利用碎屑鋯石鈾鉛定年探討臺灣中北部晚中新世至更新世沉積物來源及山脈剝蝕歷史

前人研究對於臺灣晚中新世至更新世時期的岩石地層、磁地層、沉積環境與岩象已有諸多著墨。其中根據砂岩岩象分析結果，認為晚中新世以後臺灣海峽陸棚的沉積物來源由西側的被動大陸邊緣轉為東側抬升的造山山脈所供應。近年來碎屑鋯石的年齡研究因為具有普遍性與可對比性，已被廣泛地運用在重建造山帶或古地理的演化模式。本研究利用碎屑鋯石鈾鉛定年分析晚中新世以來的沉積物，並將定年結果與源區地層鋯石年齡做統計分析，期望得知造山山脈岩層在不同時期的剝蝕概況。本研究主要選取出露於臺灣北部及中部的晚中新世至更新世地層，在不同層位共採集30個砂岩樣本分離出碎屑鋯石，每個樣本隨機分析至少120筆鋯石年齡數據；另外將屬於中砂砂岩粒徑的樣本進行岩象分析，結合兩者資料探討前陸盆地在不同時間的沉積物可能來源及山脈岩層出露的層序變化；最後再結合前人的低溫熱定年資料，探討斷層活動與山脈剝蝕-前陸盆地堆積系統的演化歷史。砂岩的碎屑鋯石年齡顯示，各時期的沉積物皆與臺灣始新世至中新世地層具有相同的主要年齡族群，惟年齡族群所佔的相對比例在不同時期有些差異。綜合岩象分析資料與源區地層碎屑鋯石年齡特性，將造山山脈被剝蝕的源區地層分為始新世、漸新世與中新世地層。利用源區地層與本研究樣本的碎屑鋯石定年結果進行統計分析，得到不同時期造山山脈出露的各個源區地層比例。臺灣北部雪山山脈於晚期中新世開始抬升，主要剝蝕中新世岩層堆積至前陸盆地中。早期更新世（2.6-2.3 Ma），雪山山脈開始出露漸新世岩層（約佔總體沉積物1-9%），而後出露的面積逐漸擴大（2.3 Ma-現代）（約佔總體沉積物17-40%）。新店溪約於晚期更新世（1.46 Ma）開始侵蝕至雪山山脈的始新世岩層，以約佔總體沉積物1-9%的比例堆積至前陸盆地，之後始新世岩層被侵蝕的面積逐漸加大（堆積林口層）（約佔總體沉積物28%），形成今日山脈的樣貌。臺灣中部雪山山脈亦於晚期中新世開始抬升，主要剝蝕中新世岩層堆積至前陸盆地。早期更新世（2.2-1.0 Ma），雪山山脈開始穩定出露漸新世岩層（約佔總體沉積物5-15%）。晚期更新世（約1.0 Ma），雪山山脈開始大量出露始新世岩層（約佔總體沉積物46%）。之後於1.0-0.7 Ma之間，沉積物來自始新世岩層的比例逐漸減少（約減少至佔22%），而來自西部麓山帶中新世岩層的比例反而增加（約佔77%），可能暗示著斷層向西側不斷發育且活動性逐漸增強。從約70萬年前（0.7-0.3 Ma）沉積物的源區混合結果，可以得知當時山脈岩層出露狀況已與今日的樣貌類似。

關鍵字

前陸盆地；碎屑鋯石；鈾鉛定年；沉積物來源；剝蝕歷史；統計分析

並列摘要

Lithostratigraphy, magnetostratigraphy, depositional environment and sandstone compositions of foreland basin deposits in western Taiwan, from late Miocene to Pleistocene strata, have been systematically researched. According to the petrographic studies of foreland basin deposits, sedimentary provenance originated from southeastern China (the Cathaysia Block) in Eocene-Miocene, but progressively switched to Taiwan orogenic belt due to the arc-continental collision since late Miocene (~6.0 Ma). Furthermore, the Hsuehshan Range and the Western Foothills were uplifted and eroded in sequence during Pliocene to Pleistocene. In this study, we focus on detrital zircon U-Pb dating and petrographic analysis of sandstones collected from late Miocene to Pleistocene strata, aiming to understand the origin of sedimentary provenance and the exhumation history of the western flank of Taiwan orogenic belt. The results of petrography show that sedimentary and low-grade metamorphic lithics fragments gradually increased from late Miocene to Pleistocene in sequence. In addition, the detrital zircon U-Pb age populations of late Miocene-Pleistocene strata are similar to those of Eocene-Miocene strata, suggesting that the synorogenic sediments from late Miocene to Pleistocene were derived from the orogenic belt. Based on the characteristics of U-Pb ages, lithology and metamorphic grade of Eocene-Miocene strata, we classified the provenances into Eocene, Oligocene and Miocene formations. We utilize statistical analysis to quantify the proportion of the exposed provenances of each sample, which can reveal the relation between source and sink thoroughly. In northern Taiwan, the Hsuehshan Range was uplifted during late Miocene. Foreland basin received the sediments from the Miocene strata of Hsuehshan Range since Pliocene. During early Pleistocene (2.6-2.3 Ma), Oligocene strata of Hsuehshan Range started to expose and had 1-9% proportion among the source area. Subsequently, Oligocene strata widely exposed after 2.3 Ma (with 17-40% proportion among the source area). The drainage system have begun to carry the sediments from Eocene strata of Hsuehshan Range (1-9% proportion among the source area) since late Pleistocene (~1.46 Ma). After that, Eocene strata widely exposed (with 28% proportion of the source area) in the meantime as Linkou formation deposited. In central Taiwan, the Hsuehshan Range was also uplifted during late Miocene. Foreland basin also received the sediments from the Miocene strata of Hsuehshan Range since Pliocene. During early Pleistocene (2.1-1.0 Ma), Oligocene strata of Hsuehshan Range started to expose and had 5-15% proportion among the source area. Eocene strata of Hsuehshan Range started to widely expose since late Pleistocene (~1.0 Ma) with 46% proportion among the source area. During 1.0-0.7 Ma, the sum of proportion of Eocene and Oligocene strata gradually decreased (from 60% to 23%), in converse, proportion of Miocene strata gradually increased (from 40% to 77%), which suggested that the Miocene strata of Western Foothills were widely eroded. According to the statistical analysis of the upper Toukoshan deposit (~0.7-0.3 Ma), the proportion of the provenances were similar to the framework of mountain nowadays.