Research On Genesis And Geological Signiifcance Of Magnetic Quiet Zone In Northern South China Sea

Marine magnetic anomalies have two kinds of distribution forms: the magneticlineation and the magnetic quiet zone (MQZ), where magnetic anomalies are greatlysubdued in amplitude with long wavelength, also probably contains information oftectonic activities and evolutions from continental margin rifting to sea floor. TheMQZs show large-area magnetic anomalies with low value in magnetic anomaly maps.The MQZ in the northern South China Sea is located in the continent–oceanboundary (COB), and is located on the south of high positive magnetic anomaly beltand on the north of the magnetic lineation in the sea basin. This article introduces thedistributions and the causes of the global MQZs. We collect the geological andgeophysical data in the Northern South China Sea and discuss the current situation ofthe study on the MQZ there.We define the distribution range of the MQZ clearly according to the△Tmagnetic anomaly data, the free air gravity anomaly data and tectonic backgroundinformation. We obtain the reduction to pole magnetic anomaly using a low magneticlatitude method. We discuss the characteristics of gravity and magnetic field in theNorthern South China Sea by the wavelet multi-scale decomposition method. Wepoint out that the gravity field characteristics of the MQZ in the northern SCS isdifferent with the gravity field characteristics of the MQZ around the Reed Bank, andinfer that their causes may be different.We obtain the distribution of fractures around the MQZ from the magnetic andgravity data using several boundary identification methods such as wavelet fractureanalysis method, Analytic Signal Amplitude method, Normalization Total HorizontalDerivative method and so on. By comparing with the seismic interpretation profiles, we identify23fractures including10lithosphere or crust fractures which strike nearNE or NW direction.We obtain the locations and depth information of gravity and magnetic fieldsource by the power spectrum method, Tilt-angle derivative method and Eulerhomogeneous equation method. We pick up the magnetic anomalies caused by the topand bottom boundary of the magnetic layer by wavelet multi-scale decomposing ofthe reducing to pole magnetic anomaly result. We obtain the depth of the magneticbasement, the depth of the Curie isothermal interface and the thickness of themagnetic layer by the Parker inversion method. We obtain the depth of Mohointerface from the Buge gravity anomaly in the north South China Sea in the sameway as above. The inversion result of the depth of Curie isothermal interface showsthat the depth of Curie isothermal interface in the MQZ is shallow. The simulationresult of temperature field in the MQZ shows that the temperature is higher thanaround areas, the heat flow value is high, and the thermal Curie isothermal interface isshallower than around areas. The computing result also shows the depth of Moho isshallow.We analyze the inversion results and the related research progress, andsummarize the genesis of the MQZ in the South China Sea. We consider that thedirect reason of the formation of the MQZ is the thinning of the magnetic layer whichhas three causes. First, the paleo-pacific plate subduction turned direction and thesubduction zone withdrew, so the South China continental margin at late Mesozoicbegan to break apart. The MQZ located in the earlier continental margin thinning andextensional faulting area. The earlier thinning and extensional faulting formatted aseries of NE directional positive huge regional fractures and a series of graben orhalf-graben basins. The basins developed thick Mesozoic era sedimentary stratum.The Mesozoic and Cenozoic sedimentary strata had teeny magnetic susceptibility andthe magnetic anomaly is very weak. The crust extension led to the old strata faulted,and the depth of the magnetic basement is increased, and the magnetic anomalyattenuated. Second, the crust extension led to the deep mantle mass rose and filled bythe gravity equilibrium effect. The Moho interface and Curie isothermal interface rose by the thermal demagnetization of the magnetic layer caused by the high temperaturematerial from the deep mantle. The thickness of the magnetic layer thinned. Third, thethermal activities aggravated in the northern South China Sea after the spreadingcenter migrated southward during the seafloor spreading period. After the seafloorspreading period, mantle materials went up and formed the high velocity layer by thestrong thermal activities. So the Curie isothermal interface rose again. The upliftedhigh temperature materials cooled and magnetized by modern geomagnetic field.Usually the magnetization direction of the high velocity layer is different with abovemagnetic layer which is magnetized earlier. Negative magnetic anomaly is added intothe earlier positive magnetic anomaly, and the difference of magnetization directionbetween the earlier magnetic layer and the magnetic layer formed by the high velocitylayer.Last we discuss the distribution of the Tethys suture zone ruins in the northernSouth China Sea and infer that the L3fracture belt may be the Tethys suture zoneruins in the northern South China Sea. Then we discuss some understanding on theformation and evolution of the South China Sea based on the reason of the MQZ.