《Chinese Science Bulletin》 2012年24期
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Magnetic recording of the Cenozoic oceanic crustal accretion and evolution of the South China Sea basin

【摘要】:We review and discuss some of the recent scientific findings made on magnetic data in the South China Sea (SCS). Magnetic anomalies bear extremely rich information on Mesozoic and Cenozoic tectonic evolution. 3D analytical signal amplitudes computed from magnetic anomalies reveal very precisely relict distributions of Mesozoic sedimentary sequences on the two conjugate continental margins, and they are also found very effective in depicting later-stage magmatism and tectonic transitions and zonation within the SCS oceanic crust. Through integrated analyses of magnetic, gravity and reflection seismic data, we define the continent-ocean boundary (COB) around the South China Sea continental margin, and find that the COB coincides very well with a transition zone from mostly positive to negative free-air gravity anomalies. This accurate outlining of the COB is critical for better tracing magnetic anomalies induced by the oceanic crust. The geometrically complex COB and inner magnetic zonation require the introduction of an episodic opening model, as well as a transform fault (here coined as Zhongnan Fault) between the East and Southwest Sub-basins, while within the East and Southwest Sub-basins, magnetic anomalies are rather continuous later-ally, indicating nonexistence of large transform faults within these sub-basins. We enhance magnetic anomalies caused by the shallow basaltic layer via a band-pass filter, and recognize that the likely oldest magnetic anomaly near the northern continental margin is C12 according to the magnetic time scale CK95. Near the southern continental margin, magnetic anomalies are less recognizable and the anomaly C12 appears to be missing. These differences show an asymmetrical opening style with respect to the relict spreading center, and the northern part appears to have slightly faster spreading rates than to the south. The magnetic anomalies C8 (M1 and M2, ~26 Ma) represent important magnetic boundaries within the oceanic basin, and are possibly related to changes in spreading rates and magmatic intensities. The magnetic evidence for a previously proposed ridge jump after the anomaly C7 is not clear. The age of the Southwest Sub-basin has yet to be further examined, most favorably with deep-tow magnetic surveys and ocean drilling. Our magnetic spectral study shows that the shallowest Curie points are located around the eastern part of the Southwestern Sub-basin, whereas within the East Sub-basin Curie depths are smaller to the north of the relict spreading center than to the south. This pattern of Curie depths is consistent to regional heat flow measurements and later-stage volcanic seamount distributions, and we therefore reason that Curie-depth variations are closely associated with later-stage magmatism, rather than with crustal ages. Although magnetic anomalies located around the northern continent-ocean transition zone (COT) are relatively quiet, this area is not a typical magnetic quiet zone since conceptually it differs markedly from an oceanic magnetic quiet zone. The relatively quiet magnetic anomalies are seemingly associated with a shallowing in Curie isotherm and thinning in magnetic layer, but our comprehensive observations suggest that the well-preserved thick Mesozoic sedimentary rocks are major causes for the magnetically quiet zone. The high similarities between various low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies clearly confirm that deeper magnetic sources (in the lower crust and the uppermost mantle) have contributions to long-wavelength surface magnetic anomalies in the area, as already inferred from magnetically inversed Curie depths. The offshore south China magnetic anomaly (SCMA) becomes more prominent on low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies, indicating very deeply-buried magnetic sources beneath it.

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