Open Session - SWISS GEOSCIENCE MEETINGs

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Symposium 1: Structural Geology, Tectonics and Geodynamics
Figure 1. Paramagnetic (white background stereonets) and diamagnetic (gray background stereonets) anisotropies are illustrated for diffe-
rent locations around the Morcles Nappe, on equal-area stereonets. Stereonet symbols refer to the maximum (squares), intermediate (tri-
angles) and minimum (circles) susceptibilities; solid-line great circles refer to the orientation of the cleavage plane; dashed great circles
indicate the bedding plane; stars indicate the mineral stretching lineation.
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Structural and kinematic analysis in the Dourbie river valley, Cévennes,
SE French Massif central
Augenstein Clemens* & Burg Jean-Pierre*
* Geological Institute, ETH Zurich, Leonhardstrasse 19, 8092 Zurich (aclemens@ethz.ch)
In Summer 2007 and 2008 two field seasons have been carried out in the SW Cévennes (SE French Massif Central) in order
to gain data for structural and kinematic analysis. During structural mapping rocks were sampled for microstructural,
crystallographic preferred orientation (CPO) and petrological investigations. Main lithologies in the mapping area are two
micaschists divided by a quartzite and the St-Guiral granitic intrusion. The structures suggest a continuous deformation
history: 1. SSW-SW directed shearing, 2. drag folds, 3. evolution of non-cylindrical folds due to heterogeneous shear, 4. buckling
of open folds 5. intrusion of the granite in a depth of 3kbar with 650°C generated small-scale kinks and flexures.
Recorded CPO patterns of the dynamically recrystallised quartzite can be classified into two types, one with a monoclinc
symmetry (type I) and one with an orthorhombic symmetry (type II). Type I is related to a simple shear deformation and type
II to an apparent constrictive coaxial deformation. These two types are consistent with the results of structural observations,
where a constrictive coaxial deformation regime could have been active during formation of non-cylindrical folds. In the
contact metamorphic aureole CPO patterns intensify towards the granite during static recrystallisation, until grain growth
and mica band spacing lead to preferred grain orientations parallel to the mica bands.