Rocks of the Western Domain: volcanic and sedimentary rocks of the Chilliwack River terrane in forground and greenschist of the Easton terrane on Mount Shuksan behind. Domains of the North Cascades Rocks of the Western Domain   To understand the rocks in the Western Domain, we all have to think big. It’s not hard to imagine a bedded sequence of sedimentary rocks where each bed on top of the next is younger. Even on the huge scale of the Grand Canyon of the Colorado River our imagination is not too taxed to envision the layers of sediment accumulating over millions of years.

Deformation and emplacement of terranes along thrust faults (TF) during collision of a tectonic plate with the North American plate as seen in diagrammatic cross section. The Western Domain in the North Cascades also has a large-scale layering, but each layer is a terrane and the layers were stacked on top of each other by faulting in a relatively short time, geologically speaking, probably on the order of a few hundred thousand to a few million years. Furthermore, the youngest layer is not on top, but at the bottom. The terranes of the Western Domain have been thrust over each other, probably at the western edge of the North American continent as it collided with another tectonic plate. Sketch of up-arched stack of overthrust terranes with erosion exposing lowest terrane in core of arch. Dashed line shows location of folded thrust plate before erosion. These stacked-up thrust plates have then been folded in a broad arch with the axis trending more or less southeastward under Mount Baker. Erosion has eaten away much of the rock bent into this arch, exposing the lowest rocks in the core, although these are today somewhat obscured by the young volcanic splotch of Mount Baker. The Western Domain is a stack of odd bedfellows, disparate terranes that formed in different settings and whose previous histories were quite different before they were all put together in what is now the western North Cascades. But the story is a little more complex than this because the geologic forces that stacked these terranes as thrust plates, gave the whole pile at least one more whack, so to speak, and sliced off a chunk of the whole pile and added it to the top. This final slice, the Gold Run Pass thrust plate, can be seen at Gold Run Pass, naturally, where rocks of the Chilliwack River terrane are on top of the Easton terrane, not under it, as is the usual arrangement. We all should be spared such a geologic complication, but perhaps that is what is to be expected in a subduction zone where an ocean floor plate has smashed against a continental plate. Something extra: Making Sense of Metamorphic Rocks and Terranes in General

Visit each of the Western Domain Terranes or Continue to the Metamorphic Core Domain

| North Cascades geology home | North Cascades National Park home | | Geologic & Field Trip Maps | Geology field trip | Site contents |

| USGS Geology in the Parks home | NPS Park Geology Tour home | This site is a cooperative endeavor of the US Geological Survey Western Earth Surface Processes Team and the National Park Service. Please share your comments and suggestions with us! parkgeology@den.nps.gov http://www.nature.nps.gov/grd/usgsnps/noca/twestdom.html This page was last updated on 12/1/99

Material in this site has been adapted from a new book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle