| Summary: | 碩士 === 國立中央大學 === 地球物理研究所 === 96 === West Philippine Basin (WPB) was generally thought to start spreading at 53 Ma and end at 33/30 Ma. In the early of spreading, the spreading rate was faster (nearly 44 mm/yr). Therefore there are some fast-spreading structures whish has been found in the region area of the basin. For example, there are many overlapping spreading centers (OSC) in the northern part of WPB. OSC seafloor structure occurs in fast-spreading ridge and replaces the transform fault of two cross ridges. The structure may also happen magma supply is larger.
We have collected bathymetric data in the north-eastern part of the West Philippine Basin. This study uses these new data to analyze the seafloor structures between Luzon-Okinawa Fracture Zone and Gagua Ridge. There are three major NE-SW oriented fracture zones in the study area. These fracture zones and abyssal hills all pointed out the direction of spreading in NE-SW, which is the same as in northern and southern part of WPB. It implies that this study area is formed in the first spreading stage of WPB. Besides, there is an OSC existing in north of the study area. We think it is because of the magma supply in fast spreading rate more than in the south. We infer that magma supply was propagating southeastward.
Spreading in this study area could be just one single stage. It is different from east of Luzon-Okinawa Fracture Zone which had axis rotation and displayed three spreading stage. Therefore, the connection between Luzon-Okinawa Fracture Zone and the second spreading stage of WPB was a transform fault in that time. Moreover, because of the saw-type character of bathymetry across any fracture zone in study area, we can expect the ocean crust in east side could be order.
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