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米崙礫岩台地上的橫移斷層系統
Strike-Slip Tectonics in the Milun Conglomerate Terrace
摘要
本文主要以地形與地質調查作爲主要的研究方法，配合地球物理資料的再整理，發現米崙臺地的活動斷層分布形態成一個倒寫的「爪」字。斷層北端由西向東分別爲北埔斷層、米崙斷層、民意斷層與南濱斷層；這些斷層於臺地南端的吳全銜接成一條左移斷層帶。斷層的走向大致與臺地長軸方向平行；均爲兼真左移與逆衝的北北東－南南西向斷層；且斷層的左移分量由西向東逐漸增加。 藉由橫移斷層的平衡模式觀念，提出米崙台地的可能構造史；在主壓應力爲西北－東南向的區域構造應力作用下，臺地兩側發育了北北東向，含有在移分量的邊界斷層。由於臺地東側斷層的左移分量遠大於臺地西倒斷層，因而造成兩斷層中間所夾的楔形塊體向斷層收斂側推擠，形成古米崙臺地。當古米崙臺地形成不久後，新的斷層於背斜軸部生成，再將蓋地切割成東、西兩楔形地塊。如此場景，隨著新生斷層的發育，再度發生。使得米崙薑地上推擠出美崙山與花崗山。由於模形體不斷地向南擠壓，其北端塊體因受南端牽引而形成應力伸張區。此種機制造成夾於兩邊界斷層問塊體產生新的斷層。這些拉張型，西北－東南向之新生正斷層，由北往南掉落，造成位於米崙臺地北方的七星潭正斷層、東民意正斷層，及位於收斂側的西北東南走向，傾角向北的逆衝盲斷層共同實現橫移斷層的體積轉換平衡。
並列摘要
On the Milun terrace is a rhomboid and lens-shaped high-standing terrace in the northem Longitudinal Valley, eastem Taiwan. The geological map of the Milun terrace shows four-braidecccd strike-slip faults converge to the major fault zone in map view. There are the Peipu fault, the Milun fault, the Minyi fault and the Nanbin fault from west to east. It is showing positive flower structure in three dimensions, which is combination of strike-slip and reverse slip on the faults in contractional strike-slip duplex. Each fault increases its left-slip rate from west to east. Because of the different horizontal slip rate, the block between two faults will be compressed and elevated where the fault convergence. All of the faults in the Milun terrace do not all move at the same time; the block between two faults compressed and elevated one by one. Such geometric relations are especially easy to envisage along braided zones of single major strike-slip faults where all slices occur in the Pleistocene conglomerate (e.g. the Milun Conglomerate). Along the northem part of the Longitudinal Valley fault, the faults bounding the slices converge at depth to rejoin the strike of the through-going fault. Four faults meet at depth under the Wuchuan, or presumably end in the lower crust in approaching to low-velocity zone. Such a phenomenon is common and important characteristic of wrench fault.