Southern margin of the Variscan belt: the north-western Gondwana mobile zone (eastern Morocco and Northern Algeria)

Stratigraphic and structural correlations between the Palaeozoic massifs of eastern Morocco and northern Algeria allow three tectonic domains to be distinguished: (1) The cratonic zone, i.e. the West African platform which remained outside the Variscan chain and its peripherical margin (Moroccan Anti-Atlas and Algerian Ougarta); (2) a WSW-ENE trending zone, over 1500 km from Marrakech to Kabylia and Calabria (in their assumed Palaeozoic location). — This zone was characterized during the Late Palaeozoic by a continuous instability indicated by the development of successive turbiditic basins and a major orogeny at the Devonian-Carboniferous boundary; and (3) central and western Morocco, which corresponds to the external zones of the European Hercynides.The Marrakech-Kabylia zone separates the Variscan domain from the stable and undeformed West African craton. During Early Palaeozoic times it began as an extensive or transtensive zone. It has been deformed by the Late Devonian orogeny and by Carboniferous and Permian reactivation. The zone represents the southern limit of the Hercynian chain and is distinguished by its transcurrent regime throughout the Late Palaeozoic. Correspondence to: A. Piqué     

Mapping and geochemical characterization of the Toarcian organic matter in the Mediterranean Tethys and Middle East

Organic matter from Toarcian outcrops and boreholes in several basins around the Mediterranean Sea and Middle East has been studied. Rock-Eval pyrolysis, elemental analyses of kerogens and gas chromatography of chloroform extracts within these different basins have been used to determine the amount, petroleum potential and type of the organic matter. The results have been used to compile a mapping of the organic matter for the Toarcian stage, which shows heterogeneities in the distribution and type of organic matter:

1. (1) Marine organic matter (type II) occurs in different environments of deposition. Whereas high contents (>5% total organic carbon) correspond to thick deposits within the West European realm (Great Britain, North Sea, Paris Basin and Germany), the organic matter is less abundant in the Mediterranean area where lower concentrations (1–2% TOC), preserved in Lower Toarcian thin levels, are limited only to restricted basins (northern Italy, Greece).

2. (2) The predominance of continental organic matter (type III) along the northern margin of the Tethys corresponds to a deltaic environment.

3. (3) There is a predominance of altered organic matter within the carbonated platforms around the Mediterranean Tethys.

These results, supplemented with the data issuing from the literature, add a geochemical dimension to the paleogeography of the Tethys.     

Palynological studies on manganese ore layers in Úrkút (Transdanubia), Hungary

Samples of manganese carbonate and oxide layers from the Úrkút ore deposit were investigated for organic remains, especially for spores and pollen grains. Light (LM), Transmission Electron (TEM), and Scanning Electron Microscopy (SEM) methods were used. LM revealed several new sporomorph taxa; a microstratigraphic standard was established by reconstructing vegetation zones surrounding the sedimentary basin. Index fossils of palynomorphs and the relative abundance of other taxa support a Liassic (Lower Jurassic) age for these layers. TEM data were used to establish: (1) important new understanding about the phylogeny of the pollen-exine ultrastructure and, in particular, the tectate columellar ectexine of the Mesozoic gymnospermous pollen grain, Spheripollenites; and (2) the degree of destruction of the exine ultrastructure helps to reconstruct taphonomical processes during sedimentation and to unravel biopolymer-type molecular structures in the walls of palynomorphs. The SEM method aided in the taxonomy of the microfossils. Combined TEM and SEM methods revealed that submicroscopic surface degradation may be a consequence of bio-degradation during deposition of manganese ore.     

Seismic anisotropy of the Pacific Ocean inferred from long-period surface waves dispersion

The dispersion of surface (Rayleigh and Love) waves in the period range 40–300 s along a large number of paths, allows the estimation of both the azimuthal anisotropy and the shear-wave polarization anisotropy. The regional dispersion is determined, taking into account simultaneously its dependence with age and an azimuthal factor. The Pacific Ocean has been divided into 5 regions for Rayleigh waves and into 3 regions for Love waves. This partition discriminates the regions of extreme age which show a fast variation of dispersion with age, from the regions of intermediate age where the variation is weak. A variation of ～ 2% of Rayleigh-wave group velocity with the azimuth of the path, measured with respect to the direction of spreading is displayed, up to very long-period. On the contrary, the azimuthal anisotropy for Love waves is difficult to resolve. For Rayleigh waves, the present-day direction of plate motion seems to agree best with the direction of maximum velocity. On the other hand, the isotropic inversion of the regional dispersion curves indicates, except for young regions, a discrepancy between Rayleigh-wave and Love-wave models. With this hypothesis, SH-velocities are higher than SV-velocities for the regions older than 23 Ma, down to a depth of 300 km, which is indicative of the presence of polarization anisotropy. The latter, very weak for the young part of the ocean, increases with age and reaches 7%, for the oldest region.     

Petroleum isoprenoid hydrocarbons derived from catagenetic degradation of archaebacterial lipids

The three GC coeluting C₄₀ bis-phytanes with head-to-tail, tail-to-tail and head-to-head linkages considered to be archaebacterial markers, have been recognized in petroleum by enhanced mass-fragmentography. Supporting evidence has been obtained by the simultaneous occurrence of two C₂₁–C₂₄ series of isoprenoid and quasi-isoprenoid hydrocarbons that are supposedly formed during the catagenetic degradation of the isomeric C₄₀ isoprenoids. The C₂₁–C₂₄ quasi-isoprenoid hydrocarbons have been identified conclusively for the first time by comparison with authentic standards.     

Sites occupancy of Fe3+ in Garfield Nontronite: A spectroscopic study

New data on the structure of Garfield nontronite have been produced by the use of different spectroscopic techniques: Mössbauer spectroscopy, optical spectroscopy, X-ray absorption edges and EXAFS and NMR. The tetrahedral iron content is found to be no higher than 1 percent. All iron atoms belong to the octahedral sheet excluding the possibility of the presence of non crystallized phases. Some ambiguities remain about the coherence of the octahedral sheet because of the presence of two doublets in the Mössbauer spectrum and at least two lineshapes in NMR spectra of OH which correspond to different environments.     

Statistical analysis of isotopic ratios in MORB: the mantle blob cluster model and the convective regime of the mantle

A statistical examination of isotopic distributions for MORB from various ocean ridges leads to the “blob cluster model”, in which the oceanic crust accreting at ridges results from the mixing of two components within the ascending mantle. These are (1) upper mantle material and (2) discrete rising blobs of more radiogenic material. The blobs are fractionated to a variable degree and are distributed in the upper mantle circulation in a manner that is related to the spreading rate.(1) Themean values of the isotopic distributions allow us to calculate the probabilities of the two types of material within the mantle. The results show that theproportion of asthenospheric material in the mixtureincreases with the spreading rate, in agreement with the hypothesis of blob dilution within the upper mantle convection.Mass fluxes can be estimated for the rising blobs from these probabilities, which depend on the respective concentrations in the sources of the two types of material. If the blobs originate in the lower mantle, this flux estimation would suggest that a significant part of the lower mantle has been injected into the upper mantle during earth history.(2) Thestandard deviations of the distributions depend on the “efficiency” of the mixing process:the more imbricated are the asthenospheric and blob materials in the mixture,the smaller is theisotopic spread. This efficiency parameter is shown to increase with the spreading rate, as already suggested by previous comparisons between the East Pacific Rise and the Mid-Atlantic Ridge. Moreover, this feature may also be correlated with other data such as ridge bathymetric variations.