Tectonics at the continental transform boundary of the South America-Scotia plates: the Magallanes-Fagnano fault system

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

Chieti. Italia

Congreso; Congreso de la federación italiana de ciencia de la Tierra; 2001

Federación Italiana de Ciencia de la Tierra (FICT)

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 of 50° S throughout the Tierra del Fuego Island. It has generated impressive topographic lineaments associated with strike-slip displacement in the central region of the Magallanes Strait. The Mid-Cretaceous Andean orogenesis produced in Tierra del Fuego regional metamorphism and NE-SW shortening, by the formation of a fold-and-thrust belt and progressive deformation of the siliciclastic sediments of the Magellan foredeep basin. The E-W left-lateral strike-slip Magallanes-Fagnano fault system (MFS)  evolved from Mid-Cretaceous to present as a component of the relative motion between South America and the Antarctic Peninsula. These regional wrench lineaments crop out mainly in the east arm of the Magellan Strait, along the shore of the Fagnano Lake, on and off the Atlantic coast for more the 400 km. The evolution of the MFS is intimately related to the complex tectonic events responsible for the late-Oligocene development of the oceanic floor of the western Scotia Sea, which definitively led to the isolation of Antarctica from the other land masses. Seismicity along the plate boundary is low (M<3.5) and mainly related to shallow crustal earthquakes showing pure strike-slip as well as trastensional fault-plane solutions. The seismic, plate tectonics (DGPS-based) and geologic slip rate  along the boundary is close to 0.5 cm/yr. A series of distinctive regional morphological ESE-WNW trending lineaments were traced on multispectral SPOT images,  in Synthetic Aperture Radar (SAR) frames and in Digital Elevation Models (DEM). They were also identified on aerial photographs and checked in the field  at the mesoscopic scale in selected outcrops. The main morphostructural feature of the Island is the 110 km long and about 7 km wide Lago Fagnano. The bathymetric map of Lago Fagnano delineates a basin profile with a highly asymmetric shape, with the steepest slope paralleling the northern shore, and reaching a maximum depth of 201 m. This trend follows the ESE-WNW narrow depression located onshore to the East, with a releasing sidestep constituted by at least two segments in an en-echelon arrangement. All the lineaments provide evidence for fault activity, with linear intermontane valley, outcrops of aligned fault scarps, truncated vegetation and sag ponds and occurrence of fault traces in Quaternary alluvial deposits. A W-E scarp of about 1 m associate with a stepped gravel barrier enclosing a sag pond on the Fagnano Lake shore was created during the M. 7.7,  1949 earthquake. The extensional fault system, with a  WNW-ESE trend, include sub-vertical structures with cumulative offset of hundreds of meters. Deformation is of both brittle and ductile types while kinematic analysis indicates a prevalent left lateral trastensional motion. These transtensional structures are superposed onto the older lineaments and suggesting that they may have reactivated pre-existing weak zones formed by Cretaceous-Tertiary shortening. The seismic lines acquired off the Atlantic coast of the Tierra del Fuego Island, and those gathered in the central and western Magallanes Strait, have imaged the cross-section geometry of the MFS. Along the aligneament there are several asymmetric basins, bounded by S or N dipping faults with complex syntectonic sedimentary infilling, probably related to the switch from strike-slip to an extensional tectonic regime.  Both onshore and offshore data support the interpretation that the MFS is remarkably transtensive in nature and the relative motion is partitioned along the diverse segments which make up the fault array, where the linkage and step-over geometry play an important role in the development of pull-apart system.