EarthScope

North America, particularly its west coast, is the best studied natural laboratory for active tectonics.  Nonetheless, the downturn in Earth Science funding in the USA has threatened an ambitious project aimed at consolidating knowledge of plate interactions there.  Nature (15 November 2001, p. 241) reports that the EarthScope initiative now has strong backing from the US National Academy of Sciences.

EarthScope has 4 elements: a mobile grid of seismometers; an observatory to monitor movement of plates below the NW Pacific Ocean; a programme aimed at drilling into the San Andreas Fault System; an interferometric radar satellite that will accurately measure ground movements in relation to tectonic and volcanic features.  The total cost is around $400 million, shared equally between NASA and the National Science Foundation, if the funding proposal wins acceptance.

Information from:  http://www.earthscope.org

Continental tectonics of eastern Eurasia

Interferometric radar remote sensing provides high precision information on Earth motions associated with earthquakes (Radar analysis of Turkish earthquake, Earth Pages August 2001), but depends on “before and after” imaging.  Continental tectonics is not just the outcome of occasional large movements on major faults, but of strains that continually occur throughout the lithosphere.  Global positioning satellites provide means of precise location, particular when operated in differential mode, in which field-station signals are matched to those at fixed, geodetically precise base stations.  Precisions to within centimetres or better are now commonplace at low cost.  Structural geologists have been using GPS receivers for over a decade to check on the annual rates of plate motion across major structures such as the Alpine Fault of New Zealand and spreading centres such as that exposed on land in Iceland.  In the 19 October issue of Science, such geodetic analysis of tectonics leaped by an order of magnitude.

The jewel in the crown of continental tectonics is eastern Eurasia, where the active collision of the Indian sub-continent with Asia drives a huge array of very large faults that separate rigid blocks and others, such as the Tibetan Plateau, that are deforming en masse.  The spreading power of the Carlsberg and Central Indian Ridges is dissipated in motion of continental crust spanning 30° of latitude and 60° of longitude.  Chinese scientists and their collaborators from the US universities of Alaska and Colorado have measure GPS positions at 354 stations throughout China, every one or two years for the last decade.  Their analysis of the interim results (Wang, Q. et al.  2001.  Present-day crustal deformation in China constrained by global positioning system measurements.  Science, v.  294, p. 574-577) helps confirm or modify ideas about crustal motions that stemmed from seismic first-motion studies and regional field evidence.  More than a third of the tectonic power accounts for crustal shortening within the Tibetan Plateau.  While the western part of the huge system involves consistent motion towards the north-north-east, driving into Eurasia’s hinterland, the “free-edge” of eastern China  and Indo-China seems to encourage the escape tectonics first proposed by Molnar and Tapponier.  That involves a massive clockwise rotation around the East Himalayan Syntaxis, which takes up a great deal of motion.  Whereas Molnar and Tapponnier proposed the shoving of south-eastern China oceanwards by the “escape” of Tibet, Wang et al’s measurements reveal that its motion to the east is only between one third and a quarter that of the adjacent east Tibetan Plateau.  The lack of any sign that Tibetan crust is overriding that of south-east China, or that the latter is being shortened, may suggest that escape is funnelled around the East Himalayan Syntaxis into Burma and South-East Asia.

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