress fields. The determined ellipsoids indicate the presence of extensional and compressional stress regimes. A well defined radial extension is deduced from our paleostress analysis. Also compressional NW-SE to NE-SW paleostress orientations can be deduced from this data set. In stations 4, 6 and 16 the stress ellipsoids have axes with different magnitudes.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

4.2. Lineament analysis

From the analysis of the aerial photographs a total of 1,259 lineaments were mapped in Byers Peninsula (Fig. 6A). Rose diagrams represent the orientation of lineaments (Figs. 6B and 6C). To avoid the influence of line-segmentation number, the statistical analysis of fracture directions was made weighting the fracture trace length (Fig. 6C). The length of lineaments varies between 31 m and 1,555 m. Their size distribution is log-normal, with a mode of 100-150 m (Fig. 7A).

Orientation

Lineaments in both igneous and sedimentary rocks in Byers Peninsula show a clear orientation maximum NWSE, with a dispersion of about 70[degrees] (Fig. 7B). This NW-SE trend is also recorded by the direction of the Ray Promontory and some segments of the pre-Holocene terrace scarps in the northern part of the peninsula. Other secondary maxima are NE-SW and ENE-WSW. Although they are not statistically representative when considering the number of lineaments with respect to the main maximum, the secondary sets include several long lineaments in the southern part (ENE-WSW set) and also in the western sector (NE-SW set) of the peninsula, where they control the direction of terrace scarps and of the coast.

The relationship between direction and length (Fig 7C) shows that the most abundant class of lineaments correspond to NW-SE orientations and lengths between 31 and 400 m. NE-SW and ENE-WSW lineaments are dominant at the interval between 400 and longer than 800 m. The NE-SW lineaments represent a secondary maximum in the 200-400 m interval. An aspect of length-orientation relationships is that NE-SW and ENE-WSW lineaments are increasingly important when considering longer lineaments.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Spatial variations in the orientation of lineaments were calculated by means of a grid of square cells, representing the length and number of lineaments in each cell (Fig. 8).The results show a consistent pattern of dominant NWSE directions, with secondary maxima similar to those obtained in the analysis of total data. Overall, the maximum NW-SE direction is in the northwestern part of Byers Peninsula, in the Ray Promontory (between horizontal reference lines 3060221 and 3057221 and between vertical reference lines 592031 and 596031); and in the southern sector (between horizontal reference lines 3053221 and 3049221 and between vertical reference lines 592031 and 604031, Fig. 8). In the southern part of the Peninsula NE to ENE direction becomes prevalent (between horizontal reference lines 3057221 and 3053221 and between vertical reference lines 595031 and 604031, Fig. 8). Occasionally, for example in the southwestern part of the Peninsula, both directions are present with the same frequency.

Density

In the study area, the mean average distance between lineaments in igneous and sedimentary rocks in Byers Peninsula is 292 m (Fig 9A), and the mode about 150 m, very similar to the mode for lineaments length (see Figs. 7A and 9A). The cumulative percentage [[PHI].sub.95] is about 600 m. The contour map of distance between lineaments shows that, in most parts of the study area, the distance between lineaments varies between 36 and 300 m (Fig 9B). To construct a density map with geological significance, the minimum cell size must be greater than the distance between fractures previously calculated (Cortes et al., 2003).

To calculate lineament density, several tests were made, with cells of different sizes. When the cell size is too small (100 m x 100 m), the contour map is not more representative than the lineaments map, since many null values appear. We chose a cell size of 1000 m x 1000 m (between four and five times the average spacing of lineaments) in order to show an outcrop-scale map with geological significance (Fig. 7). This implies that reasonable accuracy is achieved and cell with null values between lineaments are avoided.

The fracture density in the peninsula show several minima due to recent to modern beach deposits, stone fields or debris slope processes which cover the rock outcrops. The fracture density maxima within the exposed outcrops are variously located (Fig. 10): (i) near of the northwestern margin of the Ray Promontory, (ii) at the south of Punta Varadero and Chester Cone and (iii) at the north of Cerro Negro, following a WNW-ESE shape.

5. Discussion: Recent evolution of the South Shetland Block from morphostructural and paleostress analyses

By integrating morphostructural results with those obtained from the paleostress analysis we can compare previously published data from the region, and then fit these into regional geodynamic models.

