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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

:::Major Geologic Features Of Taiwan
Volcanism and Igneous Activity Major Structural Features Major Orogenies and Crustal Deformation
This chapter summarizes all major processes recorded in the geologic history of Taiwan, including igneous activities, geologic structures, and orogenic movements. These have already been described in considerable detail in the chapters on different geologic provinces; this synthesis is included in order to integrate the effects of all these processes and to emphasize their crucial roles in the geologic development of Taiwan. A comprehensive understanding of the relationship and significance of the various geologic processes that took place in different areas at different times is of utmost importance to unravel the geologic evolution of Taiwan. These processes are therefore synthesized in this chapter based on their relative ages and geographic and geologic distribution.

Volcanism and Igneous Activity
Igneous activity is first recorded in pre-Tertiary times in the metamorphic complex of eastern Taiwan. Mafic volcanic rocks are quite widespread in the original sediments of the complex. They have been largely metamorphosed into greenschist and amphibolite. Part of these rocks could be altered ancient oceanic crust. In the eastern part of the metamorphic complex, large to small blocks of mafic to ultramafic igneous rocks are widely scattered in the schists. Their original lithologic components are basalt, gabbro and peridotite, but these have been altered largely into serpentinites during late metamorphic events. These mafic rocks represent ophiolites emplaced at different times in the metamorphic series. They originated as oceanic crust and form exotic blocks enclosed in the metamorphic complex during various early stages of plate convergence. Granodiorite or granite intruded the rocks in the northern part of the metamorphic complex probably in Mesozoic times. The heat and pressure of the intruding magma converted the surrounding sediments and metamorphic rocks into gneisses.
No large magmatic bodies have been found in the Paleogene to Miocene submetamorphic argillaceous rocks in the Central Range. Small pods and lenses of basaltic tuff, basaltic flows and other minor igneous rocks are, however, present in these sediments, indicating local contemporaneous igneous activity. These volcanic rocks are more widespread in the Hsuehshan Range belt than in the Backbone Range belt of the Central Range. In the Backbone Range belt, small lenses of basalt, diabase or tuff are more abundant in the Eocene slates and phyllites at widely scattered places than the Miocene slates. Due to lack of detailed mapping, small volcanic bodies may not be well known and adequately documented. Volcanic rocks are scarce in the Miocene Lushan Formation of the Backbone Range. Small and irregular volcanic bodies have been discovered in the Wanta Reservoir near Wushe, however, and in the Laonungchi drainage area near Paolai in Kaohsiung-hsien. In eastern Taiwan, volcanic rocks are also found in the Chihpen Formation in the Chihpen hot spring area and this formation is believed to be equivalent to the Lushan Formation. In the Hsuehshan Rang belt, bedded or irregular lenses of volcanic rocks are common in the Tatungshan Formation. The distribution of these volcanic lenses, however, is still poorly known due to lack of detailed mapping. The main lithologic components are basaltic pyroclastic detritus and minor mafic flows in addition to a small amount of other volcanic rocks. They represent contemporaneous volcanic eruption from a number of small openings during the deposition of the Tatungshan rocks. Very minor volcanic lenses have also been found in the Kankou Formation underlying the Tatungshan Formation. In the southern part of the Hsuehshan Range belt, mafic volcanic bodies are found in the Eocene Shihpachungchi Formation and the basal part of the Tachien Sandstone. No detailed petrographic studies or mapping of these rocks have yet been undertaken.
In the western foothills, small-scale volcanism was active in the sedimentary basin from late Oligocene to Miocene times and ended in the Pliocene. In northern and north-central Taiwan, small lenses or irregular bodies of basaltic tuff, tuffaceous breccia and minor basaltic flows are present in all Miocene formations, but their distribution is limited to scattered local areas only. Miocene volcanism occurred in three episodes. The oldest, the Kungkuan volcanic stage in the Yehliu Group, has the widest geographic distribution. The Chiehshih volcanic stage represents volcanic activities in the Juifang Group and is very limited in distribution. Due to the scarcity of outcrops, most geologists are not aware of this stage in Taiwan. The Chiopanshan volcanic stage is the youngest, and includes all the volcanic activities that took place in the Sanhsia Group. The volcanic rocks of this stage are widely distributed in the Taoyuan-Hsinchu areas. South of Miaoli-hsien, Miocene volcanic rocks are poorly developed and are seldom found. Miocene volcanic rocks there have only been reported in subsurface sections and in wells drilled by the Chinese Petroleum Corporation. In southern Taiwan, a few small tuffaceous lenses have been found, mostly in upper Miocene rocks.
The Coastal Range in eastern Taiwan is a volcanic arc containing a large amount of volcanic rocks and their pyroclastic derivatives. The most important igneous mass is the Chimei Igneous Complex, which outcrops also in a small exposure in Changyuan southeast of Chimei. The pyroclastic rocks of the Tuluanshan Formation are also associated with this phase of volcanism. The main constituents of the Chimei Igneous Complex are andesites of different mineralogical compositions and textures. These rocks are mainly of Miocene age. The two offshore islands in southeastern Taiwan, Lutao and Lanhsu, are composed of the same andesites and pyroclastic rocks. Their ages range from Miocene to Pliocene. All the igneous rocks in the Coastal Range are related to the Luzon volcanic arc and will be discussed further in the chapter on plate tectonics.
Pleistocene volcanism is very important in the evolutionary history of Taiwan. Two Pleistocene volcanic centers are known on the northern coast, the Tatun and Chilung Volcano Groups. Andesites constitute the main lava flows in these two volcano groups. The Tatun volcanoes are more mafic and contain some basaltic flows, whereas the Chilung volcanoes are more acid, composed mainly of dacite or quartz andesite. The five islands off northeastern Taiwan, Pengchiahsu, Mienhuahsu, Huapinghsu, Chilungtao and Kueishantao, are all volcanic islands formed of either andesite or dacite. They are ascribed to the same volcanic belt and stage as those in northernmost Taiwan. These represent the western end of the Ryukyu volcanic arc, and will be discussed later. On the Penghu Island Group in the Taiwan Strait, Pleistocene fissure eruptions produced a widespread flood basalt that covers 63 islands and islets. This basaltic volcanic stage may occur at the end of the main orogeny in Taiwan. Recent unpublished studies, however, have reported that the Penghu basalt may be of Miocene age on the evidence of K-Ar dating and the contained foraminiferal fossils.

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