Electrical Tomograhy at the Eastern end of Lago Fagnano. Tierra del Fuego. Argentina.

VILAS, J. F.; TASSONE, A.; SANTOMAURO, M.

Perugia. Italia

Simposio; XXIV General Assembly. International Union of Geodesy and Geophysics (IUGG).; 2007

IUGG

A main portion of the South America-Scotia plate boundary related to the Magallanes-Fagnano Fault System (MFS) is occupied by the lago Fagnano. Just at its eastern end a sag pond area -which is the surface expression of one basin associated with the MFS- is flanking a monzodioritic intrusion (Hewhoepen pluton). The MFS overprints the fold-and-thrust-belt of the Fuegian Orogene and is associated with basin formation along its length. These basins display the greater sedimentary thickness against the main fault in the major displacement zone. Both Paleogene and Quaternary outcrops show transtensional structures associated with MFS. Evidences of Neotectonic fault activity are displayed in Quarternary outcrops on Lago Fagnano margins as in reflection seismic data from the lake-floor. In both, normal faults are recognized in continuity with the main system fault of the northern Fagnano area. An electric resistivity survey was carried on at the eastern lago Fagnano with the purpose of constraining the subsurface continuation of W-E faults related to MFS and the unexposed portion of the Hewhoepen pluton. A Syscal R1+Switch 48 resistivity-meter system was employed to perform two N-S two-dimensional electrical imaging/tomography surveys. These electric sections are parallel; the first one is 4.0 km long and the second section 0.40 km. The roll-all-on method was applied with a Wenner array using 48 electrodes (each 10 m) connected to a 480 m-long multi-core cable, with four sections and 48 channels. In the first profile, 2 sectors are distinguished by their geoelectric properties. The Northern segment presents very low resistivity, whereas the southern one presents a cover with higher resitivity and a lower part with very low resistivity. The pseudo-section and the obtained 2D inversion model from the southern segment allow inferring two lithologies in the subsurface which display at least two contrasted resistivities, i.e. the uppermost peat blanket and lower sandstones. Both litologies show a network of vertical and horizontal discontinuities defining small fragments dipping southwards. A high resistivity body was also recognized in correspondence with the magnetic anomaly of the Hewhoepen pluton. The second profile (located 1 km apart the first one) shows a sector of very low resistivity that cut all the section (about 40m) in continuity with the position in surface of the main lineaments of the MFS. The results of the resistivity tomography allow inferring the existence of a small basin in the north eastern end of the lake, and the buried eastwards continuity of the stepped transtensive fault system related to the Lago Fagnano pull-apart basin.