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1 Tectonophysics 476 (2009) Contents lists available at ScienceDirect Tectonophysics journal homepage: Geophysical modeling and structure of Ushuaia Pluton, Fuegian Andes, Argentina Javier Ignacio Peroni a,, Alejandro Alberto Tassone a, Marco Menichetti b, María Elena Cerredo c a CONICET-INGEODAV, Dpto. de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2, CP-C1428EHA, Buenos Aires, Argentina b Istituto di Scienze della Terra, Università di Urbino, Campus Scientifico Universitario, 61029, Urbino, Italy c CONICET, Dept. Cs. Geológicas, FCEyN, Universidad de Buenos Aires, Argentina article info abstract Article history: Received 6 November 2008 Received in revised form 17 July 2009 Accepted 20 July 2009 Available online 28 July 2009 Keywords: Magnetic modeling Pluton emplacement Strike-slip activity Tierra del Fuego Within the area of Ushuaia Bay (Tierra del Fuego, southernmost South America) the deformed Lower Cretaceous sedimentary rocks of Yahgán Formation host the Ushuaia Pluton. The intrusive body is oval in map view; it is compositionally varied with rocks ranging from the ultrabasic to the mesosiliceous realm. The emplacement time is constrained within the Albian Cenomanian span by new amphibole K/Ar data. Meso- and microstructures of Ushuaia Pluton and its host indicate a synkinematic emplacement with a dominant extensional component. A set of transcurrent and normal faults related to the sinistral strike-slip Beagle Channel Fault System affects the pluton and its host. On the basis of aeromagnetic data combined with field information, a new model is presented for the Ushuaia Pluton. Modeling results fit well with a laccolithic body with an estimated volume of around 111 km 3. The model pluton cross-section displays a central zone with an average thickness of 2000 m which progressively thins toward the margins ( 500 m) and a southern root which reaches 5000 m deep. The combined structural and geophysical model supports a transtensive scenario for the Ushuaia Pluton emplacement at Early Late Cretaceous boundary Elsevier B.V. All rights reserved. 1. Introduction The tectonic evolution of Tierra del Fuego has been subjected to many studies, especially the area between the Fuegian orogen and the fold and thrust belt in the southernmost region of the Andes (Cunningham, 1993; Klepeis, 1994; Mukasa and Dalziel, 1996; Diraison et al., 2000; Lodolo et al., 2002a,b, 2003; Kraemer, 2003; Gighlione and Ramos, 2005; Menichetti et al., 2008 and references cited therein). Other aspects, such as stratigraphy, related to the evolution of Tierra del Fuego, have been studied in the Argentinean part of the island, setting an appropriate framework for other geological studies in the area (Martinioni et al., 1999; Olivero et al., 1999; Olivero and Malumián, 1999; Olivero and Martinioni, 2001; Olivero and Malumián, 2008; and references cited therein), and its correlation with the off shore (Tassone et al., 2008). Also, petrologic studies of small intrusive bodies have been made in the Argentinean part (Cerredo et al., 2000; Acevedo et al., 2002; Cerredo et al., 2005). However, a multidisciplinary systematic study of these igneous bodies has not been carried out so far. That is why, in the last five years, scientist from INGEODAV and collaborators have performed several geophysical and structural studies about the nature of many intrusive Corresponding author. addresses: peroni@gl.fcen.uba.ar (J.I. Peroni), atassone@gl.fcen.uba.ar (A.A. Tassone), menichetti@uniurb.it (M. Menichetti), cerredo@gl.fcen.uba.ar (M.E. Cerredo). bodies and the environment in which they are emplaced in the Argentinean part of Tierra del Fuego, including magnetic and 2D gravimetric modeling (Tassone et al., 2002, 2005a, 2006; Peroni, 2006; Menichetti et al., 2007; Peroni et al., 2007, 2008a) in addition to the systematic survey of magnetic fabric data (anisotropy of magnetic susceptibility) and paleomagnetic studies (Baraldo et al., 2002; Rapalini et al., 2005; Esteban et al., 2007; Rapalini, 2007; Esteban, 2008). The Ushuaia Pluton is one of the few outcrops of the Fuegian Batholith in the Argentinean part of Isla Grande de Tierra del Fuego. This batholith mainly outcrops south of the Beagle Channel, in the Chilean sector, making a long belt of exposures, in Hoste and Navarino Islands (Fig. 1A) and south-western Chilean territory, and belongs to the southernmost part of the Patagonian Batholith (Fig. 1B). It is composed of multiple intrusion, where three groups were distinguished: the Gabbro Complex of lower Cretaceous age ( Ma) and two granitoid assemblages, the Beagle Channel Plutonic Group of Cretaceous age ( Ma) and the Seno Año Nuevo Plutonic Group in Paleocene times (60 34 Ma) (Hervé et al., 1984). On the northern margin of Beagle Channel, in Isla Grande de Tierra del Fuego (Fig. 1A), the exposures currently known are scarce and small, usually less than 20 km 2 such as the Kranck pluton, located on the northern margin of Fagnano Lake (Tassone et al., 2007; Peroni et al., 2008b), the Moat dioritic pluton (González Guillot et al., 2005), a few dioritic stocks in the area of Rancho de la Lata, on the Paso Spion-Kop, studied by Petersen (1949), the Jeu-Jepén intrusive (Tassone et al., 1998, 1999; /$ see front matter 2009 Elsevier B.V. All rights reserved. doi: /j.tecto

