The Colorado Plateau and a Small Model of Subduction Compression

June 04, 2021

TomMc_EPOD.CottonwoodCanyonRoadUtah copy (005)

TomMc_EPOD.DuctileAndBrittleFoldsPhylliteCT.WallRockB (005)

Photographer: Thomas McGuire  
Summary Author: Thomas McGuire  

The cause of the uplift and the compressional features on the Colorado Plateau during the Laramide Orogeny (mountain building roughly 60 million years ago) were not understood until it was proposed that the Farallon Tectonic Plate is subducting at a low angle beneath the North American Plate. This is what raised the whole plateau. Additionally, compressional folding, including dozens of asymmetric folds (Cockscomb, San Rafael Swell, Comb Ridge, Raplee Anticline, Waterpocket Fold, and many others) was caused by frictional drag with the remains of the subducting Farallon plate. Many of the folds are also related to sub-surface compressional faulting.

Small scale rocks often reflect larger regional features. The top image is The Cockscomb monocline along Cottonwood Canyon Road in Arizona. The second image is a rock from Connecticut. What is now western New England was an area of subduction in the closing of the Ordovician Pre-Atlantic Iapetus Ocean (about 400 million years ago), which created the Pangea land mass. This rock is a smaller-scale model of the Colorado Plateau’s asymmetric compressional folds, like The Cockscomb. My drawing on the image shows how this smaller rock represents The Cockscomb (a penny is used for scale).