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.     

Crustal age domains and metamorphic reworking of the deep crust in Northern-Central Tanzania: a U/Pb zircon and monazite age study

Geochronological data are presented from Northern Tanzania, where deep-crustal terranes of different age are exposed. Stacking of these terranes was diachronous with one peak around 640 Ma, defined as East African Orogeny, and final consolidation at 550–580 Ma, that is defined as Kuunga Orogeny. This later event is predominant in the Western Granulite Belt of northern Tanzania and related to thrusting onto the Tanzanian Craton. The Tanzania Craton itself experienced a polycyclic history; age domains around 2.64 Ga prevail in the studied samples. There is no evidence of the Paleoproterozoic Usagaran Belt in northern Tanzania. Here the gneisses contain relicts of reworked Archean basement and are therefore considered part of the Western Granulites. Inliers of the Western Granulites are also found in the cores of marble antiforms that are part of the upper, sedimentary sequence of the Eastern Granulites. Those inliers formed during the Kuungan orogenic phase when the Eastern Granulites have taken their final position and were folded together with the Western Granulites.     

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.     

Probabilistic Appraisal of Earthquake Hazard Parameters Deduced from a Bayesian Approach in the Northwest Frontier of the Himalayas

A straightforward Bayesian statistic is applied in five broad seismogenic source zones of the northwest frontier of the Himalayas to estimate the earthquake hazard parameters (maximum regional magnitude M _max, β value of G–R relationship and seismic activity rate or intensity λ). For this purpose, a reliable earthquake catalogue which is homogeneous for M _W ≥ 5.0 and complete during the period 1900 to 2010 is compiled. The Hindukush–Pamir Himalaya zone has been further divided into two seismic zones of shallow (h ≤ 70 km) and intermediate depth (h > 70 km) according to the variation of seismicity with depth in the subduction zone. The estimated earthquake hazard parameters by Bayesian approach are more stable and reliable with low standard deviations than other approaches, but the technique is more time consuming. In this study, quantiles of functions of distributions of true and apparent magnitudes for future time intervals of 5, 10, 20, 50 and 100 years are calculated with confidence limits for probability levels of 50, 70 and 90 % in all seismogenic source zones. The zones of estimated M _max greater than 8.0 are related to the Sulaiman–Kirthar ranges, Hindukush–Pamir Himalaya and Himalayan Frontal Thrusts belt; suggesting more seismically hazardous regions in the examined area. The lowest value of M _max (6.44) has been calculated in Northern-Pakistan and Hazara syntaxis zone which have estimated lowest activity rate 0.0023 events/day as compared to other zones. The Himalayan Frontal Thrusts belt exhibits higher earthquake magnitude (8.01) in next 100-years with 90 % probability level as compared to other zones, which reveals that this zone is more vulnerable to occurrence of a great earthquake. The obtained results in this study are directly useful for the probabilistic seismic hazard assessment in the examined region of Himalaya.     

Watershed prioritisation for soil conservation — a GIS approach

In order to check the premature siltation of the reservoirs, the Government of India has launched the schemes of soil conservation and integrated watershed management in the catchments of River Valley Projects (RVPs) and Flood Prone Rivers. However, due to lack of sufficient funds and manpower for surveys and soil and water conservation programmes, a priority approach for treatment of the watersheds was developed and followed for implementation of soil conservation and watershed management schemes in India.

This paper deals with the prioritisation of watersheds using GIS approach in a part of Musi river catchment in RR district of Andhra Pradesh. The GIS software package, ARC/INFO version 5.0, was used to digitize, edit, display, analyse and plot the maps. It was concluded that by the creation of computerised data base for the maps, the composite map generation and calculation of area statistics are performed much faster and more accurately compared to the conventional method.     

A hierarchical generalized linear model with variable selection: studying the response of a representative fish assemblage for large European rivers in a multi-pressure context

Global change calls for an understanding of how temperature and flow regimes influence aquatic ecosystems. Fish assemblages are a major component of river ecosystems and are thought to exhibit more integrative informative responses than single species to environmental variations, whether rare and sudden or gradual and continuous. The use of long-term datasets is thus of primary importance, allied to statistical modeling. For each of three previously identified species clusters, we performed Bayesian variable selection and inference within a hierarchical log Poisson Generalized Linear Model using a spike and slab normal prior to pinpoint which subset of environmental variables is of importance for each fish assemblage. Fish counts from electrofishing experiments are known to provide overdispersed data and, not surprisingly, the contribution of recorded environmental effects is found to be weak compared with those of other intra-assemblage sources of variation. The posterior distribution of the regression parameters is in coherence with what was expected from biological knowledge of the three species clusters. In particular, thermophilic species tend to benefit from warmer waters, whereas the recruitment of cold water species decreases due to global warming effects. Our study provides an example of the advantages of hierarchical modeling for quantifying interspecies ecological effects and selecting common environmental variables of importance.     

A survey of near-mean-motion resonances between Venus and Earth

It is known since the seminal study of Laskar (1989) that the inner planetary system is chaotic with respect to its orbits and even escapes are not impossible, although in time scales of billions of years. The aim of this investigation is to locate the orbits of Venus and Earth in phase space, respectively, to see how close their orbits are to chaotic motion which would lead to unstable orbits for the inner planets on much shorter time scales. Therefore, we did numerical experiments in different dynamical models with different initial conditions—on one hand the couple Venus–Earth was set close to different mean motion resonances (MMR), and on the other hand Venus’ orbital eccentricity (or inclination) was set to values as large as e = 0.36 (i = 40°). The couple Venus–Earth is almost exactly in the 13:8 mean motion resonance. The stronger acting 8:5 MMR inside, and the 5:3 MMR outside the 13:8 resonance are within a small shift in the Earth’s semimajor axis (only 1.5 percent). Especially Mercury is strongly affected by relatively small changes in initial eccentricity and/or inclination of Venus, and even escapes for the innermost planet are possible which may happen quite rapidly.