Boulder, Colo., USA Geosphere papers posted online 4 and 17 April 2013 use LiDAR, ALSM (Airborne Laser Swath Mapping), EarthScope, CHIRP (compressed high-intensity radar pulse), and IODP (International Ocean Drilling Program) data to further geoscientists' understanding of the nature of Earth. New locations studied: Sakhalin, in the Russian far east; Pea de Bernal nation monument, Mxico; and Andalshatten batholith, central Norway. Geosphere's online-only articles feature a variety of article lengths, stunning figures, and animations or 3-D digital displays.
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Constraints from magnetotelluric measurements on magmatic processes and upper mantle structure in the vicinity of Lassen Volcanic Center, northern California
Stephen K. Park and Linda C. Ostos, Department of Earth Sciences, University of California Riverside, 2258 Geology, Riverside, California 92521, USA. Published online 4 Apr. 2013; http://dx.doi.org/10.1130/GES00799.1. Themed issue: Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California.
A geophysical survey in northern California provides new images of magma source regions beneath Lassen Volcanic Center and volcanically active areas to the east. While geologists can predict the processes that form magma beneath volcanoes including depths and temperatures, they cannot deduce the geometries of the source regions. Magma is electrically conductive, and a geophysical technique called magnetotellurics can be used to create images of the distribution of electrically conductive bodies down to depths of 200 km or more. Based on how conductive are these bodies, the percentages of magma and/or fluids can be estimated. This study by Stephen K. Park and Linda C. Ostos, spanning Lassen Volcanic Center in northern California, shows a highly conductive body at a depth of 100 km and approximately 50 km east of the peak. This body is too conductive to be magmatic, and we instead suggest that it results from dewatering of the subducting Gorda plate. Zones of enhanced conductivity found in the upper mantle above the body provide conduits to active volcanic centers in the Lassen region. Park and Ostos estimate ~7% magma in the overlying mantle wedge from its average electrical conductivity. Several conductive bodies are also found in the crust and located beneath surface expressions of recent eruptions; they infer that these are the magma chambers.
Quaternary landscape evolution over a strike-slip plate boundary: drainage network response to incipient orogenesis in Sakhalin, Russian far east
Uisdean Nicholson (David Macdonald, corresponding author) et al., School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK. Published online 4 Apr. 2013; http://dx.doi.org/10.1130/GES00883.1.
In some parts of Earth, the tectonic plates move past each other horizontally, creating large strike-slip faults. The best-known strike-slip boundary in the world is the San Andreas fault of California, but on the other side of the Pacific, San Andreas has a mirror image on the Russian island of Sakhalin, where the Sakhalin-Hokkaido Shear Zone forms part of the boundary between the North American and Eurasian plates. This shear zone has two main differences with San Andreas. First, it moves much slower than San Andreas at about one tenth of the rate. Second, the Amur River has deposited a delta across the shear zone. The Amur is the tenth largest river on earth, and the delta is large -- 220 km across -- with a sediment accumulation that is up to 5 km thick. This has created a new flat landscape in the north of Sakhalin, giving us an unrivalled opportunity to study the development of hills and the response of rivers to oblique compression on a strike-slip plate boundary over the past 3 million years. This lack of landscape inheritance is very rare in active deformation, so we can study the very early stages of mountain building.
Magnetostratigraphic importance of secondary chemical remanent magnetizations carried by greigite (Fe₃S₄) in Miocene sediments, New Jersey Shelf (IODP Exp. 313)
Andreas Nilsson et al., Geology and Geophysics, School of Environmental Sciences, University of Liverpool, Liverpool, UK. Published online 4 Apr. 2013; http://dx.doi.org/10.1130/GES00854.1. Themed issue: Results of IODP Exp313: The History and Impact of Sea-level Change Offshore New Jersey.
Paleomagnetic and mineral magnetic analyses were carried out on Miocene clays from upper unit II at Sites M0027 and M0028 recovered during Integrated Ocean Drilling Program Expedition 313 on the New Jersey shallow shelf. A zone of mixed polarity in the lower section of Hole M0028A and dual overlapping magnetization components in upper Hole M0027A indicate that the sediments may have been chemically remagnetized during one or several events. Mineral magnetic investigations reveal that the magnetization is carried by the ferromagnetic iron-sulfide greigite (Fe3S4), possibly with traces of titanomagnetite. Study authors Andreas Nilsson and colleagues find that several changes in polarity coincide with variations in magnetic mineral grain size and/or concentration. They interpret these variations to be different stages of greigite growth, which were triggered by changes in pore-water chemistry and/or upward migration of methane.
