| Summary: | 碩士 === 國立臺灣師範大學 === 地球科學系 === 105 === The island of Taiwan is an active orogen resulting from the oblique collision between the Philippine Sea Plate and the Eurasian Plate, and is located at the conjunction between two subduction zones of opposite vergence that making various geological structures in Taiwan. The Coastal Range of eastern Taiwan is the accreted material composited of Luzon arcs and surrounding basins onto the Eurasian crust. To investigate the deformation pattern across the Chimei fault more precisely, this study analyzed oriented samples along the Hsiukuluan River via anisotropy of magnetic susceptibility (AMS) and paleomagnetism. It has been demonstrated that the orientation and shape of the magnetic susceptibility ellipsoid can give reliable and important information about the deformation and the tectonic history of a rock.
The Chimei fault is the only major high-angle reverse fault across the entire Coastal Range and is also a typical lithology-contrast fault thrusting the volcanic Tuluanshan Formation of Miocene over the sedimentary Paliwan Formation of Pleistocene. To decipher the deformation pattern and history across the Chimei fault, this study collected oriented samples of I-V domains in the hanging wall and of A-C domains in the footwall and conduct a series of experiences.
Results of AMS and magnetic susceptibility ellipsoids show that at the zone Ⅴ, AMS indicates northwest-southeast compression and belongs to the oblate ellipsoid of Type 2. K1 orientations of magnetic ellipsoids indicate N-S compression at Zone Ⅳ and the AMS is oblate ellipsoid of Type 2. AMS at Zone Ⅱ belongs to oblate and prolate ellipsoid of Type 2 -3. At Zone I, AMS belongs to oblate ellipsoid of Type 5 with the strongest strain among all domains. Magnetic susceptibility ellipsoids with strong deformation came from the deformation of secondary fault developed after the Chimei fault.
Results of the hysteresis loop and temperature-function magnetic susceptibility experiment illustrate that ferromagnetic mineral is main contribution to magnetic susceptibility in the samples across the Chimei fault and furthermore magnetite is major magnetic carrier. The hysteresis loop also indicates the PSD is the main domain size of magnetite. As a result of this study, magnetic mineral and domain size do not have strong influence to the distribution of magnetic susceptibly ellipsoids. The AMS and magnetic fabric can be treated as the representative of the finite deformation.
Magnetic fabric at the foot wall of Chimei fault is strongly affected by the plate convergence. The Luzon arc rotated clockwise about 30 degree by result of Lee et al., 1991 after collision that cause NW-SE compression direction of Zone Ⅴ and N-S compression direction of ZoneⅣ to ZoneⅠ. Also, it caused the rotation of bedding strike from N-S direction of Zones V to E-W direction of ZoneⅣ to ZoneⅠ. The secondary fault at Zone III developed after the formation of Chimei fault. No clear deformation trend inferred from magnetic fabrics is observed in the hanging wall due to the strong igneous rocks.
This study demonstrates that AMS pattern across the Chimei fault is the consequence of finite deformation. Our results of evolution of magnetic fabrics across the Chimei fault can provide insights into understanding the tectonic development of Chimei fault.
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