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| :::Major Geologic Features Of Taiwan |
Major Structural Features
Taiwan lies among the islands that festoon the western border of the Pacific Ocean, and exhibits structural
features characteristic of an island arc. The main structural pattern of the island is that of an elongated
arc convex toward the Asiatic continent, having a much longer south arm and a short northeast arm. The
Taiwan arc is bounded on the east by the Philippine Sea basin and on the west by the old massif of Cathaysia
that forms the southeastern part of the Asia continent. In contrast to most island arc structures in the
western Pacific, the Taiwan arc is convex toward the west or the China mainland and concave toward the
Pacific Ocean. This westerly convex arc is situated between the southeasterly convex Ryukyu arc to the north
and the Luzon arc to the south. The tectonic significance of the arc structures will be discussed in the
chapter on plate tectonics. The structural pattern and major structural lines in Taiwan conform closely to
this arcuate structure of the main island. The rocks generally trend east-northeast along the northern short
bend of the Taiwan arc, and strike north-south or north-northeast along the major south arm of the arc. On
the west, the Penghu Island Group and the coastal plain in western Taiwan constitute the shelf area or the
foreland of the western Tertiary sedimentary basin. The Penghu Island Group is covered by flat-lying Neogene
sediments and plateau-type basalt flows. The thickness of the Neogene sequence is much less than that found
to the east. Deformation is so mild that over most areas the sediments appear to lie nearly flat. Under the
alluvial cover on the western coastal plain, the Neogene sub-surface beds display a regional tilting toward
the east with very gentle folding. The records of oil exploration, however, have discovered many transverse
and longitudinal faults in the subsurface. Both normal and reverse faults are known. On the east and to the
north of the coastal plain, the rolling hills and high terraces are underlain by gently folded younger
Neogene and early Pleistocene rocks that still show the characteristics of foreland structural features.
This open and gently folded area is bounded on the east by a series of prominent thrust faults. These are
the major sole thrusts marking an abrupt change in style of deformation from the foreland into the orogenic
belt. East of these thrusts, strongly folded and faulted strata constitute the main part of the western
foothills.
The western foothills are built of sedimentary rocks of Oligocene to Neogene ages. These rocks have been
folded into a succession of synclines and anticlines, one crowded against the next. In general, synclinal
folding is more dominant than anticlinal folding and asymmetric folding prevails with axial planes dipping
toward the southeast. Faults are abundant and often form the structural boundary of most major folds so that
unbroken folds are few. Many of the faults are low-angle imbricate thrusts that have carried sheets of rocks
for some distance northwestward over strata beneath. The sediments in the western foothills underwent
thin-skinned or decollement deformation with no marked metamorphism or plutonism.
East of the foothills region rise the mountains of the Central Range. The sub- metamorphic argillaceous belt
is encountered first on proceeding toward the east. The structural style of the western foothills continues
into the argillite-slate belt. Both open and tight folds are represented, dissected by a number of
longitudinal thrust faults. The predominant argillaceous rocks and intercalated sandstone beds were thrown
into broad folds with accompanying small-scale complex folds and faults. Folding in the slightly
metamorphosed shaly sediments is mainly by shear and the rocks have been deformed mostly by laminar flow
with extensive develop- ment of slaty cleavage.
East of the submetamorphic belt lies the crystalline metamorphic complex of the Central Range. This complex
is made up of various kinds of pre-Tertiary metamorphic rocks. These rocks have a highly complex structure
so that it is difficult to resolve them into any orderly system of folding. Much flowage of the rocks and
thickening and tinning of the strata are involved. Faults have generally been deformed and congealed and
their exact traces are difficult to follow. This is a region of repeated regional metamorphism, plutonic
emplacement, and intense deformation.
The Central Range is separated from the Coastal Range to the east by a longitudinal valley. The Coastal
Range on the Pacific border is composed of non- metamorphosed Neogene clastic and volcanic rocks. The core
of the Coastal Range is formed of folded and fractured andesitic volcanic series and thick pyroclastic
rocks, quite characteristic of a volcanic arc. Extensive volcaniclastic rocks overlie this volcanic
sequence. Large-scale submarine slumping formed a typical melange widely distributed in the southern part
of this range. All these rocks were deformed into a fold-and-thrust belt. Thrust faults dip toward the east
and some extend for quite a long distance.
All the major geologic and tectonic provinces of Taiwan are separated from each other by longitudinal faults
that are mainly east-dipping thrust faults (Biq, 1972a). At least some of the thrusts have a strike-slip
component. These faults include the Chuchih fault between the western foothills and the Hsuehshan Range
(Fig. 4), the Laonungchi fault (southern continuation of the Chuchih fault) between the western foothills
and the Backbone Range in southern Taiwan (Fig. 4), the Lishan fault between the Hsuehshan Range and the
Backbone Range of the Central Range in northern and central Taiwan (Fig. 4), and the Central Range fault
and the Coastal Range fault on the west and east sides of the Longitudinal Valley in eastern Taiwan.
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:::© CENTRAL GEOLOGICAL SURVEY, MOEA
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