Switch of Mesozoic extensional tectonic style in the Lusitanian basin (Portugal): Insights from magnetic fabrics

Magnetic fabrics in the Lusitanian basin show extreme variability in the orientation of strain axes during part of its Mesozoic extensional stage. A detailed study of Anisotropy of Magnetic Susceptibility (AMS) was done in Triassic and Upper Jurassic units in the onshore part of the basin. In the Triassic samples, the magnetic lineation shows a dominant NW-SE orientation, perpendicular to the main normal fault set and also to the extension directions inferred from the analysis of brittle mesostructures. For the Late Jurassic period, the AMS analysis indicates a dominant N-S to NE-SW extension. Moreover, these main directions are complicated by perpendicular solutions co-existing in the same sites. Comparison with syn-sedimentary faults reveals the difficulty in interpreting AMS data in these settings, although magnetic stability for the obtained solutions can be tracked through different mineralogical composition for the sampled rocks. Interpretation of the obtained fabrics through a Triassic radial extension regime is consistent with axes switching and local reorientation of strain axes close to major normal faults related to the opening of the Northern Atlantic during the Jurassic.