Cenozoic Plate Tectonic Setting
In Cenozoic time, Taiwan is located on the convergent boundary between the Eurasian plate and the Philippine
Sea plate. Plate interaction is marked mainly by the collision of the Luzon volcanic arc and the Asiatic
continental margin. The Longitudinal Valley of eastern Taiwan is the suture zone between these two plates in
Taiwan. In this plate tectonic event, the Ryukyu Arc is on the east and northeast of Taiwan where
convergence is marked by northward subduction of the Philippine Sea plate beneath the Ryukyu Arc on the
Eurasian plate along the Ryukyu Trench (Figs. 9 and 10). The Luzon Arc System is south of Taiwan. In this
system, convergence is marked by eastward subduction of the Eurasian plate underneath the Luzon Arc on the
Philippine Sea plate (Figs. 9 and 10). Both volcanic arcs extend onto the island of Taiwan. The Tatun
Volcano Group, the Chilung Volcano Group, and the northeastern offshore volcanic islands represent the
western extension of the Ryukyu volcanic arc. On the other hand, the Chimei Igneous Complex and the
southeastern offshore volcanic islands, Lanhsu and Lutao, are all located on the northern extension of the
Luzon volcanic arc. Therefore the Cenozoic geology of Taiwan is closely related to the Ryukyu and Luzon
arc-trench systems.
 |
Figure 9. Cenozoic plate tectonic setting of Taiwan and its environs, showing interaction between
Philippine Sea plate and Eurasian plate and structural relations of Taiwan and Ryukyu Arc and Luzon Arc |
 |
| Figure 10. Schematic block diagram showing continent-arc collision and platc tectonic setting of Taiwan (after Angelier) |
RYUKYU ARC SYSTEM AND TECTONIC DEVELOPMENT OF TAIWAN
The Ryukyu Arc System is east and northeast of Taiwan on the margin of the Eurasian continent. This arc
system can be divided into three tectonic elements from north to south:
| Okinawa Trough |
backarc basin |
| Ryukyu Ridge |
island arc belt including both volcanic arc and non-volcanic arc; volcanoes are exposed only at the ends and are over-lapped by the Okinawa Trough in the middle. |
| Ryukyu Trench |
deep-sea trench |
In northeastern Taiwan, subduction of the Philippine Sea beneath the Ryukyu Arc is marked by an east-west
planar seismic zone about 50 kilometers thick located near Hualien at 24
oN (Tsai, 1978). This seismic zone
may represent a north-dipping Benioff zone of the subducting Philippine Sea plate. It dips about 45o
northward beneath the Ryukyu Arc to a depth of nearly 150 kilometers. The position of this depth is right
underneath the Tatun and Chilung volcano groups in northern Taiwan and the northeastern offshore volcanic
islands. The volcano groups and volcanic islands in northern Taiwan thus represent the western extension of
the Ryukyu volcanic arc and are mostly of Quaternary age.
The Okinawa Trough was formed by backarc spreading. From seismic and morphologic evidence, the Okinawa
Trough extends into the Asiatic continent and the Ilan Plain in northeastern Taiwan could be the western end
of the Okinawa Trough, However, the Ilan Plain is structurally located on the oceanward side of the Ryukyu
Arc System. The volcanic ridges in Taiwan (Tatun and other volcano groups in northeastern Taiwan) thus
overlap or are situated behind the backarc basin (Ilan Plain). This proves that backarc rifting was
initiated and took place along the weak zone of the volcanic belt and a part of the volcanic chain could be
obliterated by the opening of the backarc basin. Volcanoes or active volcanism are, however, well
represented at both ends of this volcanic belt.
The Ryukyu arc terminates about 100 to 120 kilometers east of Taiwan near
122o. It has been explained by Wu
(1970) that an important north-trending, right-lateral, trench to trench transform fault displaced the
subduction zone to the north relative to the rest of the Ryukyus. This transform fault acts as a collision
boundary with the subduction zone displaced to the north on the western side of the fault relative to the
main part of the Ryukyu Trench. This northward-moving segment of the subduction zone may be connected with
the plate boundary in Taiwan near 24o.
Such a tectonic setting indicates that the western part of the Ryukyu
Arc System may currently experience a dextral transform movement near Taiwan. However, Bowin et al. (1978)
indicate that the Ryukyu Trench structure continues up to the steep continental margin of northern Taiwan
without any transform displacement. Recent work by French marine scientists (personal communication) also
shows that the Ryukyu Trench may extend directly to the eastern coast of Taiwan, marked only by a sharp
northward bend of the trench axis near Taiwan.