[FIGURE 9 OMITTED]

5.1. Comparison between the local and regional stress fields

Several paleostress analyses were previously made in different parts of the South Shetland Block and surrounding areas. In Hurd Peninsula (Livingston Island) several authors deduce a NW-SE extensional regime (Santanach et al., 1992; Sabat et al., 1992; Willan, 1994; Gonzalez-Casado et al., 1999). Prior to this extensional regime, two orientations of oi were deduced corresponding to a wrench-faulting regime. In King George Island, a similar stress field was defined by Smellie et al. (1984), Tokarski (1991) and Uhlein et al. (1993). Galindo-Zaldivar et al. (2006) and Lopez-Martinez et al. (2006) studied recent tectonics of Elephant Island. According to these authors, the northwestern sectors are dominated by NE-SW and NW-SE compression, related to the Scotia-Antarctica left-lateral movement and to the subduction of oceanic lithosphere. The southern sector is characterized by WNW-ESE extension related to the opening of the Bransfield Basin. Maestro et al. (2007) deduced on Deception Island a recent stress field characterized by NE-SW and WNW-ESE to NW-SE extension, with local compression with NW-SE and NNE-SSW to NE-SW maximum horizontal orientations.

[FIGURE 10 OMITTED]

Our paleostress data, from brittle mesostructures at Byers Peninsula, show many sites (1, 8, 12, 13, 14, 15) with a vertical well odd axis, that probably corresponds to [[sigma].sub.1]. This supports a stage of regional extension related to the uplift of the South Shetland Block. Additionally, E-W to NE-SW odd axes were identified in many sites from the northeast (3, 14), central (2), south (5, 6, 12) and northwest (16). This corresponds to a major regional stress field that was active in the region. These ellipsoids probably represent a NE-SW compression related to the convergence of the Scotia-Antarctic plates. The western coast of Byers Peninsula is characterized by very consistent NW-SE odd axes. These may represent a NW-SE compressive stage related to the relative westward motion of the South Shetland Block in respect to the Antarctic Peninsula, or a local perturbation of the E-W oriented compressive field. Finally, N-S compression is observed in several stations (1, 11) that may be associated with subduction activity along the South Shetland Trench.

Dispersion of maximum horizontal stress trajectories is interpreted as the product of local perturbations in this regional stress field related to the nearby Hero Fracture Zone, and its interaction with several geodynamic processes acting in the region.

5.2. Relationship between density of lineaments in Byers Peninsula and the underlaying structures

The dominant orientations of lineaments are consistent with the orientation of dykes and faults measured in Byers Peninsula by Valenzuela and Herve (1972) and Smellie et al. (1984). Fault and joints measured at these 16 stations show a large dispersion, but the NW-SE orientation predominates. Also, these fractures show NE-SW and E-W secondary maximums.

The lineament density variation in the studied area can be correlated with the underlying macrostructures involving igneous and sedimentary rocks. This relationship is especially noticeable in the central and eastern parts of Byers Peninsula, where there are two lineament density maxima in N-S and WNW-ESE direction. They could be related with the extension of N-S to transfer zones that divided the Bransfield Basin (Jeffers et al., 1991; Maestro et al., 2007) and with the main trend of Livingston Island uplift from oblique convergence between the Antarctic and Pacific plates (Gonzalez-Casado et al., 2000; Maestro et al., 2007). This means that fracturing of rocks, even with different orientation, is pervasive near the large structures underlaying the studied area. The explanation for this effect may lie in the role of large structures as inhomogeneities and 'stress raising zones' (Pollard and Segall, 1987; Sassi et al., 1993; Sassi and Faure, 1997).

6. Conclusions

The tectonic imprint of recent geodynamic processes in Mesozoic rocks of Byers Peninsula is deduced from the analysis of lineaments and brittle mesostructures, which are overprinted on prior stress regimes. Lineaments, which show similar trends to fractures, are related to the underlying macrostructures developed in igneous and sedimentary rocks.

Palaeostress analysis at Byers Peninsula indicates that the area has undergone radial to NNW-SSE to NNE-SSW extension, and local compressions in a NE-SW and NWSE direction, which are in agreement with other results obtained by several research groups in the region.

The integrated study of paleostress data from the South Shetland Block and its surrounding areas indicates several stress sources: (i) the recent stress field characterized by NNW-SSE to NNE-SSW extension, related to the opening of the backarc Bransfield Basin, (2) NE-SW maximum horizontal stress field is related to the leftlateral displacement between the Antarctica and Scotia plates, and (3) NW-SE to N-S compression is related to the oceanic lithosphere subduction under the Antarctic plate along the South Shetland Trench. Perturbations of the maximum horizontal stress trajectories are produced by the Hero and Shackleton fracture zones. In addition, structural and lithological heterogeneities produce local dispersions of these orientations.

