Late Quaternary normal faulting along the western boundary fault of Peiku Co graben in southern Nyalam-Coqen rift: Implications for extensional deformation in southern Tibet and seismic hazard

Abstract

The NS-trending rifts play an important role in accommodating E-W extensional deformation in the interior of the Tibetan Plateau. Although ∼380 km long Nyalam-Coqen rift (NCR) is one of the most prominent rifts in southern Tibet, the late Quaternary fault activity, strong earthquakes and extensional deformation along the NCR remain unknown. Here, we focus on the western boundary fault of Peiku Co graben, Gangpengqing fault (GF), in the southern NCR. We use the vertical offsets obtained from the unmanned aerial vehicle (UAV) digital elevation model (DEM) and published 10Be cosmogenic surface exposure ages to determine the late Quaternary throw rates over different timescales, and combine the trenching record and AMS-14C dating to constrain the paleoseismic events. Our results demonstrate that the late Quaternary throw rates along the GF are from 0.4 ± 0.1 mm/yr to 0.6 ± 0.1 mm/yr. Combining the throw rates and dip angle of fault, we further estimate the extension rates across the southern NCR, ranging from 0.2 ± 0.1 mm/yr to 0.4 ± 0.1 mm/yr. The paleoseismic results suggest that two surface-faulting events occurred on the GF at ∼13.6–10.8 ka BP and ∼7–6 ka BP. The elapsed time of ＞6 ka since the most recent strong earthquakes (Mw 6–7) implies the potential for occurrence of Mw 6.8 or larger earthquake in the future along the GF. Relative to normal faults in the Yadong-Gulu rift, the GF has the lower throw rate (∼0.4–0.6 mm/yr) and longer recurrence interval (less than ∼6–7 ka) of strong earthquake since ∼18 ka BP, Moreover, the comparison with published extension rates across other main rifts indicates that the late Quaternary extension rates across different rifts in southern Tibet are not completely consistent. These imply that the extensional deformation and strong earthquake in different rifts in southern Tibet have the distinct difference.