The genesis, development, and evolution of original vertical joints in loess

Original vertical joints (OVJs), which are highly correlated with loess anisotropy, have been considered a non-tectonic source of extensive erosion, gully development, and landform fragmentation in loess regions such as the Chinese Loess Plateau. The existing literature discussing the origin of OVJs lacks a systematic overview of the current theories related to OVJ genetic mechanisms, development patterns, theoretical models, and relationships between OVJ morphologies and loess historical sedimentary processes (LHSP) since the Quaternary. By reviewing the key factors governing the formation and evolution of OVJs in loess, we present both a conceptual framework and a case study to demonstrate the genesis and dynamics of OVJs in the loess landscape. We propose that when the driving force produced by the hydro-mechanical coupling regime exceeds the resistance force due to tensile strength, OVJs appear along the boundaries between different structural units within the loess soils. Influenced by the loess sedimentary effects in the later period (i.e., LHSP), such as loess pile thickening, drying-wetting cycles, and weathering, OVJs formed in historical loess frequently degrade and expand. Based on the knowledge of OVJ dynamics, we reconstruct the evolutionary trajectory of OVJs in the loess profile from the LHSP perspective, which is further confirmed by field observations on OVJ morphologies under different local microtopographical and geological conditions. The new perspective on the genesis, development, and evolution of OVJs enhances theoretical understanding of the formation of anisotropy and structural evolution in loess, which helps describe and model subsurface hydrology and geohazard risks in loess regions.