Mesozoic-Cenozoic evolution of the Fueguinian Andes and Magallan fold-and-thrust belt (Tierra del Fuego Island)- a reappraisal.

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

Potsdam. Alemania

Simposio; IX ISAES; 2003

ISAES

The Fueginan Andes are considerated the orogenic belt South of the Strait of Magellan and developed along the western and southern margin of the Tierra del Fuego Island . The Andean Cordillera with a N-S-trending for 3800 km, bends progressively towards E-W  trend in correspondence of the Tierra del Fuego Island linking to the eastern northern Scotia Ridge.  This progressive rotation is marked by left lateral wrench faults of the Magallanes-Fagnano system. The belt was originated along a collisional margin by horizontal shortening and crustal thickening, with huge and widespread magmatic emplacements in the late Cretaceous and Cenozoic time. The Mesozoic-Cenozoic Andean orogenic cycle evolved from crustal stretching  and widespread silicic volcanism in Middle to late Jurassic, associated with the break-up of Gondwanaland.  The continental regions of Southern South America and Antartica Peninsula were not yet physically separated within Gondwana. In the Early Cretaceous mafic volcanism increased with the development of the Rocas Verdes back-arc basin. It was floored with the oceanic crust of the ophiolitic complex, which outcrops now from more than 800 km from Cape Horn to the North of Sarmiento Cordillera.  The change of the tectonic regime started in the Albian, probably related with the  changes of plate drift with the closure and the inversion of the Rocas Verdes basin. The uplift of the Cordillera, the emplacement of plutonic rocks and the intracontinental polyphase deformation occurred in the late Cretaceous. In the Fueginan Orogen, a high grade metamorphic rock of the Upper Paleozoic to Lower Teriary of the Cordillera Darwin, and the Ophiolitic Complexes of the Rocas Verdes basin, form the major stack of the internal thrusts. The emplacement towards the continent of the different thrusts sheets, in- or out-of-sequence, produced rapid exhumation of the basement rocks and the shortening of the 7000m-thick siliciclastic sedimentary cover of the Magellan foredeep basin, with the formation of a NE verging fold-and-thrust belt.  From Mid-Cretaceous to present E-W sinistral wrench tectonics affected the region as a component of the relative motion between South America and the Antarctic Peninsula intimately related to the complex tectonic events, responsible from the late-Oligocene of the development of the oceanic floor in the western Scotia Sea. The principal tectonic features in the Tierra del Fuego Island evidence the transtensional wrench lineaments. The main structures are asymmetric and restricted  pull-apart basins constituted by several segments in an en-echelon arrangement with a ESE-WNW trend, connected with releasing side-steps structures. The shear zones, forming part of the E-W left-lateral strike-slip Magallanes-Fagnano fault system, was identified in the field along part of the shore of the Lago Fagnano, in the Atlantic coast shore line. The outcropping rocks are strongly deformed and metamorphosed in the  N-verging fold-and-thrust belts, and splay structures related to N-S-trending faults. The extensional fault system, with a  ESE-WNW trend, includes sub-vertical structures with cumulative offset of hundreds of meters. Deformation is of both brittle and ductile types while the kinematic analysis indicates a prevalent left lateral transtensional motion. These transtensional structures are superposed onto the older lineaments and suggest that they may have reactivated pre-existing weak zones formed by Cretaceous-Tertiary shortening.