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標題: | 臺灣中央山脈北段核飛跡年代的熱構造意義 Thermo-tectonic implications of fission-track ages from the northern part of the Central Range, Taiwan |
作者: | Tzu-Tsen Shen 沈姿岑 |
指導教授: | 劉聰桂 |
共同指導教授: | 鍾孫霖 |
關鍵字: | 核飛跡定年,剝蝕速率,地溫梯度,東北台灣,琉球弧-溝系統, fission-track dating,exhumation,geothermal gradient,NE Taiwan,Ryukyu Arc-Trench system, |
出版年 : | 2019 |
學位: | 博士 |
摘要: | 台灣島是歐亞板塊與北呂宋島弧斜向碰撞擠壓形成,在台灣東部外海,菲律賓海板塊沿琉球海溝向北隱沒活動,使台灣東北部的大地應力環境從原本的弧-陸碰撞壓縮作用,轉變成琉球弧-溝系統的隱沒和伸張作用。為了釐清琉球弧-溝系統開始影響台灣東北部的時間,以及後期的隱沒和伸張作用對山脈剝蝕與地溫結構的影響,本研究在中央山脈北段區域分別採集花蓮和平的大濁水花岡片麻岩,以及宜蘭清水的廬山層和四稜砂岩樣品,分析磷灰石和鋯石核飛跡年代,利用熱定年學完整探討大地架構改變對台灣東北部山脈剝蝕以及熱構造的影響。
在3.0到1.5 Ma期間可能是琉球弧-溝系統開始影響台灣東北部的時間,此時在和平地區的地殼剝蝕速率從~4 mm/yr下降到2.3-1.6 mm/yr,推測是因為地殼失去初始的弧-陸碰撞擠壓支撐,隨後在琉球隱沒帶上方的伸張作用影響下,造成山脈的剝蝕速率逐漸變慢。而在~0.2 Ma之後,和平地區發生空間上剝蝕作用的不一致性,即在溪谷區域仍維持穩定剝蝕狀態,但是在高海拔山區的剝蝕速率增快到5.2 mm/yr,增快的速率可能因正斷層發育故在下盤 (山區) 產生快的剝蝕作用,或是因迎風面潮濕氣候,導致地表加速侵蝕造成。 此外在1.5-0.2 Ma,大濁水花岡片麻岩體的剝蝕速率維持不變,但是冷卻速率卻逐漸增快,表示地下溫度結構會隨時間改變,從鋯石和磷灰石的核飛跡封存深度結果,可分別估算在1.5 Ma時的地溫梯度為~80 °C/km,後期~0.7 Ma會增高到~110 °C/km。和平地區有罕見的高地溫梯度,造成研究的片麻岩體下方產生無震帶,亦很可能反應在短時間內,東北台灣周圍區域發生複雜的大地架構改變。 在清水地區的變質碎屑岩樣品,鋯石核飛跡皆受到部分癒合作用,其封存溫度範圍推估在245-185°C之間。從廬山層的總顆粒樣品可解析出最年輕的峰值為~4.1 Ma,視作為清水地區的冷卻年代,進一步估算冷卻速率為56-41 °C/m.y.。根據台灣島上各地質構造區的冷卻速率與地熱流線性函數關係,推估清水地區的地溫梯度為42-39 °C/km,推測是廬山層北段經歷區域變質作用的埋藏溫度。而從今日清水地區的高地熱徵兆,以及雪山山脈北段後期增快的冷卻速率,指示在北台灣的後碰撞時期,地殼拉伸所引發的岩漿活動,亦影響到台灣東北部造成區域性地溫梯度的增高。 The Taiwan mountain belt is built up by the oblique collision between the northern Luzon Arc of the Philippine Sea Plate (PSP) and the Eurasian Plate. On the offshore of NE Taiwan, the PSP subducted northwards along the Ryukyu Trench and its western edge is beneath the NE Taiwan. The stress field of NE Taiwan is transiting from the compression of the arc-continent collision into the extension induced from the Ryukyu Arc-Trench system. However, the link between the Ryukyu Arc-Trench system and the exhumed NE Taiwan mountain belt has not been well constrained by the thermochronolgic data so far. In order to clarify the timing and influence superimposed by the Ryukyu Arc-Trench system on the exhumation and geothermal state of NE Taiwan, rock samples were collected from both the Tananao Complex (TC) and the Lushan Slate (LS) of the northern Central Range to extract the apatite and zircon fission-track ages. The extension of the Ryukyu Arc-Trench system started to influence the exhumed NE Taiwan in between 3.0 and 1.5 Ma, resulting in the declined exhumation rate of the northern TC from ca. 4 mm/yr to 2.3-1.6 mm/yr. After ~0.2 Ma, the exhumation accelerated to ca. 5 mm/yr at the high elevation but remained steady at the valley bottom. The decoupling of exhumation resulted from either the relatively intense exhumation in the foot-wall due to the development of the normal fault or the rapid erosion at the high elevations due to the humid weather. In addition, the cooling rate of the northern TC has exponentially accelerated from ~190 to 480 °C/my while the exhumation rate has maintained constant for the last 1.5 Ma. Therefore, the geothermal gradient was estimated to increase with time: from ~80 °C/km in 1.5 Ma to ~110 °C/km in the last 0.7 Ma. Besides, the exceptionally high geothermal gradient in the Hoping area is probably associated with the rapidly transiting of complex tectonic configuration surrounding NE Taiwan. For the meta-clastic samples of the northern LS, the fission-tracks in zircons were partially annealed. The degree of annealing is slightly higher in the zone along the Chingshui fault than the surrounding areas. The youngest peak analyzed from hundreds of grains is reported at ~4.1 Ma. Based on the empirical relationship between heat flow and cooling rate in the different geological regime of Taiwan Island, the geothermal gradient is calculated as 42-39 °C/km in the Chingshui area when the cooling rate is 56-41 °C/m.y. The estimated geothermal gradient of northern LS is almost identical to that of the central LS, interpreted as the geothermal state induced by the regional metamorphism during the Penglai Orogeny. The extension-related magmatism in the post-collisional stage of northern Taiwan results in the regionally high geothermal gradient. Consequently, the significant geothermal activity is formed in the Chingshui area, and the increased cooling rate is observed in the northern Hsuehshan Range. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/73014 |
DOI: | 10.6342/NTU201901464 |
全文授權: | 有償授權 |
顯示於系所單位: | 地質科學系 |
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