Original paper

Complex magnetic fabrics in cleaved marls; link to deformation processes (Southern Pyrenees, Spain)

Oliva-Urcia, B.; Pueyo, E.L.; Larrasoaiia, J.E.; Gi, A.; Mata, P.; Pares, J.M.; Schleicher, A.M.; Pueyo, O.


Room Temperature AMS (RT-AMS) ellipsoids from Upper Cretaceous carbonatic rocks in a well-cleaved domain in the SouthwesternPyrenees exhibit an unusual orientation:neither all sites show the maximum axes in the intersectionlineation (LI) nor the minimum axes are at the pole of the cleavage plane. The study of the subfabrics(Low-TemperatureAMS and AARM -Anisotrpy of the Anhysteretic Remanent Magnetization-) is required to control the orientation of the different carriers of the fabric (paramagnetic and ferromagneticrespectively). The orientation of the paramagnetic grains (phyllosilicates) shows a blocking time in the early stages of the diagenesiswith the minimum axis still perpendicularto the bedding plane. The phyllosilicatesrespond to a layer parallel shortening, and their orientation is not further altered when the cleavage domain develops later in upper Eocene times, when the rocks are lithified. On the contrary, the ferromagnetic grains seem to orient in relation to the strain field, which produces also the cleavage domain. The maximum axes of the ferromagnetic ellipsoids have a NNE-SSW direction, which coincides with the stretching direction of thrusts. Despite the fact all samples come from a homogeneous structural context, the results do not follow a common pattern and especially do not conform the typical model for a cleavage domain. A careful consideration of magnetic fabrics in their particular tectonic context is necessary regarding the results we present here.