Multichannel seismic reflection profiles, gravity measurements, and bathymetric soundings, in conjunction with field geological reconnaissance and remote sensing images, reveal with unprecedented detail the morphostructure of a major segment of the South America–Scotia plate boundary in the Tierra del Fuego region. This segment, known as the Magallanes‐Fagnano fault system, is a continental transform margin arranged in an en echelon geometry, along which prominent asymmetric basins were developed. Data acquired off the Atlantic coast of Isla Grande (the main island of Tierra del Fuego), in its central and eastern part, and in the central and western Magallanes Strait image the surface and subsurface structure of the transform fault and its associated basins. The Magallanes‐Fagnano fault system is composed of distinct tectonic lineaments that are segments of the transform system and are represented by mostly near‐vertical faults. In the Atlantic sector, the fault system trends broadly N70°E and seems to be composed by a single master fault, along which a highly asymmetric basin has formed. At around 63°W, the fault terminates by splaying into secondary normal faults that dissipate the horizontal displacement along the system. In the central eastern part of Isla Grande, the fault segments have been principally identified from analyses of remote sensing images on the basis of their morphological expression. These segments are located within river valleys and are generally associated with localized gravity minima. Lago Fagnano, a 105‐km‐long, E‐W trending depression, is a large, mostly asymmetric pull‐apart basin developed within the principal displacement zone of the Magallanes‐Fagnano fault system. Restraining bends and overlapping step‐over geometry characterize the central part of the Magallanes Strait. Along the western part of the fault system, in the vicinity of the Pacific entrance of the Magallanes Strait, asymmetric sedimentary basins have also developed. The sedimentary architecture of the basins formed within the principal displacement zone of the fault, in which the thick end of the depositional wedge abuts the transform segment, suggest simultaneous strike‐slip motion and transform‐normal extension, a common feature found in other continental transtensional environments. Strike‐slip faulting in the Tierra del Fuego region is also documented along other prominent lineaments which parallel the Magallanes‐Fagnano fault system. Along at least two of these lineaments, characterized by a remarkable morphological expression, widespread Quaternary activity occurs. The present‐day motion between the South America and Scotia plates is slow (<5 mm/yr). Also the modern seismicity monitored in the Tierra del Fuego region is low (individual events <3.5 in magnitude). The low seismicity may be explained by the slow relative motion between plates and may be further affected by slip partitioning along the different segments which make up the Magallanes‐Fagnano fault array, and along the subsidiary wrench lineaments that...

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A comprehensive suite of field surveys was carried out by a team of Italian and Argentinean scientists in South America's Tierra del Fuego region to investigate the 600‐km‐long Magallanes‐Fagnano fault system (MFS), a transform‐type margin that developed on continental crust (Figure 1). Identifying and analyzing the morphological and structural elements related to the MFS and understanding mechanisms of slip along the fault are the principal objectives of an ongoing project called Tectonic Evolution of the South America‐Scotia Plate Boundary During the Cenozoic (TESAC).

It is not known whether the Patagonian Orocline, the major bend of the southern Andes at the southern tip of South America, is a primary or secondary feature. Palaeomagnetic data along the Patagonian Orocline are still too scarce to provide a reliable and unambiguous answer to this question. New palaeomagnetic results on Late Jurassic–Late Cretaceous magmatic units along the central segment of the Fuegian Cordillera are reported. Data from four Late Cretaceous small intrusions and three sites on Late Jurassic–Early Cretaceous metabasalts and metagabbros showed anticlockwise declination deviations between 21° and 46° with respect to South America. From these and previous data, a picture of a nearly homogeneous post-Late Cretaceous regional rotation of the central Fuegian Cordillera is suggested. This supports a model of nearly 30° of anticlockwise secondary bending of the Patagonian Orocline since the Late Cretaceous (72 Ma). Lack of rotation of post-50 Ma sedimentary rocks exposed to the north of our study region, and larger rotations (of c. 90°) reported to the south of it suggest that a geographical and/or temporal progression of rotation values from south to north in the Fuegian part of the Patagonian Orocline should be investigated.

High‐resolution seismic profiles acquired in the eastern sector of Lago Fagnano, the southernmost ice‐free lake in the world, have shown the presence of very shallow gas‐bearing layers in the upper sedimentary sequences. The gas‐related features observed on seismic profiles include a typical, very strong reflection with reversed polarity, multiple reflections and acoustic blanking that hide subsurface sedimentary and structural features. The top of the acoustically high‐amplitude layer is located between 0.3–1.7 m below the lake floor. It generally forms a sharp boundary, often marked by a varying offset probably due to different levels of gas penetration, which could be related to the lithology of the overlying sediments. To confirm the presence of gas, some gravity cores were recovered in places where the blanking effect was most relevant and in the supposed gas‐free zone. Sediment core analyses have highlighted the occurrence of significant organic‐rich components within the uppermost, largely unconsolidated sedimentary layers, in correspondence of the seismically‐detected gassy zone, whereas only a few organic layers were found in the gas‐free zone. We assume that the main origin of gas is linked to the presence of a shallow, thin peat‐rich layer of Middle‐Late Holocene age. In fact, the mapped gassy zone occurs in correspondence of the outlet of the Rio Turbio, the principal tributary of Lago Fagnano, which discharges waters coming from a relatively small sag pond located immediately to the east of the eastern shore of the lake. To date, this is the first evidence of shallow gas in Tierra del Fuego lakes.

