Influence of décollement friction on magnetic fabrics of sedimentary rocks: Insights from AMS analysis in the Kohat fold-thrust belt, Pakistan

Abstract

The Kohat fold-thrust belt (KFTB) in the northwestern Himalayan foreland provides a unique opportunity to investigate strain partitioning and the role of evaporitic décollement in fold-thrust belts (FTBs) propagation through magnetic fabrics analysis. Using anisotropy of magnetic susceptibility (AMS) as a strain marker through analog models of FTBs, have already demonstrated the impact of high and low friction décollement in the magnetic fabrics development. In order to validate the accuracy of such model and to enhance the understanding of strain partitioning in FTBs with low-friction décollements, an integrated framework combining AMS and structural analyses was applied to the KFTB. AMS investigations of oriented cores from the Siwalik strata of 72 sites, acquired at different structural levels within the KFTB, highlight distinct magnetic fabric patterns. The low-friction décollement-proximate Siwalik strata exhibit Type-I sedimentary magnetic fabrics, the décollement-distal strata possess intermediate fabrics and the décollement front strata exhibit Type-VI tectonic fabrics. Collectively, the AMS parameters analysis and magnetic fabrics classification reflect the influence of structural controls on magnetic fabrics development. The Type-I sedimentary magnetic fabrics in the Early Miocene Kamlial Formation signify paleo-Indus flow direction towards southeast direction in the KFTB. A new conceptual schematic model integrated with bootstrapped AMS ellipsoids along different structures of the KFTB emphasize the influence of décollement friction, intensity of deformation and FTB propagation over weak layers.