Transcript for Exotic Terrane, segment 07 of 12
Fossil discoveries in the Wallowa Mountains hinted that the Blue Mountains Island Arc was once an entire ocean away.
I'm standing at the icthyosaurs site, and ichthyosaurs are swimming marine reptiles that lived during the age of the dinosaurs. This particular icthyosaur fossil was a skull discovered by a high school student, very inadvertently, who stumbled upon it here on these shales. Icthyosaurs like this are swimming reptiles, but this particular one is known from only one place in the world and that's in China, and that supports the idea that these rocks might have originated very far out in the Pacific, far away from North America.
A fossil find in Hells Canyon proved that the ancient islands migrated far north from their tropical setting. Geologist Sidney Ash, a professor at Weber State University, found the remains of an ancient forest on the banks of the Snake River.
The forest grew a hundred and seventy-five million years ago, and it looked something like today's forest. It took about thirty-five million years at the rate of three inches per year for the old islands to move to their new setting. By modern comparison, the San Andreas Fault in California moves about two inches per year. Los Angeles, which is moving north on the Pacific plate, will line up with San Francisco, three hundred and eighty miles away, in about twelve million years.
Plate tectonic processes like faulting and subduction caused the Blue Mountains Island Arc to move across the ocean. By a hundred a thirty million years ago, the islands were close to the North American continent. At that time the west coast was located far inland from today's coast. Ocean waves broke on beaches in western Idaho. The Blue Mountains Island Arc was one of many island groups offshore. Plate tectonics gradually closed the gap. About a hundred and fifteen million years ago the Blue Mountains Arc docked with the continent. The collision occurred along a zone that passes close to Riggins, Idaho, near Hells Canyon.
Just east of Riggins, just three or four miles, you come to what's called the suture zone, and you can think of it as a zipper or something like that where the continent's been zippered together with these oceanic islands.
The rocks of the suture zone were squeezed and tilted during the collision. The force was so great that the mountains of central Idaho were pushed higher by thousands of feet.