Public Release: 

June 2011 Geosphere highlights

Geological Society of America

Boulder, CO, USA - The June 2011 GEOSPHERE announces a new themed issue: Tectonics, volcanism, and stratigraphy within the evolving transform margin north of San Francisco Bay, California. The issue also includes a contribution to an existing theme, Advances in 3D Imaging and Analysis of Geomaterials, and features several articles not associated with a specific theme.

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The Tectonics, volcanism, and stratigraphy within the evolving transform margin north of San Francisco Bay, California, issue, edited by V.E. Langenheim and D.S. Sweetkind, presents studies of the geology and tectonic history of the northern San Francisco Bay region. One paper in the themed issue was previously published in the October 2010 issue of Geosphere (Langenheim et al., 2010), and along with the introduction to the themed issue, the June issue presents "Age, composition, and areal distribution of the Pliocene Lawlor Tuff, and three younger Pliocene tuffs, California and Nevada" (Sarna-Wojcicki et al.); "Geology and geochemistry of volcanic centers within the eastern half of the Sonoma volcanic field, northern San Francisco Bay region, California" (Sweetkind et al.); and "Geology, geochronology, and paleogeography of the southern Sonoma volcano field and adjacent areas, northern San Francisco Bay region, California" (Wagner et al).

From the Introduction: The geology and tectonic history of the northern San Francisco Bay (North Bay) region, California, has not previously been studied in great detail despite the proximity of major population centers to the San Andreas Fault system. Here, the transition from subduction to transform margin tectonics east of the San Andreas Fault took place during the late Neogene and was characterized by northward-younging volcanism and northward-propagating faults. This history is recorded by a complex assemblage of sedimentary and volcanic deposits that are cut and offset by an array of active strike-slip faults, which are the northward continuation of the eastern part of the San Andreas Fault system (East Bay fault system). The East Bay fault system has accommodated up to 175 km of right-lateral strike-slip, which in the North Bay is partitioned onto a broad array of strike-slip faults east of the San Andreas Fault. The kinematic history of this fault system and the timing and nature of coeval sedimentation and volcanism have significant implications for groundwater hydrology, seismic hazard, and the overall tectonic evolution of the Pacific-North American plate margin.

The papers within this themed issue of Geosphere comprise a series of articles devoted to North Bay tectonics, stratigraphy, and volcanism. They are an outgrowth of concurrent studies in two five-year U.S. Geological Survey projects and a California Geological Survey mapping effort. Scientists from these three projects highlighted the preliminary results of their cooperative work in a technical session and associated field trip guide at the 2005 Geological Society of America Cordilleran Meeting in San Jose, California (Stevens and Cooper, 2005). This themed issue of Geosphere is an expansion and formalization of the ideas and discussions presented at the 2005 meeting. The papers demonstrate both the multidisciplinary approach and the breadth of geoscience information that is required to understand the tectonic history of the northern San Francisco Bay region. Because of their diversity, these papers will appeal to a wide range of earth scientists interested in these topics and approaches in the North Bay region and in application to analogous areas in other parts of the world.

Evolution of the northwestern margin of the Basin and Range: The geology and extensional history of the Warner Range and environs, northeastern California
Anne E. Egger and Elizabeth L. Miller, Dept. of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305-2115, USA; doi: 10.1130/GS620.1.

The Surprise Valley fault in northeastern California the westernmost fault of the Basin and Range province, where the crust is actively extending, and motion along it has resulted in the uplift of the Warner Range. Unlike the rest of the western margin of the Basin and Range, however, it lies directly inland from subduction along the Cascadia margin, where the crust is undergoing compression. As a result, it has a unique history, only sparsely studied up to this point. Currently, the Surprise Valley fault is seismically active and hosts an extensive geothermal system, so understanding that unique history has important implications. The new mapping and analysis presented in this study elucidate the long-term history of this large fault system, indicating that motion along the Surprise Valley fault occurred in at least two distinct phases over the last 15 million years separated by widespread volcanic activity. The second phase of motion and uplift continues today, though the rate of motion is lower than other parts of the margin.

From the Advances in 3D Imaging and Analysis of Geomaterials themed issue
Reduce 3D: A tool for three-dimensional spatial statistical analysis of crystals

David M. Hirsch, Geology Dept., Western Washington University, 516 High Street, Bellingham, Washington 98225, USA; doi: 10.1130/GS361.1.

This article presents Reduce3D, Render3D, and GraphCFs, which compose a computer software suite for processing of 3-D crystal data sets. Such data sets may derive from X-ray computed tomography (like a CAT scan) of rock samples, or from the computational simulation of crystallization in three dimensions. The suite is designed to allow the user to easily view the data set (Render3D), perform sophisticated statistical analysis on the dataset (Reduce3D), and visualize the statistical results (GraphCFs). Targeted toward the study of how new crystals form during metamorphism (kinetics), this research allows researchers to interpret their results much more rapidly and easily than currently possible.


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