Boulder, CO, USA – GSA's August GEOSPHERE articles are all about technology and modeling: how geographical information systems increase understanding of shear-zone growth; fault structures as fingerprints for rockslide motion and direction; a reconstruction of the Cenozoic geologic history of the southern Tobin Range, Nevada; thermal infrared and visible/short-wave infrared sensing help map metamorphic and igneous terrains in Morocco; and CT scans provide insight into the evolutionary traits of foraminifera, possibly leading to improved reconstructions of past environments.
Highlights are provided below. Representatives of the media may obtain complementary copies of articles by contacting Christa Stratton at cstratton@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Contact Christa Stratton for additional information or assistance.
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Using network analyses within geographic information system technologies to quantify geometries of shear zone networks
Prajukti Bhattacharyya, Dept. of Geography and Geology, University of Wisconsin, 800 West Main Street, Whitewater, Wisconsin 53109, USA; and Dyanna M. Czeck.
This paper describes potential ways network analyses capabilities within geographical information systems (GIS) can be used to describe and quantify anastomosing networks of ductile shear zones. Currently, no reliable technique exists for quantifying the manner in which smaller ductile shear zones interconnect to become part of a larger shear zone. The manner in which such networks form may be important in terms of deformation localization, and as such the approach outlined in this paper can be vital for understanding how shear zones develop and grow in nature.
Structural analysis and analogue modeling of the kinematics and dynamics of rockslide avalanches
Thomas Shea, Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, 63000, France; and Benjamin van Wyk de Vries.
Large-scale terrestrial rockslide avalanches involve the downward movement of colossal amounts of rocky materials that reach unexpected distances considering their mostly dry nature. These large runout distances (typically ten times the fall height) are difficult to explain with simple frictional models; thus, most scientific works have invoked fluidizing mechanisms or lubricating agents to reduce forces opposed to momentum, especially at the base of the moving rock mass. However, the properties and mechanics of low friction are still poorly understood. It is of critical importance that any model for motion and emplacement of these rockslides integrate geometric, morphologic, and structural features, all crucial in constraining the evolution of deformation in response to the environment. We present one of the first attempts to link the presence and spatial distribution of fault structures observed on the surface and in cross sections of rockslide-avalanche deposits by comparing structural maps of natural and experimentally obtained deposits. We show that fault structures are sensitive fingerprints testifying to the motion and emplacement styles of rockslide avalanches.
Timing and evolution of Cenozoic extensional normal faulting and magmatism in the southern Tobin Range, Nevada
Zachary J. Gonsior, Dept. of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA; and John H. Dilles
The authors made a geologic map, obtained isotopic ages of volcanic rocks, and reconstructed the Cenozoic geologic history of the southern Tobin Range, north-central Nevada. Volcanic rocks of late Eocene and Oligocene age were deposited in an ancient paleovalley trending east-west that was later disrupted by Basin-and-Range-style normal faults. Normal faulting evolved over time beginning at ~33 Ma and extending to the present, and in total produced about 50% east-west extension associated with tilting of crustal blocks about 25 to 30 degrees to the east. Early normal faults have an Oligocene age of ~33 Ma, and were succeeded by maximum faulting and extension in the mid-Miocene at ~15 Ma and slightly diminished faulting and extension rates to the present. The geology of the Tobin Range documents a zone of transition from more highly faulted and extended terranes to the southeast to less faulted terranes to the northwest, and also provides new data that document the temporal migration of extension from Eocene ages in northeast Nevada to mid-Miocene and younger ages in the Walker Lane belt of Nevada near the California border.
Interpretation and processing of ASTER data for geological mapping and granitoids detection in the Saghro massif (eastern Anti-Atlas, Morocco)
Matteo Massironi et al., Dipartimento di Geoscienze, Universita degli Studi di Milano-Bicocca, Italy.
Satellite remote-sensing analysis is used extensively for geological mapping in arid regions. However, it is not considered readily applicable to the mapping of metamorphic and igneous terrains where lithological contacts are less predictable. In this work, satellite ASTER data were used to clarify the geological framework of the Precambrian basement in the Saghro massif (Eastern Anti-Atlas, Morocco). A major effort was dedicated to the detection of granitoid plutons using both thermal infrared (TIR) and visible/short-wave infrared (VNIR/SWIR) data. It is well known that ASTER thermal bands are useful for discriminating granitoids with different silica content, but in the case of plutonic bodies of similar composition, such as those studied on the Saghro massif, the VNIR/SWIR bands can give even more effective results. Indeed, these wavelengths are affected by both the original content of mafic minerals (Fe, Mg-OH absorptions) and the degree of hydrothermal and surface alteration. These last two properties, generally considered to be impediments to the lithological detection of granitoid rocks, can instead be very useful because the first may directly depend on the magmatic evolution of a plutonic body, and the second depends on its textural character and may be indirectly related to its modal ratio.
Quantifying foraminiferal growth with high-resolution X-ray computed tomography: New opportunities in foraminiferal ontogeny, phylogeny, and paleoceanographic applications
Robert P. Speijer et al., Dept. of Earth and Environmental Sciences, K.U. Leuven, Belgium
Foraminifera are single-celled amoeba-like organisms living in the world's oceans and seas. During life, foraminifera build a chambered shell—generally measuring less than 0.5 mm—that after death ends up in the sediment at the seafloor. The enormous abundance of these shells in marine sediments and their more than 500-million-year-long fossil record make foraminifera extremely versatile tools in reconstructions of Earth's history. A newly developed, high-resolution X-ray computed tomography (CT) scanner at the Centre for X-ray Tomography at Ghent University, Belgium, now provides unprecedented ways to obtain quantitative insight into the interior architecture of the shells. The resolution of the CT scanner is below one micrometer and enables accurate volumetric measurements and detailed three dimensional imaging of the shells. This tool will offer better insights into evolutionary traits of foraminifera, and as such this development may lead to improved reconstructions of the oceanography and climates of the past.
Source: Geological Society of America