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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