31 research outputs found
Relevance of viscous flow in accretionary wedges
The orogenic wedge model (Davis et
al. 1983; Platt 1986) marks a conceptual
breakthrough in understanding
the growth and long-term evolution of
accretionary wedges. The characteristic
rheology of subduction-related accretionary
wedges is thought to change
from Coulomb to viscous when the
wedge becomes thicker than ca. 15 km,
a transition that may influence the stability
and dynamics of these wedges.
Platt (1986) proposed that viscous flow
may trigger extensional faulting in the
upper rear part of the wedge and Wallis
et al. (1993) argued that viscous flow
may cause vertical ductile thinning of
the rear part of the wedge.
Material fluxes control the geometric
shape of an accretionary wedge (Brandon
et al. 1998; Platt 1986). Frontal
accretion and erosion both tend to drive
the wedge into a subcritical condition
as the taper angle of the wedge is progressively
reduced. This leads to horizontal
shortening across the wedge. If
underplating is dominantly controlling
the flow field in the wedge and frontal
accretion or erosion at the rear of the
wedge are small, the wedge is supercritically
tapered and leading horizontal
extension. Horizontal extension leads
to a subhorizontal foliation and may
eventually lead to normal faulting in
the rear-part of the wedge. Despite
the importance of these issues, there
remains a paucity of detailed information about ductile deformation and how
viscous flow influences the stability of
subduction-related accretionary wedges.
Strain measurements are an instrument
to address whether viscous flow strongly
influences the deformation in accretionary
wedges. They provide direct information
about the kinematics of ancient
orogenic belts. Additionally, they
allow understanding important tectonic
processes in subduction wedges such as
the pattern of flow within the wedge.
We focus on deformation analysis on
a suite of samples from the Otago
wedge exposed in the South Island
of New Zealand. The Otago accretionary
wedge offers a unique opportunity
to study the tectonic evolution of a
typical subduction-related accretionary
complex. Its across-strike length of
ca. 600 km makes it one of the largest
exposed ancient accretionary wedges on
Earth. Pressure and temperature estimates
indicate that our samples are representative
of deformation conditions to
depths as great as ca. 35 km. This is
similar to maximum depths observed for
subducting slabs beneath modern forearc
highs.
The deformation measurements show
that the strain magnitude is generally
small in the Otago wedge. The
oct
values, a measure of the distortion a
sample experienced (independent from
the strain geometry), range from 0.34–
3.87 for the Rf /? strains, 1.01–4.28 for
XTG strains across the whole suite of
the Otago rock pile, and 0.08–0.70 for
the absolute strains obtained from low
metamorphic grade rocks. The Otago
samples are characterized by considerable
volume strain that increases from
the lower textural zones towards the
high-grade interior of the wedge.
Our strain results are inconsistent with
the models which advocate supercritically
tapering of accretionary wedges
and that supercritical tapering eventually
triggers normal faulting. Taking
averages of our strain measurements,
a residence time in the wedge
of 35 Myr, burial depths of 30 km, coaxial
deformation and a depth-dependent
rate for ductile deformation, we calculate
vertically-averaged strain rates.
Because the principal strain axes of
the tensor average are all inclined, the
vertical averaging changes the principal
stretches. The horizontal principal
stretch parallel to the 160°-striking
Otago wedge becomes 0.79, that for
across strike 0.88 and for vertical
strain 0.44. Averaged strain rates are
−1.44−16 s−1 for parallel-strike horizontal
strain, −6.2−17 s−1 for across-strike
horizontal strain, and −8.02−16 s−1 for
vertical strain. The strain rates are related
to volume loss and to the efficiency
with which dissolved chemicals
are advected away. The rates are similar
to the ones calculated by Bolhar &
Ring (2001) and Ring & Richter (2004)
for the Franciscan wedge. These strain
rates are orders of magnitude smaller
than the 1−14 s−1 strain rates assumed
by Platt (1986). Thus, our data imply
that the Otago wedge could not shorten
horizontally fast, and hence could not
have steepened up its surface slope. The
fact that shortening was accompanied
by volume loss has another important
and interesting consequence. Even if
a case was envisioned in which horizontal
shortening was fast enough to
steepen up the surface slope of the
wedge, the volume loss would not necessarily
change the wedge geometry into
a supercritical configuration triggering
normal faulting. As a consequence of
the slow strain rates and the high volume
loss, viscous flow probably was not
fast enough to significantly influence the
stability of the wedge and to form a supercritically
tapered wedge.conferenc
Application of Photogrammetry in Geology: 3D Investigation of Rock Fracture Distributions
Geology as a science has an important
visual component and the knowledge of
any geologist is deeply linked to visual
experience of rock outcrops, thin sections
and analytical images. One of the
shortcomings of most geological images
such as maps, cross sections and outcrop
photographs is that they are 2D, while processes geologists are interested
in are typically occurring in 3D space.
The 3D geometry of faults, fractures
and joints is crucial to quantify geological
processes related to fracture mechanics,
such as hydrothermal mineralization
and ground water flow, but also geotechnical
problems such as rock mass stability.
