Deep seismic-reflection data from the BABEL and FIRE profiles across the Fennoscandian Shield image Svecofennian crust that is made up of a collage of terranes. The data suggest sequential accretion of island arcs and microcontinents to the Karelian craton (1.9–1.8 Ga). These accretionary events may have caused temporary changes to arc geometries, accretionary episodes or collisional phases, and westward growth of the continent. The accreted terranes experienced gravitational collapse that stabilized the crust and exhumed the medium- to high-grade rocks to their present positions. The structures froze after isostatic balance was achieved and thus have been protected from later tectonic...