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North Cascades Geology

Shifting The Pieces

About 90 million years ago, tectonic plate movements had brought together the terranes of the North Cascades and thickened the Earth’s crust, profoundly metamorphosing its deeper part-as seen today in in the Metamorphic Core Domain. And the plates continued their slow rearrangement of crustal pieces. About 55 million years ago, in the Pacific Northwest, directions and rates of movement changed enough to cause local extension or stretching of the crustal rocks, and in places this extension continued until about 40 million years ago. Manifestations of the continued shifting of the pieces that now make up the North Cascade Range were northward drift of western crustal blocks relative to their eastern neighbors, sinking of some crustal blocks to form depressions where sediment accumulated, and in conjunction with the extension that caused the sinking intrusion of igneous plutons that caulked some of the cracks, that is, the faults created by the extension and northward drift.

Northward Drift

Three lines of evidence suggest that rocks in the North Cascades that are older than about 100 million years were translated northward from where they originally formed.

Nason terrane offset by movement along the Straight Creek Fault.
Nason Terrane offset by movement along the Straight Creek Fault.

Evidence from old faults

The most direct evidence of past northward movement of rocks in the North Cascades can be seen in the distribution of the Nason terrane. Two outcrop areas of the Nason Terrane one in Washington east of the Straight Creek Fault, and the other in British Columbia, west of the Straight Creek Fault are separated by at least 63 miles. Rocks of the two separate areas were born in similar places if not the same place and share similar metamorphic and deformational histories. Because of the similarities, geologists reason that rocks of the two areas once must have been adjacent. By making similar comparisons of rocks on opposite sides of many other faults in the North Cascades and Canada, geologists have gathered enough evidence to conclude that even if the northward movement of crustal blocks on the west side of major faults has not been a hard rule over the last 100 million years or so, it has at least been a significant habit.

Such northward migration is still happening today all along the west coast of North America. It is most dramatic in central and southern California, where a large block of crust on the west side of the infamous San Andreas Fault is slowly moving northward, carrying Los Angeles towards San Francisco at a rate of about 1 1/2 inches per year. In the North Cascades, movement along several faults the Straight Creek Fault in particular was at one time similar to that of along the San Andreas Fault. Unlike along the San Andreas, movement has not occurred along the Straight Creek Fault at least in Washington for some 35 million years. At that time subduction began to generate the Cascade Volcanic Arc, described in From the Fiery Furnace. Collision between the oceanic plate and the North American plate had become more head-on, decreasing the tendency of parts of the continent to split off and move north. Geologists speculate that by this time, also, another fault farther west than the Straight Creek fault, perhaps separating the Olympic Mountains from the North Cascades, had become the major fault accomodating northward drift of the west edge of the continent. Such northward drift continues today at a low rate and is a major cause of shallow earthquakes in the vicinity of Seattle.

Evidence from tectonic plate motions

Additional evidence suggesting that the North Cascades originated far south of their current location comes from known patterns of plate movement. By studying the present position of all the Earth’s tectonic plates, measuring their present rates and direction of movement, and taking into account the geologic history of certain well-studied areas, geologists and geophysicists have been able to pretty well reconstruct tectonic plate movements over the last 150 million years. Most such reconstructions show a steady northward motion of plates in the Pacific Ocean relative to the North American plate over the last 80 million years. Even when and where the oceanic plate has subducted beneath the continental plate, rather than simply sliding past it, the constant northward push on the edge of North America tends to break off pieces of continent and move them northward. Probably part of the Nason terrane drifted north along the Straight Creek fault into British Columbia during a later stage of this process.

Black Peak tonalite
Rocks of the Black Peak batholith forming the main ridge here south of Easy Pass have been thought by some geologists to have crystallized many hundreds of miles to the south before plate movement brought them to the North Cascades.

Evidence from paleomagnetism

Finally, the most dramatic evidence of northward movement of the terranes that now make up the North Cascade mosaic is the old magnetism recorded in some rocks. This paleomagnetism records, in a rough way, the distance from, and direction to the Earth ’s magnetic pole when the rock formed. If paleomagnetists (ordinary geologists call them paleomagicians) can find a rock that retains its original magnetism, and if they can determine how much it has tilted or twisted, then they can also determine at what latitude the rock was born. Several paleomagnetic studies have found that rocks now in the North Cascades and nearby in British Columbia moved north some 1800 miles between 100 million years ago and about 50 m.y. ago. This conclusion is quite controversial, but each additional study supports the idea that North Cascade rocks formed far to the south of where they are now.

Something extra: Paleomagnetism: Finding a Rock’s Place of Birth

On to Stretching the crust

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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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