台灣地震地研究-台灣西南部活斷層研究─台灣西南部活斷層的大地構造分析

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行政院國家科學委員會專題研究計畫成果報告

台灣西南部活斷層的大地構造分析

計畫類別：整合型計畫計畫編號： NSC92-2116-M-002-005- 執行期間： 92 年 08 月 01 日至 93 年 12 月 31 日執行單位：國立臺灣大學地質科學系暨研究所計畫主持人：鄧屬予報告類型：精簡報告報告附件：出席國際會議研究心得報告及發表論文處理方式：本計畫可公開查詢

中華民國 94 年 5 月 17 日

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Abstract

The continental margin to the east and south of China comprises an active marginin the East China Sea, a collision mountain belt in Taiwan, and a passive margin in the South China Sea. These three segments were generally regarded as separate tectonic entities and their interrelations have long been the subject of debate. Here we synthesize available information to outline the tectonic and geological background of the China margin, examine the link between Taiwan and the neighbouring China margins, and thereby establish a Cenozoic evolutionary model.

The China margin is floored with a pre-Cenozoic continental basement covered with an up to 10-km-thick pile of Cenozoic sedimentary strata. The continental basement has been invariably stretched and molded into a series of northeast-trending horsts and grabens. Except in the Okinawa Trough of the East China Sea, the Cenozoic sedimentary cover typically exhibits a two-tier tectonostratigraphic structure, with narrow Palaeogene rift basins draped by a blanket-like Neogene-Quaternary sequence. The two-tier structure prevails in the entire inner part of the China margin, including the Taiwan Strait off western Taiwan. In the outer China margin, however, the two-tier structure persists only in the South China Sea, and is in stark contrast with the collisional orogen of Taiwan and the Ryukyu arc of the East China Sea.

By untangling the contractional deformation of the northern Taiwan mountain belt, it has been possible to reconstruct a pre collisiona tectonostratigraphic section with a distinctive two-tier structure shown by a Palaeogene half-graben covered with a Miocene drape sequence. When put together with Palaeogene rift basins of the Taiwan Strait, it becomes clear that the precollisional continental margin of Taiwan resembles that of the South China Sea, characterized by two lines of Palaeogene rift basins. Hence before the collision started in Late Miocene times, Taiwan was part of the passive South China margin that extended northward to the southern Ryukyu area.

Ever since the end of the Cretaceous, the China continental margin has been dominated by extensional tectonics, regardless of the presence or absence of subduction zones. In the Early Cenozoic, extensive crustal attenuation resulted in region-wide subsidence and formation of rift basins. Extension in the South China Sea culminated in Late Oligocene times, when part of the outer margin was drifted away by the opening ocean basin. In the East China Sea, the margin remained intact and became separated from the South China Sea margin by a trans-form fault. From the Miocene onward s, the South China Sea margin has been passively subsiding,

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sporadically punctuated with basaltic volcanism. In the East China Sea margin, the Okinawa Trough has opened and the Ryukyu volcanic arc thrived. The NE edge of the South China Sea margin was deformed as the Taiwan orogen.

Keywords: Tectonics, Cenozoic, Stratigraphy, Southeast Asia, Collision

中文摘要

中國大陸的緣海包括三個構造區塊：東海的活動大陸邊緣、台灣的碰撞造山帶及南海的被動大陸邊緣。這三個區域彼此間的相互關係長久以來都是不斷地被爭論的議題。我們在此彙整已有的資料，先描繪中國大陸邊緣的構造和地質背景，再檢視台灣與鄰近的中國大陸邊緣之間的連接關係，並藉此建立新生代的的演化模型。中國大陸邊緣以先新生代的大陸岩盤為基底，上覆近10公里厚的新生代沉積層。大陸基盤受到拉張作用，形成一系列東北向的地壘與地塹。除了東海的沖繩海槽外，其他地區大多呈現一個兩層架構，下部為一狹窄的古第三紀張裂盆地，上為被覆型的新第三紀-第四紀地層。這兩層架構遍布在中國大陸邊緣的內部，包括台灣海峽。然而在中國大陸邊緣的外部，這兩層架構卻只出現在南海。這與台灣的碰撞造山帶和東海的琉球島弧呈現出明顯的對比。在解除台灣北部的壓縮變形後，我們重建出一個古第三紀的半地塹盆地。若與台灣海峽的古第三紀張裂盆地配合，則台灣碰撞前的大陸邊緣就有兩條古第三紀張裂盆地，類似南海大陸邊緣。因此在晚中新世開始碰撞之前，台灣應該是南海被動大陸邊緣的一部份，這個被動邊緣可能往北延伸至南琉球地區。自白堊紀末期以來，中國大陸邊緣一直受到地殼拉張作用的影響。在新生代早期，地殼大規模地變薄，導致區域性的下沉並形成張裂盆地。漸新世初期，張裂活動達到最高潮，造成南海的擴張，並使得大陸邊緣的外部飄開。不過在東海，大陸邊緣則仍維持原貌，並以轉型斷層與南海區隔。中新世之後，南海大陸邊緣的張裂活動逐漸平息，僅有零星的玄武岩漿噴發，南海大陸邊緣的東北角則被碰撞作用擠壓成台灣的造山帶。與此同時，東海大陸邊緣上的沖繩海槽開始張裂，琉球火山島弧則變得更加活躍。關鍵字：構造、新生代、地層學、東南亞、碰撞

報告內容

論文詳見全文後，已發表於，Geological Society, London, Special Publications,

226, 313-332, 2004.

成果自評

台灣西南部的斷層活動啟源於呂宋島弧和中國大陸之間的碰撞造山運動。由於過去學者對碰撞造山運動的機制有許多不同的看法，因此在推測台灣西南部的斷層活動之前，我們必須先確定弧陸碰撞的模式。從整合已有的地質資料中，我們發現一個被動大陸邊緣和一個火山島弧之間

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的碰撞最符合台灣的實際情況。我們不但重建出碰撞前台灣地區的構造地層形貌，並且清楚地勾劃出它和南海大陸邊緣的關係。由於這個被動大陸邊緣如今仍保留在南海的北坡，它和台灣西南部之間的變化，清楚地顯示當碰撞發生時大陸邊緣會如何變形。這個漸變的過程奠定了探究台灣西南部斷層活動的基礎。就現階段的成果而言，我們相信被動大陸邊緣和島弧碰撞是最合理的工作假想，然而其他的觀點，如弧弧碰撞、二次碰撞等，尚未能完全排除。透過本年度的資料整理，我們找到了一些可以驗證的關鍵性疑點，將作為未來更深入研究的方向。