Late Quaternary slip rate of the northern Lancangjiang fault zone in eastern Tibet: Seismic hazards for the Sichuan-Tibet Railway and regional tectonic implications

Abstract

The Sichuan-Tibet Railway in eastern Tibet runs across numerous seismogenic faults with high seismic risks. The Lancangjiang fault (LCJF), one of the essential strike-slip faults within the Qiangtang block, traverses the railway. However, the geometry and late Quaternary activities of the LCJF remain unclear, limiting the assessment of seismic hazards on the Sichuan-Tibet Railway and the understanding of kinematic patterns in eastern Tibet. Based on the interpretation of high-resolution satellite imagery, field observations, and shallow geophysical exploration, we first mapped the detailed geometry of the northern LCJF. It consists of two branches: Yangda-Yaxu fault (YYF) and Baqing-Leiwuqi fault (BLF), which merge eastwards onto the main LCJF. Displaced landforms and multiple numerical dating techniques reveal that the YYF is a mainly sinistral fault with a late Quaternary slip rate of ~2.2 + 0.6/−0.5 mm/yr and a minor vertical component (~0.1 mm/yr). The BLF undergoes an oblique motion with a left-lateral slip rate of ~1.2 + 0.5/−0.3 mm/yr and a northeast-vergent reverse rate of ~1.2 + 0.4/−0.2 mm/yr in the Holocene. Vector analysis of slip rates suggests that the LCJF has a sinistral rate of ~3.5 + 1.1/−0.8 mm/yr with a reverse component of ~1.6 + 0.4/−0.3 mm/yr, displaying an essential role in strain partitioning of eastern Tibet. Together with slip rates of other intra-block faults, the kinematic pattern of eastern Tibet probably applies to the sub-block model. Offset geologic units yield a total displacement of ~10–11 km for the YYF. Using the slip rate of the YYF, the LCJF was initiated at ~5.9–3.9 Ma, probably related to the establishment of the present-day strain pattern and the onset of crustal thickening in eastern Tibet during the late Miocene. In addition, the active YYF and BLF across the Sichuan-Tibet Railway have a seismic potential of Mw ~7.2–7.3 that will produce a ~ 1.8–2.0 m sinistral slip on the YYF, and a ~ 2.0 m horizontal, and ~ 1.1–1.3 m reverse motion on the BLF. They will pose a high seismic hazard potential on the railway. This study provides quantitative data for regional seismic hazard analysis and shows insights into the deformation mechanism of the plateau.