| 摘要: | 台灣西南部二仁溪地是一個變形活躍且有抬升現象的動態區域，這些現象主要集中在幾個逆斷層的下盤。本研究旨在了解驅動這些現象的地質過程。地主要由晚中新世至早更新世的古亭坑泥岩（Gtk）組成，其厚度超過4000公尺。細顆粒的泥岩加上巨大厚度的條件下，在地底深處可能產生異常的壓力導致泥岩的塑性變形（頁岩構造）。本研究區域主要的斷層包括旗山斷層、龍船斷層、古亭坑斷層和小崗山斷層，這些斷層均向東傾。地球物理數據顯示，沿海平原存在著褶皺構造，如中洲背斜等。大地測量結果亦表明，二仁溪流域中游地區的擠壓速率約為每年30毫米。另外，干涉合成孔徑雷達（InSAR）、水準測量數據和全新世侵蝕速率的證據也支持該地區明顯抬升的說法。值得注意的是，高度變形的地方集中在古亭坑斷層下盤處。為了解構造抬升的原因，我們訂了幾個研究目標，首先，針對變形下盤上的二仁溪河階進行了放射性碳定年，以量化下切速率。而後，對變形區進行了全面的結構分析。此外，根據急折法(kink method)的概念構建立一個地質剖面，我們發現其下切速率約為每年18毫米和34毫米，揭示抬升速率十分快速。超微化石數據顯示其年齡為上新世（NN15）。由於古亭坑斷層沿斷層跡的上盤年齡並不同，因此我們利用較大範圍的岩層位態來定義超微化石的（NN15）年代位於古亭坑斷層其上盤。透過對古亭坑泥岩詳細的微化石層序分析，我們將其分為三個年代單元：中新世古亭坑層、上新世古亭坑層和更新世古亭坑層。在古亭坑斷層帶的野外調查中，發現一個明顯東南傾的剪切帶，其剪切面位態為080°/51S°。這條東南傾的剪切帶及斷層出現在泥質相，周遭為粉質相，表示有可能發生了數十公尺的位移，推測該斷層為橫切過一個西傾逆斷層的古亭坑斷層。除此之外，基於相似的超微化石年代，剖面繪製龍船斷層和古亭坑斷層來自同一個脫斷面，其深度在3-4公里處，而木柵斷層則從更深的滑脫面（約4.5公里）發育，將中新世古亭坑層推向地表，故上盤有來自較深部的長枝坑層（Mcc）。晚中新世的一個或多個正斷層導致中新世古亭坑層在“古亭坑盆地” 的中沉積，這可解釋為何古亭坑斷層以東的中新世古亭坑層厚度會較厚的原因。沿著龍船山???由北至南存在相同年代的淺海和深海相，支持了正斷層盆地的觀點。基於地層生長不對稱性的，斷層深度達5公里，中洲構造可用斷層相關褶皺 (fault-related fold) 來解釋。本研究中還提出深度為8-9公里處，存有大陸邊緣再活化結構，使得裂谷地層變形，且此構造可能仍然活躍中。此外，由於古亭坑斷層西側的古亭坑泥岩較厚，頁岩構造可能扮演著一定的角色，下盤因擠壓造成褶皺，可以解釋下盤的異常變形的現象。;The Erhjen River in southwestern Taiwan is a dynamic region characterized by active deformation and uplift clustered in the footwall of the several reverse faults. This study focuses on understanding the geological processes driving this phenomenon. The region predominantly comprises the Late Miocene to Early Pleistocene Gutingkeng Mudstone (Gtk), which exhibits an impressive thickness exceeding 4000 meters. The fine-grained composition and enormous thickness may trigger abnormal overpressure at depths, potentially resulting in the ductile deformation of the mudstone (shale tectonics). Major faults in the study area are the Chishan Fault, Lungchuan Fault, Gutingkeng Fault, and Hsiaokangshan Fault, which all dip eastward. Geophysical data reveal the presence of folded structures, such as the Chungchou Anticline in the Coastal Plain. Geodetic measurements indicate that the midstream of the Erhjen River experiences compression at a rate of approximately 30 mm/yr. Additional evidence from InSAR, leveling data, and Holocene incision rates support significant uplift in the region. Notably, the high deformation concentrates along the footwall side of the mapped Gutingkeng Fault. To understand the structures responsible for this pronounced uplift, several research objectives were pursued. Radiocarbon dating of the Erhjen River terraces in the deforming footwall was carried out to quantify incision rates. A comprehensive structural analysis was undertaken in the highly deforming area using bedding and shear zone orientations and nannofossil data. Additionally, we constructed a geological cross-section using the kink method. We found that incision rates from the terraces located along the deforming area, T1a and T2a are 19 mm/yr and 34 mm/yr, respectively, indicating a rapid uplift rate. Our nannofossil age from terrace T2a revealed a Pliocene age (NN15). The age of hanging wall of Gutingkeng Fault is not similar along the fault trace, therefore we utilized broader scale bedding attitude to define that the nannofossil age of Pliocene (NN15) is located on the footwall side of the Gutingkeng Fault. Additionally, using published nannofossil data in the Gutingkeng Mudstone, we classify it into three age units: The Miocene Gtk, Pliocene Gtk, and Pleistocene Gtk. Field observations in the Gutingkeng Fault zone unveiled a prominent southeast dipping shear zone with its shear plane oriented 080°/51S°. This southeast dipping shear zone/ fault exhibited muddier facies adjacent to silty facies and might have undergone a displacement of tens of meters. It is proposed that this fault might be the Gutingkeng Fault which cross-cuts a west-dipping reverse fault. Furthermore, our cross-section revealed that the Lungchuan Fault and Gutingkeng Fault lie on the same detachment at a depth of ~4 km. This interpretation is based on the similar nannofossil age of the strata on the immediate hanging wall of both the faults. Furthermore, the Mucha Fault detaches from a depth of ~4.5 km, thrusting Miocene Gtk to surface and having Changchihkeng Formation (Mcc) of Middle-Late Miocene on its hanging wall. Greater thickness of the Miocene Gtk to the east of Gutingkeng Fault can be explained by the presence of a normal fault/faults in the Late Miocene that led to the thicker deposition of Miocene Gtk in a basin we called as ‘’Gutingkeng Basin’’. The presence of shallow and deep marine facies of the same age along the Lungchuan Ridge from north to south support the idea of normal-fault basin. ChungChou structure can be interpreted as a fault-propagation fold based on the asymmetry of the growth strata and the fault goes as deeper as 5 km. Furthermore, we propose the presence of a continental margin reactivated structure at a depth of 8-9 km that deforms the post rift strata and the foreland sequence and is probably active. Additionally, owing to the thicker Gutingkeng Mudstone to the west of Gutingkeng Fault, the phenomenon of shale tectonics might also be playing a role and the footwall might be folded in response to compression that could explain the anomalous footwall deformation. |
