High mobility of the channelized ancient Linka rock avalanche within the Bangong - Nujiang suture zone, SE Tibetan Plateau

Highlights

• The Linka rock avalanche is an ancient, large, channelized, and long runout event.

• The rock avalanche originated from a buckling failure of two dip-slope slabs.

• Superelevation by trimline around valley bend suggests a velocity of 30–40 m/s.

• Inverse remnant stratigraphy indicates a cascading failure sequence instead of a single massive event.

• Channelized flow, rock fragmentation and low basal shear probably contribute the high mobility.

Abstract

Large rock avalanches often result in loss of life and catastrophic damage to infrastructure far from their initial failure positions, predominantly due to their tremendous volumes, high velocities and long runouts. Studies of the surface morphologies and internal structures of rock avalanche deposits can reveal evidence of their movement and deposition mechanisms. Through field survey, satellite image interpretation and particle-size distribution analysis, we studied the ancient, long-runout Linka rock avalanche, which is next to the Nujiang River, southeast Tibetan Plateau. The Linka rock avalanche, with a source volume of ~310 Mm³ originated from a high, steep dip slope and then transformed into a channelized rock avalanche, showing high mobility with a superelevation back-calculated maximal velocity of 42 m/s, a travel distance of ~6200 m and a Fahrboschung of 0.24. The slope failure sequence consisted of an initial failure of an outer limestone slab and a second failure of an inner sandstone slab. The limestone avalanche had 2 or 3 surges, while the sandstone avalanche probably had only one surge. The high mobility of the Linka rock avalanche predominantly resulted from a buckling failure occurred on a high relief source, low-energy dissipation during the channelized flow, intensive rock fragmentation in the basal facies and possible water lubrication when propagating bi-directionally along the Waqu River. Our findings for the Linka rock avalanche support previous knowledge that multiple mechanisms have different roles at different stages in a long runout rock avalanche.

Introduction

Rock avalanches are the extremely rapid, massive and flow-like movements of fragmented rocks from a large rock slide or rock fall (Hungr et al. 2014). Their volumes generally range from 0.5 Mm³ to 10s Bm³, they can reach velocities as high as 100 m/s, and their travel distances can be up to tens of kilometers (Davies et al. 1999; Legros 2002; Crosta et al. 2007). Due to the tremendous hazards they pose, and their complicated behaviors and deformation patterns, the large, highly mobile rock avalanches have attracted worldwide attention. Various competing hypotheses on the mechanisms of movement have been proposed based on field investigations, physical experiments, numerical simulations and theoretical analyses (see summaries in Legros 2002; Davies and McSaveney 2012; Dufresne et al. 2016). However, due to their natural complexity and the rarity of direct case studies, understanding of the high mobility of rock avalanches is still limited and widely debated (Johnson et al. 2016; Davies and McSaveney 2016; Iverson 2016).

The surface morphologies and internal structures of rock avalanche deposits are a key to understand the mechanisms of long runout rock avalanches (Strom 2006; Shea and van Wyk de Vries 2008; Weidinger et al. 2014; Iverson et al. 2015; Dufresne et al. 2016). Morphologic features, such as Toreva blocks, scarps, flowbands, levees, trimlines, ridges, grid grooves, block belts, hummocks, and splashes provided evidence from which to deduce failure sequence and movement kinematics (Dufresne et al. 2016; Zeng et al., 2019, Zeng et al., 2020). Numerous large rock avalanches have remnant stratigraphy with characteristic features (Davies 1982; Strom 2006), i.e., lithologies maintaining their identity and arranged sequentially outward from the source. Strom (2006) summarizes two types of stratigraphy of landslide deposits. Internal structures, such as jigsaw-fractured clasts, inner shear zones, faults, inverse grading, rock fragmentation, frictionite, and basal mélange have been used to characterize the dynamics during stages of runout and emplacement (Weidinger et al. 2014; Dufresne et al. 2016). A three-part facies model for rock avalanche deposits has become commonly accepted, i.e., a coarse carapace, a finer-grained body facies and a basal facies (Weidinger et al. 2014; Dufresne et al. 2016). Many case studies focusing on rock avalanche deposits have been carried out (e.g. Guthrie et al. 2012; Paguican et al. 2014; Iverson et al. 2015; Reznichenko et al. 2017; Strom and Abdrakhmatov 2018; Zeng et al., 2019, Zeng et al., 2020; Dufresne and Geertsema 2020).

Velocity is a key parameter of rock avalanche mobility, but its actual measurement is quite rare, except for the nuclear-induced rock avalanches at Novaya Zemlya (Adushkin 2006) and the Mount St. Helens volcanic debris avalanche (Voight et al. 1983). Some rock avalanches while propagating along their flow paths, can exhibit spectacular mobility in overtopping substantial topographic obstacles (Coe et al. 2016), in running up opposing slopes (Eisbacher 1979; Geertsema et al. 2006; Guthrie et al. 2012), and in superelevating while traversing valley bends (Evans et al., 1989, Evans et al., 2001). Those features can be used to back-calculate the peak velocity of the front of a rock avalanche using the potential energy equation v = (2gh)^1/2 and the forced vortex equation v = (R_chgcosθ/kB)^1/2, respectively (Chow 1959; McClung 2001; Hutchinson 2006; Iverson et al. 2016). In addition, due to increasing seismic data sharing and seismic network coverage, the real-time records of seismic signal of rock avalanches have been widely used to analyze their dynamic processes and estimate their mean velocities (e.g. McSaveney and Downes 2002; Allstadt 2013; Ekstrom and Stark 2013; Iverson et al. 2015; Hibert et al. 2019; Yan et al., 2020).

We present a case study of the ancient, large, channelized, long-runout Linka rock avalanche (~ 310 Mm³ in source volume, herein abbreviated as the LKRA) within the Bangong – Nujiang Suture Zone of the southeast Tibetan Plateau, based on field investigations, interpretation of high-resolution Google Earth images, UAV surveying and particle size analyses (Fig. 1, Fig. 2). We describe the basic characteristics and examine mobility features of the LKRA based on the observations of surface morphologies and inner structures of the deposits. We also present a buckling-failure model for the LKRA and discuss a channelized emplacement mechanism, and then discuss our study's implication for hazard assessment of long runout, channelized rock avalanches.