| 摘要: | 台灣西南外海為一有快速沉積、侵蝕與構造抬升作用的造山帶前緣，地質與地球物理資料顯示了區域中有大量的天然氣水合物賦存，天然氣水合物和沉積、侵蝕與構造抬升作用在這區域形成了動態平衡的狀態。因此，本研究選擇一處可能富含天然氣水合物的區域，利用MCS735震測剖面、OR1-759與OR1-769變頻聲納剖面，並比對海底地形，研究該區域近代的沉積與構造活動，進一步研究海底仿擬反射的深度與分布，解析此動態平衡。 震測剖面解析區域構造，發現在區域東側有出露逆斷層分布，為後段下部斜坡；西側則都為背斜，為前段下部斜坡區。變頻聲納提供了高解析度的海床淺層資料，最深可達50米，從變頻聲納剖面辨識出了數種迴聲型態，各自對應了不同的沉積或侵蝕狀態。將深層的震測剖面、淺層的變頻聲納資料及海床的數位地型對比，發現在研究區域的前段下部斜坡處，除了澎湖海底峽谷支流附近區域外，靠近壽山峽谷北側的區域，近代沉積速率比構造抬升速率高；西北側較遠離壽山峽谷的區域則構造抬升較沉積快速。後段下部斜坡則構造抬升速率較沉積速率快。 海底仿擬反射(BSR)的分布和構造分布有關。在前段下部斜坡區分布在海脊附近，後段下部斜坡區則廣泛地分部在區域中。將BSR走時轉換為深度，並計算熱流值，發現熱流值的分布可以壽山峽谷分界，北側較南側低。推測南側因後段下部斜坡的出露逆斷層，以及前斷下部斜坡部分區域未出露到海床表面的逆斷層發育，海床下深處的熱流體可從被傾斜的地層往較淺處流通，造成較高的熱流值；北側則因無逆斷層分布，熱流體較難往淺部流通，且部分區域因沉積速率較高，堆積了厚層的沉積物，更使得熱難以往海床傳，造成較低的熱流值。 The frontal orogenic wedge off southwest Taiwan is characterized by spatially varied rapid sedimentation, erosion, and tectonic uplift. A large amount of geophysical data indicates the prevalent existence of gashydrates beneath the seafloor. The interactions between the processes of sedimentation, erosion, and tectonic uplift may result in a dynamic equilibrium system in which gas hydrates are preserved in the strata. In order to understand this system, we use chirp sonar and reflection seismic data to interpret the processes of sedimentation, tectonic activities, and bottom simulating reflectors distributions off southwest Taiwan. Seismic data reveals that the frontal orogenic wedge is dominated by a series of folds cored by blind thrusts in the west and an array of emergent thrusts in the east with the first emergent thrust, separating these two structural domains. Chirp sonar data provide high-resolution stratigraphic configurations (i.e., echo types) of up to ~50 m beneath the seafloor. Through a joint analysis of seafloor topography and echo types of chirp sonar data, we recognized six echo types in the study area. They are distinct type, indistinct type, steep-slope type, slide-slump type, ridge type, and hyperbola type, each corresponding to various sedimentary and erosional processes. These echo types indicate that major erosional processes occur in the drainage of the Penghu canyon as well as at the hangingwalls of emergent thrusts, whereas deposition takes place in rest of the areas. There is a correlation between subsurface structures, topography and the areal distribution of BSRs in the study area. High concentration of BSRs is found along the bathymetric ridges formed by folding and thrusting. BSRs are more prevalent in the east domain dominated by emergent thrusts, while the west domain with folds cored by blind thrusts only found scarce BSR distribution especially along bathymetric ridges. We have computed the magnitude of heatflows using subsea depths of BSRs and invoking theoretical pressure and temperature conditions for the existence of gas hydrates. Our analysis indicate that higher heatflow of ~60 mW/m2 exists in the east domain. By contrast, lower heatflow of ~40 mW/m2 is found in the west domain. There is an area in the west domain characterized by anomalously low heatflow of ~30 mW/m2, which we interpret to result from the sediment blanking effect due to rapid sedimentation. Our BSR-derived heatflow pattern also correlated well with published direct heatflow measurements using heat probes. Our analysis shows that the existence of gas hydrates in the orogenic wedge is in a state of dynamic equilibrium between sedimentation/erosion and tectonic uplift. |
