The Rocky Mountains may be famous for their ski fields, dramatic scenery and Yogi bear but how the mountains formed has been a long-standing geological puzzle. Dr Maria Seton and Professor Dietmar Muller from the University of Sydney, in partnership with a team from the California Institute of Technology, present the solution today in the April edition of Nature Geoscience.

Professor Muller from the University’s School of Geosciences said most mountain belts in the world of this type owe their existence to two continents colliding with each other head on.

“This is the case for the tallest and most spectacular mountains, such as the Himalayas” he said.

“In contrast the Rocky Mountains formed while an oceanic plate was subducted, or forced under, the North American continent. About 70 million years ago, a shallow sea along the west coast of North America was gradually lifted up and rose to form a nearly 5000km long mountain belt.

“The uplifted area stretches inland to well over 1500km east of the coastline. This is extremely unusual. Normally, mountain building between an ocean basin and a continent reaches only 300 to 600km inland.”

As all conventional theories had failed to explain the Rockies, the team combined three methodologies to investigate the cause, including plate tectonic reconstructions, a high performance computer code, and seismic images of the deep Earth underneath North America.

“We found that two separate ocean plateaus successively hit the west coast of North America,” Professor Muller said.

“These collisions started a period of crustal shortening and shoved the submarine plateaus underneath western North America.

“Once the plateaus were no longer stuck underneath North America, normal mantle material was allowed to take their place. This material was hot and buoyant leading to further uplift of the Rockies.”

“The significance of our work is that we have uncovered the solution to one of the major geological enigmas of North America – how the Rockies formed. But more importantly, our methodology, which is at the forefront of geodynamics research, may help to explain similarly anomalous regions in other parts of the world,” Dr Seton said.

Media inquiries: Rachel Gleeson 02 9351 4312 or rachel.gleeson@sydney.edu.au

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