A magnetic fabric study of the Aigoual–Saint Guiral–Liron granite pluton (French Massif Central) and relationships with its associated dikes.

International audienceIn the southeastern French Massif Central, the Aigoual–Saint Guiral–Liron pluton consists of porphyritic and microgranitic types. The latter is encountered within dikes forming the northern end of the pluton. Both types show prefull crystallization microstructures indicating weak subsolidus deformations. An anisotropy of magnetic susceptibility (AMS) study has been carried out to determine the granite fabric. Biotite, local hornblende, and small grains of magnetite are the main carriers of AMS in both types. Porphyritic granite and dikes display different AMS patterns related to magma dynamics and regional deformation recorded during crystallization. In the porphyritic type, the AMS lineation is mainly consistent with the regional, NW-SE to E-W trending, extensional event coeval with emplacement and crystallization of the pluton indicating an influence of regional tectonics on the linear fabric development. The dome shaped foliation pattern of the Saint Guiral massif likely corresponds to internal deformation-related processes within the pluton. In the microgranite dike swarm, the NNE-SSW trending lineations with shallow plunges record magmatic flow processes within dikes, i.e., magma injection and filling of dikes from a probable source located southwestward. Regional tectonics played a significant role in the pluton geometry and fabric development. For example, the NE-SW trend of the dikes suggests that extensional fractures took place in the same extensional strain field as elsewhere in the pluton. Different fabric development modes were therefore responsible for the contrasted fabric patterns between the microgranite and the porphyritic granite

Mechanisms of extension at nonvolcanic margins: Evidence from the Galicia interior basin, west of Iberia

We have studied a nonvolcanic margin, the West Iberia margin, to understand how the mechanisms of thinning evolve with increasing extension. We present a coincident prestack depth‐migrated seismic section and a wide‐angle profile across a Mesozoic abandoned rift, the Galicia Interior Basin (GIB). The data show that the basin is asymmetric, with major faults dipping to the east. The velocity structure at both basin flanks is different, suggesting that the basin formed along a Paleozoic terrain boundary. The ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension. At the rift flanks (stretching factor, β ≤ 2) the ratio is fairly constant, indicating that stretching of upper and lower crust was uniform. Toward the center of the basin (β ∼ 3.5–5.5), fault‐block size decreases as the crust thins and faults reach progressively deeper crustal levels, indicating a switch from ductile to brittle behavior of the lower crust. At β ≥ 3.5, faults exhume lower crustal rocks to shallow levels, creating an excess of lower crust within their footwalls. We infer that initially, extension occurred by large‐scale uniform pure shear but as extension increased, it switched to simple shear along deep penetrating faults as most of the crust was brittle. The predominant brittle deformation might have driven small‐scale flow (≤40 km) of the deepest crust to accommodate fault offsets, resulting in a smooth Moho topography. The GIB might provide a type example of nonvolcanic rifting of cold and thin crust

Redox and Lewis acid-base activities through an electronegativity-hardness landscape diagram

International audienceChemistry is the science of bond making and bond breaking which requires redistribution of electron density among the reactant partners. Accordingly acid-base and redox reactions form cardinal components in all branches of chemistry, e.g., inorganic, organic, physical or biochemistry. That is the reason it forms an integral part of the undergraduate curriculum all throughout the globe. In an electronegativity (chi)- hardness (eta) landscape diagram the diagonal chi = eta line separates reducing agents from oxidizing agents as well as Lewis acids from Lewis bases. While electronegativity is related to the degree of electron transfer between two reactants, hardness is related to the resistance to that process. Accordingly the electronegativities of oxidizing agents/Lewis acids are generally greater than the corresponding hardness values and the reverse is true for reducing agents/Lewis bases. Electrophiles and nucleophiles are also expected to follow similar trends