Constructing a Craton

Geophysics![Figure][1] CREDIT: USGS The cores of continents, or cratons, make up the stable lithosphere. Their formation probably occurred billions of years ago, though the mechanisms responsible remain unclear. One way to seek further insight is to look below the present-day continents for clues, such as where the lithosphere ends and the asthenosphere begins. Miller and Eaton used receiver function seismic analysis to study a transect across the width of Canada, revealing the lithosphere-asthenosphere boundary in addition to tilted discontinuities in the mid-lithosphere. These discontinuities may be the scars of an ancient collision of several smaller microcontinents that ultimately fused together to form the North American craton. In a related study, Yuan and Romanowicz modeled the anisotropy of seismic waves across nearly the entire North American continent and observed not only a continuous lithosphere-asthenosphere boundary, but another shallower, laterally variable boundary within the middle of the lithosphere. Based on its thickness and a correlation with geochemical evidence, this new boundary may separate the older part of the craton from a relatively younger layer. Although the two studies present a contrasting view of the evolution of the lithosphere beneath North America, they suggest that accretion processes, and not hot mantle plumes, were primarily responsible for its formation. Geophys. Res. Lett. 10.1029/2010GL044366 (2010); Nature 466 , 1063 (2010). [1]: pending:yes