Geophysical Study of the Magallanes Fault System in the Tierra del Fuego Region (Argentina).

LODOLO E.; COREN, F.; VIDMAR, R.; ZANOLLA, C.; TASSONE A.; LIPPAI, H,

Strasbourg

Simposio; European Union of Geosciences. EUG 10; 1999

EUG

The Magallanes Fault System (MFS) is interpreted as the westward prosecution of the present-day strike-slip boundary between the South America and Scotia plates, extending for over than 600 km across the Tierra del Fuego in both the Argentinean and Chilean territories. Sparse and poorly documented geological evidences of transcurrent fault and associated thrusting found onshore have not yet allowed a precise mapping of this fault system, and prevailing geometrical extrapolations (onshore westwards prosecution of the South Scotia Ridge) have been consid­ered for tracing the MFS from the Atlantic coast of the Tierra del Fuego to the southern Chile Trench. The precise age of the deformation associated to die presence of the MFS, and the distribution of relative movement between the two plates along its length, are also poorly kiiown. In particular, it is not clear the role that the MFS had in the more general context of the Drake Passage opening and the successive development of the western Scotia plate during Oligocene time. A geophysical and geological investigation have been designed in order to clarify diese aspects, and two campaigns were carried out in February-April 1998 and October-November 1998 in die Tierra del Fuego Island, with the collection of GPS-fixed gravimetric and magnetic data points, the execution of field structural geology tran­sects and petrologic samplings, in an area 30 km x 60 km wide, located just on the east of the Fagnano Lake. This research project, calle'd TESAC (Tectonic Evolution of the South America-Scotia plate boundary during the Cenozoic), is part of a scientific collaboration between Argentina and Italy for the study of the Antarctic region and adjacent seas. Main aim of mis program is to analyze the geological structure of the segment of the MFS in the Tierra del Fuego region bom onshore and offshore, and to recon­struct the principal aspects and timing of die strike-slip activity occurred in the area during the Cenozoic. The offshore part of die survey is planned for die Austral summer 1999-2000. Preliminary magnetic and gravity maps of the studied area have been produced, and a deep structural model was constructed across the supposed location of the MFS, along die eastward prosecution of the Fagnano Lake, in some parts deeper than 500 m, where the Soum America-Scotia plate boundary is supposed to be located. The topographic correction for the Bouguer anomaly map has been computed using the digital elevation model derived from interferons try of a pair of Syndietic Aperture Radar (S AR) images acquired by ERS-1 and ERS-2 (Earth Resource Satellite), because of the unavailability of extensive and precise altitude information. Analyses performed on die acquired data furnished important indications on die pres­ence of a main tectonic lineation, accompanied by sub-parallel structures, elongated in an ENE-WSW direction. prosecuting eastward on the Atlantic Ocean in correspon­dence of a regional gravity minima, as seen on satellite-derived free-air gravity data. This evidence seems to confirm Uiat die MFS effectively represents the onshore prosecution of the structures associated to the South America-Scotia plate boundary, even if significant aspects related to its role in the tectonic development of the western Scotia Sea region are not yet well understood.