Tectonic Taphrogenesis and Paleoseismic Records from the Yishu Fault Zone in the Initial Stage of the Caledonian Movement

Abstract:The Yishu fault zone (mid‐segment of the Tanlu fault zone) was formed in the Presinian. Periodic tectonic activities and strong seismic events have occurred along the fault zone. During the initial stage of the Caledonian Movement, with the proceeding of the marine transgression from the Yishu paleo‐channel to the western Shandong, uneven thick sediments, composed mainly of sand, mud and carbonates of littoral, lagoon, and neritic facies, were deposited in the Yishu fault zone and western Shandong, and constructed the bottom part of the Lower Cambrian consisting of the Liguan and Zhushadong formations. Through field observations and the lab‐examinations, various paleoseismic records have been discovered in the Liguan Formation and the Zhushadong Formations of the Yishu fault zone and its vicinity, including some layers with syn‐sedimentary deformation structures that were triggered by strong earthquakes (i.e. seismite, seismo‐olistostrome, and seismo‐turbidite). Paleoseismic records developed in the Zhushadong Formation are mainly seismites with soft‐sediment deformation structures, such as liquefied diapir, small liquefied‐carbonate lime‐mud volcano, liquefied vein, liquefied breccia, convolute deformation (seismic fold), graded fault, soft siliceous vein, and deformation stromatolite, as well as seismites with brittle deformation structures of semiconsolidated sediments. Paleoseismic records preserved in the Liguan Formation are not only seismo‐olistostrome with a slump fold, load structure, and ball‐and‐pillows, but also seismo‐turbidite with convolution bedding, graded bedding and wavy‐bedding. However, in the western Shandong area, the closer to the Yishu fault zone, the greater the thickness of the Liguan Formation and the Zhushadong Formation, the greater the number and type of layers with paleoseismic records, and the higher the earthquake intensity reflected by associations of seismic records. This evidence indicates that tectonic taphrogenesis accompanied by strong earthquake events occurred in the Yishu fault zone during the initial stage of the Caledonian Movement, which embodied the break‐up of the Sino‐Korean Plate along the Paleo‐Tanlu fault zone at that time.