The Effects of Disequilibrium and Deformation on the Mineralogical Evolution of Quartz Diorite During Metamorphism in the Eclogite Facies

In the Sesia Zone, Western Alps, a large volume of orthogneiss formed as a result of eclogite fades metamorphism and deformation of quartz diorite during early Alpine underthrusting and subduction. Rare lenses of undeformed metaquartz diorite, preserved within the orthogneiss, represent an early stage in the evolution of this latter rock type. The metamorphic and microstructural evolution of the orthogneiss in the eclogite fades has been reconstructed from studies of gradational contacts between undeformed and strongly deformed rocks. High pressure transformations of the original igneous plagioclase + biotite + quartz assemblage to jadeitic pyroxene (Jd0.95 -0.85 + zoisite + quartz + garnet + 2 muscovites developed prior to deformation. Slow intergranular diffusion resulted in a state of disequilibrium between small textural domains in the metaquartz diorite. The compositions of the phases of the undeformed metaquartz diorite do not reflect the bulk rock composition, but were controlled by their position relative to reactant phases. The jadeitic pyroxenes, for example, formed in localized domains which originally consisted of sodic plagioclase whereas omphacite was the equilibrium pyroxene for the bulk rock composition. Mineralogical changes which occurred during subsequent deformation of the metaquartz diorite are interpreted as resulting from a progressive enlargement of equilibrium domains and the partial equilibration of mineral compositions to the bulk rock composition rather than from changes in pressure and temperature. Initially during high-strain deformation, fine-grained aggregates of jadeitic pyroxene + quartz + zoisite (originally pseudomorphing plagioclase) are inferred to have deformed by a mechanism of grain boundary sliding accommodated by diffusive mass transfer. Muscovite and garnet compositions homogenized during the deformation but due to slow intracrystalline diffusion, pyroxene compositions (Jd0.95 -0.80) remained metastable. The coarsening of pyroxene eventually terminated deformation by grain boundary sliding and this mineral subsequently deformed by intracrystalline plastidty. This latter process was accompanied by and perhaps catalysed a change in pyroxene composition from metastable jadeite towards omphacite by a reaction involving the resorption of garnet and the nucleation and growth of paragonite. The resulting orthogneiss consists of quartz + omphadte + garnet + phengite + paragonite + zoisite. The rock is characterized by a broad range of pyroxene compositions (Jd0.8 -0.5) due to the incomplete equilibration of this mineral to the bulk rock composition and a lack of Fe-Mg exchange equilibrium between pyroxene and garnet. However, in contrast to the undeformed metaquartz diorite, there are no obvious textural indications of disequilibrium between phases in the orthogneis

Three-Dimensional Mechanics of Yakutat Convergence in the Southern Alaskan Plate Corner

Three-dimensional numerical models are used to investigate the mechanical evolution of the southern Alaskan plate corner where the Yakutat and the Pacific plates converge on the North American plate. The evolving model plate boundary consists of Convergent, Lateral, and Subduction subboundaries with flow separation of incoming material into upward or downward trajectories forming dual, nonlinear advective thermal/mechanical anomalies that fix the position of major subaerial mountain belts. The model convergent subboundary evolves into two teleconnected orogens: Inlet and Outlet orogens form at locations that correspond with the St. Elias and the Central Alaska Range, respectively, linked to the East by the Lateral boundary. Basins form parallel to the orogens in response to the downward component of velocity associated with subduction. Strain along the Lateral subboundary varies as a function of orogen rheology and magnitude and distribution of erosion. Strain-dependent shear resistance of the plate boundary associated with the shallow subduction zone controls the position of the Inlet orogen. The linkages among these plate boundaries display maximum shear strain rates in the horizontal and vertical planes where the Lateral subboundary joins the Inlet and Outlet orogens. The location of the strain maxima shifts with time as the separation of the Inlet and Outlet orogens increases. The spatiotemporal predictions of the model are consistent with observed exhumation histories deduced from thermochronology, as well as stratigraphic studies of synorogenic deposits. In addition, the complex structural evolution of the St Elias region is broadly consistent with the predicted strain field evolution. Citation: Koons, P. O., B. P. Hooks, T. Pavlis, P. Upton, and A. D. Barker (2010), Three-dimensional mechanics of Yakutat convergence in the southern Alaskan plate corner, Tectonics, 29, TC4008, doi: 10.1029/2009TC002463

