Geology and geodynamics of the Tierra del Fuego.

MARCO MENICHETTI; E. LODOLO; A. TASSONE

19-20 de noviembre. Santiago. Chile.

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

GEOSUR

Despite more than a century of researches, the geological framework and structural setting of the Isla Grande de Tierra del Fuego is only partially known, mostly because a sparse and incomplete data set. Large areas of the region remain unknown, such as the western (Cordillera Darwin) and eastern (Peninsula Mitre) sectors of the Island, as well as some other areas of difficult access. Moreover, is still largely unconstrained the role played by strike-slip tectonism in the geodynamic evolution of the Tierra del Fuego region. There is much field evidence that the pre-Jurassic rocks were involved in the Andean compressional phase but the magnitude of the shortening is very difficult to estimate along the Fuegian Andes. The outcrops on the western part of the Island, together with the location of the boundary between the Andean chain and the Magallanes foreland, suggest the presence of Meso-Cenozoic strike-slip faults that need to be confirmed along the Cordillera. Strike-slip faults parallel to the trench are a common feature in various sectors of the Andean Cordillera, and their presence in the Tierra del Fuego needs to be further investigated. The Cenozoic E-W sinistral wrench tectonics characterizing the morphostructure of the region is superposed to pre-existing tectonic weaknesses and possibly inverted or reactivated many structures. The presence of major belts of mylonitic rocks along the Fuegian Cordillera associated with strike-slip shear zones could indicate the presence of strike-slip activity since the Mesozoic. Several pull-apart basins developed along the Magallanes-Fagnano fault system, present complex sedimentary architectures with a different rate of subsidence. The geometry and the distribution of positive and negative structures along the principal deformation zone of the strike-slip fault arrays, the alternation of restraining and releasing structures, seem to be controlled by old basement structures. The predominance of transtensional features in the eastern part of the Tierra del Fuego Island could be related to the northward propagation of the extensional strain field associated to the Scotia Sea Neogene evolution. The mechanism of the progressive bending of the Andean Cordillera from a N-S trending towards an E-W orientation in correspondence with the Tierra del Fuego Island could be related to an orocline process. Paleomagnetic and structural data indicate that such a progressive rotation could also be interpreted as a tectonic bend marked by the left-lateral wrench faults along the northern Scotia Ridge. The tectonic evolution of the Fuegian Andes since the Mesozoic is linked to the general plate tectonic scenario of Gondwana break-up where intracontinental deformation, bending, and strike-slip faulting played an important role in shaping the southernmost tip of South America. Several open questions on the geology of the Tierra del Fuego region and on its geodynamic evolution are here presented, as a contribution for further explorations and investigations.