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 Earths 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.
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.
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 Earths
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.
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 Rocks Place of Birth