Abstract
Introduction. – The Variscan basement of the French Massif Central is considered [Faure, 1995] to have suffered (i) a southwestward nappe stacking during the early Carboniferous ; (ii) a NW-SE trending extension during the late Visean and granite emplacement during Namurian and Westphalian time ; (iii) a NE-SW trending extension during late Carboniferous-early Permian. The structure of the Veinazès and Marcolès monzogranitic plutons, located in the Châtaigneraie (southern French Massif Central), is studied in this paper through the anisotropy of magnetic susceptibility technique (AMS) to better understand the effects of the Namurian-Westphalian phase which is poorly documented in this region.
Magnetic and structural study. – The long axis K1 of the AMS ellipsoid represents the magnetic lineation and the short axis K3 is normal to the magnetic foliation. This magnetic fabric may be equated with the mineral fabric. Km, the mean magnetic susceptibility, corresponds to the arithmetic mean (K1+K2+K3)/3. Pp %, the magnetic anisotropy ratio, corresponds to ((K1 – D/K3 – D) – 1) × 100, D being the diamagnetic component.
Magnetic susceptibility (K) values (fig. 3 ; table I) range from 3.5 to 18.9 × 10−5 SI (mean 12.3 × 10−5 SI) for Veinazès, and from 0.7 to 13.1 x 10−5 SI (mean 8.1 × 10−5 SI) for Marcolès. Such values are typical of dominantly paramagnetic rocks. This is confirmed by the very good correspondence between these measured values and the magnetic susceptibilities calculated from sites for which chemical analyses are available (table II). For Veinazès the highest K values are mainly located in the western part of the pluton, whereas the weakest are in the northeastern part (fig. 3). This corresponds roughly to the distribution of the main petrographic facies determined by Joubert [1978] and Bogdanoff et al. [1989a].
The anisotropy of the magnetic susceptibility ratio (Pp %) (fig. 4 ; table I) ranges from 0.9 % to 5.3 % (mean 2.2 %) for Veinazès and from 0.8 % to 4 % (mean 2.2 %) for Marcolès. The part of the Veinazès pluton located to the south-west of the Sansac-Montsalvy line displays a mean anisotropy (2.7 %) clearly higher than the northeastern part of the pluton (1.6 %). For Marcolès, the map of Pp % does not display significant distribution heterogeneities. Three types of microstructures were determined in thin-sections in the Veinazès and Marcolès granites (fig. 5) : 1–undeformed magmatic textures ; 2– late magmatic weakly deformed textures ; 3– late magmatic deformed textures. There is a good correspondence between the mapped distribution of these microstructures (fig. 6) and the Pp % values (fig. 4).
Magnetic foliations (fig. 7 & 8 ; table I). Veinazès : to the north-east of the Sansac-Montsalvy line, the foliations are roughly organized in a dome. To the south-west of this line, the foliations are mainly WNW-ESE trending with a medium to steep northward dip. Near the borders of the pluton the foliations become WSW-ENE. Marcolès : the foliations are concentrated around a NW-SE trending mean direction, with generally steep (≥ 60o) and northeast dips. Near the extremities of the pluton the foliations are closer to an E-W trend. In both plutons the pattern of foliations tends to form Z-sigmoids.
Magnetic lineations (fig. 7 & 9 ; table I). Veinazès : to the north-east of the pluton, the lineations display generally shallow plunges and N-S trending directions. In the central part of the pluton they display very shallow plunges and are mainly E-W trending. To the west, the lineations display shallow to steep plunges mainly trending either E-W or NW-SE. Marcolès : most lineations are NW-SE to E-W trending, with middle SE or eastward plunges. These lineations form a Z-sigmoid pattern in plan view.
Discussion and interpretation. – The shallow plunging lineations carried by rather steep foliations and the sigmoidal features observed in plan view, show that most magmatic fabrics of both plutons may correspond to strike-slip movements. Moreover, the organization of the main cleavage in the country rocks around the Veinazès pluton could correspond to an E-W-trending left-lateral transcurrent zone (fig. 1). In the western part of the Châtaigneraie, the mean direction of the cleavage becomes N150oE-165oE, with eastward middle to steep dips, indicating a clockwise rotation with respect to the regional trend. Dextral shearings are linked to this NNW-SSE band.
Gravimetric data indicate that the Veinazès pluton is characterized by a rather flat floor and a single zone of relative deepening (2.5 km) between Ladinhac and Sansac-Veinazès (fig. 10), interpreted as a possible root. For the Marcolès pluton the maximum depth (1.25 km) is located beneath the village of Marcolès.
We propose the following model (fig. 11) for the emplacement of the Veinazès and Marcolès plutons during the late Namurian-early Stephanian time-span:
initial opening of the crust corresponding to the present deepest parts of the plutons floor, led by ENE-WSW-trending left-lateral strike-slip faults (fig. 11a) ; magmatic ascent and accretion of the plutons (fig. 11b) ; about 40o clockwise rotation of the Marcolès pluton, the SW end of the Veinazès pluton and their country rocks, linked to a NNW-SSE right-lateral strike-slip band (fig. 11c).
We consider that these events predate the low temperature sinistral movements along the NNE-SSW-trending Sillon Houiller during the Stephanian time.
Conclusion. – The magmatic fabrics measured in the Veinazès and Marcolès plutons, combined with previous data from the country rocks, show the probable importance of transcurrent movements for the emplacement of these plutons. The rotation suffered by the western part of this region after granites were emplaced would need to be better contrained by palaeomagnetic measurements on the Marcolès pluton.
Determining the Plio-Quaternary uplift of the southern French massif-Central; a new insights for intraplate orogen dynamics
