When the subducted plate reaches the depth of about 80 kilometers melting occurs. As magmas move upwards, a small island arc is formed by the underwater volcanic eruption. Island arcs are characterized by paired metamorphic belts. The Japanese islands are a modern example. Shallow marine and subaerial deposition dominate in this setting.
Sediments are flat lying, unmetamorphosed and undeformed.
Shallowing marine sediments on unstable shelves, 3. Slope-covering and trough filling turbidites. A marginal sea is a basin with the marine sedimentation bounded by the volcanic arc on one side and the remnant arc on other side. Formation of a marginal sea can be caused by the rifting of an island arc. This process may be related to tensional tectonics, leaky transform faults or secondary convection cells in the mantle wedge overlying aBenioff zone.