Analysis of the anisotropy of magnetic susceptibility in Paleozoic and Cenozoic sedimentary sequences affected by non-coaxial deformation in the Central and Eastern Precordillera, San Juan, Argentina

Abstract

Anisotropy of magnetic susceptibility (AMS) is an important tool for understanding the magnetic fabric of rocks,such as depositional conditions or information about strain ellipsoid. This contribution presents AMS results andmagnetic mineralogy analysis of sedimentary units from Central Precordillera (Agua Hedionda anticline) andEastern Precordillera (Las Salinas Norte, Sur Wedge anticlines), located in the central Andes of Argentina.The AMS ellipsoids do not preserve a sedimentary magnetic fabric, suggesting that it has been overprinted bytectonic deformation. The AMS ellipsoid presents a clockwise deviation of the maximum susceptibility axes (K1)by an average of 14–34◦ from the strike of the bedding plane and a pattern of deviations of the minimum axes(K3) from the normals to the stratification planes. These features are interpreted as the product of non-coaxialdeformation associated with interstratal shear under a transpressional regime. Integration of the AMS resultswith the structural characteristics of the study area provides a comprehensive model linking interstratal shearand transpressional deformation to the development of AMS fabrics. This approach indicates that the foldsformed through flexural-slip mechanisms and allows the reconstruction of the relative timing of the structuresinvolved. Accordingly, the Agua Hedionda and Las Salinas Norte anticlines exhibit AMS patterns consistent witha stage of non-coaxial deformation that postdated structural uplift. In contrast, the structural geometry of the LasSalinas Sur Wedge anticline, together with the observed AMS fabrics, suggests that folding developed synchronouslywith deformation during the regional transpressional event.