花東海盆南部沉積環境演化與地體構造研究

Abstract

花東海盆位於台灣東部外海，為菲律賓海板塊最西端的部份，由周圍數個高區圈合成一個幾乎封閉的海盆，在台灣周圍海域是為一較獨特的沉積環境。作為菲律賓海板塊於台灣附近隱沒、碰撞的前導，其在地體構造中的重要性不言而喻。本研究利用在花東海盆南部收集來之多頻道反射震測剖面資料進行該地區之沉積構造解釋，藉由判識出的沉積層序及內部構造與特徵，釐清影響花東海盆南部沉積環境的因素，進一步整合前人研究，探討整個花東海盆沉積環境演化以及地體構造的變動。 在花東海盆南半部地區（台東峽谷以南），震測剖面上可以辨識出兩個主要的層序界面X與Y，此二層序界面大致可對應到前人在花東海盆北部發現的層序界面，將整個花東海盆的沉積物區分為三個層序：S1、S2、S3，各代表不同的沉積階段。在海盆南部Y代表北呂宋島弧舊島鏈停止活動的時間，X則是北呂宋島弧新島鏈開始活動的時間；層序S1則代表北呂宋島弧屬於洋間島弧活動時期所堆積的沉積物，此階段沉積物大致平整地上覆於基盤上；層序S2為北呂宋島弧火山活動中斷期間堆積的沉積物；層序S3代表北呂宋島弧新島鏈形成後堆積的沉積物，而透過剖面的沉積特徵也可以看出來自台東峽谷，亦即源自台灣造山帶的沉積物於研究區域北部也已有堆積。 花東海盆無論南北，其沉積物主要來自西邊的呂宋島弧及台灣造山帶。在花東海盆南部，呂宋島弧並未碰撞到歐亞大陸邊緣，沉積物來源主要受呂宋島弧活動演化的控制；在花東海盆北部，呂宋島弧碰撞上歐亞大陸邊緣，沉積物的來源因此受到台灣造山帶演化的控制。在時間上，不論弧陸碰撞或呂宋島弧活動演化，其不同活動階段發生的年代相近，可能是造成X、Y兩主要層序界面分布於整個花東海盆的原因。 透過反射震測剖面亦可發現北呂宋島弧東側有一被沉積物覆蓋、走向約為330度的海脊，稱為弧東海脊，使島弧東翼的弧坡區向南變寬；花東海盆沉積層序則在近弧東海脊東側有凹陷現象，並伴隨許多階段性活動的小正斷層，且不同時期沉積中心會變動。至於加瓜海脊西側則有一系列南北走向的小海脊，可能與花東海盆中的古破裂帶有關，這些海脊並持續有抬昇現象。 本研究由認為影響花東海盆南部沉積環境的因素，主要為北呂宋島弧的火山活動，而弧東海脊、加瓜海脊及其西側小海脊這些原已存在的地形特徵，便受北呂宋島弧的撓曲效應影響使海盆的沉積中心因而改變。在島弧形成過程中，由於岩石圈承受不停增長的火成岩體而產生撓曲，於北呂宋島弧旁的基盤低區形成槽溝（moat）並產生凸起（bulge），然而由於有海盆形成初期造成加瓜海脊等脊槽系統的古破裂帶存在，使得隆起地區古破裂帶的斷層再度活動，這樣的過程促成花東海盆的沉積中心有因島弧隆起而向東偏移或受撓曲下陷與小海脊活動影響而向西偏移的改變。

Offshore eastern Taiwan, Huatung Basin is the most western part of the Philippine Sea Plate (PSP). Surrounding by the Yaeyama Ridge to the north, Gagua Ridge to the east, and Luzon Arc to the south and west, it forms a special sedimentary environment adjacent to Taiwan. It may also heavily influence by the collision and subduction processes of the Luzon Arc, this is important in tectonic studies. Due to lack of geophysical data in the southern Huatung Basin (SHB), previous studies often discussed southern and northern Huatung Basin as a whole. In this study, recently acquired multichannel seismic reflection data in the SHB were used to study the evolution of sedimentary environment and tectonics in Huatung Basin. Two major sequence boundaries, namely X and Y are identified on the seismic profiles which divided basin sediments into three seismic sequences. Similar stratigraphy could extend to northern Huatung Basin. In the SHB, Y is interpreted to be the time when the old Luzon Arc volcanism ceased at about 4~5 Ma, while X represents the time when the new Luzon volcanic arc started to form. Thus, S1 is the oldest sediment sequence in the Huatung Basin which lay directly above the basement. The sediment sources are likely to be derived from the North Luzon Arc when it was still an intra-ocean arc. Sequence S2 is the sediment deposited after the old Luzon volcanism stopped, and sequence S3 was deposited after a new volcanic chain erupted, with additional source from Taiwan arc-continent collision zone. In both northern and southern Huatung Basin, most of the sediments came from North Luzon Arc and the Taiwan mountain belt to the west. In the SHB, North Luzon Arc has not collided with the Eurasia continental margin yet, sediment input was mainly influenced by the volcanic activities of the North Luzon Arc. In the north Huatung Basin, North Luzon Arc has already collided with the Eurasia continental margin, the influence of orogenic sediments therefore were be controlled by the evolution of the Taiwan orogen. Just beneath the eastern flank of the North Luzon Arc, we observed on the seismic profiles that there exists a buried ridge which widens the east flank of the Luzon arc slope to the south. This ridge is named the East Ridge of Luzon Arc （ERLA）in this study. East of ERLA, sediment sequences in the Huatung Basin appear to be subsided with minor faults. The depocenters changed with time in the Huatung Basin, and there are 2~3 north-south trending small ridges lie to the west of the Gagua Ridge, these ridges could be formed along old fracture zones in the Huatung Basin, and has been uplifting through time. The main factor influences the sedimentary environment in SHB is the volcanic activities of the North Luzon Arc. The Huatung Basin basement, the small ridges west of the Gagua Ridge, and the ERLA, have all been influenced by the flexure of the Philippine Sea Plate due to loading of the North Luzon Arc. Locations of the depocenters at different times were affected by the loading processes. When the North Luzon Arc formed, a depression in southwestern Huatung Basin was formed just east of the volcanic arc due to loading of the arc, and moat and bulge developed where the small ridges west of the Gagua Ridge lie. As these ridges were uplifted, old faults which were associated with the ridge formation reactivated and formed ridge-trough systems west of the Gagua Ridge. The sedimentary processed in SHB is controlled by the evolution of the North Luzon Arc, plate flexure and evolution of the Luzon Arc volcanic activities strongly influenced these strata.