(A) Schematic illustration of classic back-arc basin experiencing trench-parallel extension. (B and C) Back-arc basin opening at high angle to the trench. The latter represents, for example, the Gulf of Mexico and the Canada Basin, and reveals why such basins are prone to confinement; these small oceans are surrounded by continental crust. EP—Euler pole. Preexisting Late Paleozoic sutures are shown schematically with a purple hatched pattern and are interpreted to have caused weaknesses that were exploited. Modified after Heuret and Lallemand (2005).