利用反射震測以及海底地震儀震測資料探討南海北部地殼構造

Abstract

南中國海(以下簡稱南海)是觀察大陸地殼張裂、破裂以及海底擴張的理想地點，南海的海底擴張大約始於早漸新世(32Ma)止於晚中新世(15Ma)，南海北部是南海最早期擴張的位置，因此，了解此處的地殼構造能夠讓我們更進一步認識南海的初期大陸地殼張裂到海底擴張的演化。 本研究利用2007年海研一號多頻道反射震測(MCS)840航次以及2008年海研一號883航次之多頻道反射震測來解析基盤以上的淺部地殼形貌，再輔以21顆海底地震儀(OBS)資料探討深部地殼構造(莫荷面以上)。MCS840測線位在東沙島東南方，其範圍橫跨東沙隆起、潮汕凹陷、南部隆起區，而MCS883全測線皆位在南中國海東次海盆內。在建構地殼速度模型方面，我們先利用速度分析法求得淺部(基盤以上)速度後，再將此結果作為接下來層析成像模擬法(tomography)的初始速度模型以增加淺部速度的可信度，最後再進行正演模擬，在正演模擬內藉由我們挑選出更多的反射及折射的波相，更加提高速度模型的可信度。 在反射震測剖面MCS840中發現有許多張裂時期形成的高角度正斷層及海底擴張後形成的火成岩體，MCS883中則遍布火成岩體以及火成岩基盤，上覆的沉積層非常平整；而在海底地震儀速度構造結果顯示莫荷面深度由東沙島漸漸向東南方變淺(24公里到14公里)，在OBS840剖面的陸海過渡帶(continent-ocean transition, COT)之下部地殼處有高速帶(high velocity layer, HVL,P波速度大於7.3~7.5 km/s)出現，推測與大陸地殼張裂時及海底擴張結束後東沙島附近區域火成活動所造成的底侵作用(underplating)有關，OBS883為典型的海洋地殼，地殼厚度約為9公里厚，因為玄武岩質岩漿的關係，此測線之基盤P波速度較OBS840快。

The South China Sea (SCS) is an ideal place to examine the nature of continental rifting, breakup, and the onset of seafloor spreading. The SCS basin was formed by seafloor spreading from early Oligocene (~32Ma) to the middle Miocene (~15.5Ma),to know the crustal structures of the northern SCS continental margin may help us to understand the mechanism of the SCS rifting processes. In this study, a total of 21 ocean bottom seismometer (OBS) records along 2 multichannel seismic reflection profiles are analyzed to investigate the upper crustal structure and crustal velocity profiles along this margin. There are two seismic profiles MCS840 and MCS883 which were collected by Ocean Research I are located in the northern continental margin of SCS, and they form a NW-SE trending line southeastern of the Dongsha island. This line starts from Dongsha rise, runs across the Chaoshan depression and south uplift area to the northern SCS East Sub-basin. To construct a more reliable structure model, we extract shallow velocity structure from the MCS profile data, to build an initial model to derive a 2D velocity model of the crustal structure with tomographic inversion technique using PROFIT code, then, we do forward modeling using RAYINVR to get a convincingly hybrid velocity model. MCS840 seismic reflection profile shows that there are many high angle normal faults and some igneous bodies which have been dated at 1.23 MA. However, the igneous basement spread all over in profile MCS883, and the sediment strata above the basement are flat. OBS840 results show that the Moho depth decrease from north to south, and there exist a high velocity layer which is interpreted to be underplating material under the continent-ocean transition(COT). OBS883 shows a classic oceanic crustal structure with a crustal thickness about 9 km. Because of the basaltic magmatism, the basement velocity in OBS883 is higher than that of the OBS840.