Boulder, CO, USA - The February 2011 Geosphere includes two articles designated for the latest Geosphere theme, "New developments in Grenville geology: In honor of James McLelland." Other topics include 3-D characterization of rocks, ash, minerals, lava, and so forth through various technical means: X-ray computed tomography, Stereo Scanning Electron Microscopy, X-ray micro-fluorescence, X-ray diffraction, and terrestrial LiDAR. Also studied: the La Silla and Todos Santos Formations, Mexico, Death Valley, the south-central Andes, and the Nellis Dunes Recreation Area.
Keywords: Grenville Province, James McLelland, Adirondack Mountains, X-ray tomography, Stereo Scanning Electron Microscopy, Plinian eruptions, vesicles, X-ray, LiDAR, Nazas arc, Todos Santos Formation, La Silla Formation, Mexico, Death Valley, Andes, Nellis Dunes Recreation Area
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Geosphere announces a new themed issue: "New developments in Grenville geology: In honor of James McLelland," edited by Graham B. Baird, Bruce W. Selleck, and Catherine H. Shrady, honors the work of James McLelland in the advancement in understanding of Grenville geology. Two papers in the February issue of Geosphere (Chiarenzelli et al., 2011, and Lupulescu et al., 2011) are part of this special issue, as are two previously published papers that appeared in the December 2010 issue (Seifert et al., 2010, and Chiarenzelli et al., 2010).
Introduction to the Theme: The Grenville Province exposes the interior of an ancient mountain belt whose scale rivals the modern Himalayan-Alpine Orogen. The geology of this vast tectonic collage holds a record of mid-Proterozoic continental margin dynamics, including the assembly of the supercontinent of Rodinia. This themed issue of Geosphere addresses the recent developments in Grenville geology and originated with the 2008 GSA Annual Meeting in Houston, Texas, sessions: Recent Advances in the Understanding of Adirondack and Southern Grenville Province Tectonics I and 2: In Honor of James McLelland. For over 40 years, James McLelland has been a major contributor to the advancement in understanding of Grenville geology, with particular emphasis on the Adirondacks. This is an exciting time to be working in tectonics and the Grenville province. No longer do the Adirondacks (or the Grenville Province as a whole) seem like an undecipherable puzzle of multiple deformed and metamorphosed rocks. Despite the complexity of the geology of the Grenville, Jim McLelland and many collaborators and colleagues have significantly advanced our understanding of this important global-scale orogen. This themed issue recognizes Jim McLelland's creative fervor, unrelenting capacity for hard work, and dedication to the science of geology.
Using pegmatite geochronology to constrain temporal events in the Adirondack Mountains
Marian V. Lupulescu et al., Research and Collections, New York State Museum, Albany, New York 12230, USA; doi: 10.1130/GS596.1.
Marian V. Lupulescu of the New York State Museum and colleagues discuss results from initial attempts to date zircon and monazite crystals from pegmatites from the Adirondack Mountains, New York, by Laser Ablation-Multiple Collector-Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) and by electron microprobe. These new data are used to constrain the timing of the Adirondack igneous, metamorphic, and deformational events and to test the current tectonic models.
3D characterization of sandstone by means of X-ray computed tomography
V. Cnudde et al., Dept. of Geology and Soil Science-UGCT, Ghent University, Krijgslaan 281–S8, 9000 Gent, Belgium; doi: 10.1130/GS563.1.
By using high-resolution X-ray CT systems, it is possible to look inside sandstone without destroying it. This paper by V. Cnuddle of Ghent University and colleagues focuses on the possibilities of analyzing structures in detail by means of X-ray CT in combination with 3-D analysis software.
A new 3D method of measuring bubble size distributions from vesicle fragments preserved on surfaces of volcanic ash particles
Alexander A. Proussevitch, Complex Systems Research Center, University of New Hampshire, Durham, New Hampshire 03824, USA; doi: 10.1130/GS559.1.
