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The period of time that follows the Wrangellia collision is a time of NEW tectonic relationships. There is no evidence of volcanism, plutonism, subduction, convergence, thrusting or any related activity during the period 100 million until about 54 million years ago. Thus, the new Northwestern coastline was not a convergent subducting boundary during this period. Way off to the west in the NEW PACIFIC OCEAN BASIN, a large ocean plate rifted into a THREE-PLATE CONFIGURATION. At 80 million years ago, the Pacific Basin was made up of -- the KULA PLATE, THE FARALLON PLATE, AND THE PACIFIC PLATE. These three plates were separated respectively by the Kula-Farallon ridge (the divergent zone between the Kula and Farallon plates); the Kula-Pacific ridge (the divergent zone between the Kula and Pacific plates); and, the Pacific-Farallon ridge (the divergent zone between the Pacific and Farallon plates). This three-plate configuration was recorded in the marine magnetic anomalies (“magnetic stripes”) that develop at mid-oceanic ridges. As we've already discussed, the Pacific Ocean basin 80 million years ago was the site of a three-plate configuration -- The Kula, Farallon, and Pacific plates. As the North American drifted westward (from the divergent motion on the Mid-Atlantic Ridge), it drew closer to the Kula-Farallon ridge and the Kula Plate. At about 70 million years ago the Kula plate was essentially next to North America and heading northward. This motion created a transform-fault motion that "tugged" at the edge of the continent. The tugging pulled the accreted terranes northward along weak zones in the local geologic structure -- thus creating a number of transform faults. These included the Straight Creek Fault (between the Skagit Crystalline Core and the Northwest Cascades Thrust System. |
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