October 2010 Geosphere highlights

October 01, 2010

Boulder, CO, USA - This month's themed issue, "Advances in 3D imaging and analysis of geomaterials," edited by Guilherme A.R. Gualda, Don R. Baker, and Margherita Polacci, features papers from the 2009 AGU Joint Assembly session "Advances in 3-D Imaging and Analysis of Rocks and Other Earth Materials." Studies include 3-D imaging and analysis techniques for Wild 2 comet material returned from the NASA Stardust mission and the first 3-D X-ray scans of crystals from the Dry Valleys, Antarctica.

Introduction: Advances in 3D imaging and analysis of geomaterials
Guilherme A.R. Gualda, Vanderbilt University, Earth & Environmental Sciences, Station B #35-1805, Nashville, Tennessee 37235, USA

Excerpt: Beginning in the 1970s, the availability of computers led to the development of procedures for computer-assisted acquisition and reconstruction of 3-D tomographic data, in particular using X-rays. X-ray tomography is now a mature technique that is used routinely. It has been applied to a wide array of geomaterials, from rocks to fossils to diverse experimental charges, to name a few. The ability to create 3-D maps with millions to billions of volume elements (voxels) created the challenge of processing and analyzing such large amounts of data. While qualitative observations in 3-D yield significant insights into the nature of geomaterials and geological processes, it is in the pursuit of quantitative data that 3-D imaging shows its greatest potential. The continued improvements in computer capabilities have led to ever more sophisticated procedures for 3D image analysis. The papers in this issue encompass a wide range of topics, from applications of established techniques to a variety of materials, the development of new imaging techniques, and the description of improved imaging and analysis techniques.

3D imaging of volcano gravitational deformation by computerized X-ray micro-tomography
M. Kervyn et al., Dept. of Geology and Soil Science, Ghent University, Krijgslaan 281/S8, 9000 Gent, Belgium

Volcanoes are known to be unstable constructs that can deform gravitationally when they build upon weak sedimentary layers. The structures and velocity of deformation depend on the volcano loading and the properties of the underlying layers. These processes can be studied with scaled laboratory experiments in which volcanoes are simulated by a mixture of sand and plaster. Silicone is used to simulate the weak underlying layers. This team from Belgium and France, lead by M. Kervyn of Ghen University, presents the results of imaging such experiments with X-rays. The micro-tomography technology used in imaging these experiments enables the virtual re-construction of the 3-D shape of the deformed experiment. Virtual cross-sections through the experiment provide a new way to characterize the faults and fissures forming within the experimental volcano during its deformation. Results from a range of experiments with different geometrical characteristics provide a better understanding of the impact of such gravitational deformation, currently recorded at several well-known volcanoes on Earth (e.g. Etna, Kilauea), on the construct's structure at depth and its potential zones of weakness.

Three-dimensional measurement of fractures in heterogeneous materials using high-resolution X-ray computed tomography Richard A. Ketcham et al., Dept. of Geological Sciences, Jackson School of Geosciences, 1 University Station C1100, The University of Texas at Austin, Austin, Texas 78712-0254, USA

When present, fractures tend to dominate fluid flow though rock bodies, and characterizing fracture networks is necessary for understanding these flow regimes. Specialized CAT scanning has long been an important tool in imaging fractures in 3-D in rock samples. However, a number of factors have reduced the fidelity of such data, including the natural heterogeneity of real rocks and the limited resolution of CAT scanning. Richard A. Ketcham of The University of Texas at Austin and colleagues present new, general methods for overcoming these problems and extracting the best-quality information possible concerning fracture aperture, roughness, and orientation, even in highly heterogeneous rocks. The methods are also general enough that they can be applied to similar situations, such as measuring mineral veins. This work was funded in part by U.S. National Science Foundation grants EAR-0113480 and EAR-0439806.

Laser scanning confocal microscopy of comet material in aerogel
Michael Greenberg and Denton S. Ebel, Dept. of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, USA

The NASA Stardust mission returned extraterrestrial material from the comet Wild 2 -- the first solid sample-return mission since the Apollo era. Particles from the tail of Wild 2 were captured in aerogel, low-density, translucent, silica foam at a relative velocity of 6.1 km per second. Upon impact into the aerogel, particles from the tail of the comet were fragmented, melted, and ablated, creating cavities, or tracks -- each of which is unique to the original particle before capture. Michael Greenberg and Denton S. Ebel of the American Museum of Natural History present nondestructive 3-D imaging and analysis techniques for comet material returned from the NASA Stardust mission. The methods described in this paper represent the highest resolution 3-D images of Stardust material to date. The procedures described here will easily extend to other translucent samples in the geosciences.

