What geological feature formed the deep basin of Lake Superior?

The Midcontinent Rift formed the deep geological basin underlying Lake Superior.

This ancient continental rift began about 1.1 billion years ago, when the North American continent started to split along a vast curved zone. Molten rock rose through the crust, producing extensive volcanic activity and thick layers of basalt. The rifting stopped before the continent fully divided, leaving a major geological scar that later influenced the shape and depth of Lake Superior.

Much later, repeated continental glaciations widened and deepened the basin, while also scraping and reshaping its surrounding landscape. The modern lake therefore reflects both very ancient tectonic activity and comparatively recent Ice Age erosion.

The Midcontinent Rift is not an active rift like the East African Rift today. It is a failed or aborted rift system: the crust began pulling apart but the process ended. Its rocks and structure can be traced in parts of the Lake Superior region and beneath surrounding areas.