西菲律賓海盆一般認為從53 Ma開始擴張，在33/30 Ma時停止。擴張之初，海床擴張的速率較快(速率約為44 mm/yr)，因此在盆地的邊緣區域，留下一些海床快速擴張所產生的構造，例如在西菲律賓海盆的北部區域，有許多重疊的中洋脊擴張中心(Overlapping Spreading Center, OSC)的出現。重疊中洋脊擴張中心指的是一種海床構造，發生在快速擴張的中洋脊，在轉形斷層無法產生於兩個錯動的海脊之間時，取而代之的一種海脊輕微重疊的構造，此種構造也可能由擴張時的岩漿供應量較大所產生。 我們在西菲律賓海盆西北部(123°E以東)的地區收集多頻道水深及磁力資料，本研究主要是利用這些最新收集的多音束水深資料研究位於呂宋─沖繩破裂帶(Luzon-Okinawa Fracture Zone)和加瓜海脊(Gagua Ridge)之間的海床構造。研究區域內有三條明顯為東北─西南向的破裂帶，此破裂帶走向和研究區內的深海丘陵走向皆顯示為東北─西南方向擴張，與其東邊的西菲律賓海盆的北部和南部之第一期海床擴張走向一致，此意味本研究區的海盆同為第一期擴張時所形成。另外在研究區北部有明顯重疊擴張中心(OSC)的存在，我們認為可能是快速擴張海盆的岩漿供應量較南部為多所造成，且推測當時中洋脊的岩漿不但向兩側推動，也向東南方前進。 研究區的海盆擴張可以用單一個階段表示，並不像呂宋─沖繩破裂帶以東的地區，因為擴張軸的旋轉而有三個擴張階段，因此，在呂宋─沖繩破裂帶與第二期海床擴張接觸面當時應為轉形斷層帶。另藉由研究區內破裂帶兩側的鋸齒狀水深特性，推測東側海洋地殼年齡應較西側為老。 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.
