Seismic study of the northwestern boundary of the Scotia plate on the Argentinean continental margin.

TASSONE, A.; YAGUPSKY, D.; LODOLO, E.; COMINGUEZ, A.; MENICHETTI, M.; VILAS, J. F.; GELETTI, R.; LIPPAI, H.

Potsdam. Alemania.

Simposio; IX ISAES.; 2003

ISAES

In the central northern side of the Fueguian Andes, the structures of the external thrust front propagate through to the Magallanes thin-skinned foreland fold-and-thrust belt. In onshore areas, the main structures consist of a system of N-verging, ESE-WNW-trending, asymmetric folds-and-thrusts, originated by a mostly N-S Late Cretaceous to Tertiary shortening. Within the surveyed area the tectonostratigraphic sequence is dominated by marine sequences of the Rocas Verdes marginal basin (Yaghan Formation) and minor outcrops of its volcaniclastic substrata that constitute an imbricate wedge thinning northwards. The overlaying syntectonic sedimentary wedge consists of the siliciclastic succession of the Magallanes foreland basin. The Magallanes-Fagnano Fault System (MFS), interpreted as the result of shear stresses produced along the transcurrent South America-Scotia plate boundary, is a E-W left-lateral strike-slip fault with an extensional component that overprints the contractional structures and is responsible for the main depression of the Lago Fagnano. About 1.200 km of multichannel seismic reflection profiles were acquired off the Atlantic coast of the Tierra del Fuego Island. The profiles cut across the South America-Scotia plate boundary, a transform margin which traverses in an E-W direction the Island. In the area located to the north of Isla de los Estados, the N-S trending seismic lines show a complex superposition of different tectonic structures, with extensional, compressional and transtensional features. The profiles shows the seismic geometry of the deep duplex with buried leading edge is geometrically similar to the structures exposed in the western part of the Island (Alvarez Marrón et al., 1993; Klepeis & Austin, 1997). The model geometry of passive roof thrusts and back-thrust that develop the triangle zone north of the Lago Fagnano area is well constrained with the geological field data and well marked by the shear zones seismic reflectors. The geometry of the MFS is reveled by the seismic images in the Atlantic off-shore as well as in the Magallanes Strait (Lodolo et al., 2002a,b) where half-graben basins can be associated with the same deformation event. The near parallelism among the younger (transtensive) and older (contractive) lineaments, suggests that the development of the transtensional structures may have reactivated pre-existing weakened zones formed by the shortening. On the basis of acoustic fabrics and seismic discontinuities four main units have been recognized; from bottom to top they are: the acoustic basement (Seismic unit 1), which is overlain by a unit (Seismic unit 2) which displays a tabular geometry and some reflector packages of high amplitude; this unit might be related to the volcanic and volcaniclastic sequences of Tobífera/Lemaire Fms. The Seismic Unit 3 displays an irregular shape and internal reflector configurations of moderate amplitude and continuity and low- to moderate frequency; the onshore Yaghán/Beauvoir Fms could be the surface equivalent of this unit. The uppermost seismic unit (4), located between the ocean floor and the 0.5 sec twt, displays a fairly uniform thickness, it is clearly distinguished from the underlying unit by the low frequency of reflectors with low- to moderate amplitude and continuity. The unit 4 may be correlated with the Tertiary sediments of the Magallanes foreland basin which were involved in the fold and thrust belt.