The observational and semi-quantitative sedimentological analyses of the lithofacies assemblage contribute to describe at different scales the breccia matrix along the sheared and transformed contact between rock avalanche deposits and trailing debris flow deposits emplaced by a large landslide at Mt. Meager volcano in British Columbia in 2010. An inverted cataclastic gradient of crushed breccias in a fluidized and mixed matrix implies a structurally controlled flow regime and rapid deposition. The coating matrix changed the initial polymodal distribution of the debris-flow lithofacies. Clast shape evolution helps to characterize the cataclastic sorting during transport and fluidized disaggregation. A plot of matrix percent against matrix/gravels helps to distinguish primary hot fracturing of about 61% from initial dilation and sheared fracturing involving ~22% matrix. Extensional disaggregation between 73 and 79% matrix is related to hydromagmatic fragmentation within an epithermal system. Clayey mineral assemblages identified by XRD patterns are related to colloidal aluminium gel, cataclastic shear bands, and quartz microstructures in epithermal breccia zones (pH = 2‐–3, 200‐–350 °C, <10‐– 20 GPa).Microstructural analysis differentiates the inner rim of coated clasts from their border and the surrounding matrix in impact melt breccias. Sequential coating stages are inferred during the propagation of the shock wave with an oscillatory relative speed during the inter-seismic period. We differentiate: 1) shock faulting which contributes to the impacted quartz (10‐–35 GPa) in a devitrified matrix and pseudotachylite coating related to frictional melting at the margin of a conduit; 2) shock response (85 GPa) in epithermal vein with calcic spheroids, CO 2 dissociation, and basaltic melt (70‐–101 GPa, >1500 °C); and 3) the secondary fracturing with flash heating and pressure pulse during cavitation (ΔP ~ 10 GPa, >1000‐–1500 °C), which generates pockets of partial melting, quartz spheroids, and a roll-over effect for the inner rim of coated clasts. The formation of impact melt breccias and the debris flow are related to the slowing elastic impact wave with an oscillatory relative speed during the inter-seismic period along a proximal strikeslip fault.This study helps identify how the proximal rock avalanche transformed into a highly mobile debris flow, larger examples of which pose a hazard to the town of Pemberton, at a distance of 65 km from Mt. Meager.