從晚中新世里龍山層磷灰石核飛跡定年研究探討恆春半島隆起的起始年代

Abstract

本研究目的為討論恆春半島在造山時期地殼隆升過程的冷卻歷史。因此，針對晚中新世未變質的里龍山層進行磷灰石的核飛跡定年 (低溫熱定年)，來了解台灣南部造山演化史。本研究的樣本採自里龍山層上部砂礫岩層，包含砂礫岩層中的變質砂岩礫石與基質的砂岩。里龍山層屬於超微化石NN11帶的晚中新世地層 (5.6-8.3 Ma)，其沉積環境為淺海陸棚至斜坡的沖積扇，古水流方向由北向南，沉積物來源自北方的造山帶，推測由變質砂岩礫石的磷灰石核飛跡年代可用以討論源區造山帶的冷卻歷史。然而本研究結果顯示，封存溫度僅135±20˚C的磷灰石核飛跡已經在里龍山層被深埋的過程中被完全癒合，因此無法記錄源區的剝蝕歷史，不過深埋後癒合的核飛跡年代可解釋恆春半島的抬升剝蝕歷史。 本研究指出里龍山層的磷灰石核飛跡集合年代介於2.6±0.5至3.5±0.6 Ma，砂岩與變質砂岩單顆粒年代頻譜相似，並且核飛跡年代皆小於里龍山層的沉積年代 (NN11帶，>5.6 Ma)，代表磷灰石核飛跡在里龍山層沉積後被完全癒合 (totally reset)。由於里龍山層在沉積以後未受變質，埋藏溫度可能接近於磷灰石核飛跡封存溫度 (~135°C)，因此推測磷灰石核飛跡冷卻年代接近於初始抬升年代，故此定年結果可視為恆春半島的初始隆起年代為2.6±0.5至3.5±0.6 Ma。從研究結果可以得知里龍山層於晚中新世到上新世期間上覆地層持續增厚至2.1-3.8公里，初始抬升以來的冷卻速率為34-46°C/ Ma，平均剝蝕速率為0.6-1.5 mm/yr。

This study aims to investigate the thermal history regarding the Late Miocene strata in the Hengchun Peninsula with low-temperature thermochronology. Those samples collected were from the Lilungshan Formation, including quartzite (pebbles) and sandstones (matrix). Apatite fission-track thermochronology is a radiometric dating method that refers to thermal histories of the sedimentary basin within the closure temperature range of 135±20˚C. Our study shows that fission-track ages of quartzite pebbles and the sandstones obtain pooled ages between 2.6±0.5 Ma and 3.5±0.6 Ma, and the age distributions indicate total reset. Nevertheless, the formation age of the Lilungshan Formation falls within nannofossil zone NN11 (5.6-8.3 Ma). If the fission-track age record is less than the depositional age, which represents a post-deposition annealing age, the strata should be buried deeply to over the closure temperature of apatite fission-track. Therefore, we confirm that the Lilungshan Formation has encountered a thermal event caused by tectonic burial in the foreland basin. In conclusion, the apatite fission-track ages of the Lilungshan Formation have been totally reset due to paleo-burial. Besides, the onset of uplifting age in the Hengchun Peninsula was between 2.6±0.5 Ma and 3.5±0.6 Ma. Furthermore, if we assume the geothermal gradient is 45±13°C/km, the thickness of overlying strata must at least 2.1-3.8 km to reach the closure temperature of apatite fission-track (~135°C). It shows that the cooling rate of the basin is 34-46℃/Myr, and the exhumation rate of the basin is 0.6-1.5 mm/yr.