August 2009 Geology and GSA Today media ...(during the mid-Cretaceous (80-120 millio...)

Date:7/31/2009

during the mid-Cretaceous (80-120 million years ago): sea level was higher, continents were inundated, the climate was warmer, ocean basins were oxygen poor, voluminous volcanism occurred, and Earth's magnetic field underwent a period of relative stability. In the past, the cause of these substantial changes was believed to be due to a rapid pulse of seafloor spreading. However, it has also been suggested that new mid-oceanic ridges, created after the breakup of the supercontinent Pangea, were responsible. To resolve this debate, Seton et al. have reconstructed the world's ocean basins and obtained estimates of seafloor spreading rates and mid-oceanic ridge lengths from the Cretaceous to the present day. They have found that these planetary-wide changes were not related to a sudden change in global mid-oceanic ridge length, but rather were related to higher seafloor spreading rates and the changing age-area distribution of oceanic lithosphere.

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Salt tectonics and collapse of Hebes Chasma, Valles Marineris, Mars
J.B. Adams et al., Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. Pages 691-694.

Planetary scientists have debated the origin of the great canyons of Valles Marineris on Mars ever since their discovery. Hypotheses have included erosion by giant floods, tectonic rifting, and collapse. Intense interest has been focused on the huge flood channels that drained some of the canyons, and on the materials that are exposed to depths of over 8 km. At stake in the debate is whether large volumes of water shaped the early history of Mars, and how much of that water might still be trapped below the surface. This study by Adams et al. presents evidence that Hebes Chasma, a 310-km-long canyon that has no surface outlet, was formed by collapse and draining of brines and entrained solids into sink holes. The study was based on scaled laboratory models of Hebes and on satellite images and
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Contact: Christa Stratton cstratton@geosociety.org 303-357-1093 Geological Society of America Source:Eurekalert

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