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 $^{39}$Ar-$^{40}$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 $^{39}$Ar-$^{40}$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