Correlating magnetic fabrics with finite strain: Comparing results from mudrocks in the Variscan and Appalachian Orogens

• Published in: Acta geologica hispanica Vol. 2; no. 3; pp. 213 - 220
• Main Authors: Parés, Josep Mª, Van der Pluijm, Ben A.
• Format: Journal Article
• Language: English
• Published: Barcelona Universitat de Barcelona 2004; Universitat de Barcelona Revistes Cientifiques de la Universitat de Barcelona; Institut de Ciències de la Terra "Jaume Almera"; Universitat de Barcelona (UB), Geociències Barcelona (Geo3BCN), Institut de Diagnosi Ambiental i Estudis de l'Aigua (IDAEA), Universitat Autònoma de Barcelona (UAB)
• Subjects: Anisotropy; Anisotropy of Magnetic Susceptibility; Cantabrian Arc; Finite strain; Rocks; Valley and Ridge Province
• ISSN: 1695-6133; 0567-7505; 1696-5728
• DOI: 10.1344/105.000001428

New magnetic anisotropy data from Variscan mudstones collected in the Cantabrian Arc, N Spain constrain the corresponding strain (shortening). The results are based on our previous study of mudrocks from the Valley and Ridge Province (Appalachians) where independent strain quantification of pencil structures permitted a correlation between magnetic fabric and tectonic strain. An exponential relationship between the AMS shape parameter T and tectonic shortening was found for the interval of 10-25% shortening: shortening (%)=17*exp(T), relationship that appears to be supported by tectonic strains up to 40%. The T parameter describes the shape of the magnetic susceptibility ellipsoid, which in pelitic rocks appears more sensitive to strain than the degree of anisotropy parameter P (or P’). In mudrocks from the Cantabrian Arc a positive correlation between T parameter and deformation intensity, reflected by cleavage domains spacing, is observed. Using the above relationship, we estimate the range of tectonic shortening for the Cantabrian mudstones. The correlation between strain and AMS offers a robust estimate of strain magnitude of 10-40% in weakly to moderately deformed clay-rich rocks, where other strain indicators are often lacking or are poorly preserved.