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    题名: 利用曾文溪沿岸階地及碳14定年法分析臺灣西南部崙後斷層及口宵里斷層之活動特性;Analyzing the Holocene activity of the Lunhou fault and Kouhsiaoli fault in southwestern Taiwan based on the Tsengwen River terraces
    作者: 石智偉;Shih, Jhih-Wei
    贡献者: 應用地質研究所
    关键词: 崙後斷層;口宵里斷層;烏山頭背斜;河階;曾文溪;Lunhou fault;Kouhsiaoli fault;Wushantou anticline;terrace;Tsengwen River
    日期: 2022-08-22
    上传时间: 2022-10-04 12:22:28 (UTC+8)
    出版者: 國立中央大學
    摘要: 崙後斷層與其東側的口宵里斷層位於台灣西南部的台南嘉義一帶,兩者為比鄰的南北走向逆移斷層,前者向東傾,後者向西傾。該區的InSAR影像解析結果顯示跨兩斷層每年約有20毫米的垂直變位量,因此本研究利用流貫本區的曾文溪,其沿岸的階地對比分析,探討崙後斷層及口宵里斷層的構造關係及活動性對此抬升量的可能貢獻。進行的步驟方法有兩項,其一藉由高精度數值模型進行地形判釋,再輔以野外調查及14C定年相互對比繪製河階圖,其二是透過地質資料,繪製出地質剖面,將兩者結合分析與解釋,藉此瞭解兩條斷層之關聯與活動特性。本研究於野外調查時,藉由全站儀和VBS-RTK與無人機(DJI Mavic 2 pro)測量河階階面、基岩頂部的侵蝕面、採樣樣本之海拔高程以及所獲得之14C定年結果及前人定年資料分析將河階階面依河拔高程的高低分成十一階,由老至年輕分別為T0 (15-18ka)、T1a (8 ka)、T1b、T2a、T2b、T3 (3-4 ka)、T4a、T4b (2 ka)、T4c、T4d、T5,由西向東至烏山頭背斜河流階地地表變形逐漸抬升,並且將測量到的高程資料與定年資料進行下切速率的計算,可見研究區最西邊與最東邊之T3階地其下切速率相近,這代表著兩側的T3階地沉積環境相當,且根據河階變形判釋與下切速率的分布,視為未變形區,崙後斷層上盤T4b階地年代為1998 -1878 cal BP,下盤T4b階地年代為1867-1615 cal BP,且階地高程相似,故推斷崙後斷層可能在2千年以來無活動跡象。口宵里斷層上盤新社T3階地(年代為3820-3584 cal BP),與下盤劉陳灣T3階地(年代為4219-3926 cal BP),階地高程差約30公尺,且於劉陳灣露頭可見口宵里斷層切穿上覆河階沉積物,於下盤處河階沉積物內由西向東採集到四個樣本,分別為近口宵里斷層基岩侵蝕面的15256-14854 cal BP和18480-17887 cal BP,以及距口宵里斷層東側約260公尺處上部細粒料內採集到的8976-8595 cal BP與3212-3005 cal BP,造成上述四個樣本皆沉積於同一處河階的原因,可能是下盤因口宵里斷層的活動導致基岩抬升,致使曾文溪流向改變由西向東遷,或是此處沉積相當穩定,河階不斷地沉積、侵蝕於同一處,致使底部較頂層的樣本老。根據定年樣本以及河階的基岩侵蝕面之河拔高,是有利於計算下切速率,計算下切速率後可以觀察到距口宵里斷層西側約6公里的頭社T3河階其下切速率為6.0-8.4 mm/yr與口宵里斷層東側的劉陳灣T3河階其下切速率為3.8-5.3 mm/yr,兩者的下切速率相當,但於兩者之間的新社T3河階下切速率卻是15.0-21.0 mm/yr,下切速率快於前兩者許多,主要原因是新社的T3河階應該是受構造活動影響。根據以上野外調查以及下切速率對比以及定年結果的解釋,都說明口宵里斷層至少於3-4千年內有活動跡象,這也說明著口宵里斷層可能為第一類活動斷層。綜合以上所述得已知曉兩條斷層的活動性,因此本研究欲探討兩斷層之間的關係以及該區的變形貢獻,故將繪製完成之河階階面的座標投影於N75°W的剖面上,並結合Lu et al.的InSAR資料(2020)分析研究區之地表變形與垂直變位量,可發現河階分布呈不對稱的背斜形(antiform)且口宵里斷層西側至少6公里內皆有向上抬升的跡象,為解釋構造對於該區的影響,本研究提出兩種可能的解釋,其一為崙後斷層因淺部傾角過高而鎖住無法繼續活動,形成口宵里背衝斷層向東逆衝;另一為緊鄰崙後斷層西側的烏山頭背斜因曲滑褶皺作用持續地增長,然而背斜於地表下一、兩公里的範圍,兩翼過度緊密而鎖住,致使層間滑動面於背斜西翼的反曲點,順勢往上擴展截切背斜東翼及崙後斷層並切穿地表河階沉積物,形成向東逆衝的口宵里斷層。由本研究的河階剖面,可觀察到口宵里斷層以西,跨崙後斷層及烏山頭背斜,約六公里的範圍,相對口宵里斷層東側的下盤區,有明顯的抬升,呈現背斜形的變位,且最大抬升量約在烏山頭背斜軸跡的位置,故本研究認為口宵里斷層為一逆移斷層,截切烏山頭背斜與崙後斷層,烏山頭背斜的增長及口宵里斷層活動使得口宵里斷層以西的地表不斷抬升,故根據野外調查與河階對比及河流下切速率,證明口宵里斷層為第一類活動斷層,而斷層的活動性,於烏山頭背斜西側新社T3階地的相對抬升速率為13.7-15.7 mm/yr,且新社和劉陳灣的T3之相對抬升速率差異,搭配劉陳灣的口宵里斷層露頭的斷層傾角42°,可推得最大的斷層滑移速率為20.0-23.0 mm/yr。;The Lunhou fault (LHF) dips to the east and Kouhsiaoli fault (KSLF), which is to the east from LHF, dips to the west. These two faults are striking north-south and are located in southwestern Taiwan through Chiayi and Tainan area. InSAR Images show 20 mm/yr of uplift across these two faults. Therefore, this study aims to reveal if LHF and KSLF have any contribution to this vertical component, based on the river terraces along the Tsengwen River. Firstly, we used a 5m DEM and a Lidar DTM for terrain interpretation and terrace mapping. Then, we conducted field investigations to measure the elevation of terrace surface, terrace strath and dated samples using total station, VBS-RTK and UAV, and to collect samples for 14C dating to complete the terrace correlation. Secondly, we draw a geological profile based on geological data. We integrated these two approaches to better analyze and explain the structural relationship and activity of these two faults. Eleven levels of terraces, which are T0 (15-18 ka), T1 (8 ka), T1b, T2a, T2b, T3 (3-4 ka), T4a, T4b (2 ka), T4c, T4d and T5 from oldest to youngest, have been classified according to their elevation difference from the modern river, and radiocarbon dating. In the field, the LHF dip angle