 This subduction zone records another subduction zone called the Intermontane Trench under an ancient ocean between the Intermontane Islands and the former continental margin of Western Canada called the Slide Mountain Ocean.This arrangement of two parallel subduction zones is unusual in that very few twin subduction zones exist on Earth; the Philippine Mobile Belt off the eastern coast of Asia is an example of a modern twin subduction zone.As the Intermontane Plate drew closer to the pre-existing continental margin by ongoing subduction under the Slide Mountain Ocean, the Intermontane Islands drew closer to the former continental margin and coastline of Western Canada, supporting a volcanic arc on the former continental margin of Western Canada.