Tectonic fabrics vs. mineralogical artifacts in AMS analysis: A case study of the Western Morocco extensional Triassic basins

• Magnetic fabrics of 42% of the sampled sites have primary extensional fabric.
• Secondary magnetic fabric occurs in sites with axes switching or axes tilted with respect to bedding.
• The main carrier is hematite and to a minor extent, phyllosilicates.
• Post-sedimentary hematite hexagonal platelets carry the secondary magnetic fabric.

New magnetic fabric data from 48 sites in Upper Triassic red beds from the Argana, Asni and Tizi n’Tichka areas in the western High Atlas, in combination with rock magnetic analyses, SEM observations and qualitative chemical analyses, reveal that mineralization processes can affect the primary (extensional) or secondary (post-depositional) magnetic fabrics. Twenty out of the 48 analyzed sites show tectonic-related fabrics consistent with the rifting stage (primary). Their orientation suggests that the extensional Atlasic (for the Asni area) and Atlantic (for Argana area) distinct directions prevailing during Liassic times are already present in the Upper Triassic sediments. The other 28 sites show axes switching (including different possibilities, kmax-kmin or kint-kmin), indicating their secondary development related to mineralogical changes after deposition. However, orientation of magnetic susceptibility axes (without considering their relative value) is consistent with the main directions obtained for the rifting stage. This magnetic fabric study also suggests that (i) extension had a small transtensional component and (ii) there is a limited influence of compressional inversion tectonics.

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