The Canadian Rocky Mountains were formed when the North American continent was dragged westward during the closure of an ocean basin off the west coast and collided with a microcontinent over 100 million years ago, according to a new study by University of Alberta scientists.

The research, based on high resolution data of Earth's subsurface at the Alberta-British Columbia (BC) border, favours an interpretation different from the traditional theory of how the Canadian Rocky Mountains formed. The traditional theory, known as the accretion model, suggests that a gradual accumulation of additional matter eventually formed the Canadian Rockies-unlike the sudden collision event proposed by this research.

"This research provides new evidence that the Canadian section of this mountain range was formed by two continents colliding," said Jeffrey Gu, professor in the Department of Physics and co-author on the study. "The proposed mechanism for mountain building may not apply to other parts of the Rocky Mountains due to highly variable boundary geometries and characteristics from north to south."

The study involved seismic data collected from a dense network of seismic stations in western Alberta and eastern BC, combined with geodynamic calculations and geological observations. The results suggest that an ocean basin off North America's west coast descended beneath the ribbon-shaped microcontinent, dragging North America westward, where it collided with the microcontinent.

"This study highlights how deep Earth images from geophysical methods can help us to understand the evolution of mountains, one of the most magnificent processes of plate tectonics observed at the Earth's surface," said Yunfeng Chen, who conducted this research during his PhD studies under the supervision of Gu. Chen received the Faculty of Science Doctoral Dissertation Award in 2018.

"There are other mountain belts around the world where a similar model may apply," said Claire Currie, associate professor of physics and co-author on the study. "Our data could be important for understanding mountain belts elsewhere, as well as building our understanding of the evolution of western North America."

Alberta and British Columbia communities supported these research efforts by hosting seismic stations on their land. This research is also supported by the Alberta Energy Regulator.

The paper, "Seismic evidence for a mantle suture and implications for the origin of the Canadian Cordillera," is published in Nature Communications (doi: 10.1038/s41467-019-09804-8).