Observations on the sedimentary structure of prehistoric rock avalanches on the Tibetan Plateau, China

doi:10.13745/j.esf.sf.2020.9.12

Abstract

Abstract:

Rock avalanche kinematics is a forefront major challenge in the field of engineering geology globally. To explore the kinematics of rock avalanches, three typical prehistoric cases occurring on the Tibetan Plateau were investigated in this study. Detailed field surveys and analyses of their surface and internal sedimentary structures revealed the following: (1) Obvious sequential distributions of toreva blocks, longitudinal/transverse ridges, and hummocks were observed, and the rock avalanches could be divided into three zones from rear to front: the source area, transition zone, and accumulation zone, according to the spatial distribution of these landforms. (2) Inverse grading was observed in the deposits, with tree facies, carapace facies, body facies, and basal facies, divided vertically from top to bottom. In the carapace and body facies distribute retained stratigraphic sequences, jigsaw structures, etc.; in the basal facies, diapiric structures, convoluted laminations, etc. can be observed, indicating an intensive interaction between the basal facies and substrate. Based on these observations, we propose a plausible mechanism for the emplancement and accumulation of rock avalanches as follows: As avalanche mass became detached from the source area, it underwent first a rapid laminar flow-like motion, then an extensional sliding motion in the transition zone, followed by an extensional sliding motion as it arrived at the accumulation zone; and if the water content of the substrate were high, a rapid radial spreading process could occur at its frontal edge due to the rapid decrease in basal friction.

Key words: Tibetan Plateau, rock avalanche, surface sedimentary landforms, internal sedimentary structures, transport chronology

CLC Number:

P694
P642

WANG Yufeng, CHENG Qiangong, LIN Qiwen, LI Kun, SHI Anwen. Observations on the sedimentary structure of prehistoric rock avalanches on the Tibetan Plateau, China[J]. Earth Science Frontiers, 2021, 28(2): 106-124.

Figures/Tables 14

Fig.1 Geological settings of the study area (a), and plan-views of the Tagarma rock avalanche (b), the Nyixoi Chongco rock avalanche (c) and the Luanshibao rock avalanche (d). Modified after Topographic Map of China, No. GS(2016)1609.

Fig.2 Remote sensing image of the Tagarma rock avalanche (a), and rose diagrams of the counted sites of megablocks (b)

Fig.3 View of the toreva block (a) with an enlarged-view of the jigsaw structure (b), and view of the right lateral levee (c)

Fig.4 Exhibition of megablocks deposited on the top surface of the transverse ridges

Fig.5 Directional arrangement of layered masses composed of megablocks (a) versus runout (b-g)

Fig.6 Vertical profile of the deposit (a) with an enlarged view of the substrate (b-e)

Fig.7 Remote sensing image of the Nyixoi Chongco rock avalanche (a), and histograms (b) and rose diagrams (c) of the counted ridges

Fig.8 The toreva blocks (a, b), and longitudinal ridges (c) featured by steep frontal accumulation slope (d) of the Nyixoi Chongco rock avalanche

Fig.9 Subzone surfacial landforms of the accumulation zone of the Nyixoi Chongco rock avalanche

Fig.10 Photos of megablocks deposited on the top surface of the Nyixoi Chongco rock avalanche

Fig.11 Photos showing the internal structures of the Nyixoi Chongco rock avalanche

Fig.12 Remote sensing image of the Luanshibao rock avalanche

Fig.13 Photos showing the surficial landforms of zones Ⅱ (a,b) and Ⅲ (c-e) and deposited structures of megablocks (f) of the Luanshibao rock avalanche

Fig.14 Photos showing the internal structures of the Luanshibao rock avalanche

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