April GEOLOGY and GSA TODAY media highlights

Unzipping Long Valley: An explanation for vent migration patterns during an elliptical ring fracture eruption

Eoghan P. Holohan et al., Department of Geology, School of Natural Sciences, Trinity College, Dublin D2, Ireland. Pages 319-322.

California is home to the still-active Long Valley caldera volcano, which formed during the third largest eruption in Pleistocene North America. Stratigraphic and petrologic studies of this eruption had deciphered an intriguing pattern of vent migration during the caldera's collapse. This vent migration was thought to mirror the lateral propagation (“unzipping”) of the magma-conducting ring fractures, along which the Long Valley magma chamber roof subsided by some 2–3 kilometers. Reasons for this migration pattern, however, were ill-understood. From simple, scaled analog models, easily re-run in the classroom, Holohan et al. show that this unzipping pattern inferred at Long Valley was intrinsically related to the highly elliptical plan-view shape of the pre-collapse magma chamber roof. They also explain how the elliptical roof shape systematically influences the initial location and subsequent lateral propagation of a caldera's ring fractures. Such a systematic development of ring fracturing above an elliptical magma chamber may help explain the distribution of major vents at other elliptical calderas, and it enhances our ability to predict how such hazardous volcanoes may behave in the future.

Impact effects and regional tectonic insights: Backstripping the Chesapeake Bay impact structure

Travis Hayden et al., Western Michigan University, Department of Geosciences, 1187 Rood Hall, 1903 W. Michigan Avenue, Kalamazoo, Michigan 49008, USA. Pages 323-326.

Studies of impact structures provide a powerful window into a number of aspects of geology and tectonics. Hayden et al. use a modeling technique known as backstripping to model the long-term effects of the Chesapeake Bay Impact Structure on the tectonics and sedimentary history of coastal Virginia. This impact occurred 35 million years ago, and this study represents the first time this backstripping technique has been applied to an impact structure. The results of this study indicate that the crustal-scale effects of the impact persisted for 7 million years, longer than previously thought. In addition, these results increase our understanding of sedimentary processes along the passive margin of eastern North America, as well as increasing our understanding of the evolution of passive margins around the world.

Toasting the jelly sandwich: The effect of shear heating on lithospheric geotherms and strength

Ebbe H. Hartz, Oslo University, Physics of Geological Processes, P.O. Box 1048 Blindern, Oslo 0316, Norway, and Aker Exploration, Oslo, Norway; and Yuri Y. Podladchikov, Aker Exploration, Oslo, Norway. Pages 331-334.