2 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 1. A) Geological units within the study area and surroundings: 1 Undifferentiated Fuegian Batholith, 2 Tortuga complex, 3 Lemaire Fm., 4 Yahgán Fm. SCB: Beagle Channel Fault System, CF: Cadic Fault, AF: Andorra Fault, EBF: East Beagle Fault, MFS: Magallanes Fagnano Fault System (modified from Menichetti et al., 2007). Box indicates the area of Fig. 2. B) Location of the area of Fig. 1A within the southern tip of South America. Arrowed lines indicate the extension of different segments of the Patagonian Batholith (after Hervé et al., 2007). BPN: North Patagonian Batholith. BPA: South Patagonian Batholith. BF: Fuegian Batholith. Acevedo et al., 2000; Cerredo et al., 2000, 2005; Tassone et al., 2005a, b) and the Ushuaia Pluton (Acevedo et al., 1989, 2002; Acevedo, 1996; Tassone et al., 2005b, 2006; Peroni, 2006; Cerredo et al., 2007; Menichetti et al., 2007). Since most parts of Isla Grande de Tierra del Fuego is covered by forest and peat which restrict the exposed surface of the intrusive bodies, the information provided by geophysics becomes a fundamental tool for the study of these poorly exposed plutons. The goal of this study was to obtain the model of the Ushuaia Pluton in order to extrapolate in depth the surface observations. This model was obtained using geological information (lithology integrated to structures), geophysical data (aeromagnetic anomalies, magnetic susceptibilities and values of remanent magnetic fields). 2. Regional tectonic frame The Isla Grande de Tierra del Fuego is located in the southernmost portion of the Andes where a steep flexion of almost 90 eastward deviates it from the N to S axis along the western edge of South America. It has been proposed that this sharp orogenic curvature is due to major strike-slip offsets (Cunningham, 1993). The current tectonic arrangement of Tierra del Fuego is the result of a long and complex evolution which involved the southernmost part of South America, South Africa and Antarctica, beginning in the Paleozoic when these three plates were part of Gondwana. Towards the beginning of the Mesozoic, a subduction zone along the active western margin of the super continent was developed (Dalziel and Elliott, 1973; Dalziel, 1982). Due to the regional extension associated with the break-up of Gondwana, during lower to Middle Jurassic (Bruhn, 1979), there was an extensive emission of silicic volcanic material all over Patagonia, including Fuegian Andes, the Tobífera Formation, which is known as the Lemaire Formation in Argentina. This extensional period was associated with the formation of oceanic crust (represented by the Tortuga complex in the Southern Archipelago, and the Sarmiento Ophiolite emplacements in Southern Patagonia) due to the development of Rocas Verdes marginal basin (Dalziel and Elliott, 1973) with the consequent deposition of marine sequences (Dalziel, 1981) known in the Argentinean part as Yahgán Formation. Approximately, towards 100 Ma (Dalziel et al., 1974), a compressional stage began in the southern Andes, probably as a result of a shift in the convergence velocity of the plates, related to the opening of the South Atlantic Ocean. This generalized compressional stage led to the closure of the Rocas Verdes marginal basin with a contemporary development of the Magallanes basin as foreland basin (Natland et al., 1974). Since the Late Mesozoic to the Tertiary, the area of Tierra del Fuego experimented the continent-ward development of the fold and thrust belt with a thick-skin versus thin-skinned structural style (Winslow, 1982; Biddle et al., 1986; Alvarez-Marrón et al., 1993; Gighlione and Ramos, 2005; Menichetti et al., 2008; among others). During this period several plutonic emplacements occurred in the study area spreading from the southern archipelago to the Tierra del Fuego inland. From the Paleogene to the present, EW sinistral strike-slip tectonics with the development of several extensional and compressive structures along the different fault strands, affected the region. From 30 Ma ago the relative motion between South America and the Antarctic Peninsula led the Drake Passage opening and the development of Scotia Plate (Cunningham et al., 1995). The boundary between South America and Scotia Plates, represented in Tierra del Fuego by strike-slip faulting, is known as the Magallanes Fagnano Fault which runs from North Scotia ridge to the Chile trench at 50 S (Fuenzalinda, 1972; Dalziel, 1989). The strike-slip activity is well documented from the Carbajal valley to the Canal de Beagle region south of the Magallanes Fagnano transform fault system (Fig. 1A). The deformation is partitioned in several E W structures with a sinistral strike-slip kinematics with an important extensional component where several pull-apart basins are developed (Lodolo et al., 2001).

