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05/11/2005
End of Mesozoic Tectonics
– A reorganization of plates.
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 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).
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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