Geometry and Quaternary slip behavior of the San Juan de los Planes and Saltito fault zones, Baja California Sur, Mexico: Characterization of rift-margin normal faults
Melanie M. Coyan et al., School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, Arizona 85287, USA. Published online 4 Apr. 2013; http://dx.doi.org/10.1130/GES00806.1. Themed issue: Origin and Evolution of the Sierra Nevada and Walker Lane.
An array of north-striking, left-stepping, active normal faults cuts the southwest margin of the Gulf of California and across the southern tip of the Baja California peninsula. This is the gulf margin fault system of the oblique-divergent plate boundary within the Gulf of California. Detailed geologic and geomorphic mapping along the onshore San Juan de los Planes and Saltito fault zones allow authors Melanie M. Coyan and colleagues to delineate geometric sections and to infer the tectonic history of the fault zones. To achieve a more complete understanding of these individual normal faults within a larger array, Coyan and colleagues mapped faults to ~10 km offshore using seismic CHIRP (compressed high-intensity radar pulse) profiling. Both onshore faults slip at a low rate and have a low total offset. Along the San Juan de los Planes fault zone, which is entirely onshore, the young, scarp-forming fault reactivated older faults to rupture a broad, low-relief pediment surface with thin Quaternary cover, reflecting a two-stage slip history along this fault zone. The offshore study suggests a northward continuation of the onshore Saltito fault, and a complex fault array north of the La Gata fault on the east side of the San Juan de los Planes basin extending northward to the west Cerralvo fault. Study results suggest relatively low rates of active faulting of less than 1 mm/yr across the San Juan de los Planes system of faults compared to high rates on the active gulf-axis system, and relatively higher rates on earlier Neogene gulf margin faults in other areas along the southwest Gulf of California margin.
Geologic setting of the Pea de Bernal Natural Monument, Quertaro, Mxico: An endogenous volcanic dome
Gerardo J. Aguirre-Daz et al., Centro de Geociencias, Universidad Nacional Autnoma de Mxico, campus Juriquilla, Boulevard Juriquilla 3001, Juriquilla, Quertaro, C.P. 76230, Mxico. Published online 4 Apr. 2013; http://dx.doi.org/10.1130/GES00843.1.
This work presents for the first time the geologic, geochronologic, and geochemical characteristics of the rock known as Pea de Bernal, at Quertaro, central Mxico. In spite of being an important natural monument and that in 2009 it was included in the list of protected worldwide patrimonies of United Nations Educational, Scientific, and Cultural Organization (UNESCO), very little was known about this prominent peak that, with a 433 m height, is apparently the highest monolith of the world. Study results lead Gerardo J. Aguirre-Daz and colleagues to conclude that Pea de Bernal is a volcanic spine-type dome with a dacitic composition (SiO2 = 67 wt%), which was forcefully intruded practically as a solid plug in the country rock about 8.7 million years ago. This is the first scientific study carried out on this geologic feature. In addition, it should be emphasized that Pea de Bernal is a sacred site for indigenous peoples celebrations, which pre-Hispanic traditions still continue at present, that Pea de Bernal is a reference site for thousands of tourists and mountain climbers, and that Pea de Bernal is also known as the main entrance to the Sierra Gorda mountain area, which was declared a Biosphere Natural Reserve by UNESCO in 2001.
Point-Based Computing on Scanned Terrain with LiDAR Viewer
Oliver Kreylos et al., W.M. Keck Center for Active Visualization in the Earth Sciences, University of California, Davis, California 95616, USA. Published online 4 Apr. 2013; http://dx.doi.org/10.1130/GES00705.1. Themed issue: Seeing the True Shape of Earth's Surface: Applications of Airborne and Terrestrial LiDAR in the Geosciences.
Point-based computing is an alternative approach to processing and visualization of very large unstructured 3-D point clouds, such as those collected by high-resolution laser scans of terrain, via LiDAR (Light Detection and Ranging) or ALSM (Airborne Laser Swath Mapping). Using data structures and algorithms that access points directly from hard disk, point clouds much larger than a computer's main memory can by processed very efficiently. Examples of such processes include visualization, vegetation removal, hill-shading, or feature extraction. LiDAR Viewer is an open-source software for desktop and virtual reality environments combining off-line processing and real-time visualization.
