February Geosphere media highlights

Boulder, CO, USA – The February issue of Geosphere, published by the Geological Society of America, is now available online. Geology topics of interest include: revised dating of Grand Canyon lava flows; Missouri's Reelfoot Rift and its impact on the central Mississippi River Valley; LIDAR mapping of active surface faults in Houston, TX; a paleo-continental divide in northeastern Nevada; and a cybergeology approach to geologic mapping. Two articles address north-central Nevada's Caetano caldera.

Large-magnitude Miocene extension of the Eocene Caetano caldera, Shoshone and Toiyabe Ranges, Nevada
Joseph P. Colgan et al., U.S. Geological Survey, Menlo Park, California 94025, USA.
Keywords: Basin and Range Province, Miocene, extension tectonics, calderas.

The late Eocene Caetano caldera formed about 34 million years ago during eruption of the Caetano Tuff (described in a companion paper by John and others in this issue of Geosphere). Remnants of the caldera are presently exposed in a series of north-trending, east-tilted (about 40 degrees), fault-bounded blocks that crop out across 40 km (east-west) of the Shoshone and Toiyabe Ranges in north-central Nevada. Restored geologic cross-sections indicate that the caldera was originally about 12-18 km (north-south) by 20 km (east-west) and has therefore been stretched to about twice its original width. The authors interpret Miocene sedimentary rocks exposed between these fault blocks to represent material shed from rising mountain ranges into adjacent basins while the faults were moving, indicating that deformation began about 16 million years ago and continued until 10-12 million years ago. These older basins and ranges were broken up by younger, locally active faults that formed the modern basins and ranges seen in north-central Nevada today. It is likely that Miocene faulting and tilting was not confined to the former caldera, but also affected surrounding Paleozoic rocks-and, potentially, large, nearby Carlin-type gold deposits.

History of Quaternary volcanism and lava dams in western Grand Canyon based on lidar analysis, 40Ar/39Ar dating, and field studies: Implications for flow stratigraphy, timing of volcanic events, and lava dams
Ryan Crow et al., Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA.
Keywords: Grand Canyon region, Uinkaret, basalt flows, lava dams, volcanic history.