Intermontane Belt

The Intermontane Belt is a physiogeological region in the Pacific Northwest of North America, stretching from northern Washington into British Columbia, Yukon, and Alaska. It comprises rolling hills, high plateaus and deeply cut valleys. The rocks in the belt have very little similarities with the North American continent.

The formation of the Intermontane Belt is complex. It began forming during the early Jurassic period when an island arc called the Intermontane Islands collided against a pre-existing continental margin. This island arc is believed to have formed on a pre-existing tectonic plate called the Intermontane Plate about 245 million years ago by subduction of the former Insular Plate during the Triassic period. 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 North America called the Slide Mountain Ocean. This arrangement of two parallel subduction zones is unusual because 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 North America, supporting a pre-existing volcanic arc on the former continental margin of North America. As the North American Plate drifted west and the Intermontane Plate drifted east to the ancient continental margin of western North America, the Slide Mountain Ocean eventually closed by ongoing subduction under the Slide Mountain Ocean. The subduction zone eventually jammed and shut down completely about 180 million years ago, ending the volcanic arc on the ancient continental margin of western North America and the Intermontane Islands collided, forming the Intermontane Belt. This collision crushed and folded sedimentary and igneous rocks, creating a mountain range called the Kootenay Fold Belt which existed in far eastern Washington State and British Columbia.

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