Miocene long-runout debris-avalanche deposits in the Eastern Pamir foreland basin record cataclasis and fragmentation mechanisms

Abstract

The Miocene Aertashi Debris-Avalanche Deposits at the southwestern Tarim Basin, the foreland basin of the Eastern Pamir (NW Tibetan Plateau), have an incredibly long-runout distance (much longer than 130 km) and provide critical constraints on the evolution of drainage patterns, tectonics, and geochronological framework of the region. To investigate when and how the deposits formed, we conducted extensive field and microstructural observations, as well as geochronologic analyses. The deposits are massive, poorly sorted, and matrix-supported, containing massive blocks, jigsaw-fractured clasts, and fluidal-shaped cataclastic lava bodies. Some cataclastic lava bodies up to ~25 m across are compact on the hand-specimen scale but are pervasively fragmented on the microscale. Microstructural observations of particles show abundant diagnostic features of highly-energetic collisions, such as conchoidal fractures and microcracks. Both field and microstructural observations indicate that the deposits were derived from a debris avalanche. Pervasive cataclasis of many fluidal-shaped lava bodies is interpreted to record intense shear of hot, semisolid lavas close to the glass-transition temperature, in which phenocrysts deformed brittlely while the glassy components mainly deformed plastically and controlled the rheology of the bodies as ductile material. The mechanical properties of clasts are important in controlling their deformation within debris avalanches. The presence of abundant hot, semisolid lavas in the studied deposits may be critical for the survival of megaclasts after a long-runout distance. The field and microstructural observations and geochronologic analyses show that debris-avalanche deposits can aid in constraining regional geochronologic framework. These results consistently confirm that the Aertashi Debris-Avalanche Deposits were deposited at ~11 Ma, during rather than long after the eruptive period.