In plate tectonics, a constructive margin is the boundary between two lithospheric plates (see lithosphere), which are moving apart and new crust is being formed. Constructive margins are associated with shallow-focus earthquakes, high heat flow and eruptions and injections of basalt. Characteristically they produce mid-ocean ridges, but Iceland is an example where a mid-ocean ridge has surfaced above sea-level.

See also destructive margin.