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| 題名: | 臺灣西南海域深水區褶皺逆衝斷層帶：以震測成像、地層演育、天然氣水合物及地熱特徵觀點探討;Deepwater Fold-and-Thrust Belts off Southwestern Taiwan: A View on Seismic Imaging, Stratigraphic Development, Gas Hydrates, and Associated Thermal Signatures |
| 作者: | 費索;Dirgantara, Feisal |
| 貢獻者: | 國際研究生博士學位學程 |
| 關鍵詞: | 深水;海洋地質;海洋地球物理學;地層學;天然氣水合物;地震成像;熱性能;造山帶;台灣;折疊推力帶;折疊推力帶;deepwater;marine geology;marine geophysics;stratigraphy;gas hydrates;seismic imaging;thermal properties;orogen;Taiwan;fold-and-thrust belts;Feisal Dirgantara |
| 日期: | 2021-08-23 |
| 上傳時間: | 2021-12-07 12:17:03 (UTC+8) |
| 出版者: | 國立中央大學 |
| 摘要: | 臺灣西南海域位於馬尼拉隱沒帶的北緣，為一地體活動的過渡帶，地體活動由南往北逐漸由隱沒作用轉變為弧陸碰撞。因特殊的地體架構，使得此區域關於震測影像處理與解釋、深水層序演化，以及天然氣水合物系統與其相關熱流特徵的研究有諸多挑戰。本研究重新處理兩條2009年收集，通過南中國海北坡張裂大陸邊緣深水區與台灣增積岩體的2維多頻道反射震測剖面MGL0905-10與MGL0905-27。此外，亦配合深鑽資料、震測層序地層與震測相分析，解析南中國海北部的層序演化。除上述MGL0905-10 與 MGL0905-27兩條測線外，本研究亦使用了MGL0905-05與MGL0905-20這兩條2維多頻道反射震測時間剖面，進行研究區域地層與構造演育的研究。本研究依據震測訊號的特徵，劃分出6個關鍵層面與11個震測相，並建立新的年代地層以解釋南中國海東北部深水區層序的演化。結果顯示，古近紀張裂活動形成的半地塹系統，以分離不整合為面為界，與晚漸新世的淺海沉積物接觸。於後張裂早期之晚期漸新世至早中新世之間，有局部的海底火山與岩床發育。於晚期中新世，有廣泛的海底峽谷發育，在中新世最晚期，弧陸碰撞開始發育。在更新世時期，有許多的崩積物與沉積物波堆積。張裂大陸邊緣之外緣，因弧陸碰撞作用，廣泛發育褶皺逆衝斷層帶和斜坡盆地。以前陸基底不整合面為界，可分開下伏的後張裂時期地層與上覆的前陸盆地堆積。相較於張裂大陸邊緣區域，於台灣增積岩體區之斜坡盆地，多為各自獨立且少有水道發育，以半遠洋沉積物為主。除上述特徵外，在增積岩體上有可能為活動斷層的分岐斷層(其常切穿海床並且造成明顯的坡折)，以及位於海床下約3 - 12公里深處、角度平均在 4–6 度之間的滑脫面。在距離海床深度約 300–600公尺處，廣泛的海底仿擬反射，顯示了無論是張裂大陸邊緣或台灣增積岩體上皆有天然氣水合物富集，可能受控於活躍的流體活動。弧陸碰撞的聚合作用則造成了增積岩體內泥貫入體的發育與墾丁高原基盤岩的隆起。
根據震測解釋的結果，發現在增積岩體的上部斜坡區域，有廣泛的海底仿擬反射及泥貫入體分布，暗示這區域存在許多天然氣水合物及游離氣。下枋寮盆地為增積岩體上部斜坡區的一個半封閉盆地，由3維 MCS937與2維MGL0908-TST震測資料得知，下枋寮盆地的海底仿擬反射大多橫切此區域傾斜的地層與泥貫入體，而海底仿擬反射可能代表著天然氣水合物穩定帶的底部界面。由3維震測屬性顯示，海底仿擬反射的上方有天然氣水合物與游離氣的富集，而海底仿擬反射的下方則富集游離氣。泥貫入體及其相關的斷層可能為深處熱成熟甲烷氣的通道，甲烷氣可沿著此通道向上移棲至天然氣水合物穩定帶 (GHSZ)。本研究以蒙特卡羅模擬為估算的基礎，初步估計在 60 平方公里的研究區域內，天然氣總體積為 2,048 Bcf。
海底仿擬反射在增積岩體上通常位於較淺的深度，而下枋寮盆地的海底仿擬反射深度與其他上部增積楔斜坡區域相比則較深。地溫梯度和熱流估計值分別為 33 oC.km-1 和 41 mW.m-2，顯示熱毯作用仍為控制當今盆地溫度的主因；此外，以熱探測器和紅外成像儀對淺層活塞岩心密集量測的結果，顯示此區域地溫梯度和熱流平均值分別為 55 °C.km-1 和 62 mW.m-2，此一量測結果與上述估計的結果有明顯的差異。造成此一差異的原因，可能與直接熱測量對淺層流體通量的敏感度有關，區域性的淺層地溫梯度會隨著流體向上移棲而增加。由高解析度的海底地形、震測相解釋（包含有3維的MCS937，以及2維的 MW9006-01與 MW9006-02測線），以及熱異常分佈的資料顯示，古高屏峽谷曾流經現在的下部枋寮盆地，並堆積了一系列的砂質濁流沉積物。由於沉積物快速的堆積，可能會導致古高屏峽谷區域沉積物的孔隙水來不及排出，造成了孔隙壓力處於超壓的狀態，因而導致了此區域有較低的熱流與地溫梯度，有助於加深天然氣水合物穩定帶 (GHSZ)的底部界面。另外，古高屏峽谷由於後來的泥貫入體入侵與區域海床的抬升改變了原本的流向，逐漸往南遷移演變成現今的高屏峽谷，而原本的古高屏峽谷則被廢棄。;Offshore southwestern Taiwan lies in the northern end of Manila Subduction System (MSS) where normal subduction progressively evolves into initial arc-continent collision. The tectonic configuration offers the area with challenges pertaining to the comprehension of subsurface imaging, deepwater stratigraphic development, gas hydrates systems, and associated thermal signatures. Two legacy 2D multichannel seismic data (MGL0905-10 and MGL0905-27), crossing the deepwater rifted Chinese continental margin to submarine Taiwan accretionary wedge, were re-processed to image the depth-domain subsurface. Together with age-controlled deep drilling, seismic stratigraphy and seismic facies analyses were utilized to redefine the stratigraphic development in the northeastern South China Sea (SCS). Additional 2D MCS time-domain data (MGL0905-05 and MGL0905-20) were included to better constraint the key stratigraphic correlation from the rifted continental margin. Six key horizons and eleven seismic facies were identified. A new chronostratigraphic column was established to explain the stratigraphic development in the study area. Paleogene fault-bounded half graben systems underlie the break-up unconformity, followed by late Oligocene shallow-marine sediments. Localized buried seamounts and sills developed during the early post-rift in late Oligocene to early Miocene. Deepwater canyons extensively developed in late Miocene, followed by the inception of arc-continent collision. Mass transport deposits (MTDs) and sediment waves deposited during the Pleistocene. In the distal rifted