Mesozoic Geothermal Anomaly in Western Hills of Beijing and Origin of Fangshan Metamorphic Core Complex

摘要

The configuration of the Fangshan metamorphic core complex consists of three layers i.e. between the Archean basement and the upper part of the cover consisting of Jurassic and Cretaceous sedimentary sequence there is a ductilely deformed Middle Proterozoic to Lower Paleozoic low-grade metasedimentary sequence. The structures of this sequence, such as intrafolial to interformational recumbent folds, bedding subparallel shear zones and mylonites, bedding subparallel foliation, were formed by subhorizontal ductile shear under an extensional regime. The spatial distribution of these ductile structures is inhomogeneous, but generally coincident with an area of paleo-geothermal anomaly with tempratures above 300X. Stretching lineations in the ductilely deformed sequences plunge to the SEE except around the Cretaceous Fangshun pluton where the lineations dip outward from the pluton, a pattern that indicates reorientation by bollooning of the pluton . The Mesozoic geothermal anomaly and SEE-directed extension are the main factors causing the formation of those ductile flow structures and the thinning of the crust > while the thermal uplift and plutonism during the Yanshanian stage have played an important role in the uplift of the metamorphic core complex, and a thermal event of mantle origin maybe the main cause of the puleo-gcothermal anomaly.