Recent publications

Three sets of crystallographic sub-planar structures in quartz formed by tectonic deformation

Tine DEREZ, Gill PENNOCK, Martyn DRURY & Manuel SINTUBIN
Earth and Planetary Science Letters 442, 157-161 – DOI: http://dx.doi.org/10.1016/j.epsl.2016.03.005

In quartz, multiple sets of fine planar deformation microstructures that have specific crystallographic orientations parallel to planes with low Miller–Bravais indices are commonly considered as shock-induced planar deformation features (PDFs) 1 diagnostic of shock metamorphism. Using polarized light microscopy, we demonstrate that up to three sets of tectonically induced sub-planar fine extinction bands (FEBs), sub-parallel to the basal, γ, ω, and π crystallographic planes, are common in vein quartz in low-grade tectonometamorphic settings. We conclude that the observation of multiple (2–3) sets of fine scale, closely spaced, crystallographically controlled, sub-planar microstructures is not sufficient to unambiguously distinguish PDFs from tectonic FEBs.


Mechanisms of flexural flow folding of competent single-layers as evidenced by folded fibrous dolomite veins

Koen TORREMANS, Philippe MUCHEZ & Manuel SINTUBIN
Journal of Structural Geology 69, 75-90 – DOI: http://dx.doi.org/10.1016/j.jsg.2014.10.002

Cross-polars thin section of folded fibrous dolomite vein.

Cross-polars thin section of folded fibrous dolomite vein.

Flexural flow is thought unlikely to occur in naturally deformed, competent isotropic single-layers. In this study we discuss a particular case of folded bedding-parallel fibrous dolomite veins in shale, in which the internal strain pattern and microstructural deformation features provide new insights in the mechanisms enabling flexural flow folding. Strain in the pre-folding veins is accommodated by two main mechanisms: intracrystalline deformation by bending and intergranular deformation with bookshelf rotation of dolomite fibres. The initially orthogonal dolomite fibres allowed a reconstruction of the strain distribution across the folded veins. This analysis shows that the planar mechanical anisotropy created by the fibres causes the veins to approximate flexural flow. During folding, synkinematic veins overgrow the pre-folding fibrous dolomite veins. Microstructures and dolomite growth morphologies reflect growth during progressive fold evolution, with evidence for flexural slip at fold lock-up. Homogeneous flattening, as evidenced by disjunctive axial-planar cleavage, subsequently modified these folds from class 1B to 1C folds. Our study shows that the internal vein fabric has a first-order influence on folding kinematics. Moreover, the fibrous dolomite veins show high viscosity contrasts with the shale matrix, essential in creating transient permeability for subsequent mineralising stages in the later synkinematic veins during progressive folding.

 

 

The Mousny massive quartz occurrence – the vestige of a late-orogenic dilational jog in the High-Ardenne slate belt (Belgium)

Simon DEPOORTER, Dominique JACQUES, Kris PIESSENS, Philippe MUCHEZ & Manuel SINTUBIN
Geological Belgica 17(3-4), 293-310 – URL: http://popups.ulg.ac.be/1374-8505/index.php?id=4660

P1020495

Mousny massive quartz occurrence

Near the village of Mousny, Belgium, a peculiar massive quartz occurrence, composed of multiple large, >m³-size, bodies of milky quartz can be found at the locality known as “Les Blancs Cailloux”. Strikingly, the quartz bodies contain elongated, cleaved, host-rock fragments, still oriented parallel to the regional cleavage attitude.
A detailed petrographical, microstructural and mineralogical study of the vein quartz and a geochemical analysis of fluid inclusions has revealed that the Mousny massive quartz occurrence is genetically linked to the regionally common cleavage-parallel quartz veins. They both show a fluid evolution typical of the metamorphic fluids in the central, epizonal part of the High-Ardenne slate belt.
While the cleavage-parallel veins can be considered to result from mode I extensional fracturing, the genetically linked massive quartz occurrence is seen as being formed in a dilational jog. Within the late-orogenic context of the High-Ardenne slate belt, we favour a model in which the dilational jog is comprised within a weakly south-dipping, extensional shear zone, related to the late-orogenic extensional destabilization of the slate belt, causing a transient enhancement of the structural permeability in this low-permeability mid-crustal environment. The Mousny massive quartz occurrence may in this respect be exemplary for massive quartz occurrences throughout the High-Ardenne slate belt.
 
 
 

Regional significance of non-cylindrical folding in the northwestern part of the High-Ardenne slate belt (Redu, Daverdisse, Belgium)

Dominique JACQUES, Tine Derez, Philippe MUCHEZ & Manuel SINTUBIN
Geological Belgica 17(3-4), 252-267 – URL: http://popups.ulg.ac.be/1374-8505/index.php?id=4641

Saddle reef quartz vein.

Regional mapping and a detailed geometric analysis of complex, mixed brittle-ductile, fold-related accommodation structures, along the well-exposed banks of the Lesse river between Redu and Daverdisse (Belgium), reveal that the finite strain in the predominantly incompetent Lower Devonian rock sequence in the northwestern part of the High-Ardenne slate belt deviates from pure flattening and approximates plane strain. This finite strain is materialised by different manifestations of non-cylindrical folding, i.e. regional en-echelon and periclinal fold geometries, mesoscale non-cylindrical folds with hinges showing variable plunges, oblique flexural slip and local non-axial planar, transecting cleavage. The observed non-cylindrical folding fits in the regional framework of the Meuse Valley Recess, a transpressional corridor in the Ardenne allochthon that developed on top of a buried, buttressing oblique ramp in the pre-structural basement. Differential propagation in the overriding Ardenne allochthonuous domain to the east and west of this buttressing oblique ramp led to a component of lateral shortening on top of the ramp, resulting in the rotation of the overall structural grain and the development of en-echelon, non-cylindrical folds. Our study suggests that the Meuse Valley Recess can be continued towards the southeast, at least affecting the northern parts of the High-Ardenne slate belt.

 

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