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| 題名: | 台灣西南部曾文溪一帶跨越西部麓山帶山麓前緣的活動構造及全新世變形速率;Active structures and Holocene deformation rates across the piedmont of the Western Foothills, Tsengwen River, southwestern Taiwan |
| 作者: | 陳長志;Chen, Chang-Chih |
| 貢獻者: | 應用地質研究所 |
| 關鍵詞: | 曾文溪;河階;活動斷層;全新世變形;斷層相關褶皺;TsengWen River;river terrace;active fault;Holocene deformation;fault-related folding |
| 日期: | 2023-05-03 |
| 上傳時間: | 2024-09-19 17:25:34 (UTC+8) |
| 出版者: | 國立中央大學 |
| 摘要: | 在台灣西南部曾文溪一帶，GPS水平速度從烏山頭背斜東側的25毫米/年急劇減少至海岸平原東側約4毫米/年。因此，在造山帶前緣預期應有活動構造存在。然而，以往的研究並未清楚探討構造的幾何形貌和活動情況。為了進行地震危害評估，了解該區域的幾何形貌和變形速率可能非常重要。本研究旨在通過地質剖面、變形全新世沉積物和大地測量來確定地質結構的幾何形貌和全新世變形速率。在本研究中，我使用曾文溪露頭和海岸平原東側已發表的淺層鑽孔資料來計算全新世變形速率，同時建立了地質剖面以確定構造的幾何形貌。
我使用我在曾文溪河階的觀察和相關的碳14定年結果，結合前人在曾文溪沿線進行的研究成果。此外，我還利用公開的淺層鑽孔資料，嘗試比較這兩個數據集。由於兩者沉積環境不同，我分別進行計算。我根據侵蝕速率、沉積速率、下盤的沉降速率來推估河階的抬升速率。因此，我使用大地測量數據和從淺層鑽孔中選擇的等年代線來決定相對穩定的地點進行沉積速率計算，並且我選擇靠近造山帶以西約10公里的總爺淺層鑽孔作為基礎，獲得河階的抬升速率。在海岸平原，我使用淺層鑽孔的碳14定年結果，並考慮海平面變化來獲得抬升速率。最終我結合河階和淺層鑽孔的抬升速率，得出一個與現代大地測量數據趨勢相似的剖面。抬升速率從變形帶西部到中部逐漸增加，在麓山帶持續上升直到烏山頭背斜和口宵里斷層。同時，我使用已發表的地表地質資料、超微化石地層學、深層鑽孔、深層反射震測線以及從淺層鑽孔中選擇的等年代線，在研究區北部和南部建立了剖面。此外，我利用剖面的結果來了解構造在不同緯度上的幾何形貌差異。我認為在麓山帶深度約3到4公里處可能為口宵里斷層向西延伸。此外，口宵里斷層可能是引起麓山帶全新世變形的主要構造並延伸至海岸平原形成了一個背斜。
我比較了全新世變形率和地質剖面的結果並將它們分成三個不同的區域：海岸平原、台南麓山帶和麓山帶前緣進行討論。在海岸平原，根據震測剖面及全新世抬升率的變化，可瞭解到在平原區有著正在活動之背斜之構造，且形成背斜之構造可能由麓山帶延伸至此。在台南麓山帶，我認為全新世變形可能是由口宵里斷層所引起的。在這個前提下，我利用斷層傾角和曾文溪河階的抬升速率，然後基於斷層彎曲褶皺模型估計斷層滑動速率。我得到的斷層滑動速率為26.2-27.9毫米/年。此外，根據現今的大地測量資料所示，變形速率的峰值在北側地區並不是在靠近口宵里斷層處，而是在靠近烏山頭背斜軸，此現象可能是由北方的高傾角地層限制了口宵里斷層的滑動，並將斷層滑動轉換成在烏山頭背斜處的褶皺作用。在麓山帶前緣，我利用了口宵里斷層在麓山帶的滑動速率和我所繪製剖面中的斷層傾角來確定抬升速率約為4.5-4.8毫米/年。因此，根據研究區域的結果，在海岸平原、麓山帶前緣和台南山麓帶的烏山頭褶皺帶的抬升速率變化分別為-4毫米/年、4.5-5.2毫米/年和19.3毫米/年。
現有的研究成果更多支持口宵里斷層向西延伸，且在西南側的研究區延伸至人口較多的沿海平原，並形成一個背斜。此類型正在活動且有著長變形帶的構造對地震危險性評估有著重要的影響。
;In Southwestern Taiwan, at the latitude of the TsengWen River, westward GPS horizontal velocities dramatically decrease from 25 mm/yr east of the WuShanTou anticline to about 4 mm/yr in the eastern coastal plain. Therefore, active structures are expected at the topographic front of the mountain belt. However, geometry and activity of structures are not very well addressed in the previous studies. It might be important to understand the geometry and the deformation rate in this area for seismic hazard assessment. This study aims to determine the geometry of the geological structures and to quantify the Holocene deformation rates based on geological cross-sections, deformed Holocene deposits, and geodesy. In this study, I use TsengWen River terraces and published shallow boreholes data in the eastern coastal plain to calculate the Holocene deformation rates. I also build the geological cross-sections to determine the geometry of the structures.
