| 摘要: | 台灣北部造山帶往外海延伸成為以正斷層為主的垮塌造山帶，本研究建立橫跨北部造山帶兩條構造平衡剖面，以釐清台灣造山帶之構造形貌，並藉以推測其構造演化與壓縮量。另外，在海域地區利用新蒐集的多頻道反射震測與變頻聲納迴聲資料，結合前人研究，繪製台灣東北海域構造分佈情形，並探討陸上與海域構造的相關性。最後，本研究由構造平衡剖面與海域構造剖面，整合探討台灣東北海域與陸上臺北盆地等區域從造山至垮山之演變過程。
東北外海的古造山帶因為壓縮應力轉為伸張應力，形成一系列正斷層造成古造山崩垮，形成一系列的半地塹盆地。由震測資料解釋分析後半地塹盆地內所堆積的沉積物厚度由外海向陸地方向逐漸減薄，顯示外海的正斷層是由海域發育並往陸上延伸。震測資料顯示數個構造，由西北往東南，本文將其依序命名為J1背斜、T1向斜、A正斷層、B正斷層、C正斷層、D正斷層、E正斷層、L背斜、F正斷層、G正斷層、H正斷層、I正斷層。研究顯示海域構造與陸上構造高度相關。經構造分析與對比顯示，金山西北外海的J1背斜即為陸上尖山湖背斜外海延伸；海域T1向斜對比跳石向斜；A斷層對比陸上磺溪(正)斷層；B斷層對比崁腳斷層；D斷層對比基隆斷層；E斷層對比台北斷層；L背斜對比鼻頭角背斜；貢寮外海的F斷層對比屈尺斷層；三貂角外海的G斷層對比大礁溪斷層；頭城外海的H斷層對比梨山斷層；宜蘭市外海的I斷層對比濁水斷層。其中A斷層、G斷層、H斷層由外海的震測資料與陸上地質顯示，這些正斷層可能已發育至台灣陸上，其中尤以海域A斷層延伸至陸上的磺溪斷層最為明顯。
綜合研究區域之構造分布及前人研究所得之構造發育年代，本研究將台灣北部構造發育史分為下列五個時期：(1) 6.5 Ma ~ 3 Ma：台灣造山運動造成東北－西南向的新莊-金山、崁腳、台北、新店、屈尺等向東傾的覆瓦狀逆衝斷層，這些逆衝斷層延伸至今日的東北外海，成為造山帶的一部分。(2) 3 Ma ~ 2 Ma：菲律賓海板塊隱沒方向開始反轉，造成北部的壓縮應力消失，逆衝斷層停止發育，開始發育正斷層。2.8~2.5 Ma時，有小規模侵入火成活動。(3) 2 Ma ~ 0.8 Ma：北部外海開始受到南沖繩海槽張裂的影響，張裂性斷層此時開始大量發育。(4) 0.8 Ma～0.4 Ma：較劇烈的張裂與火成活動，於北方外海形成了廣布的地塹盆地以及北方諸島（基隆嶼、花瓶嶼、棉花嶼和彭佳嶼等）；更使得大屯火山群大量噴發。金山逆斷層轉為磺溪正斷層後，新莊斷層也從此時轉為山腳正斷層。(5) 0.4 Ma～0 Ma：台灣北方海域以及台灣北部持續張裂。台灣北部山腳斷層與磺溪斷層活動，形成臺北盆地與金山盆地。大屯火山噴發則趨緩。; The orogenic belt in northern Taiwan extends along-strike and offshore to become collapsed and submerged terrane. This study establishes two structural balanced cross sections across the northern Taiwan mountain belt to clarify the spatial structure distribution, and to deduct structural development and amount of compression for this mountain belt. In addition, with newly collected reflection seismic and chirp sonar data, together with published results, I map out major structures in the offshore area. The spatial relation between offshore and onshore structures is also discussed. Finally, tectonic evolutionary stages from mountain building to mountain collapsing in the study area are proposed according to results from this study and literature.
Offshore northern Taiwan, it is in a state of post-collisional collapse, where collision orogen collapsed as a result of normal faulting. The development of normal faults has brought about the development of a series of half-grabens. Reflection seismic data shows those half-grabens feature a westward-thickening and wedge-shaped sediment package which lying unconformity on top of tilted strata. The underlying tilted beds were deformed during the compressional orogenic phase before late Pliocene. They were then subjected to extension since late Pliocene. Seismic data show that the sediment thickness of those graben infills thins southwardly toward Taiwan from offshore to onshore area. This indicates that normal faults first develop in offshore area and propagate toward Taiwan in a later stage.
Seismic data show a few major offshore structures. From NW to SE, we herein name those structures as J1 anticline, T1 syncline, normal faults of A, B, C, D, and E, L anticline, normal faults of F, G, H, and I, respectively. Study results show that most of offshore structures are spatially correlatable to onshore structures. For examples, J1 anticline and T1 syncline lying NW offshore Chinshan area are the offshore extension of the onshore Jianshanhu anticline and Tiaoshi syncline, respectively; A fault is the offshore extension of the onshore Huanghsi (normal) fault. Faults B, D and E are spatially correlatble to their onshore counterparts of Kanchiao fault, Keelung fault and Taipei fault, respectively. L anticline is the offshore extension of the onshore Pitouchiao anticline. The F fault, offshore Kungliao is spatially correlatble to the onshore Chuchih fault; G fault (offshore Santiaochiao) to the onshore Tachiaohsi fault; H fault (offshore Toucheng) to the onshore Lishan fault; and I fault (offshore Ilan) to the onshore Jhuoshuei fault. Offshore seismic data and onshore geology show that offshore normal faults of A, G, and H may already extend to the onshore region. The offshore A fault connecting to the onshore Huanshi fault is the most prominent onshore-offshore fault correlation and development in the studied region.
The study results and published literature allow this study to propose five tectonic evolutionary stages to depict how the study area evolved from initial mountain building to final collapsing with an emphasis on fault development and igneous activities. (1) 6.5~3 Ma: Initial collision resulted in a series of NE-SW trending, and east dipping imbricated thrust sheets in northern Taiwan, and these thrust slices extend along-strike to the present-day offshore area. (2) 3~2 Ma: Extension commenced in the offshore area because of the flip of subduction polarity. Early volcanism was seen at the Tatung volcano around 2.8~2.5 Ma. (3) 2~0.8 Ma: Rifting of the South Okinawa Trough resulted in the extensive development of normal faults offshore northern Taiwan. Igneous activities are found both in onshore Keelung and Tatun volcanoes and some offshore islets such as Penchiayu, Mianhuayu, and Keelungyu and (4) 0.8~0.4 Ma: Extensive normal faulting and magmatic activities resulted in a series of half-grabens in offshore areas and some main edifices of volcanic islets, such as Keelungyu, Huapingyu, Mianhua, and Pengjiayu, and the main body of onshore Tatun volcano. Onshore Chinshan thrust fault was inverted to become the Huanghsi normal fault and Hsinchuang thrust fault inverted as Shanchiao normal fault. (5) 0.4~0 Ma: Continued extension in both offshore and onshore areas with subdued volcanic eruption of the Tatung volcanoes. Normal faulting on Shanchiao fault and Huangshi fault results in the formation of Taipei and Chinshan half-grabens, respectively. |
