Morpho-structure of Lago Fagnano (Tierra del Fuego) and adjacents areas. Bollettino di Geofísica teorica ed applicata.

LIPPAI, H,; LODOLO, E,; TASSONE, A,; HORMAECHEA, J. L.; MENICHETTI, M.; VILAS, J. F.

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Lugar: Trieste; Año: 2004 vol. 45 p. 142 - 144

0006-6729

Lago Fagnano occupies an important part of the Magallanes-Fagnano continental transform system (MFS), which represents a major segment of the South America-Scotia plate boundary across the Isla Grande de Tierra del Fuego (Lodolo et al., 2003). The lake, trending broadly E-W, is about 110-km-long, with average width of about 7 km. Only its westernmost tip (about 15 km) is located in Chilean territory. In order to analyze the morphological expression of the submerged segment of the MFS, a bathymetric map of the entire Lago Fagnano was derived from data collected during four separate field campaigns (2000, 2001, 2002 and 2003) along D-GPS-fixed transversal profiles and longitudinal tie-lines. A total of forty-three profiles were acquired, in spite of the often prohibitive meteorological conditions, with a 200 kHz echo-sounder and an ECHOTRAC-DF-3200 MK II installed on a Zodiac boat of the Prefectura Naval – Dpto. Lago Fagnano. The bathymetric map has been integrated with the available topography of the surrounding region provided by the Planification Subsecretary of the Tierra del Fuego Govern. The acquired bathymetric profiles have delineated the main morphological features of the Lago Fagnano. The floor is divided into distinct parts, which suggests that the basin is composed of different sub-basins. In most areas, the basin floor is highly asymmetric in shape, with flat depocentral areas. The most pronounced asymmetry of the basin is seen in the eastern end of the lake, where there is also the deepest depression (maximum water depth of 206 m). The steeper slope of the basin, along the northern shore of the Lago Fagnano, also coincides with the most pronounced regional topographic gradient. The total throw along the northeastern shore of the Lago Fagnano, considering also the submerged part of the lake, is of the order of 600 m. The drainage system in this area shows a peculiar pattern: a very short surface stream system flowing toward the lake, with the separation boundary paralleling the shoreline and running along the ridge summit. This anomalous stream network configuration was possibly generated by the action of the fault strand in this sector of the Lago Fagnano, with rapid uplift of the hanging wall of the ridge as a consequence of isostatic rebound. A prominent secondary fault (the Rio Claro fault), forming an angle of about 130° with the master segment of the eastern Lago Fagnano, laterally offsets the principal fault. This splay fault aligns with the eastern flank of a wedge-shaped, graben-like depression occupied by the Rio Claro river, and it is very clear in the remote-sensing maps. Strands of the transform on both sides probably border the central, symmetrical part of the lake floor. The eastern-half of the lake presents a maximum water depth of 165 m. This part of the basin is broadly symmetric in shape and, like the eastern part of the lake, presents a relatively flat depocentral area. Along the westernmost part of the Lago Fagnano, two major, sub-parallel tectonic lineaments control the shape of the floor. Onshore, those lineaments define large graben structures, and bound on both sides a tilted sliver of crust (Monte Hope). An elevated area (about 35 m below the lake level) is found in the central part of the Lago Fagnano, morphologically separating the western and eastern part of the lake. This structure may represent a pressure ridge formed within the principal displacement zone of the MFS.