Fragmentation and shear zones in rock avalanches
Final Report Abstract
For about 130 years, scientists have been trying to understand the processes by which large rock avalanches move and cover unexpectedly high runout distances. One approach is to study their deposits for clues to their movement history, and much progress has been made in describing their morphology and sedimentology. The challenge is to provide data that can be used to infer the processes acting during runout and to ground-truth existing emplacement hypotheses. To achieve these data, we developed two sampling techniques. The first is designed for the analyses of grain size distributions, which can be used to infer breakage processes. This method is based on detailed facies mapping and the sampling of material from each facies alone; as opposed to bulk sampling that destroys the signals contained in individual facies. In the second method, the samples taken are undisturbed sections of outcrops encased in epoxy; in situ clast arrangements and degrees of fragmentation can thus be studied. We found that the facies we identified are present throughout the entire deposit length and depth, suggesting that all processes act within the entire rock avalanche mass and during the entire runout duration. Comparing the facies’ grain size distributions with those produced under controlled conditions in the laboratory by studies from colleagues, we identified (i) breakage along existing planes of weakness, (ii) fragmentation that creates new grain surfaces, and (iii) shear concentration in narrow bands as the dominant processes acting at increasing stresses during runout. Just this observation alone rules out any exotic emplacement hypothesis and supports those that are in agreement with heterogeneous stress distributions in the moving rock avalanche mass. Next, the development of each facies was investigated with regards to external influences. As saturated sediments in the rock avalanche’s path are loaded by the overriding debris, pore pressures are increased and the material seeks to escape upwards, finding easily accessible routes into the rock avalanche mass along shear zones near the base. These injections interfere with the shear zone and fragmentation within it ceases prematurely. Thinsections prepared from the undisturbed samples show grains in the process of breakage; their pieces still so close together that the original grain can be visually reassembled. The different stages and processes of grain breakage, as identified for each facies in their grain size distributions, are also reflected in the evolution of grain shapes from one facies to another. Although a couple of problems still exist in image acquisition techniques, the method is another promising tool for process identifications in deposits of unknown origin. Further work in developing the presented techniques is carried out by the applicant and her collaborators.
Publications
- (2017). Process-dependence of grain size distributions in rock avalanche deposits. Landslides 14(5): 1555-1563
Dufresne A, Dunning S
(See online at https://doi.org/10.1007/s10346-017-0806-y) - (2017). Rock avalanche sedimentology – recent progress. In: Mikos M, Tiwari B, Yin Y, Sassa K (eds) Advancing Culture of Living with Landslides 2(1): 117-122
Dufresne A
(See online at https://doi.org/10.1007/978-3-319-53498-5_14) - (2018). River-damming, late-Quaternary rockslides and rock avalanches in the Ötz Valley region (Tyrol, Austria). Geomorphology 310: 153-167
Dufresne A, Ostermann MA, Preusser F
(See online at https://doi.org/10.1016/j.geomorph.2018.03.012) - (2018). Sedimentology and geomorphology of a large tsunamigenic landslide, Taan Fiord, Alaska. Sedimentary Geology – Special Issue ‘Evidence of Geohazards’ 364: 302-318
Dufresne A, Geertsema M, Shugar D, Koppes M, Higman B, Haeussler P, Stark C, Venditti J, Bonno D, Larsen C, Gulick SPS, McCall N, Walton MA, Loso MG
(See online at https://doi.org/10.1016/j.sedgeo.2017.10.004) - (2019) Angepasste Sedimenttransferpräparate-Anfertigung und Bergung von Lockersedimentproben unter extremen Bedingungen. Der Präparator : Präparationstechniken für Naturwissenschaften, Museum, Medizin, Kultur- und Kunstgeschichte 6580-87
Kraus W, Dufresne A, Reicherter K
- (2019). Sample-bonding. Der Präparator 64: 6-20
Kraus W, Reicherter K, Urai JL
