Structural Features and Orogenic Events
The pre-Tertiary basement terrain involves a number of different tectonic and metamorphic stages as
delineated by recent studies (Y. Wang, 1979; Ernst et al., 1981; Stanley et al., 1981). The effects of
multi-stage recrystallization and deformation are shown by the divergence or superposition of foliation
trends, incompatible mineral associations, and folding of schistosity and axial surfaces in the basement
rocks. The presence of conglomeratic schists may also attest to different phases of orogenic pulsations.
Thin pods and lenses of chloritoid rocks in the foliated marble and greenschist of the metamorphic complex
represent metamorphosed lateritic soils (P. Y. Chen and Liou, 1979). The soils may indicate a stratigraphic
break or unconformity on top of the marble and greenschist series.
The various metamorphic rocks in the basement have been deformed into a number of open, asymmetrical, or
overturned folds and intricate flow folds after different phases of orogenic deformation and metamorphism.
Because the stratigraphic sequence of the rocks remains obscure, it is difficult to resolve these highly
deformed rocks into an orderly pattern of folding and faulting. According to Yen (1967), however, the
surface structures of the Tailuko belt are characterized by open folding, whereas those of the Yuli belt are
dominated by overturned folds with eastward vergence. Faults older than the regional metamorphism have been
largely obliterated by later deformation, whereas faults that post-date metamorphism are readily recognized.
These faults can be inferred at abrupt changes in lithology, orientation of foliation, or topographic relief.
Tracing these faults across the whole metamorphic belt, however, is often handicapped by lack of key beds and
by inaccessibility. Consequently no thorough-going faults of significant magnitude have been documented in
the metamorphic complex, although some may exist.
In the northern part of the metamorphic complex, bedding and schistosity are nearly parallel, and trend
generally east-west. The strike deviates locally to east-northeast, but more often to west-northwest. Dips
of foliation and bedding vary considerably over wide areas. South of the Ilan-Hualien border along the
Hopingchi stream (Tachoshuichi), major structural trends in the schists rotate to the northeast and
north-northeast, in general conformity with the arcuate structural pattern of the Taiwan orogenic belt. Dips
vary from 20o to
65o both to the northwest and the southeast.
The precipitous rocky cliff along the eastern coast between Suao and Hualien is interprated to be a fault
scarp with the west side up.
Deformation of the metamorphic complex must have taken place at great pressures and temperature with abundant
magmatic fluids. No single plutonic process and no single structural episode could be responsible for the
formation of this metamorphic complex. It is likely this is the result of the sum of many structural and
plutonic processes, evolving over a long period of time.
A network of multiple fold generations and axial-plane foliations was established in recent field mapping
and structural analysis in some parts of the metamorphic complex by Y. Wang (1979) and Stanley et al.
(1981). The tectonic significance and ages of these diverse foliations and folds remain uncertain. They
could be related to differences in position within major folds, original differences in lithology, or
differences in degree of metamorphism. Some of the divergent trends could be the result of one single
movement or they may indicate different orogenic pulses. Y. Wang (1979) attempts to associate each foliation
trend with some phase of reported orogeny in the geological literature. The correlation of foliation trend
with orogenic phase remains largely speculative, however, for lack of positive evidence, especially
radiometric dates for the rocks involved. A more detailed classification and delineation of the different
orogenic events in the metamorphic complex is therefore quite difficult. Thus all the orogenic phases
suggested to date for the Tananao metamorphic terrain are conjectural to some extent.
The chloritoid-rich rocks in the marble and greenschist of the metamorphic complex in the Liwuchi may
represent the end of an orogenic phase. They are considered a possible metamorphosed aluminous lateritic
deposit. As mentioned above, P. Y. Chen and Liou (1979) postulated that the chemical properties and mode of
occurrence of the chloritoid rocks indicate that they are a metamorphosed old soil deposit developed at an
unconformity over the marble and greenschist. The distribution of the chloritoid rocks, however, is very
limited. It is still difficult to depict the exact nature, extent, magnitude, and age significance of this
postulated structural and stratigraphic break in the basement complex. It may represent the unconformity
contact suggested by Wang-Lee between the Changchun Formation and the Chiuchu Formation.
One of the most notable orogenic events in Taiwan is the Late Mesozoic Nanao Orogeny, the diastrophism affecting
the pre-Tertiary metamorphic basement before deposition of the Tertiary argillite-slate cover sequence. This
is the main orogeny during which the pre-Tertiary rocks were tectonized to form the metamorphic basement
of the Tertiary fold-and-thrust belt on the Eurasian plate. The Nanao Orogeny, named after the village of
Nanao on the northeastern coast of Taiwan, may correspond to one phase of the important Yenshan orogeny in
mainland China.
There is no argument about the existence of a major unconformity between the metamorphic basement and its
subjacent Tertiary cover series. There is current controversy, however, over whether this unconformable
contact has been overprinted or obscured by a later thrust fault, and whether this fault, if present, is of
local or regional significance. Yen et al. (1956) and Suppe et al. (1976) demonstrated an unconformable
contact between the metamorphic basement and the slaty cover rocks at Suao on the northeast coast and at one
locality on the central cross-island highway. The evidence includes the presence of quartz-bearing basal
conglomerate, truncation of units at the contact, and multiple folding in the basement in contrast to
single- phase folding in the cover rocks. The basal conglomerate is called the "M Conglomerate" by Yen and
others (1956) to manifest the unconformity of the Nanao orogeny. However, many other workers have pointed
out that this unconformity has been largely overprinted and masked by a later thrust fault in most places
(Tsan, 1977 Chao-Hsia Chen, 1979a; Hashimoto et al., 1975). A discussion of this debate is given in the text
"Tectonic evolution of Taiwan" (Ho, 1982) and will not be discussed in detail here. Field evidence of the
basement/slate unconformity is rather obscure, as the older schist series structurally overlies the younger
slate series in general, both dipping in the same direction along a large extent of the contact. In some
areas, shearing, recrystallization and fabric reorientation may have wiped out the evidence of unconformity
and may have created a false semblance of conformity. It is fairly certain, however, that the thick slate
series that constitutes the main part of the Central Range is unconformable with respect to the metamorphic
basement. The thrust faulting along the contact is a secondary tectonic feature superimposed on the
unconformity, which can be observed only where it has not been obscured by later faulting.
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