Bernard Henry

S U M M A R Y Classical progressive unfolding makes a distinction between pre-folding, post-folding or syn-folding magnetization. It yields the direction of magnetization only in the two first cases. To determine the direction of synfolding magnetization, possible different unfolding percentages during remagnetization have to be assumed according to the site. The small circle approach of Surmont et al. not only leads to this direction, but also to the dip value at each site during remagnetization. In the Cévennes area, extensive palaeomagnetic study reveals syntectonic remagnetization enabling the reconstruction of the shape of a fold for the time of the magnetic overprinting as well as analysis of the rotations undergone by some sites after the remagneti-zation.

Heating produces changes, which does not always correspond to simple enhancement of the magnetic fabric. Two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating. The first diagonalizes the tensor from the difference between each tensor term before and after heating. The second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment. When the same magnetic fabric is obtained from several thermal steps, it cannot be related to randomly oriented ferrimagnetic minerals. Instead, the newly formed fabric must be related to characteristics of the pre-existing rock. By comparing this ferrimagnetic minerals fabric with the initial whole rock fabric, we can distinguish cases where heating simply enhances pre-existing fabric from those where thermal treatment induces a different fabric. Relative to the pre-heating fabric, this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction, related to petrostructural elements other than those defining the initial fabric.

The magnetic zone axis is the common direction of the intersection of the magnetic foliations of different samples. It is easily determined, using tensors, from the minimum susceptibility axes. It provides useful information for the interpretation of the magnetic fabric, particularly of the maximum axis. A bootstrap method reveals small local disturbance of the magnetic foliation. It yields also the confidence zone of the magnetic zone axis.

AbstrAct this study describes the deformation in zones affected by regional shearing, and its relation with local factors, in particular rock compositions. the tihali-ouine and teg Orak plutons were emplaced close to a major shear zone of the tuareg shield. their magmatic to sub-magmatic fabrics were determined by using measurements of anisotropy of magnetic susceptibility; they are similar to those of some other late Panafrican plutons of the tuareg shield. the eastern part of the teg Orak pluton displays a coherent fabric with a sub-horizontal lineation oblique to the 4°50 major shear zone located just to the east. this fabric is clearly related to shearing by a dextral strain-slip movement along the shear zone during magma crystallization. the fabric in the western part of the teg Orak pluton and in the tihaliouine massif presents much more scattered principal axes. It was much less affected by shearing along the shear zone. this difference strongly depends on the nature of the host-rocks: Gra-nitic host-rocks around the tihaliouine and the western part of the teg Orak acted as a rigid block, protecting the intrusions from regional deformation, while basic plutonic and metamorphic host-rocks around the eastern part of the teg Orak pluton had a more plastic behavior and transmitted the regional strain to the intrusion.

A study carried out on widespread sites of the Aptian– Albian formations in Lebanon led to two palaeomagnetic directions corresponding to the primary magnetization (N ¼ 37 sites, D ¼ 307.18, I ¼ 23.78, k ¼ 18 and a 95 ¼ 5.58 after tilt correction and to a post-folding remagne-tization (N ¼ 18 sites, D ¼ 346.38, I ¼ 49.28, k ¼ 108 and a 95 ¼ 3.28 before tilt correction). Comparison of these data with previous palaeomagnetic results for the Jurassic age in Lebanon and expected directions from African apparent polar wander path yields evidence of three different counterclockwise regional rotations, of the order of 338 before Aptian deposition, of 118 during Late Miocene times, and of 188 since Miocene period. The two last rotations are related to the relative displacement of the African and Arabian plates. A model is proposed for the evolution of this particular Middle East area, in which the Dead Sea Transform shows a strong deviation relative to its main north–south orientation.

– Une réaimantation " syntectonique " éocène a été mise en évidence dans toute la virgation des Corbières, des Fenouillèdes à l'arc de Saint-Chinian. Son analyse montre que le synclinorium de Saint-Paul de Fenouillet a conservé son orientation E-W originelle, alors que son extrémité orientale a subi une torsion anti-horaire importante à l'amorce de la virgation. La branche méridienne de la virgation est composée de blocs plurikilométriques qui présentent une rotation senestre homogène sur toute leur surface, mais différente d'un bloc à l'autre. Le paléomagnétisme a permis de mettre l'accent sur le caractère majeur de l'accident de Vingrau, orienté NE-SW comme le réseau de failles des Céven-nes, qui limite des blocs ayant subi, de part et d'autre, des rotations de valeurs différentes. En outre, il a été démontré que localement le socle avait subi la même rotation au même titre que la nappe des Corbières qui le surmonte. La com-paraison des données paléomagnétiques avec la réaimantation, déterminée dans les Cévennes et ayant permis de dater de l'Eocène les minéralisations sulfurées liées aux strates (MVT), montre que la même réaimantation est présente dans l'avant-pays pyrénéen. Regional rotations evidenced by the paleomagnetism in the Corbières orocline (France) Abstract. – Eocene " syntectonic " remagnetization has been documented throughout the Corbières orocline, from the Fenouillèdes area to the Saint-Chinian arc. Analysis of the Saint-Paul de Fenouillet synclinorium shows that, although the main part of this area retained its initial E-W orientation, its eastern end was affected by a strong anticlockwise rotation. The N-S part of the orocline is composed of blocks that are several kilometres in size, which display homogeneous anticlockwise rotations that differ from one block to another. Paleomagnetism brought out the importance of the Vingrau structure that trends NE-SW like the fault network of the Cévennes, and which itself separates blocks affected by differing amounts of rotation. Moreover, it has been shown that the basement has also been affected by the same rotation as the overlying Corbières thrust sheet. Comparison between the paleomagnetic data and the remagnetization obtained in the Cévennes area (which dated the MVT ore deposit to the Eocene), shows that similar remagnetization also affects the Pyrenean foreland.

The anisotropy of low field susceptibility (AMS) represents the orientation distribution of all minerals in a rock, whereas the anisotropy of magnetic remanence (AMR, preferably anhysteretic) isolates that of the accessory remanence-bearing minerals. The subtraction of normalized AMR from AMS, in theory and under limited practical circumstances, may isolate the paramagnetic+diamagnetic anisotropy contribution and thus the orientation distribution of the

A paleomagnetic study has been carried out on three sedimentary formations of the Permian Rodez basin in the southern France. Two of them yield paleomagnetic poles of Saxonian and Thuringian age showing counterclockwise rotation of moderate amplitude, during or after the Thuringian deposition. For the French Massif Central, contrary to its stable southern (Lodève basin) and eastern (Largentière basin) borders,

Two different paleomagnetic directions have been obtained in several formations in northwestern Bulgaria. One of them, found only in Upper Permian–Lower Triassic red sandstones, is very likely a primary magnetization. A secondary magnetization of Eocene age appears as either pre-, syn- or post-folding depending on the site. The identification of the timing of remagnetization with respect to folding allows us

We report a detailed paleomagnetic and rock magnetic study of 17 independent lava flows belonging to the Trans-Mexican Volcanic Belt; 175 oriented samples were collected in the Ceboruco-San Pedro volcanic field. These sites were previously dated by means of a state-of-the-art 40 ...