Lake Level Changes and Holocene Extension Rate Across the NS‐Trending Peiku Co‐Gyirong Faults, Southern Tibet

Abstract

Precise studies of Quaternary extension rates across NS‐trending normal faults in southern Tibet are important to understand the recent tectonic evolution of the high plateau. As one of the six main, southernmost Tibetan rifts, the ∼NS‐trending Peiku Co‐Gyirong rifts and intervening horst (PGR), co‐involve the west‐dipping Gyirong normal fault (GF) and the east‐dipping Peiku Co normal faults (PKF). Using unmanned aerial vehicle (UAV) high‐resolution topographic data, 14C dating of lake shorelines, and 10Be cosmogenic exposure dating of alluvial surfaces, we assess the offsets and constrain the ages of fans and lake level variations at four sites along these rift faults. Across the PKF's southern segment, our measurements at three sites are consistent with offsets of ∼4.7–6.5 m during the last ∼12.3 ka. Across the GF, a larger total offset of ∼8.5 m accrued since ∼10.4 ka. Using the average, exposed, 60° dip angle of the PKF implies a total Holocene EW extension rate of 0.72 ± 0.08 mm/yr across the PGR. Overall, our results are in keeping with published extension rates of ∼0.6–1.7 mm/yr across other southern Tibetan rifts, supporting the inference that the total, ≈EW, late Quaternary extension rate throughout southern Tibet is 9 ± 2 mm/yr. That there has been no surface faulting since at least ∼6.9 ka implies that the Gyirong County area, west of Peiku Co, may be at risk of Mw ≈ 6.9? event.