The Magallanes-Fagnano Fault is a major left-lateral strike-slip system that cuts across Tierra del Fuego at the southern tip of South America, running roughly 600 kilometres from the Magellan Strait eastward across Lago Fagnano and into the Atlantic. It forms the on-land boundary between the South American Plate and the smaller Scotia Plate.
The fault accommodates the eastward motion of the Scotia Plate relative to South America, sliding at a rate of about 6 to 7 millimetres per year. Its trace is beautifully expressed by the elongated, fault-controlled basin of Lago Fagnano, one of the largest lakes in Patagonia, whose linear shape follows the fracture directly.
The most notable event was the December 1949 Tierra del Fuego earthquake sequence, with two shocks of magnitude around 7.5 that shook the region and were felt across Patagonia. Because the area is sparsely populated, damage was limited, but the fault remains capable of generating large earthquakes near Ushuaia and Río Grande.