Morphostructure of a transform continental margin in the Tierra del Fuego region. (Southern South America).

LODOLO E.; BEN-AVRAHAM, Z.; MENICHETTI, M.; LIPPAI, H,; TASSONE A.

Moscow-St. Petersburg. Russia.

Conferencia; International Tectonic Conferences; 2001

Tectonic Society of Russia

The Tierra del Fuego region, the southern end of South America, is one of the few places on Earth which offers the opportunity to observe the surface and subsurface geological features related to a continental transform plate boundary, but it is one of the least known because its remoteness and the difficult access in most of its territory. A comprehensive suite of field surveys was carried out from early 1999 to late 2000 in the Tierra del Fuego region, to investigate the 600-km-long Magallanes-Fagnano fault system (MFS), a transform-type margin developed on continental crust. This fault boundary, runs from the western arm of the Magallanes Strait to the Atlantic offshore, and substantially splits the Tierra del Fuego Island in two continental blocks. The identification and analysis of the morphological and structural elements related to the MFS, and the understanding of mechanisms of slip along the fault, are the principal objectives of an ongoing project called TESAC (Tectonic Evolution of the South America-Scotia plate boundary during the Cenozoic). An integral part of the project is focused on the 3-dimensional architecture of the pull-apart basins, the elongated depocenters which generally form in correspondence of the principal displacement zone of the fault system. Multichannel seismic reflection measurements and bathymetric profiles acquired off the Atlantic coast of the Island, along Lago Fagnano and central Magallanes Strait, and at the western end of the Magallanes Strait, in conjunction with field structural analyses, have imaged the surface and sub-surface structure of the MFS and its associated basins. The fault system is composed by distinct tectonic lineaments that are segments of the transform system, and are represented by mostly near-vertical faults, with polarities that change along the strike of the fault. The sedimentary architecture of the asymmetric basins formed within the principal displacement zone, suggests simultaneous strike-slip motion and transform-normal extension, a common feature found in other continental transtensional environments. Analyses of remote-sensing images and the Digital Elevation Model (DEM) derived from Synthetic Aperture Radar (SAR) data, reveal with unprecedented detail the structures identified in the field, which are characterized by a noticeable morphological expression, and have allowed to reconstruct at a regional scale the morphostructure of the central-eastern part of the Tierra del Fuego Island. The timing of the initiation of the transtensional deformation in the MFS is problematic because of the lack of distinctive information, but field geological and structural data support the idea that the strain mechanisms along the MFS are intimately related to the complex tectonic events responsible for the late-Oligocene development of the oceanic floor of the western Scotia Sea, which has led to the definitive separation of Antarctica from the other land masses.