AGU Journal Highlights -- Aug. 14, 2007

On rocky coastlines, abrasion, dissolution and biological activity can create an erosion notch where the ocean meets land. Given that sea level has generally risen since the end of the last glacial period 20,000 years ago, Cooper et al. hypothesize that notches stay at the coastline if sea level rise is matched by tectonic uplift. However, if sea level rise is outpaced by tectonic uplift, notches will be stranded above the coastline. Noting that stranded notches can reveal uplift rates in seismic areas, the authors study four known paleoshorelines along a stretch of coastline in Greece. Prior work indicates that marine fauna found in these four notches are 650, 1900, 3700 and 6500 years old, respectively, correlating to known periods of sea level stability. The new study’s authors use the notches' elevations to calculate uplift rates. These values correspond well with known uplift rates during the last ice age, suggesting that notch sequences could be used in some locations to characterize long-term patterns of uplift, slip-rate, and seismic hazards on active faults.

Title: A comparison of 10³ – 10⁵ year uplift rates on the South Alkyonides Fault, central Greece: Holocene climate stability and the formation of coastal notches

Authors: Frances J. Cooper: University of Southern California, Los Angeles, California, U.S.A.;

Gerald P. Roberts and Charlie J. Underwood: Research School of Earth Sciences, Birkbeck College, University of London, London, U.K.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL030673, 2007

12. Imaging the magma beneath New Zealand's Taupo Volcanic Zone

The Taupo Volcanic Zone is a region of intense volcanism associated with the subduction of the Pacific Plate beneath the continental crust of New Zealand's North Island. Heat output through hydrothermal venting in this region is high, but studies suggest that a much greater amount of magma is cooled within the crust. Because little is known about the properties and extent of this magma, Heise et al. map subsurface structures within the Taupo zone. Through magnetotelluric imaging methods, which plot spatial variations in electric currents induced by natural fluctuations in Earth's magnetic field, the authors generate a map of the zone’s conductivity structure. They find a steep increase in conductivity at a depth of 10 kilometers (6 miles), corresponding to a region about three kilometers (1.9 miles) beneath the base of the seismogenic zone and eight kilometers (5 miles) above the base of the continental crust. The authors hypothesize that this increased conductivity marks the presence of molten material within the lower crust.

Title: Melt distribution beneath a young continental rift: The Taupo Volcanic Zone, New Zealand

Authors: Wiebke Heise, Hugh M. Bibby, T. Grand Caldwell, and Stephen C. Bannister: GNS Science, Lower Hutt, New Zealand;

Yasuo Ogawa: Volcanic Fluid Research Center, Tokyo Institute of Technology, Tokyo, Japan;

Shinichi Takakura and Toshihiro Uchida: Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL029629, 2007

13. Modeling large-scale inundation on the Amazon River's seasonally flooded wetlands