Geophysical study on Lago Fagnano area. Tierra del Fuego

SANTOMAURO M.; TASSONE, A.; LIPPAI, H.; MENICHETTI, M.; LODOLO, E.; VILAS, J. F.

19-20 de noviembre. Santiago. Chile.

Simposio; International Geological Congress on the Southern Hemisfere (Geosur 2).; 2007

GEOSUR

The South America-Scotia plate boundary is located on the southern region of Tierra del Fuego Island and is determined by the Magallanes-Fagnano fault system (MFS), which is a transtensive tectonic complex. A series of pull-apart basins, such as Lago Fagnano, have been detected along this east-west echelon fault system. An electric resistivity survey along the eastern region of Lago Fagnano was carried out in order to determine the subsurface geometry of the main left lateral strike-slip faults. The resistivity survey was accompanied by a field geological survey of Lago Fagnano margins. Quaternary outcrops on the lake region show evidence of neotectonic activity which is displayed by a system of E-W left lateral strike-slip faults with a south-dipping extensional component. These normal faults are in continuity with the main fault system of the northern Fagnano margin and its subsurface continuity is recognized in reflection seismic data from the lake floor and on the resistivity tomographies. The geoelectric survey was performed along two N-S parallel profiles in order to get two correlative tomography sections. The first one, along the eastern shore of the lake is 4 km long and the second one (located 2.5 km to the east of the first one) is 0.4 km long. Resistivity measurements were acquired with a Syscal R1 resistivity-meter system. The roll-along method was applied with a Wenner-Schlumberger array using 48 electrodes (each 10 m) connected to a 480 m-long multi-core cable, with four sections and 48 channels. The obtained resistivity pseudosections were processed with inversion software to get 2D models of the study area, where the investigation depth reaches 70 m approximately. The first model section (eastern shore) shows both horizontal and vertical resistivity variations. The first case corresponds to a resistivity stratification associated with a decrease of such values with depth. This could be related to the conductivity contrast between the uppermost peat blanket and the lower sandstones (glacilacustrine deposits). The second case (vertical variation) is verified by a conspicuous resistivity contrast in the northern region which defines a low resistivity area on the south and a less conductive area on the north. On the north - central region low resistivity south-dipping narrow areas are recognized over which a smooth thickness increase of the different resistivity levels is observed. Furthermore the thickness of the levels increases remarkably to the south. The second model section (2.5 km east) also shows a resistivity decrease with depth as well as a thickness increase of the different levels to the south. The results of the tomography sections allow inferring the existence of south dipping faults in continuity with the main fault system of the northern Fagnano margin, verifying the eastern continuity of one of the Magallanes-Fagnano fault segments. A small basin can also be inferred in the northern region of the lake shoreline as indicated by the smooth increase of the resistivity levels in this region. The most important thickness increase of the different levels to the south, also recognized on seismic sections, is interpreted as mass flow deposits generated by destabilized and redeposited material which could also be associated with tectonic activity pulses along Magallanes-Fagnano fault system.