Rheologic properties and kinematics of emplacement of the chaos jumbles rockfall avalanche, Lassen Volcanic National Park, California

Abstract

The Chaos Jumbles is a rockfall avalanche deposit that was emplaced by three separate events ∼ 300 years ago. Deposits from each event are distinguishable on the basis of morphology, size variation of large dacitic surface clasts, and by the color of both the matrix and entrained dacitic blocks. Steep lateral and distal deposit margins and surface features such as folds and apparent strike‐slip faults indicate that each rockfall avalanche had a finite yield strength and was being actively deformed and sheared throughout the body of the moving deposit, rather than strictly along a basal surface. Kinematic analysis of the three deposits indicates that each had a very low apparent coefficient of friction and was emplaced at velocities of up to ∼ 100 m/s. These data suggest that each rockfall avalanche can be modeled as a pseudoplastic material undergoing flow parallel compression above a frictionless base. This model allows calculation of deposit volumes ranging from ∼1.2 to 1.7 × 10 8 m 3 and also suggests that a future rockfall avalanche from the same location would have a more restricted runout than the previous events.