BASIC CHARACTERISTICS OF THE GEOLOGICAL STRUCTURES OF THE YANSHAN AREA,HEBEI PROVINCE

The"Zanshan area"refers to northern Hebei, Beijing and Tianjin in general, covering a broad area largely between latitudes 39°40’—42°N. The area is characterized by complex geological structure and rich mineral resources. The exposed strata include the Archean, lower Proterozoic, Sinian suberathem, Paleozoic （the Silurian and Devonian absent） and Mesozoic. The magmatic activity has been widespread and strong, characterized by a multistage history in time and distinct orientation and zoning in space. There have been volcanic-intrusive processes of different characters at various geological stages from Archean to Cenozoic times. According to the history of geological development, the tectonic evolution of the area may involve four stages: the stage of formation and folding of the early Proterozoic geosyncline, the stage of inheritance and development of the late Proterozoic （Sinian suberathem） parageosyncline, the stage of continuous development of the Paleozoic platform and the stage of reactivation of the Mesozoic platform. The four evolutional stages exhibit marked changes in nature and direction of tectonic movements and in deformation. The main fracture zones in the Yanshan area have the following characteristics: generally the E-W zones are older, while the NE-NNE zones are younger （except a few older NE-NNE-trending fractures）; some E-W fracture zones gradually swing northwards from east to west;and the magmatic activity is very closely related to the fracture zones. The authors hold that regularities in time and space are available governing the formation and development of the geological structures of the Yanshan area located at northeastern margins of the north China plate. In time, pre-Mesozoic diastrophism was dominated by E-W trend, while postMesozoic diastrophism was dominated by NE-NNE trend; in space, the area has been affected by both the Mongolian plate to its north and the Pacific plate to its east, thus forming the peculiar fault-block structural features characterized by the intersection of two different directions.