晚更新世以來花東海岸南段的階地演化

Abstract

縱谷斷層上盤的海岸山脈受板塊擠壓作用下，於東側海岸產生快速的抬升，因而形成一系列的階地地形。但是，同時期各地區形成的海階階數、階面寬度及階崖高度都不相同，除了受控於全球海水面變化之外，亦受區域的構造活動、岩性等因素影響，使各地區的海階產狀具有差異。本研究除了彙整前人定年資料之外，亦重新採樣及調查階地的沉積層序與產狀，透過測量海階底岩面的高度重新建立剖面與估算花東海岸的地殼抬升速率，藉由相對海水面變化探討海階的成因與演化。 本研究彙整前人海水面資料重建兩萬年以來近臺灣地區的海水面變化曲線，16-7 ka絕對海水面快速上升，速率約13 mm/yr，~7 ka海水面趨於穩定，研究區域內各區域抬升速率皆不同，三仙台至都蘭地區抬升速率約3-7 mm/yr，富山及富岡地區速率約9-13 mm/yr，因此在花東海岸大部分區域至少經歷一次相對海進與海退。海階產狀與相對海水面變化息息相關，都蘭鼻海階沉積層序顯示，~17-16 ka相對海水面變化速率較慢，此時波浪開始侵蝕底岩八里灣層形成海蝕平台，~16-8 ka相對海水面上升近50公尺，海浪不斷地向陸地侵蝕而形成寬廣的海階面，並在侵蝕面上方堆積厚層海相沉積物，此階段後期大量沉積物朝海側堆積，於海相沉積物上方堆積厚層沖積扇沉積物，~8 ka相對海水面開始下降，~8-7 ka下降速率較緩慢，形成的海階面相對較寬，而~7 ka以後相對海水面快速下降，階面停留於海水面的時間較短，所以形成的海階面寬度較窄，且因沉積空間較小而堆積厚度較薄的海灘沉積層，中期全新世以後形成的海階多呈現此樣貌，與晚期更新世形成的海階具有很大的差異。 雖然海階面寬度與相對海水面變化有極大的關係，但在研究區域內海退時形成的海階卻顯示兩者無明確的相關性，而造成上述現象應為海岸線後退侵蝕速率的差異所導致。研究區域內觀察到的海階崖多為潛移抬升加上一次以上同震抬升所累積的高度差，而因海岸線後退侵蝕速率差異，使其累積不同的抬升量，造成各區域海階崖高度呈現很大的不同。另外，依據延續性良好的古海岸線角，其形成時間與其他研究縱谷斷層地震曾發生的時間點相近，顯示這些階崖與同震抬升事件相關。海階產狀受控於岩性、構造活動特性、抬升速率及海岸線後退速率等因素，且通常是多種因素共同作用，造成花東海岸各區域的海階呈現不同樣貌。

In Taiwan, the Philippine Sea Plate collided and overridden on the Eurasian Plate along the Longitudinal Valley Fault (LVF). The southern segment of LVF consists of three faults─Chihshang fault, Lichi fault and Luyeh fault, which behave as creeping thrust fault. On the hanging wall of these active faults, the well-preserved and extensive emergent marine terraces and coral reefs along the Huatung coast are the result of sea level fluctuations superimposed on tectonic uplift since the postglacial period. In this study, we collected previous research of sea level oscillations around Taiwan and reconstructed sea level curve during the past 20,000 yrs. Between 16 and 7 ka, sea level rose from ca. -110 m to ca. 0 m at a rate of 13 mm/yr, and then became stable after 7 ka. At the same time, the average tectonic uplift rate remained quite stable in the study area, ranging from 3-7 mm/yr to 9-13 mm/yr in Sanxiantai-Dulan and Fushan-Fukang, respectively. The interaction between sea level fluctuation and tectonic uplift, also known as relative sea level change (RSL), resulted in the environment change from a relative transgression to a relative regression, and was recorded completely in the depositional sequence in Dulan area. (1) ~17-16 ka: The rate of eustatic sea level rise was slightly lower than the crustal uplift rate. The bedrock of Paliwan Formation formed a wave-cut platform. (2) ~16-8 ka: RSL rose about 50 m, the nearshore to offshore deposits uncomformably overlaid on the wave-cut platform. As RSL rose, the platform extended landward and sedimentary accommodation space increased which resulted in the deposition of thick-bedded marine deposits. (3) ~8-7 ka: The similar rate of tectonic uplift and eustatic sea level change caused the marine terrace formed in this period broad. (4) After ~7 ka: The crustal uplift rate surpassed eustatic sea-level rise rate which led to form a series of Holocene marine terraces. Due to fast and active crustal uplift, time for terrace formation is much shorter and accommodation space is much smaller. Holocene marine terraces display a characteristic of narrow terrace surface and thin beach-face deposits. Although RSL plays an important role in the evolution of terrace formation, the relationship between Holocene marine terraces in Huatung coast and RSL seems not related. The width of marine terraces may be narrower when RSL falls faster, but there is not so obvious in the study area because of coastline recession which is affected by lithology and amount of sediment supply, thus the rate of retreating coastline changes with regions. Coastline recession of a subsequent sea cliff into the landmass might narrow the width of higher marine terraces even remove it. Lower terraces are uplift by coseismic deformation and interseismic creeping, and multiple steps of small terrace could be merged together because of retreating coastline, displaying as higher sea cliff. The characteristic of deformation deduced from marine terraces denote that heights of sea cliff in Huatung coast are accumulated from several coseismic uplift and interseismic creeping uplift. In conclusion, occurrence of marine terrace is controlled by several factors, such as lithological character, oscillation of global sea level, crustal uplift rate and coastline retreat. These factors altogether formed various Holocene marine terraces.