Diffuse bifurcations engraving diverse shear bands in granular materials

Shear bands with characteristic spatial patterns observed in an experiment for a cubic or parallelepiped specimen of dry dense sand were simulated by numerical bifurcation analysis using the Cam‐clay plasticity model. By incorporating the subloading surface concept into the plasticity model, the model became capable of reproducing hardening/softening and contractive/dilative behavior observed in the experiment. The model was reformulated to be compatible with the multiplicative hyperelasto‐plasticity for finite strains. This enhanced constitutive model was implemented into a finite‐element code reinforced by a stress updating algorithm based on the return‐mapping scheme, and by an efficient numerical procedure to compute critical eigenvectors of elastoplastic tangent stiffness matrix at bifurcation points. The emergence of diamond‐ and column‐like diffuse bifurcation modes breaking uniformity of the materials, followed by the evolution of shear bands through strain localization, was observed in the analysis. In the bifurcation analysis of plane strain compression test, unexpected bifurcation modes, which broke out‐of‐plane uniformity and led to 3‐dimensional diamond‐like patterns, were detected. Diffuse bifurcations, which were difficult to observe by experiments, have thus been found as a catalyst creating diverse shear band patterns.     

Fluid-assisted granulite metamorphism: A continental journey

Lower crustal granulites, which constitute the base of all continents, belong to two series: high-pressure granulites generated by crustal thickening (subduction) and (ultra)high-temperature granulites associated with crustal extension. Fluid inclusions and metasomatic features indicate that the latter were metamorphosed in the presence of low-water activity fluids (high-density CO₂ and brines), which have invaded the lower crust at peak metamorphic conditions (fluid-assisted granulite metamorphism). High-pressure and (ultra)high-temperature granulites commonly occur along elongated paired belts. They were formed, from the early Proterozoic onwards, during a small number of active periods lasting a few hundreds of m.y. These periods were separated from each other by longer periods of stability. Each period ended with the formation of a supercontinent whose amalgamation coincided with low- to medium pressure (ultra)high-temperature granulite metamorphism, immediately before continental break-up. It is proposed that large quantities of mantle-derived CO₂ stored in the lower crust at the final stage of supercontinent amalgamation, are released into the hydro- and atmosphere during breakup of the supercontinent. Fluid-assisted granulite metamorphism, therefore, appears to be an important mechanism for transferring deep mantle fluids towards the Earth's surface. Possible consequences were, for example, the sudden end of Proterozoic glaciations, as well as the post-Cambrian explosion of life.     

Phillipsite formation in nephelinitic rocks in response to hydrothermal alteration at Mount Etinde,Cameroon

Fresh nephelinitic rocks and hydrothermally altered rocks at Mount Etinde (Cameroon Volcanic Line, West Africa) have been studied by combined whole rock geochemistry (ICP-MS), mineralogy and mineral chemistry (SEM-EDS, WDS, XRD) techniques. The nephelinites have feldspathoids, clinopyroxene, perovskite and titanomagnetite as the principal minerals in the mode with subsidiary apatite and sphene. The mineralogy of their hydrothermally altered counterparts includes phillipsite, calcite and analcime which are secondary phases developed in response to hydrothermal fluid events. Correspondingly, the bulk rock geochemical data show elevated SiO₂, CaO, Na₂O and K₂O concentrations with increasing alteration and Al₂O₃ and Fe₂O₃ depletion while MgO, MnO and TiO₂ concentrations are largely unaffected. The nephelinites also have high concentrations of LILLE, HFSE and REE and upon hydrothermal alteration they show an enrichment of LREE and MREE over HREE. Phillipsite is the principal alteration mineral in the rocks and it occurs along cracks, vesicles and also forms alteromorphs after feldspathoids. The Ce content of these categories of phillipsite varies. Phillipsite along cracks is richer in Ce while phillipsite associated with calcite has lower Ce concentration and the phillipsite alteromorphs very little or no Ce. Various stages of fluid circulation are proposed hereby to explain the variations in phillipsite generation and composition.     

Modeling policy and agricultural decisions in Afghanistan

Afghanistan is responsible for the majority of the world’s supply of poppy crops, which are often used to produce illegal narcotics like heroin. This paper presents an agent-based model that simulates policy scenarios to characterize how the production of poppy can be dampened and replaced with licit crops over time. The model is initialized with spatial data, including transportation network and satellite-derived land use data. Parameters representing national subsidies, insurgent influence, and trafficking blockades are varied to represent different conditions that might encourage or discourage poppy agriculture. Our model shows that boundary-level interventions, such as targeted trafficking blockades at border locations, are critical in reducing the attractiveness of growing this illicit crop. The principle of least effort implies that interventions decrease to a minimal non-regressive point, leading to the prediction that increases in insurgency or other changes are likely to lead to worsening conditions, and improvements require substantial jumps in intervention resources.     

