Boulder, Colo., USA – New Geosphere articles posted online 16 July 2012 include additions to three themes: "Seeing the True Shape of Earth's Surface: Applications of Airborne and Terrestrial LiDAR in the Geosciences"; "Neogene Tectonics and Climate-Tectonic Interactions in the Southern Alaskan Orogen"; and "Origin and Evolution of the Sierra Nevada and Walker Lane." Locations studied include Baja California; Cofre de Perote volcano, Mexico; Mammoth Mountain and Long Valley, California, USA; the Sevier hinterland; and the Wrangell volcanic belt.
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Applications of airborne and terrestrial laser scanning to paleoseismology
David E. Haddad et al., School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA. Posted online 16 July 2012; doi: 10.1130/GES701.1.
Earthquakes that rupture Earth's topography can cause significant loss of human life and destruction to anthropogenic structures, such as schools, hospitals, and office buildings. Fortunately, these large earthquakes leave telltale marks about their sizes and distributions over geologic time. For example, large earthquakes can displace stream channels, river terraces, fluvial ridges, and even topple fragile geologic features. The purpose of this paper by David E. Haddad and colleagues is to demonstrate how laser scanning, also known as LiDAR, can be applied in paleoseismic research to measure the magnitude and extent of past large earthquakes. The paper presents three case studies that include millimeter-scale imaging of sedimentary deposits and faults in paleoseismic trenches, measuring decadal-scale erosion rates of fault scarps, and high-resolution imaging of precariously balanced rocks. All case studies show that laser scanning is an effective analytical tool for documenting past earthquakes and has the potential for being an integral component of the paleoseismic toolbox.
Miocene basin development and volcanism along a strike-slip to flat-slab subduction transition: stratigraphy, geochemistry, and geochronology of the central Wrangell volcanic belt, Yakutat-North America collision zone
Jeffrey Michael Trop et al., Bucknell University, Dept. of Geology, 701 Moore Avenue, Lewisburg, Pennsylvania 17837, USA. Posted online 16 July 2012; doi: 10.1130/GES762.1.
The Wrangell volcanic belt extends more than 500 km across eastern Alaska, northwestern British Columbia, and southwestern Yukon Territory and makes up much of the high topography of the Wrangell-St. Elias Mountains, including ice-capped volcanoes with elevations exceeding 4600 m. Volcanism, mountain building, and sedimentary basin development resulted in the deposition of diverse volcanic and sedimentary rocks and was driven by subduction of oceanic crust and collision of crustal fragments against the North American continent. Jeff Trop of Bucknell University and colleagues have obtained extensive field and analytical data from the essentially unexplored central part of the volcanic belt to better constrain the timing and nature of tectonic processes and landscape evolution. They integrate their new datasets with previous studies from the western and eastern parts of the volcanic belt to present a tectonic model that documents diachronous volcanism and basin development along the northern Pacific margin in response to progressive insertion of the Yakutat terrane into the arcuate continental margin of southeastern Alaska.
Magnitudes and spatial patterns of erosional exhumation in the Sevier hinterland, eastern Nevada and western Utah: Insights from a Paleogene paleogeologic map
Sean P. Long, Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada, 89557, USA. Posted online 16 July 2012; doi: 10.1130/GES783.1.
Extension of the crust in the Basin and Range province has complexly overprinted the hinterland of the Sevier fold-thrust belt in Nevada and Utah, which is a region often interpreted as an ancient, high-elevation orogenic plateau. In this paper, Sean P. Long has constructed a paleogeologic map of this region that shows the distribution of rocks and structures that were at the surface between 25 and 45 million years ago, and therefore offers a simplified view of the bedrock geology prior to the majority of Basin and Range extension. In addition, using records of thickness patterns of sedimentary rocks that once overlay this region, Long presents an additional map that shows the amount of rock that was eroded off of the Sevier hinterland prior to extension. These paleogeologic and erosion maps greatly aid in our understanding of the geometry and distribution of thrust faults and folds in the Sevier hinterland, the erosion processes that operate in high-elevation orogenic plateaus, and the timing of the subsequent extensional collapse of this region.
