Rocky mountains originated from previously unknown oceanic plate

The mountain ranges of the North American Cordillera are made up of dozens of distinct crustal blocks. A new study clarifies their mode of origin and identifies a previously unknown oceanic plate that contributed to their assembly. Geologists were able to locate the remnants of several deep-sea trenches that mark subduction sites at which oceanic plates plunge at a steep angle into the mantle and are drawn almost vertically into its depths.
Source: http://www.sciencedaily.com/releases/2013/04/130403141402.htm