構造與氣候作用對於臺灣西南部泥岩惡地演育的影響

Abstract

本論文整合遙測工具－雷達與光學衛星影像、近景測量技術與颱風期間的懸浮質與河水化學等資料，從集水區到邊坡尺度討論臺灣西南部泥岩惡地地形對於區域構造抬升、地震與不同強度降雨型態的地形反應。根據研究結果，分為以下三部分說明: 首先，構造抬升是影響地形演育的營力之一。本研究透過合成孔徑雷達(synthetic aperture radar, SAR) 影像與河階的碳14定年資料，重建研究區的構造抬升歷史。並輔以光達 (LiDAR) 產製之數值高程模型，討論龍船斷層活動與惡地地形演育的關係。結果顯示，當地表抬升速度高於7毫米/年時，惡地分布與構造抬升速率有空間相關。此外，山脊密度 (hilltop density)、山脊曲率 (hilltop curvature) 與坡度在海拔50-60公尺間均達到最大值，根據河階定年結果，海拔50公尺的河階對應研究區內最大構造抬升量的時期，推測此區域的惡地成因可能與2千年間快速構造抬升導致的河流下切作用有關。 第二，岩石風化侵蝕作用是影響地球化學循環的重要環節，矽酸鹽風化速率更是調節地質時間尺度中大氣二氧化碳濃度的關鍵。分析2017年尼莎-海棠颱風期間南雄橋集水區的河水化學與懸浮質濃度之時間序列資料。結果顯示，矽酸鹽風化速率為18 噸/平方公里/日，高於全球年平均值 24噸/平方公里/年。由於鈉吸著比（sodium adsorption ratio）與河水流量、懸浮質濃度共變，推測因風化泥岩內，鈉離子析出造成的土壤團粒崩解，增強物理侵蝕量，造成懸浮質濃度提高。 第三，本研究透過無人飛行載具產製30公分解析度的數值高程模型，得到2016-2018年間共計4個時期的地表變動量。結果顯示，美濃地震造成惡地坡度的中位數增加1度、颱風事件造成惡地坡度的中位數減少4度。另外，邊坡計測指標 (morphometric slope index)與侵蝕量(速率)、坡度有高度正相關，與水系長度具有負相關，反應研究區內惡地之侵蝕型態，屬於重力侵蝕而非單純水力侵蝕。

This dissertation combines data compilation of satellite-based remote sensing, close range photography and high temporal resolution of suspended sediment and riverine chemistry records to address how tectonic and climatic drivers impact on the mudstone badland landscape evolution from catchment to hillslope scale in southwestern Taiwan. First, tectonic uplift is the element of landscape evolution. The dissertation constrains the history of tectonic uplift by combining synthetic aperture radar method and 14C dating of river terrace. Plus, the LiDAR - driven digital elevation model can help us to understand the correlation of Long-Chuan fault and badland landscape evolution. I find that badlands occur within the most rapidly uplifting areas. Surface velocity positively correlates with the fraction of badland area within the landscape, once uplift rates exceed 7 mm/year. In the time dimension, higher hilltop density, higher hilltop curvature and higher gradient are maximum at 50-60 m of altitude, which coincides with rapidly uplifted history and therefore I infer that badlands are caused by meso-scale rapid incision of the river channel at a time scale of 2 k yBP. Second, rock weathering and erosion is the key to geochemistry of Earth, and the silicate weathering is the key to regulating compress of atmospheric carbon dioxide in geological time. I find that water chemistry is dominated by silicate weathering at 18 t/km2/day during the 2017 typhoon period, which is higher than the global mean weathering rate at 24 t/km2/year. Sodium adsorption ratio is covariant with river-discharge and sediment concentration, which can be assigned to sodium-induced dissolution, which would increase physical erosion. Further, sodium of suspended sediment account for about 10.6 % of mass loss in this even, and the current-induced dissipation may be responsible for it. Third, the dissertation conducted unmanned aerial vehicle - driven DEM of four survey period from 2016 to 2018. I find evidence that 2016 Mino earthquake steepen 1 degree median gradient of hillslope, typhoons decrease 4 degree median gradient of hillslope and low-intensity precipitation can either steepen or flatten a landscape. The pattern of change of hillslope gradient distributions observed in badlands was mirrored in the response of the Taiwan mountain topography to typhoon Morakot in 2009, confirming that badlands offer special opportunities to quantify common, natural landscape dynamics on observational time scales. Last, the morphometric slope index (MSI) has a strong positive correlation with erosion and its rate but shows a negative correlation with drainage length and a positive correlation with inclination. This suggests that the erosion pattern is due to gravitational mass wasting instead of hydrological erosion.