Study On The Mesozoic Structural Analysis To The South Of The Middle Segment Of The Yanshan Belt In East China

Malanyu-dome or Malanyu gneiss dome, at the south of the middle segment of the Yanshan orogenic belt, is composed of Archean metamorphic rocks. On the southern and western sides of the dome, significant fold deformations can be observed clearly in their sedimentary covers. Previously, the above structures were called as Malanyu anticlinorium or Malanyu anticline (MA), which is a multi-fold with a metamorphic core. Lately, some geologists named the anticline like as the metamorphic core complex. They suggested that the"Malanyu metamorphic core complex"is a represent of extensional tectonics within the North China Craton during the early Mesozoic time. However, the evolution of the Malanyu is one of important questions to the evolution of the North China Craton, which would provide a key mechanism to the denudation and exhumation of the crystalline basement.From filed investigations, we found an unconformity between the cover and the crystalline basement. There is no low-angle detachment fault within the dome, or no ductile shear zone between coverage and gneissic core. Moreover, no high angle normal faults of upper plate co-exist yet in the covers. Therefore, we think the complex'recognition is puzzled being called as the metamorphic core complex (MCC).Based on the filed studies, we found a weak polarity which extended from north to south in the folds. We called it as sub-polarity here. The deformation to these folds had stronger detachment in several layers. We could find many box-folds, chervon folds, and more conjunate kink-bands in lamella sedimentary rocks, such as the Tiechang chervon anticline, and the Zhuangguoyu chervon syncline. The main detached layer is the Mn-bearing shale in the second group to the Gaoyuzhuang Formation and the shale to the Hushuizuhang Formation. Deformation of these folds is in a crust's compressional stress. It belongs to a type of the buckle fold. The N-S-shortening reaches up to 20% in the south lamb of the Malanyu area. These folds can be classified as the detachment folds. The relationship between Panshan granitic pluton and its surrounding folds indicates that the granitic intrusion has resulted in certain deformations to the former folds. Therefore, the covered folds are likely to be constituted prior to the emplacement of the Panshan granitic pluton. In this study, the"Malanyu anticline"is used, but its exact name should be"Malanyu folds or Malanyu detachment folds".There are several faults should be emphasized. The detachment under the Tiechang chevron anticline is not a low-angle extensional detachment, but a thrust detachment fault. The Yangzhuang fault is an out-of-syncline thrust, syn-folding with the Fujunshan fold. The Huangyaguan fault is a basement fault, exposed by the differential uplift of the blocks controlled by the weakness zones of the crystalline basement. The timing of the Huangyaguan fault occurred in late Jurassic when the Malanyu Dome has begun to be uplifted.The Malanyu Dome is not a metamorphic core complex which formed in the Early Mesozoic. Syn-tectonic sedimentation indicates that this dome initially developed during the late Jurassic. Between the uplift and the depressed blocks, or the margins of the dome, there are three big faults which have been considered as basement faults. In the south margin of the dome, the covers take a monocline. The tectonic setting of the uplift is not an extension, and the simple vertical uplift is farfetched to explain the case of the uplifting. The form of the dome had an important consistence to tectonic re-activation of the basement under the contraction motion, and with a syn-kinematic deformation between the vertical movement and horizontally crustal movement.