當山脈碰撞抬升時,造山前緣會逐漸下陷,形成前陸盆地。因此海岸山脈的後前陸盆地於上新世以來持續堆積了厚層的造山帶沉積物,這些沉積物記錄了山脈的剝蝕歷史。本研究目的為利用海岸山脈後前陸盆地的沉積物,以核飛跡定年法研究,探討中部脊樑山脈在造山時期地殼抬升的冷卻歷史,以了解脊樑山脈的剝蝕歷史。本研究的樣本採集自鹽寮坑溪(十號橋),地層為蕃薯寮層和八里灣層,採集砂礫岩層中的變質砂岩礫石和基質的砂岩。蕃薯寮層屬於晚上新世的中-下部深海沖積扇、八里灣層為晚上新世至早更新世深海上部沖積扇,皆來自西側的脊樑山脈。由於前人探討海岸山脈盆地的地溫梯度僅有約14˚C/km,故其核飛跡並不會因深埋作用而被癒合,利用盆地地層中碎屑鋯石和磷灰石的核飛跡年代可以計算脊樑山脈的抬升與剝蝕歷史。 本研究指出蕃薯寮層和八里灣層的磷灰石核飛跡的最年輕峰值介於6.6Ma至2.1Ma,越年輕的地層其核飛跡年代也隨之年輕、而礫石中鋯石核飛跡皆為部份癒合(partial annealing),代表其源區地層並沒受到超過250度以上的熱事件、砂岩的鋯石核飛跡則有一癒合年代峰值4.9Ma。而利用磷灰石和鋯石的核飛跡年代得知當時有兩種不同冷卻路徑的源區,為源自太魯閣帶及玉里帶,及得出於中期上新世時玉里帶脊樑山脈的冷卻速率約52-60˚C/Ma,到了4-3Ma時玉里帶和太魯閣帶有一明顯的加速冷卻。同時利用磷灰石和鋯石核飛跡可以探討源區的剝蝕岩性,約在3.4Ma時(蕃薯寮層)脊樑山脈開始已經出露(被剝蝕)極低度變質的硬頁岩岩層,約3.3Ma時(八里灣層)開始出露低度變質岩層(板岩)。
This study aims to investigate the exhumation history of the Backbone Range from the orogen-derived sediments which were deposited in retro-foreland basins in the Coastal Range since Pliocene. Samples are collected from Yenliaokeng river at the northern Coastal Range which include the Fanshuliao Formation and the Shuilien Conglomerate of Paliwan Formation. 9 samples of sandstones and boulders are dated by apatite and zircon fission track thermochronometries which are referred to the thermal history of rocks from the closure temperature for 135˚C and 250˚C of apatite and zircon, respectively. Fission-track ages of apatites are between 6.6Ma and 2.1Ma, and zircons are all partial annealed, suggesting no post annealing during burial. Apatite fission-track ages of the Fanshuliao Formation and the Shuilien Conglomerate have been totally reset. The zircon fission-track ages of conglomerate in Shuilien Conglomerate were partial annealing but the sandstone get a 4.9Ma peak age which means these two different cooling paths were indicating 2 provenances (Tailuko Belt, Yuli Belt) during Pliocene. Our data show the cooling rate of the Yuli Belt was about 52-60˚C/Ma at Pliocene. After that, Tailuko Belt and Yuli Belt accelerated since 4-3Ma, indicating the unroofing of the Taiwan orogenic belt, and revealed the changes of provenance from very low-grade to low grade metamorphic terrane since Pliocene.