| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 94 === The South China Sea (SCS) is one of the marginal sea basins in the western Pacific area. The SCS is bounded by Eurasia continental crust in west, south and Manila Trench in east. In past two decade, several models were tried to describe the tectonic and its evolution of the SCS. Tapponnier et al (1986) proposed so-called “collision-escape” model to demonstrate the possible process of the opening of the SCS. Briais et al (1993) re-compiled then analyzed the magnetic data collected by Mainland China and the result shows the SCS probably started spreading at 30.1 Ma ago (magnetic lineation C11) and stopped spreading at 16.7 Ma ago (magnetic lineation C5c) including three time spreading direction change. Whatever the model mentioned previous, is not given a comprehensive tectonic of view further 19 °N in latitude. The crust further 19 °N in latitude is few study and probably belongs to oceanic domain. What role of the tectonic this part play in the whole SCS evolution history and relationship to the Taiwan orogen ? This thesis is tried to answer that.
The past collected and resent collected marine and land bathymetry, multichannel seismic, gravity and magnetic data were combined to analyze in this study. The newly compiled magnetic anomaly map is from 116° E ~ 123°E and 16°N ~ 26.5°N that shows several E-W trending magnetic lineations which implied the existence of the oceanic crust. The magnetic dating result was showed the existence of the older oceanic crust in the northern SCS that is 37 Ma (magnetic lineation C17) and carried 44 mm/yr half spreading rate in the older spreading stage. Based on the forward gravity model across the northern SCS margin with magnetic inversion result, the continent and ocean boundary is probably dropped at the highly crustal deformation and relative low magnetization area. Furthermore, the Luzon-Ryuku Transform Boundary (LRTPB) probably is the tectonic boundary that revealed different acoustic basement relief from 3.5 km ~ 4km depth in south to 4.5 km ~ 6.5 km depth in north. The forward gravity model also shows the thinned crust distributed beneath LRTPB and the upper crust is also performed highly shearing deformation accurrence close to basement. Therefore, LRTPB probably is a northern boundary of the oceanic crust of SCS.
Besides, the part of C11~C17 oceanic crust could be prolongated southwestern ward and linked to the central part of the SCS. In the early spreading stage, the spreading direction was changed from NW-SE to N-S between 27.9 Ma to 28.7 Ma (magnetic lineation C9~C10) that corresponded with the tectonic event T1. Otherwise, at 17.6 Ma (magnetic lineation C5d), the LRTPB was ceased but active subduction still occurred at north that probably resulted in the compressed the sedimentary layers in south of the LRTPB. That probably is linked to the development with the tectonic event T2.
Finally, the inversion of the magnetic anomaly is showed the relative high magnetization belt distributed along the northern SCS margin that extended to southern Taiwan. Combined with multichannel seismic and well log analysis, the late Paleocene to mid-Eocene basaltic type eruption is probably related to the high magnetization and the seafloor spreading of the SCS. Besides, the distribution of the great earthquakes in Taiwan area is probably controlled by the distribution of the magnetized crust. I put over magnitude earthquakes (including historic earthquakes before 1990) on the magnetization map. The result is shown the good correlation between earthquake occurrence and low magnetization area in west Taiwan area. Most of earthquakes was distributed along the boundary of the high magnetization area called Lukang high (LHM). The collision stress probably released along the LHM and resulting lots of earthquakes.
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