臺灣東北部之海階地形及其新構造意義
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2013
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海階為沿海岸發育的階梯狀地形，是經由海水侵蝕作用的海蝕平台受到地殼抬升、海水面的變化或兩者共同作用之下所形成，成為一平緩而略向海洋傾斜的階地。海階主要由階面與階崖所構成，階面代表昔日的海水侵蝕或沈積之平面，階崖則是連結兩個相鄰階面的陡坡，其高度則代表著地殼或海水面的變動量。透過海階地形的研究，將有助於了解區域地體構造的抬升作用及推論與重建海域的海準面變化。臺灣位於歐亞板塊與菲律賓海板塊的碰撞帶，所造成的地體構造活動極為複雜，為環太平洋海階最發達的地區之一。然而由過去的研究顯示，臺灣東北部由於處於碰撞造山的後期，抬升作用已逐漸轉變為沉降作用。在這種情況下，臺灣東北部海岸卻仍然有發達的海階地形，其可能代表之地殼抬升與臺灣北部整體的構造型態似有矛盾。因此本研究針對臺灣東北部沿海的海階地形，利用地形分析的方法，對其型態與演育進行詳細的調查及研究。
本研究之東北部區域包括新北市瑞芳區、貢寮區及宜蘭縣頭城鎮的沿海。首先利用航空照片、1：5000像片基本圖初步描繪東北部海階地形之分布圖、同時利用臺灣四十公尺數值地形進行室內分析，其次至野外針對海階利用即時動態定位(Real Time Kinematic, RTK)-GPS進行地形剖面的量測，最後利用構造地形學之概念進行整體資料的綜合處理與分析，針對階面高度、沉積物厚度、底岩的出露及地形剖面等特徵加以對比，希望可以了解各區域間變動的差異。
臺灣東北部海岸除了廣泛分布的現生海蝕平台之外，可分為低位階與高位階兩群海岸階地。由於本區的高位階地因年代久遠以及侵蝕作用盛行，分布非常零星，不易對比，因此在本研究中不予討論。然而本區最低位的階地位於海蝕平台與高位階末端階崖之間，沿著海岸呈帶狀分布，具有連續性良好且面積廣大等特性。藉由野外觀察顯示，此一低位階地為一侵蝕性階地，底岩主要為臺灣東北部廣泛分布之砂岩及頁岩，上覆厚薄不一之海岸沈積層。經由詳細測量各階地的比高之下，發現由於階面上覆沉積物厚薄不一，而導致區域間階面高度出現差異變化，無法準確代表海階的抬升量，然而若是改以階地底岩面的高度進行比較，則研究區內的海階抬升量整體而言差異並不大；此外，本研究利用龍洞地區的海階碳十四定年資料顯示，推測本研究區海階可能在四至五千年前開始發育，平均上升速率約為0.62－0.75mm/yr，屬中全新世海階地形。因此，本研究經由臺灣東北部的海階地形映證此一地區的海階並非由斷層構造活動形成，此一緩慢的抬升速率是長時間以來岩石圈均衡理論之下的整體抬升作用，並逐漸形成今日分布於沿海的海階地形。
Marine terraces include raised wave-cut platforms and sea cliffs. Therefore, they indicate regional tectonics, paleo-climate and former sea-level changes. They may not be only influenced by eustatic changes, but also by regional tectonic movements. The Taiwan Island is located on the collisional boundary between the Philippine Sea plate and Eurasian plate. The mountain belt in northeastern Taiwan is undergoing extension and the mountain belt is collapsing. But there are many well developed marine terraces that seem to be contradictory with the northern Taiwan tectonics. In this study, we utilized aerial photographic interpretation, geomorphic analysis of 40m-DTM, high-resolution topographic profiles obtained from RTK-GPS easurements and data analysis to characterize the coastal geomorphic features, mincluding the elevation of terrace, thickness of sediments, bedrock outcrops and topographic profiles. Along the coast of northeastern Taiwan, there are two steps of marine terraces. The higher terraces belong to Pleistocene terraces, and their distribution is very scattered and difficult to analyze, thus we will not discuss them in this study. The other step are lower terraces, and their elevations are averaged from 5-10m after measurement of RTK-GPS. They are well developed along the coastal lowland and with better continuity in this area. Based on radiocarbon dating, these lower terraces belong to middle Holocene terraces, and the average uplift rate is about 0.62-0.75mm/yr from about 4-5ka. The result shows that this area has very low uplift rate. The slow uplift pattern is generally consistent with isostatic hypothesis, in which topography on the earth surface is compensated at depth. The crust of northern Taiwan may be gradually rising and forms continuous marine terraces along the coast today, after a large amount of materials are eroded away after a longer history of mountain building.
Marine terraces include raised wave-cut platforms and sea cliffs. Therefore, they indicate regional tectonics, paleo-climate and former sea-level changes. They may not be only influenced by eustatic changes, but also by regional tectonic movements. The Taiwan Island is located on the collisional boundary between the Philippine Sea plate and Eurasian plate. The mountain belt in northeastern Taiwan is undergoing extension and the mountain belt is collapsing. But there are many well developed marine terraces that seem to be contradictory with the northern Taiwan tectonics. In this study, we utilized aerial photographic interpretation, geomorphic analysis of 40m-DTM, high-resolution topographic profiles obtained from RTK-GPS easurements and data analysis to characterize the coastal geomorphic features, mincluding the elevation of terrace, thickness of sediments, bedrock outcrops and topographic profiles. Along the coast of northeastern Taiwan, there are two steps of marine terraces. The higher terraces belong to Pleistocene terraces, and their distribution is very scattered and difficult to analyze, thus we will not discuss them in this study. The other step are lower terraces, and their elevations are averaged from 5-10m after measurement of RTK-GPS. They are well developed along the coastal lowland and with better continuity in this area. Based on radiocarbon dating, these lower terraces belong to middle Holocene terraces, and the average uplift rate is about 0.62-0.75mm/yr from about 4-5ka. The result shows that this area has very low uplift rate. The slow uplift pattern is generally consistent with isostatic hypothesis, in which topography on the earth surface is compensated at depth. The crust of northern Taiwan may be gradually rising and forms continuous marine terraces along the coast today, after a large amount of materials are eroded away after a longer history of mountain building.
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臺灣東北部, 海階, 新構造運動, 構造地形學, 即時動態差分GPS, Northeastern coast of Taiwan, marine terraces, neotectonics, tectonic geomorphology, RTK-GPS