Microstructural Insights into Strain Partitioning During Incipient Folding in Mechanically Layered Strata: Evidence from Deformation Bands in the Outer Carpathians, Poland

Deformation bands provide a microscale record of strain localisation within sandstones and offer key insights into deformation mechanisms and conditions. This study integrates detailed field observations with optical microscopy and three-dimensional X-ray microtomography (µCT) to characterise deformation bands in thick-bedded sandstones of the Krosno Formation (Silesian Nappe, Outer Carpathians). Two sections within a regional first-order fold were examined: an upper, mudstone-rich and mechanically weak unit, and an underlying sandstone-dominated competent unit. The contrasting kinematics of the deformation bands reflect layer-parallel strain partitioning during the onset of folding. Normal-shear bands developed in the weaker upper unit, whereas compaction bands formed pervasively in the competent unit. Microstructurally, shear bands are sharply bounded, organised in arrays, and dominated by grain rearrangement with local cataclasis, while compaction bands exhibit diffuse margins, tight grain packing, and disaggregation through progressive cataclasis. These features indicate that the bands formed under shallow-burial (<500 m) conditions. µCT imaging reveals the bands as darker, low-attenuation zones relative to the host rock, reflecting post-deformational cementation and the absence of cement within the bands. This diagenetic contrast enhanced mechanical heterogeneity and promoted later reactivation and fracture development. The study provides a three-dimensional microstructural assessment of early strain localisation in mechanically layered rocks in the buckle fold limb.