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USGS Geology in the Parks

North Cascades Geology

Annealing the Parts: Metamorphism

The foundation of the North Cascades is a complex mosaic of terranes whose early history includes deposition of sediments and volcanic rocks in oceans and volcanic arcs. Plate collisions brought the pieces together (accreted them), probably somewhere along the west side of North America by about 90 million years ago (Cretaceous). Only at this point can these assembled rocks properly be regarded as the block of crust today known as the North Cascades. After about 90 million years ago, the speed of movement lessened significantly, although even today pieces of the mosaic are still moving.

Uplifted metamorphic core

Deeply buried metamorphic rocks uplifted and exposed in the core of the North Cascades (To enlarge image click here).

As demonstrated by the rocks of the Western Domain, the accretion process stacks up a considerable thickness of rock by thrusting one terrane over another. A consequence of such overthrusting (along reverse faults), as it is called, is burial of rocks on the bottom of the thrust stack. If such rocks remain buried long enough, they heat up and become metamorphosed. The rocks of the Metamorphic Core Domain, the most rugged and mountainous part of the North Cascades, were created or modified during such a metamorphism at the bottom of a thrust stack, most of which began about 90 million years ago.

Much of the rock in the Metamorphic Core Domain began as magma intruded just before or during the metamorphism. These invading plutons may have supplied much of the heat for the recrystallization that occurred during the metamorphic episode. The remaining rock is schist of the Chelan Mountains and Nason Terranes. Almost all these rocks exhibit metamorphic foliation and/or lineation, and the alignment of minerals tells us that the rocks, both the original material of the terranes and the invading plutons were being squeezed and stretched deformed as they were recrystallized.

Click here for more information on foliation and lineation in metamorphic rocks.

View of the Eldorado Orthogneiss.
View of the Eldorado Orthogneiss, a 90 million-year-old stitching pluton in the Metamorphic Core Domain. The view of Forbidden Peak is from a joint on Mount Johannesburg.

Stitching Plutons

Because the plutons invaded most of the terranes that make up the North Cascades mosaic and extend from one to another like a staple, geologists call them stitching plutons. Most of the plutons do not really "stitch" the terranes together, but by invading rocks of many of the terranes all at about the same time, they show that the mosaic was by then assembled. The terranes may not yet have been attached to the North American continent in northwest Washington. For a discussion of that part of the puzzle, see Shifting the Pieces. Although stitching plutons are not found in the Western Domain at the present level of erosion (who knows what lies below?), some rocks there were also metamorphosed about the same time as those in the Metamorphic Core Domain, but to a much lesser degree.

Radiometric dating indicates that many stitching plutons are between about 65 and 90 million years old (Cretaceous)(For a discussion of dating a stitching pluton, click here). The origin of all this magma is not completely clear although many geologists think that it represents the roots of an old volcanic arc. Few of the ancient arc volcanoes or their immediate products have survived erosion, except perhaps in the Methow Domain, so we cannot be sure that all the plutons represent a single volcanic arc.

The Skagit Gneiss Complex

The metamorphic Skagit Gneiss Complex is derived from the metamorphosed strata of the Chelan Mountains terrane by intrusion of many igneous sills and dikes, which were then metamorphosed. Metamorphism occurred about 90 million years ago and continued or renewed about 45 million years ago (Cretaceous and Eocene).
Banded gneiss.
Banded gneiss.

In the Metamorphic Core Domain, including much of the wilderness of North Cascades National Park, two kinds of gneiss, banded gneiss and orthogneiss, dominate the scene. Banded gneiss is made up of layers of granitic orthogneiss alternating with schist. The schist layers in the banded gneiss appear to be remnants of the Chelan Mountains Terrane's Cascade River Schist and the Napeequa Schist. The gneiss layers are mostly deformed igneous sills.

Large tracts of land began as igneous plutonic rock, now commonly with metamorphic foliation and/or lineation; Such plutonic rocks are called orthogneiss (literally meaning "straight gneiss"), by which geologists mean gneiss derived from igneous plutonic rocks rather than from the thorough metamorphism of sedimentary or volcanic rocks. Geologists have lumped the banded gneisses and many of the more extensive orthogneiss bodies together under the name Skagit Gneiss Complex. Most of the Skagit Gneiss Complex consists of what geologists call migmatite.

Visit field trip stops Ruby Mountain and Diablo Dam to learn more.

Cross section of small pluton beneath a volcano.
Cross section of small pluton beneath a volcano. Rocks around the pluton are baked.

More Metamorphism

As if this invasion by stitching plutons and metamorphism were not confusing enough, beginning about 10 million years later, when the Cascade Volcanic Arc awakened, arc-root plutons invaded the rocks of the North Cascades. Near these hot magmas, intense heat caused the older schists and gneisses to recrystallize once again, but this time, the rocks were not squeezed or stretched. The resulting thermal, or contact, metamorphism tended to obscure the foliation, lineation, and other effects of earlier metamorphic episodes. The rocks baked in the zone around the plutons are harder and more resistant to erosion than their unbaked (or even half-baked) neighbors. For more on contact metamorphism click here.
Something extra: Orthogneisses and Migmatites
On to Shifting the pieces northward

Material in this site has been adapted from a 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.

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