台灣北部麓山帶的磷灰石核飛跡定年分析及其大地構造隱示

Abstract

北部台灣麓山帶有著極為複雜的構造歷史，早期受到歐亞板塊隱沒於菲律賓 海板塊之作用，造山作用發育得最早。其後，地體環境演變為菲律賓海板塊向西 北隱沒於歐亞板塊之狀況；近期，整個北部台灣麓山帶則處於發育張裂性構造環 境之下。為釐清台灣北部麓山帶構造演化，本研究工作利用磷灰石核飛跡定年分 析台灣北部麓山帶發育時間、熱歷史與構造形態。 本研究工作在台北、基隆、宜蘭等地共採41 個樣本，僅有16 個標本具有足 夠量的磷灰石，可得合理的核飛跡年代。由分析結果顯示大寮層磷灰石核飛跡屬 部分癒合，呈現10~50Ma年代分布；而木山層及五指山層的磷灰石核飛跡年代 為完全癒合，呈現為2~4Ma，推估西部麓山帶的癒合帶出現在大寮層及木山層 之間。其中金瓜石一帶的木山層(0.7Ma)及大寮層(5.5Ma)的年代相較附近相同地 層年代年輕，應係火成作用影響所致。除此之外，在不同地區出露之大寮層磷灰 石核飛跡系統的癒合程度比較，可知由北向南癒合程度有漸增之趨勢。反觀北部 的雪山山脈地層，澳底層媽岡段、大桶山層僅達部分癒合程度、到四稜砂岩則呈 現完全癒合，年代為2.4Ma。 相對於台灣北部西北端木山層即已癒合，東側較老的漸新世地層並未呈現癒 合現象，其界限似乎受控於位處前陸盆地邊界之屈尺斷層的活動，屈尺斷層以東 位處斷層上盤，或因缺乏前陸盆地沉積物，導致漸新世地層變質溫度未達癒合溫 度。相對下盤的木山層之上有較厚的前陸盆地沉積，因此變質溫度超過磷灰石癒 合溫度，在斷層上盤的木山層的癒合年代出現於3Ma左右，顯示北部褶皺逆衝 帶開始時間約3Ma，且由於火成作用開始於2.8~2.5Ma，考量地層由癒合溫度到 地表時間，本研究工作結果認為在火山作用開始的2.5~2.8Ma，此時區域應力應 仍處於壓縮狀態，其後在約0.8Ma時轉變為伸張環境。

Taiwan mountain belt was resulted from the collision between NE trending continental margin of the Eurasia and N-S trending Luzon arc on Philippine Sea plate.The Philippine Sea plate moves in NW direction that results in an oblique collision and the mountain building propagates southwardly, with a rate of 50-90 km/Ma. This model suggests that the development of the fold and thrust propagates southwardly. With the cessation of active folding and thrusting during the collision, northern Taiwan is now undergoing post-collisional extension, followed by the eruption of the Tatun volcanoes since 2.6Ma. In this study, we examine the fission track systematics of apatites from the fold and thrust belt in northern Taiwan. Forty-one rock samples were successfully collected from the Tertiary sequences but only sixteen of them yield enough apatites with reliable ages. The results show that the reset boundaries of the apatite fission tracks are quite different between the eastern and western sides of the Chushih fault. In the western side, the reset boundary appears in the Miocene Mushan Formation. In contrast, it locates in the Oligocene to Eocene formations on the eastern side. The difference of reset boundary may reflect the thickness difference of the sedimentary sequences on both sides. On the hanging wall the sequence thickness above the Oligocene Formation may have been less than 4~5 km, as reflected by the partial reset ages. By contrast, the resetting boundary is located in the Miocene Mushan Formation, which is much younger than that on the hanging wall, due to the thicker foreland basin deposits. The oldest total reset ages appear to be near 3 Ma from the formations on hanging wall of the Kanchiao fault and Chushih fault, indicating that the fault-and-thrust belt in northern Taiwan may have been developpned at near 3 Ma. This age appears to be similar to that discovered in southern Taiwan. This similarity further implies that the southward propagation model of the fold-and-thrust belt in Taiwan has not gained supports from the fission track data.