ELECTRICAL TOMOGRAHY AT THE EASTERN END OF LAGO FAGNANO. TIERRA DEL FUEGO. ARGENTINA

Alejandro Tassone1, Melina Santomauro1, Juan Francisco Vilas1

1CONICET-INGEODAV. Dpto. de Ciencias Geológicas.Universidad de Buenos Aires. Aires. Argentina.

Aim of the research

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 Collect new geological data

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The studied area is located in the southernmost South America.

Geodynamic contest of the southern tip of South America

Regional tectonic map with the plate boundary.

The Tierra del Fuego region, which occupies the southern extremity of the South America Continent, is cut across by a 600-km-long tectonic lineament here termed Magallanes-Fagnano fault System (MFS).This lineament runs from the Pacific entrance of the Magallanes Strait to the offshore Atlantic, and splits the Isla Grande in two continental blocks.

Pull-apart basins developed within the principal displacement zone of

the Magallanes-Fagnano Fault System

The Lago Fagnano, located in the central area of Tierra del Fuego Island, is the result of trantensive tectonics responsible for several pull-apart basins

associated with the Magallanes-Fagnano Fault System (MFS).

The resistivity survey was accompanied by a geophysics and geological survey of Lago Fagnano region.

Geological map of Lago Fagnano area and Geological map of Lago Fagnano area and surroundingssurroundings..

Bathymetric map of Lago Fagnano

Complete Bouguer anomaly map of the Isla Grande Complete Bouguer anomaly map of the Isla Grande de Tierra del Fuegode Tierra del Fuego.

Complete Bouguer anomaly map of the Isla Grande de Tierra del Fuego, Complete Bouguer anomaly map of the Isla Grande de Tierra del Fuego, computed from a total of more than 500 DGPS-fixed gravity measurements.computed from a total of more than 500 DGPS-fixed gravity measurements.

from Lodolo et al., 2007

The gravity map has shown that, superimposed on a regional N-S negative gradient, is present an E-W-trending gravity anomaly low, which is composed by various and distinct relative minima, and is interpreted as the expression of elongated and narrow sedimentary basins formed within the principal displacement zone of the MFS.

from Lodolo et al., 2004

Bathymetric map of Lago Fagnano (top), and representative profiles (bottom).

The most pronounced asymmetry of the basin is seen in the eastern end of the lake (see, for example, profile LF-13), where there is also the deepest

depression (maximum water depth of 206 m).

The floor is divided into distinct parts, which suggests that the basin is composed of different sub-basins. In most areas, the basin floor is highly

asymmetric in shape, with flat depocentral areas.

Geological map of Lago Fagnano area and surroundingsGeological map of Lago Fagnano area and surroundings..

The geological map summarizes the most important observations The geological map summarizes the most important observations taken from the outcrops present in the central-eastern part of the taken from the outcrops present in the central-eastern part of the TdF. The map integrates the results derived from the analyses of TdF. The map integrates the results derived from the analyses of

SPOT and SAR images.SPOT and SAR images.

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.

Quaternary outcrops show transtensional structures associated with MFS.

Line drawing of an outcrop located just to the east of the Lago Fagnano eastern shore where a glacio-fluvial sand and gravel barrier shows several sets of sub-vertical, S-dipping normal faults.

Previous seismic data pointed to the probable presence of neotectonic in the the Lago Fagnano

Evidences of Neotectonic fault activity are displayed in Quarternary outcrops on Lago Fagnano margins as in reflection seismic data

from the lake-floor

N

Electric resistivity survey

-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.

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 geoelectric data were acquired, in spite of the often prohibitive meteorological conditions.

Furthermore the thickness of the levels increases remarkably to the south.

N S

The 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.

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.

RESULTS

El Torito on The Lago Fagnano south margin

Thank you