Magnetic fabrics of weakly-deformed mudrocks from the Jaca-Pamplona basin (Pyrenees); new constrains on the sensitivity of magnetic fabrics and the tectonic evolution of the Southern Pyrenees

Abstract

The anisotropy of magnetic susceptibility (AMS) is a sensitive marker for studying the spatial and temporal evolution of orogens. In weakly deformed rocks deposited in compressive contexts the AMS signal mostly reflects the preferential alignment and deformation of paramagnetic phyllosilicates during early layer parallel shortening (LPS) related to both near and/or far-field structures. Moreover, localized deformation and post-depositional shearing in the vicinity of thrusts can alter early LPS fabrics. An AMS analysis of 651 samples was conducted along two stratigraphic sections, Izaga and Berdún (2800 m-thick), in the Jaca-Pamplona basin (Southern Pyrenees). The sampled sections record the tectonosedimentary evolution of the basin from the Late Lutetian to the Middle Priabonian. In both sections, the magnetic fabric is controlled by LPS. The lower and upper part of the Izaga section shows triaxial to oblate and oblate ellipsoids, respectively, which are interpreted to indicate a decrease of tectonic activity associated to the Larra thrust system. The Berdún section begins with oblate ellipsoids, which subsequently become triaxial and prolate before returning oblate towards the top of the succession. Ellipsoids in the lower part of the section exhibit an incipient girdle between the minimum and intermediate axes, which signal an undetected thrust, further corroborated, by structural analysis. The remainder of the Berdún section shows a magnetic fabric derived from LPS but it is not possible to associate it to any structure. The uppermost part of the section displays prolate ellipsoids, which we link to enhanced tectonic activity leading to continentalization of the basin.

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