Weekend 2: North Cascades/Methow Valley

7/31 Stop 1: Dalles Bridge

This stop began with a lecture on the plate tectonics of the North Cascade region.  Going from east to west (oldest to youngest), it began with the Okanogan Terrane (which is a superterrane).  The Okanogan Terrane and the Methow Terrane is divided by the Pasayten Fault.  The western boundary of the Methow has two faults with the Hozameen to the north and Ross Lake to the South.  The next terrane to the west is Crystal Lake.  The Straight Creek Fault is between this set of superterranes and the Northwestern Cascade terranes.  The Dalles Bridge is along the Easton Terrane which is part of the Northwestern Cascades.  The Easton Terrane has Darrington Phyllite and Shuksan Greenschist/Blueschist.  The primary component of Darrington Phyllite is metasediments that were deposited on the seafloor, while the Shuksan Schist are metabasalt that was previously part of an ancient seafloor.

As subducting layers shoved sediment down the trench, the force caused some of the older sediment to regurgitate back out.  With the Easton Terrane, basalt and sediments were deposited into the trench 160-130 million years ago.  As the pressure and temperature of the sediments increased, metamorphism occurred.    Approximately 105 million years ago, the basalt was transformed into greenschist at 375 ^oC. Basalt has a random mineral pattern.  When metamorphism occurs, the temperature and pressure cause the minerals grow and line up by densities.  Some of the minerals changed into a different form, while others stayed the same.

After the initial metamorphism, the phyllite folded as Wrangella collided with North America 90 million years ago.  It warped the phyllite's foliation.  This formation was altered again when quartz veins were deposited 30-40 million years ago.

Shawn and James looking at the Darrington Phyllite exfoliation.

Scott explaining phyllite foliation to James with Doug looking at the formation in the back.

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