3 438 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 2. A) Geological map of the Ushuaia Pluton, North margin of Beagle Channel (modified from Menichetti et al., 2007). B) Geological sketch showing the facies of the Ushuaia Pluton and the extensión of contact metamorphic aureole with indication of characteristic mineral assemblage. Numbers indicate the location of sampling sites for MRN measurements. 3. Ushuaia Pluton 3.1. Local geologic framework The Ushuaia Pluton (UP) mainly outcrops on the eastern shore of Ushuaia Bay where the exposed area of around 11 km 2 displays an elliptical shape with a major axis trending WNW ESE; minor exposures of the UP are found in Ushuaia Peninsula (Fig. 2A). The pluton intrudes with clear-cut contacts the turbidite sequence of the Early Cretaceous Yahgán Formation. UP is an epizonal intrusive body with typical subvolcanic textures of mainly andesitic composition Table 1 New amphibole K/Ar ages for Ushuaia Pluton. Sample %K 40 Ar rad, nl/g % 40 Ar air Age, Ma GMP ±2.8 GM ±3.1 Age determination was carried out at Actlabs (Canada) on amphibole separated from whole rock. (GMP 27 hornblendite of the ultrabasic heterogeneous domain and GM36 andesite of the roof area); the potassium concentration was performed by ICP and the argon analysis was performed using the isotope dilution procedure on noble gas mass spectrometer.

4 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 3. A) Outcrops of the ultrabasic facies on Beagle Channel coast with hornblendite rock cut across by syenite veins, where, on the left, there is a sharp contact between the two lithologies and, on the right, there is a suite of veins of syenitic composition. The stereogram shows the normal faults affecting the complex (lower hemisphere equal area plot). Rose diagram shows prevalent N-NE orientation of syenite veins. Black dots indicate the drilling sites where the oriented drilling-cores were obtained. B) Dome structure generated by an intrusive apophysis in the Ushuaia Peninsula. Lower hemisphere equal area plot with main fault planes associated with regional lineaments. Kamb contours of fault planes with striae are located 2 sigma-apart. Orientation of maximum stress axis (Sigma 1: triangle, Sigma 2: diamond, Sigma 3: star) was calculated using the Angelier inversion method. Sample size (n) is indicated for each structural population.

5 440 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 4. A) Extended mafic dyke within syenite of the heterogeneous domain. B) Boudinage of quartz vein in pelites of the Yaghan Formation. close to the borders and in the roof facies exposed at Peninsula Ushuaia. Two main facies are distinguished within the inner areas of UP (Fig. 2B): an ultrabasic facies mostly exposed along the eastern Ushuaia Bay shoreline and a mesosiliceous facies cropping out at the northeastern half of the pluton. The ultrabasic facies is split in a homogeneous domain made of coarse-grained cumular hornblendite with minor thin veins of syenites and a heterogeneous domain where magma mixing processes are evident. The hornblenditic rocks occur in minor proportions within this domain, accompanied by basic pillows and enclaves (ranging from centimeter to meter) with typical crenate outlines hosted in mesocratic to leucocratic rocks of mozonitic to syenitic composition. The mesosiliceous facies is dominated by hornblende-monzonites with variations towards monzodioritic to diorite and minor tonalite compositions. The rock suite of UP is metaluminous and spans from the medium-, through the high-k up to the shoshonite series. Mg# (33 45) and Ni, Cr and Co contents of basic and ultrabasic rocks indicate some Fe Mg silicate fractionation at the source area. Sr spykes on multielement plots along with smooth to null Eu/Eu indicate that no significant plagioclase fractionation controlled liquid evolution. Trace element contents and interelemental ratios clearly indicate the subduction-related nature of UP (Cerredo et al., 2007, 2008). Previously reported chronological data for UP (K/Ar whole rock data from hornblendite, Acevedo et al., 2002) has yielded 113±5 Ma. New acquired amphibole K/Ar mineral ages provide a more tightly constrained time span for UP emplacement (Table 1) in the latest Early Cretaceous (Albian Cenomanian within errors). The Ushuaia Pluton is hosted in the sedimentary rocks of Yahgán Formation which consists of 6000 m-thick of deep-marine mudstones including volcaniclastic andesite-rich turbidites and tuffs of Low Cretaceous age (Olivero and Malumián, 2008). This volcaniclastic material had its source in the WNW ESE oriented volcanic arc located further south (Dalziel et al., 1974). Regionally the Yahgán Fm. displays a very low grade metamorphism, and an intense deformation related to the Andean tectonic phase. Around UP an extended metamorphic contact aureole is recognized reaching up to the K-feldspar zone where local pockets of partial melts were produced