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ABSTRACT. Lago Fagnano, an E-W elongated basin located in the central part of Isla Grande de Tierra del Fuego, occupies a structural depression originated along a segment of the Magallanes-Fagnano fault system. Its evolution was mostly conditioned by tectonic processes, and later was affected by glacial and glacio-lacustrine depositional events. New high-resolution single-channel seismic data, integrated with previous seismic profiles, and geological information acquired in the surroundings of the Lago Fagnano, allows us reconstructing the basement surface of the lake, and the geometry, distribution, and thickness of the glacial and glacio-lacustrine sequences. We recognized three main sub-basins within the Lago Fagnano: 1. a medium-size (ca. 21x5 km), deep (373 m), and asymmetric basin to the east; 2. an E-W trending (44x3 km), shallower (150 m) central sub-basin; and 3. a smaller (3.5x1.3 km), shallow (128 m) sub-basin to the west. The isopach sediment map shows that the most pronounced deposition occurred along the E-W axis of the lake, with a gradual increase in thickness towards east (from 100 to 150 m). The glacial deposits are widespread along the basin. The lacustrine sediments are preferentially localized along the E-W axis of the lake filling topographic lows. The shape of the sub-basins and their location in relation with the Magallanes-Fagnano fault system, along with the distribution, geometry, and thickness of the sedimentary units, show that the general morphology of the Lago Fagnano was mostly controlled by pre-existing and syntectonic features. Based on the structural data observed in the outcrops around the Lago Fagnano and the geophysical data, we proposed that the lake is composed by 4 amalgamated pull-apart sub-basins. RESUMEN. Geometría del basamento y espesores sedimentarios del lago Fagnano (Tierra del Fuego). El lagoFagnano, una cuenca elongada E-W en la parte central de la isla Grande de Tierra del Fuego, ocupa una depresión estructural originada a lo largo de un segmento del sistema de fallas Magallanes-Fagnano. Su evolución estuvo principalmente condicionada por procesos tectónicos y, luego, fue afectada por eventos deposicionales glaciales y glaciolacustres. Nueva sísmica monocanal de alta resolución, integrada con perfiles sísmicos previos e información geológica adquirida en los alrededores del lago Fagnano, nos permite reconstruir la superficie del basamento del lago, y las geometrías y espesores de las secuencias glaciales y glaciolacustres. Reconocimos tres subcuencas principales dentro del Lago Fagnano: 1. una subcuenca asimétrica profunda (373 m) de tamaño medio (ca. 21x5 km) en el este; 2. una gran subcuenca central (44x3 km) más somera (150 m); y 3. una pequeña subcuenca occidental (3,5x1,3 km) somera (128 m). El mapa isopáquico muestra que los mayores depósitos se localizan a lo largo del eje E-W del lago, con un incremento gradual del espesor hacia el este (de 100 a 150 m). Los depósitos glaciales están ampliamente distribuidos a lo largo de la cuenca. Los sedimentos lacu...

ABSTRACT. An anisotropy of magnetic susceptibility (AMS) transect was carried out across the Fuegian Andes, in Argentina, with the aim of studying its tectonic evolution. Two-hundred and forty oriented samples were collected from 27 sites distributed between the Paso Garibaldi to the north and the Canal Beagle to the south. The study was restricted to the Upper Jurassic Lemaire Formation, with a single site located in the Lower Cretaceous Yahgán Formation. Studied rocks comprised basaltic, andesitic, volcaniclastic and sedimentary rocks affected by low-grade metamorphism. AMS measurements were complemented with thin section analyses of representative samples in order to characterize the microstructures and metamorphic assemblages. In general, the magnetic fabric shows dominant oblate shapes and a large variation in the anisotropy degree from 1.04 up to 2. The anomalously high values were observed to be associated to growth of secondary pyrrhotite, which was identified by rock magnetic tests. Magnetic foliation was generally consistent with slaty cleaveage as observed in the field, confirming the tectonic origin of the magnetic fabric. Three geographic domains were distinguished in the study region on the basis of the pattern of the AMS axes distribution. In the northern domain, from Paso Garibaldi to Valle Carbajal, the orientation of the maximum susceptibility axis (k 1 ), or magnetic lineation, is N-S to NE-SW with moderate plunge towards the S-SW and coincides with previous determination of mineral lineations associated with the Andean deformation and very low grade metamorphism. The magnetic fabric pattern can be correlated with the main deformational phase responsible for the development of slaty cleavage (main Andean deformational phase) and the tectonic transport due to progression of the Fuegian fold and thrust belt in the Late Cretaceous. A different character is shown along the Valle Carbajal domain, where subvertical E-W magnetic foliation planes and roughly E-W to ESE-WNW subhorizontal magnetic lineations are more difficult to correlate with the main folding phase and suggest its relation to an E-W, possibly localized, strike-slip regime during the main deformational and metamorphic phase. The magnetic fabrics in the third domain, close to the Canal Beagle, displays a more heterogeneous character with both E-W and N-S striking foliations; in this case a population of subhorizontal E-W magnetic lineation (k 1 ) suggests the existence of a significant component of strike-slip deformation. Keywords: AMS, Microstructures, Mesozoic, Andean tectonics, Tierra del Fuego, Argentina. 65 Esteban et al./ Andean Geology 38 (1): 64-81, 2011 RESUMEN. Fábrica Magnética y Microestructuras a través de los Andes de Tierra del Fuego, Argentina.Se presentan los resultados de una transecta de Anisotropía de Susceptibilidad Magnética (ASM) realizada en los Andes Fueguinos, Argentina. Se midieron 240 muestras orientadas provenientes de 27 sitios de muestreo distribuidos entre Paso Garibaldi, al norte y el Canal Beagle al su...