A number of studies have shown
that some geological structures can be
described with a scale invariant, fractal
distribution. So far these observations
on which these findings are based were
restricted to one and two dimensions
and has been difficult to obtain a full
spatial geometric picture of fracture sets
from rock outcrops, because much of the
rock is not directly accessible. However,
without taking into account the spatial
distribution of geological structures the
true geometry of joint patterns cannot
be fully described and scaling laws, fractal
or not, cannot be derived.
We present images of joint patterns
based on datasets acquired by digital
photographs which are processed
to three dimensional images using the
photogrammetry software Siro3D. This
technique allows to obtain a highly accurate
3D picture of the visible outcrop.
The spatial pattern of joints in
nature is investigated using the software
SiroJoint. For the analysis of joint
systems a large data set was collected
from the Heavitree Quarzite at Ormiston
Gorge, near Alice Springs. The
Heavitree Quartzite is fragmented by a
spectacularly regular three-dimensional
joint pattern, which is repeated at different
scales and therefore represents a
perfect laboratory for our investigations
(Hobbs 1993). Siro3D generates a spatially
fully referenced 3D image from
overlapping digital images, such that
each pixel of the image is assigned spatial
coordinates. The software SiroJoint
routinely constructs planes from the intersection
of the rock-face with the linear
trace of planar features (Poropat
2001). It provides stereographic plots
of structural elements and additionally
measures joint persistence, area, and
joint spacing. Our measurements allow
to analyse geometrical scaling relationships
of joint sets with high accuracy
and will help explore the character of
their 3D complexity.
Several hundred joint planes were defined
with SiroJoint in an Ormiston
Gorge outcrop. Three different joint
sets can be distinguished. Joint set one
and two are characterized by steeply inclined
planes with joint spacings ranging
between 2 cm to 40 cm and 2 cm to
10m respectively. Both joints sets depict
a power law distribution in joint
spacing/frequency plots. The third set
is defined by a subhorizontal orientation.
It shows a very regular spacing in
the meter scale and lacks an exponential
distribution. We intend to use the
results as a basis to compare observed
fracture pattern with those generated
by computational methods like Iterated
Function Systems. This might help to
understand how physical rock properties
influence the spatial complexity of
fracture systems and develop constitutive
scaling relationships for certain rock
types.conferenc
Sphingosine-1-phosphate links glycosphingolipid metabolism to neurodegeneration via a calpain-mediated mechanism
We have recently reported that the bioactive lipid sphingosine-1-phosphate (S1P), usually signaling proliferation and anti-apoptosis induces neuronal death when generated by sphingosine-kinase2 and when accumulation due to S1P-lyase deficiency occurs. In the present study, we identify the signaling cascade involved in the neurotoxic effect of sphingoid-base phosphates. We demonstrate that the calcium-dependent cysteine protease calpain mediates neurotoxicity by induction of the endoplasmic reticulum stress-specific caspase cascade and activation of cyclin-dependent kinase5 (CDK5). The latter is involved in an abortive reactivation of the cell cycle and also enhances tau phosphorylation. Neuroanatomical studies in the cerebellum document for the first time that indeed neurons with abundant S1P-lyase expression are those, which degenerate first in S1P-lyase-deficient mice. We therefore propose that an impaired metabolism of glycosphingolipids, which are prevalent in the central nervous system, might be linked via S1P, their common catabolic intermediate, to neuronal death
Genetic Evidence for Involvement of Neuronally Expressed S1P1 Receptor in Nociceptor Sensitization and Inflammatory Pain
Sphingosine-1-phosphate (S1P) is a key regulator of immune response. Immune cells, epithelia and blood cells generate high levels of S1P in inflamed tissue. However, it is not known if S1P acts on the endings of nociceptive neurons, thereby contributing to the generation of inflammatory pain. We found that the S1P1 receptor for S1P is expressed in subpopulations of sensory neurons including nociceptors. Both S1P and agonists at the S1P1 receptor induced hypersensitivity to noxious thermal stimulation in vitro and in vivo. S1P-induced hypersensitivity was strongly attenuated in mice lacking TRPV1 channels. S1P and inflammation-induced hypersensitivity was significantly reduced in mice with a conditional nociceptor-specific deletion of the S1P1 receptor. Our data show that neuronally expressed S1P1 receptors play a significant role in regulating nociceptor function and that S1P/S1P1 signaling may be a key player in the onset of thermal hypersensitivity and hyperalgesia associated with inflammation
Quantification of deformation processes in the Torlesse accretionary wedge, New Zealand
Zusammenfassung:In dieser Studie werden Deformationsprozesse im mesozoischen Torlesse Akkretionskeil (Neuseeland) quantifiziert, um Aufschluß über die Dynamik in Akkretionskeilen zu erhalten. Absolute und relative Verformungsmessungen zeigen sowohl im lokalen als auch regionalen Maßstab eine stark heterogene Deformation des Torlesse Keils. Die regionale Deformation wurde mit Hilfe einer Tensordurchschnittsberechnung, unter Benutzung einzelner lokaler Verformungsdaten, als uniaxiale Verkürzung entlang einer subvertikalen, maximalen Verkürzungsachse charakterisiert. Absolute Verformungsmessungen an niedriggradigen Metasandsteinen belegen darüber hinaus durchschnittliche Volumenverluste von ca. 20% SiO2. Volumenveränderungen in tieferkrustalen Aufschlüssen wurden mittels einer
