Formation and timing of the extensional ductile shear zones in Yiwulü Mountain area, Western Liaoning Province, North China

Abstract

The NNE-trending ductile shear zones in Yiwulu Mountain area were formed in relation to two successive extensional events. The Yiwulu High Temperature Extensional Ductile Shear Zone and the Waziyu Low Temperature Extensional Ductile Shear Zone were related to ductile deformation at higher temperatures and brittle-ductile deformation at lower temperatures, respectively. Both deformations were accompanied by large scale volcanic eruptions and magmatic intrusions. Based on structural analysis of macroscopic and microscopic deformations, and quartz lattice preferred orientations, we show that the early Yiwulu High Temperature Extensional Ductile Shear Zone was resulted from a NE-SW extension at amphibolite facies in the middle crust, whereas the Waziyu Low Temperature Extensional Ductile Shear Zone was due mainly to a NWW-SEE extension at greenschist facies in the upper crust. The SHRIMP zircon age of a syn-tectonic granitic dike emplaced at the late stage of high temperature extension is 155±2 Ma, indicating that the early extensional event took place in the Middle-Late Jurassic. 40Ar-39Ar age of muscovite from tectonic schists in the low temperature extensional ductile shear zone is 131.6±1.0 Ma, suggesting that the late extension occurred in the Early Cretaceous. Subsequent overall uplifting succeeded the late extension. The new discovery of the Middle-Late Jurassic NNE-trending extensional ductile shear zone provides evidence constraining the switch of tectonic regimes and Middle Jurassic thinning of lithosphere in the eastern North China Craton.