Major Orogenies and Crustal Deformation
The orogenic movements in Taiwan have been discussed in many published papers and a large number of orogenic
phases have been proposed by different authors. Many of these suggested movements have not been well
documented, and some are only conjectural. Others are represented merely by volcanic effusion of limited
extent. With the exception of possible crustal movements in the metamorphic complex, which are still
imperfectly known, only three orogenic events are really significant to the geologic evolution of Taiwan.
They are the Nanao Orogeny, the Miocene/Eocene movement in the slate series, and the Penglai Orogeny. Other
local stratigraphic and structural breaks are small scale and of limited tectonic significance, and will not
be discussed in this chapter.
OROGENIES IN THE TANANAO METAMORPHIC COMPLEX
A complex history of orogenic and metamorphic events is recorded in the Tananao metamorphic complex as
discussed in detail earlier. These rocks must have experienced more than one phase of deformation and
metamorphism during their development. However, many details of the pre-Tertiary tectonic history are still
poorly known due to lack of stratigraphic and structural data, and especially of critical radiometric dates.
All the orogenic phases suggested for the Tananao metamorphic terrane are somewhat conjectural in the
general absence of reliable evidence.
The chloritoid-rich rocks in the marble and greenschist of the metamorphic complex in the Liwuchi deserve
special mention. They are considered a possible metamorphosed aluminous lateritic deposit by P. Y. Chen and
Liou (1979). Chen and Liou postulated that the chemical properties and mode of occurrence indicate that the
chloritoid rocks unconformably overlie the marble and greenschist. The distribution of the chloritoid rocks
is very limited, however, and it is difficult to depict the exact nature, extent, magnitude, and age
significance of this possible structural break in the basement complex.
NANAO OROGENY
This inferred orogenic movement is one of the most notable orogenic events in Taiwan, the diastrophism
affecting the pre-Tertiary metamorphic basement before the deposition of the Tertiary cover of
argillite-slate sequence. This is the main orogeny during which the pre-Tertiary geosynclinal rocks were
tectonized to form the metamorphic basement of the Tertiary fold-and- thrust belt on the Eurasian plate.
The major movement is inferred to have taken place in late Mesozoic time and is called the Nanao Orogeny.
A more elaborate discussion of this movement has already been given in the chapter on metamorphic basement.
OROGENIES IN THE ARGILLITE-SLATE SERIES
The study of orogenic events in the argillite-slate belt is most controversial and challenging. Although
several orogenic episodes of different ages have been suggested by various workers, only one orogenic
episode can be validated at present on the basis of extended mapping. This is the structural and
stratigraphic break between Eocene and Miocene slates in the Backbone Range belt, supported by the absence
of Oligocene strata between the two.
The N conglomerate was named by L. S. Chang (1972) as marking a possible Oligocene hiatus between Miocene
and Eocene in the slate belt. At the type locality Nantawushan on the western side of the southern Central
Range, Eocene Discocyclina- limestone structurally overlies Miocene slates in an inferred inverted sequence.
The absence of Oligocene fossils was used to indicate the Eocene/Miocene break. At the type section, no
conglomerate has been found at the contact although limestone blocks containing Discocylina species are
scattered on the mountain slopes. Thus physical evidence of the N conglomerate and of the Miocene/Eocene
hiatus is very weak at the type section where overturning of the strata without faulting must first be
established to substantiate this unconformity. If the N conglomerate can be validated by future studies,
this conglomerate may mark the hiatus between Miocene and Eocene or between Miocene and pre-Tertiary
basement in the slate belt.
Another faunal gap was cited by T. C. Huang (1980) as indicating an un- conformity between Eocene and
Miocene or late Oligocene rocks in the slate belt. His study of calcareous nannofossils along the southern
cross-island highway yielded biostratigraphic data supporting an Eocene/Miocene or Oligocene hiatus in
which late Oligocene transgressive strata (Likuan Formation) unconformably overlie the Eocene strata
(Pilushan Formation). This unconformity, if present, could be related to the Eocene/Miocene hiatus reported
by L. S. Chang (1972) and be considered part of the same orogenic event.