Explosive volcanic eruptions pose a serious threat to human security and infrastructure. Plinian eruptions, the most energetic (and thus devastating) class of volcanic activity, lead to fragmentation of bubbly magma into fine ash particles. Because it is the growth of bubbles within the magma that drives the eruption, it is important to be able to document the growth history and final size distribution of bubbles to better understand eruption dynamics. The morphology of ash fragments retains a record of the eruption process at the moment the magma explodes out of the volcanic vent, a time not safely observable in situ during an eruption. The curvature of concave surfaces of simple ash fragments, and concave imprints on the surface of larger compound ash particles can be used to reconstruct the size of bubbles that burst during eruption, so Alexander A. Proussevitch of the University of New Hampshire and colleagues developed a novel method to measure the sizes and distributions of these bubbles. Stereo Scanning Electron Microscopy (SSEM) was used capture images of the ash particle surfaces from two angles so that their 3-D topography could be determined digitally. By extrapolating the curvature in orthogonal cross sections of each bubble, the complete surface of each bubble can be reconstructed and bubble volume calculated using specialized software newly developed for this purpose. The bubble size distribution (BSD) thus compiled reflects the state of bubble growth that led to explosive eruption and fragmentation of the magma and can be used in future research to better characterize the processes involved in specific historic eruptions.
A study on the reproducibility of counting vesicles in volcanic rocks
Don R. Baker et al., Dept. of Earth and Planetary Sciences, McGill University, 3450 rue University, Montreal, Quebec H3A 2A7, Canada; doi: 10.1130/GS553.1.
Reproducible results are the hallmark of science, but investigations of such reproducibility are rare in geosciences, especially in studies of rock textures. Don R. Baker of McGill University and colleagues undertook an investigation of the reproducibility of counting the three-dimensional vesicle size distributions in a volcanic rock and in a synthetic glass foam that were imaged by X-ray tomography; three researchers independently counted the same samples using two different software programs without discussing the techniques applied. The results are encouragingly similar, although some differences in the results were obtained. Nevertheless, the results all produced the same distribution law for the vesicle sizes. Thus this study supports the reproducibility of counting vesicle size distributions by different researchers and allows us to compare results obtained by different scientists.
Three-dimensional phase separation and identification in granite
Matthieu Boone et al., UGCT, Department of Physics and Astronomy, Ghent University, Proeftuinstraat 86, 9000 Gent, Belgium; doi: 10.1130/GS562.1.
This paper by Matthieu Boone of Ghent University and colleagues describes a combination of three X-ray based techniques (X-ray computed microtomography, X-ray micro-fluorescence, and X-ray diffraction) to allow a three-dimensional separation and identification of the different minerals in a granite. Despite the complexity of the sample, a good separation and identification can be performed, proving the power of the combination of these techniques. Since two of these techniques are non-destructive, they can also be used for characterization of valuable objects.
Reconstructing flood basalt lava flows in three dimensions using terrestrial laser scanning
Catherine E. Nelson et al., Dept. of Earth Sciences, Durham University, South Road, Durham, DH1 3LE, UK; doi: 10.1130/GS582.1.
Catherine E. Nelson of Durham University and colleagues present a method of reconstructing the 3-D size and shape of ancient basaltic lava flows using terrestrial LiDAR equipment. The equipment allows us to capture a virtual cliff section and interpret the position of the lava flows in the outcrop. The data we obtained are used to build 3-D geological models of lava flows, which give us information on flow sizes, shapes and how they stack up. These models can then be used to improve seismic imaging in areas covered by flood basalt lava flows, as well as improving our understanding of flow geometries.
Jurassic volcanic and sedimentary rocks of the La Silla and Todos Santos Formations, Chiapas: Record of Nazas arc magmatism and rift-basin formation prior to opening of the Gulf of Mexico
Antonio Godinez-Urban et al., Posgrado en Ciencias de la Tierra, Universidad Nacional Autonoma de Mexico, Campus Juriquilla, Queretaro, 76100, Mexico; doi: 10.1130/GS599.1.
The study by Antonio Godinez-Urban of UNAM and colleagues of volcanic and sedimentary rocks of Jurassic age in central Chiapas (southern Mexico) suggests a close relationship with a continental volcanic arc that existed prior to opening of the Gulf of Mexico. The record of this volcanic arc in northern Mexico (in the states of Durango, Zacatecas, San Luis Potosi, and Nuevo Leon) is extensive; it has been called the Nazas arc. The age, geochemistry, and lithology of the rocks in Chiapas indicate that volcanic rocks were replaced by rifts deposits. These rift deposits have been called the Todos Santos Formation, and they represent the initial stages of the separation of Yucatan from Texas and northern Mexico and the subsequent creation of the Gulf.
