(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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clicking on the link provided at the end of each Highlight. You can also read the
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engine the full doi (digital object identifier), e.g. 10.1002/jgrb.50175. The doi is
found at the end of each Highlight below.
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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.
###
Contact:
Mary Catherine Adams
Phone (direct): +1 202 777 7530
E-mail: mcadams@agu.org
AGU journal highlights -- May 7, 2013
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