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4. Siletz Terrane: last exotic arrival

Exotic Terranes
(Lighter shading indicates original extent.)

Beginning about 60 million years ago, another exotic terrane moved into the subduction zonesee Plate Tectonics diagram and smashed into North America. Like Hawaii, this exotic terrane formed where a hot spot developed beneath the oceanic crust. These rocks formed a pile too thick to slide under North America; instead, they stuck on the edge of the continent, creating the foundation of the Coast Range. Over the next few million years the subduction zonesee Plate Tectonics diagram moved westward into the Pacific Ocean, to the position the zone now occupies. The era of exotic terranes is not over. All of California west of the San Andreas fault will arrive in Oregon 20 million to 40 million years from now.

OGDC units: Eocene Siletz Terrane in northwest Oregon

Age Range: 60 million to 48 million years ago
Rock Types: basalt, pillow basalt
Did You Know? Much of Siletzia is composed of pillow basalt, lava that is squeezed out in long tube-shaped blobs underwater.


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