Access the August issue via http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=1553-040X. A summary of contents follows.
LETTER FROM THE EDITOR
Randy Keller, University of Texas at El Paso, El Paso, TX 79968-0555, USA.
RESEARCH PAPER
Cenozoic tectonics of Macedonia and its relation to the South Balkan extensional regime
Nikola Dumurdzanov, et al. Corresponding Author: B. Clark Burchfiel, Massachusetts Institute of Technology, Earth, Atmospheric and Planetary Science, Cambridge, MA 02139-4307, USA.
This paper describes the deformation of Macedonia for the past 45 million years, showing the evolution of this area to the present. It also relates the deformational history relative to its regional setting within the Balkans and Aegean Sea area. Emphasis is also given to the present day deformational setting with respect to active faulting and seismic activity in the region.
RESEARCH PAPER
Viewing the subsurface in three dimensions: Initial results of modeling the Quaternary sedimentary infill of the Dundas Valley, Hamilton, Ontario
K.E. MacCormack, McMaster University, Hamilton, Ontario L8S 4K1, Canada; et al.
The Dundas Valley is a deep bedrock valley underlying the Hamilton-Wentworth region of southern Ontario that has been infilled with as much as 180 m of Quaternary sediment. These sediments contain a valuable record of past environmental change, as well as control groundwater and contaminant migration pathways throughout the region. Unfortunately, the nature, origin and spatial distribution of the sedimentary units comprising the infill are poorly understood. This paper presents the initial results from the compilation and three-dimensional modeling of subsurface geological data obtained from water-well and borehole records, and engineering and construction reports from the Hamilton region. Sediments identified within the Dundas Valley include clays and silty clays, sands and gravels, and silty sands that record deposition under glacial/glaciolacustrine, shoreline, and lagoonal conditions. RockWorks v.2002 was used to model and create three-dimensional images describing the bedrock topography and valley infill stratigraphy. These images can be used not only to better constrain the late Quaternary depositional history of the region, but also to identify and delineate the major aquifers and aquitards essential for groundwater protection and remediation planning.
RESEARCH PAPER
Computational methods for quantitative analysis of three-dimensional features in geological specimens
Richard A. Ketcham, University of Texas at Austin, Geological Sciences, Department of Geological Sciences, Jackson School of Geosciences, Austin, TX 78712, USA.
Volumetric imaging techniques, such as high-resolution X-ray computed tomography, allow investigators to "see" inside solid volumes and observe interior features without harming the object being studied. However, there has been a lack of software tools for extracting measurements relevant to earth scientists, such as the distribution of sizes, positions, shapes, orientations, and contact relationships of crystals, bubbles, metal grains, and other features of interest. The particular challenge is that there may be hundreds or thousands of such items, and they may be touching, but they still must be measured as individual objects. This paper introduces a new computer program, called BLOB3D, that allows these measurements to be made efficiently, accurately, and verifiably. This program enables new avenues for research across a range of disciplines, as it allows features to be measured in their true three-dimensional state, rather than extrapolated from two-dimensional observations.
SOFTWARE CONTRIBUTION
Improved methods for quantitative analysis of three-dimensional porphyroblastic textures
Richard A. Ketcham, University of Texas at Austin, Geological Sciences, Department of Geological Sciences, Jackson School of Geosciences, Austin, TX 78712, USA; et al.
Because metamorphic crystallization occurs on geological time scales, it cannot be observed directly. Instead, the conditions under which it takes place and the interplay of physical mechanisms responsible for its occurrence must be inferred from rocks in which crystallization has already run to completion. This study utilizes three-dimensional textural analysis of crystal size and spatial distributions to address this issue. If there is evidence that a growing crystal tended to inhibit nucleation or slow growth of other crystals in its immediate neighborhood, it implies localized chemical communication, leading to the conclusion that intergranular diffusion of crystal-building materials was the rate-limiting process. Earlier work in the 1990s suggested that this chemical communication has taken place in some geological settings, but improvements in imaging, measurement, and statistical methods in the intervening decade permit this analysis to be undertaken much more rigorously now. This paper utilizes the BLOB3D software described in the companion paper, along with improved tomographic data and statistical analysis techniques, to examine a suite of metamorphic rocks that had been studied previously with less developed methods. The improved analysis corroborates the previous work, but also illustrates various subtleties that serve to raise the bar for the data quality and specimen favorability required to provide unambiguous information.
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