129 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
Oligocene emplacement of the Eclogite Zone of the central Tauern Window, Eastern Alps, Austria
The EZ is an approximately 20km long
and 2–3km wide coherent unit of the
Tauern Window in the Eastern Alps. It
is sandwiched between the Venedigerand
the Glockner Nappe. While rocks
in the EZ experienced HP metamorphic
conditions (24 kbar/650°C), rocks
from the underlying Venediger Nappe
and the overlying Glockner Nappe only
record lower alpine metamorphic conditions
with peak pressures not exceeding
10 and 8 kbar, respectively. While metamorphism
in the EZ is well dated with
an average age of 31.5±0.7Ma (Glodny
et al. 2005) the final emplacement of
these different nappes is still under debate.
Our Rb-Sr-data indicate that top-N thrusting
at the base and large-scale folding
of the EZ was coeval with sinistral
strike-slip faulting at its upper boundary
and eclogite-facies metamorphism
in the EZ. The data also indicate that
today’s nappe architecture must have
been established in less than 2Ma after
the eclogite facies metamorphism in
the EZ. Very fast exhumation of the EZ
was accomplished in a transpressional
setting, which might explain why the exposed
EZ is such a small unit.conferenc
Constraining the long-term evolution of the slip rate for a major extensional fault system in the central Aegean, Greece, using thermochronology
The brittle/ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary. In this study we examine the slip history of the large-scale Naxos/Paros extensional fault system (NPEFS), Cyclades, Greece, by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime. Based on apatite and zircon fission-track (AFT and ZFT) and (U–Th)/He dating we observe variable slip rates across the brittle/ductile transition on Naxos. ZFT and AFT ages range from 11.8 ± 0.8 to 9.7 ± 0.8 Ma and 11.2 ± 1.6 to 8.2 ± 1.2 Ma and (U–Th)/He zircon and apatite ages are between 10.4 ± 0.4 to 9.2 ± 0.3 Ma and 10.7 ± 1.0 to 8.9 ± 0.6 Ma, respectively. On Paros, ZFT and AFT ages range from 13.1 ± 1.4 Ma to 11.1 ± 1.0 Ma and 12.7 ± 2.8 Ma to 10.5 ± 2.0 Ma while the (U–Th)/He zircon ages are slightly younger between 8.3 ± 0.4 Ma and 9.8 ± 0.3 Ma. All ages consistently decrease northwards in the direction of hanging wall transport. Most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of 6–8 km Myr− 1 on the NPEFS across the brittle/ductile transition. Even the intrusion of a large granodiorite body into the narrowing fault zone at 12 Ma on Naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate. The data also show that the NPEFS accomplished a minimum total offset of 50 km between 16 and 8 Ma
Structural contacts in subduction complexes and their tectonic significance: the Late Palaeozoic coastal accretionary wedge of central Chile
Understanding the contact between the very low-grade metagreywacke of the Eastern Series and
high-pressure metamorphosed schist of the Western Series in the Late Palaeozoic accretionary wedge of
central Chile is fundamental for the understanding of the evolution of ancient accretionary wedges. We show
the progressive development of structures and finite strain from the least deformed rocks in the eastern part of
the Eastern Series of the accretionary wedge to high-pressure schist of the Western Series at the Pacific coast.
