We used computer models to study how mountains formed by the collision of tectonic plates can later affect the breakup of these same plates. Our results show that in large, warm mountain belts, new faults form due to the orogen being  weak overall, while in smaller, colder belts, breakup follows old fault zones. Microcontinents that were accreted during collision can create new continental fragments during extension. These findings help explain how past geological events shape continent margins.