geochemischen Massenbilanzanalyse abgeschätzt. Chemische Zusammensetzungen höhergradiger Zonen weichen je nach Grad der Volumenverformung von der Protolitzusammensetzung ab und zeigen somit Verluste von 15% SiO2 an. Da Speicherorte für das gelöste Material nicht bekannt sind, muss angenommen werden, dass das Material aus dem Keil abtransportiert wurde. Die Verformungsergebnisse geben weiterhin Aufschluß über den Grad der Kopplung zwischen Akkretionskeil und subduzierter Platte. Die ermittelten Scherwerte in den Gesteinen liegen deutlich unter den zu erwartenden Scherwerten, die mittels eines einfachen Modells berechnet wurden, das sowohl verschiedene Konvergenzgeschwindigkeiten als auch Exhumierungsraten berücksichtigt. Dies belegt, dass der Torlesse Keil stark von der subduzierten pazifischen Platte entkoppelt war und die Deformation hauptsächlich durch den Fluß der Sedimente in und aus dem Keil bestimmt wurde.Abstract:In this study deformation processes in the Torlesse accretionary wedge (New Zealand) are quantified in order to get information on the dynamics of accretionary wedges. Absolute and relative strain measurements show a heterogeneous deformation in the Torlesse wedge. The regional deformation was estimated by using a tensor average calculation, that takes into account single local strain measurements. The calculation reveals an uniaxial shortening along a subvertical maximum shortening axis. Absolute strain measurements on low grade metasandstones additionally prove an average volume loss of c. 20 SiO2. A geochemical mass balance analysis was used to estimate volume change in deeper crustal levels. Chemical compositions in higher grade rocks differ from the protolith compositions depending on the degree of volume deformation and thus show volume loss of 15% SiO2. Sinks for the dissolved material are not known, therefore a transport of the material out of the wedge must be assumed. Strain results
also reveal information on the degree of coupling between the accretionary wedge and the subducting plate. Measured shear strains in the rocks are clearly lower than the expected shear values, which were calculated by using a simple model that takes into account different plate convergence velocities and exhumation rates. This proves that the Torlesse wedge was strongly decoupled from the subducted pacific plate and the deformation was mainly influenced by the flow of sediments in and out of the wedge
Seismic radiation from wind turbines: observations and analytical modeling of frequency-dependent amplitude decays
In this study, we determine spectral characteristics and amplitude decays of wind turbine induced seismic signals in the far field of a wind farm (WF) close to Uettingen/Germany. Average power spectral densities (PSD) are calculated from 10 min time segments extracted from (up to) 6-months of continuous recordings at 19 seismic stations, positioned along an 8 km profile starting from the WF. We identify 7 distinct PSD peaks in the frequency range between 1 Hz and 8 Hz that can be observed to at least 4 km distance; lower-frequency peaks are detectable up to the end of the profile. At distances between 300 m and 4 km the PSD amplitude decay can be described by a power law with exponent b. The measured b-values exhibit a linear frequency dependence and range from b = 0.39 at 1.14 Hz to b = 3.93 at 7.6 Hz. In a second step, the seismic radiation and amplitude decays are modeled using an analytical approach which approximates the surface-wave field. Since we observe temporally varying phase differences between seismograms recorded directly at the base of the individual wind turbines (WTs), source-signal phase information is included in the modeling approach. We show that phase differences between source signals have significant effects on the seismic radiation pattern and amplitude decays. Therefore, we develop a phase-shift-elimination-method to handle the challenge of choosing representative source characteristics as an input for the modeling. To optimize the fitting of modeled and observed amplitude decay curves, we perform a grid search to constrain the two model parameters, i.e., the seismic shear wave velocity and quality factor. The comparison of modeled and observed amplitude decays for the 7 prominent frequencies shows very good agreement and allows to constrain shear velocities and quality factors for a two-layer model of the subsurface. The approach is generalized to predict amplitude decays and radiation patterns for WFs of arbitrary geometry
Sphingosine-1-Phosphate: Boon and Bane for the Brain
Sphingosine-1-phosphate (S1P), an evolutionary conserved bioactive lipid, is essential for brain development, but might also exert detrimental effects in terminally differentiated post-mitotic neurons. Its concentration in the brain is tightly regulated by specific kinases and phosphatases, and mainly by the S1P degrading enzyme, S1P-lyase (S1PL). The role of S1P in neurons was initially studied in primary cultures by using structural analogues. During the last 3 years generation of a S1PL deficient mouse model substantially promoted our knowledge on the functional role of S1P metabolism in the brain, and its potential relation to neurodegenerative diseases. However, our understanding of the molecular mechanisms that underlie the physiological and pathophysiological actions of S1P in neurons remains rather scarce