東沙陸緣高原之構造地層特徵與大地構造演化

Abstract

陸緣高原係指於大陸棚與下部陸坡間、突出而平坦之地形特徵。他們的成因或與大陸分離之減薄的地殼碎片，以及富岩漿陸緣分裂過程中因熱點等之岩漿作用而形成的火成體有關。陸緣高原在大西洋、印度洋等區域的大陸邊緣皆有發現，但在邊緣海則較少相關討論。在南中國海(South China Sea, SCS)的北部陸緣，我們在東沙環礁周邊辨識出了一個陸緣高原(後續以東沙陸緣高原稱呼，Dongsha marginal plateau)。為了了解東沙陸緣高原的構造與地層之發展，本研究使用多頻道反射震測(multi-channel reflection seismic, MCS)資料並且整合過去發表之文獻，來進行高原地下地層與構造之辨識。 透過與過去發表文獻之剖面進行關聯，我們在震測剖面中辨識了數個層序邊界，包含Tg (聲學基盤頂部反射面，~66 Ma)、T7 (分裂不整合面，~33.9 Ma)、T6 (~23 Ma)、T4 (~16 Ma)、T32 (~11.6 Ma)以及T2 (~5.3 Ma)。依照不整合面的分布，本研究以東沙斷崖為界，將東沙陸緣高原分為東、西兩段，並且認為東、西兩段具有不同的構造特徵以及陸緣架構。高原東段為隆起基盤為主，其西北以及東南兩側基盤之深度較深並且呈旋轉斷塊狀，表示經歷強烈的拉張變形。整體而言高原東段呈分離陸緣(divergent margin)特徵，而根據隆起基盤缺乏同張裂沉積物、基盤內古背斜以及較厚的地殼特徵，本研究認為隆起基盤在新生代張裂時期應為張裂減薄程度較低的地殼碎片，並且係受到中生代隱沒作用造成岩石圈異質性而不易張裂減薄所致。高原西段為轉換斷層區，該區基盤受西北-東南走向的斷層強烈變形，係具調適(accommodate)陸緣張裂程度側向變化之作用，因此高原西段的張裂模式應是以走滑伸張為主，與轉形陸緣(transform margin)更為相似。 除了張裂作用，根據高原上方地層的變形、T2的侵蝕不整合以及海床的廣泛侵蝕，我們辨識出了一期後張裂時期抬升事件—即東沙運動。東沙運動造成高原全區強烈的斷塊升降以及下部陸坡的岩漿活動。此外，從高原西側地層被東沙斷崖向上拖曳之情形可知，高原東段受東沙運動抬升較高原西段明顯。整體而言，東沙運動具有加強東沙陸緣高原突出形貌之作用，但其貢獻程度與張裂時期的影響相比或為相對次要。 最後，東沙陸緣高原的東西兩側在張裂時期的岩漿活動上有強烈差異，此或因高原西段的轉換斷層區在陸緣張裂時期具有作為屏障、阻止了岩漿向西傳播延伸(propagate)之效用。本研究進一步引用大西洋以及印度洋的陸緣高原來與東沙陸緣高原進行比較，比較結果顯示聖保羅高原同樣具有以轉形陸緣分隔貧、富岩漿陸緣之情形，然而德梅拉拉高原以及埃克斯茅斯高原則無，因此陸緣高原、轉換斷層以及貧、富岩漿陸緣交界三者是否具有相依性，仍需透過更多觀測以及討論。

The marginal plateaus indicate those relatively flat and elevated reliefs between continental shelf and lower slope. Their formations are interpreted including thinned continental crust and magmatic activities from hotspots during the rifting process of magma-rich margins. Marginal plateaus are found along the continental margins of regions such as the Atlantic and Indian Oceans, but very little observation in marginal seas has been made. In the northern South China Sea (SCS), a marginal plateau is recognized in the northern margin around the Dongsha Atoll (hereafter Dongsha marginal plateau, DMP). To identify the distribution of the subsurface strata and structures, and tectonic development of the DMP, archived multi-channel seismic (MCS) reflection data are reprocessed, and the published profiles in the vicinity of the Dongsha Island are integrated. Several sequence boundaries in the seismic profiles are identified, including Tg (~65 Ma, the top of the basement reflector), T7 (~32 Ma, breakup unconformity), T6 (~23 Ma), T5 (~20 Ma), T4 (~16 Ma), T3 (~10 Ma), and T2 (~5 Ma). Based on distributions of these unconformities, we divided the DMP into eastern and western segments with different structural characteristics by the Dongsha Escarpment. The eastern segment of the DMP is dominated by uplifted basement bounded by rifting basins with rotated fault blocks at both the northwest and southeast sides, showing structural features of a divergent margin. Absence of the syn-rift sequences upon the uplifted basement, the presence of the paleo-anticlines within the basement, and the relatively thicker crust collectively suggests that the uplifted basement was a crustal fragment with lower degree of crustal stretching during the rifting phase due to the underlying lithospheric heterogeneity most likely caused by Mesozoic subduction. In the western segment of the DMP, the basement is highly deformed by northwest-southeast trending strike-slip faults, showing the lateral variations in the extent of continental necking. Accordingly, this strike-slip fault is likely to indicate that this segment is a structural accommodation to differential degree of crustal rifting, showing a structural feature of a transform margin. The distribution of the unconformities T32 and T2 across the DMP, along with magmatic activities in the lower slope, allow us to identify the uplift event known as the Dongsha Movement. This event may also contribute to the uplifting of both eastern and western segments of the DMP, although the uplift of these segments seems to be differential across the Dongsha Escarpment. Due to the existence of this differential uplifting, we consider that the seaward protrusion of the DMP may be morphologically enhanced by Dongsha Movement, while the contribution of the Dongsha Movement shall be subordinate rather than the that of structural inheritance in earlier rifting phase. Last, there is a significant east-west differences of the DMP in the magmatism during the rifting phase in the northern margin of the South China Sea. In this study, we suggest that the transfer fault zone in the western segment of the DMP may act as a barrier to westward propagation of syn-rift magmatism. A similar case is found in Sao Paulo Plateau offshore Brazil, whereas this is not observed in the Demerara Plateaus in Austral Atlantic and Exmouth Plateaus offshore northwest Australia. The interconnection among marginal plateaus, transfer faults, and the boundaries of magma-poor and magma-rich margins requires further observations and discussions.