A review of the geology of the Argentinian Fuegian Andes

Abstract

Seven stratigraphic units reflect the tectonic evolution of the Argentinian Fuegian Andes: Basement (Paleozoic–Jurassic); Lemaire Formation (Upper Jurassic); Yahgan-Beauvoir formations (Lower Cretaceous); Cerro Matrero Formation (Upper Cretaceous); Rı́o Claro Formation (Paleocene); La Despedida Group (Eocene); and Cabo Peña Formation (uppermost Eocene–Lower Oligocene). Basement rocks (garnet, quartz-sericite, and chlorite schists; and amphibolites) are unconformably covered by the Lemaire Formation (rhyolites; basalts; slates; and acidic volcaniclastic breccias, tuffs, conglomerates, and turbidites), formed during extensional tectonism. The post-rift Yahgan Formation (deep-marine black mudstones, andesitic volcaniclastic turbidites and tuffs) interfingers northward with the Beauvoir Formation (slope and platform black mudstones), and covers the Lemaire Formation unconformably. The Yahgan Formation represents an andesitic, volcaniclastic apron, coeval with a Pacific volcanic-arc, filling a marginal basin floored with oceanic crust. The Late Cretaceous compressional orogeny resulted in tectonic inversion, closure of the marginal basin, peak metamorphism and folding, and initial uplifting of the Fuegian Andes. By the latest Cretaceous–earliest Paleogene, the Andes were exposed to subaerial erosion, and the lowest Danian Rı́o Claro Formation bears clear evidence of an Andean clastic provenance. The Rı́o Claro Formation represents the first molasse deposits of the foreland stage of evolution of the Fuegian Andes. Earliest Paleogene north-verging thrust propagation deformed the Rı́o Claro Formation and older units, producing northward depocenter migration. La Despedida Group rests unconformably on the Rı́o Claro Formation and is involved in the thrust and fold belt. Important Eocene compression resulted in thrusting of central Andean basement schists and the Lemaire Formation over Lower Cretaceous and continental Paleogene rocks, respectively. In the Argentinian Fuegian Andes, the compressional orogeny ceased by the Late Eocene and the subhorizontal Cabo Peña Formation unconformably overlies folded La Despedida Group strata. Field mapping suggests a left-lateral offset of 20–30 km of Cretaceous–Eocene rocks along the Magallanes–Fagnano Fault System; this is clear evidence that the strike-slip regime has been active since at least the latest Eocene.