Neotectonics of the Tierra del Fuego region.

M. MENICHETTI; A. TASSONE; J. FLORES

19-20 de noviembre. Santiago. Chile

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

GEOSUR

The Tierra del Fuego region is characterized by important morphostructural lineaments associated with fault ruptures in the Holocene sediments distributed in different parts of the Island. The present day deformation pattern, as measured by the reoccupation of DGPS stations located on both sides of the main wrench faults of the South American and Scotia plates, suggests a horizontal slip of about 6 mm/y, while plate tectonics models indicate about 22 mm/y. The localization of geological seismically active structures is a powerful tool for planning social and strategic infrastructures as gas and oil pipelines. In the southern part of the Tierra del Fuego region, the continental transform boundary between the South American and the Scotia Plates is characterized by low seismicity (M<3.5) and is associated with several shallow crustal earthquakes. The available focal solutions give rise to a strike-slip component as well as transtensional one. In 1949 the region was affected by an M 7.5 earthquake that produced several landslides and local tsunami waves in the westernmost arms of the Magallanes Strait. During the 1949 earthquake on the eastern bank of the Lago Fagnano shore a W-E scarp of about 1m formed, associated with a stepped gravel barrier. Subvertical WNWESE extensional faults, with a significant sinistral strike slip component, are surveyed in the area of the Lago Fagnano and Lago Deseado area, where scarps a few meters within Pleistocene glacial deposit, display extensional deformations. In the eastern part of Lago Fagnano abundant morphological evidence of the Quaternary activity of the fault is found, with linear truncation of river meanders, drag of river valleys, and hang valleys. Along the Atlantic coast a system of N-S trending, subvertical dextral strike slip faults shift the actual shoreline and the coastal cliffs a few tens of meters. The main morphostructural lineaments mapped for the whole of the island on the panchromatic SPOT mosaic images, are mostly E-W oriented, with secondary N-S lineaments. In the northern part of the island, around the area of the Magallanes Strait, strong morphostructural evidence of normal faulting is exposed and recognizable both on the field and in satellite and digital elevation model maps, as displayed in the drainage network, as well as in the segmented and aligned scarps, triangular facets, and slope breaks. They are part of a system of ENE-WSW trending normal faults, with one of the main structures represented by the depression running from Bahia San Sebastian on the Atlantic coast to Bahia Inútil in the Magallanes Strait. The Quaternary activity of these structures is also visible in the Magallanes Strait’s and Atlantic off-shore seismic lines. These extensional structures have been interpreted as an expression of a Neogene rift system initiated by a counter clockwise rotation of the foreland in the concave side of the Andean Cordillera. The geometry of the N-S dextral strike slip faults, together with the NW-SE trending fold-and-thrust belt and NE-SW trending extensional features, is consistent with the general E-W left lateral transform system that has affected the whole southern tip of the South American Plate since the Paleogene.