at thin layers and screens of the host within the marginal areas of the ultrabasic facies and in discrete rafts enclosed in the mesosiliceous facies. The contact mineral assemblages are oriented defining an S 2 foliation which overprints the former S 1 pressure solution cleavage. Oriented biotite+garnet±amphibole associations characterize the inner contact aureole which extends up to 1 km from pluton margin (Fig. 2B), whereas the more restricted external aureole is characterized by the presence of S 2 -oriented biotite. Reverse faults and low-angle backthrust with North vergence affect the pluton and its host, cross cut by a sub-vertical (southdipping), strike-slip and normal fault systems, associated with the Beagle Channel Fault System (BCFS) (Menichetti et al., 2004). These faults pertain to the E W left-lateral strike-slip faults that define, together with Magallanes Fagnano Fault, the transform boundary between South America and Scotia plates (Lodolo et al., 2003; Menichetti et al., 2008). In Ushuaia Peninsula, there are three quarries exposing the uppermost levels of the intrusive body. The pluton-host contacts are sharp, showing apophysis of the intrusion penetrating the Yahgán Fm. in a dike-like fashion (Fig. 3B). From a structural point of view, the Ushuaia Peninsula exposures represent the boundary of a dome structure generated by the intrusion of the Ushuaia Pluton. The central part of this outcrop is cut across by a sub-vertical E W sinistral strike-slip fault, which is affected by a normal NW-SE fault, dipping 60 NE. The geometries indicate that these faults cross cut the compressional features related to the Andean orogeny. The shear zones show brittle features with S/C structures in the marly levels of the host. The kinematic analysis of these faults evidences a subvertical maximum compression vector with a weak strike-slip component, which can be related to a SW NE extensive stress field (Fig. 3B). Along the NE shore of the Ushuaia Bay, the whole intrusive body is affected by SW dipping normal faults with an angle from 30 to subvertical. These same faults cut other E W sinistral strike-slip faults and reverse faults. The kinematic analysis of the fault population indicates a N S extensional stress field (Fig. 3A). The rotation of the stress fields in the two localities is related to the partitioning of the wrench deformation along the different fault strands that in the Ushuaia Bay are clockwise rotated. In any case these extensional structures are the younger faults related to the left-lateral transtension that affect both the western and eastern arms of the Beagle Channel Meso- and microstructures of Ushuaia Pluton The macroscopic fabrics within the pluton range from foliated to weakly foliated or locally unfoliated. Along the northeastern border the foliation is steep to vertical parallel to the foliation in the host. Instead, within the homogeneous domain of the ultrabasic facies the foliation is mostly subhorizontal; within the heterogeneous domain, in turn, a roughly E W gently inward dipping foliation/banding predominates. Brittle extensional meso structures dikes, veins and boudinage can be observed in several outcrops. A complex rhombic pattern of centimetric wide syenite veins in hornblendite facies with a prevalent N S direction in the Ea. Tunnel is associated with larger syenite dikes (Fig. 3A). Extension in E W direction can be inferred also from the boudinage of quartz veins within the host Yahgán Formation (Fig. 4B) and from brittle extension of basic dykes within syenite of the heterogeneous domain of the hornblendite facies (Fig. 4A). Information gained from microstructure recognition indicates that magmatic intrusion took place in a tectonically active setting. Different magma batches intervening in pluton assembly display deformation features which span from the early synmagmatic to the waning stages of liquid crystallization. Further brittle low-t deformation is unevenly

6 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 5. Photomicrographs of the microstructures in Ushuaia Pluton. A) Magmatic microstructure in the ultrabasic facies characterized by amphibole SPO. XPL light, base of the photo 3.6 mm B) Submagmatic microstructure in subvolcanic marginal facies. Microboudinage in amphibole phenocryst in andesite; note the prominent magmatic amphibole orientation. PL light, base of the photo 3.6 mm C) Magmatic plagioclase tiling and subparallel biotite orientation, with interstitial fine-grained quartz pockets in tonalite of the mesosiliceous facies. XPL light, base of the photo 3.6 mm D) Healed microfracture in hornblendite of the ultrabasic facies. Fine-grained amphibole welds the microfracture. PL light, base of the photo 1.5 mm E) Brittle microshear zone with lozenge plagioclase pophyroclast set in a fine-grained comminuted quartz feldspar matrix. XPL light, base of the photo 3.6 mm F) Drag microfold in amphibole associated with a brittle