Neotectonics at the continental transform boundary of the South America-Scotia plates: the Magellanes-Fagnano fault system

MENICHETTI, M.; LODOLO E.; TASSONE A.; GELETTI, R.

Siena. Italia

Workshop; Antartic Neotectonic; 2001

Terra Antartica Publ

The continental transform boundary between the South America and Scotia plates runs from the western sector of the North Scotia Ridge to the Chile Trench south 50°S, through the Tierra del Fuego Island. The mid-Cretaceous Andean Orogen developed in the southern sector of the Tierra del Fuego with a regional metamorphism. A mid-Tertiary shortening involved the siliciclastic sedimentary cover of the Magellan foredeep basin, with the formation of a fold-and-thrusts belt NE verging. The E-W left-lateral strike-slip Magallanes-Fagnano fault system evolved as a component of the relative plate motion between the southern South America and the Antarctic Peninsula. This regional wrench lineament outcrops mainly in the eastern arm of the Magellan Strait, along the north shore of the Lago Fagnano, and along the Atlantic coast (Winslow, 1989). The present-day deformation pattern measured by re-occupation of DGPS stations located in both sides of the main wrench faults of the South America and Scotia plates, gives a slip of about 0.5 cm/yr ( Del Cogliano et al., 2000). The seismicity along the fault system is low (M<3.5) and mainly shallow. The available focal solutions give a strike-slip with a component of extension (Pelayo and Wiens, 1989). The Tierra del Fuego Island was affected in the 1949 by a M 7.7 earthquake, that produced several landslides in the bank of the Lago Fagnano and local tsunami waves in the westernmost arms of the Magellan Strait (Jaschek et al., 1982). The Magallanes-Fagnano fault system outcrops in the Chilean side of the Tierra del Fuego Island in correspondence of the Mt. Hope and in the Lago Deseado area, where aligned fault scarps, truncated vegetation and sag ponds evidence the fault trace in the quaternary alluvial cover. In the Upper-Jurassic Tobifera Formation are present subvertical WNW-ESE extensional fault arrays with significant sinistral strike-slip component (Kepleis, 1994). In the eastern part of Lago Fagnano, there are several morphological evidences of the quaternary activity of the fault, with linear truncation of river meanders, deflection of stream direction and hang valley. In the southern shore of the Lago Fagnano, outcrops of Pleistocene glacio-lacustrine sequences consist of proglacial delta facies of silts, cross-stratified gravel and coarse sands and fine gravel lacustrine deposits (Lopez et al., 1997). A W-E scarp of about 1 m, associate with a stepped gravel barrier on the Lago Fagnano shore was created during the 1949 earthquake, enclosed a sag pond of the lower Rio Turbio Valley. This scarp merges in a small hill where sand and gravel cross-stratified beds of the glacio-fluvial plain facies, outcrop in a small quarry. These sediments are quite deformed by several sets of subvertical, S-dipping normal faults. The kinematic analyses of fault populations in the area show a prevalent left-lateral trastensional component