| 摘要: | 台灣造山帶的形成被認為是源自於菲律賓海板塊的呂宋島弧與歐亞大陸板塊之間傾斜碰撞的結果，過程中許多關於弧陸碰撞的紀錄被完好地保留下來，為全世界的碰撞帶研究提供了一個特殊的研究場域。多數研究者認為隨著時間演進，台灣的造山過程可藉由一條由北到南的剖面來表示碰撞過程的不同演化階段：位於台灣北部海域、沖繩海槽最南端、琉球隱沒系統的碰撞後期階段（post-collision stage）；位於中台灣的碰撞階段（collision stage）；以及位於馬尼拉隱沒帶北邊的碰撞前期階段（pre-collision stage）。其中關於台灣北部海域，雖然前人普遍認同該區域的張裂構造環境，但對於該區域的地質演化過程卻缺少整體的認知。本研究透過多頻道反射震測及既有鑽井資料所建構起來的層序與變形解釋，建構出台灣北部海域從擠壓到張裂各個階段的構造演化模型，在該模型的基礎上，我們將研究區域劃分為兩塊具有不同構造環境的區域：為台灣造山帶的一部分，經歷造山階段並轉變為張裂垮塌階段的垮塌造山帶（collapsed zone）；與相對穩定，既非碰撞也非張裂階段的陸棚盆地（shelf basin）。陸棚盆地與垮塌造山帶之間被一條與陸上山腳斷層相連的斷層分隔，我們稱之為「海域山腳斷層（Offshore Shanjiao Fault）」。在6百萬年以前，垮塌造山帶屬於東海陸棚盆地的一部分，當呂宋島弧北端與歐亞大陸邊緣於台灣北部海域碰撞時，海域山腳斷層為整個系統最西北邊的逆衝斷層，也就是古台灣造山帶的逆衝前緣邊界。此時歐亞大陸邊緣因持續受到菲律賓海板塊的聚合擠壓而縮短增厚，進而產生了一系列的東北-西南走向的逆衝斷層，如野柳斷層、崁腳斷層-F1、台北斷層-F2、新店斷層-F3等，接著並形成數條西北-東南走向的右移走向滑移斷層，以調節造山帶兩側區塊之間的應力差。當碰撞造山運動由東北往西南方向遷移後，台灣北部海域地殼便失去了擠壓應力的支持，進而轉變為地殼拉張的環境，並在大約2.6百萬年前形成垮塌造山帶。向台灣西南方向逐漸變淺的基底不整合面也意味著台灣北部海域的拉張系統開始由東北往西南方向推進。而既存的東北-西南走向逆衝斷層（海域山腳斷層、野柳斷層、F1、F2、F3）也開始在垮塌造山帶內轉變為傾動地塊（tilted block faulting），形成一系列半地塹構造。而沿著F4及F5的地區則形成轉換拉張帶（transtensional zone），促成了西北-東南走向的海底峽谷發育。由於碰撞後期陷落與南沖繩海槽張裂的雙重影響，拉張區域持續演進成為現今的台灣北部區域，許多鄰近海域山腳斷層的活動斷層表示拉張活動並不僅發生於南沖繩海槽的弧後盆地，也同時存在於中國大陸邊緣區域。
;The Taiwan mountain belt is known as a result of the oblique collision between the Luzon arc of the Philippine Sea Plate and the Eurasian Plate, where some of the best records on arc-continent collision have been preserved. It thus offers a unique example for the study of collision belts worldwide. Most authors suggest that the mountain building has evolved through time and could be represented by a series of cross-sections from north to south as the following stages: the post-collision stage in the offshore area of northern Taiwan, southernmost Okinawa Through and Ryukyu subduction system; the collisional stage in middle Taiwan; and the pre-collision stage in the northern Manila subduction zone. Although the extensional tectonism in northern offshore Taiwan is generally agreed, a holistic understanding of its geological evolution is lacking. Based on the interpretation of stratigraphy and deformation from multi-channel seismic profiles and the existing borehole data, we use the geological features above to construct the tectonic history of the area off northern Taiwan from the compressional to extensional stages. On the basis of the model, we divided into two regions with different tectonic events: The collapsed zone is the part of the Taiwan mountain belt during the collision stage and become to collapse during the extensional stage. Whereas, the shelf basin is quite stable and involves neither collision nor extensional stage. The shelf basin and the collapsed zone are distinguished by a fault that is connected to the onshore Shanjiao Fault, which is named as Offshore Shanjiao Fault. Before 6 Ma, the collapsed zone was a part of the East China Sea shelf basin. When the northern tip of the Luzon arc collided with the Eurasian continental margin off northern Taiwan, the Offshore Shanjiao Fault is a northwesternmost thrust fault and is known as a frontal thrust boundary of the proto-Taiwan mountain belt. The Eurasian continental margin continued to be shortened and thickened by the converging Philippine Sea plate, forming several NE-SW trending thrust faults such as Yehliu Fault, Kanjiao Fault – F1, Taipei Fault – F2, Hsintien Fault – F3, and so on. NW-SE trending dextral strike-slip faults also formed to accommodate the differential stresses on both sides of the belt segments. When the collisional orogen migrates from NE to SW direction, the crust has lost its compressional support and becomes an area of crustal stretching, forming collapsed zone at ~2.6 Ma. The southwestwardly shallowing of Basal Unconformity surface toward Taiwan implies that the extensional regime has propagated from NE to SW direction in offshore northern Taiwan. The pre-existing NE-SW trending thrust faults (Offshore Shanjiao Fault, Yehliu Fault, F1, F2, and F3) became tilted block faulting in the collapsed zone, forming a series of half-grabens. The area along the F4 and F5 becomes transtensional zones, facilitating the development of the NW-SE trending submarine canyons. Due to both post-collisional subsidence and rifting of the southern Okinawa Trough, the extensional regime continues to propagate to present-day northern Taiwan. Several active faults occur near the Offshore Shanjiao Fault, indicating that the active rifting not only exists in the back-arc basin of the southern Okinawa Trough but also in Chinese continental margin. |