microfault in hornblendite of the heterogeneous domain. XPL light, base of the photo 1.5 mm. Table 2 M e : Median magnetic susceptibility values employed in the magnetic modeling for the different lithologies in Ushuaia Bay area measured in the laboratory. Geological unit Age M e magnetic susceptibility [CGS] UP Ultrabasic facies Albian Cenomanian Mesoliceous facies Yahgán Fm. Early Cretaceous Lemaire Fm. Late Jurassic ( ) Metamorphic basement Paleozoic ( ) UP: Ushuaia Pluton. σ: standard deviation. N: number of measurements ( ). Susceptibilities after Peroni (2006) and Tassone et al. (2005a). σ N developed. Except for localized zones of high deformation, the preservation of euhedral crystal shapes in the feldspars as well as in the mafic minerals (mainly amphibole, rarely biotite and clinopyroxene), indicates that UP did not undergo extensive sub-solidus deformation. Macroscopic shape-preferred orientation (SPO) is common in K-feldspar megacrystals of syenitic rocks within the heterogeneous domain of ultrabasic facies, SPO is also observed in amphibole crystals of hornblendites (Fig. 5A). Magmatic plagioclase tiling accompanied by subparallel alignment of hornblende (±biotite, ± clinopyroxene) is common in rocks of the mesosiliceous facies (Fig. 5C). Microstructures at high solid fraction, i.e. at the submagmatic stage are unevenly developed and represented by microfractures in feldspars and microboudinage in amphibole of the subvolcanic marginal andesitic facies (Fig. 5B), often submagmatic cracks are

7 442 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 6. Stereograms showing the directions of remanent magnetization in the seven sampling sites located in Fig. 2B. Black symbols: lower hemisphere, white symbols: upper hemisphere. normal/subnormal to primary magmatic crystal alignment. Finegrained quartz-plagioclase-k-feldspar±myrmekite often occur in the mesosiliceous facies as pockets filling interstitial spaces. Although these fine-grained pods do not show a particular shape, they tend to display a subtle shape-preferred orientation parallel to the magmatic foliation (Fig. 5C). Limited high to moderate-t sub-solidus overprint is represented by flame perthites in K-f, fine myrmekite lobes, chess-board twinning in plagioclase, either diffuse or elongated subgrains in feldspars, healed cracks in amphibole welded by fine-grained (neo?) amphibole (Fig. 5D). Brittle microstructures are very restricted, associated with discrete microshear zones where locally a porphyroclast/matrix texture develops (Fig. 5E) with lozenge shaped plagioclase set in a mass of comminuted quartz feldspar. In hornblendites, when brittle shear cut across large amphibole crystals often drag microfolds are developed (Fig. 5F). Straight brittle fractures with epidote infillings (likely a posthumous magmatic product) are widespread in all lithologies and in turn are affected by brittle shearing. 4. Geophysical study With the aim of achieving an estimation of the unexposed portions of the Ushuaia Pluton a geophysical modelling was performed. It combined the information of aeromagnetic anomalies with new magnetic data acquired in the field. In addition, the obtained modelled pluton was inserted in a schematic N-S section integrating structural, lithological and regional magnetic data Sampling and processing for magnetic properties determination In the field, hand samples and drill cores of the main lithologies were obtained with a portable drilling machine and oriented with sun and magnetic compasses. Eighty-five oriented samples were collected at seven sites indicated in Fig. 2B. The values of magnetic susceptibility and remanence data necessary for the modeling were obtained in the laboratory. Magnetic susceptibility determination was made using a Bartington MS2 susceptibility meter in selected drill cores. The magnetic susceptibility values employed for the modeling were obtained by averaging 38 samples of the ultrabasic facies, 35 samples of the mesosiliceous facies Table 3 Mean remanent magnetization intensity (Jr), declination (Decl.) and inclination (Incl.) for each site and its confidence intervals (α 95 ). Site Jr (ma/m) Dec. Incl. α 95 n Q The great difference in sampling site no. 7, where hornblendite prevails, is outstanding compared with the rest of the sites. and the 12 samples of host rock (Yahgán Fm.) from the seven sampling sites in the Beagle Channel area (Table 2). Natural remanent magnetization was determined using a DC-squid cryogenic magnetometer (2G-750R). Stereograms in Fig. 6 show the direction of the remanent magnetization for each sample site (Fig. 2B). For most of the sites, magnetization was weak and dispersed (Table 3), except for site no. 7, in the ultrabasic facies, which shows the highest mean intensity (2420 ma/m) and a mean declination of 90 and inclination of 3.8. For all other sites, Q is less than one, thus remanent magnetization is not very important in the outcrops, where