海底仿擬反射估算熱流-以宜蘭海脊南北嘴為例

Abstract

南沖繩海槽是位於臺灣東北方海域的弧後張裂盆地，過去認為南沖繩海槽在構造上有延續至宜蘭平原，從斷層構造走向以及熱流分布特徵顯示雖然宜蘭近岸的龜山島周遭有高熱流分布，但在南沖繩海槽與宜蘭近岸間的宜蘭海脊區域地熱流資料相對有限，且在此區域的構造特徵也與近岸的宜蘭平原及更外海的南沖繩海槽略有差異，故南沖繩海槽的熱構造上是否延續至宜蘭平原之議題，仍有進一步探討的空間。本研究利用多頻道反射震測剖面及利氏探針所量取的地熱流資料，震測剖面顯示宜蘭海脊的南、北嘴區域海床下的震測反射訊號普遍具有海底仿擬反射現象，出現的深度分別約在海床下250公尺和100公尺處，而且集中在地形高區，之後再以海底仿擬反射之分佈，推導南沖繩海槽西緣宜蘭海脊的南、北嘴區域的熱流特徵，熱流值推導結果顯示宜蘭海脊南嘴約為28-67 mW/m2，而宜蘭海脊北嘴約為73-159 mW/m2，宜蘭海脊北嘴熱流值較高的原因推測是受斷層構造作用影響，深部高溫的流體沿斷層移棲到海床淺部；反觀宜蘭海脊南嘴少有斷層構造，因此，流體移棲的作用較不發達，熱流值較低，再以利氏探針量測的熱流值比較海底仿擬反射推導的熱流值結果，兩者變化趨勢大致符合。比較兩種地熱流特徵的研究方法，雖然利氏探針可測得海床現地的熱流值，但當海床底的質較硬或沈積物砂質含量較高時，便不利於其地熱流量測工作之進行施做，因此利用海底仿擬反射來推導熱流，可補足利氏探針不易施做的問題，透過兩者調查方法的相互搭配及整合，不僅可相互驗證地熱流的量測結果，也可在調查區中獲得更大範圍的地熱流特徵分佈資訊。而從震測資料中的斷層構造與宜蘭海脊的地熱流分佈特徵分析結果，本研究認為在宜蘭海脊南、北嘴區域構造上受造山後垮塌影響，熱流資料上則顯示尚未出現如南沖繩海槽的火成活動熱流值，本研究推導宜蘭海脊的南、北嘴的熱流值顯示此區尚未有成熟弧後張裂的特徵，推測本研究區域可能仍處於弧後張裂早期大陸裂谷階段。

The South Okinawa Trough (SOT) is a back-arc spreading basin located in the northeastern sea area of Taiwan. In the past, it was believed that the structural continuation of the South Okinawa Trough extended to the Yilan Plain. However, based on the fault structure trends and heat flow distribution characteristics, high heat flow is observed around Turtle Island near Yilan, there is relatively limited heat flow data between the SOT and the coastal area of Yilan. Additionally, the structural features in this area slightly differ from those of the nearby Yilan Plain and the offshore SOT. Therefore, further investigation is required to determine whether the thermal structure of the SOT extends to the Yilan Plain.This study utilized multi-channel seismic profiles and heat flow data obtained from Lister type probes. The seismic profiles revealed that the seismic reflection signals beneath the seafloor in the North Yilan Ridge Spur (NYRS) and South Yilan Ridge Spur (SYRS) generally exhibit seafloor bottom simulating reflectors (BSRs). The depths of these BSRs are approximately 250 meters and 100 meters below the seafloor, respectively, and are concentrated in high relief areas. By examining the distribution of BSRs, the thermal characteristics of the NYRS and SYRS, which is located at the western edge of the SOT, were inferred. The derived heat flow values indicate that the SYRS is approximately 20-65 mW/m², while the NYRS is approximately 70-150 mW/m². The higher heat flow values in the NYRS are attributed to the influence of fault structures, where high-temperature fluids migrate along the faults and settle in the shallow layer. Conversely, the SYRS has fewer fault structures, resulting in less developed fluid migration and lower heat flow values. Comparing the heat flow values measured by Lister type probes with those derived from BSRs, the two methods generally exhibit similar trends. While Lister type probes can measure in situ heat flow values on the seafloor, the measurement becomes challenging when the seafloor substrate is harder or contains a higher proportion of sandy sediment. Therefore, using BSRs to derive heat flow can complement the limitations of Lister type probes. Through the combination and integration of these two investigation methods, not only can the measurement results of heat flow be mutually verified, but also a larger range of heat flow distribution information can be obtained in the study area. Based on the analysis of fault structures in the seismic data and the distribution characteristics of heat flow in the Yilan Ridge, this study suggests that the structural features in the SYRS and NYRS have been influenced by post-orogenic collapse. However, the heat flow data indicates that the volcanic activity-related heat flow values observed in the South Okinawa Trough have not yet appeared in the Yilan Ridge. The inferred heat flow values of the SYRS and NYRS in this study indicate that this area does not exhibit mature characteristics of back-arc spreading, suggesting that the research area may still be in the early stage of a continental rift in the back-arc spreading phase.