晚新生代台灣西部前陸盆地之震測地層學、前凸起隆升與前陸演化

Abstract

本研究藉由台灣西部前陸盆地系統之震測剖面及水深資料，了解台灣西部盆地自被動大陸邊緣環境而轉進入前陸盆地系統之過渡時期之沈積地層，與前陸層序演化之大地構造意義。 以前陸盆地系統之觀點，台灣海峽中線之彰雲砂脊為一現生之前凸起堆積區，而烏坵凹陷可能係一相對應的後凸起堆積區；澎湖北側有數個平移構造複合體(strike-slip duplex) ，可視為撓曲應力作用在被動大陸邊緣的張裂斷層所造成的次生逃脫(escape)構造；而在前陸層序之分布上，早期的前陸層序主要受到造山楔前緣荷重張裂體系之正斷層影響，而晚期之前陸層序則因應造山帶的遷移而有沈降中心的遷移。前凸起的運動模式應為以垂直向上抬升為主，而非有向陸遷移之現象之外，而其發生時間應為6.5 - 5.6Ma；由於桂竹林層仍符合全球海水面變化，則其表示了一個在前陸盆地系統撓曲行為中一個較不被影響的構造單元，則此應為前凸起之樞紐帶位置。 晚中新世時由於呂宋弧逐漸撞上歐亞大陸之邊緣，原陸坡沈積物與外來岩體對歐亞岩石圈之荷重使其撓曲，造成了現今台灣海峽處出現了前凸起；前凸起緩慢抬升的過程中遭到侵蝕，遂向抬升樞紐帶的東側沈積了未受構造影響而仍係主控於海水面變化影響的的桂竹林層，與受構造影響而逐漸沈降深化的錦水頁岩，唯其分布上皆仍受撓曲體系正斷層之影響；一直到更新世以來台灣造山帶快速且大量抬升，雪山山脈成為主要的沈積物供應源後，始有造山帶物源的卓蘭層出現；最後中央山脈的出現，除了形成頭嵙山層之沈積，沈積中心亦隨著造山活動而遷移。持續的構造活動及伴隨著造山帶沈積物供應，形成今日的台灣西部前陸盆地系統。

This study try to understand the depositional sequences of the West Taiwan Foreland Basin which is a transition between passive margin setting and uplift orogen setting and of evolution of foreland sequences, using seismic profiles, bathymetry data and previous researches. Within the foreland basin system, the Changyun Sand Ridge located at the middle of Taiwan Strait could be regard as a modern forebulge depozone, and the Wuchiu Depression might be a corresponding backbulge depozone; Strike-slip duplex at the north of Penghu Island might be regard as a second-order escape extrusion under flexure tectonics influencing on passive margin; The thickness of foreland sequence is affected by flexural faulting system in front of orogeny load at early stage, and migration of depositional center going on with propagation of orogen; Instead of landward migration, the behavior of forebulge should be uplifted vertically and onset between 6.5 - 5.6 Ma. The tectonic effect by foreland basin system might not be significant since Kueichulin Formation corresponded to eustatic sea level change and the Kueichulin Formation might means an unaffected tecton - sediemntary unit. The locality might be the hinge of forebulge flex. The Luzon Arc has collided collide with Euro - Asia continent margin since late Miocene. The slope sediments and Allochthon bended the lithosphere and formed the forebulge in the middle of Taiwan Strait. The eroded part of forebulge was transported eastward and deposited Kueichulin Formarion which was controlled by eustatic sea level than tectonics on the hinge of forebulge and Chinshui Formation which is recognised as a initial deepening of foreland basin and not correspond to eustatic sea level change. The thickness is controlled by flexure faulting system. After Pleistocene the uplift of Taiwan orogen was fast and Hsuehshan Range became the prominent sediment source, the Cholan Formation was formed; At later stage and deposition center shift after Central Range appeared, forming the Tuokoshan Formation. Lasting, active tectonics accompanied with sediment supply from Taiwan orogen, results in modern West Taiwan Foreland Basin System.