Acknowledgements

Financial support for this work was provided by the research projects REN2001-0643, CGL2005-03256 and CGL2007-28812-E/ANT of the Spanish R & D National Plan. The authors thank to the colleagues and logistic personnel that provided support for the field work. The cooperation of the Limnopolar project directed by Dr. A. Quesada is specially appreciated. We also thank Drs. Moratti and Arche for their useful comments.

Received: 22/12/09 / Accepted: 25/01/10

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P. Alfaro * (1), J. Lopez-Martinez (2), A. Maestro (2,3), J. Galindo-Zaldivar (4), J.J. Duran-Valsero (3), J.A. Cuchi (5)

(1) Departamento de Ciencias de la Tierra y del Medio Ambiente. Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain. pedro.alfaro@ua.es (2) Departamento de Geologia y Geoquimica. Universidad Autonoma de Madrid. 28049 Madrid, Spain. jeronimo.lopez@uam.es (3) Instituto Geologico y Minero de Espana, Rios Rosas, 23. 28003 Madrid, Spain. a.maestro@igme.es; jj.duran@igme.es (4) Departamento de Geodinamica. Universidad de Granada. IACT. 18071 Granada, Spain. jgalindo@ugr.es (5) Escuela Politecnica Superior de Huesca. Universidad de Zaragoza. Carretera de Cuarte s/n, 2207' Huesca, Spain. cuchi@unizar.es

* Corresponding author
Table I.--Summary of stress tensors and stress orientations obtained
from fault population analysis. Sites are located in figure 2;
Formation name; So bedding orientation in site,
strike/dip;[[sigma].sub.1], [[sigma].sub.2] and [[sigma].sub.3]
values of principal stress axes; R stress ratio =
([[sigma].sub.2]-[[sigma].sub.3])/([[sigma].sub.1]-[[sigma].sub.3])
(Wallace, 1951).

Tabla I.--Resumen de los tensores y orientaciones de esfuerzo
obtenidos a partir del analisis poblacional de fallas. Las estaciones
estan localizadas en la figure 2; nombre de la Formacion; So:
estratificacion en la estacion, direccion/buzamiento; G1, G2 y G3:
valores de los ejes principales de esfuerzo; R relacion de
esfuerzos = ([[sigma].sub.2]-[[sigma].sub.3])/([[sigma].sub.1]-[[sigma].sub.3]) (Wallace, 1951).

                                                Most probable

                                               [[sigma].sub.1]

                                    So            (plunge/
Site        Lithology          (strike/dip)        strike)

 1           Igneous                               16/354

                                                   58/224

 2           Igneous                               20/258

 3           Igneous                               20/098

 4           Igneous                               20/105

                                                   62/049

 5           Igneous                               33/296

 6        Cerro Negro Fm        N70E 20NW          18/044

                                                   14/205

 7           Igneous                               08/131

                                                   32/095

 8           Igneous                               72/192

 9     President Beaches Fm     N115E 18SW         09/310

 10          Igneous                               08/302

 11          Igneous                               06/000

 12      Chester Cone Fm        N98E 24NW          04/072

 13          Igneous                               77/350

 14          Igneous                               06/082

 15    President Beaches Fm     N124E 25SW         76/294

                                                   66/158

 16          Igneous                               15/251

                                                   16/201

                 Most probable

       [[sigma].sub.2]   [[sigma].sub.3]

                                                R
Site                                       axial ratio

 1         29/094            56/239           0.05

           30/067            10/331           0.08

 2         40/005            44/148           0.12

 3         68/248            10/004           0.03

 4         70/297            04/196           0.49

           18/176            21/273           0.48

 5         53/149            16/037           0.15

 6         70/199            08/311           0.09

           70/339            14/111           0.48

 7         16/039            72/246           0.09

           58/271            02/004           0.35

 8         04/090            18/359           0.01

 9         28/045            60/204           0.21

 10        02/212            82/108           0.29

 11        20/268            69/106           0.01

 12        26/340            64/170           0.35

 13        10/131            08/222           0.06

 14        12/173            77/326           0.23

 15        02/032            14/122           0.06

           15/286            18/021           0.11

 16        16/157            68/021           0.49

           00/291            74/021           0.46
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Author:Alfaro, P.; Lopez-Martinez, J.; Maestro, A.; Galindo-Zaldivar, J.; Duran-Valsero, J.J.; Cuchi, J.A.
Publication:Journal of Iberian Geology
Article Type:Report
Date:Jan 1, 2010
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