A major suture exists in North America between largely Archean rocks (older than 2.5 billion years) and younger Paleoproterozoic rocks (between 1.6 and 2.5 billion years old), running approximately from northern Utah to the Great Lakes region. The suturing, generally interpreted as reflecting the accretion of one or more island arcs to the Laurentian continent, took place approximately 1.80 to 1.74 billion years ago. The suture region remained stable for about 100 million years as the continent grew and plate tectonic activity shifted southward. Work by Daniel S. Jones and colleagues shows that around 1.65 billion years ago a segment of this suture was reactivated in the Sierra Madre of southern Wyoming in response to compressional events far to the south. This reactivation caused the crust near the suture to thicken and eventually melt, producing 1.63-billion-year-old granite. Building on other studies, Jones and colleagues suggest that the entire transcontinental Archean-Paleoproterozoic suture remained a line of mechanical weakness long after it became interior to the Laurentian plate and that it was reactivated along its length during the Mazatzal orogeny approximately 1.65 billion years ago.
Predicting grain size in gravel-bedded rivers using digital elevation models: Application to three Maine watersheds
Noah P. Snyder et al., Earth and Environmental Sciences Dept., Boston College, 140 Commonwealth Ave., Chestnut Hill, MA 02467, USA. Posted online 25 October 2012; http://dx.doi.org/10.1130/B30694.1
River bed grain size -- from tiny particles of sand to gravel and boulders -- plays a crucial role in establishing habitat for
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Geological Society of America