Jianmin Hu, Chinese Academy of Geological Sciences (China)
Yue Zhao, Chinese Academy of Geological Sciences (China)
Xiaowen Lin, National Geological Samples Center of the Ministry of Land and Resources of China (China)
Gang Xu, Chinese Academy of Geological Sciences (China)
The Yanshan thrust belt, "Yanshan" for short, is located at the northern edge of the North China plate. Yanshan is a perfect place to study continental tectonics in China, because that Yanshan was formed by several different periods of contraction deformations. The crustal shortening was the result of the subduction of Pacific plate towards North China plate. The northern edge of the North China plate had a strong uplift with several tectonic events from late Paleozoic to early Mesozoic and constructed a Mesozoic eastern China plateau. Yanshan is developed at the southern part of this plateau. Recent investigations show that Yanshan experienced strong deformations in early Mesozoic. The tectonic events had borne upon sequential closures of the paleo-oceans on both south and north edges of Yanshan; such as the paleo-Qinling ocean in late Paleozoic to Triassic at south, the paleo-Asian ocean in late Paleozoic and the Mongol-Okhotsk ocean in Mid-Late Jurassic at north, respectively.
Three stages of early Mesozoic deformations are identified in eastern Yanshan, Lingyuan area. The first stage was characterized by the out-of-sequence westward thrust structure (D1), the second stage was the eastward in-sequence thrust system (D2), and the third-stage was extensional gravity-induced collapse and landsliding (D3). Timing of deformations is constrained by crosscutting relationships of faults and veins, and by isotopic geochronology dating of volcanic rocks and gravels. These geological records show that the first two thrusting events took place in the Late Triassic and Early Jurassic, respectively, whereas extensional deformation occurred at the end of Middle Jurassic. The D1 deformation caused a quick uplift in the eastern Yanshan area. The large geomorphological relief in this area induced collapsing, landsliding, and mud-flow. Dengzhangzi formation was formed during the D1 to D2 period and recorded this quick uplift, erosion, and deposit condition. In this paper, the authors obtained some SHRIMP U-Pb ages of zircons from the volcanic rocks of the Shuiquangou Formation, the volcanic gravels of Dengzhangzi Formation, the volcanic rocks of Tiaojishan Formation, and the diorite-porphyrite vein intruded into the early thrust system. They are 230.4 ± 3.1 Ma, 210.7 ± 5.6 Ma, 160.1 ± 4.3 Ma, and 167 ± 7 Ma respectively. The ages constrain the times of the early Mesozoic deformations in Yanshan intraplate orogen. These early Mesozoic contractional deformations in Yanshan probably related to the closure of ancient Asian ocean and ancient Qinlingocean. The later crustal extension induced large gravitational collapse of the eastern China plateau during early Mesozoic.
Key Words Yanshan thrust belt; Early Mesozoic deformation; Thrust; Sliding