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.
This study was supported by the Israel Science Foundation (ISF grants 868/17) and a grant from the Israeli Government under Geological Survey of Israel DS project 40706. We thank Catalina Luneburg and Stephen Laubach for efficient editorial handling, together with Ruth Soto and Manish A. Mamtani for helpful reviews and constructive comments. RW was inspired by John Ramsay while participating in a fieldtrip to the Alps led by John in 2002. GIA would like to take this opportunity to acknowledge John Ramsay's support while a post-doc at ETH Zurich in the late 1980's. TL had the privilege of showing John Ramsay outcrops of slump horizons and seismites in the lacustrine sediments of the Dead Sea region during John's visit to Israel in 2008.
- Magnetic fabrics
- Strain ellipsoid