From paleontologic evidence, a Eocene/Miocene or Eocene/Oligocene break may occur in the argillite-slate
belt. The proposed orogenic phase has no formal name yet but may represent one of the major crustal
movements in Taiwan.
PENGLAI OROGENY
The orogenic climax of the Cenozoic movement in Taiwan is in the Plio-Pleistocene. This diastrophism is
named the Penglai Orogeny and is the most important and widespread one in Taiwan. This orogenic episode
began in western Taiwan near the end of the Pliocene when thick conglomerate accumulated in many places in
the subsiding basin of western Taiwan. Much of the deformation was late in early Pleistocene time and
reached its climax in the middle Pleistocene. During the climax the western geosynclinal basin collapsed and
folded and faulted mountain chains gradually emerged. The post-orogenic sediments are represented by late
Pleistocene lateritic to non-lateritic gravel beds that lie unconformably on the eroded surface of all
deformed older rocks. This angular discordance is the only distinct and conspicuous unconformity that can be
observed in the Cenozoic rocks of Taiwan.
The Penglai Orogeny is not restricted to the Tertiary rocks in western Taiwan, but affected eastern Taiwan
and the metamorphic complex as well. The pre-Tertiary basement may thus have been remobilized. In plate
tectonic interpretation, this strong orogeny was caused by the gradual collision between the Luzon Island
Arc and the continent of Asia during the on-going convergence of the Eurasian and Philippine Sea plates.
LATE PLEISTOCENE TO RECENT CRUSTAL MOVEMENTS
After the climax of the Penglai Orogeny in the western foothills, mild and regional uplifting, block
faulting, broad warping, and regional tilting characterize the late Pleistocene tectonic history, indicating
that crustal deformation continues in Taiwan. The Pleistocene orogeny has not yet ended as shown by the
frequent earthquake tremors and local folding and uplifting of the tableland gravel on some places.
The most significant tectonic event of recent time is the strong vertical movement which has taken place
from late Pleistocene to the present. The tectonic uplift rate in Taiwan is one of the highest in the world.
Although several estimates of the Holocene uplift rate of Taiwan have been reported based mainly on
radio-metric dating of raised coral reefs, the most up-to-date one is presented by Peng and others (1977).
Their reported average uplift rate over the last 9,000 years in southern Taiwan and the Coastal Range is in
the range of 5+0.7 mm/year. Rates of uplift in the northern coastal area have been lower recently: from 5.3
mm/year (8500 BP to 5500 BP) to 2 mm/year (5500 BP to 1500 BP). The rates in northern Taiwan are similar to
those of the Ryukyu Islands. The high uplift rate is consistent with the island arc collision and high
seismicity in eastern Taiwan. The study of Li (1976) shows that the average erosion rate of the Central
Range is about 5.5 mm/year. Therefore, because the uplift rate and the erosion rate are balanced, the net
change of altitude of the mountains in Taiwan may be negligible.
Taiwan lies in a mobile belt of young and on-going tectonic activity. A continuation of crustal mobility
from the Plio-Pleistocene orogeny to the present is manifest. The frequent occurrence of earthquake shocks
also indicates crustal unrest up to recent time. After repeated earthquakes, surface faulting has been
observed in many places (Alien, 1962; Biq, 1976; Bonilla, 1977; Hsu and Chang, 1979). Quaternary faults are
mostly west-vergent thrusts in Taiwan. Both NNE left-lateral and WNW right-lateral strike-slip faults are
also developed. Some of the Quaternary faults in the hilly and mountainous areas of western Taiwan could be
detachment faulting from gravity tectonics (Bonilla, 1977; Hsu and Chang, 1979); but a deep- seated regional
stress system predominates in the coastal plain area (Bonilla, 1977). Recurring displacements along these
existing ruptures are also quite common.
|
