It is observed that active interac basins are found only where the subducted plate can interact with the plate behind the trench. This condition exists where trenches are terminated or offset by transform faults or where large variations in subduction rates occur. Shear coupling between the two juxtaposed plates across a transform or strike-slip fault creates a tensional regime behind the island arc. Differential subduction velocities will couple the plate motions across both the more slowly subducting trench and across the intervening fracture zone. If the trench axis is assumed to move more slowly than a plate, inter-arc basin spreading may be initiated to conserve area.