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Terranes of the North Cascades

Terranes of the North Cascades

Nason Terrane

Location of Nason Terrane rocks
Location of Nason Terrane rocks shown in green.
Summary: Mostly metamorphosed sandstone and shale derived from a distant volcanic island arc. Original sediments deposited about 210 million years ago (Triassic).
spacer image The Nason terrane underlies much of the Glacier Peak Wilderness and parts of British Columbia. It does not crop out in the North Cascades National Park Complex. The Nason terrane appears to be metamorphosed submarine fans derived from distant arc volcanoes. The rocks are now mostly mica schist, hornblende-mica schist, and amphibolite, and have been called the Chiwaukum Schist after exposures in the Chiwaukum Mountains, southeast of Stevens Pass (south of the area covered by this site). Large tracts of the Nason terrane are gneisses derived from the Chiwaukum Schist and metamorphosed granitic materials. Geologists do not know for sure the age of the original fan deposits that make up the Nason terrane, but some evidence indicates that the fans were deposited about 210 million years ago (Triassic). The fragments of Nason terrane in British Columbia appear to have gotten there by faulting along the Straight Creek Fault.
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US Geological Survey Western Earth Surface Processes Team
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This page was last updated on 11/30/99
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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