foredeep

In earth science, an elongated structural basin lying inland from an active mountain system and receiving sediment from the rising mountains. According to plate tectonic theory, a mountain chain forming behind a subduction zone along a continental margin develops a foredeep or gently sloping trough parallel to it on the landward side. Foredeeps form rapidly and are usually so deep initially that the sea floods them through gaps in the mountain range. As the mountain system evolves, sediments choke the foredeep, pushing out marine water. As marine sedimentation stops, only nonmarine deposits from the rapidly eroding mountains are formed. These consist of alluvial fans and also rivers, flood plains, and related environments inland.

Before the advent of plate tectonic theory, such foredeep deposits and changes in sediments had been interpreted as sedimentary troughs, called geosynclines, that were supposed ultimately to build upward into mountains.