The Algodões amphibolite–tonalite gneiss sequence,Borborema Province,NE Brazil: Geochemical and geochronological evidence for Palaeoproterozoic accretion of oceanic plateau/back-arc basalts and adakitic plutons

Field relationships, geochemistry and U–Pb zircon and Sm–Nd whole-rock geochronology are used to constrain the genesis of Palaeoproterozoic massive amphibolites and orthogneisses of the Algodões Sequence, Central Ceará Domain, Northeastern Brazil. The 2236 ± 55 Ma-old Algodões amphibolites show trace element geochemical signature and positive εNd(t) values similar to Phanaerozoic oceanic plateau basalts and less often to back-arc basalts. The amphibolites are intruded by tonalitic to quartz-dioritic gneisses of the Cipó unit which were dated at 2160–2170 Ma and 2130 Ma. Neodimium isotope data and whole-rock geochemistry for these gneisses show they are akin to juvenile arc plutonics of the adakitic suite. The likely occurrence of Palaeoproterozoic plateau basalts and arc plutonics in this part of northeastearn Brazil render comparisons with similar terranes elsewhere in South America and Africa, which in turn indicate a significant contribution of accretionary orogens in the early assembly of the Columbia Supercontinent.     

A combined modelling and geochemical study of the fate of terrigenous inputs from mixed natural and mining sources in a coral reef lagoon (New Caledonia)

Open-cast mining for Ni, Cr and Co was conducted in the south-west part of New Caledonia during the 20th century. Abandoned mining and prospecting sites were severely affected by erosion, resulting in an increase in the load of terrigenous particles transported to the coral reef lagoon. This article assesses the impact of a typical small catchment area (La Coulée River, 85 km2 watershed) on two bays (Boulari and Sainte Marie) located near Noumea, New Caledonia's main city. This multi-disciplinary study combines geochemical, sedimentological, and hydrodynamic approaches. Ni and Cr concentrations contained in the geochemical matrix phase of the pelitic fraction were determined. The study of the geochemical signatures together with sedimentological data and 3D numerical simulations of the transport of non-settling particles throughout the lagoon demonstrated that terrigenous inputs from the Coulée River were mainly transported and deposited along the shoreline, reaching areas as distant as Sainte Marie Bay. Although quantitatively low (about 3% of the pelite mass of the bay sediments), the terrigenous inputs in Sainte Marie Bay originating from the Coulée River could be traced. The metal content in suspended matter was over 7000 mg kg(-1) for Ni and 4200 mg kg(-1) for Cr.     

Super-parameterization in ocean modeling: Application to deep convection

We explore the efficacy of “super parameterization” (SP) in ocean modeling in which local 2-d non-hydrostatic plume-resolving fine-grained (FG) models are embedded at each vertical column of a coarse-grained (CG) hydrostatic model. A general multi-scale algorithm is described in which tendencies from the FG models are projected onto the CG model which in turn constrains the average state of the FG models, coupling the two models together. The approach is tested in the context of models of open ocean deep convection and compared with a pure hydrostatic, coarse resolution model using convective adjustment (HYD) and a full 3-d non-hydrostatic plume-resolving simulation (NH). The SP model is found to be greatly superior to HYD at much less computational cost than the fully non-hydrostatic calculation.     

Structure and metamorphism of the Karakorum gneisses in the Braldu-Baltoro Valley (North Pakistan)

Abstract

The Karakorum gneisses outcrop north of the complex suture separating the Indian-Pakistan plate from the Europe-Asia block; they grade to deformed earlier members of the Karakorum batholith ranging in age from Cretaceous to Miocene and are cross-cut by its later members. The main interest of the region lies in the fact that very young high-grade gneisses (Miocene), outline the southern edge of the Europe-Asia Plate. The tectonic and metamorpic evolution of the Braldu-Baltoro region is interpreted here as resulting from a poly phased history. The following structural sequence has been defined : - (1) A Dl isoclinal folding was accompanied by subparallel healed shear zones and by intense boudinage, and cross-cut by a dense net of post-Dl hetero-geneous leucogranitic veins and stocks; - (2) a major phase of EW trending recumbent folds (D2), is followed by (3) large open D3 folds generating EW trending domai structures (Dassu and Panmah domes); and (4) a late set of brittle to locally more ductile structures such as the southern thrust contact of the Karakorum gneisses with the Shyok suture zone. The sequence proposed here differs from other interpretations (Rex et al. 1988). We consider that the Dl event only may be attributed to the main India-Asia collision and that the D2-D3 events, interpreted as having occurred in a continuum, correspond to a late reactivation of the major thrusts and sutures related to continuing continental subduction.

A Dl-related intermediate pressure assemblage is preserved (Grt-St-Ky) in the upper levels of the tectonic pile; the estimated PT conditions determined are 10-4 Kb and 700°--525°C. In the core of the large D3 domes, late granoblastic recrystallization is widespread together with almost complete S1-S2 transposition, incipient melting and development of a low-pressure sillimanite-bearing assemblage where relicts of higher pressure minerals are locally preserved. Corresponding PT conditions are 650°-550°C and a lower pressure (5.5 to 2.5 Kb). As most of the observed structures at the lower levels (mineral lineations, boudinage) are clearly associated with (or reworked by) D2 and accentuated by D3 which was accompagnied by partial melting, D2 and D3 are interpreted as representing a continuum developed in the same PT field. It can be assumed also that the Baltoro granite was emplaced by the end of this combined D2-D3 event. From the Miocene ages published for the Baltoro granite (20 Ma to 8 Ma), the low-pressure evolution of the Karakorum gneisses may represent a very young high-grade assemblage. The age of Dl is less defined but at least older than 36 Ma old leucogranites.

