A sweeping synthesis of two decades worth of data from Canada's major earth sciences project has yielded an unprecedented geological map of North America that offers researchers a glimpse back in time.
"This is the most detailed cross-section of a continent ever put together," says Ronald Clowes, a seismologist at UBC and director of Lithoprobe. "It's a 6,000-kilometer snap shot of four billion years of crustal assembly, recycling and reorganization that took place over much of the planet's geological history."
Released as the cover story in GSA Today's June issue, the curved profile--to account for the curvature of Earth--crosses Canada from west to east, starting at the Cascadia subduction zone and traversing the mountain belts, Canadian Shield, Appalachians, to the Grand Banks continental shelf and the Atlantic passive margin.
The section extends down to depths as great as 270 kilometers to the very base of the North American tectonic plate. The profile reveals how the structure of the rock below our feet still preserves a record of how the continent developed.
"The continental crust and the underlying mantle roots of the continent bear the scars of ancient continental collisions and oceanic subduction," says UBC researcher Phil Hammer, lead author of the paper. "The work of the Lithoprobe research community has shown that the processes that continue to shape the continent today have been active through more than three billion years of Earth’s history."
The cross section highlights some of the puzzles that emerged from the data over the past six years. Thick continental crust as is observed beneath the Himalayas was not imaged beneath either the modern or ancient mountain belts in Canada. Small crustal ‘roots’ are noted in several places, but "We found a surprisingly uniformity of depth in the crust at between 33 and 43 kilometers across the continent, with very few exceptions,” notes Hammer.
"That is one of the really surprising features," adds Clowes. "Crustal roots clearly do form in some places, but these results show that they can be removed over time and in many situations, aren't formed at all."
Understanding the fate of the crustal roots below ancient mountain systems, and learning how the boundary between the crust and mantle changes over time, remains but one of numerous puzzles of continental evolution that have yet to be resolved.
The data from which the cross-section was assembled has already proven invaluable for learning about earthquake hazards, for industry exploration for minerals, oil and other resources, and for demonstrating the value of technologies not previously used in industry.
The Lithoprobe project was funded primarily by the Natural Sciences and Engineering Research Council of Canada and the Geological Survey of Canada.
GSA Today
www.geosociety.org/gsatoday/archive/21/6/article/i1052-5173-21-6-4.htm
