Paleoseismic Activities Recorded By Dammed Lakes Along Longmenshan-Mingshan Fault Zone In The Upper Reaches Of The Minjiang River

The paleo-dammed lake sediments in the high mountains and canyons are good object on paleoearthquake research, thereinto, soft-sediment deformational structures are important diagnostic features of paleoseismic research. The north–south-trending upper reaches of the Minjiang River run along the Longmen Shan–Min Shan fault zone, a zone of abrupt topographic change along the eastern margin of the Tibetan Plateau. Multiple levels of well-preserved soft-sediment deformation structures (seismites) occur in sediments deposited in paleo-dammed lakes in the upper part of the Minjiang River Valley. The research on the type and chronology of the soft-sediment deformational structures indicate as:These deformation structures include liquefied convolute deformation, water-escape structures, flame structures, pseudonodules, ball-and-pillow structures, sedimentary dykes, mud lenses, and large-scale folds.Several kilometers from the Diexi paleo-dammed lakes, seven deformed structural layers were identified at different heights in late Quaternary stratigraphic sequences near Shawan Village, Maoxian County. Analyses of the deformation structures, landforms, and the structural environment indicate that these deformation structures were caused by earthquakes, slumps, and landslides. They are Quaternary seisimites.OSL (optical stimulated luminescence) and 14C dating of soft-sediment layers from the Shawan site indicate that intense earthquakes occurred during the period 24 ka B.P. and 19 ka B.P., which indicate the approximately period of paleoearthquake and significantly enrich our knowledge of the Longmen Shan–Min Shan fault zone paleo-earthquake history.