On the use of magnetic techniques for stratigraphic purposes : examples from the Lower Palaeozoic Anglo-Brabant Deformation Belt (Belgium)

Within the Lower Palaeozoic Anglo-Brabant Deformation Belt, magnetic susceptibility on its own does not allow for a straightforward distinction between different lithostratigraphic units, except for the high-susceptibility levels of the Lower Cambrian Tubize Formation. Moreover, the variation in magnetic susceptibility within individual lithostratigraphic units is often larger than that between different units, but at the same time, this internal variation in susceptibility may show no clear relationship to features obvious in outcrop or hand specimens. Hence, the applicability of magnetic susceptibility for stratigraphic purposes in the Anglo-Brabant Deformation Belt is low. Better results are obtained using the temperature-dependent variation in terms of percentage of magnetic susceptibility within the "room temperature interval". Also the anisotropy of magnetic susceptibility allows for a better distinction between different lithostratigraphic units than does magnetic susceptibility. The best results are obtained by a comparison of thermal demagnetisation curves of magnetic remanence, used for determining ferromagnetic mineralogy. This method even allows distinguishing lithostratigraphic units in which ferromagnetic carriers do not contribute to overall magnetic susceptibility and its anisotropy. Ideally, each magnetic technique should be used for stratigraphic purposes only in combination with other magnetic techniques. Moreover, knowledge about the magnetic carriers (s.l.) facilitates this use of magnetic techniques and strongly improves the accuracy of the interpretations

Injection triggering mechanisms inferred from AMS and microtectonic Analysis. Example of Injectites in the Blue Marls formation, SE basin of France

International audienceUnderstanding the way fracture networks evolve in space and time in low-permeability rocks is required in several applied problems. The turbiditic system of the SE Basin of France is associated with injectites that constitute markers of early deformation in the Blue Marls formation. It thus offers a unique opportunity to characterize the onset dynamics and evolution through time of such a 3D fracture network into low permeability rocks. This study explores the relative timing between (i) Blue Marls deposition, (ii) hostrock deformation, (iii) sand injection into fractures and (iv) regional changes in paleostresses, thanks to both microtectonics and magnetic fabric (Anisotropy of Magnetic Susceptibility) analyses in both the hostrock and injectites. These two coupled studies put forward that the hostrock was submitted to a N150-N160 layer parallel shortening which was not recorded or preserved by injectites. Injection, probably triggered in several pulses, was synchronous with a N/S compression responsible for strike-slip faulting in the study area. This compression persisted after cementation of injections triggered by the first pulse. The AMS signal in injections is more characteristic of hydrodynamic processes at the time injection took place than compaction. Fracturing in injections is decoupled from that recorded in hostrock

On the use of magnetic techniques for stratigraphic purposes: examples from the Lower Palaeozoic Anglo-Brabant Deformation Belt (Belgium)

Within the Lower Palaeozoic Anglo-Brabant Deformation Belt, magnetic susceptibility on its own does not allow for a straightforward distinction between different lithostratigraphic units, except for the high-susceptibility levels of the Lower Cambrian Tubize Formation. Moreover, the variation in magnetic susceptibility within individual lithostratigraphic units is often larger than that between different units, but at the same time, this internal variation in susceptibility may show no clear relationship to features obvious in outcrop or hand specimens. Hence, the applicability of magnetic susceptibility for stratigraphic purposes in the Anglo-Brabant Deformation Belt is low. Better results are obtained using the temperature-dependent variation in terms of percentage of magnetic susceptibility within the "room temperature interval". Also the anisotropy of magnetic susceptibility allows for a better distinction between different lithostratigraphic units than does magnetic susceptibility. The best results are obtained by a comparison of thermal demagnetisation curves of magnetic remanence, used for determining ferromagnetic mineralogy. This method even allows distinguishing lithostratigraphic units in which ferromagnetic carriers do not contribute to overall magnetic susceptibility and its anisotropy. Ideally, each magnetic technique should be used for stratigraphic purposes only in combination with other magnetic techniques. Moreover, knowledge about the magnetic carriers (s.l.) facilitates this use of magnetic techniques and strongly improves the accuracy of the interpretations.status: publishe

On the use of magnetic techniques for stratigraphic purposes: examples from the Lower Palaeozoic Anglo-Brabant Deformation Belt (Belgium)

status: publishe

Geometrical 3D characterisation of sand injections in a fracture network and onset dynamics, Bevons (South East France)

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Anisotropy of elastic, magnetic, and microstructural properties of the Callovo-Oxfordian argillite

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A singe method for the inversion of anisotropic data sets with application to structural studies

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