南海北部地體構造之研究; Tectonics of the northern South China Sea

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题名:	南海北部地體構造之研究;Tectonics of the northern South China Sea
作者:	葉一慶;Yi-Ching Yeh
贡献者:	地球物理研究所
关键词:	演化;磁力;震測;海洋地殼;構造;南海;oceanic curst;South Chian Sea;tectonic;evolution;magnetic;seismic
日期:	2006-06-27
上传时间:	2009-09-22 09:54:38 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	南海（South China Sea）為西太平洋地區之一的邊緣海（marginal sea basin）。其西、南兩面為歐亞大陸地殼圍繞，其東為以馬尼拉海溝（Manila Trench）為界。由於南海的張裂與整個東南亞地體構造演化有關，過去二十年來，有許多學者提出了許多模型試圖可合理解釋南海的地體構造及其演化歷史。其中，Tapponnier 等人提出了著名的碰撞-逃逸理論，南海的張裂與此運動很可能相關。另外，Briais等人整編了南海海域的磁力資料並重新分析，其結果顯示南海應於距今30.1百萬年前（磁力條帶 C11）開始張裂並於距今16.7百萬年前（磁力條帶C5c）時停止張裂，其間經歷了三次張裂方向的改變。但無論是上述那個模型由於其運用的地球物理資料僅限於北緯19度以南，其研究成果實難清楚描述北緯19度之地體構造。而北緯十九度以北（相當於台灣西南外海）之北部南海應為更老之南海海洋地殼，此部分在南海演化史佔了何角色且與台灣造山帶的關係為何，皆為本論文之研究重點。本研究使用了舊有及新蒐集的所有種類的海洋及陸上地球物理資料，包含了水深、多頻道震測、重力、磁力等資料。並將上述資料進行綜合分析。其中，新整編的磁力異常圖西由東經116度起至東經123度; 南由北緯16度起至北緯26.5度。其結果顯示了南海北部確實有數條近乎東西走向的磁力條帶分佈，這暗示著海洋地殼張裂的證據。而磁力定年的結果顯示南海北部存在著更老的海洋地殼，其年代可追溯至距今37個百萬年前（磁力條帶C17），其半擴張速率約44 mm/yr。此外，根據兩條跨越大陸邊緣的震測線之重力模擬結果與磁化強度分析結果顯示，南海北部的大陸海洋邊界（Continent Oceanic Boundary）約落在地殼開始強烈的減薄處且此處恰好磁化強度正位於由最低區開始轉變為正之處。另外，多頻道震測資料分析的結果顯示，琉球-呂宋轉形邊界（Ryuku-Luzon Transform Boundary, LRTPB）兩側的聲波基盤（acoustic basement）深度有顯著不同：由南側的3.5 km ~ 4 km至北側的4.5 km ~ 6.5 km而重力順推模型的結果亦顯示了LRTPB下方的地殼確實有減薄的趨勢。此外，其上部地殼的震測結果顯示了位於LRTPB處地殼確實受到相當顯著的變形，而此變形僅限於基盤附近。此可說明LRTPB可能為一構造邊界可進一步推測為南海海洋地殼的最北邊界。另外，南海北部的C11至C17的磁力條帶可往西南與Briais等人所整編的磁力條帶連接，此也同時意味著南海北部為中部南海的延伸亦為整個南海的一部份。且在早期南海張裂時期約在距今27.9百萬年前?28.7百萬年前（磁力條帶C9?C10）間南海由西北東南的張裂方向改為南北向張裂。此張裂方向的改變的事件可能可對應到此區域一近基盤的強反射面T1。此外，約17.6百萬年前（磁力條帶C5d）LRTPB已停止張裂，但LRTPB北方菲律賓海岩石圈仍然向西北方向隱沒，此作用很可能使得南海北部地層受擠壓作用，而這擠壓作用很可能與另一強反射面T2的形成有關。將南海北部的磁力異常資料進行逆推，逆推結果顯示南海北部大陸邊界存在一高磁化強度高區，此高區可由南海北部大陸邊緣一直進入台灣的西南部陸上。與過去蒐集的震測及鑽井資料比對，此磁化強度高區很可能為過去玄武岩質岩漿噴發凝固後受磁化的結果，而此岩漿噴發年代約由曉新世（Paleocene）晚期至史新世（Eocene）中期，此很可能與早期南海張裂相關。此外，本區域的地震分佈很可能與磁性地殼基盤分佈有一定關係。如將過去發生的歷史地震（1990年前）與中央氣象局的短週期目錄規模超過6的地震繪於磁化強度圖上。其結果顯示台灣西部的大規模災害地震幾乎都發生在磁化強度的低區，而恰好均圍繞著一相對高磁化強度地塊-鹿港高區（LHM）。這說明著LHM很可能為一個相對較堅硬的地塊，而板塊聚合的應力均順著這個邊界釋放。 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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