No abstract

Although Quaternary deposits within the Ganyanchi (Salt Lake) pull-apart basin along the eastern Haiyuan fault preserve a record of both the paleoenvironmental and tectonic evolution of the northeastern Tibetan Plateau, this sedimentary archive has yet to be dated. Here we report a paleomagnetic study of a 328-m-long sediment core drilled near the modern depocenter of the basin, and use this record to both date the onset of sedimentation and reconstruct the depositional history of the basin. The observed magnetic polarity sequence comprises 13 normal and 12 reversed polarity zones, and we explore two possible correlations to the geomagnetic polarity time scale. Our preferred correlation minimizes changes in sedimentation rate and extends from the Brunhes normal polarity chron to the Gauss normal polarity chron. This correlation indicates that the Ganyanchi Basin began to develop by at least ca. 2.76 ± 0.03 Ma. Sediment accumulation rates (SAR) were elevated in two intervals, from ca. 1.92 to 1.78 Ma, when they were ~130.3 m/m.y., and from ca. 0.77 Ma to present, when they were ~234.6 m/m.y. We attribute these enhanced depositional episodes to both Northern Hemisphere cooling and local tectonic effects, with SAR values increasing as the regional climate has shifted toward overall drier and cooler conditions.

According to historical earthquake records, the Luhuo segment of the Xianshuihe fault has produced two large earthquakes: the 1816 M7.5 earthquake and the 1973 M7.6 earthquake. The surface ruptures caused by these events remain well preserved. This study focused on the rupture behavior of the Luhuo segment. Based on field investigations, trench excavations and analysis of historical earthquakes, we identified six seismic events that occurred within the past 3000 years, which are dated at 769 BC, 318-545 AD, 677-833 AD, 1008-1444 AD, 1816 AD and 1973 AD. The recurrence intervals of these events, from oldest to youngest, are approximately 1200, 324, 471, 590 and 157 years. Thus, the recurrence behavior of the fault segment appears inconsistent with time-or slip-predictable models, whereas, the revealed seismic sequence appears consistent with clustering and abnormal accelerating stress release behavior. The fault strike-slip rate during the period of anomalous stress release is approximately 2-3 times faster than the average rate of 8.4 mm/a. Moreover, the Luhuo segment has experienced ongoing high levels of seismic activity over the past 3000 years, and the entire Xianshuihe fault currently shows a high degree of seismic activity. Therefore, we suggest there was a long period of earthquake quiescence prior to 3000 years ago, which might have balanced the high activity and accelerating stress release of current earthquakes.

In mainland China, approximately 86% of M≥7 earthquakes have occurred in the block boundary zone, which has been well explained by active block theory. However, a few large earthquakes have occurred within the active block, which provides us with an opportunity to better study the deformation of the Tibetan Plateau. The Litang Fault (LTF) is a strike-slip fault within the Sichuan-Yunnan Active Block and produced the 1948 Litang M7.3 earthquake. We presented the Holocene rupture behavior of the LTF based on detailed field investigations, paleoearthquake trenching, and radiocarbon dating. Specifically, we revealed 13 Holocene paleoearthquake events at four trenching sites and divided these events into 3 rupture cycles at the whole-fault scale. The seismic rupture behavior of the LTF is characterized by recurrent southeastward migration, and since the Holocene, the period of each rupture cycle has decreased rapidly from 8000 years to 500 years. Our results may provide geologic evidence for understanding the intrablock stress patterns and material transfer of the southeastern region of the Tibetan Plateau. The rapidly enhanced fault activity of the LTF since the late Holocene indicates that the LTF may have played an important role in accommodating the deformation of the southeastern region of the Tibetan Plateau.