Re-evaluation of the late-Pleistocene slip rate of the Haiyuan fault near Songshan, Gansu province, China

WENQIAN YAO, Jing Liu, Michael E. Oskin, Veronica B. Prush, Wei Wang, & Zhanfei LI

Submitted August 13, 2018, SCEC Contribution #8463, 2018 SCEC Annual Meeting Poster #220

Well-constrained fault slip rates are important for understanding strain partitioning within a fault system and the associated seismic hazard. The Haiyuan fault is an ~1000 km-long active strike-slip fault in the northeast margin of the Tibetan Plateau with ongoing controversy over its late Pleistocene slip rate. Previous work by Lasserre et al. (1999) suggested a slip rate of 12+/-4 mm/yr, which is higher than recent geodetically determined rates on adjacent fault sections. In this paper, we reanalyze and re-evaluate the slip rates at their two sites, located north of the Songshan village, using airborne LiDAR data, field mapping, and Quaternary geochronology. A suite of loess-capped terraces document the slip history of this fault. At the Majia Wan site, using microtopographic analysis of these offset terraces, we document a sinistral displacement of 124 ± 15 m of the upper edge and 95 ± 15 of the lower edge of T1/T2 terrace riser, respectively, on the left bank of the channel. On the right bank, T1/T4 terrace riser indicates a 129 ± 15 m offset, similar to that previously documented. Combining optically stimulated luminescence, cosmogenic 10Be, and carbon-14 dates, we assess the abandonment age of T2 as 20.0 ± 2.8 ky and T1 as 9445 ± 30 yr. An upper terrace reconstruction of displacement yields a slip rate of 6.5 ± 1.6 mm/yr since ~ 20 ka, whereas the lower terrace reconstruction indicates a rate of ~10 mm/yr since ~ 9,500 ka at the site. The configuration of offset channels favors an upper terrace reconstruction of displacement accumulation. Our results, in conjunction with other geologic, paleoseismic, and geodetic slip rates along the Haiyuan fault, indicate an average slip rate of ~6 mm/yr, compatible with geodetic data and with no secular variation. Our re-evaluation supports that apparent slip rate discrepancies in northern Tibet share a common bias due to lower-terrace reconstruction used in interpreting the age of offset. The ephemeral channels of semi-arid northern Tibet do not vigorously refresh terrace risers to obliterate offset accumulated during channel downcutting.

Citation
YAO, W., Liu, J., Oskin, M. E., Prush, V. B., Wang, W., & LI, Z. (2018, 08). Re-evaluation of the late-Pleistocene slip rate of the Haiyuan fault near Songshan, Gansu province, China. Poster Presentation at 2018 SCEC Annual Meeting.


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