| 摘要: | 臺灣島為既年輕又活躍，且有良好監測的造山帶，在這樣的條件下提供了觀察及更深入地了解板塊聚合邊界及相關的地震災害機制的機會。在臺灣南部，這造山帶被認為是介於中臺灣弧陸碰撞造山帶與南台灣外海馬尼拉海溝板塊隱沒增積楔形岩體間的過渡。並行地，先前台灣西南外海的研究及近期地殼震波層析影像顯示大陸斜坡邊緣的方位從外海的東北西南向，往南台灣轉變為近乎東西向，強化了板塊被動邊緣的幾何形貌影響台灣造山帶的發育之認知。論及地殼變形的活躍度，台灣西南部為台灣最高應變率的數處之一。充裕的大地測量資料顯示在橫越台南地區的麓山帶具有每年約4.5公分西向的縮短量，往南至高雄的緯度則轉變成西南向的快速大地構造脫逸。然而，調適如此活躍變形之地質構造卻為人瞭解地極其有限。這個計劃的目地即是想瞭解台灣南部的西部麓山帶，現今到更長的幾個百萬年尺度的演育。我將著眼在探究底下的幾個關鍵問題: 南臺灣的西部麓山帶的地體架構為何?現今調適這變形及潛在產生大地震的活動構造為何?在這些鑑識出的斷層及褶皺的長期累積縮短量為何?及這麓山帶過去的這幾個百萬年是如何演變的?為了回答這些問題，我提議利用地表與地下地質資料、生物地層、震測層析剖面及大地測量資料，繪製從台南市到高雄港的近岸相當的緯度區間橫跨西部麓山帶的五幅平衡地質剖面。在這個階段將預期對地質構造及目前活躍的構造在地表及剖面上有全面性的瞭解。下一個階段，我將回復這些剖面以決定剖面上已鑑識構造的長期累積縮短量。生物地層及同期沉積地層的證據，對解析上新世到更新世變形歷史提供了時間上的制約。此計畫的研究成果，預期將對台南高屏地區的大城市之地震災害評估極其重要，也將對更長時間尺度的研究問題有所貢獻，如弧陸碰撞造山帶與海溝板塊隱沒間的過渡帶是如何地運作，及板塊被動邊緣的幾何形貌如何影響台灣造山帶的發育，亦或跨南台灣從東部外海到西部變形前緣的總縮短量與板塊大地構造重建的暗喻等。 ;Taiwan is an actively growing and well-monitored orogen that provides opportunities to observe and better understand the mechanisms at converging plate boundaries and the associated seismic hazard. In southern Taiwan, the orogen is considered as a transition between the product from an arc-continent collision, as observed in central Taiwan, and an accretionary wedge associated to a subduction zone, as observed offshore southern Taiwan along the Manila Trench. In parallel, earlier studies conducted offshore southwestern Taiwan and recent seismic tomography imaging of the crust showed that the orientation of the continental shelf edge evolves from NE-SW offshore to nearly E-W beneath southern Taiwan, reinforcing the idea that the passive margin geometry influenced the development of the Taiwan mountain belt. Regarding active deformation, southwestern Taiwan is among the places with the highest strain rates in Taiwan. A wealth of geodetic data shows about ~4.5 cm/yr of westward shortening across the foothills of Tainan, evolving southward to a rapid southwestward tectonic escape at the latitude of Kaohsiung. Yet, the geological structures that accommodate this deformation remain poorly understood.The goal of this proposal is to understand how the Western Foothills of southern Taiwan evolved with time, from the present day to the longer timescale of a few million years. The key questions I aim at answering are as follows: What is the architecture of the Western Foothills of southern Taiwan? What are the active structures that currently accommodate deformation and that would potentially generate large earthquakes? What is the cumulative long-term shortening on the identified faults and folds? How did the foothills evolve at the scale of a few million years? To answer these questions, I propose to construct five balanced geological cross-sections across the foothills, based on surface and subsurface geology, nannostratigraphy, tomography, as well as geodesy, from the latitude of Tainan to the near-shore of Kaohsiung harbor. The expected outcome of this step is a comprehensive understanding of the geological structures and of the currently active structures in map view and in sections. In a second step, I will restore the cross-sections to determine the long-term cumulative shortening on the structures identified on the cross-sections. Biostratigraphy and evidence for syntectonic sedimentation are expected to provide time constraints to decipher the deformation history during the Plio-Pleistocene. The expected outcome of this project participates to local seismic hazard assessment in a densely populated area and to longer-term research questions such as how the transition from arc-continent collision to subduction has been operating, how the passive margin geometry influenced the development of the mountain belt, or how much the total shortening across southern Taiwan is, from offshore eastern Taiwan to the western deformation front, and implications for plate tectonics reconstructions. |
