| Popis: |
Debris avalanches (DA) are large landslide events characterised by long runouts and high mobility that poses a great hazard to communities close to volcanoes. Although many theories have been proposed to explain the excessive runout phenomenon, the mechanisms enabling the mobility remain unresolved and poorly constrained. As a result, it is still challenging for models and theoretical concepts to encompass DA deposit field observations.DA deposits are complex; however, detailed study of their sedimentary architecture can provide information regarding their propagation processes. In this study, the deposits of two DAs in the Canary Islands: Tenteniguada DA, located on the east of Gran Canaria; and Abona DA on the southeast of Tenerife have been examined. Although they are located in nearby volcanic islands they occurred in different environments with different triggering processes, scale, material and their deposits suggest different propagation rheology. A detailed field study of the deposits was carried out in September 2021, mapping their facies and feature distribution and sedimentology. Structure from motion photogrammetry methodology has been used to generate high accuracy 3D models of outcrops and sample windows to quantify facies distribution. The data collected allow for evaluation of the effects of material properties, substrate and its geometry, and to assess aspects of the dynamics of the DAs. Therefore, it was possible to generate conceptual models for the transport and emplacement mechanisms of the two events corresponding to the observations and to relate them to the two debris avalanche distinctive characteristics by comparison.In the Tenteniguada DA deposit, the degree of disaggregation is low, with large portions of the original edifice preserved along with their original stratigraphy, although displaced relative to each other by brittle deformation. In contrast, Abona DA is much more disaggregated. Monolithological blocks are microfractured and cataclased, and original stratigraphy is not preserved. There is no evidence of brittle deformation. The highly comminuted material has been elongated in a fluidised spreading flow, achieving a long runout on an erodible pumice substrate. Conversely, the Tenteniguada DA did not fully transition from a slide to a flow and has not generated a long runout while propagating in an active fluvial ravine. These findings suggest that the behaviour and the distribution of stresses was very different during propagation, owing to the properties and volume of the material in the flow and potentially the substrate properties and triggering mechanisms.The present study highlights how the field examination of sedimentological, morphological, and structural features is vital in fully understanding DA propagation and emplacement mechanisms. |
