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Terranes of the North Cascades

Terranes of the North Cascades

Little Jack terrane

Location of Little Jack Terrane rocks
Location of Little Jack Terrane rocks shown in green.
Summary: Metamorphosed sediments deposited at the toe of a submarine fan derived from a volcanic arc. Contains blocks of metamorphosed mantle.
spacer image Another mystery terrane is the Little Jack terrane, which is mostly exposed in the Ross Lake Recreation area. Fine-grained mica schist and layers of fine-grained amphibolite make up the Little Jack terrane. Most of the rock probably began as deep-sea sediments at the toe of a submarine fan. In these metamorphosed sediments are blocks of ultramafic rock; that is, metamorphosed mantle. This peculiar assemblage suggests that as the sediments accumulated, blocks of mantle material, perhaps previously uplifted at a subduction zone, slid off into the ocean. The Little Jack terrane might be metamorphosed rocks of the Methow Domain, except that the Methow rocks contain no ultramafic blocks.
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Visit Western Ross Lake Overlook to catch a glimpse of the Little Jack terrane

On to the Methow Domain
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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