Paleomagnetism of the Todos Santos and La Silla Formations, Chiapas: Implications for the opening of the Gulf of Mexico
Antonio Godinez-Urban et al., Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, 76100, Mexico; doi: 10.1130/GS604.
A second study by Antonio Godinez-Urban of UNAM and colleagues reports paleomagnetic data for Jurassic rocks in central Chiapas, southern Mexico. Paleomagetic data, a kind of fossil magnetism in the rocks, support the origin of the Gulf of Mexico as the result of rotation of Yucatan. This hypothesis had been suggested on the basis of plate reconstructions, but the data for central Chiapas refines the timing and amount of rotation required to open the Gulf.
Two-stage formation of Death Valley
Ian Norton, Jackson School of Geosciences, Institute for Geophysics, University of Texas at Austin, J.J. Pickle Research Campus, Bldg. 196, 10100 Burnet Road (R2200), Austin, Texas 78758-4445, USA; doi: 10.1130/GS588.1.
Death Valley represents an area with some of the most extreme topography in the United States. Vertical relief in the valley is more than twice that of the Grand Canyon and the valley floor, at 282 feet below sea level, is less than 100 miles from Mount Whitney, the highest point in the lower 48. This extreme topography has previously been assumed to have developed in the last 15 million years as a combination of extension related to the widespread Basin and Range that forms the Great Basin in the western U.S., and strike slip faulting. This paper by Ian Norton of the University of Texas at Austin suggests that almost all of this extreme topography formed just in the last 3 million years. This young deformation is a result of eastward migration of the Pacific-North America plate boundary. The San Andreas fault forms the present-day plate boundary, but motion across this fault is decreasing. Already, 25% of the motion between the Pacific and North America is taken up in faults east of the San Andreas. Death Valley is part of this newly evolving fault system that will eventually form the full plate boundary when motion along the San Andreas Fault ceases.
Pre-Andean deformation of the Precordillera southern sector, southern Central Andes
Laura Giambiagi et al., CONICET-CCT Mendoza-IANIGLA (Instituto Argentino de Nivologia, Glaciologia y Ciencias Ambientales), Parque San Martin s/n, 5500 Mendoza, Argentina; doi: 10.1130/GS572.1.
This article by Laura Giambiagi of CONICET Argentina and colleagues explores deformational processes that occurred in the Southern Central Andes before its final uplift and deformation. It reconstructs the deformational history of this sector of the Andean Mountains, since the Paleozoic times to the Present, through kinematic studies of geological structures.
Combining surface mapping and process data to assess, predict, and manage dust emissions from natural and disturbed land surfaces
Brett T. McLaurin et al., Dept. of Geography and Geosciences, Bloomsburg University of Pennsylvania, 400 East Second Street, Bloomsburg, Pennsylvania 17815, USA; doi: 10.1130/GS593.1.
The health and environmental effects of dust emission on air quality is a growing concern in many areas, particularly the southwestern United States. One aspect of dust emission is determining the source of the dust load and whether it can be attributed to natural or anthropogenic causes. To address this aspect of dust emission and air quality, Brett T. McLaurin of Bloomsburg University of Pennsylvania and colleagues examined the Nellis Dunes Recreation Area (NDRA), located north of Las Vegas, Nevada. The NDRA is a popular area for off-road vehicle (ORV) activity and a prime location to address such impacts of human-generated versus natural dust emission. This study first involved the development of a 1:10,000 scale map to delineate dust generating surface units. These seventeen surface units contain varying amounts of rock, sand, silt, and clay. Next, the > 500 km network of unpaved tracks in the NDRA was digitized on a high-resolution satellite basemap to assess the density of disturbance and to quantify the amount of track within each surface unit. From this information, wind erosion measurements and experiments on the emission of dust from ORVs were combined to generate dust emission risk maps for ORV activity and wind erosion. It was determined that areas of surface units dominated by silt, clay, and desert pavements were most susceptible to dust emission when disturbed by ORV activity. Once these units are disturbed, then wind erosion of the material becomes more pronounced. In contrast, sandy areas such as the dunes produce high dust emissions through natural wind erosion. The dust emission risk maps are an important tool in developing long-term mitigation plans that balance the interests of access to public lands with the goal of improving air quality.
Journal
Geosphere