Evolution And Regional Tectonic Implications Of The Lidao Detachment Fault In Jiaodong Peninsula

There are a lot of extensional structures in North China craton because of tectonic regime transformation in late Mesozoic, e.g. basin and range tectonics, metamorphic core complexes and detachment faults, etc. This paper will take the Lidao detachment in Rongcheng area located at Jiaodong Peninsula as an example to analyze the characteristics of this transformation via macroscopic and microscopic analysis of tectonites formed at different depths of the crust, and statistics of geometric data based on microcosmic stuctures. Emphasis will be laid on discussion of the effects of extension on the detachment fault after the transformation. The evolution model will be established and its regional tectonic implications will be discussed.Different from common detachment fault zone, the Lidao detachment fault was formed by extension superimposed on an early thrust which formed a ductile shear zone——the Rongcheng shear zone before detachment faulting. Therefore, the tectonites along the detachment fault zone mostly show deformation characteristics of two different temperatures, either in macro or in micro-scales. Early stage deformation shows coarse recrystallization minerals, relatively straight tectonic gneissosity and asymmetric thrust folds, and polycrystalline quartz bands and recrastallized polygonal feldspar aggregates. Late stage deformation is characterized by mica fishes, shrust folds and asymmetric quartz veins and undulatory extinction in quartzs, asymmetric tails in feldspars and ductile-britile fractures. Both microscopic deformation characteristics and EBSD fabrics of quartz account for about 500℃ in the main deformation stage of the shear zone and uneven low temperature deformation in the late stage. Strain rate obtained by method of fractal dimension indicates a rapid ductile deformation history in the early stage to form the Rongcheng shear zone. The analysis of differential stress obtained by dislocation density in sheared quartzs reveals that the rocks farther from the brittle fault seem to be sheared at higher deformation temperature and deeper deformation level. The late, shallow ductile deformation superimposed on the early, deeper flow deformation to form the current distribution pattern of Lidao detachment fault zone, and it consist of brittle tectonites,high temperature tectonites which overprinted by uneven low temperature deformation and low temperature tectonites on the local.Based on deformation evolution of Lidao fault, formation of the upper plate volcanic basin and the lower plate Mesozoic Weideshan pluton, this paper preliminary considers that the formation of the Lidao detachment fault covers four stages:220-200Ma, the rapid exhumation of subbucted Yangtze plate gave rise to partial melting of middle-lower crust in Rongcheng area, so the ductile flow deformation happened and high temperature mylonites were formed; 200-122Ma, middle-lower crust uplifted to upper crust at a low exhumation rate; 122-124Ma, the upper crust began to be detached, producing late stage low temperature ductile deformation; 110-105Ma, intrusion of the Weideshan pluton and the formation of the Lidao volcanic basin.Tectonic extension and detachment faulting in Rongcheng area was under the same stress field as other extensional structures of the region. They are both the result of the rollback of the subducting Pacific plate but formed by a late stage of deformation after the main period of extension. Combined with results from the Guojialing detachment fault in Jiaodong and the Wanfu and Dandong detachment faults in Liaodong, the Lidao detachment fault shows a progressive evolution deformation to the main stress period in eastern North China and may be a subsidiary detachment fault developed by reaction of early fault zone along a weakness plane.