Complex structure of an upper-level shale detachment zone: Khao Khwang fold and thrust belt, Central Thailand

•A newly identified upper-level shale detachment zone in the Khao Khwang fold and thrust belt of central Thailand is an ideal natural laboratory to study the deformational behaviour and mechanisms of shale at a high detail.•Deformation is a mixture of viscous and brittle mechanisms but there is little evidence of classically ‘mobile shale’ through ductile deformation.•Deformational mechanisms are influenced is characterised by competency contrast of rocks as well as proximity to the thrust at the base of the damage-zone.•Deformation is complexly three-dimensional, with duplexes forming egg-carton like geometries in a system of anastomosing faults. The damage-zone exhibits significant and complex along-strike variation.

Shale detachment zones have been previously described as largely mobile in character; however, increasing resolution of seismic images and understanding of these zones suggest brittle deformation may have a significant role in their deformational behaviour and in the deformation of overlying fold and thrust belts. Dependence on seismic imaging and other indirect and low-resolution study methods has resulted from the lack of outcropping shale detachment zones, both active and ancient, for detailed study. However, a newly described upper-level detachment zone in a shale unit in the Khao Khwang Fold and Thrust Belt in Central Thailand is exceptionally well-exposed. We use detailed structural analysis to investigate deformational mechanisms of this ancient, exhumed detachment zone, as an analogue to active modern-day examples. Through detailed field mapping we were able to construct multiple cross-sections through the detachment zone. These cross-sections were subdivided into structural domains, which display an increase in density and complexity of deformational structures (folds, faults), and heterogeneity of their orientations toward the thrust at the base of the detachment zone. Contrasting strain rates in zones of continuous-discontinuous deformation result from contrasting rock-competencies through the sections. We conclude that the fault damage-zone in this study constitutes an upper-level detachment and is potentially analogous to those in active collisional fold and thrust belts. It displays mixed brittle–ductile deformation and a complex, three-dimensional anastomosing fault-system.