When people traverse North America’s Great Plains, they tread atop ancient continental blocks dating back billions of years. This old part of the continent is also very stable and is made up of three distinct cratons: the Wyoming, Superior, and Medicine Hat cratons, all of which are surrounded by slightly younger chunks of crust that latched on to them.
Typically, when something ages, it moves more slowly and begins to fade. But the mantle underneath an ancient craton rejects geriatric stereotypes and conducts seismic waves at higher velocities than neighboring youngsters. Here Hopper and Fischer examine how velocities vary underneath these extremely old hunks of crust and try to tease out the origins of the variations.
The researchers used converted seismic waves, specifically the Sp phase, to image the mantle beneath those relic cratons. A seismic event releases two main waves of seismic energy into the body of the Earth: slower-moving S waves and faster-moving P waves. Scientists can detect and use the waves to image interior regions of the Earth. When an S wave hits a boundary in velocity, some of it is converted to a P wave (called an Sp phase), which will hit a seismic detector before the S wave. Researchers can use these S wave precursors to image regions of the crust or mantle.
Wave imaging revealed four distinct types of velocity interfaces—or layers—in the upper mantle. One type is a drop in velocity that lies horizontally about 70–90 kilometers deep; another is a drop in velocity between 85 and 150 kilometers deep that dips downward. In these first two zones, volatile-rich minerals and/or relict oceanic crust from ancient subduction slow the propagation of seismic waves. The third type of layering exists near the base of the lithosphere—the rigid plate at the top of the mantle—and actually shows that seismic waves speed up, although it’s unclear why.
Finally, the researchers looked for a velocity change at the base of the plate—the boundary between the lithosphere and asthenosphere. This boundary is clearly seen via Sp waves in young, active regions like the western United States; beneath these old cratons, however, it is generally invisible because it lacks the sharp negative velocity gradients that exist beneath young plates.
Between 85 and 150 kilometers in depth, the dipping gradients provided clues that cratonic mantle formed from subduction—their drooping shape may represent the remains of old subducting slabs. Large amounts of eclogite, a type of metamorphic rock often linked to subduction, have been found in areas above these dipping structures. The proportions of different oxygen isotopes in the eclogites suggest they were altered by seawater, implying they were part of an oceanic slab that subducted underneath the craton. These facts suggest that a person standing in the Great Plains is standing not only on a billion-year-old piece of Earth’s crust but also over the remains of ancient seafloor. (Geochemistry, Geophysics, Geosystems, doi:10.1002/2015GC006030, 2015)
—Cody Sullivan, Writer Intern
Citation: Sullivan, C. (2016), Variable mantle lies below ancient pieces of Earth’s crust, Eos, 97, doi:10.1029/2016EO047777. Published on 16 March 2016.