On the influence of magnetic mineralogy in the tectonic interpretation of anisotropy of magnetic susceptibility in cataclastic fault zones

Of the several factors involved in the development of magnetic fabrics in fault zones at shallow crustal levels, lithology and deformation intensity have probably the most important consequences for the reconstruction of their kinematic history. The basement-involved Cenozoic thrusts in the Demanda Massif (N Spain) provide the opportunity for testing the applicability of anisotropy of magnetic susceptibility (AMS) to the study of deformation in cataclastic fault rocks belonging to shallow fault zones. The Rastraculos thrust is a relatively minor basement thrust (dip-slip movement of 2km defined from cross-sections and geological maps) of Cenozoic age. This thrust contains a re-activated fault zone involving different rock types both belonging to its hangingwall (Palaeozoic) and its footwall (Triassic sandstones and dolostones and Jurassic limestones). AMS results show magnetic foliations parallel or slightly oblique to the fault zone, and both transport-parallel (projected onto the foliation plane) and transport-perpendicular (parallel to the observed intersection lineation) magnetic lineations. The two types of strain/magnetic fabric relationships can be related to deformational and mineralogical features inferred from the direct analysis of thin and polished sections under the microscope and the naked eye, respectively. Analysis of fault rocks in the Rastraculos fault zone indicates that in cataclasites, magnetic fabrics are particularly dependent on lithology and hence magnetic mineralogy. The results obtained prove the usefulness of AMS in fault zones where kinematic indicators are scarce and also give clues on the number of samples necessary to define magnetic susceptibility axes, depending on grain size, ellipsoid shapes and magnetic mineralogy

Emplacement and deformation of mesozoic Gabbros of the High Atlas (Morocco): paleomagnetism and magnetic fabrics

A paleomagnetic and magnetic fabric study is performed in Upper Jurassic gabbros of the central High Atlas (Morocco). These gabbros were emplaced in the core of preexisting structures developed during the extensional stage and linked to basement faults. These structures were reactivated as anticlines during the Cenozoic compressional inversion. Gabbros from 19 out of the 33 sampled sites show a stable characteristic magnetization, carried by magnetite, which has been interpreted as a primary component. This component shows an important dispersion due to postemplacement tectonic movements. The absence of paleoposition markers in these igneous rocks precludes direct restorations. A novel approach analyzing the orientation of the primary magnetization is used here to restore the magmatic bodies and to understand the deformational history recorded by these rocks. Paleomagnetic vectors are distributed along small circles with horizontal axes, indicating horizontal axis rotations of the gabbro bodies. These rotations are higher when the ratio between shales and gabbros in the core of the anticlines increases. Due to the uncertainties inherent to this work (the igneous bodies recording strong rotations), interpretations must be qualitative. The magnetic fabric is carried by ferromagnetic (s.s.) minerals mimicking the magmatic fabric. Anisotropy of magnetic susceptibility (AMS) axes, using the rotation routine inferred from paleomagnetic results, result in more tightly clustered magnetic lineations, which also become horizontal and are considered in terms of magma flow trend during its emplacement: NW-SE (parallel to the general extensional direction) in the western sector and NE-SW (parallel to the main faults) in the easternmost structures

Mécanismes d'intrusion des granites supracrustaux : Modèles analogiques et exemples naturels.

