Magnetic fabrics as strain markers in folded soft-sediment layers

Abstract

We exploit the potential of magnetic fabrics acting as strain markers in folded layers, by analysing an exceptionally well-exposed, recent (<1 kyr) slump horizon in unlithified lake deposits within the Dead Sea basin. The ∼3-m-long folded soft-sediment layer, together with an underlying basal detachment, and an ‘undeformed’ reference layer are extensively sampled (n = 97) for an anisotropy of magnetic susceptibility (AMS) analysis. This analysis reveals deformation fabrics within the folded layer which are significantly different from fabrics detected in the ‘undeformed’ layer. The maximum magnetic susceptibility axes (K1) show a hinge-parallel orientation, and the minimum magnetic susceptibility axes (K3) show a trail of orientations directed eastward parallel to the direction of downslope slumping toward the depocenter of the basin. In terms of shape of the AMS, samples from the ‘undeformed’ layer are oblate, while the majority of samples from the fold backlimb are oblate to neutral, and those from the forelimb and hinge zones are more prolate. We postulate that the deformation shown by the AMS analysis approximates well to sections through the strain ellipsoid in the folded layer, suggesting that magnetic fabrics serve as strain markers that are invisible to the naked eye. The deformation fabrics are created by particles moving relative to one another and reorganising during hydroplastic deformation. Particles physically rotate in the hinge zone, resulting in shortening of the intermediate axes and creation of more prolate shapes. The combination of two types of fabrics (deposition and deformation) in the hinge zones increases the intensity of the lineation due to the intersection of the primary and secondary fabrics (foliations). Based on the dense sampling scheme, we produce GIS-based interpolation maps that show the spatial distribution of the AMS parameters in the folded layer. These maps are compared to data from classical strain analyses, providing a benchmark for combining traditional structural methods and AMS analyses in studying folding and soft-sediment deformation.