Distinctive shear zones demonstrate pervasive laminar cataclastic flow throughout the gigantic Iymek rock avalanche

Intensive fragmentation is a pervasive process during rock avalanche propagation, with a series of typical shearing characteristics being generated, indicating the occurrence of differential shear-induced comminution of clasts. However, much less is known about how shearing evolves within a rock avalanche over long runout. In the Iymek rock avalanche (IRA), pervasive shear zones characterized by multistoried, multiscale, and multistyle shear structures are well developed along the travel path of the IRA, providing an invaluable opportunity to decipher the evolution of shear-induced fragmentation occurred in rock avalanches. In these shear zones, a series of indicative structures, including preserved stratigraphic sequences, jigsaw structures, Riedel shear arrays, sigmoidal shear planes, asymmetrical folds, bookshelf and boudin-like structures, and anastomosing shear networks, are identified. These structures indicate that a laminar-like flow regime characterized by low disturbance but intensive shearing should dominate the emplacement of the IRA, contributing to the formation of these pervasive shear structures and material facies. As indicated by the progressive evolution of these pervasive shear structures, three facies are determined from the transition zone to the accumulation zone, i.e., low-fragmented facies, ductile-dominated facies and brittle-dominated facies. The low-fragmented facies is mainly distributed in the transition zone, similar to a semisolid phase rather than the solid phase of the bedrock in the source area. Conversely, the brittle-dominated facies features a high fragmentation degree and is predominant in the accumulation zone, which mainly consists of a semifluid to fluid phase. This indicates that the rock avalanche mass should evolve from a solid phase to a fully fluid phase during propagation, which involves clast fracturing and comminuting, frictional sliding and rotation. Therefore, we propose that the rock avalanche is a classical cataclastic flow driven by progressive and differential shearing comminution during its laminar-like propagation instead of a granular flow. These results provide a significant basis for interpreting similar structures in other rock avalanche deposits and yield new insights for understanding the emplacement mechanisms of rock avalanches.