Magnetostratigraphic Research On The Structural Activities Of The Xiadian Fault In Beijing Plain

Xiadian fault is an important, hidden and active normal fault which locates in thenorthern part of the Huabei plain, and dips50°to70°to the SE, separating theDachang depression and Tongzhou uplift in the eastern part of Beijing as theboundary fault. Affected by the normal dip-slip faulted movement of Xiadian fault,Dachang depression continues to subside and has deposited thick sediments, whileTongzhou uplift is relatively uplifted. But it’s hiding prevents us from furtherelaborately studying of it’s activity, bringing hazards and inconvenience to human life,property safety and urban layouts. By magnetostratigraphic research on the sedimentsof all the drills near the northern part of Xiadian fault in the Mafang area, Pinggu,we provide new insights into the structural activities of the Xiadian fault.By magnetostratigraphic research on the sediments of four drills, we discoverthat: the Brunhes-Matsuyama Boudary of ZK07in the northwestern plate appears inthe depth of84m and two normal polarity zones founded in the Matsuyama reversedpolarity zone are Jaramillo(122-131m) and Olduvai(151-164.4m) polarity subchron,respectively; the Matsuyama-Guass Boundary of ZK08in the northwestern platewhich is closer than ZK07to Xiadian fault appears in the depths of184.5m, and thenormal polarity zone(170.5-180.5m) founded in the Matsuyama reversed polarityzone is Olduvai normal polarity subchron; the Matsuyama-Gauss Boundary of ZK09in the southeastern plate appears in the depth of221m, the normal polarityzone(200-214m) founded in the Matsuyama reversed polarity zone is the Olduvainormal polarity subchron.Those date indicate Xiadian fault began to activate since the Pliocene,meanwhile this region began to deposit sediments. By comparison of the difference ofsediments’ thickness during the same period between two plates, we conclude thatXiadian fault activated periodically and had variable intensity:1. During2.58Ma-1.95Ma, sediments in the lower plate is3m thicker than thosein the upper plate, it’s activity was relatively weak with vertical movement rate of0.476cm/ka;2. During1.95Ma-1.77Ma, sediments in the lower plate is4m thicker comparedto those in the upper plate, it’s activity was relatively enhanced compared with that in the period of2.58Ma-1.95Ma and reached2.22cm/ka;3. During1.77Ma-1.51Ma, sediments in the lower plate is1.5m thicker thanthose in the upper plate, it’s vertical movement rate reached0.577cm/ka and wasevidently weaker compared to that in the period of1.95Ma-1.77Ma;4. During1.51Ma-0.64Ma, the difference between the thickness of thesediments in the two plates reached25.5m, it’s activity reached the highest rate of2.93cm/ka since2.58Ma ago;5. During0.64Ma-now, sediments in the lower plate is2.5m thicker than those inthe upper plate, it’s activity weakened and reached the lowest value of0.39cm/kasince2.58Ma age.