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:::
Introduction
Earlier Geologic Maps of Taiwan
Geographic Setting
General Geology And Geologic Provinces Of Taiwan
Explanation Of Legend And Representation Of Geologic Data
Eastern Central Range
Western Central Range Backbone Ridges
Western Foothills
Eastern Coastal Range
Geology Of The Hengchun Peninsula
Major Geologic Features Of Taiwan
Plate Tectonic Setting
References


:::Eastern Coastal Range
General Description Stratigraphy and Lithology Geologic Structure Geologic History Longitudinal Valley of Eastern Taiwan
Longitudinal Valley of Eastern Taiwan
The Central Range is separated from the Coastal Range by a long and narrow valley between the cities of Hu alien and Taitung in eastern Taiwan, the Longitudinal Valley. It is approximately 150 kilometers long and 3 to 6 kilometers wide, averaging 4 kilometers m width. The valley is the suture between the Eurasian continent on the west and the Luzon Island Arc on the Philippine Sea plate on the east, the two important tectonic elements of Taiwan. This valley is a tectonic through valley drained by three streams with topographically indistinct divides.
The Longitudinal Valley is linear and narrow, with negligible relief, odd drainage patterns, and high seismicity. All these features indicate that the Longitudinal Valley is structurally controlled (T. L. Hsu, 1976a; York, 1976) and related to faulting (Alien, 1962; T. L. Hsu, 1962b). This valley contains an exceptional thickness of alluvium and is fringed with an extensive series of alluvial fans, mostly from streams draining the Central Range. These spectacular morphological features are clearly shown on the geologic map of Taiwan and on aerial photography or satellite imagery.
The fault origin of the Longitudinal Valley has been recognized for a long time. Active faulting is indicated by high seismicity, many historical great earthquakes, and fault scarps that cut alluvial deposits of the valley floor (Alien, 1962). Biq (1965) postulates that the Longitudinal Valley is a ramp valley in accordance with the hypothesis that each side of the valley is bounded by a high-angle upthrust. These thrusts generally had sinistral strike-slip components as well. The fault on the eastern side is called the Coastal Range fault, and is east-dipping; whereas the western side is bounded by the Central Range fault, which is a west-dipping thrust uplifting the Central Range to its present height. These two major tectonic lines have been very active in recent times and are very significant in the Quaternary tectonics of Taiwan. Nevertheless, due to rapid erosion rate and thick alluvial cover, exposures of the actual fault surface can rarely be observed (T.L. Hsu, 1976; York, 1976). The few fault scarps or scarplets and other evidence of faulting are so scattered and dissected that little information about the overall sense of fault displacement can be gained, accounting for the present debate about the exact nature of the fault tectonics in the Longitudinal Valley.
The most powerful proof concerning the nature of fault movement of the Coastal Range fault is that of the major earthquakes of October 22 and November 25, 1951, each associated with well-documented faulting on the surface (T. L. Hsu, 1962b). Both fault breaks are east-dipping upthrusts with a sinistral strike-slip component. A number of other fault scarplets have also been found discontinuously along the trace of the Coastal Range fault between Hualien and Taitung (T. L. Hsu, 1976a). Arguments still exist, however, between geologists and geophysicists on the existence of the Central Range fault along the western margin of the Longitudinal Valley. More investigations are needed to study this intricate tectonic problem.
Judging from the extreme linearity of the Longitudinal Valley, Alien (1962) visualized that the main boundary faults could be largely vertical transcurrent fractures. Recent studies of fracture patterns by Barrier and Angelier (1986) support the idea that the faults along the Longitudinal Valley are mainly thrust faults, under a compressional stress in the direction between 090 and 140, with only twenty percent of the movement representing the component of sinistral displacement. The sinistral movement of the faults is also shown by the offset of streams in the Longitudinal Valley (Biq, 1971; York, 1976). Re-triangulation of the northern part of the Longitudinal Valley (C. Y. Chen, 1974) also shows an average left-lateral movement of 3.65 meters between surveys made in 1971 and 1909 to 1942, a sinistral displacement rate of more than 6 cm/year. The significance of the Longitudinal Valley is further discussed in the last chapter of this text.

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