||11 April 2007
GSA Release No. 07-15
April Media Highlights: Geosphere
Boulder, Colo. - The April issue of GEOSPHERE, published in electronic format only by the Geological Society of America, is now available online. Geology topics of interest include the White River area of Washington state, which may have potential for precious and/or base metal deposits at shallow depths. Articles are open access and may be viewed or downloaded at www.gsajournals.org/.
- Variable structural style along the Karakoram fault explained using triple-junction analysis of intersecting faults
Nickolas S. Raterman et al., University of California Davis, Davis, CA 95616, United States of America. Pages 71-85.
- Keywords: strike-slip faults, Himalayan orogeny, kinematics, fault zones, triple junctions.
- The Altyn Tagh and Karakoram faults are two of the largest strike-slip faults in the world. This article presents a study of the intersection between these two faults in the western Tibetan plateau. The study shows that the direction and sense of movement along the Karakoram Fault is directly related to the evolution of the western Altyn Tagh Fault. This has significant implications for understanding similar fault intersections in zones of active faulting throughout the world.
- Gravity reduction spreadsheet to calculate the Bouguer anomaly using standardized methods and constants
Derek I. Holom and John Oldow (corresponding author), University of Idaho, Department of Geological Sciences, Moscow, ID 83844-3022, USA. Page 86-90.
- Keywords: Bouguer anomalies, gravity, anomalies, gravity methods, spreadsheets.
- With the use of the Global Positioning System (GPS) for surveying station locations and altitudes, availability of digital terrain models, and enhanced computational capability, gravity modeling is a cost-effective tool in subsurface analysis ranging from basin to continental scale studies. Current standards for reduction and modeling of gravity measurements largely are unregulated and vary among geophysical textbooks, commercial software programs, and academic research spreadsheets available for download from the Internet. Using new standards established by the U.S. Geological Survey and the North American Gravity Database Committee, the authors developed a spreadsheet for reduction of raw data to the Bouguer anomaly and, with the use of terrain correction, the Complete Bouguer anomaly. The spreadsheet is available for free download via text links. We view the spreadsheet as particularly useful for field data reduction and modeling where internet access is limited or unavailable.
- Crustal controls on magmatic-hydrothermal systems: A geophysical comparison of White River, Washington, with Goldfield, Nevada
Richard J. Blakely et al., U.S. Geological Survey, 345 Middlefield Rd, M.S. 989, Menlo Park, CA 94025, USA. Pages 91-107.
- Keywords: magnetic anomalies, gravity anomalies, hydrothermal, Cascade Range, faults.
- In Miocene time, the Cascade arc extended across much of what is now western Nevada, northeastern California, Oregon, and Washington. Ore-forming systems were common in the arc, and this paper compares two far-separated examples: the White River altered area near Enumclaw, Washington, and the Goldfield mining district in Nevada. Both areas now have large exposures (>20 km2) of shallow hydrothermal alteration that formed about 20 million years ago. A geophysical comparison of the two areas using various techniques, suggests that both areas formed in similar tectonic settings. Overlapping strike-slip faults established a region of tension between the two faults that facilitated emplacement of plutons at shallow depth. Extensional faults also opened in the overlap region, providing pathways for hydrothermal fluids. Goldfield is the largest known high-sulfidation epithermal gold deposit in North America, whereas silica is the only commodity exploited at White River to date. However, based on geophysical, tectonic, and mineralogical similarities of the two areas, White River has potential for concealed precious and/or base metal deposits at shallow depth.
- Analysis of clustering in three-dimensional grain fabric
Atsushi Yamaji et al., Kyoto University, Graduate School of Science, Kyoto City, Kyoto 606-8502, Japan. Pages 108-119.
- Keywords: X-ray tomography, aspect ratio, anisotropy, cluster analysis, shape analysis.
- The statistics of grain orientations is an important clue to a variety of geological problems. For example, that of sedimentary grains suggests depositional environments: the dominant orientations of grains are clues to hydraulic conditions when the grains are deposited. Conventional sedimentological analysis investigates only long-axis orientations. This paper points out that the information of the orientations cannot be separately dealt with the information of grain shapes in the three-dimensional space, and proposes a statistical method to recognize the dominant orientations jointly with dominant grain shapes. The utility of the method was shown by the application to the sand grains that were deposited in an experimental flume and were visualized by X-ray computed microtomography. It was found that the dominant long- and short-axis orientations had correlations with grain shapes and with paleocurrent direction in the flume.
To view the current issue of GEOSPHERE,
go to www.gsajournals.org/.