Holocene seismically induced soft-sediment deformation structures in the Qingshuihe Basin, northeastern Tibetan Plateau

Abstract

This study focuses on the extensive development of soft-sediment deformation structures (SSDS) within the Holocene fluvial-lacustrine strata in the central and southern Qingshuihe Basin, located on the northeastern margin of the Tibetan Plateau. Through basin-wide sedimentological analysis, the study systematically examines the development characteristics, formation mechanisms, and triggering factors of SSDS across various sedimentary facies, while also exploring their relationship with regional seismic events.

The results reveal that a wide variety of SSDS are present in the basin, including small-scale faults, pseudofaults, load structures, water-escape structures, clastic dikes, deformed laminations, tension veins, and mixed accumulations. Most SSDS occur alongside syn-sedimentary faults (small-scale faults or pseudofaults) and exhibit significant directional features. SSDS in fluvial facies are more prominent than those in lacustrine facies, with the former displaying clear stratification, while the latter shows multi-phase deformation. Both fluvial and lacustrine SSDS are closely correlated in stratigraphic position and formation age with tension veins observed in loess deposits on the western side of the basin. Furthermore, the intensity of deformation exhibits a significant negative correlation with the distance from boundary faults. Most SSDS display characteristics of liquefaction, fluidization, and brittle deformation, consistent with seismic shaking.

Radiocarbon dating of 35 samples and optically stimulated luminescence (OSL) dating of 7 samples indicate that the SSDS in the southern part of the basin (6220 ± 95 a BP to 717 ± 24 a BP) are strongly associated with three major seismic events along the Haiyuan-Liupan Mountain Fault (a paleo-earthquake from 6600 ± 500 a BP to 5640 ± 540 a BP, the 1219 Guyuan M7 earthquake, and the 1306 Guyuan M7 earthquake). Meanwhile, SSDS in the central basin (11,270 ± 1100 a BP to 7728 ± 36 a BP) likely reflect three significant seismic events along the southeastern segment of the Xiangshan-Tianjingshan fault (XS-TJSF). These earthquakes are estimated to have magnitudes ranging from 6.5 to 7.3, with epicenters located in the central region of the southeastern segment of the XS-TJSF.

This study refines the criteria for identifying earthquake-induced SSDS and provides critical evidence for reconstructing the regional paleo-seismic history, thereby enhancing the understanding of the complex tectonic activity in the area.