the induced magnetization prevails due to its high susceptibility Processing of aeromagnetic information The 1:250, II (total magnetic field; SEGEMAR, 1998) aeromagnetic chart of anomalies not reduced to pole (Fig. 7A) and reduced to pole (Fig. 7B) was used. These charts were obtained from a regional aeromagnetic survey, with N S flight lines, 500 m-apart, E W control flights, 5000 m-apart and a 120 m constant height above ground. Digitalization of the available hard copy of aeromagnetic charts was performed; the magnetometric contour lines were digitalized by means of more than 10,000 points (referred to geographic coordinates) in each chart. Therefore, the software required parallel profiles could be obtained. The modeling of Ushuaia Pluton was carried out with a total of 20, 18 km-long, NS-oriented profiles with 500 m of equidistance between sections. Values of latitude, longitude, value of the magnetic Fig. 7. Comparison between not reduced to pole (A) and reduced to pole (B) 5569-II aeromagnetic chart and its position with respect to the modeled intrusive (Gauss Krüger projection). Contour lines 5 nt-apart. W E cross-section showing the relationship among topography (grey area), lithology, reduced-to-pole aeromagnetic anomaly (segmented line) and magnetic susceptibility recorded in the field (solid line). The difference between the pluton susceptibility values and those of the host rock is remarkable. USH: Ushuaia city. The location of the geological section of Fig. 10 is shown in Fig. 7A.

8 J.I. Peroni et al. / Tectonophysics 476 (2009)

9 444 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 8. A: Bathymetric isolines of modeled intrusive body. Contour lines every 1000 m. Note the southern deep tail and the northern shallowing of the modeled pluton. B: Isolines of percentage differences between measured and modeled grids. See text for explanation of the calculation of %Error. field and topographic data (obtained from DEM) characterize the 1025 points that were used to define each one of the 20 parallel sections. The digital version of the 5569-II reduced to pole chart (Fig. 7B) allowed a better appreciation of the intrusive location as well as its approximate shape and size. The chart displays an elliptical positive anomaly in the Ushuaia Bay area of 70 km 2, with a WNW ESE major axis of 9.6 km-long and a 8.6 km-long minor axis. The contour lines, ranging from 1200 to 60 nt, show an asymmetric distribution with the steepest gradient in the eastern half of the anomaly. The unreduced anomaly, in turn (Fig. 7A), shows a maximum located N with an asymmetrical bell shape and an intensity of 350 nt and an associated minimum of 320 nt with an elliptical shape located further S. It is worth emphasizing the presence of another magnetic anomaly of minor intensity, located E of the above described one, on the NE margin of Encajonado River, near Cerro Trapecio (Fig. 7B), with a maximum of 20 nt surrounded by values around 40 nt. Unlike the anomaly produced by the Ushuaia Pluton, this other one does not show an important variation in its shape in the reduced to pole chart (Peroni et al., 2009) Modeling The modeling was performed with the Encom Model Vision Pro 7.0 software (Encom Technology, 2002), using the information from the International Geomagnetic Reference Field (IGRF, revision 2004) ofthe study area, magnetic susceptibility and remanent magnetization of the modeled intrusive body. This software is based on the Won and Bevis (1987) algorithm which calculates the magnetic anomaly by means of an n-side polygon within a bi-dimensional space. The software performs the magnetic modeling from simple bodies (cubes, spheres, cylinders, etc.) as well as from a series of polygons in order to achieve a better correlation between the collected data and the obtained model. In order to differentiate local from regional anomalies, a regional field was calculated from a grade one polynomial following the equation: R = a + bx + cy where: a=33,050; b= and c= In order to obtain the better fit of the modeled curve, the total volume of the body was divided into 500 m-thick, N S oriented parallel vertical sheets. The geometry of these sheets was modified throughout the modeling process to attain a better fit between the measured curve and the calculated one, using a trial and error method. The modeled intrusive body is represented by 20 parallel sections composed of two lithologies (ultrabasic and mesosiliceous facies) and displays a laccolithic shape with an average thickness of 2000 m in its central area coincident with the pluton exposures in the Ushuaia Bay and a southern root reaching 5000 m deep. The body thickness decreases towards the edges, reaching 500 m (Fig. 8A). The modeled intrusive body is composed of 1130 facets. The Q factor was not employed because during modeling, an extreme scenario was tested by setting a magnetic remanence for the whole pluton equal to the one recorded in site no. 7 (Fig. 6), which would mean Q factor=1.23. Such a vector, with that orientation and intensity, did not give a noticeable change in the modeled curve. Therefore, for this model, the magnetic anomaly would depend mostly on the magnetic susceptibility (therefore, on the induced magnetization). Primary and preliminary evidence of the similarity between the calculated and measured anomalies is provided by the comparable shape and size of the reduced-to-pole anomaly and the modeled one (Fig. 7B). In order to properly test the fitness between original and

