AGU journal highlights -- May 7, 2013

(Press-News.org) The following highlights summarize research papers that have been recently published in Journal of Geophysical Research-Solid Earth (JGR-B), Journal of Geophysical Research-Oceans (JGR-C), Journal of Geophysical Research-Earth Surface (JGR-F) and Tectonics.

In this release:

Graphite lubricates fault zones Sediment processes can be significant source of ambient noise Evolution of the Qin Mountains as part of the supercontinent Rodinia Studying how flocculation affects acoustic reflection How do braided river dynamics affect sediment storage? Charting the growth of the Turkish-Iranian plateau

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1. Graphite lubricates fault zones

Graphite is known to be a low-friction material, and rocks rich in graphite are often found in fault zones. Oohashi et al. conducted laboratory studies to determine how much graphite is needed to reduce the frictional strength of a fault. Their experiments included samples with various mixtures of graphite and quartz, as well as pure quartz and pure graphite, and they covered large displacements (up to 100 meters (328 feet)), a range of slip rates (from 200 micrometers (0.0079 inches) per second to 1.3 meters (4.27 feet) per second), and shear strains (up to several tens of thousands.)

The authors find that the coefficient of friction decreases nonlinearly with increasing graphite fraction for any given shear strain and slip rate. Friction decreases quickly as graphite fraction increases between 5 percent and 20 percent by volume; at concentrations of 30 to 50 percent graphite, frictional levels were similar to that with pure graphite. They suggest that graphite in natural fault zones can effectively reduce the fault strength.

Source: Journal of Geophysical Research-Solid Earth, doi: 10.1002/jgrb.50175 http://onlinelibrary.wiley.com/doi/10.1002/jgrb.50175/abstract

Title: Graphite as a lubricating agent in fault zones: an insight from low- to high- velocity friction experiments on a mixed graphite-quartz gouge

Authors: Kiyokazu Oohashi: Department of Earth Sciences, Graduate School of Science, Chiba University, Chiba, Japan;

Takehiro Hirose: Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kochi, Japan;

Toshihiko Shimamoto: State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China.

2. Sediment processes can be significant source of ambient noise

Many studies of ambient ocean noise have focused on anthropogenic, biological, and weather- related sources, but collisions of sediment grains can also generate a significant amount of background noise. In an observational study linking noise and sediment processes, Bassett et al. measured sediment-generated noise in Admiralty Inlet, Puget Sound, Washington, a site where peak tidal currents exceed 3 meters per second (6.7 miles per hour). They find that sediment- generated noise is the dominant noise source with frequency between 1 kilohertz and 30 kilohertz during periods of strong currents. Peak sediment-generated noise levels from 4 to 20 kilohertz were associated with mobile gravel and pebbles. In fact, the researchers find that during periods of strong currents, sediment- generated noise actually exceeded the noise from local ship traffic, and that intermittent events can increase noise levels by more than 10 decibels over periods of seconds.

Source: Journal of Geophysical Research-Oceans, doi:10.1002/jgrc.20169, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrc.20169/abstract

Title: Sediment-Generated Noise and Bed Stress in a Tidal Channel

Authors: Christopher Bassett and Brian Polagye: Department of Mechanical Engineering, University of Washington, Seattle, Washington, USA;

Jim Thomson: Applied Physics Laboratory, University of Washington, Seattle, Washington, USA.

3. Evolution of the Qin Mountains as part of the supercontinent Rodinia

The Qinling-Dabie orogenic complex, part of a large east-west mountain range in the heart of China, plays a key role in helping scientists understand the formation and breakup of the supercontinent Rodinia. But the exact configuration and geodynamic history of the Qinling-Dabie orogenic complex and the surrounding region are not fully known. Contributing a piece to the puzzle, Bader et al. used uranium-thorium-lead geochronology-from both published studies and new data-to investigate the Neoproterozoic (1 billion years ago to 700 million years ago) evolution of the Qinling-Dabie orogenic collage. The authors outline a tectonic model for evolution of the Qinling-Dabie orogen, placing it into the context of the evolution of Rodinia, which formed around 1 billion years ago and broke up around 700 million years ago. Their synthesis could help researchers gain a more complete understanding of the geologic history of one of Earth's supercontinents.

Source: Tectonics, doi: 10.1002/tect.20024, 2013 http://onlinelibrary.wiley.com/doi/10.1002/tect.20024/abstract

Title: The Heart of China revisited, I. Proterozoic tectonics of the Qin Mountains in the core of supercontinent Rodinia

Authors: Thomas Bader: Mineralogisch-Petrographisches Institut, Universität Basel, Basel, Switzerland, and Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing, China;

Lothar Ratschbacher: Geologie, Technische Universität Bergakademie Freiberg, Freiberg, Germany;

Leander Franz: Mineralogisch-Petrographisches Institut, Universität Basel, Basel, Switzerland;

Zhao Yang: Geologie, Technische Universität Bergakademie Freiberg, Freiberg, Germany, and State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China;

Mandy Hofmann and Ulf Linnemann: Geochronologie, Senckenberg Naturhistorische Sammlungen, Dresden, Germany;

Honglin Yuan: State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China.

4. Studying how flocculation affects acoustic reflection

In inland estuaries and shallow coastal waters, small particles of organic matter, such as organic waste and debris or bacteria, clump together to form an aggregate known as floc. Flocculated particles can span a range of sizes, from a few micrometers to a few millimeters, and the properties and concentration of floc have a strong influence on water quality. To infer the properties of floc particles, researchers have proposed using acoustic backscatter measurements, a common technique for estimating sediment concentrations. To do so, however, requires an understanding of how the properties of floc particles affect acoustic wave reflection.

