Structural characteristics and formation process of pseudotachylyte in the Wulian detachment fault zone on the Jiaodong Peninsula, China

Exploring pseudotachylytes (PSTs) within detachment fault zones provides valuable insight into the kinematic processes of these fault systems. Nevertheless, significant controversy persists regarding the origin and formation mechanisms of PST veins. The PST was initially identified within the Wulian detachment fault zone (DFZ) on the Jiaodong Peninsula by delineating the deformation characteristics of the fault zone's transition from ductile to ductile-brittle behavior. Macroscopic and microstructural features of the PST within the Wulian DFZ were examined using geological field surveys. Using quantitative and high-resolution techniques, such as scanning electron microscopy, TIMA-X mineral composition analysis, and transmission electron microscopy, variations in mineral composition between the veins and detritals of the PST were investigated. Furthermore, this study has investigated the progressive deformation processes and underlying causes contributing to PST formation within the DFZ. This study revealed the presence of both type D and B-type PSTs in the Wulian DFZ formed under ductile or ductile-brittle conditions. These PSTs exhibit fracture and fusion structures resulting from fault frictional fragmentation and frictional melting and undergo progressive deformation. During the initial stages of the D-type PST formation, biotite fragmentation in poorly cohesive foliation was the primary contributor, supplemented by frictional melting. As the deformation progressed, quartz fragmentation and partial melting became dominant, constituting up to 81.9% of the matrix. The formation of the B-type PST aligns with deformation under brittle or ductile-brittle conditions, with frictional fragmentation and fragmentation flow playing significant roles. Vein formation is initiated by biotite fragmentation or partial melting, followed by quartz, potassium feldspar, albite fragmentation, or partial melting. Mature PST veins exhibited distinct zoning characteristics, with a diffuse quartz distribution comprising up to 59.87% of the vein composition. Intermittent and abrupt uplift of the detached fault zone, along with the resultant fault friction, contributed to fine graining and melting, serving as pivotal factors in PST formation.