由鶴岡地區的礫岩特性探討縱谷中部的構造演化

Abstract

位於臺灣東部的縱谷斷層為花東地區重要的活動構造，頻繁的構造活動常使縱谷內的第四紀礫石沉積物產生變形並形成特殊的構造地形特徵。過去研究曾利用縱谷內數塊被抬升變形的第四紀礫岩單位探討縱谷斷層的活動特性。位於縱谷中部的鶴岡地區也有相似的礫岩單位出露，然而它們沉積學相關的基本性質和與活動構造間的關係尚不清楚。為了釐清這些礫岩的沉積物來源及其沉積環境，本研究於野外依岩相對礫岩進行初步分類並檢驗其礫石的岩性組成，再將其與兩側山脈的現生河流沉積物進行比較。後續利用高精度的數值地形資料進行階地面的成因判釋，檢視構造與地形之間的關係。最終統整以上資料，探討鶴岡礫岩的構造演化歷史。 本研究由野外調查成果確認鶴岡礫岩的分佈與基本特性，發現鶴岡礫岩的礫石岩性包含了中央山脈與海岸山脈來源的沉積物，與現今縱谷主流河道內的礫石岩性組成相似。此結果指示鶴岡礫岩之沉積環境過去為縱谷主流的一部分，過去應位於縱谷斷層的下盤區域。然而鶴岡礫岩現在卻位於縱谷斷層的上盤，本研究因此認為應存在一舊縱谷斷層於鶴岡礫岩的東側。此斷層為鶴岡礫岩與海岸山脈地層的岩性邊界，而現今活躍的縱谷斷層是此構造系統下新產生的前緣分支。西側縱谷斷層在發育過程中於大肚滑溪區域形成了斷層擴展褶皺，並造成了原先主流河道面的抬升，在海岸山脈西緣形成南北向的主流階地系統。由階地累計的抬升量與前人提出的構造抬升速率推估此分支構造應在7400年以前即開始發育，海岸山脈側遭錯動的年輕沖積扇面顯示此分支構造至今仍持續活動。本研究提出的構造演化將有助於進一步了解縱谷中部之活動構造與地形演育。

In eastern Taiwan, the east-dipping Longitudinal Valley fault is a major active structure, along which the strata of the Coastal Range thrust onto younger Longitudinal Valley deposits. The activity of this structure also caused deformation and uplift of Quaternary gravels within the valley, leading to the formation of distinctive tectonic landforms. Along the valley, previous studies have utilized several uplifted and deformed Quaternary conglomerate units to identify and characterize active structures and their properties. However, in Hegang area in the central segment of the valley, some similar conglomerate units have been mapped, but their sedimentary environment and their relationship with active structures remain unknown. To understand the source of these uplifted and deformed conglomerates and their sedimentary environment, this study examined the composition of the conglomerate grains and compared it with modern river sediment from different parts of the valley. Combined with a refined mapping of river terraces in the area, we were able to obtain a better understanding of the structural evolution in the central Longitudinal Valley. Based on results of field investigations, this study proposed and defined the distribution and basic characteristics of a new Hegang Conglomerate. The results show that the Hegang Conglomerate is composed of materials from both the Coastal Range and the Central Range, similar to the composition of present-day river deposits found in the trunk rivers flowing in the Longitudinal Valley floor. This indicates that the Hegang Conglomerate was once part of the main trunk river system of the valley, likely located in the footwall block of the Longitudinal Valley fault. However, these conglomerates are currently located at the hanging-wall block of the present-day Longitudinal Valley fault. Therefore, we propose the existence of an old Longitudinal Valley fault on the eastern side of the Hegang Conglomerates, serving as the lithological boundary between the Hegang Conglomerate and the Coastal Range strata. The presently active Longitudinal Valley fault is considered a young frontal branch of this fault system. During the development of this young Longitudinal Valley fault, fault propagation fold formed in the area of Daduhua River, leading to the uplift and the formation of the trunk river terrace systems along the western flank of the Coastal Range. Based on the calculation using the accumulated uplift amount estimated from the terrace surfaces and the previously proposed uplift rate of the fault system, this frontal branch likely began to develop before 7400 ka. Scarps on young alluvial fans on the Coastal Range side indicates ongoing activity of this structure. The proposed structural evolution in this study provides additional insights into the active structures and landform evolution in the central part of the Longitudinal Valley