Quantifying 3D crystal populations, packing and layering in shallow intrusions: A case study from the Basement Sill, Dry Valleys, Antarctica
Dougal A. Jerram et al., Dept. of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK

This innovative paper presents the first 3-D X-ray analysis from the Dry Valleys in Antarctica. Dougal A. Jerram of the University of Dunham and colleagues use high resolution 3-D imaging to identify and quantify the crystals that form classic layered structures in the Basement Sill, Antarctica. This article not only reports on the key findings but contains a number of animations to highlight the detail that such X-ray scans can produce and shows how the use of such modern techniques, along with classic petrology, is a powerful tool to study rock. The U.S. National Science Foundation funded this expedition (grant OPP-02 29306).

Geophysical framework of the northern San Francisco Bay region, California
V.E. Langenheim et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA

V.E. Langenheim of the U.S. Geological Survey and colleagues investigate faulting and basins north of San Francisco Bay, California, by analyzing small variations in Earth's gravity and magnetic fields. Gravity data reveal deep sedimentary basins that underlie the Santa Rosa Plain and Napa and Sonoma Valleys and are important for assessing groundwater resources and seismic hazard. The geophysical data show that much of the 175 km of right-lateral slip from the East Bay fault system (to the south) is partitioned around Napa and Sonoma Valleys, slip that produced pull-apart basins beneath the valley floors. The magnetic data suggest that the Maacama fault has reactivated older basement structures that appear to influence patterns of microearthquakes in the region.

Late Triassic stratigraphy and facies from northeastern Mexico: Tectonic setting and provenance
José Rafael Barboza-Gudiño et al., Universidad Autónoma de San Luis Potosí, Instituto de Geología, Manuel Nava No. 5, Zona Universitaria, 78240, San Luis Potosí, México

Authors Jose Rafael Barboza Gudino and colleagues of the Universidad Autonoma de San Luis Potosi, Mexico, discus stratigraphic revision and facies distribution of Upper Triassic strata in northeastern Mexico based on detrital zircon geochronology data, supporting a new model for tectonic evolution and sedimentation along the western equatorial margin of Pangaea after the Laurentia-Gondwana collision.

Late Triassic Texas uplift preceding Jurassic opening of the Gulf of Mexico: Evidence from U-Pb ages of detrital zircons
William R. Dickinson et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85718, USA

William R. Dickinson of the University of Arizona and colleagues use U-Pb ages for 2655 individual detrital zircon grains in 30 samples of Upper Triassic (Carnian-Norian) sandstones of the southwestern USA (our data) and northern Mexico (other data) to infer regional Late Triassic provenance relations and tectonic features, which included a prerift uplift in Texas precursory to Jurassic opening of the Gulf of Mexico. A preliminary version of this paper was presented in the theme session on "Permian to Triassic Tectonics, Magmatism, and Sedimentation in the NE Mexico and South-Central USA Region" at the South-Central Section meeting of the Geological Society of America in Dallas (March 2009). This work was funded in part by U.S. National Science Foundation grant EAR-0443387.

Development of an igneous rock database with geologic functions: Application to Neogene bimodal igneous rocks and mineral resources in the Great Basin
Douglas B. Yager et al., Central Mineral and Environmental Resources Science Center, U.S. Geological Survey, Denver Federal Center, Box 25046, MS 973, Denver, Colorado 80225, USA

This database contribution builds on a well-designed rock geochemistry database developed by Lehnert et al. in Lehnert, K.A., Su, Y., Langmuir, C.H., Sarbas, B., and Nohl, U., 2000, A global geochemical database structure for rocks: Geochemistry, Geophysics, Geosystems, v. 1, 16 p. This new database schema presented by Douglas B. Yager of the U.S. Geological Survey and colleagues focuses on capturing information acquired at multiple scales of observation. Geologists routinely use sample data (descriptive, qualitative, quantitative) to characterize a hierarchy of large and small geologic features that each have their own independent attributes, use physical relationships between geologic features to establish their relative ages, combine this information with dated features to understand evolutionary histories of study areas at various scales, and produce maps to display such information in space and time relative to other features of interest. The resulting database is utilized to capture information on Neogene bimodal igneous rocks, primarily in northeastern Nevada and make queries to generate GIS displays that elucidate the time-space-composition relationships of volcanic centers to one another and associated geophysical anomalies, structural features, and mineral deposits. Geochemical plots and variation diagrams constructed from stored rock geochemistry for areas within the Great Basin are provided as examples of how the database can be used to address geologic questions. Yager and colleagues welcome participation and future collaborative efforts to build on the database design and ideas presented in this paper.

InSAR observation of the strike-slip faults in the northwest Himalayan frontal thrust system
Lize Chen and Shuhab D. Khan, Dept. of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA

This work used the Interferometric Synthetic Aperture Radar (InSAR) technique to estimate slip rates along an important strike-slip fault in the northwest Himalayan frontal thrust system. InSAR results suggest that the current active deformation is occurring mostly within the Potwar-Salt thrust wedge instead of at the thrust front. The findings presented in this work are helpful in earthquake prediction and mitigation in this seismically very active region.
Keywords: Voxels, microtomography, fractures, NASA Stardust Mission, Wild 2, aerogel, Dry Valleys, Antarctica, geophysics, microearthquakes, Mexico, zircon dating, database, InSAR.

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