臺灣東北部外海的震測地層學與正斷層構造分析

Abstract

臺灣北方海域隨著臺灣造山帶逐漸向南遷移，自碰撞造山進入由伸張系統主導的垮山階段，而形成了一系列東北 西南向的正斷層系統。為更深入了解臺灣北方海域正斷層系統於垮山時期之構造特徵及其發育過程，本研究將利用臺灣東北外海已發表的剖面、重新處理的剖面和新收集的多頻道反射震測資料，對正斷層分布進行重新檢視，同時利用各斷層滑移量隨時間的分布變化對正斷層進行構造分析，以了解斷層在生長過程各階段的活動性，並有助斷層是否具備持續延伸的潛勢評估。本研究透過震測資料辨識出三個層序邊界（盆地基盤面PRSB及上覆層序中的PU1及PU2），以及九個主要的東北 西南向正斷層（外山腳斷層1、外山腳斷層2、外崁腳斷層、外基隆斷層2、外基隆斷層1、外台北斷層、外龍洞斷層、外蚊子坑斷層以及外三貂角斷層），並藉由三個層序邊界的構造等時圖，劃分出陸棚區、近岸斷塊區、離岸地塹區及地塹以東地區共四個構造區。再透過正斷層之滑移量-斷層跡線距離（T-x）等構造分析，我們發現九條斷層發育以南遷為主（外山腳斷層2、外崁腳斷層、外基隆斷層2、外基隆斷層1、外台北斷層、外龍洞斷層），少部分原位（in situ）發展（外山腳斷層1、外三貂角斷層），亦有少部分往北向發展（外蚊子坑斷層）。本研究區域構造演化可劃分為四個階段。第一階段（2.6 Ma之前）處於壓縮系統主導的逆斷層構造環境；第二階段（2.6 Ma之後）時轉為伸張系統，造成PRSB開始沉降且臺灣北方海域轉為正斷層系統開始發展；第三階段（約2 1 Ma）或因沖繩海槽再次向西延伸，產生新一期斷層活動並形成PU1，此期正斷層活動較前期趨緩；第四階段（1 Ma至今）為最晚近期火山活動，正斷層活動再次活躍，形成PU2並呈現明顯的南向遷移趨勢。然而關於正斷層發育的精確時序，仍需更多近岸海域地層年代與分布以進一步進行驗證與討論。

The collapsed orogenic belt in northern Taiwan experienced stress changes from a compressional to an extensional environment, showing a series of normal fault structures offshore northern Taiwan. To better understand the structural features and development of the normal fault system, we revisit the distribution of offshore normal faults by newly collected multi-channel seismic (MCS) reflection data, combined with published and reprocessed archived MCS profiles northern offshore Taiwan. Furthermore, we take advantage of structural analyses on the basis of the fault throw distribution over time to understand fault activities in each stage and to estimate the possible future extension of these faults. In our seismic dataset, 3 seismic sequence boundaries(the lowermost PRSB, and PU1 and PU2 within the overlying sequence) and along with 9 major NE–SW-trending normal faults including Outer Shanchiao Fault 1 (OSAC1), Outer Shanchiao Fault 2 (OSAC2), Outer Kanchiao Fault (OKAC), Outer Chilung Fault 2 (OCHG2), Outer Chilung Fault 1 (OCHG1）, Outer Taipei Fault (OTP), Outer Longdong Fault (OLD), Outer Wentzukeng Fault (OWTK), and Outer Sandiaojiao Fault (OSDJ). Based on the structural contour map of the seismic sequence boundaries, 4 distinct structural regions are recognized, including the Shelf Zone, the Nearshore Fault Blocks area, the Offshore Graben area, and the East of the Graben. The fault throw to distance along the fault trace plots (T-x plot) reveal that most of the observed faults migrated southward (OSAC2, OKAC, OCHG2, OCHG1, OTP, OLD). Only few faults developed in situ (OSAC1 and OSDJ) and one (OWTK) migrated northward. Then, a 4-stage tectonic evolution model of the northern off Taiwan is proposed. The first stage (2.6 Ma~) was characterized by a compressional regime and associated reverse fault structures. In the second stage (after 2.6 Ma), the stress regime changed from compression to extension, resulting in the PRSB to subside and the normal fault system in the northern offshore Taiwan to initiate. During 2–1 Ma, northern offshore Taiwan was influenced by westward extension of the Okinawa Trough, leading to a new phase of faulting and the formation of PU1; however, fault activities during the third stage were more reduced than previous stages. The fourth stage (1 Ma to present) was marked by the most recent volcanic activities and reactivated normal faults, showing a pronounced trend of southward migration. However, the precise ages of normal fault development remain uncertain and future investigation of the nearshore stratigraphic distributions are required.