利用解壓密方法探討末次最大冰期以來臺灣西南部褶皺逆衝斷層帶前緣與前陸盆地的構造特性

The tectonic rates of the frontal fold-and-thrust belts and foreland basins in the southwestern Taiwan since the Last Glacial Maximum by using the decompaction method

鄭紹安(Shao-An Cheng)

指導教授 ： 陳文山

國立臺灣大學/理學院/地質科學研究所/碩士(2020年)

https://doi.org/10.6342/NTU202000553

摘要

臺灣西南部外麓山帶至平原區分別位於褶皺－逆衝斷層帶前緣與前陸盆地，此區域發育了一系列約南北走向的逆斷層系統，並被一些約東西走向的平移斷層所截切。末次最大冰期以來海平面上升，此區域覆蓋了晚更新世至全新世的沉積層。本研究即透過解析該地層（資料來源主要為中央調地所的各項研究計畫中總計210個鑽井、槽溝剖面與野外露頭），計算垂直方向的地殼變動速率，進一步分析斷層的活動特性。
　　以往計算垂直地殼變動速率時，經常忽略地層因為自身荷重所產生的壓密沉陷作用。當計算過程沒有扣除壓密因素時，地殼垂直變動量將包含額外的壓密沉降量，因此本研究期望計算出此壓密量並予以排除。壓密量計算包含兩大過程分別是「地層狀態分析」與「地層厚度還原」。地層狀態分析中，會將地層由頂面至基盤切分為數層極小單位厚度的次層（sublayer），由上至下逐層計算各次層的應力狀態、孔隙比與固體含量。地層厚度還原部分，則是將特定深度以上的地層全數移除，重建該深度的沉積層在堆積當時的厚度。透過固體體積守恆的概念，還原地層在當時的應力狀態，並且重新分配地層中固體與孔隙之間的比例，計算當時的地層厚度再減去現今的厚度即為壓密量。
　　算出各個點位的垂直地殼變動速率後，再進行探討構造的活動速率。本研究區域被數條平移斷層與逆斷層分成四個褶皺－逆衝斷層系統，由北至南分別為(I) 嘉義地區：北界為梅山斷層，南界為嘉義斷層與新營斷層之間的構造轉換帶。斷層包括嘉義斷層與梅山斷層；(II) 北臺南地區：北界接續著上述構造轉換帶，南界為新化斷層。斷層包括六甲斷層、木屐寮斷層、新營斷層與該構造轉換帶；(III) 臺南－高雄地區：北界為新化斷層，東南界為車瓜林斷層。斷層包括臺南斷層、後甲里斷層、中洲斷層、小崗山斷層與新化斷層；(IV) 高雄－屏東地區：西北界為車瓜林斷層，東界為潮洲斷層。斷層包括旗山斷層、潮洲斷層與車瓜林斷層。本研究將針對上述區域內的斷層進行探討，包括其垂直錯移速率、斷層滑移速率與構造形貌等。
　　從本研究計算的垂直變動速率來看，這些斷層大致可分成兩種活動模式。第一類為三角剪切模式，此模式的特徵為上盤與下盤之間存在一廣大的變形帶，速率從下盤、變形帶至上盤區域的變化趨勢為：下盤呈現極緩慢且穩定的變動，進入變形帶內速率開始呈現逐漸增加的趨勢，上盤區域速率趨於緩慢且穩定的變動。計算此類斷層的活動速率可透過上下盤速率差而獲得，或透過三角剪切模式進行擬合；第二類為斷層尖端破裂近地表的斷層，其特徵為變形帶狹小，變動速率從下盤、變形帶至上盤區域的變化趨勢為：下盤速率呈現緩慢且穩定的變動，變形帶內的速率變化大且呈現不規則的跳動，上盤區域呈現緩慢且穩定的變動。計算此類斷層的活動速率可透過上下盤速率的差而獲得。
　　本研究總結出各斷層的活動速率如下：(1) 梅山斷層：具壓縮性的右移斷層，東側的錯移速率最高，向西逐漸遞減。好收一帶的垂直錯移速率約2.6 mm/yr，至水林－竹圍減至1.2 mm/yr。(2) 嘉義斷層：屬於第一類，透過擬合得到的滑移速率為23 mm/yr。(3) 木屐寮斷層：屬於第二類，垂直錯移速率約6.4±1.4 mm/yr，滑移速率約6.4±1.4 mm/yr。(4) 六甲斷層：屬於第二類，垂直錯移速率約5.0±1.3 mm/yr，滑移速率約10.0±2.6 mm/yr。(5) 新營斷層：屬於第一類，垂直錯移速率在北側約5.4±1.2 mm/yr，南側約6.6±0.8 mm/yr。(6) 臺南斷層：屬於第一類，透過擬合得到的滑移速率約22.2 mm/yr。(7) 後甲里斷層：屬於第二類，垂直錯移速率約3.6±1.6 mm/yr，滑移速率約4.0±1.8 mm/yr。(8) 中洲斷層：屬於第二類，垂直錯移速率約3.9±0.9 mm/yr，滑移速率約4.5±1.0 mm/yr。 (9) 小崗山斷層：屬於第二類，垂直錯移速率約4.5±1.3 mm/yr，滑移速率約6.4±1.8 mm/yr。(10) 新化斷層：右移斷層，垂直方向的分量不明顯。(11) 旗山斷層：屬於第二類，垂直錯移速率在大社約1.7±1.5 mm/yr，仁武約2.2±1.8 mm/yr。滑移速率在大社約2.2±2.0 mm/yr，仁武約2.9±2.6 mm/yr。(12) 潮州斷層：屬於第二類，垂直錯移速率2.6±1.2 mm/yr，滑移速率約2.8±1.3 mm/yr。(13) 車瓜林斷層：本研究區段垂直錯移量不大。