The sharp contact along the Shyok Suture zone, interpreted as a large thrust (Main Karakorum Thrust - MKT) of this young high-grade metamorphic terrene against the older (older than 30-45 Ma from late undeformed intrusives) Kohistan-Ladakh island-arc domain, is interpreted, following Mattauer (1985), as resulting from the interaction between the still-ongoing northward movement of the Indo-Pakistan plate and an opposite southward continental subduction, seismically active, operating in Pamir.     

Chemical and dynamical evolution of spiral galaxies

We investigate the consequences of the hypothesis of the secular evolution (growth of the bulge from disc material via a bar and temporal evolution of the Hubble sequence) on the chemical evolution of a galaxy. We present the first dynamical and chemical results of our 3D tree-SPH simulations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Groundwater occurrence in cold environments: examples from Nunavik,Canada

Water availability and management issues related to the supply of drinking water in northern communities are problematic in Canada. While rivers and lakes are abundant, they are vulnerable to contamination and may become dry in winter due to freezing. Groundwater can often provide a more secure and sustainable water source, however its availability is limited in northern Canada due to the presence of permafrost. Moreover, the exploitation of northern aquifers poses a dual challenge of identifying not only permafrost-free areas, but also permeable areas which will allow groundwater recharge and exploitation. Suitable aquifers are not as common in northern Canada since the shallow subsurface is mostly composed of low-permeability crystalline rocks or unconsolidated sediments of glacial origin that are highly heterogeneous. In order to investigate groundwater occurrence and associated geological contexts in Nunavik (northern Quebec, Canada), along with exploring how these resources will evolve in response to climate change, field and compilation work were conducted in the surroundings of the four villages of Salluit, Kuujjuaq, Umiujaq and Whapmagoostui-Kuujjuarapik. These villages are located in different permafrost zones, ranging from continuous to discontinuous, as well as in different geological environments. It was found that despite the ubiquitous presence of permafrost, unfrozen aquifers could be identified, which suggests that groundwater may be available as a source of drinking water for small communities. Expected climate change, with predicted permafrost thawing and increases in temperature and precipitation, should enhance groundwater availability and may contribute to a more secure source of drinking water for northern communities.     

A kinematic GPS methodology for sea surface mapping,Vanuatu

During the past few decades, satellite altimetry has brought tremendous new knowledge about the spatial and temporal variations of sea surface heights over the Earth’s oceans. However, the precision is limited over short wavelengths and in coastal areas, and other methods such as kinematic GPS may be needed to fill in this information. We present kinematic GPS work aimed at mapping the sea surface height, with special attention to the precision one can expect. Active marine subduction zones, like the Vanuatu archipelago, may present short wavelength, high amplitude undulations of the sea surface height that are difficult to map with satellite altimetry. This paper presents the methodology used around Santo Island, in Vanuatu, to obtain a well-resolved local sea surface map with a precision of 5–15 cm limited by the sea conditions and the distance from the coastal reference station. We present the results of three campaigns in 2004, 2006 and 2007. Careful observation of the ship behaviour along the surveys as well as simultaneous recording of the ship attitude variations is mandatory to obtain reliable results. We show that the ship GPS antenna height varies with the ship’s velocity and we suggest a method to correct this effect. The final precision is estimated using the crossover differences method.     

Exploring image data assimilation in the prospect of high-resolution satellite oceanic observations

Satellite sensors increasingly provide high-resolution (HR) observations of the ocean. They supply observations of sea surface height (SSH) and of tracers of the dynamics such as sea surface salinity (SSS) and sea surface temperature (SST). In particular, the Surface Water Ocean Topography (SWOT) mission will provide measurements of the surface ocean topography at very high-resolution (HR) delivering unprecedented information on the meso-scale and submeso-scale dynamics. This study investigates the feasibility to use these measurements to reconstruct meso-scale features simulated by numerical models, in particular on the vertical dimension. A methodology to reconstruct three-dimensional (3D) multivariate meso-scale scenes is developed by using a HR numerical model of the Solomon Sea region. An inverse problem is defined in the framework of a twin experiment where synthetic observations are used. A true state is chosen among the 3D multivariate states which is considered as a reference state. In order to correct a first guess of this true state, a two-step analysis is carried out. A probability distribution of the first guess is defined and updated at each step of the analysis: (i) the first step applies the analysis scheme of a reduced-order Kalman filter to update the first guess probability distribution using SSH observation; (ii) the second step minimizes a cost function using observations of HR image structure and a new probability distribution is estimated. The analysis is extended to the vertical dimension using 3D multivariate empirical orthogonal functions (EOFs) and the probabilistic approach allows the update of the probability distribution through the two-step analysis. Experiments show that the proposed technique succeeds in correcting a multivariate state using meso-scale and submeso-scale information contained in HR SSH and image structure observations. It also demonstrates how the surface information can be used to reconstruct the ocean state below the surface.     

Geochemical characteristics of an intracontinental shoshonitic association: the example of the Damavand volcano,IranCaractéristiques géochimiques d'une association shoshonitique intracontinentale : exemple du Damavand,Iran

Lavas of the Quaternary Damavand volcano (Central Alborz, Northern Iran) have been analysed for major and trace elements. The shoshonitic character of this magmatic series is definitively confirmed and most samples correspond to banakites. The Nb, Ta and Ti negative anomalies observed in all lavas suggest that they derive from of a sub-continental lithosphere metasomatised in a subduction setting. Their variable trace element enrichments are explained both by mineral fractionation and by crustal contamination in a shallow magmatic chamber. To cite this article: H. Mehdizadeh et al., C. R. Geoscience 334 (2002) 111–117.