Wrench tectonics in the Fueginan Andes and in the Magellanes fold-and-thrust belt (Tierra del Fuego Island).

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

Santiago, Chile

Simposio; VII Simposio Internacional de deformaciones recientes de la corteza terrestre en Sudamérica y alrededores.; 2002

SEGEMIN Chile.

Impressive topographic lineaments associated with strike-slip displacement highlight the continental transform boundary between the South America and Scotia Plates. They run from the western sector of the South Atlantic North Scotia Ridge to the Chile Trench South of 50° S through the Tierra del Fuego Island and the arms of the Magallanes Strait. The Mid-Cretaceous Andean Orogen developed in the Tierra del Fuego region and involved the Late Paleozoic-early Mesozoic rocks of the accretionary complex of the Pacific margin of Gondwanaland. The shortening  and regional metamorphism affected the shallow marine sequences of the Rocas Verdes marginal basin and the vulcanic arc . The external foreland 7000m-thick siliciclastic sedimentary cover of the Magellan foredeep basin, was progressively involved in the NE verging fold-and-thrusts belt, while the metamorphic hinterland was affect by denudation tectonics. From the Late-Cretaceous to present a E-W sinistral wrench tectonics developed in 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, to the development of the oceanic floor in the western Scotia Sea. The main tectonics features in the Tierra del Fuego Island show evidence for the transtensional nature of the E-W wrench faults, superposed onto the older lineaments and if possible  the reactivated pre-existing weak zones of the Cretaceous-Tertiary shortening. In the Atlantic coast,  the Quaternary system of  N-S faults, with right-lateral strike-slip attitude crosses all the previous features. 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 sidesteps structures. The extensional fault systems, include sub-vertical structures with cumulative offset of hundreds of meters. The outcropping rocks are strongly deformed and metamorphosed mainly in the  N-verging fold-and-thrust belts, with several splay structures. In the field these structures are mapped along part of the Lago Fagnano and in the Atlantic coast where shear zones with both brittle and ductile deformations outcrops. The kinematics analysis indicates a prevalent left lateral transtensional motion. The structural architecture of fault populations fit into a left-lateral  strike-slip tectonic framework along the ESE-WNW striking Magallanes-Fagnano master fault. The Magallanes-Fagnano fault system outcrops in the Chilean part of the Tierra del Fuego Island in correspondence with Mt. Hope and in the Lago Deseado area, where aligned fault scarps, truncated vegetation and sag ponds show evidence for the fault trace in the Quaternary alluvial cover. In the Upper Jurassic Tobifera Formation sub vertical ESE-WNW extensional fault arrays with significant sinistral strike-slip component are present. In the Tierra del Fuego Island, both onshore and offshore data, support the interpretation that the Magallanes-Fagnano wrench fault system is remarkably transtensive in nature and the relative motion is partitioned along the diverse segments which make up the fault arrays, where the linkage and step-over geometry play an important role in the development of pull-apart system.