The QBK Dome And NW Structures In Western Hills Of Beijing: Feature And Significance

Western Hills of Beijing located in the intersection part of the Taihang Mountaintectonic belt and the Yanshan orogenic belt, which strike NNE and nearly W-E,respectively. For a long time, researchers’ attention focused on the tectonic frameworkcontrolled by EW, NNE and NE structures. However, few attention had been paid onthe QBK-dome and NW structural features, which are uncoordinated with the mainstructure in larger scale, and with a lower study degree. Representative NW structures’geometry characteristics, the relationship with other structural features, and itsmechanism haven’t been revealed comprehensively in Western Hills of Beijing.Secondary asymmetric folds have been observed in the NE and SW wings ofthe QBK-dome,its geometry characteristics reflect that the QBK-dome has thecharacteristics of buckle fold deformation, which indicates NE-SW conteaction. AndIn the core of the dome,sills along the beds folded with it. Therefore, the QBK-domewas formed in buckle fold deformation mechanism, and it’s a asymmetric short axisanticline with SW wing steeper than NE wing. According to the zircon ages of thedike folded with the dome and the Nandaling volcanic rocks covered the dome, Thedeformation age is limited between late episode of early Jurassic and early episodeof middle Jurassic（177-174Ma）.The AJX anticline and the QBK anticline have a similar geometry, both of whichare asymmetric anticline with SW wing steeper than NE wing. And their hinges arealmost paralleled. So the AJX anticline is an extension of the QBK anticline, and theQBK anticline might be the southeast plunging of this regional NW anticline.Apart from the AJX anticline and the QBK anticline, NW structures have alsobeen found in the SE wing of TJS syncline (SWD anticline) and Zhaitang toDaanshan. As a result, NW structures are also one of the widely distributed structurecomponents of Western Hills of Beijing.The NW structures, which have been ignored，formed before the Episode A ofthe Yanshan Movement, and after Indo-China Movement. They have experiencedcomplex tectonic evolution in Western Hills of Beijing, and were reformed intenselyby subsequent NEE-NE compression and extensional structures. They are no longercontinuous or clear. The discovery of NW structures is much helpful to re-understandthe process of the complex tectonic evolution in Western Hills of Beijing area.