I use my TsengWen River terraces observation and the dating results and combine them with the previous studies along the TsengWen River. Further, I also take the published shallow boreholes data and try to compare these two data sets. Since the sedimentary environments are different, I do the calculations of the uplift rates separately. I estimate the uplift rates of the terraces based on the incision rates, sedimentation rates, and the footwall subsidence rate. I use geodesy data and the Isochrones selected from the shallow borehole to select a relatively stable location for sedimentation rates calculation. I choose the shallow boreholes in TsungYeh which is west of the deformation zone and around 10km away from the piedmont as the reference to obtain the terraces uplift rates. In the coastal plain, I take the dating results in the shallow boreholes and consider the sea level changes to obtain the uplift rates. Lastly, I combine the uplift rate from terraces and the shallow boreholes to obtain a profile that has a similar trend to the contemporary geodesy data. These uplift rates increase from the west to the middle of the deformation zone and keep rising in the foothills until the WuShanTou anticline and KouhSiaoLi fault. In parallel, I use the published surface geological data, nannostratigraphy, deep boreholes, seismic reflection lines that I did the depth-converted, and Isochrones which I selected from shallow boreholes to build the cross-sections at the north part and the south part of the study area. Further, I used the results of the profiles to understand the geometrical differences of the structures in different latitudes. I propose that at a depth of approximately 3 to 4 km in the foothills the KouhSiaoLi fault may extend westward. Furthermore, the KouhSiaoLi fault may be the main structure causing Holocene deformation in the foothills, and it extends to the coastal plain forming an anticline.
I compare the results from Holocene deformation rates and the geological cross-sections. Then, I separate them into three different areas; the coastal plain, the Tainan foothills, and the piedmont for discussion. In the coastal plain, it is an active anticline in the coastal plain area according to the seismic lines and variation of the Holocene uplift rate. The structure that forms the anticline may extend from the foothills. In the Tainan foothills, I propose that the Holocene deformation may be caused by the KouhSiaoLi Fault. Under this premise, I used the fault dip angles and the uplift rates of the TsengWen River terraces to estimate the fault slip rates based on the fault-bend fold model. The fault slip rates I obtained are 26.2-27.9 mm/yr. In addition, according to the current geodetic data, the peak of deformation rate in the northern region is not near the KouhSiaoLi fault but near the WuShanTou anticline fold axis, which may be due to the high-dip angle formations in the north restricting the slip of the KouhSiaoLi fault and transforming the fault slip into the folding at the WuShanTou anticline. At the piedmont, I determined the uplift rate to be about 4.5-4.8 mm/year based on the slip rate of the KouhSiaoLi fault in the foothills and the fault dip angle in the geological cross-sections I drew. Therefore, according to the results of the study area, the uplift rate changes in the coastal plain, to the piedmont, and to the WuShanTou fold belt in the Tainan foothills are -4 mm/yr, 4.5-5.2 mm/yr, and 19.3 mm/yr, respectively.
The research results support the westward extension of the KouhSiaoLi fault, which extends to the heavily populated coastal plain on the southwest side of the study area and forms an anticline. Such structures that are active and have a long deformation zone have a significant impact on seismic hazard assessment. |
| 顯示於類別: | [應用地質研究所] 博碩士論文
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