Tectono-thermal evolution of the detachment fault along the southeastern margin of the Yunmeng Shan metamorphic core complex in the northern part of Beijing.

In this paper the Hefangkou low-angle detachment fault on the east side of the Yunmeng Shan metamorphic core complex is chosen as the object of research. According to the deformation features and formation environment of biotite (which is the main mineral of foliation and lineation in the Hefangkou ductile shear zone), the temperature and pressure conditions for the tectono-thermal evolution at the fault zone were 780-860℃ and-0.73-0.86 GPa. The genetic mechanism for the metamorphic core complex is as follows: in the setting of regional extension, the semi-solid Yunmeng Shan granite was involved in the Hefangkou-Shuiyu extensional ductile shear zone at the southeast side, thus resulting in ductile deformation of granites under intermediate- to high-temperature conditions, and afterward, this ductile shear zone passed progressively to the shallow level in the uplift process, forming some shallow-level brittle extensional structures.