| 摘要: | 南海北坡為一新生代的張裂性大陸邊緣，在其上發育了一系列的張裂盆地，如台南盆地、珠江口盆地、瓊東南盆地等，其中台南盆地位在南海最東北側的大陸邊緣。本研究利用在南海東北部張裂邊緣的大陸斜坡區及深水區收集的反射震測剖面，搭配台南盆地陸棚區的鑽井資料，對台灣西南海域的南海東北部張裂性大陸邊緣的構造與沉積演育及地熱流進行研究。
地熱流研究共用了9口位在陸棚區的鑽井資料與通過大陸斜坡區的反射震測剖面。在陸棚區使用了Horner plot法對鑽井量測到的井底溫度進行溫度修正，獲得該深度的正確地層溫度，斜坡區則利用海底仿擬反射(BSR)在海床下的深度反推地層溫度。以海床下的深度與溫度的關係求得地溫梯度後，將地溫梯度與熱傳導率相乘即得到熱流值。地溫梯度與熱流值分別在28至128℃/km及40至159 mW/m2之間，其中陸棚區的平均地溫梯度與熱流值為34.5℃/km和62.7 mW/m2，斜坡區則為56.4℃/km與70.9 mW/m2。
構造與沉積演育研究則利用陸棚區的鑽井資料，將重要的地層面或不整合面對比到震測剖面。共對比出四個重要的層面，分別為更新世底部(約1.8百萬年)、上新世底部(約5.3百萬年)、中期中新世最大海漫面(約17百萬年)、以及分離不整合面(新第三紀底部)。震測剖面解釋工作則發現了：(1)分離不整合面下為古第三紀的地塹及沉積物。(2)數群被掩埋的海底火山，另有一座出露海床的早期中新世海底火山。火山熔岩流的反射訊號都位於中期中新世最大海漫面之下。(3)福爾摩沙峽谷上游下方的中新世地層中有大規模的疊瓦狀地層訊號。(4)研究區域東南部有大量的峽谷侵蝕與堆積形貌的反射訊號。(5)福爾摩沙峽谷下游兩側堆積了厚層的沉積物波。最後則進行震測相分析，共辨識出了7種震測相，分別為平行連續反射訊號震測相、波狀震測相、混亂震測相、U形峽谷侵蝕震測相、疊瓦狀震測相、強震幅地層反射組震測相、以及噴出火山活動震測相。
根據地層對比、震測剖面解釋及震測相分析，本研究重建了研究區域的演育模型：(1)在古第三紀同張裂時期，研究區域發育多條正斷層，形成數個地塹-地壘構造。其中在大陸斜坡上、九龍海脊下方，往北傾沒且可能切穿下部地殼的大型半地塹邊界斷層，在此時期已開始發育。(2) 早期中新世南海張裂以後，在現在的深水區發生了區域性的火山活動，這些火山活動一直持續到早期中新世結束前。(3)中期中新世到晚期中新世時期，火山活動結束後，研究區域的構造活動趨緩。位於下部斜坡區的古福爾摩沙海底峽谷系統在晚期中新世時開始發育。(4)上新世時期除了九龍海脊下的大型半地塹邊界斷層可能有活動外，其餘地區幾無構造活動。古福爾摩沙峽谷在研究區域東南部有多期峽谷侵蝕與堆積形貌，推測此區域有大量的砂質濁流沉積物堆積。(5)福爾摩沙峽谷下游的位置在更新世時期，從西北/東南向改道成為今日的東西向樣貌，並往東匯流至澎湖峽谷與馬尼拉海溝。原福爾摩沙峽谷下游流域上方則堆積沉積物波，從沉積物波底部推測其形成在早期更新世至中中期更新世之間。
;A series of Cenozoic rifted basins developed in the northern margin of the South China Sea. Tainan Basin is one of these rifted basins near Taiwan lying in the outer margin. We employ reflection seismic data both in the shelf and deep-water areas and boreholes drilled in the shelf to understand the tectonic and sedimentary development, and heat flows in the northern SCS near Taiwan. Temperature measurements carried out on 9 hydrocarbon exploration boreholes together with Bottom Simulating Reflectors (BSRs) from reflection seismic images are used in this study to derive geothermal gradients and heat flows. The method of Horner plot is applied to obtain true formation temperatures from measured bottom-hole temperatures in the boreholes, which are disturbed by drilling processes. Sub-seafloor depths of BSRs are used to calculate sub-bottom temperatures using theoretical pressure/temperature phase boundary that marks the base of gas hydrate stability zone. Our results show that the geothermal gradients and heat flows in the study area range from 28 to 128 ℃/km and 40 to 159 mW/m2, respectively. There is a marked difference for geothermal gradients and heat flows beneath the shelf and slope regions. It is cooler beneath the shelf with an averaged geothermal gradient of 34.5 ℃/km, and heat flow of 62.7 mW/m2, respectively. The continental slope shows a higher averaged geothermal gradient of 56.4 ℃/km, and heat flow of 70.9 mW/m2, respectively. Low heat flow on the shelf is most likely caused by thicker sediments that have accumulated there compared to the thinner sediment thickness beneath the slope.
Four key stratal surfaces (i.e. base of Pleistocene, base of Pliocene, 17 Ma Maximum Flooding Surface (MFS), and break-up unconformity of 30 Ma) and 7 seismic facies (i.e. continuous and parallel layer seismic facies, wavy seismic facies, chaotic seismic facies, U-shape canyon-cut seismic facies, imbricated layer seismic facies, HARPs seismic facies, and extrusive volcanism seismic facies) are recognized from seismic data with ages constrained by borehole stratigraphy drilled in the shelf. A model for the Cenozoic tectonic and sedimentary development in the rifted northern margin of the South China Sea near Taiwan is established. The occurrence of Paleogene fault-bounded grabens/half-grabens topped by break-up unconformity indicates that these rift basins develop on continental crust, attesting that thinned continental crust underlies the deep-water study area, rather than oceanic crust as reported in some literature. High heat-flow values in the continental slope may also result from this thinned continental crust. Extrusive volcanic bodies, of early Miocene age, are buried by thick deep-water sediments showing features of buried seamounts. Fairly continuous stratal surfaces of base Pliocene and base Pleistocene reveals that faulting and volcanic activities almost ceased to be active since middle Miocene. A series of channel cut-and-fills is observed in the late Miocene, Pliocene, and Pleistocene strata beneath or to the south of the modern Formosa Canyon. We name this channel system as Paleo-Formosa Canyon. Two distinct fields of deep-water sediment waves developed since middle Pleistocene are found lying to the west of modern deformation front/Manila Trench, and to the north and south of the Formosa Canyon, respectively. |