關鍵字

褶皺－逆衝斷層帶 ； 垂直變動速率 ； 活動斷層 ； 壓密

並列摘要

In this study we utilized 210 boreholes, trenches and outcrops to analyze the tectonic rates of the frontal fold-and-thrust belts and foreland basins in the southwestern Taiwan (south of Meishan fault), through the method of decompaction. Because the process of compaction would cause subsidence, we tried to calculate the amount of compaction to eliminate this influence. Through the restoration of compaction, we are able to estimate tectonic rate more accurately.
The method of decompaction includes two processes, analysis of stratum and reconstruction. First we divide the stratum into several thin layers which we call sublayer and analyze each sublayer individually. By the relationship between effective stress and void ratio, we are able to obtain the void ratio and solid content of each sublayer. Next, following the rule of conservation of mass, the solid content of each sublayer won’t change after compaction. Therefore, we can redistribute the ratio of solid and void among each sublayer, obtaining the previous thickness before compaction.
In general, the thrust faults discussed in this study can be classified into two types by its activity characteristics. Type 1 have a wide deformation zone, and the tectonic rate in the zone change gradually following the trishear model. In contrast, Type 2 have a narrow deformation zone and the tectonic rates change irregularly.
The following are the slip rates and the activity characteristics of the faults: (1) Meishan Fault: Belonging to right-lateral strike-slip, the vertical slip rate in the west is higher than east. (2) Chiayi Fault: Belonging to type 1, the slip rate is 23.0 mm/yr. (3) Muchiliao Fault: Belonging to type 2, the slip rate is 6.4±1.4 mm/yr. (4) Liuchia Fault: Belonging to type 2, the slip rate is 10.0±2.6 mm/yr. (5) Hsinying Fault: Belonging to type 1, the slip rate is 6.6±0.8 mm/yr. (6) Tainan Fault: Belonging to type 1, the slip rate is 22.2 mm/yr. (7) Houchiali Fault: Belonging to type 2, the slip rate is 4.0±1.8 mm/yr. (8) Chungchou Fault: Belonging to type 2, the slip rate is 4.5±1.0 mm/yr. (9) Hsiaokangshan Fault: Belonging to type 2, the slip rate is 6.4±1.8 mm/yr. (10) Hsinhua Fault: Belonging to right-lateral strike-slip, the slip rate in vertical direction is not distinct. (11) Chishan Fault: Belonging to type 2, the slip rate is 2.2±2.0 mm/yr in Dashe and 2.9±2.6 mm/yr in Renwu. (12) Chaochou Fault: Belonging to type 2, the slip rate is 2.8±1.3 mm/yr. (13) Chegualin Fault: The slip rate in vertical direction is not distinct in our study area.

並列關鍵字

fault-and-thrust belt ； tectonic rate ； active fault ； compaction

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