June 2011 GSA Today Science Article Includes Exclusive Lithoprobe Poster

May 27, 2011

What would we see and what would we learn if we were able to cut North America in half, pull it apart, and look at the resulting cross section through the continent, from the surface all the way down to its very deepest mantle roots? Although it sounds like an impossible undertaking, Philip Hammer of the University of British Columbia and colleagues have done just that.

Utilizing geological and geophysical data collected over more than 20 years as part of the Canadian LITHOPROBE project, Hammer and colleagues have constructed a curved cross-section (to account for the curvature of Earth’s surface) that extends from the Cascadia subduction zone on the west coast, east to the Atlantic margin, and down to depths as great as 270 km to the very base of the North American tectonic plate.

A detailed, 36-by-16-inch poster illustrating this cross section accompanies the issue and can also be downloaded from the GSA Today website, www.geosociety.org/gsatoday/.

What can be learned from this cross section? Both the mantle roots of the continent, as well as the overlying continental crust, bear the scars of ancient continental collisions and eons of oceanic subduction, two processes that remain such a recognizable part of modern plate tectonics. It appears that the processes that continue to shape the continent today have been active through more than three billion years of Earth’s history.

But there are mysteries hidden in the cross section as well. Those deformed and faulted segments of crust that record the growth of our continent are now devoid of the deep crustal roots produced during continental collision. Instead of deep mountain roots, it appears that the base of the continental crust is flat. Understanding the fate of the crustal underpinnings of ancient mountain systems, and determining the processes that flatten the base of the crust, remains but one of numerous puzzles of continental evolution that have yet to be resolved.

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