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TheIntermontane plate was an ancient oceanictectonic plate that lay on the west coast ofNorth America about 195 million years ago. The Intermontane plate was surrounded by a chain ofvolcanic islands called theIntermontane Islands, which had been accumulating as a volcanic chain in thePacific Ocean since theTriassic period, beginning around 245 million years ago. The volcanism records yet another subduction zone. Beneath the far edge of the Intermontane microplate, another plate called theInsular plate was sinking. This arrangement with two parallel subduction zones is unusual. The modernPhilippine Islands are located on thePhilippine Mobile Belt, one of the few places onEarth where twin subduction zones exist today. Geologists call the ocean between the Intermontane islands and North America theSlide Mountain Ocean. The name comes from theSlide Mountain terrane, a region made of rocks from the floor of the ancient ocean.
During the earlyJurassic period, the Intermontane Islands and thePacific Northwest drew closer together as the continent moved west and the Intermontane microplate subducted. On the continent, subduction supported a new volcanic arc that again sent intrudinggranite-type rocks into the ancient continental sediments. Eventually, about 180 million years ago in the middle Jurassic, the last of the microplate subducted, and the Intermontane Islands collided with the Pacific Northwest.
The Intermontane Islands were too big to sink beneath the continent. The subduction zone of the Intermontane plate shut down, ending the volcanic arc. As theIntermontane Belt accreted to the edge of thecontinent, the subduction zone of the Insular plate became the active subduction zone along the edge of the continent.
