Palaeoseismicity recorded in soft-sediment deformation structures within a 166-m-long drill core from Diexi Palaeolake, eastern Tibetan Plateau

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Abstract
Soft-sediment deformation structures (SSDS) in water-saturated, unconsolidated sediments are the product of various causes and provide a valuable record of environmental and geological perturbations. We report a record of SSDS preserved in a ∼166 m-long drill core (DX-2) extracted from the Late Pleistocene Diexi Palaeolake, eastern Tibetan Plateau. Two factors make this an outstanding site for studying SSDS: (1) it is among the most seismically active regions on Earth, and (2) it has experienced extremely fast sedimentation rates (∼15 mm/yr) thanks to the prodigious sediment supply from seismically perturbed hillslopes and rivers upstream. We describe and interpret 13 SSDS types within the DX-2 based on detailed sedimentological, morphological, and lithological analysis. We consider the genesis of the abundant SSDS observed in the DX-2 core with several possibilities: mass movement, rock avalanche-driven tsunamis, rapid sedimentation, and seismic shaking as the most probable triggering mechanisms. We suggest high-intensity earthquakes of VI (Modified Mercalli Intensity) or greater as drivers of SSDS in the DX-2. Based on our observations, we propose a conceptual model that attempts to explain the transition from ductile to brittle SSDS behaviour with progressive accumulation and consolidation of the sediment pile. This paper highlights the value of SSDS analysis in palaeo-earthquake identification, aiming to improve the applicability of SSDS as a palaeo-earthquake marker in alpine and canyon areas.

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