margin, arc-continent collision commenced in late Miocene, promoting the extensive development of fold-and-thrust belts and slope basins. Basal foreland unconformity (BFU) marks the transition from the underlying post-rift sequences to the overlying foreland sequences. As opposed to the rifted-margin domain, modern stratigraphy in the accreted slope basins of Taiwan accretionary wedge suggested isolated, hemipelagic-dominant sediments, and less of channel development. Splay fault cutting through the seafloor and the fault lying at the toe of steep slope with significant slope break, indicating that the splay fault is most likely an active fault. Decollement in the lower accretionary wedge is estimated at depths of 3 – 12 km beneath the seafloor with average angles between 4 – 6 degrees. Bottom simulating reflectors (BSRs) at depth around 300 – 600 m from the seafloor suggests active fluid expulsion controlling the gas-hydrate accumulations in the rifted margin and the offshore Taiwan fold-and-thrust belts. The convergence also promotes the development of mud diapirs and uplifted bedrocks in the Kenting Plateau of the upper accretionary wedge.
Regional seismic interpretation suggests dispersed distribution of BSRs and mud diapir in the upper slope domain, inferring pervasive gas-hydrate systems and associated free-gas distribution, including in the Quaternary Lower Fangliao Basin (LFB), a semi-enclosed slope basin in the upper wedge slope of Taiwan. BSRs are present to cross-cut both inclined stratigraphy and intruding diapirs as hinted from 3D MCS937 and 2D MGL0908-TST seismic data. The BSRs are interpreted to represent the basal phase boundary of the gas hydrate stability zone (GHSZ). 3D seismic attributes suggest that free gas and gas hydrates may be located above BSRs, and free gas below BSRs. Mud diapirs and associated faults may act as pathways along which thermogenic methane from a deep and as-yet unidentified source may migrate up into the GHSZ. First order volumes of free gas and gas-hydrates in place were estimated on the basis of geobody extraction, geophysical approximations, and Monte Carlo simulation and suggest 2,048 Bcf of total gas volume over a study area of 60 km2.
As the depth of BSRs are generally shallowing towards the accretionary wedge, the depth of BSRs in the LFB suggests anomalously deeper BSRs depth compared to other region in the upper wedge slope domain. The low geothermal gradient and heat-flow estimation, 33 oC.km-1 and 41 mW.m-2, respectively, suggest the role of thermal blanketing in controlling the present-day basin temperature. Closely spaced thermal probes and infrared imaging from piston cores revealed average values for geothermal gradients and heat flows of 55 °C.km?1 and 62 mW.m?2, respectively. Discrepancies between both measurements are related to the sensitivity of direct thermal measurements over shallow fluid flux, where shallow geothermal gradients increase locally as the fluid migrates upward. An array of data, including high-resolution seafloor bathymetry, seismic facies interpretation (from 3D MCS937, 2D MW9006-01, and 2D MW9006-02), and distribution of thermal anomaly, reveal that a paleo-Gaoping canyon had flowed through the LFB and deposited a stacked series of turbidite sands. Rapid deposition and sediment burial in offshore southwestern Taiwan had caused insufficient dewatering process in the paleo-Gaoping canyon sediments, leaving high water saturation within pore spaces and overpressured sediments. These, together, lead to lower heat flows and thermal gradients and contribute to deepen the base of GHSZ. Further mud diapiric intrusions and uplifting of seafloors had blocked the course of paleo-Gaoping canyon. The LFB was abandoned following the channel course shifted to the south along the present-day Gaoping Canyon course. |
| 顯示於類別: | [地球系統科學國際研究生博士學位學程] 博碩士論文
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