Mémoires de Géosciences Rennes, n° 62, 270 p. ISBN: 2-905532-61-0Ce travail est une contribution à l'étude de l'intrusion des granites supracrustaux dans les domaines orogéniques. Les objectifs sont de déterminer comment ces intrusions se mettent en place, et comment se crée l'espace nécessaire pour l'accommodation du magma dans l'encaissant. Notre étude a consisté en deux travaux complémentaires : (1) Une revue bibliographique des exemples naturels fournis par les granites hercyniens de l'arc ibéro-armoricain (chaîne varisque), ainsi qu'une étude structurale d'un massif sur le terrain, et (2) une étude expérimentale des conditions de formation des laccolites et de la géométrie des intrusions mises en place en contexte de tectonique active. L'exemple des granites syntectoniques de l'arc ibéro-armoricain souligne que beaucoup de ces massifs ont généralement des géométries de type laccolite. Ils se son~ mis en place par expansion latérale, soit dans une couverture sédimentaire ductile au-dessus d'un socle plus résistant, soit le long de zones de cisaillement ductile majeures. L'étude expérimentale concerne les mécanismes de mise en place des granites dans une croûte supérieure. Les résultats permettent d'expliquer beaucoup des caractéristiques des granites syntectoniques, en montrant en ~articulier que: (1) La formation d'un laccolite nécessite l'existence d'une couche de faible résistance entre deux unités résistantes, (2) l'épaisseur critique de la couche de faible résistance nécessaire à la formation d'un laccolite diminue avec l'augmentation de la profondeur, (3) la géométrie des intrusions reflète le contexte tectonique régional régnant lors de leur mise en place. La mise en place de granites le long de zones de décrochement d'échelle crustale ne nécessite pas la création de vides ou l'existence de zones d'extension locale. Dans nos modèles, les intrusions en régime décrochant se mettent en place dans la direction d'élongation maximale de l'ellipsoïde de déformation, les structures fragiles ne contrôlant que d'unl3 manière secondaire la géométrie des intrusions. Les intrusions en régime extensif montrent une forme asymétrique avec un front à fort pendage et une langue basale cisaillée à l'arrière.No abstrac

Statistical significance of magnetic fabric data in studies of paramagnetic granites

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Evidence for the Permo-Triassic transtensional rifting in the Iberian Range (NE Spain) according to magnetic fabrics results

Anisotropy of magnetic susceptibility (AMS) techniques are applied to Permo-Triassic red beds from the Castilian Branch (Iberian Range, NE Spain) that were deposited in an extensional basin inverted during Cenozoic times. The main goal of this work is to characterize the tectonic evolution of the basinal stage by differentiating synsedimentary to early diagenetic magnetic fabrics from the secondary tectonic fabrics related to compression, which are scarcely developed because no penetrative structures related to compression have been recognized. Oblate magnetic fabrics, with k(min) axes perpendicular to bedding,are observed in most cases. Magnetic lineations are variable, showing a dominant ENE-WSW maximum, which fits with a dextral transtensional regime acting on NW-SE master faults during the Triassic. We propose that variations in the orientation of the magnetic lineation are associated with transfer faults which fragment the basin and trigger strain partitioning in different areas. Magnetic fabrics are locally modified by Cenozoic compression, with intermediate and minimum axes distributed along girdles perpendicular to fold axes. Comparing all these results with macrostructures and mesostructural kinematic indicators, we conclude that the fine-grained hematite-bearing rocks carry a consistent magnetic fabric which can be used to reconstruct the basin history. (C) 2015 Elsevier B.V. All rights reserved

Emplacement and Deformation of Mesozoic Gabbros of the High Atlas (Morocco): Paleomagnetism and Magnetic Fabrics

A paleomagnetic and magnetic fabric study is performed in Upper Jurassic gabbros of the central High Atlas (Morocco). These gabbros were emplaced in the core of preexisting structures developed during the extensional stage and linked to basement faults. These structures were reactivated as anticlines during the Cenozoic compressional inversion. Gabbros from 19 out of the 33 sampled sites show a stable characteristic magnetization, carried by magnetite, which has been interpreted as a primary component. This component shows an important dispersion due to postemplacement tectonic movements. The absence of paleoposition markers in these igneous rocks precludes direct restorations. A novel approach analyzing the orientation of the primary magnetization is used here to restore the magmatic bodies and to understand the deformational history recorded by these rocks. Paleomagnetic vectors are distributed along small circles with horizontal axes, indicating horizontal axis rotations of the gabbro bodies. These rotations are higher when the ratio between shales and gabbros in the core of the anticlines increases. Due to the uncertainties inherent to this work (the igneous bodies recording strong rotations), interpretations must be qualitative. The magnetic fabric is carried by ferromagnetic (s.s.) minerals mimicking the magmatic fabric. Anisotropy of magnetic susceptibility (AMS) axes, using the rotation routine inferred from paleomagnetic results, result in more tightly clustered magnetic lineations, which also become horizontal and are considered in terms of magma flow trend during its emplacement: NW-SE (parallel to the general extensional direction) in the western sector and NE-SW (parallel to the main faults) in the easternmost structures