A review of the Mesozoic-Paleogene geology of the Andes Fueguinos, Isla Grande de Tierra del Fuego.

E. B. OLIVERO; D.R. MARTINIONI

Mesozoic Paleogene geology of the Marginal-Austral basin of Tierra del Fuego

Laboratorio de Geología Andina (CADIC-CONICET)

Lugar: Ushuaia; Año: 1998; p. 1 - 13

CONFERENCE ABSTRACT:   New results of field and laboratory studies produced during the last five years in the Andean region of the Isla Grande de Tierra del Fuego by the authors and associated researchers allow for a better understanding of the geological evolution of the Argentinean Andes Fueguinos. The Alpine geology and tectonics of the Andes Fueguinos excerpt a strong control on the stratigraphic evolution of the region. Seven major stratigraphic units reflecting the main features of the evolving tectonic regimes can be differentiated in the area. From base to top, these are: 1) Paleozoic- Jurassic basement rocks; 2) Late Jurassic Lemaire Formation; 3) Early Cretaceous Yahgan-Beauvoir formations; 4) Late Cretaceous Cerro Matrero Formation; 5) latest Cretaceous?-Paleocene Rio Claro Formation; 6) Eocene La Despedida Group; and 7) late Eocene-Oligocene Cabo Peña-Rio Leona formations. At the southern main axis of the Andes, the Paleozoic-Jurassic basement (1), consisting of garnet, quartz-sericite, and chlorite schists, greenstones and amphibolites, is only exposed in a small area of the southwestern Argentinean Andean area. The basement rocks are unconformable covered by a complex submarine unit of rhyolitic lavas and domes, acidic volcaniclastic breccias, tuffs, conglomerates, turbidites, and slates, and basaltic rocks of the Late Jurassic Lemaire Formation (2). The later unit is unconformable covered in the southern Andean area by Early Cretaceous (3) deep marine black slates, andesitic volcaniclastic turbidites and tuffs of the Yahgan Formation, which grades laterally to the north into the Beauvoir Formation. These three units form a distinct tectonic-stratigraphic interval of the Central Cordillera of the Andes Fueguinos. The tectonic interpretation of the basement rocks (1) is not well known, and they are loosely referred as an accretionary prism of deep marine sedimentary and basaltic rocks on the panthalassic margin of Gondwana. The Lemaire (2) volcanic rocks are interpreted as a syn-rift unit formed during the main extensional tectonic regime related to the initial opening of the South Atlantic Ocean. The Yahgan-Beauvoir formations (3) are interpreted as post-rift units. The Yahgan Formation represents an andesitic, volcaniclastic apron, coeval with the main activity of a Pacific volcanic arc, filling a deep-marine marginal basin partially floored with oceanic rocks to the south. To the north, the Yahgan Formation interfingers with the slope and platform mudstones of the Beauvoir Formation. During the Late Cretaceous, the compressional Patagonidic Orogeny resulted in the tectonic inversion and closure of the marginal basin, peak metamorphism, folding, and initial uplifting of the Andes Fueguinos. The Rio Claro Formation (5) represents the first molasse deposits of the foreland stage of evolution of the Andes Fueguinos. By the latest Cretaceous-earliest Paleogene, the already uplifted central Andean rocks were exposed to subaerial erosion, and the lowest, Danian part of the sediments of Río Claro Formation presents the first clear evidence of an Andean clastic provenance. The geology and tectonic interpretation of the Late Cretaceous Cerro Matrero Formation (4) stilI remains unclear. During the earliest Paleogene, northward thrust propagation involved deformation of the Río Claro Formation and older units and migration of the depocenters to the north. The Eocene La Despedida Group (6) unconformable rests on the Río Claro Formation but during the northward migration of the deformation it was itself involved in the thrust and fold belt. During the Eocene an important compressional event resulted in uplifting of deeper rocks and northward, basement-involved, thrusts transport of central Andean schists or Late Jurassic Lemaire rocks over Cretaceous or Paleogene rocks, respectively. The compressional orogeny finished by the end of the Paleogene, and the mostly Oligocene Cabo Peña-Río Leona formations (7) are represented by subhorizontal beds, unconformable overlying folded strata of the La Despedida Group. First, clear evidence of initiation of a strike-slip regime is found in the latest Paleogene, and field mapping suggest s a left-lateral offset in the order of 25-30 km of Cretaceous-Paleogene rocks along the Magallanes-Fagnano fault system.