June 2008 Geology and GSA Today media highlights

Oskin et al. studied six active faults in the eastern California shear zone and found that the sum of fault slip rates is at most 6.2 ± 1.9 mm per year, which is only half the present-day displacement rate of 12 ± 2 mm per year as measured from global positioning system (GPS) and triangulation surveys. Oskin et al.'s study indicates that displacement rate, and thus seismic hazard, has varied over time in the eastern California shear zone. The dates of past earthquakes on these faults, determined from trenching studies, are consistent with the slower long-term displacement rate. These previously published ages of prehistoric earthquakes also show that earthquake activity occurs in clusters, with increased activity over periods that last approximately 1,000 years. The latest cluster of activity, which includes the 1992 Landers and 1999 Hector Mine earthquakes, occurred while the rate of far-field displacement was double the average rate of fault slip. Far-field displacement rates, usually assumed to be constant, are used to determine the rate of stress increase (loading) on faults and estimate the chance of an earthquake occurring. Because a higher rate of loading has been shown to directly increase the likelihood of an earthquake, the elevated present-day displacement rate determined by this study implies that seismic hazard is currently high. Likewise, an area where the present-day displacement rate is diminished could have a decreased seismic hazard.

GSA Today Science Article

Canada's Craton: A bottom's-up view

Dante Canil, School Earth & Ocean Sciences, University of Victoria, PO Box 3055 STN CSC, Victoria, British Columbia V8W 3P6, Canada. Pages 4-11.

How did the continents first form, and how have they evolved" For answers to these fundamental earth science questions, we have to look to the ancient cratons that lie at the heart of many of today’s continents. The stable heart of the North American continent is the Canadian Shield, an ancient craton that includes the oldest continental crust on Earth. The Canadian Shield is rich with mineral deposits, including diamonds, and is underpinned by a 200-km-thick keel of cold, strong mantle. In the June GSA Today, Dante Canil of the University of Victoria-British Columbia presents an extensive compilation of geological and geophysical data that shows that this mantle keel consists of rocks that originally lay at the base of the primordial seas that covered the early Earth. Even more surprising is his assertion that this durable keel, which is largely responsible for the longevity and the stability of the continent, didn’t develop until 500 to 1000 million years after the formation of the craton. Much further research is required to determine how rocks that lay at the surface of Earth ended up being inserted beneath and adhering to an already ancient craton.

To review the abstracts for these articles, go to www.gsajournals.org. View the complete table of contents for the current issue of GEOLOGY at www.gsajournals.org/perlserv/?request=index-html&issn=0091-7613.

www.geosociety.org