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Liegeois J.-P.,Royal Museum for Central Africa | Abdelsalam M.G.,Missouri University of Science and Technology | Ennih N.,Chouaib Doukkali University | Ouabadi A.,University of Science and Technology Houari Boumediene
Gondwana Research

In this paper, we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events, leading to the transformation of their margins. In some cases, craton interiors can also be shaped by intense collisional processes, leading to the generation of intracratonic orogenic belts. We propose to call these events "metacratonization" and the resulting lithospheric tract "metacraton". Metacratons can appear similar to typical orogenic belts (i.e. active margin transformed by collisional processes) but are actually sharply different. Their main distinctive characteristics (not all are present in each metacraton) are: (1) absence of pre-collisional events; (2) absence of lithospheric thickening, high-pressure metamorphism being generated by subduction, leading to high gradient in strain and metamorphic intensity; (3) preservation of allochthonous pre-collisional oceanic terranes; (4) abundant post-collisional magmatism associated with shear zones but not with lithospheric thickening; (5) presence of high-temperature-low-pressure metamorphism associated with post-collisional magmatism; (6) intracontinental orogenic belts unrelated to subduction and oceanic basin closures. Reactivation of the rigid but fractured metacratonic lithosphere will cause doming, asthenospheric volcanism emplacement, and mineralizations due to repetitive mineral enrichments. This paper provides several geological cases exemplifying these different metacratonic features in Scandinavia, Sahara, Central Africa and elsewhere. A special focus is given to the Saharan Metacraton because it is where the term "metacraton" originated and it is a vastly expanded tract of continental crust (5,000,000km 2). Metacratonization is a common process in the Earth's history. Considering the metacraton concept in geological studies is crucial for understanding the behavior of cratons and their partial destruction. © 2012 International Association for Gondwana Research. Source

Liegeois J.-P.,Royal Museum for Central Africa | Stern R.J.,University of Texas at Dallas
Journal of African Earth Sciences

Neoproterozoic gneisses at Meatiq and Hafafit in the Eastern Desert of Egypt give Rb-Sr and U-Pb zircon ages of 600-750Ma. These gneisses are interpreted by different workers to represent deeper levels of juvenile Neoproterozoic crust or Archaean/Palaeoproterozoic crust that was remobilized during Neoproterozoic time. Geochemical and Sr-Nd isotope compositions for these gneisses reported here are remarkably homogeneous: Initial 87Sr/86Sr (0.70252±0.00056) and Nd (+6.4±1.0). These values are best explained as reflecting derivation from depleted asthenospheric mantle sources during Neoproterozoic time, consistent with mean Nd model ages of 0.70±0.06Ga. The increasing recognition of old, xenocrystic zircons in juvenile ANS igneous rocks can be explained in several different ways. The participation of ancient crust is allowed as one of the explanations, but it is the isotopic composition of radiogenic elements such as Sr and Nd for whole-rock specimens that are the most reliable indicators of whether or not a given crustal tract is juvenile or reworked older crust. These isotopic data indicate that the protolith for the Meatiq and Hafafit gneisses were juvenile Neoproterozoic igneous rocks and sediments derived from them. There is no support in the isotopic data for any significant contribution of pre-Neoproterozoic crust in these two sections of Eastern Desert crustal infrastructure. © 2009 Elsevier Ltd. Source

Beeckman H.,Royal Museum for Central Africa
IAWA Journal

The largest part of forest biomass consists of wood. A global estimate of carbon stored in lignified tissues rises up to 400 Pg. Given these quantities, there is a growing interest of implementing wood research in diagnoses and evaluations of the carrying capacity of the global ecosystem and its forests. The question arises how disciplines like wood anatomy could respond to the increasing demands of a trait-based ecology, understood as a paradigmatic shift in addressing global changes. Dendrochronology and ecological wood anatomy, traditionally operating within the paradigm of species-based ecology, developed robust methods to address ecological questions. However, sampling strategies and database design will likely be different when wood traits are to be used to study individual tree performance, including responses to stress. Aiming at optimally involving wood research in trait-based ecology, some trait concepts are analysed. The value of the IAWA standard lists of wood anatomical features as starting points for trait databases is recognized. A summary of the functionality of wood is given to inform the trait-research community of basic aspects of tree performance. The time dimension is highlighted, as well as the foundations for understanding bio-hydraulics, bio-mechanics and metabolism of wood and relevant traits. Guidelines are given for sampling strategies and database concepts. Prospects of time axis construction and system integration are discussed, as well as the importance of standardizing for size. © 2016 International Association of Wood Anatomists. Source

Fannes W.,Royal Museum for Central Africa
Bulletin of the American Museum of Natural History

The goblin spider genus Zyngoonops Benoit, 1977 is revised. The type and hitherto only species, Z. clandestinus Benoit from the Democratic Republic of Congo (D.R.C.), is redescribed on the basis of topotypical material. Nine new species are described from the D.R.C. (Z. moffetti, Z. redii, Z. goedaerti, Z. rockoxi, Z. beatriceae, Z. chambersi, Z. swammerdami, Z. marki) and the Central African Republic (Z. walcotti). Members of Zyngoonops show remarkable modifications of the sternum and pedicel tube: the sternum has two heavily sclerotized posterior extensions, and the pedicel tube has a protruding ventral lip. The female genitalia are complex, consisting of an epigastric scape, two uterine sclerites, and a long genital duct. In most species, the distal part of the duct is highly coiled. Males resemble those of Antoonops Fannes and Jocqué and Coxapopha Platnick in having elaborately modified endites, a pair of pleural flaps, and a carapace with extended anterolateral corners. The type species of Coxapopha is redescribed, and new images of the female genitalia of Antoonops and Coxapopha are provided. A cladistic analysis of morphological characters provides support for the monophyly of Zyngoonops and identifies Z. redii as the most basal species of the genus. © 2013 American Museum of Natural History. Source

Delvaux D.,Royal Museum for Central Africa | Barth A.,University of Heidelberg

The kinematic models and the associated orientation of extensional stress of the East African Rift System have been subjected to much debate since a long time. In the past decades, the proposed models relied on the interpretation of the overall rift geometry, geological fault-slip data and the few focal mechanisms available. These models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space. In the recent years, there has been a significant increase in the number of focal mechanisms available for the entire rift system, and it is now possible to estimate the present-day stress field in relative detail based on seismotectonic data alone. We compile 347 focal mechanism data from the Global/Harvard CMT catalogue and various other sources and grouped 332 of them in 24 distinct regions (boxes) on the basis of their geographical proximity, kinematic homogeneity and tectonic setting. For each box and for the same data set, reduced stress tensors have been obtained by formal stress inversion using both the TENSOR program (Delvaux & Sperner, 2003) and the SLICK method (Michael, 1984/1987). Both inversion methods show in comparable results in terms of horizontal stress axes orientations and tectonic stress regimes, which stem for the robustness of the approach. The obtained stress pattern reflects a complex interaction between 1st order effects as different driving forces, including plate boundary forces, and 2nd and 3rd order effects as gravitational potential of topography, intra-lithospheric processes, and the influence of structural heterogeneities of the rift structures. The evidence present in the stress orientations of the 2nd and 3rd order stress pattern as the variations in the horizontal stress axes along the axis of the rift are of particular interest as they were not yet captured or shown in earlier numerical models of the stress field. Additional sources of tectonic forces in supplement to the gravitational potential energy forces as considered in the earlier models are necessary to explain the observed patterns. © 2009 Elsevier B.V. All rights reserved. Source

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