Analysis And Age Restriction Of Northeast-trending Structure In Northern Part Of North China

The Yanshan tectonic belt lies in the northern margin of the North China plate, which is one of the cradles of geologic work of China. It's famous for the intense intracontinental deformation in the Mesozoic and Cenozoic. Quite a few geoscientists have done researches on structural geology and tectonics in Precambrian Mesozoic, and Cenozoic, and have achieved many agreements. However, there are several disputes about the chronology and mechanism of deformation in the eastern part of Yanshan tectonic belt during the Mesozoic.During the period of Indosinian Movement, Yanshan tectonic belt was under approximately NS- compression in Middle Early Triassic. As a result of tectonism of the Palaeo-Pacific plate, a significant transformation took place in the tectonic framework of east China in Jurassic. The tectonic framework in Yanshan tectonic belt was changed from EW-trending to NE-NNE-trending. Previors geologis did many detailed investigations in study area. They mainly focused on EW-trending tectonic framework in middle part of the Yanshan tectonic belt, and the formation mechanism , tectonic evolution of NE-trending thrust zones in western Liaoning area. However, there is little literature on the tectonic deformation in Kuangcheng, which is the key area for the tectonic framework transformation from EW-trending to NE-trending. This paper takes Jixian-Kuangcheng-Lingyuan area as the key area for studying the tectonic framework inversion from EW-trending to NE-trending, focuses on tectonic deformation (mainly folds) when the tectonic framework transformation occurred, whether there was detachment between Archean metamorphic rocks in deep level and Middle Proterozoic and the upper unmetamorphic strata, whether the basement was involved in folding deformation and their chronological constraints.Through this paper, we've achieved some new understandings, including:1. Many folds in study area have an axial plane dipping SE, not dipping NW as early described. Thrusts in western Liaoning area shows SE-trending in the field, while actually they are influenced by the SW-trending compression and make their adjustments inside.2. According to the U-Pb zircon chronology of dykes, NE-trending deformation occurred at 160Ma. There are NE-trending structures influenced by NW-trending compression in Western Hills of Beijing, which also occurred at ca. 160Ma. That's also in agreement with Mesozoic magmatism in the Taihangshan Mountain belt3. The detachment between Archean-Early Proterozoic and Middle-Late Proterozoic strata just exist in study area, In Jixian, western Liaoning Province and other places, it is angular unconformity or thrust fault.