Dynamic Evolution of Multiple-generation Structures within Failed Granular Slopes: Analogue Models and Natural Landslides

Abstract:

Results of a series of analogue models, and both scanned data and sections of natural landslides are used to investigate the spatial and temporal distribution of the 3D internal structures during the collapse of granular slopes. Model results show that the collapse of granular slopes results in formation of different-generation normal faults in the rear of the failure mass and shortening structures in the front of the failure mass. The first-generation, steep normal faults (dipping about 60º) cut across the entire stratigraphy of the slope, and involve the bulk volume of the failure mass. In contrast, the later-generation normal faults (dipping about 40º) cut only across the shallow units, and involve less amount of the failure mass. These faults differ also in amount of displacement they accommodate. Results of homogeneous slope models show that later-generation faults accommodate significantly larger displacement than the first-generation faults. In contrast, results of non-homogeneous slope models embedding a weak horizon show that, the first-generation faults, which form within the weak horizon, accommodate significantly larger displacement than the later-generation faults. In other words, sliding is facilitated along the first-generation faults in the presence of a weak horizon. Beds rotate along these different-generation faults. Analysis of model results show that stratigraphically deeper beds rotate more than shallow ones. Our field observations confirm the presence of at least two generations of failure surfaces where the older ones, which are cut by the gentler ones, are steeper. Sections through natural slides show some of the key features observed in the toe of the model slopes. Model results also show that the failure mass deforms not only in the dip direction, but also in the strike direction. More normal faults generate within the middle section of the failure mass, whereas more shortening structures generate within the side sections of the failure mass. Within the seemingly chaotic structural pattern of the failure mass, superimposed structures are observed in front of the failure mass. These structures include faulted folds, folded fault surfaces, refolded folds and displaced fault surfaces.

Keywords: Granular slopes; faults; folds; Analogue models; Natural landslides