Transtensional deformation and structural control of contiguous but independent magmatic systems: Mono-Inyo Craters, Mammoth Mountain, and Long Valley Caldera, California
P. Riley et al., ExxonMobil Production Company, 800 Bell Street, Houston, Texas 77002, USA. Posted online 16 July 2012; doi: 10.1130/GES00662.1.
This paper by P. Riley and colleagues provides two tectonic models to explain recent (younger than three million years old) magmatism around Mammoth Mountain and Long Valley, California, USA. First, we suggest that magmatism associated with Mammoth Mountain, and extending north along the Mono-Inyo volcanic chain is part of a ridge-transform-ridge system, similar to known rift margins. Second, we suggest that fault patterns, block rotations, and plate motions produced a region of enhanced extension, creating space for the formation of the Long Valley Caldera. The two tectonic models are both a consequence of transtensional deformation occurring between the Sierra Nevada microplate and the North America plate, and suggest that the area may be an incipient rift margin.
The origins of Late Quaternary debris avalanche and debris flow deposits from Cofre de Perote volcano, Mexico
Rodolfo Díaz-Castellón et al., Centro de Geociencias, Universidad Nacional Autónoma de México (UNAM), Campus Juriquilla, 76230 Querétaro, México. Posted online 16 July 2012; doi: 10.1130/GES708.1.
Cofre de Perote volcano is a compound, shield-like volcano located in the northeastern Trans-Mexican volcanic belt. Large debris avalanche and lahar deposits are associated with the evolution of Cofre. The two best preserved of these debris-avalanche and debris-flow deposits are the ~42,000-year-old Los Pescados debris flow deposit and the ~11,000- to 13,000-year-old Xico avalanche deposit, both of which display contrasting morphological and textural characteristics, source materials, origins and emplacement environments. Rodolfo Días-Castellón and colleagues used multiple analytical techniques to identify the most abundant clay, sulfate, ferric-iron, and silica minerals in the deposits, which were either related to hydrothermal alteration or chemical weathering processes. They next used cloud-free Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) remote sensing imagery, supporting EO-1 Hyperion image spectra, and field-ground truth samples to map the mineralogy and distribution of hydrothermally altered rocks on the modern summit of Cofre de Perote. The results show that the older Los Pescados debris-flow deposit contains mostly halloysite and hydrous silica minerals, which match the dominant mineralogy of soils and weathered volcanic deposit in the surrounding flanks of Cofre de Perote. Its source materials were most likely derived from initially noncohesive or clay-poor flows, which subsequently bulked with clay-rich valley soils and alluvium in a manner similar to lahars from Nevado del Ruiz in 1985, but on a larger scale. The younger Xico avalanche deposit contains abundant smectite, jarosite, kaolinite, gypsum, and mixed-layered illite/smectite, which are either definitely or most likely of hydrothermal alteration origin.
Middle Miocene to early Pliocene oblique extension in the southern Gulf of California
Fiona H. Sutherland et al., Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093, USA. Posted online 16 July 2012; doi: 10.1130/GES770.1.
Seismic reflection profiling reveals an early history of oblique rifting in the Gulf of California that highlights earlier detachment of Baja California away from mainland Mexico. Capture of the Baja California microplate was thought to be a two-step process that began about 12 million years ago but did not gather steam until six million years ago, when extension was focused in what we now call the Gulf of California or Sea of Cortez. New research by Fiona H. Sutherland and colleagues shows that extension focused behind Baja California began in earnest 12 million years ago and has been continuing ever since. This process of nascent rifting is similar to what is happening today in the Walker Lane deformation belt, a zone of trans-tension and shear, just east of the Sierra Nevada mountain range, although this process has not reached full continental-scale rupture (in Alta California) just yet.
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Geosphere