July 2009 Geology and GSA Today Media Highlights

There is a significant difference in the population of radar-bright craters, 1 to 16 km and larger in diameter, between regions of the southeastern nearside lunar highlands north and south of about 48 degrees south latitude. This is consistent with a lunar megaregolith thickness of about 1.5 km in the north and about 2.5 km in the south, a difference Thompson et al. attribute to South Pole-Aitken basin ejecta.

Megathrust earthquakes can nucleate in the forearc mantle: Evidence from the 2004 Sumatra event
J.-X. Dessa et al., GeoAzur, UPMC, CNRS-IRD-UNS, 2 quai de la Darse, BP 48, F-06235 Villefranche-sur-Mer, France. Pages 659-662.

Dessa et al. have investigated the deep geological structures offshore Sumatra, where the giant 2004 earthquake triggered a tsunami that killed over 220,000 people. Current models predict that fault rupture associated with such events can only occur under specific physical conditions that restrict them to a depth range controlled by temperature and rock types. They demonstrate that, in this case, the rupture started and partly propagated in a zone that was not expected to promote brittle failure. This finding requires a revision of some geological models and a reassessment of the size of earthquakes and resulting tsunamis that could occur in similar geodynamic settings elsewhere in the world, as each could prove larger in some places than currently anticipated.

Porphyroblast rotation and strain localization: Debate settled!
Scott E. Johnson, Dept. of Earth Sciences, University of Maine, Orono, Maine 04469-5790, USA. Pages 663-666.

Porphyroblasts are relatively large metamorphic minerals that commonly show chemical zoning and trap preexisting structural fabrics as inclusion trails during their growth. The information contained in porphyroblasts makes them centrally important for studies that examine deformation and metamorphic histories, rates of diffusion and chemical reaction, deformation kinematics, finite strain, kinematic vorticity, pluton emplacement, and folding mechanisms, among other topics. A long-running debate has centered on the rotational behavior of porphyroblasts. The debate is important, because porphyroblasts that have undergone little or no rotation, relative to a fixed kinematic reference frame, may preserve the original orientations of structural fabrics present at the time of porphyroblast growth. The paper by Johnson provides a convincing case of rotation of plagioclase porphyroblasts relative to one another and a fixed kinematic reference frame during ductile deformation, thus settling the debate.

Were deep cratonic mantle roots hydrated in Archean oceans?
Dante Canil and Cin-Ty A. Lee, School of Earth and Ocean Sciences, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia V8W 3P6, Canada. Pages 667-670.

The mantle beneath ancient parts of Earth's continents, called "cratons," is old, hosts diamonds, and has a deep root (like an iceberg) that differs in composition from mantle underlying other parts of the crust. The study by Canil and Lee shows that the unique composition beneath the cratons may be due to chemical exchange when such mantle rocks resided on the seafloor billions of years ago. This mantle rock chemically exchanged with the old seawater and was then thrust under to become the deep mantle root to the cratons. The study shows how some chemical features in rocks deep in the mantle today owe their existence to geological processes that happened on the surface of our planet billions of years ago.