Abstract
Lianfeng fault zone (LFZ) in Southwest China has great significance for understanding the seismogenic environment, but its nature and latest active age is still poor constrained up to now. The Qiaojia-Jinyang segment (QJS) of LFZ intersects with Jinsha River; so the well-developed river terraces provide a potential spatio-temporal constrains for recent faulting of QJS. Based on investigation of the terrace deposits along river, this paper makes a detailed logging and dating of the faulting and liquefaction of QJS. Combined previous data, the spatio-temporal sequence of the Late-Quaternary river terraces in the area was redetermined. It is considered that the river terraces T1 (lowest) and T2 at QJS (~ 10–20 m and 60–70 m, respectively, above the local river level) are roughly developed in the Middle Holocene and the Late Pleistocene, indicating that the valley along QJS was strongly undercut since the Late Pleistocene. Based on the analysis of the morphological characteristics, spatial distribution, material composition and intersecting relationship between the sand veins and the layers, the strong ground motions is preliminarily detected, which indicates the strong seismic activity of the during the Quaternary. Combined with the faulting characteristics and the dating data of strata within the profiles of terrace deposits, it is considered that the LFZ is active at least at QJS, and the latest active time is not earlier than the Early-Middle Holocene. These findings provide a clear geological evidences for the seismicity assessment at LFZ, and help to the understanding of regional tectonic environment and the prevention of earthquake disasters.
Highlights
Fault activity is an basis for understanding regional seismotectonic environment and establishing seismotectonic model
It is considered that the first and second grade river terraces at Qiaojia-Jinyang segment (QJS) (~10-20m and 60-70m, respectively, above the local river level) are roughly developed in the middle Holocene and the late Late Pleistocene, indicating that the valley along QJS was strongly undercut since the Late Pleistocene
The OSL dating results of the top of the first terrace in the southwest of Shanjiang Township is 7.1 ± 8 ka (Table 1); The OSL age at the top of the tributary alluvial fan corresponding to the second terrace of Xiaoniulan Village is 13.4 ± 8 ka (Table 1), which is similar to the terrace age of Qiaojia made by Li et al (Li et al 2016)(Table 1), indicating that the first and second river terraces at QJS were formed in the early and middle Holocene and Late Pleistocene (Fig. 3)
Summary
Fault activity is an basis for understanding regional seismotectonic environment and establishing seismotectonic model. This paper focuses on the investigation of structural fault characteristics recorded by terrace deposits within the Laojie east profile, the profile near confluence of Niulan river, the profile to southwest of Shanjiang township, and the profile to southwest of changpingzi, etc. The OSL dating results of the top of the first terrace in the southwest of Shanjiang Township is 7.1 ± 8 ka (Table 1); The OSL age at the top of the tributary alluvial fan corresponding to the second terrace of Xiaoniulan Village is 13.4 ± 8 ka (Table 1), which is similar to the terrace age of Qiaojia made by Li et al (Li et al 2016)(Table 1), indicating that the first and second river terraces at QJS were formed in the early and middle Holocene and Late Pleistocene (Fig. 3)
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