Hybrid submarine flows comprising turbidity current and cohesive debris flow: Deposits, theoretical and experimental analyses, and generalized models
Peter J. Talling, National Oceanography Centre, European Way, Southampton, Hampshire SO14 3ZH, UK. Published online 17 Apr. 2013; http://dx.doi.org/10.1130/GES00793.1. Themed issue: Exploring the Deep Sea and Beyond.
This paper by Peter J. Talling summarizes a new framework for understanding debris flows that occur on the sea floor, which include some of the largest mass flows on our planet. These flows can break strategically important sea floor cables that carry 95% of transoceanic data traffic, including the internet. The flows help to form the largest sediment accumulations on earth, whilst ancient flows produced rocks that now hold important hydrocarbon reserves. Talling analyses how their behavior and deposits change as debris flow strength increases, and encompasses field observations from ancient rocks and the modern sea floor, laboratory tank experiments, and quantitative numerical modeling. It is the broadest ranging summary of submarine debris flows yet published.
Epeirogenic transients related to mantle lithosphere removal in the southern Sierra Nevada region, California: Part II. Implications of rock uplift and basin subsidence relations
J. Saleeby et al., Caltech Tectonics Observatory, California Institute of Technology, Pasadena, California 91125, USA. Published online 17 Apr. 2013; http://dx.doi.org/10.1130/GES00816.1. Themed issue: Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California.
The uplift history of the Sierra Nevada mountain range and the subsidence history of the adjacent San Joaquin Basin of California are integrated with numerical modeling results on the physical processes attendant in the ongoing removal of mantle lithosphere from beneath the region. The geological data are leveraged against the modeling results as a means of better constraining the physical processes simulated in a number of model runs. A class of models is resolved that can satisfy both the uplift and subsidence histories, as well as a broad array of other geological constraints. These results are further developed into an integrated geological-geodynamic model of mantle lithosphere removal and its surface manifestations as it progressed over time in three dimensions.
Batholith tectonics: Formation and deformation of ghost stratigraphy during assembly of the mid-crustal Andalshatten batholith, central Norway
Heather S. Anderson (Aaron S. Yoshinbu, corresponding author) et al., Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA. Published online 17 Apr. 2013; http://dx.doi.org/10.1130/GES00824.1.
This work utilizes superb 3-D exposures of crystallized igneous rock to evaluate how magmas migrate through Earth's crust and how they ultimately solidify beneath arc volcanoes. The research uses high-precision geochronological dating methods to establish the timing of magmatism as well as structural analysis to explain the mechanisms by which different batches of magma were collected within a subterranean reservoir. The research highlights the complexity of magma emplacement beneath volcanoes and demonstrates how crustal rocks behave during high-temperature deformation.
Dynamic topography of the western Great Plains: Geomorphic and 40Ar/39Ar evidence for mantle-driven uplift associated with the Jemez lineament of NE New Mexico and SE Colorado
A. Nereson et al., Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Published online 17 Apr. 2013; http://dx.doi.org/10.1130/GES00837.1. Themed issue: Investigations of North America as EarthScope Reaches Its Maturity.
The iconic, windswept landscapes of the western Great Plains are renowned for their vast expanses of low-relief topography. But in a unique sub-region of the High Plains located in northeastern New Mexico and southeastern Colorado, this flat surface has been extensively modified in recent geologic history by the processes of erosion and volcanism. The resultant landscapes are characterized by networks of deep bedrock canyons, towering mesas capped by volcanic rocks, and a stunning array of dormant volcanoes and cooled lava flows. Previous scientific studies suggested that these dramatic landscape modifications began at the end of the Miocene epoch, around five million years ago, but explanations for why these processes occurred when they did, remain controversial. This study by A. Nereson and colleagues examines a variety of data sets, including radiometric ages of lava flows and patterns of erosion, to try to distinguish between the two leading drivers of landscape change: climate versus tectonic processes. Their analysis suggests that tectonic uplift of the surface, as driven by convective flow in the Earth's mantle and magmatic inflation of the Earth's crust, is the primary cause for the deeply eroded topography in our study area. The influences of past climate changes on the erodibility of landscapes probably played an important, but secondary role in shaping the modern topography there.
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