is high and the KSLF dip angle is 42°. Combining 14C ages from this study and previous studies, we found that the age of the terrace T4b on the hanging wall of LHF is 1998-1878 cal BP and the age of the terrace T4b on the footwall is 1867-1615 cal BP. Additionally, the strath elevation of T4b on both sides of LHF are similar. This indicates that LHF has been inactive since 2 thousand years. The age of the terrace T3 on the hanging wall of KSLF is 3-4 ka, which is similar to the footwall, but the elevation difference of the strath between them is 30 m. These results suggest that KSLF is active. The activity of the KSLF is further demonstrated by our observation of the Liuchenwan outcrop, where the KSLF cut through the river deposit. There, four 14C samples were collected from west to east in the river terrace sediments on the footwall of the KSLF. Ages of 15256-14854 cal BP and 18480-17887 cal BP were obtained near the strath, and 8976-8595 cal BP and 3212-3005 cal BP were obtained in the upper fine-grain part of the terrace deposits at the east side of fault at about 260 m distance. These four samples are all deposited in the same terrace sequence and we have two explanations for that. First, the fault activity led the bedrock to uplift, which caused the path of the Tsengwen River to migrate towards the east. Second, the riverbed here was quite stable, with limited incision and deposition between 18 ka and 3ka. The terrace and age dataset were also used to calculate river incision rates. For the terrace T3, we observed fairly similar incision rates of 6.0-8.4 mm/yr 6 km west of KSLF and of 3.8-5.3 mm/yr 500 m east of KSLF. But the terrace T3 located 2.2 km west of KSLF shows a faster incision rate of 15.0-21.0 mm/yr that indicates that this is area is affected by tectonic activity. Therefore, our observations indicate that the KSLF has been active since at least 3-4 ka, and we can classify it as a type I active fault. In a next step, we project the coordinates of the river terrace surface on the N75°W section and analyze the study area in combination with existing InSAR data. From our terrace deformation profile the distribution of river terraces exhibits an asymmetrical antiformal geometry, and there are signs of uplift within at least 6 km to the west of the KSLF. This study proposes two possible explanations. First, the LHF is locked because of the high dip angle at its shallow part and hence it cannot continue to move, forming the KSLF back thrust toward the east; Second, Wushantou anticline (WSTA) is adjacent to the west of the LHF. However, the anticline is 1 or 2 km below the surface, and the two limbs are too tight and locked, so that the interlayer sliding is at the flexural point of the anticline western limb. The KSLF expanded and cut through the LHF and terrace deposits. From the terrace section of this study, it can be clearly observed that the KSLF hanging wall uplifts relative to the footwall with a range of about 6 km across WSTA and LHF. To summarize, this study revealed that the KSLF is a reverse fault that cut through the WSTA and the LHF. The WSTA growth and KSLF activity generate surface uplift to the west of KSLF. According to the correlation of river terraces and the incision rate, the relative uplift rate between terrace T3 near the WSTA and terrace T3 east of KSLF is 13.7-15.7 mm/yr. Using the relative uplift rate of terrace T3 across the KSLF, the fault dip angle (42°) of the KSLF outcropping in Liuchenwan, we inferred the maximum fault slip rate to be 20.0-23.0 mm/yr.
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