Active tectonics of the Ordos Plateau, China

This thesis is concerned with the active tectonics of the Ordos Plateau in northern China, an apparently non-deforming portion of continental lithosphere that sits at the northeastern edge of the India-Eurasia collision zone. Numerous, large-magnitude, historical earthquakes have occurred at the margins of the plateau, including some of the deadliest earthquakes in recorded human history. Furthermore, the adjacent North China Plain is one of the most densely populated regions on the planet, meaning that reliable seismic hazard assessments are of paramount importance. We investigate the 1739 Yinchuan earthquake, on the western side of the Ordos Plateau, using 14C dating and a new high-resolution Pleiades DEM of the entire rupture. We find that the event took place on the East Helanshan Fault and had a magnitude in the range Mw 7.1 to Mw 7.6, smaller than previously suggested. We then use IRSL and 10Be dating, along with high-resolution Pleiades DEMs, to determine a right-lateral slip rate on the Luoshan Fault of 4.3±0.4 mm/a and a throw rate on the East Helanshan Fault of < 0.8±0.1 mm/a, indicating that north-south right-lateral shearing is the predominant tectonic motion on the western side of the Ordos Plateau. In the northern Shanxi Grabens, at the northeastern corner of the plateau, we observe good agreement between Quaternary, seismic and geodetic rates of strain, and we find that the region is extending at around 1 to 2 mm/a at an azimuth of approximately 151°. However, there is a notable seismic strain rate deficit since 1750, which could indicate that we are quite late in the earthquake cycle with the potential for large earthquakes in the relatively near future. Finally, we combine our results with information from the literature to propose a kinematic model for the whole Ordos Plateau. We suggest that the plateau behaves as a pinned block, rotating anticlockwise within a large-scale WNW-ESE left-lateral shear zone. The previously enigmatic normal faulting at the margins of the Ordos block can then be explained as wedge-shaped grabens that develop during widening of the overall zone, or en echelon features that accommodate principally right-lateral motion.