Upright chevron folds of sedimentary layering are associated with an axial-plane foliation, S1. As the F1 folds
became slightly overturned to the west, S1 was folded about west-vergent open F2 folds and an S2 axial-plane
foliation developed. Near the contact between the Western and Eastern Series S2 represents a penetrative
subhorizontal transposition foliation. Towards the structurally deepest units in the west the transposition
foliation becomes progressively flattened. Finite-strain data as obtained by Rf /ϕ analysis in metagreywacke
and X-ray texture goniometry in phyllosilicate-rich rocks show a smooth and gradual increase in strain
magnitude from east to west. Overturned folds and other shear-sense indicators show a uniform top-to-the-west
shear sense in moderately deformed rocks, whereas the shear sense is alternating top-to-the-west and top-tothe-
east in the strongly flattened high-pressure rocks of the Western Series near the Pacific coast. We interpret
the progressive structural and strain evolution across the contact between the two series to reflect a continuous
change in the mode of accretion in the subduction wedge. Initially, the rocks of the Eastern Series were
frontally accreted to the pre-Andean margin before c. 300 Ma. Frontal accretion caused horizontal shortening,
and upright folds and subvertical axial-plane foliations developed. At c. 300 Ma the mode of accretion
changed and the rocks of the Western Series were underplated below the Andean margin. This basal accretion
caused a major change in the flow field within the wedge and gave rise to vertical shortening and the
development of the penetrative subhorizontal transposition foliation. Subsequent differential exhumation was
resolved gradually over a wide region, implying that exhumation was not tectonically controlled.researc
Structural contacts in the Late Paleozoic accretionary wedge of central Chile and their tectonic significance for the evolution of the accretionary complex
The Chilean accretionary wedge is part
of a Late Paleozoic subduction complex
that developed during subduction of the
Pacific plate underneath South America.
The wedge is commonly subdivided
into a structurally lower Western Series
and an upper Eastern Series. Understanding
the contact between both series
has been a long standing problem
and is fundamental for the understanding
of the evolution of the wedge system.
We show the progressive development
of structures and finite strain from
the least-deformed rocks in the eastern
part of the Eastern Series of the accretionary
wedge to higher grade schist of
the Western Series at the Pacific coast...conferenc
Extensional faulting on Tinos island, Aegean sea, Greece: How many detachments?
Zircon and apatite fission track (ZFT and AFT) and (U-Th)/He, 40Ar/39Ar hornblende, and U-Pb zircon ages from the granites of Tinos Island in the Aegean Sea, Greece, suggest, together with published ZFT data, that there are three extensional detachments on Tinos. The Tinos granites crosscut the Tinos detachment. Cooling of the granites was controlled by the Livadi detachment, which occurs structurally above the Tinos detachment. Our U-Pb zircon age is 14.6 ± 0.2 Ma and two 40Ar/39Ar hornblende ages are 14.4 ± 0.4 and 13.7 ± 0.4 Ma. ZFT and AFT ages go from 14.4 ± 1.2 to 12.2 ± 1.0 Ma and 12.8 ± 2.4 to 11.9 ± 2.0 Ma. (U-Th)/He ages are from 10.4 ± 0.2 to 9.9 ± 0.2 Ma (zircon) and 11.9 ± 0.5 to 10.0 ± 0.3 Ma (apatite). All ages decrease northeastward in the direction of hanging wall transport on the Livadi detachment and age-distance relationships yield a slip rate of 2.6 (+3.3 / −1.0) km Ma−1. This rate is smaller than a published slip rate of 6.5 km Ma−1 for the Vari detachment, which is another detachment structurally above the Tinos detachment. Because of the different rates and because published ZFT ages from the footwall of the Vari detachment are ∼10 Ma, we propose that the Vari detachment has to be distinguished from the older Livadi detachment. We discuss various models of how the extensional detachments may have evolved and prefer a scenario in which the Vari detachment cut down into the footwall of the Livadi detachment successively exhuming deeper structural units. The thermochronologic ages demonstrate the importance of quantitative data for constraining localization processes during extensional deformation
Petrochronology of polygenetic white micas (Naxos, Greece)