10 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 9. Perspective view from SE of the modeled intrusive and the local topography (topographic isolines each 100 m ). The 20 vertical sections in which the body was divided can be distinguished. Each vertical sheet is composed of a mesosiliceous and an ultrabasic facies (lower left corner sketch). modeled grids, the percentage differences between the measured and calculated data were computed: ðx o X c Þ 100 X o where X o represents the value of a specific point in the original grid and X c is the value of the corresponding point in the modeled grid. In this way, negative values indicate that the modeled grid shows higher magnetic anomaly than the original grid. The calculated differences were represented by isolines in the map of Fig. 8B displaying the excellent fit between modeled and measured grids. The percentage differences are generally lower than ±0.1%. Higher differences are found towards the body margins, especially southwestwards. Furthermore, there is a good fit between the 0 m isoline and the boundary of Ushuaia Pluton outcrops at Ushuaia Bay (Fig. 9), which is an additional proof of model accuracy. On plant view, the proposed model has a surface of 111 km 2,of which the outcrops in Ushuaia Bay area (0 m isoline) would only account for 11%. It has an oval shape with N S major axis around 12 km-long and 10 km-long E W minor axis. The modeled volume is 150 km 3 of which 86.2 km 3 would correspond to the ultrabasic facies and the remainder 63.8 km 3 to the mesosiliceous facies Geological section elaboration The aeromagnetic survey (SEGEMAR, 1998) provides important information which after processing was a powerful tool to decipher the distribution and geometry in depth of intrusive bodies. These charts combined with the data obtained in laboratory of magnetic susceptibility and remanence, were used to generate a mathematical model which was combined with the main structures giving a modeled geological N S section of 30 km length and 10 km depth (Fig. 10). In the emplacement environment, the Beagle Channel was represented as a 4 km-wide and 7 km-deep basin filled by the Lemaire Fm. overlain by the Yahgán Fm. Tierra Mayor valley and the associated positive flower structures (making a piece of basement raised up to 1500m depth) were also represented (modified from Menichetti et al., 2004). The main structures were defined through a regional geological section merging geological data and seismic reflection analysis (Menichetti et al., 2004, 2008); the Digital Elevation Model (DEM) was used and for the topographic profile. The location of the faults on surface matches what was seen and measured in the field. A series of thrusts with north vergence can be distinguished in the section of Fig. 10, located in the uppermost portion of the sedimentary succession. These structures are cut by the modeled intrusive, which itself is affected by the E W and WNW ESE strike-slip and normal fault system, with north vergence, associated with the Beagle Channel fault system. The thrust complex includes two south-dipping duplexes, with sole thrusts rooted in the pre-jurassic basement. The sole thrusts merge northwards onto the Jurassic volcanoclastic sediments of the Lemaire Fm. Northward, a deeper slice of basement forms the leading edge of the Andes range in the Magallanes fold and thrust belt. The displacement of the major décollement thrust sheets is distributed with slip forming ramp anticlines. Cumulative shortening of these structures could be estimated in tens of kilometers across the region (Menichetti et al., 2008). The extensional features in the Ushuaia area are splays of the Beagle Channel strike-slip fault that shows a structure with double vergence in both sides. The cumulative extensional fault offset is few kilometers while the strike-slip displacement reach many tens of kilometers. In the northern part of the section, it is noticeably the positive structure of the Carbajal valley where reverse faults prevail. 5. Discussion and conclusions Ushuaia Pluton displays several meso- and microstructures related to an E W extensional stress field during its emplacement in Albian Cenomanian times. The presence of centimetric to metric syenitic dikes and veins of dominant NS orientation and E W extended veins and dikes both in the intrusive as in the host, support this interpretation.

