Illite K‐Ar Dating of the Leibo Fault Zone, Southeastern Margin of the Tibetan Plateau: Implications for the Quasi‐Synchronous Far‐Field Tectonic Response to the India‐Asia Collision

Whether tectonic strain from the early stage India‐Asia collision has synchronously affected the far‐field margin of the Tibetan Plateau is crucial for understanding plateau deformation and growth processes. However, direct evidence for early far‐field deformation remains scarce. Utilizing illite K‐Ar dating of three fault gouge samples, we established the faulting history of the Leibo fault zone (LFZ) at the southeastern margin of the Tibetan Plateau (SEMTP). Consistent authigenic illite ages of 52 ± 2, 54 ± 12 and 55 ± 6 Ma suggest the reactivated thrust faulting of the LFZ in the Early Cenozoic. Positioned ∼700 km east of the collisional boundary and at the intersection of three blocks with distinct lithospheric rheology in strength/viscosity, this event suggests a quasi‐synchronous far‐field tectonic response in the SEMTP to the India‐Asia collision. Plain Language Summary Understanding how tectonic strain resulting from the collision between the Indian and Eurasian plates in the Early Cenozoic can spread to the edges of the Tibetan Plateau is crucial for comprehending the plateau's tectonic development. To investigate this question, we used fault gouge, which resulted from the sliding of brittle faults, to directly reveal faulting information. By conducting illite K‐Ar dating on three samples from the Leibo fault zone (LFZ) at the southeastern margin of the Tibetan Plateau (SEMTP), we found consistent ages of newly formed illite, 52 ± 2, 54 ± 12, and 55 ± 6 million years ago. Since this type of minerals form simultaneously with faulting, these ages reveal a reactivated thrust faulting event of the LFZ, suggesting a nearly simultaneous response of faulting in the SEMTP to the early stage of the India‐Asia collision. Key Points Fault gouge illite K‐Ar dating is used to establish the faulting history of the Leibo fault zone, SE margin of the Tibetan Plateau Three consistent authigenic illite ages of ∼55, ∼54, and ∼52 Ma suggest a reactivated thrust faulting event of the Leibo fault zone This event indicates a quasi‐synchronous far‐field response of the SE margin of the Tibetan Plateau to the India‐Asia collision