How reliable are growth strata in interpreting short-term (10 s to 100 s ka) growth structures kinematics?

High-frequency stratigraphic cycles (10 s to 100 s ka) often show, at a specific location, an alternation of ‘dynamic’ (proximal-energetic), and ‘non-dynamic’ (distal-pelagic) processes with time. When sedimentation is syn-deformation, these processes tend respectively to fill-up tectonically-induced topography or to drape it. As a consequence, growth strata are alternatively thickened and isopach across the growth structure. High-resolution kinematic studies of growth structures (folds and faults), which assume that sedimentation always fills up topographies (‘fill-to-the-top’ model), may therefore mistake sedimentary cyclicity for tectonic cyclicity. We address this problem with one example of growth anticline in the Spanish Pyrenees, and we discuss the fill-to-the-top model. To cite this article: S. Castelltort et al., C. R. Geoscience 336 (2004).     

Les sillons plissés du Burundien supérieur dans la chaı̂ne Kibarienne d'Afrique centrale

The Kibaran belt that extends all over the central Africa, from the Katanga to the southern Uganda, straddles the African great lakes area. The Kibaran stratigraphy is one of the most debated questions. Some geologists favoured two different systems: the Ruzizian system (1800–2000 Ma) and the Burundian or Kibaran system (1600–960 Ma). Some others favoured a single Burundian system with large areas granitised and metamorphosed during several Burundian tectonic events and with a lot of sedimentary fold belts of the Burundian age. Recent geological data and new remote sensing interpretations allow us to favour the second hypothesis but with a major disconformity within the Burundian system which is separating the Upper and the Lower Burundian. To cite this article: M. Villeneuve, J. Chorowicz, C. R. Geoscience 336 (2004).     

华南陡山沱期古地理环境及"雪球地球"研究新进展

华南地块陡山沱组的古地磁和C同位素研究揭示了其形成于赤道附近的古地理环境。对冰期后的盖帽白云岩的C同位素研究表明，δ^13C从-5‰开始有缓慢升高的趋势，与世界上Marinoan冰期其他剖面的c同位素变化相一致。陡山沱期大气中CO2含量的升高可能与kodinia超大陆的裂解及火山活动等密切相关。     

L'érosion des chaînes de montagnes : influence de la sédimentation de piedmont

The applanation of mountain belts that results in peneplain is generally considered to be caused by the long-term activity of erosion. Peneplanation has been previously defined as the lowering of an elevated topography and the concomitant subduing of its relief. We propose a model following which piedmont sedimentation induces the base level rise, allowing applanation to develop at high elevation and resulting in an elevated ‘peneplain’. This model is illustrated by the morphological evolution of the southern flank of the Pyrenees during the Cainozoic. To cite this article: J. Babault, J. Van Den Driessche, C. R. Geoscience 337 (2005).     

Ilmenite, magnetite, and peraluminous Mesoproterozoic anorogenic granites of Laurentia and Baltica

Emplacement of 1.6 to 1.3 Ga Mesoproterozoic plutons in Baltica and Laurentia formed an immense belt of A-type granite batholiths that include (1) low-fO₂, ilmenite-series granite intrusions from the Baltic region to Wyoming, (2) high-fO₂, magnetite-series granite intrusions of the central to southwestern U.S., and (3) peraluminous, two-mica granite intrusions from Colorado to central Arizona. These mineralogic divisions are mirrored by substantial elemental and oxygen isotopic differences. The ilmenite-series granites, which often contain classic rapakivi textures, have the highest Fe/Mg ratios and are highest in LIL element enrichment. They also have the lowest whole-rock δ¹⁸O values at 5.7‰ to 7.7‰. The magnetite-series granites are less potassic, less LILE-enriched, and have higher whole-rock δ¹⁸O values, ranging from 7.6‰ to 10.8‰. Although they retain A-type characteristics, the peraluminous granites are the least LILE-enriched and have the lowest Fe/Mg ratios. They also have the highest whole-rock δ¹⁸O values ranging from 8.8‰ to 12.0‰. Feldspar, where strongly reddened, can exhibit elevated δ¹⁸O values, which is interpreted to indicate subsolidus exchange with surface-derived aqueous fluids. Quartz δ¹⁸O values are interpreted to generally retain their magmatic values. The transcontinental mineralogic, chemical, and oxygen isotopic variations are interpreted as indicative of broad changes in the composition of a lower crustal source, which is compatible with a reduced mantle-derived crustal source for the ilmenite-series granites and a more oxidized crustal source for the others, including a metasedimentary component in the source for the two-mica granite subprovince.

Widespread thermal metamorphism at 1.4 Ga is present throughout much of the magmatic province and is viewed as a consequence of this immense event. Compressional deformation associated with several western 1.4 Ga Laurentia granite batholiths, alternatively interpreted as the distal expressions of a presumed 1.4 Ga orogeny, have at least in part been shown to be localized on preexisting Paleoproterozoic zones of deformation. Thus, we do not find compelling evidence for a 1.4 Ga orogeny related to the formation of most of these granites. Renewed intrusions at 1.0–1.1 Ga between and immediately following phases of the Grenville orogeny indicate that situations leading to their formation need to be more broadly considered.

