|A popular and extensively studied exotic
terrane identified in the Pacific Northwest is named Wrangellia. As you
might expect, this in depth studying has resulted in a great deal of data,
and some disagreements.
Stretching from the Wrangell Mountains east of Anchorage, Alaska, through Vancouver Island, to Hells Canyon, Idaho, Wrangellia is one of the most extensively displaced exotic terranes in North America. Studies have yielded distinctive fossils in this terrane. Particularly interesting is the fossilized presence of a species of clam, Daonella, also found in rocks native to Asia but nowhere else in North America!
Wrangellia is made of volcanic rocks, capped with shallow marine shales with interbedded limestones. The thick volcanic basalt found over these layers indicates possible formation in a rift zone, or an oceanic plateau. Subsequent volcanic layers built the terrane until the late Triassic. Initial sinking allowed the carbonate depositions of shallow water to accumulate; further sinking allowed deep-water sediments to collect. There is no indication of continental material in the depositional layers from this time, but some are found in layers from the Cretaceous. This indicates these layers were accumulated after the terrane began to collide with North America.
Some geologists believe paleomagnetic and paleobiogeographic information indicates that Wrangellia, which is part of the Insular superterrane, migrated from subtropical latitudes to northern temperate latitudes in the eastern Pacific between Late Triassic and Late Jurassic time (Dickinson, W. R., 1992). Others state it is not clear from the paleomagnetic data whether Wrangellia was derived from north or south of the equator (Moores, Eldridge and Robert J. Twiss, 1995). Jones, Cox, Coney and Beck (1982) have Wrangellia forming simply near the equator. Any theory would support that this terrane may have drifted for 4,000 kilometers or more before colliding with the continent.
Another area of uncertainty involves the
site of the initial collision. One opinion bases its view on paleomagnetic
data and proposes 25 degrees N latitude as the impact site (near present
day Baja, California). After collision, the fractured terrane spread out
and migrated north along a continental fault line. Rocks taken from two
parts of Wrangellia which are now 2,500 kilometers apart, support the
theory of post-docking strike-slip faulting. The opposing view points
out that no one has found the mega-fault that would be responsible for
the displacement. David G. Howell (1985) contends Wrangellia crashed into
Oregon 70 million years ago - the subsequent faulting spreading fragments
of the terrane northward through Vancouver and Queen Charlotte islands
and on to the Wrangell Mountains of southern Alaska. The debate keeps