To find out, MacDonald et al. conducted a series of controlled laboratory experiments studying how high-frequency acoustic waves reflect off floc particles of differing composition, density, and size. They find that floc particles reflect acoustic signals differently from particles of the component organic material alone. The reflected signal depends on the base material, but also on the degree of flocculation and the size of the particle. Previous research found that as floc particles grow larger, they become less dense, so that very large floc has nearly the same density as the surrounding liquid. The authors suggest that the flocculation process itself alters the particle's reflection profile.

The authors' study explores how acoustic waves scatter off floc particles and details how sound can be used to study floc. They find that theoretical models using conventional scattering assumptions were capable of only partially describing the observed scattering properties. They suggest that future models should better align with the observed scattering characteristics, therefore allowing acoustic observations to be used to routinely measure sediment properties in flocculating marine environments.

Source: Journal of Geophysical Research-Oceans, doi:10.1002/jgrc.20197, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrc.20197/abstract

Title: Acoustic scattering from a suspension of flocculated sediments

Authors: Iain T. MacDonald: University of East Anglia, Norwich, United Kingdom, and National Institute of Water and Atmospheric Research, Hamilton, New Zealand;

Christopher E. Vincent: University of East Anglia, Norwich, United Kingdom;

Peter D. Thorne and Benjamin D. Moate: National Oceanography Centre, Liverpool, United Kingdom.

5. How do braided river dynamics affect sediment storage?

Braided rivers, with their continuously changing network of channels, are highly dynamic systems. Four mechanisms of channel change and evolution are considered the classic mechanisms of braided river formation: development of central bars, conversion of single transverse bars to mid-channel braid bars, formation of chutes, and dissection of multiple-braid bars. There have been few studies on how each of these braiding mechanisms contributes to changes in sediment storage and to the dynamics of a river. In one of the first field studies on the topic, Wheaton et al. analyzed repeat topographic surveys conducted over a 5- year period of the River Feshie, an active, braided, gravel-bed river in the United Kingdom.

They find that collectively, the four classic braiding mechanisms accounted for most of the change in sediment storage. However, their results highlight the critical role that bank erosion and other non-braiding mechanisms play in facilitating net increases in sediment storage by braiding mechanisms through providing an important local supply of sediment to feed those braiding mechanisms and through creating accommodation space where central bars can develop.

Source: Journal of Geophysical Research-Earth Surface, doi:10.1002/jgrf.20060, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20060/abstract

Title: Morphodynamic signatures of braiding mechanisms as expressed through change in sediment storage in a gravel-bed river

Authors: Joseph M. Wheaton and Alan Kasprak: Department of Watershed Sciences, Utah State University, Logan, Utah, USA;

James Brasington: School of Geography, Queen Mary University of London, London, UK;

Stephen E. Darby and David Sear: Geography and Environment, University of Southampton, Highfield, Southampton, UK;

Damia Vericat: Fluvial Dynamics Research Group (RIUS), Department of Environment and Soil Sciences, Forestry and Technology Centre of Catalonia, University of Lleida, Spain, Alcalde Roviraroure, Lleida, Catalunya, Spain.

6. Charting the growth of the Turkish-Iranian plateau

Stretching from the Persian Gulf up through Turkey, the northwest-southeast running Zagros fold-and-thrust belt is a region of extensive crustal deformation and seismic activity. Near the Zagros Mountains the structure of the Middle Eastern region is the result of the intersection of three tectonic plates, with the Eurasian plate being squished on both sides by the Arabian and Indian plates. Convergence of the plates is driving the formation of the Turkish-Iranian plateau, a high-elevation expanse of relatively smooth terrain reaching in some places more than 2 kilometers (1.24 miles) into the sky and lying northeast of the Zagros belt.

How the Turkish-Iranian plateau formed and attained its dramatic height, however, is relatively unknown. Researchers are unsure whether the plateau grew vertically at one spot and then expanded laterally, or if the entire surface area of the plateau rose concurrently. Furthermore, research suggests that the uplift related to seismic thrusts (a process largely occurring within the Zagros fold-and- thrust belt) can only account for surface elevations of up to about 1,250 meters (0.78 miles), because after this point the gravitational potential energy of the crust counterbalances the horizontal compressive forces. Also, scientists are uncertain whether the Turkish-Iranian and other similar plateaus (such as the Tibetan plateau) grow incrementally, or through periodic bursts of activity.

Combining in-the-field measurements with existing seismicity data and global positioning system observations of surface motion, Allen et al. describe in detail the complex interactions in and around the Zagros belt to better understand the formation of the Turkish-Iranian plateau. Based on their findings, the authors suggest that the Turkish-Iranian plateau grew incrementally, with the rate of uplift varying over time, and that both aseismic basement shortening and seismic upper- crustal thickening drove the plateau's ascent.

Source: Tectonics, doi:10.1002/tect.20025, 2013 http://onlinelibrary.wiley.com/doi/10.1002/tect.20025/abstract

Title: Orogenic plateau growth: expansion of the Turkish-Iranian Plateau across the Zagros fold-and-thrust belt

Authors: M. B. Allen and C. Saville: Department of Earth Sciences, University of Durham, Durham, United Kingdom;

E. J-P. Blanc: Statoil, Drammensveien, Vækerø¸ Oslo, Norway;

M. Talebian: Research Institute for Earth Sciences, Geological Survey of Iran, Tehran, Iran;

E. Nissen: Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado, USA.

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