The origin of this red granite-forming event in Laurentia and Baltica is considered as part of a global magmatic event that was coeval with intrusion of massif anorthosites and associated charnockites. Most are viewed as anorogenic, but it is recognized that the same conditions leading to their formation may have occurred during extensional phases of orogens. The immense volumes of red granites produced are also essentially unique to the Mesoproterozoic and appear to be tied to the stabilization and eventual break up of supercontinents of both Paleoproterozoic and Mesoproterozoic age.     

La branche orientale de l'arc de Chypre. Morphostructure d'une frontière de plaques d'après les résultats de la campagne BLAC (2003)

The eastern bend of the Cyprus Arc, at the transition between the submerged Mediterranean subduction and the onshore fault zones that underline the Eurasian, African and Arabic plates boundaries is a submarine feature undergoing a complex tectonic deformation. The BLAC marine geophysical survey helps to better assess the type of the deformation that affects the Messinian to Quaternary sediments along this plate boundary. The deformation, focussed between two tectonic corridors, displays compressive and transpressive features in the central part, becoming thrusting when moving westward in connection with the Cyprus accretionnary wedge. The northeastern end of this submarine range connects with the Latakia Ridge, which is, together with its continental extension, under a tensional tectonic regime. To cite this article: J. Benkhelil et al., C. R. Geoscience 337 (2005).     

Nouvelle interprétation tectonique de la montagne Sainte-Victoire (Provence,France)

The explanation normally given for the tectonics of Sainte-Victoire Mountain, a dozen kilometres east of Aix-en-Provence, to the north of the limestone Provence, is incorrect. To the east, the morphology of the Sainte-Victoire is subdued, whereas to the west, before the mountain breaks savagely, the morphology is that of a young mountain as appears in Alpine landscapes. This unusual aspect in the region and the large subvertical faults with vertically striated surfaces that mark the massif to the south and to the west, induce the idea of strong vertical uplifts and caste doubt on the tectonic interpretation given in 1962 by Corroy et al. According to those authors, the Sainte-Victoire is a unit of Jurassic and Cretaceous formations overthrusting 1800 m to the south conglomerates of the Late Cretaceous or Palaeocene. New observations about the conglomerate transgression over the Jurassic and Cretaceous beds, and about the faults around and on the massif do not give evidence of an overthrusting but, on the contrary, induce the idea of a uplift, perhaps still active, in the form of a ‘piano key’ inclined to the northeast. To cite this article: J. Ricour et al., C. R. Geoscience 337 (2005).     

Describing the Geometry of 3D Fracture Systems by Correcting for Linear Sampling Bias

Analyzing the geometric bias inherent to linear sampling of natural fracture systems is a prerequisite to any attempt of structural modeling. In this paper, the basic parameters of 1D-sampled fracture sets, i.e. orientation, density, and size, are interpreted in terms of geometric probabilities. Weighting factors are derived which allow the 3D restitution of a moderately variable fracture network from a single borehole. The proposed method is applied to well core data from a granitic rock mass, and the efficiency of the proposed corrections is illustrated through random disc simulations tested by virtual scanlines analogous to the real borehole. This approach aims to reduce the prospecting effort in exploration, and to criticize assumption of structural homogeneity by rigorously comparing fracture populations collected from nonparallel boreholes. Then a parametric study of fracture size is performed and a range of mean size leading to fully connected networks is identified.     

Estimation of PM20 emissions by wind erosion: main sources of uncertainties

The physics of the two processes (saltation and sandblasting) leading to fine mineral dust emissions by wind erosion in arid or semi-arid areas has been detailed and modeled. The combination of these two models has led to a physically explicit Dust Production Model (DPM). In this work, sensitivity tests are performed with the DPM to determine the nature of the main soil parameters that control dust emissions by sandblasting. It is found that the soil roughness length and the dry size distribution of the soil aggregates constituting the loose wind erodible fraction of the topsoil have the greatest influence on the soil potential for mineral dust production. Contrary to what is often assumed, soil texture is not a relevant parameter.In the light of these new findings, results of vertical flux measurements performed over a wide variety of sources in Niger and the US south west (14 soils) have been reanalyzed. Results show (1) that for the tested soils the DPM, and hence sandblasting, explain all dust emissions, and (2) that 13 of the 14 soils that had been selected a priori for their high potential for dust emissions contained a fine soil-aggregate component. This is consistent with the sensitivity tests indicating that the presence of such a component could enhance dust emissions by one order of magnitude. Finally, it can be concluded that most of the apparent scatter in the experimental results was in large part due to an inappropriate choice of soil parameters to interpret them.