MAGNETIC FABRIC AS A STRUCTURAL INDICATOR OF THE DEFORMATION PATH WITHIN A FOLD THRUST STRUCTURE - A TEST CASE FROM THE CORBIERES (NE PYRENEES, FRANCE)

In order to define the deformation path and the distribution of internal deformation in a foreland fold-thrust structure, the anisotropy of magnetic susceptibility was measured in 150 samples taken from 11 sites throughout the Vitrollian sequence of the Lagrasse structure (most external Pyrenean Zone). Whereas surrounding rocks seem completely undeformed, the Vitrollian Formation exhibits a complete set of microstructural markers thereby allowing fault striation measurements at sites also sampled for magnetic fabric analysis. Structural analysis shows that two mechanisms acted successively during thrust motion. The first one resulted from a NW-SE layer parallel shortening (LPS). It was responsible for a well defined vertical magnetic foliation parallel to the regional cleavage, in which a vertical magnetic lineation developed in conjunction with an increase in the shortening intensity towards the zone which would subsequently fail, leading to the development of an imbricate thrust system. During this second event, internal deformation was concentrated around the hinge and in the forelimb of the frontal fold. Around the hinge, the magnetic lineation rotated to an horizontal NE-SW trend parallel to the fold axis, indicating a significant component of longitudinal stretching during the development of the frontal ramp-anticline. Subsequently, the magnetic lineation swung again and became parallel to the NW-SE tectonic transport direction beneath out-of-sequence forelimb thrusts. This complex deformation history is not accompanied by a linear increase in the degree of anisotropy. On the contrary, we observe a cycle of construction and destruction of the magnetic fabric during LPS and imbricate thrusting.