Tectonic stress regimes,rift extension and transform motion: the South Iceland Seismic Zone

Abstract

The South Iceland Seismic Zone (SISZ) is located at the junction of three rift segments in southwestern Iceland. The presence of different types of faulting and of differently orientated subgroups in Upper Pliocene to Holocene formations indicate polyphase tectonism. We measured 736 minor faults at 25 sites. Two types of relationships between stress regimes are represented. The first type, named IDS (inhomogeneous data set), is characterized by the presence of two types of fault mechanisms, normal and strike-slip, consistent with a single direction of extension. The second type, named OSR (opposite stress regimes), is characterized by the presence of perpendicular directions of extensions for a single type (normal or strike-slip) of faulting. Because of contradictory chronological criteria, we infer that the OSR alternated during the brittle tectonic activity of the SISZ. Two stress regimes, primary and secondary, are characterized by directions of extension NW-SE and NE-SW, respectively. The general fracture pattern characterized for the primary stress regime in the SISZ includes NNE-SSW trending right-lateral strike-slip faults, conjugate ENE-WSW trending left-lateral faults and NE-SW normal faults. This distribution is quite consistent with a Riedel- type model of fault pattern in a left-lateral shear zone. The stress states characterized based on analysis of both the earthquake focal mechanisms and the recent faulting sow great similarity in terms of stress directions. The main difference is the larger ratio of strike-slip motions representing 71% of the total population in the case of earthquake focal mechanisms, whereas for the whole set of faults the proportion of strike-slip faulting was 50 %. We explain that a témpora evolution of the tectonic regime in the SISZ region, accompanied by a gradual change in stress field, starts with rift-type pure extension and progressively leads to development of preferentially strike-slip structures in the kinematic context of left- lateral transform motion. © Elsevier, Paris     

Geology and biostratigraphy of the Jurassic and lower cretaceous series to the north of the Lhasa Block (Tibet,China)

Abstract

— The Lhasa Block (s.l.) is bounded to the South by the Tertiary Yarlung Zangbo suture zone and to the North by the terminal Jurassic/earliest Cretaceous Bangong Nu Jiang suture zone. Several tectonostratigraphic units have been recognized in the central-northern part of the Lhasa Block. These are from bottom to top : 1) a thick turbiditic series with a few lenses of allodapic limestones which have yielded an Aalenian — Bajocian foraminiferal assemblage. This series is tectonieally overlain by the Donqiao ophiolite; 2) the continental to shallow marine late Malm to lowermost Cretaceous Zigetang Formation which disconformably overlies the Donqiao ophiolite and 3) continental red detrital rocks or marine Early/Late Aptian boundary to Early Albian foraminifera-rich bedded limestones in which some volcanic rocks are locally interbedded.

We discuss the palaeogeographical distribution and biostratigraphical meaning of some foraminifera (Gutnicella cayeuxi (LUCAS), Palorbitolina fen<ícu/o?(Bl .LMKNBACIl), Praeorbitolina cormyi SCHROEDER and Palor-bilolmoides hedini CHREREHI and ScilKOKDK.lt) and their bearing on the radiometric age of the Aptian-Albian boundary.     

De la distribution des diaclases dans des flexures à la chronologie tectonique : exemples dans le Plateau du Colorado (Utah-Arizona-Nouveau Mexique,U.S.A.)

Résumé

L’établissement de la chronologie relative de systèmes de diaclases ou joints est difficile à établir, surtout si les conditions d’observation ne permettent pas d’analyser de très nombreux recoupements de joints. On peut toutefois situer dans le temps, grâce à une étude principalement géométrique, la formation de familles de joints affectant des structures flexurées d’âge connu.

Les flexures monoclinales, d’âge laramien, sont les structures les plus caractéristiques du Plateau du Colorado. Les multiples systèmes de joints qui les affectent ont pour orientations : N000° à N020°, NO40°, N080°-N090°, Ν110^ο-Ν120°. N140°-N150° et Ν170°. L’étude des relations géométriques entre ces joints et les flexures, complétée par des analyses qualitatives d’intersection des joints et de rugosité de surface, permettent la classification suivante : 1) des joints anté-laramiens N080°-N090° puis N000° et Ν150°, 2) des joints N110° et N020° à N040 contemporains de la compression laramienne, 3) des joints post-laramiens Ν170° dus aux mécanismes extensifs Basin & Range.

A partir de cette étude, plusieurs idées plus générales sur la structuration des zones peu déformées sont proposées. La chronologie de la facturation ainsi définie peut également être appliquée aux zones tabulaires du Plateau du Colorado.     

Long-term evolution of the spin of Mercury: I. Effect of the obliquity and core-mantle friction

The present obliquity of Mercury is very low (less than 0.1°), which led previous studies to always adopt a nearly zero obliquity during the planet’s past evolution. However, the initial orientation of Mercury’s rotation axis is unknown and probably much different than today. As a consequence, we believe that the obliquity could have been significant when the rotation rate of the planet first encountered spin-orbit resonances. In order to compute the capture probabilities in resonance for any evolutionary scenario, we present in full detail the dynamical equations governing the long-term evolution of the spin, including the obliquity contribution.The secular spin evolution of Mercury results from tidal interactions with the Sun, but also from viscous friction at the core-mantle boundary. Here, this effect is also regarded with particular attention. Previous studies show that a liquid core enhances drastically the chances of capture in spin-orbit resonances. We confirm these results for null obliquity, but we find that the capture probability generally decreases as the obliquity increases. We finally show that, when core-mantle friction is combined with obliquity evolution, the spin can evolve into some unexpected configurations as the synchronous or the 1/2 spin-orbit resonance.     

几次大震前的地面和空间电磁场变化

本文应用非线性谱估计方法(MEM方法)、电/磁场脉冲能量统计方法,处理了四川汶川MS8.0 (2008)、新疆于田MS7.3 (2008)和青海玉树MS7.1 (2010) 地震及强余震附近的地面电/磁场和Demeter卫星电离层磁场观测数据,研究了地震电/磁场变化,得到:在震中附近及周围出现了震前地电/地磁场极低频成...