CONICET | Buscador de Institutos y Recursos Humanos

The study area comprises the Fueguian Andes which is characterized by a sequence of asymmetric ESE-WNW trending folds associated to north verging thrusts. Compressive events took place during Middle Cretaceous-Tertiary leading to the closure of the Early Cretaceous Rocas Verdes marginal basin. A geophysical survey was carried out in order to better understand the geometric contact relationships between the different units that constitute the fold and thrust belt. This comprised a resistivity tomography of the interest area which can add information to and be compared with field geological studies. The geoelectric survey was performed along Bahía Ensenada path (Beagle Channel) in Tierra del Fuego National Park. Nearest outcrops in the area correspond to basaltic rocks located 0.3 km from the bay in Monte Susana and very low-grade metamorphic rocks on Ensenada and Lapataia bays. The measurement profile was 1.28 km long with an approximate N-S strike. 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 resistivity pseudosections obtained were processed with inversion software to get 2D models of the study area, where the investigation depth reaches 70 m approximately. The analysis of the tomography section allows identifying three regions with very different electric properties. The first one, located on the north (more than 0.9 km from the bay) shows a resistivity decrease with depth. On the contrary, the central region displays a general increase of resistivity values with depth while lateral variations that define a series of discontinuities are also present. In that sense we can mention the existence of narrow regions with high values (more than 3000 ohm.m) at depths of 60 m which are separated by other regions with resistivity values < 400 ohm.m. The third region, which extends from the bay to 0.48 km north shows a conspicuous resistivity increase with depth, reaching values near 6000 ohm.m at 25 m of depth. The interpretation of the electric tomography of the first region may suggest the existence of black shales of Yahgan Formation in the subsurface (marine sequences of Rocas Verdes marginal basin) and the central region could correspond to basaltic dykes intruded in Yahgan Formation. Finally, the high resistivity values at shallow depths registered on the southern region could be interpreted as metamorphic rocks similar to those outcropping in Ensenda and Lapataia bays. Therefore, it has been probed that the geoelectric method was a useful tool for determination of subsurface continuity of the exposed units and the contact relationship between them.

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