Seismic structure of the southern Magallanes Basin off the Tierra del Fuego Island

TASSONE A.; LODOLO E.; GELETTI, R.; FOSTER, M.

Buenos Aires

Workshop; Workshop on Continental Shelf; 2000

COPLA

The Magallanes (or Austral) Basin, located at the southern tip of South America, extends in southern Patagonia from latitude 48° to 55° south. It lies between the Andean foothills and the Deseado Massif in an elongate shape, covering a surface of about 200,000 km2. Thirty percent of it lies offshore, west of the Dungenness Arc. In this Basin, flanked to the east by a stable margin, and to the west and south by an active orogenic belt, a sequence of mainly terrestrial sediments (more than 6,000 m thick) were deposited during the Cretaceous and Cenozoic, and later deformed during successive tectogenetic episodes. Igneous and volcanic rocks are predominant in the western Cordillera; the foothills belt is mostly formed by folded Cretaceous sediments, and in the eastern lowlands, relatively flat or gently folded Tertiary-to-recent sediments are present. A joint Argentinean-Italian geophysical survey has been conducted on October 1999 off the Atlantic coast of the Tierra del Fuego Island, with the collection of about 900 km of multichannel seismic reflection, bathymetric and magnetic profiles, to study the sedimentary setting and the geological structure of the southern margin of the Magallanes Basin. This work is part of the TESAC project, whose main objective is the reconstruction of the geological structure and Cenozoic geodynamic evolution of the South America-Scotia plate boundary in the Tierra del Fuego region. Seismic data image the progressive deformation affecting the siliciclastic sedimentary sequences of the Magallanes Basin and the shallow marine sequences of the Rocas Verdes marginal basin, which gradually becomes more remarkable proceeding from north to the south, and originated by a mostly N-S Late Cretaceous to Tertiary shortening. A peculiar asymmetric basin, bounded by a sub-vertical discontinuity on one side and a normal fault on the other, has been found along the eastern prosecution of the E-W-trending, left-lateral strike-slip Magallanes-Fagnano fault system, which represents one of the major segments of the South America-Scotia plate boundary in the Tierra del Fuego region. This shear zone, identified and studied onshore in the vicinity of the Lago Fagnano, is remarkably transtensive in nature, and is structurally and temporally superimposed on the older tectonic style of the Tierra del Fuego Island. Its development may be linked to the Late Miocene cessation of seafloor spreading in the western Scotia Sea, which marked a significant increment in the South America-Scotia relative plate motion.