August 2009 Geology and GSA Today media ...(sponding to the shallower depths and pro...)

Date:7/31/2009

sponding to the shallower depths, and proportions of sulfur occurring as hydrogen sulfide should be relatively low. Such fluid behavior appears to be consistent with characteristics of the quartz vein-related high-grade zones. At greater depths, fluids would exist as one supercritical phase, with a higher proportion of hydrogen sulfide, which appears to be consistent with characteristics of high-grade zones related to alteration halos.

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Elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust, Nankai Trough subduction zone
Harold J. Tobin and Demian M. Saffer, Dept. of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA. Pages 679-682.

Global observations indicate that the faults that lie between tectonic plates slip far more easily than is predicted based on measurements of rock strength. The cause of this mechanical weakness may be a key to understanding earthquake rupture processes and tsunami generation. The presence of trapped, pressurized fluids is one hypothesis widely invoked to explain these "weak faults," yet it has been notoriously difficult to test. Tobin and Saffer have detected such pressurized fluids and quantified their extent, using acoustic images of a plate boundary fault system from a subduction zone under the Pacific Ocean offshore Japan. Their results show that the strength of the plate boundary fault zone remains very low and almost constant for at least 20 km as it slides down into the subduction zone. Like a puck on an air-hockey table, the upper plate glides over the lower one on a cushion of high-pressure pore water trapped in sediments thrust beneath the fault zone.

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Mid-Cretaceous seafloor spreading pulse: Fact or fiction?
M. Seton et al., EarthByte Group, School of Geosciences, Madsen Building F09, University of Sydney, NSW 2006, Australia. Pages 687-690.

Profound changes occurred on our planet
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Contact: Christa Stratton cstratton@geosociety.org 303-357-1093 Geological Society of America Source:Eurekalert

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