Quantifying extreme behaviour in geomagnetic activity

Understanding the extremes in geomagnetic activity is an important component in understanding just how severe conditions can become in the terrestrial space environment. Extreme activity also has consequences for technological systems. On the ground, extreme geomagnetic behavior has an impact on navigation and position accuracy and the operation of power grids and pipeline networks. We therefore use a number of decades of one-minute mean magnetic data from magnetic observatories in Europe, together with the technique of extreme value statistics, to provide a preliminary exploration of the extremes in magnetic field variations and their one-minute rates of change. These extremes are expressed in terms of the variations that might be observed every 100 and 200 years in the horizontal strength and in the declination of the field. We find that both measured and extrapolated extreme values generally increase with geomagnetic latitude (as might be expected), though there is a marked maximum in estimated extreme levels between about 53 and 62 degrees north. At typical midlatitude European observatories (55–60 degrees geomagnetic latitude), compass variations may reach approximately 3–8 degrees/minute, and horizontal field changes may reach 1000–4000 nT/minute, in one magnetic storm once every 100 years. For storm return periods of 200 years the equivalent figures are 4–11 degrees/minute and 1000–6000 nT/minute

First results of material charging in the space environment

A satellite experiment, designed to measure potential charging of typical thermal control materials at near geosynchronous altitude, was flown as part of the SCATHA program. Direct observations of charging of typical satellite materials in a natural charging event ( 5 keV) are presented. The results show some features which differ significantly from previous laboratory simulations of the environment

An evaluation of the TRIPS computer system

The TRIPS system employs a new instruction set architecture (ISA) called Explicit Data Graph Execution (EDGE) that renegotiates the boundary between hardware and software to expose and exploit concurrency. EDGE ISAs use a block-atomic execution model in which blocks are composed of dataflow instructions. The goal of the TRIPS design is to mine concurrency for high performance while tolerating emerging technology scaling challenges, such as increasing wire delays and power consumption. This paper evaluates how well TRIPS meets this goal through a detailed ISA and performance analysis. We compare performance, using cycles counts, to commercial processors. On SPEC CPU2000, the Intel Core 2 outperforms compiled TRIPS code in most cases, although TRIPS matches a Pentium 4. On simple benchmarks, compiled TRIPS code outperforms the Core 2 by 10% and hand-optimized TRIPS code outperforms it by factor of 3. Compared to conventional ISAs, the block-atomic model provides a larger instruction window, increases concurrency at a cost of more instructions executed, and replaces register and memory accesses with more efficient direct instruction-to-instruction communication. Our analysis suggests ISA, microarchitecture, and compiler enhancements for addressing weaknesses in TRIPS and indicates that EDGE architectures have the potential to exploit greater concurrency in future technologies

Extreme energetic electron fluxes in low Earth orbit: Analysis of POES E > 30, E > 100 and E > 300 keV electrons

Energetic electrons are an important space weather hazard. Electrons with energies less than about 100 keV cause surface charging while higher energy electrons can penetrate materials and cause internal charging. In this study we conduct an extreme value analysis of the maximum 3-hourly flux of E> 30 keV, E> 100 keV and E> 300 keV electrons in low Earth orbit as a function of L∗, for geomagnetic field lines that map to the outer radiation belt, using data from the National Oceanic and Atmospheric Administration (NOAA) Polar Operational Environmental Satellites (POES) from July 1998 to June 2014. The 1 in 10 year flux of E> 30 keV electrons shows a general increasing trend with distance ranging from 1.8×107 cm−2s−1sr−1 at L∗ = 3.0 to 6.6×107 cm−2s−1sr−1 at L∗ = 8.0. The 1 in 10 year flux of E> 100 keV electrons peaks at L∗= 4.5 - 5.0 at 1.9×107 cm−2s−1sr−1 decreasing to minima of 7.1×106 and 8.7×106 cm−2s−1sr−1 at L∗ = 3.0 and 8.0 respectively. In contrast to the E> 30 keV electrons, the 1 in 10 year flux of E> 300 keV electrons shows a general decreasing trend with distance, ranging from 2.4×106 cm−2s−1sr−1 at L∗ = 3.0 to 1.2×105 cm−2s−1sr−1 at L∗= 8.0. Our analysis suggests that there is a limit to the E> 30 keV electrons with an upper bound in the range 5.1×107- 8.8×107 cm−2s−1sr−1. However, the results suggest that there is no upper bound for the E> 100 keV and E> 300 keV electrons

Extreme relativistic electron fluxes in the Earth's outer radiation belt: Analysis of INTEGRAL IREM data

