Alejandro Tassone

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. 21 x 5 km), deep (373 m), and asymmetric basin to the east; (2) an E-W trending (44 x 3 km), shallower (150 m) central sub-basin; and (3) a smaller (3.5 x 1.3 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 pull-apart sub-basins amalgamated.

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 km3. 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.

Geophysical and geological field surveys carried out in the central and eastern part of the Argentinean Tierra del Fuego Island on the Magallanes–Fagnano fault system (MFS) delineate its main structural features and tectonic setting. Gravity and magnetic data provide critical information for those areas lacking good exposures and support a present-day transtensional tectonic regime for the MFS. In the surveyed area, the MFS segments have a clear morphological expression and are associated with localized gravity minima interpreted as pull-apart basins. In the southeastern corner of Lago Fagnano, the magnetic data suggest a prominent crystalline body in the subsurface, partially exposed in Cerro Hewhoepen. The shape and position of this intrusive body suggest that its emplacement was localized in a releasing bend. Two-dimensional density modeling along a 40 km long N–S section east of Lago Fagnano suggests a deep duplex similar to that exposed in the western part of the island. The obtained model, combined with available surface data, implies the subsurface configuration of geological units and structures, in which the structure of the deep duplex is similar to that exposed in the western part of the Island. The model indicates the southward deepening of the basement from 5 to 7 km and provides further support for the piston shape of the Hewhoepen intrusive.Se ha desarrollado un estudio geofísico y geológico, en la parte central de la Isla de Tierra del Fuego en sector argentino, a través del sistema de Fallas Magallanes–Fagnano (MFS), con el objeto de delinear sus rasgos estructurales y ambiente tectónico. Los datos de gravedad y magnetismo obenidos aportan información crítica para este sector casi totalmente cubierto por sedimentos cuaternarios. Nuestros datos apoyan un modelo tectónico transtensivo para el actual límite de placas representado por el MFS. En el sector estudiado el MFS, tiene una clara expresion morfológica superficial y está asociada con la localización de mínimos gravimétricos, interpretados como cuencas de pull-apart. En la parte SE de la cabecera del Lago Fagnano, los datos de anomalía magnética sugieren la presencia de al menos un cuerpo intrusivo en subsuelo, parcialmente expuesto en el cerro Hewhoepen. Su forma y localización sugieren que se habría emplazado en correspondencia con un área de distensión. Se ha realizado un modelado numérico, utilizando los datos de anomalía de Bouguer, a lo largo de una sección de unos 40 km de largo y 10 km de profundidad, localizada en el extremo Este del Lago Fagnano. El modelo obtenido combinado con nuestros datos geológicos de superficie nos permitieron inferir: (a) la disposición de las diferentes unidades geológicas y continuidad de estructuras principales en subsuelo; (b) la estructura en duplex en profundidad que resulta similar a la expuesta en la parte oeste de la isla; (c) el modelo en profundidad también aporta datos acerca de la profundización hacia el Sur del basamento de 5 a 7 km; y (d) permite caracterizar el intrusivo del Cerro Hewhoepen como un cuerpo cilíndrico.