Shear deformation bands in clastic dikes: a case study from the Miocene Pohang Basin, SE Korea

Abstract

Clastic dikes and deformation bands are common geological structures in the shallow parts of sedimentary basins. Although their co-occurrence is relatively rare, some cases have been reported, but their detailed characteristics and formation mechanisms remain unclear. This study investigates a clastic dike in the Pohang Basin, southeastern Korea, where deformation bands are prominently developed, using structural observations and material analysis from outcrop to microscopic scales. At the outcrop scale, 1-2 mm-thick shear bands with normal displacement up to 1.3 cm were observed within the dikes. Some bands terminate within the dike, while others propagate into the surrounding mudrock, forming wedge structures. Microscopically, the bands in the dike show cataclastic textures in quartz and feldspar clasts, whereas those in the mudrock consist of well-aligned platy minerals with ultrafine quartz and feldspar grains, accompanied by fold structures. Stress conditions inferred from these structures indicate formation during crustal extension, with a subvertical maximum principal stress (sigma 1) and a subhorizontal, E-W-trending minimum principal stress (sigma 3). The nucleation and propagation of shear deformation bands in the dike and their termination in the mudrock are attributed to the lower shear strength of the sand in the dike compared to the mudrock, as well as the shear hardening behavior of the mudrock. Notably, shear deformation bands terminating in the mudrock display asymmetric bifurcation near their tips, similar to synthetic branch faults. These findings provide insights into shear deformation in sedimentary strata composed of alternating sediments with different physical properties.