Relativistic electrons (E > 500 keV) cause internal charging and are an important space weather hazard. To assess the vulnerability of the satellite fleet to these so-called “killer” electrons, it is essential to estimate reasonable worst cases, and, in particular, to estimate the flux levels that may be reached once in 10 and once in 100 years. In this study we perform an extreme value analysis of the relativistic electron fluxes in the Earth's outer radiation belt as a function of energy and L∗. We use data from the Radiation Environment Monitor (IREM) on board the International Gamma Ray Astrophysical Laboratory (INTEGRAL) spacecraft from 17 October 2002 to 31 December 2016. The 1 in 10 year flux at L∗=4.5, representative of equatorial medium Earth orbit, decreases with increasing energy ranging from 1.36 × 107 cm−2 s−1 sr−1 MeV−1 at E = 0.69 MeV to 5.34 × 105 cm−2 s−1 sr−1 MeV−1 at E = 2.05 MeV. The 1 in 100 year flux at L∗=4.5 is generally a factor of 1.1 to 1.2 larger than the corresponding 1 in 10 year flux. The 1 in 10 year flux at L∗=6.0, representative of geosynchronous orbit, decreases with increasing energy ranging from 4.35 × 106 cm−2 s−1 sr−1 MeV−1 at E = 0.69 MeV to 1.16 × 105 cm−2 s−1 sr−1 MeV−1 at E = 2.05 MeV. The 1 in 100 year flux at L∗=6.0 is generally a factor of 1.1 to 1.4 larger than the corresponding 1 in 10 year flux. The ratio of the 1 in 10 year flux at L∗=4.5 to that at L∗=6.0 increases with increasing energy ranging from 3.1 at E = 0.69 MeV to 4.6 at E = 2.05 MeV

A new free-surface stabilization algorithm for geodynamical modelling:Theory and numerical tests

The surface of the solid Earth is effectively stress free in its subaerial portions, and hydrostatic beneath the oceans. Unfortunately, this type of boundary condition is difficult to treat computationally, and for computational convenience, numerical models have often used simpler approximations that do not involve a normal stress-loaded, shear-stress free top surface that is free to move. Viscous flow models with a computational free surface typically confront stability problems when the time step is bigger than the viscous relaxation time. The small time step required for stability (<2. Kyr) makes this type of model computationally intensive, so there remains a need to develop strategies that mitigate the stability problem by making larger (at least ~10 Kyr) time steps stable and accurate. Here we present a new free-surface stabilization algorithm for finite element codes which solves the stability problem by adding to the Stokes formulation an intrinsic penalization term equivalent to a portion of the future load at the surface nodes. Our algorithm is straightforward to implement and can be used with both Eulerian or Lagrangian grids. It includes α and β parameters to respectively control both the vertical and the horizontal slope-dependent penalization terms, and uses Uzawa-like iterations to solve the resulting system at a cost comparable to a non-stress free surface formulation. Four tests were carried out in order to study the accuracy and the stability of the algorithm: (1) a decaying first-order sinusoidal topography test, (2) a decaying high-order sinusoidal topography test, (3) a Rayleigh-Taylor instability test, and (4) a steep-slope test. For these tests, we investigate which α and β parameters give the best results in terms of both accuracy and stability. We also compare the accuracy and the stability of our algorithm with a similar implicit approach recently developed by Kaus et al. (2010). We find that our algorithm is slightly more accurate and stable for steep slopes, and also conclude that, for longer time steps, the optimal α controlling factor for both approaches is ~2/3, instead of the 1/2 Crank-Nicolson parameter inferred from a linearized accuracy analysis. This more-implicit value coincides with the velocity factor for a Galerkin time discretization applied to our penalization term using linear shape functions in time

Robust statistical properties of the size of large burst events in AE

Geomagnetic indices provide a comprehensive data set with which to quantify space climate, that is, how the statistical likelihood of activity varies with the solar cycle. We characterize space climate by the AE index burst distribution. Burst sizes are constructed by thresholding the AE time series; a burst is the sum of the excess in the time series for each time interval over which the threshold is exceeded. The distribution of burst sizes is two component with a crossover in behavior at thresholds ≈1000 nT. Above this threshold, we find a range over which the mean burst size varies weakly with threshold for both solar maxima and minima. The burst size distribution of the largest events is exponential. The relative likelihood of these large events varies from one solar maximum and minimum to the next. Given the relative overall activity of a solar maximum/minimum, these results constrain the likelihood of extreme events of a given size