Comment on ‘Gigantic rockslides induced by fluvial incision in the Diexi area along the eastern margin of the Tibetan Plateau’ by Zhao et al. (2019) Geomorphology 338, 27–42

Abstract

Zhao et al. (2019) examine gigantic landslides in the Diexi area along the eastern margin of the Tibetan Plateau and propose their successive failure based on a knickpoint migration conceptual model. They postulate that a major river knickpoint (KpMJ) identified by them in the middle reaches of the Minjiang River (Sichuan, China) was initiated at the Longmenshan Fault and subsequently propagated ~ 85 km to its present position upstream of the Diexi lake. They then argue that this retreating knickpoint left in its wake an inner gorge that undercut and destabilized hillslopes, triggering a series of large landslides in the Diexi area. We question this interpretation based on our high-resolution landslide mapping, an analysis of knickpoints (i.e., profile convexities >30 m high) in the Minjiang channel network, and field observations of lacustrine sediments and epigenetic gorges associated with the Diexi landslides. We confront the model proposed by Zhao et al. (2019) with three key arguments: 1) Major profile convexities in the Diexi area, including KpMJ, are associated with landslide or debris flow deposits and there is no basis for connecting explicitly any of these to long-distance knickpoint retreat; 2) the giant Diexi paleolandslide predates the debris avalanches at KpMJ, therefore the latter cannot have been the trigger for landsliding in this area; and 3) the spatial distribution of 666 mapped knickpoints in the Minjiang River mainstem and tributaries is not consistent with simple long-distance propagation of an ‘incisional wave’ initiated at the Longmenshan Fault.