LUZON ARC SYSTEM AND TECTONIC DEVELOPMENT OF TAIWAN
The Luzon Arc System in the Philippines is south of Taiwan and trends northerly. From west to east the three
major tectonic elements in this arc system are:
| Manila Trench |
deep-sea trench and modem subduction zone. |
| North Luzon Trough |
fore-arc basin. |
| Luzon Volcanic Arc |
active volcanic belt on Luzon island. |
The Manila Trench is a subduction zone along which the South China Sea on the Eurasian plate is subducting
eastward underneath the Luzon volcanic arc on the Philippine Sea plate. The volcanoes aligned northerly on
the Luzon island are the volcanic arc formed by this subduction. Extending north from Luzon, this volcanic
belt continues to the active volcanic islands of Batan and Babuyan in the Bashi Channel between Luzon and
Taiwan. Further northward the Luzon volcanic belt reaches Lutao and Lanhsu, two offshore volcanic islands
southeast of Taiwan and terminates in the volcanic complex of Chimei and Changyuan in the Coastal Range of
eastern Taiwan. Along the entire volcanic belt, volcanism is of Miocene age in the northern extremity at
Chimei, but is gradually younger toward the south and is composed mainly of recent volcanoes on Luzon. Thus
volcanic activity in this belt is progressively younger from north to south. Active volcanoes are now
confined largely to Luzon and the small volcanic islands in the Bashi Channel. No active volcano is known in
eastern Taiwan or the nearby offshore area. This proves that plate convergence may first begin in Taiwan or
the northern part of the Luzon Arc System in the Miocene. It then gradually proceeded southward to the
Philippines where volcanism and plate subduction is still active today.
The Manila Trench in the Luzon Arc System gradually disappears near
21o-22oN.
in the north and entirely
disappears further northward in Taiwan. This is attributed to the fact that plate subduction along the
Manila Trench cannot continue in Taiwan because the Asiatic continental crust in Taiwan is too buoyant to
sink down in contrast to the oceanic crust of the South China Sea. Therefore the Manila Trench is most
likely closed or sealed halfway between Taiwan and Luzon (Bowin et al., 1978). This also explains the lack
of well-defined sea trench, Benioff zone or subduction zone and active volcanic system in Taiwan, which is
quite anomalous in the island arc systems in the western Pacific.
Plate convergence in Taiwan is marked by eastward subduction of the Eurasian plate underneath the Philippine
Sea plate. This is indicated by alignment of the volcanic arc on the east side of the suture zone in the
Coastal Range. When subduction took place in eastern Taiwan in the early stage, the continental margin that
now forms the bulk of Taiwan was bordered on the east by the northern extension of the South China Sea on
the Eurasian plate. Eastward subduction of this oceanic crust beneath the Luzon Arc can be easily
accommodated in plate interaction. As time passed by, however, the South China Sea adjacent to the
continental margin in Taiwan was gradually consumed and the Asiatic margin encountered the north end of the
Luzon Arc. The Asiatic continent was then in direct contact with the Luzon Arc. Due to the strong
continental character of the silica crust in Taiwan, a large part of the continental crust refused to follow
the South China Sea in descending beneath the Luzon Arc. Instead the crust was structurally compressed,
thickened and uplifted to form the lofty Central Range and the island of Taiwan by impact of the two
converging plates. An enlarged accretionary prism was thus formed between the Manila Trench and the Luzon
Trough, composed of abundant west-verging imbricate thrust sheets in the mountain systems of Taiwan.
One recent approach to explain the tectonic complexity is that subduction and plate convergence in Taiwan
may not take place along a simple plate boundary or subduction zone as commonly conceived due to the
difficulty of subducting buoyant continental crust. A broad distributed shear system may have developed
during the arc-continent collision. This shear motion apparently moved westward with time. Finally a broad
zone of deformation was formed rather than a discrete well-defined plate boundary in Taiwan. It is possible
that subduction, collision, and plate consumption may occur across this broad belt of deformation, which may
extend over a distance of more than 100 kilometers from western Taiwan to the eastern offshore region. This
deformation belt could be indicated by the widespread distribution of shallow-focus earthquakes in Taiwan
and surrounding areas. Such a plate tectonic model may explain the anomalous features of Taiwan in the
Cenozoic island systems in the western Pacific region.
|
