臺東鹿野溪巨視折彎背斜之構造分析與機制探討

Taiwan is a classical arc-continent collision orogen due to Cenozoic oblique convergence between the Eurasian and Philippine Sea Plates. Its basement, the Tananao Metamorphic Complex (TMC), is expected to be experienced various orogenic events, and could still obtain the relic structures corresponding to the previous orogenic metamorphic event, especially the rock of Yuli belt is believed to record the high pressure metamorphism. Although recent results from the metamorphism and chronological dating suggest that somehigh pressure terrane in the Yuli belt of TMC could form during Cenozoic subduction and exhume to the surface by arc continent collision, so far no distinct structures can explain how the high pressure exhume back to the surface. In order to avoid the possibility of pre-Cenozoic relic structures in the TMC, this study will mapping the Cenozoic slate belt along the Luyeh creek of Taitung for deciphering the possible structures and exhumation mechanism corresponding to the exhumation and uplift processes of high pressure terrane. Preliminary results indicated that obvious mesoscale folds, which folded cleavages, were found around the area between Hongye hotspring and Songfeng bridge at the Luyeh creek of Taitung. Those meso-folds are not commonly ZMS-type parasitic folds but they are more like the Christmas-tree folds slipped downward along the limbs of macro-anticline. Some crenulation cleavages developed parallel to axial plane of meso-folds. Quartz vein and crystals were filled within the fractures parallel to the subvertically axial plane of macro-anticline. Also, listric normal faults cut both limbs of macro-anticline. These observations illustrate that this macro-anticline is like the bending dome structure, not the buckling fold. Therefore, this study will focus on (1) detail field mapping, including attitude measurement of bedding, cleavage, and lineation, geometry parameter collection of meso-folds and en echelon veins; (2) fold analysis based on fold geometry parameters; (3) strain analysis based on orthogonal photograph of folded layers; (4) analysis of 3D resistivity pattern from magnetotelluric results. Based on works and information conducted by this study, the characteristics and origin of this macro-doming anticline can be evaluated and furthermore the possible exhumation and uplift mechanisms of high pressure terrane in the Yuli belt can be investigated. At the same time, the evaluation and understanding of this doming marco-anticline can shed light on geothermal model of Hongye hotspring for the exploration and development of geothermal energy. Preliminary results displayed that vertical shortening across the Tailuko belt, Yuli belt and Eastern Slate belt did not showed distinct difference, suggesting that the folds associated with sub-horizontal axial surface could be formed by the late stage of exhumation along the eastern Central Range. Therefore, the doming structures in the Luyeh drainage could be developed in the late stage of exhumation and associated fractures and normal faults could be fluid conduits of geothermal fluid and one of mechanisms for deep-seated landslide.