| Summary: | 碩士 === 國立中央大學 === 應用地質研究所 === 103 === Lai-Lai wave-cut platform is the geological continuation of Hsuehshan mountain range in the northeastern coast of Taiwan. The Lamprophyric dikes stand in right-stepping en-echelon form with faults, joints and folds in different scales on the platform. The dike formed in 9±1.1 Ma ago and the arrangement of the dikes records the stress state during volcanic activity in the late Miocene. Faults and fractures in the dikes and the country rocks show the strain state of Penglai orogeny since Pliocene, and also response the different deformation behavior caused by the difference of rock strength. In this study, I made detailed structural geologic maps in different scales (up to 1:20), and used unmanned aerial vehicle (UAV) to take orthophotography to display the detailed structural distribution. Electrical resistivity imaging (ERI) profile was performed to decipher the arrangement of the dikes underground and the characteristics of the faults. East-west trending dikes can reach 3 meters in maximum height and 190 meters in total length. 19 dike segments are exposed on the platform and range from 2 to 30 meters in length f. The strata of country rocks orientated north-south, dipping to west 10~20 degree are Oligocene Tatungshan formation composed mainly of argillite. My field observation reveals that N-S-trending right-lateral strike-slip faults which dislocated dikes less than 2 meters are the latest faulting event. After restoring the results from faulting, dikes remain right-stepping en-echelon arrangement in 3 segments at least. Based on field observation and stratigraphic analysis, I conclude that magma ascended along en-echelon joints and solidified into three dikes in ‘Tatungshan formation’ at depth of 2.6 kilometers. There are west dipping reverse faults and vertical right-lateral strike-slip faults in dikes. The strike-slip faults intersect with dikes in an acute angle, which makes the dikes appear as horizontal duplex structures. The structural evolution of dikes in Lailai platform are: ‘Tatungshan formation’ formed in passive continental shelf and en-echelon joints formed in late Oligocene; magma ascended along en-echelon joints and en-echelon dikes formed in late Miocene. After the Penglai orogeny took place in Pliocene, reverse faults and duplex structures took place in dikes. Then the N-S-trending right-lateral strike-slip faults cut the dikes and displaced them. At last, tectonic uplift and unroofing formed extension joints. Dikes are exposed in right-stepping en-echelon form in the platform now.
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