Andrés Gil Imaz

Factors influencing magnetic susceptibility in the southern Pyrenees

We analyse paramagnetic, ferrimagnetic and frequency-dependent susceptibility in the southern Pyrenees and northern Ebro basin. The analyzed rocks show a wide range of values for the three parameters within the different lithological groups. Sandstones, shales and marls exhibit middle to high values of paramagnetic susceptibility while mudstones usually show low values. However, a general correlation can be established for the different rock types with a wide variability for each group (for example, Eocene marls show average values of paramagnetic susceptibility of 82% but within a range between 12 and 99%, or Flysch materials, with mean values of 47–52% within a range between 15 and 90%). These changes in the paramagnetic contribution to the susceptibility are due to the change in the ferrimagnetic susceptibility. This wide variability is independent of the sedimentological characteristics and can be related to the higher ferrimagnetic contribution to the susceptibility with similar ferrimagnetic mineral phase assemblages. The ferrimagnetic and superparamagnetic contributions, independent of site location with respect to individual macrostructures (thrusts and nappes), show a sharp increase at the occurrence of the macroscopic regional Pyrenean cleavage, probably relating both aspects with changes in pressure-temperature (P-T) conditions.