New theory may better explain origin of western North American mountains

Nature : For decades, geologists believed that western North America’s mountains were created by rock scraped off the ancient, eastward-moving Farallon tectonic plate as it slid under the North American plate. In 2008, Karin Sigloch of Ludwig Maximilians University in Munich and her colleagues found evidence that one of the large slabs underneath the mountains was not connected to the Farallon plate. A recent study led by Sigloch and Mitchell Mihalynuk of the British Columbia Geological Survey in Victoria, Canada, used seismic data to create three-dimensional images of more of the subducted slabs. They found the slabs formed several nearly vertical walls between 800 and 2000 km below the surface, and none of them were connected to the Farallon plate. They also compared the images with plate reconstructions and found a similar discrepancy. Sigloch and Mihalynuk propose that the mountains were formed from arcs of islands that piled on top of each other and then had their tops scraped off as the North American plate moved over them from the east. Only after that process finished did an eastward-moving plate begin subducting under the North American plate.