造山運動期間地殼動力演化：以華南沿海金門島為例
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2017
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造山運動為形成造山帶的大規模板塊構造運動事件，可以利用溫度、壓力、岩漿訊號、應力與液壓等不同條件將其區分為同造山期、後造山期及非造山期。前人研究預期在同造山時期以逆斷層應力場，後造山時期為走向滑移斷層應力場至非造山時期的正斷層應力場。雖依據野外露頭可初步判別應力場形式，但缺乏良好材料去重建造山帶應力場，因此鮮有相關報導討論造山帶應力演化情形，導致難以更細節地重建地體構造演化模型。
金門位於中國大陸華南沿海地區，白堊紀期間華南沿海發生東北－西南方向山系的晚燕山造山運動，將中下部地殼抬升至地表，同時各造山階段各自有不同的岩漿侵入，形成獨特而豐富的岩脈景觀。同造山期的角閃岩侵入體，侵入年代約138-132Ma；後造山期侵入體包含偉晶岩與細晶岩，侵入年代約110-100Ma；非造山期的輝綠岩侵入體，侵入年代約94-76Ma。這些岩脈侵入體位態則可反映當時最小主應力方向與應力場以及液壓相關資訊，因此可利用不同階段岩脈的資料重建金門地區應力場演化史。
岩脈侵入體形成時，岩漿液壓至少要達到最小正應力，方能將岩體撐裂並於裂隙中冷卻，形成侵入體。統計充分的岩脈資料，將能判斷侵入當下三維應力場及液壓在其中所扮演的角色。利用岩石力學參數與地質壓力計的平均應力或鉛直應力，可進一步三維應力規模與液壓數值，進而可比較不同造山時期的應力場演化關係。
本研究於金門島與烈嶼島沿海地區量測岩脈位態，評估地殼尺度應力場，重建金門地區於晚燕山造山運動地殼尺度應力場演化史。
1.不同造山時期所對應的地殼尺度應力場為：同造山時期為逆斷層應力場，後造山時期與非造山時期則是正斷層應力場，但後造山時期應力場數值結果卻較為集中於走向滑移斷層應力場形式。
2.同造山時期以低角度的角閃岩與英雲閃長岩侵入體為做應力反演之素材，結果顯示為斜交東北－西南向山系之東西向擠壓逆斷層應力場，應力比值為0.54±0.18，液壓比值約為0.59，侵入深度約24.9km。呈現低角度橢球狀逆斷層應力場，岩脈形成於液壓狀態略高於靜岩壓，地溫梯度約為30.1℃/km。結合野外環繞金門太武山與東北角外海之葉理面，可能在後造山岩體侵入前便轉移至走向滑移斷層應力場或正斷層應力場。
3.後造山時期，以偉晶與細晶岩脈為材料，地質壓力計則採用華南內陸後造山岩體鋁角閃石壓力計，結果顯示應力比值為0.69±0.14，液壓比值約為1.02，形成深度約5.5公里，地溫梯度約為93.5℃/km。屬於西北－東南向擠壓之平板狀正斷層應力場。岩脈形成於液壓狀態為大於靜岩壓。
4.非造山期是平行東北－西南山系方向擠壓之正斷層平板狀應力場，應力比值0.68±0.08，液壓比值約為0.72或0.17，地溫梯度約為77.7℃/km。呈現平板狀正斷層應力場。岩脈形成於液壓狀態接近靜岩壓。
5.藉由金門地區在晚燕山期鉛直應力轉換為絕對深度，結合前人定年結果，進一步評估同造山時期到後造山時期的剝蝕速率為0.82-0.49mm/yr，而後造山期至非造山期的剝蝕速率則減慢至0.35-0.04mm/yr。
Orogeny refers to the event of making mountain belt. During orogeny, a mountain belt experienced different orogenic stages, including syn-orogeny, post-orogeny, and an-orogeny, with various conditions of temperature, pressure, geochemical signature, stress and fluid pressure. Researchers usually expect to observe different stress regimes corresponding to different orogenic stages. So far, no document had reported the phenomena of stress evolution from reverse faulting via strike-slip faulting to normal faulting stress regimes in stages of syn-orogenic, post-orogenic and an-orogenic, respectively. However, Study of dikes from Kinmen Island can shed light to show the stress evolution of orogeny. The Kinmen Island, located in the southeastern continental margin of Mainland China, cropped out the middle-lower continental crust, which was experienced different deformation and metamorphism during Late Yenshanian Orogeny. Based on previous studies of geochemistry, geochronology, and P-T condition, different types of dikes are identified. They are syn-orogenic dikes of amphibolite (138-132Ma), post-orogenic dikes of pegmatite and aplite (110-100Ma), and an-orogenic dike of gabbro (94-76Ma). The mechanism of dike development is when magma pressure overcomes the minimum stress, magma can create the intrusive dike perpendicular with the minimum stress. By investigating the distribution and attitude of dikes with different lithologies, stress orientation corresponding to the different orogenic stage can be estimated. With the constraint of rock strength, mean stress from geobarometer and vertical stress in each stage, the magnitude of stress field and magma pressure for each stage can be further calculated. This research restructured crustal dynamics evolution during Late Yanshanian Orogeny by measuring the attitude of dike around Kinman and Leiyu island. (1) Compared with orogenic stage and crustal stress regime: syn-orogeny was reverse faulting stress regime, post-orogeny and an-orogeny were normal faulting stress regime. But, the value of post-orogenic stage stress field was strike-slip faulting stress regime. (2) As the syn-orogenic stage, amphibolite and tonalite dike intrusion appeared as low dip angle, which reflected that reverse faulting regime and horizontal maximum stress direction in E-W orientation. This orientation was oblique the orientation of mountain belt, NW-SE. The stress ratio was 0.54±0.18. The fluid ratio was 0.59. The intrusive depth was 24.9km. These result reflected ellipsoid reverse faulting stress regime. The geothermal gradient was 30.1℃/km. Dikes formed in the environment, which fluid pressure were higher than lithostatic pressure. Integrate with outcrop result, the stress regime would change to strike-slip faulting stress regime or normal faulting stress regime. (3)As the post-orogenic stage, this study uses the attitude of pegmatite and aplite dike. Geobarometer uses Al-amp geobarometer inland, SE China. The str ess ratio was 0.69±0.14. The fluid ratio was 1.02. The intrusive depth was 5.5km. These result reflected plate normal faulting stress regime and horizontal maximum stress direction in NW-SE. The geothermal gradient was 93.5℃/km. Dikes formed in the environment, which fluid pressure was higher than lithostatic pressure. (4)Finally, an-orogenic dike intrusion struck NE-SW with steep dip angle direction, which reflected that normal faulting regime and NE-SW horizontal maximum stress direction. The stress ratio was 0.68±0.08. The fluid ratio was 0.72 or 0.17. The intrusive depth was 4.5km. The geothermal gradient was 77.7℃/km. These result reflected plate normal faulting stress regime. Dikes formed in the environment, which fluid pressure were lower than lithostatic pressure. (5)The vertical stress variation, a.k.a. erosion velocity ,in Kinmen area during Late Yanshanian Orogeny, syn-orogenic stage to post-orogenic stage erosion velocity was 0.82-0.49mm/yr, and post-orogenic stage to an-orogenic stage was 0.35-0.04mm/yr.
