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Maurizot P.,Bureau de Recherches Geologiques et Minieres
New Zealand Journal of Geology and Geophysics

The Adio Limestone at the base of the Palaeogene flysch in the central part of New Caledonia has long been attributed to the Late Eocene. New data now enable us to assign it a Palaeocene age based on a characterisation of its benthic foraminiferal microfauna and also on the presence of an overlying sequence of typical calciturbidite dated as Early Eocene by its planktonic microfauna. This new age assignation indicates a major Palaeocene reorganisation of this northern part of the Norfolk Ridge, which can be associated with the beginning of the convergence phase and the onset of the northeast-dipping subduction in the oceanic South Loyalty Basin. The Adio area in this scenario would represent the initial expression of a foreland bulge uplifted in response to the onset of subduction to the northeast. © 2013 The Royal Society of New Zealand. Source

Tournassat C.,Bureau de Recherches Geologiques et Minieres | Appelo C.A.J.,Hydrochemical Consultant
Geochimica et Cosmochimica Acta

The accessible porosity for Cl- in bentonite is smaller than the the total porosity due to anion repulsion (exclusion) by the surface of montmorillonite, the main mineral in bentonite. The accessible porosity is a function of the bentonite density and the salt concentration. Anion exclusion data were gathered from the literature, reprocessed in a coherent data set, and modelled using four different models. Very simple models, with or without anion access to the interlayer space, are successful in reproducing trends in anion exclusion in bentonite as a function of ionic strength in the external solution and montmorillonite bulk dry densities in the bentonite. However, a model that considers clay microstructure changes as a function of bentonite compaction and ionic strength is necessary to reproduce observed trends in the data for all experimental conditions within a single model. Our predictive model excludes anions from the interlayer space and relates the interlayer porosity to the ionic strength and the montmorillonite bulk dry density. This presentation offers a good fit for measured anion accessible porosities in bentonites over a wide range of conditions and is also in agreement with microscopic observations. © 2011 Elsevier Ltd. Source

Rohmer J.,Bureau de Recherches Geologiques et Minieres
Geophysical Journal International

Fluid injection in deep sedimentary porous formations might induce shear reactivation of reservoir bounding faults. Here, we focus on 'blind' 1000-m-long normal faults (with shear displacement ≤ 10 m), which can hardly be detected using conventional seismic surveys, but might potentially induce seismicity felt on surface. The influence of the dissymmetry in the internal structure of the fractured damage zone DZ is numerically investigated by using 2-D plane-strain finite-element simulations of a 1500-m-deep fluid injection into a porous reservoir. The problem is solved within the framework of fully saturated isothermal elasto-plastic porous media by both accounting for fault slip weakening and shear-induced degradation of fault core permeability. The numerical results show that the presence of a thick fractured hanging wall's DZ (with Young's modulus decreasing with the distance to the fault core due to the presence of fractures) strongly controls the magnitude M of the seismic event induced by the rupture. In the case modelled, M changed by more than 1.0 unit when the DZ thickness is varied from 5 to 50m (M ranges from ~0.1 to ~1.5, i.e. from a 'low' to a 'low-to-moderate' seismicity activity). However, further extending DZ up to 90mhas little effect and the relationship reaches a quasi-horizontal plateau. This tendency is confirmed considering other initial conditions and injection scenarios. Finally, the presence of a thicker footwall DZ appears to lower the influence of hanging wall's DZ, but with lesser impact than the degree of fracturing. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society. Source

Thieblemont D.,Bureau de Recherches Geologiques et Minieres
Quaternary International

Field, granulometric and geochemical data are reported for a set of twenty-one samples collected at six sites spread over Gabon in the extensive lateritic clayey to sandy layer (Cover Horizon) which constitutes the upper part of the profiles of surficial formations. Granulometric analyses show that this material of homogeneous aspect is a mixture of two fractions: a clay fraction and a medium- to coarse-grained sandy fraction. Relative proportions of these two constituents show strong variations from one site to another but are almost constant from the bottom to the top of a given profile. The coarse-grained fraction includes lithic elements of local provenance. Irrespective of their underlying basement, the samples display many common geochemical features including a lack of Ca, Na and Mg. Their major element compositions may be accounted by the combination of a discrete number of mineralogical phases, namely quartz, kaolinite, gibbsite, Fe-oxides/hydroxides and subordinate K-minerals (micas). The normative contents of aluminous and ferrous phases regularly increase with the weight-percent of clays. Conversely, the quartz regularly decreases. This inverse relationship is consistent with mixing between an Al- and Fe-bearing clay fraction and a coarse-grained fraction dominated by quartz. Correlations between most immobile trace elements and the weight-percent of clays indicate a control of those elements by Al- and Fe-bearing phases. The main exception is for Zr and Hf, as those elements are probably associated with zircons in the coarse fraction. Because of the control of the trace element distribution by secondary phases, the geochemical signatures of all samples are clearly distinct from those of the upper Continental Crust and any basement rocks. Thus, the trace element characteristics of the Cover Horizon carry no sign of any source-material, being the sole consequence of mineralogical sorting prior to deposit. From field, granulometric and geochemical data, an aeolian origin appears as the most probable for the Cover Horizon. Geochronological constraints, including archaeological data and 14C dating, indicate that this aeolian event took place during the Holocene and probably occurred in less than 3000 y. Additional works will be necessary to assess the global context which made possible this very fast and extended event over large areas of Central Africa. © 2012 Elsevier Ltd and INQUA. Source

de Martin F.,Bureau de Recherches Geologiques et Minieres
Bulletin of the Seismological Society of America

This article presents the verification of a spectral-element method (SEM) code using the so-called layer over half-space 3 (LOH.3) benchmark model of the Southern California Earthquake Center to 7 Hz with a minimum S-wave velocity of 2000 m/s. First, the approach of Liu and Archuleta (2006) is successfully implemented in an explicit SEM, then, it is shown that the SEM code displays excellent goodness of fits (GOFs) with five Gauss-Lobatto-Legendre (GLL) nodes per wavelength at 7 Hz. A parametric study on the influence of the number of GLL nodes per wavelength shows that more than five GLL nodes per minimum wavelength do not significantly increase the accuracy; however, a decrease of the accuracy starts to be seen from four GLL nodes. A parametric study of the influence of the number of relaxation mechanism on the accuracy of the numerical response shows that, for this specific case, six to eight relaxation mechanisms are sufficient to reproduce the reference solution, whereas below six the viscoelastic response is strongly affected and tends to an elastic response when using two relaxation mechanisms only. Source

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