THE BEIJING GRABEN——A SYNTHETIC STUDY

Through a synthetic study of the geological and geophysical data from the strata below the Quaternary sediments in the Beijing plain, the authors found a typical hidden graben striking NE, which is bounded by step normal faults. On its eastern and western sides lie the Daxing uplift and the Western Beijing uplift. Along the boundary faults there occur polyphase Cenozoic tho- leiite of continental rift type derived from the upper mantle. The geophysical field has revealed that the Bouguer anomaly and the isostatic gravity anom- aly strike parallel to the strike of the graben and that two gravity highs and one gravity low are arranged alternatively, with the latter sandwiched in between the formers. As revealed by the deep seismic sonding the Moho below the crust is a bit higher in level, the crust is generally 34--35km thick and thickens towards both sides, and the boundary faults are discontinuous, the velocity Pn wave is slightly lower, and the terrestrial heat flow is high, aver- aging 1.8--1.95 HFU. The telluric electro-magnetic sonding has reflected that there exists an intracrust low-resistivity layer at a depth of about 20 km and an upper mantle low-resistivity at 40--60km. The Curie isothermal surface in the axial region of the graben is obviously shallower than at both sides of it. All the data show that the Beijing graben exhibits the geology, geophysical field and features of the crust and upper mantle of a typical continental rift, so is the product of the extensional environment, and the inherited extension is still active now.