Strain Localization: Analog Modeling and Anisotropy of Magnetic Susceptibility

Abstract

Abstract The majority of the strain in Earth crust and upper mantle is localized to the high strain zones developed at ductile‐to‐brittle condition at kilometer‐to‐micrometer scale. Therefore, they represent the key to understanding the deformation evolution of the lithosphere. The finite strain pattern recorded within these zones has been therefore a subject of research in geology. The methods studying rock magnetism such as the anisotropy of magnetic susceptibility (AMS) are frequently used techniques to characterize and quantify deformation and flow record in rocks. Numerous sedimentary, subsolidus and submagmatic deformation zones exhibit typical evolution of the AMS ellipsoid across the strain gradient suggesting indirect not straightforward correlation between AMS and strain ellipsoids. To document spatiotemporal and internal fabric evolution during strain localization, pure shear, simple shear, and shear zone (SZ) analog experiments were performed using shear‐thinning thixotropic material of plaster of Paris. The experimental results closely resemble the record from natural SZs in sedimentary rock systems but also in subsolidus SZs and submagmatic mushy systems. The magnetic fabric evolution across deformation zones is interpreted to be associated with the intersection and transposition of preexisting primary fabric with shear fabrics and evolution of synkinematic subfabrics. Their development is attributed to localization of deformation at microscale due to the self‐organized slip of anisometric particles forming microshear planes reflecting the symmetry of deformation. The experimental results when confronted with the natural examples implies that the localization and partitioning of deformation is one of the most important factors for the interpretation of AMS in deformation zones.

Keywords

• shear zone
• anisotropy of magnetic susceptibility
• analog modeling
• strain