Orogeny refers to the event of making mountain belt. During orogeny, a mountain belt experienced different orogenic stages, including syn-orogeny, post-orogeny, and an-orogeny, with various conditions of temperature, pressure, geochemical signature, stress and fluid pressure. Researchers usually expect to observe different stress regimes corresponding to different orogenic stages. So far, no document had reported the phenomena of stress evolution from reverse faulting via strike-slip faulting to normal faulting stress regimes in stages of syn-orogenic, post-orogenic and an-orogenic, respectively. However, Study of dikes from Kinmen Island can shed light to show the stress evolution of orogeny. The Kinmen Island, located in the southeastern continental margin of Mainland China, cropped out the middle-lower continental crust, which was experienced different deformation and metamorphism during Late Yenshanian Orogeny. Based on previous studies of geochemistry, geochronology, and P-T condition, different types of dikes are identified. They are syn-orogenic dikes of amphibolite (138-132Ma), post-orogenic dikes of pegmatite and aplite (110-100Ma), and an-orogenic dike of gabbro (94-76Ma). The mechanism of dike development is when magma pressure overcomes the minimum stress, magma can create the intrusive dike perpendicular with the minimum stress. By investigating the distribution and attitude of dikes with different lithologies, stress orientation corresponding to the different orogenic stage can be estimated. With the constraint of rock strength, mean stress from geobarometer and vertical stress in each stage, the magnitude of stress field and magma pressure for each stage can be further calculated. This research restructured crustal dynamics evolution during Late Yanshanian Orogeny by measuring the attitude of dike around Kinman and Leiyu island. (1) Compared with orogenic stage and crustal stress regime: syn-orogeny was reverse faulting stress regime, post-orogeny and an-orogeny were normal faulting stress regime. But, the value of post-orogenic stage stress field was strike-slip faulting stress regime. (2) As the syn-orogenic stage, amphibolite and tonalite dike intrusion appeared as low dip angle, which reflected that reverse faulting regime and horizontal maximum stress direction in E-W orientation. This orientation was oblique the orientation of mountain belt, NW-SE. The stress ratio was 0.54±0.18. The fluid ratio was 0.59. The intrusive depth was 24.9km. These result reflected ellipsoid reverse faulting stress regime. The geothermal gradient was 30.1℃/km. Dikes formed in the environment, which fluid pressure were higher than lithostatic pressure. Integrate with outcrop result, the stress regime would change to strike-slip faulting stress regime or normal faulting stress regime. (3)As the post-orogenic stage, this study uses the attitude of pegmatite and aplite dike. Geobarometer uses Al-amp geobarometer inland, SE China. The str ess ratio was 0.69±0.14. The fluid ratio was 1.02. The intrusive depth was 5.5km. These result reflected plate normal faulting stress regime and horizontal maximum stress direction in NW-SE. The geothermal gradient was 93.5℃/km. Dikes formed in the environment, which fluid pressure was higher than lithostatic pressure. (4)Finally, an-orogenic dike intrusion struck NE-SW with steep dip angle direction, which reflected that normal faulting regime and NE-SW horizontal maximum stress direction. The stress ratio was 0.68±0.08. The fluid ratio was 0.72 or 0.17. The intrusive depth was 4.5km. The geothermal gradient was 77.7℃/km. These result reflected plate normal faulting stress regime. Dikes formed in the environment, which fluid pressure were lower than lithostatic pressure. (5)The vertical stress variation, a.k.a. erosion velocity ,in Kinmen area during Late Yanshanian Orogeny, syn-orogenic stage to post-orogenic stage erosion velocity was 0.82-0.49mm/yr, and post-orogenic stage to an-orogenic stage was 0.35-0.04mm/yr.
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Keywords
岩脈, 晚燕山造山運動, 應力場, Dike, Late Yenshanian Orogeny, Stress regime