Naxos in the Greek Cyclades preserves a type example of polymetamorphism. The southern and northern parts of the island record different Tertiary P–T histories between Eocene and Miocene times, including a blueschist facies event, one or more amphibolite/greenschist facies overprint(s) and contact metamorphism. Age attributions for these events are inconsistent in the literature. Here, we propose a new approach that combines electron probe microanalyzer (EPMA) characterization of the white mica (WM) with Ar-Ar–Rb-Sr multichronometry. Textural–petrographic–compositional observations reveal that the polygenetic WM consists of five different generations: pre-Eocene relicts, paragonite, high-Si phengite, low-Si phengite and muscovite. EPMA mapping of four WM samples, previously analysed by Rb-Sr, reveals major element compositions heterogeneous down to the μm scale. Each WM consists of chemically distinct generations, documenting submicron-scale retrogression of high-pressure (HP) phengite grains to muscovite. Four WM samples from a N-S traverse across the island were analysed by Ar-Ar stepheating, comparing coarse and fine sieve size fractions to obtain overdetermined K-Ar systematics. Fine sieve fractions are richer in Cl than coarse ones. Linear arrays in Cl/K-age isotope correlation diagrams show two predominant WM generations (one Cl-poor at ca. 38 Ma and one Cl-rich at <20 Ma). A lower-grade sample from southern Naxos was less pervasively recrystallized, provides older ages and preserves at least three WM generations, including a relict WM with a pre-Palaeocene K-Ar age, consistent with the high Ar retentivity of WM in the absence of complete recrystallization. The age of the Cl-poor end-member WM approximates the age of the HP event, 38 Ma. Ar inheritance in Cretaceous mica relicts is heterogeneous at the single-grain scale. Comparing the degassing rates of the WM fractions rules out ‘multidomain’ diffusion. As no sample is monomineralic, the degassing rate of each polygenetic mica is instead controlled by the mass balanced sum of the unrelated rate constants of its constituent minerals. Given the commonness of zoned and composite micas, the approach detailed here is potentially useful for reconstructing polyphase metamorphic histories worldwide
Bacteremia and Antimicrobial Drug Resistance over Time, Ghana
Bacterial distribution and antimicrobial drug resistance were monitored in patients with bacterial bloodstream infections in rural hospitals in Ghana. In 2001–2002 and in 2009, Salmonella enterica serovar Typhi was the most prevalent pathogen. Although most S. enterica serovar Typhi isolates were chloramphenicol resistant, all isolates tested were susceptible to ciprofloxacin
Physiology and pathology of eosinophils: Recent developments: Summary of the Focus Workshop Organized by DGAKI.
Over the last century, eosinophils have been regarded ambiguously either as 'friends' or 'foes'. Recent developments have greatly enhanced our understanding of the role and function of eosinophils in health and disease. Pathogenic eosinophilic inflammation can lead to severe diseases in various organs, such as the gastrointestinal tract, airways, heart and skin. In a 2-day focus workshop of the German Society for Allergology and Clinical Immunology (DGAKI), the state of the art was discussed and practical recommendations for diagnosis and treatment of eosinophilic diseases, with a particular focus on new biologics, such as anti-interleukin 5 and anti-interleukin 5R, were derived
Electron penetration in the nucleus and its effect on the quadrupole interaction
A series expansion of the interaction between a nucleus and its surrounding
electron distribution provides terms that are well-known in the study of
hyperfine interactions: the familiar quadrupole interaction and the less
familiar hexadecapole interaction. If the penetration of electrons into the
nucleus is taken into account, various corrections to these multipole
interactions appear. The best known one is a scalar correction related to the
isotope shift and the isomer shift. This paper discusses a related tensor
correction, which modifies the quadrupole interaction if electrons penetrate
the nucleus: the quadrupole shift. We describe the mathematical formalism and
provide first-principles calculations of the quadrupole shift for a large set
of solids. Fully relativistic calculations that explicitly take a finite
nucleus into account turn out to be mandatory. Our analysis shows that the
quadrupole shift becomes appreciably large for heavy elements. Implications for
experimental high-precision studies of quadrupole interactions and quadrupole
moment ratios are discussed. A literature review of other small quadrupole-like
effects is presented as well
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