Imaging the Magallanes Fault System in the Tierra del Fuego Region (Southermost South America): Results from a Geophysical Reconnaissance.

LODOLO E.; TASSONE A.

California

Simposio; Fall Meeting; 1998

AGU

The present-day boundary between the South America and Sax plates is located along a 3000 km-long strike-slip fault, running from t: western part of the North Scotia Ridge, to the southern Chile trenc Sparse geological evidences of transcurrent fault and associated thru^ ing found onshore in the Tierra del Fuego Island have been interprets as the surface evidence of this plate boundary. The largest of the fault zones is know as the Magallanes Fault System (MFS), extendii for over than 600 km across the Tierra del Fuego in both the Arge; tinean and Chilean territories. This interpretation is based main on seismological data (few and low-magnitude earthquakes) and ge> metrical extrapolations (onshore westwards prosecution of the Soui Scotia Ridge), but lacks of detailed and systematic geological invesi gations and petrologic information. The precise age of the deformans associated to the presence of the MFS, and the distribution of r<! tive movement between the two plates along its length, are also p . known. In particular, it is not clear the role that the MFS had . more general context of the Drake Passage opening and the succ. development of the western Scotia plate during Oligocene time. A geophysical and geological investigation have been carried out !io: 19th February to 4th April 1998 in the Tierra del Fuego Island, with t;; collection of 225 GPS-fixed gravimetric and magnetic data points, id execution of field structural geology transects and petrologic samplings in an area 25 km x 40 km wide, located just on the east of the Fagnan* Lake. This research project, called TESAC (Tectonic Evolution of th South America-Scotia plate boundary during the Cenozoic), is part o a scientific collaboration between Argentina and Italy for the study o the Antarctic region and adjacent seas. Main aim of this program i to analyze the geological structure of the segment of the MFS in th> Tierra del Fuego region both onshore and offshore, and to reconstruc: the principal aspects and timing of the strike-slip activity occurred ii the area during the Cenozoic. The offshore part of the survey is planner for the Austral summer 1999-2000. Preliminary magnetic and gravity maps of the studied area have bee:. produced, and a deep structural model was constructed across the sup posed location of the MFS. Two Synthetic Aperture Radar (SAR) im­ages, acquired by ERS-1 and ERS-2 (Earth Resource Satellite), haw been used to recognize the main structural lineations of the area. Tiu topographic correction for the Bouguer anomaly map has been com puted using the digital elevation model derived from interferometn of a pair of SAR images, because of the unavailability of extensive and precise altitude information. Analyses performed on the acquire,: data furnished important indications on the presence of a main liii eation elongated in an E-W direction in prosecution of the trend v\ the Fagnano Lake, in some parts deeper than 500 m, where the Sour: America-Scotia plate boundary is supposed to be located. Preliminary correlation between the structures found onshore and those pr. -,;: offshore, as seen from available seismic profiles, have allowed u : the main strike of the MFS in the surveyed area, and tentative construct the tectonic framework of the South America-Scoti; boundary in the Tierra del Fuego region.