11 446 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 10. A) N S Magnetic profile (for location see Fig. 7A). Solid line: aeromagnetic anomaly. Dotted line: magnetic anomaly value calculated through the modeling. Segmented line: regional magnetic field. B) 2D numeric model profile obtained with the Modelvision 7.0 software and magnetic susceptibility values of the different units used in the modeling process. C) Geological section, based on the numeric model, where the main Fuegian Andes structures can be observed (structural section modified from Menichetti et al., 2004). The primary magmatic crystal alignment is a common microstructure in rocks of all facies within the Ushuaia Pluton. Moreover, the coexistence of parallel magmatic, submagmatic and high-temperature solid state microstructures implies a continuous deformation history synchronous with pluton emplacement and cooling. Additionally, the S 2 -oriented contact metamorphic assemblages support also the synkynematic nature of Ushuaia Pluton. In this connection, it is important to recall the general normal to subnormal relation found between the magmatic/high-t sub-solidus alignments and the submagmatic cracks and the very low-temperature microfractures and microshear zones, which points to a protracted and persistent stress field that would have controlled magma ascent, emplacement and cooling. This extensional scenario is consistent with a strike-slip partitioning in an oblique convergent setting where the deformation is accommodated simultaneously by strike-slip fault systems and contractional structures (Teyssier and Tikoff, 1998) (Fig. 11). The relationship between plate motion and deformation is quite complex especially in a zone of oblique convergence because the collisional vector can be decomposed in a normal and tangential component. Many collisional settings display partitioning of the deformation in the contractional orogen wedges and along important strike-slip faults strands (Mann, 2007). The collisional margin from which the Andean Cordillera in Tierra del Fuego emerged after Lower Cretaceous times underwent a

12 J.I. Peroni et al. / Tectonophysics 476 (2009) Fig. 11. Block diagram for the Fuegian Cordillera region showing possible geometries related to the transpressional deformation regime during Cretaceous. The inset in the lower right shows the angular divergence between the plate motion vector and the fastest horizontal shortening direction (H min ) responsible of the rotation, toward a margin parallel orientation, of the finite strain long axis (H max ) to the fastest horizontal stretching direction (after Fossen and Tikoff, 1998 ). The northern strike-slip structure sketches Magallanes Fagnano Fault System and the southern one the Beagle Channel Fault System. combination of oblique horizontal shortening and crustal thickening of the basement rocks which produced the emplacement of several thrust sheets involving the sedimentary cover. In this scenario of oblique convergence, the infinitesimal strain axes and the finite strain axes are not parallel (Fossen and Tikoff, 1998). The angular divergence between the plate motion and the fastest horizontal shortening direction is responsible for the rotation, toward a margin parallel orientation, of the finite strain long axis to the fastest horizontal stretching direction (Fig. 11 inset). Late Cretaceous transtensive structures, with brittle ductile leftlateral strike-slip faults have been already reported in other areas of the Tierra del Fuego, especially in the western arm of the Beagle Channel where the plutons of the Yamana suite were interpreted to represent left-lateral transtensional zones (Cunningham, 1995). Lowangle extensional shear zones were also described in the north and west of the Cordillera Darwin (Dalziel and Brown, 1989; Klepeis and Austin, 1997) and can be related to the exhumation of the metamorphic core complex (Menichetti et al., 2008). The obtained model for the Ushuaia Pluton displays a laccolithic shape around 2 km maximum thick in the central area thinning outwards to 0.5 km (Fig. 9). The size and shape of the modeled pluton would match analogical models of intrusive bodies in zones of strikeslip faults (Corti et al., 2005). The prominent aeromagnetic anomaly in the Ushuaia area might be mainly caused by the high magnetic susceptibility of the ultrabasic facies (hornblendite dominated) and, to a lesser extent, of the mesosiliceous facies of Ushuaia Pluton. This is evident in the W E profile of Fig. 5, where the magnetic anomaly reduced to pole is compared to the susceptibility values recorded in the field. This fact is also supported by the low value of Koenigsberger coefficient in the measured samples. There is a striking contrast in the size of intrusive bodies outcropping to the north and to the south of Beagle Channel. While in Hoste, Navarino and surrounding Islands there are several large outcrops of plutonic rocks, on the northern side of the channel, there are only minor outcrops (Fig. 1A). The modeling of aeromagnetic anomaly associated with the Ushuaia Pluton allowed to infer a mostly buried intrusive body of similar size as the plutonic units of the southern archipelago. Acknowledgements This study was supported by grants from CONICET PIP 5782 and Agencia Nacional Científica y Tecnológica PICT The authors thank CADIC-CONICET for the logistical support in Ushuaia city and Estación Astronómica Río Grande (EARG) for the logistical and technical support in the field. The authors would like to thank Horacio Lippai for his constant help in the field and laboratory work. We are grateful for the thorough and constructive reviews of the anonymous referees. References Acevedo, R.D., Los mecanismos sustitutivos y los factores de evolución en los anfíboles de la Hornblendita Ushuaia, Tierra del Fuego. 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