| 摘要: | 台灣西南海域的構造特徵為台灣增積造山帶跨騎於南中國海張裂大陸邊緣東北部區域。在本研究裡，為解析在不同地體構造環境下，塊體搬運堆積(MTDs)和沉積物波(SWs)的來源，進行南海大陸斜坡及相鄰的台灣海域增積岩體之 MTDs 與SWs 的時空分布研究。本研究使用多音束水深、多頻道反射震測、井下資料來描述MTDs 和 SWs 的特徵。在張裂大陸邊緣，台南盆地主要由新近紀和第四紀沉積物組成，特別在深水的區域，上覆於古近紀同張裂沉積物。在張裂期間，許多正斷層陷落造成半地塹和地壘塊體構造，凹陷處充填了古近紀沉積物。深水區域的中新
世沉積物的特徵主要為半遠洋沉積物偶夾濁流岩。上新世時期有典型的斜坡沉積物向前加積的形貌及海底水道特徵。在更新世早期的格拉斯期(Gelasian Age)所形成的 MTDs、更新世 1.8 百萬年後的 MTDs 和沉積物波分別由 1.8 百萬年及 0.76百萬年等兩地層面區隔。有 4 個大尺度的更新世 MTDs (MTDa-MTDd)形成於格拉斯期；1.8 百萬年至 0.76 百萬年則發育 8 套 MTDs (MTD1-MTD8)。其中 MTD5 為最大的 MTD，範圍為 4,045 平方公里，體積約 208 立方公里。這些 MTDs 觸發的原因可能為相鄰陸棚邊緣的高沉降速率、全球海水面降低、大陸斜坡變陡、源自於初始弧陸碰撞帶的地震。從大約 0.76 百萬年後，大型 MTDs 不再發育，被廣布的 SWs 取代。現代的 SWs 僅形成於變形前緣以西，並且在兩個區域顯示不同的方向性和幾何形狀特徵。北部區域位於福爾摩沙峽谷以北和以東的區域，南部區域則位於福爾摩沙峽
谷以南。在增積岩體中，MTDs 僅出現於少數的斜坡盆地。尤其靠近分歧斷層(splay fault)。在墾丁盆地，MTD 的累積厚度達約 500 公尺厚。MTDs 在南海大陸邊緣的空間分布範圍與體積大小，遠大於增積岩體區域。這表示大陸邊緣較易發育大尺度和廣布的塊體運動，而增積岩體區域塊體運動則較為局部。增積岩體區域及相鄰的主要斷層(亦即分歧斷層)有最厚的 MTDs 發育，顯示增積岩體區更容易因地震觸發塊體運動，儘管只是區域性的沉積物崩落，仍可導致巨厚的沉積物堆
積;The Taiwan orogenic wedge overrides the northeast rifted continental margin of the South China Sea (SCS) in the offshore area of southwest Taiwan. In this work, I study the temporal and spatial distribution of Quaternary mass transport deposits (MTDs) and sediment waves (SWs) both in the SCS continental slope and its adjacent submarine Taiwan orogenic wedge in order to decipher the origins of MTDs and SWs in both tectonic regimes. Multibeam bathymetry, boreholes and multichannel reflection seismic data were used to characterize the MTDs and SWs. In the rifted continental margin, the Tainan Basin is mainly composed of Neogene and Quaternary sediments underlain by Paleogene synrift infills, especially in the deepwater area. During the rifting phase, many normal faults formed half-graben and graben blocks which were filled in by Paleogene sediments. Miocene sediments are characterized by hemiplegites with packets of turbidites in the deepwater areas. The slope progradational configuration is typical for Pliocene units with submarine channels which became well developed. During the Pleistocene Epoch, Gelasian MTDs, post 1.8 Ma MTDs, and sediment waves, which are separated by stratigrahic horizons of 1.8 Ma and 0.76 Ma, respectively. There are 4 large complexes of MTDs occurred during the Gelasian and 8 MTD sheets are recognized post 1.8 Ma boundary in the lower slope. MTD5 is the largest MTD which covers 4,045 km2, reaching a total volume of ~ 208 km3. Triggers of those MTDs are probably due to high sedimentation rates in the vicinity of the shelf edge, lowering of global sea-level, steepening of the continental slope, and earthquake shaking originated in the incipient arccontinent collision zone. The MTDs were then overlain by widespread SWs since around 0.76 Ma. The modern SWs occur only to the west of the deformation front and are characterized by two fields showing different orientation and geometry. The northern field lies to the north and to the east of the Formosa Canyon, while the southern field lies to the
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south of the Formosa Canyon. Though MTDs occur only in a limited number of slope basins in the accretionary wedge, they seem to occur near the splay fault, with a cumulated thickness up to ~ 500 m. The spatial extent of MTDs in the SCS continental margin is larger than that in the accretionary wedge. This indicates that large-scale and widespread mass movements tend to occur in continental margins, while the mass movements in the accretionary wedge tend to be more localized. The thickest MTDs are found in the accretionary wedge and adjacent to a major fault (i.e., the splay fault), indicating that the mass movements occurring in the accretionary wedge are most likely triggered by earthquake shaking, leading to gigantic thick, albeit localized sediment failures. |
