General Structural Features
The Neogene rocks in the western foothills have been stacked up by a combination of folds and thrust faults
during the Pleistocene orogeny. The rocks are not metamorphosed and no deep-seated intrusion is exposed.
Tight and asymmetric folds and low-angle thrusts are particularly predominant in the eastern part. Thrust
sheets have slid a considerable distance toward the west or northwest. The intensity of deformation
decreases from the eastern foothills toward the western rolling hills and terraces, where gentle and open
folds prevail. Faults also decrease in number and magnitude toward the west. The western foreland area
underlying the coastal plain has been affected mostly by epirogenic movements and mild stresses whose
intensity also decreases westward.
The Tertiary strata of the western foothills have been flexed into folds that trend east-northeast to
north-northeast, each one to several kilometers across. Most folds are asymmetric and have been pushed over
toward the northwest by a strong force coming from the east or southeast. These folds have steep to
overturned northwest-dipping limbs and gently dipping southeast-dipping limbs. With increase of the
northwestward push, the strata on the steep limbs have commonly been broken through by west-verging thrust
faults. Many of these faults are great low- angle thrusts which have carried sheets of rocks for kilometers
northwestward, resulting in a characteristic imbricate fault system (see Fig. 7).
 |
| Figure 7. Structure cross sections of western foothills (southern 4 sections adopted from J. T. Chou. 1986) |
At least seven or eight major thrusts have been recognized in the imbricate system of the western foothills,
all with northwestward vergence. Although not every thrust fault has the same characteristics, they have an
overall similarity and, hence, probably belong to one episode of thrust faulting. Most of these thrusts are
great low-angle thrusts and some are bedding thrusts over certain distances. They generally die out
laterally at varying lengths along strike or are truncated by later thrusts. These thrusts are characterized
by salient-and-recess or sinuous surface fault traces, abrupt termination of transverse strike-slip faulting
in meeting the major thrusts, and long distances of translation with great amounts of stratal shortening.
The planes of the major thrusts intersect the surface at relatively high angles, however, because it is a
common characteristic for many thrust sheets to steepen as they approach the surface.
In the complexly folded and faulted rocks, thrust slices bring up mostly Tertiary clastic rocks, but fail to
reveal basement. Apparently the deformation affected only the Tertiary sedimentary cover, leaving basement
rocks undisturbed. Thrusting and folding in the western foothills were active at only shallow levels,
confined to the sedimentary rocks above the basement. This is characteristic of decollement or gravitational
gliding tectonics. Decollement-style deformation in western Taiwan has been recognized and reported in many
previous papers by Biq (1966, 1969, and 1972b), Eleshewitz (1963), Ho (1967a and 1971), Meng (1965), Pierce
(1975 and 1977), Suppe (1976), and many others. The characteristic features of decollement-related
deformation in western Taiwan are: (1) the lack of basement cores in structures, (2) the limited areal
extent of the folds and limited thickness of the strata involved in the detached masses, (3) the lack of
apparent roots of thrust blocks, (4) the presence of tight folds immediately above each frontal/basal
thrust, and (5) the abundance of faulted folds and overturned folds.
The great abundance of shaly and muddy beds in the Neogene strata of western Taiwan provides adequate
horizons for detachment or decollement faulting. The best known decollement horizon, however, is at the base
of the upper Oligocene Wuchihshan Formation. Suppe (1976) suggested that the upper Miocene Nanchuang
Formation is also a major decollement horizon in southwestern Taiwan. Several other decollement horizons
also have been recognized, mostly in the shaly beds of middle to upper Miocene strata, and some even in
Pliocene strata (Suppe, 1980).
It has long been suggested that gravity is the most likely driving force producing the folding and faulting
in fold-and thrust belts. Biq (1969 and 1972b) has proposed gravity glide tectonics to explain the thrust
sheets, especially in the inner foothills of western Taiwan. He cited many criteria and examples to
illustrate the gliding mechanism and suggested that the uplifted cordillera of the Central Range of Taiwan,
now bared to the slate belt, was the uplifted source of the gravity napes. According to this hypothesis, the
imbricate thrust sheets are mostly free-sliding slabs of roof rocks detached from the uplifted cordillera.
The lateral compression responsible for the production of asymmetric folds and imbricate thrusts in the
foothills is effective only to shallow depth.
Pierce (1975) generally supports the theory of gravity tectonics and indicates that the detachment fault
masses moved westward from the uppermost part of the Central Range. However, he suggests further (Pierce,
1977) that a combination of gravity and seismic oscillations may be the mechanism for the emplacement of the
thrust masses in the western foothills. He believes that earth tremors are important factors in
gravitational gliding of rock masses. Taiwan is a very active seismic region today and probably was much
more active in the Pleistocene when the Central Range was vigorously uplifted. Seismic activity in
association with this rapid uplift may have triggered detachments in the western foothills and aided their
westward gravitational gliding, with the stratigraphically higher detachments moving first.
Suppe (1976) favors a forceful "push from the rear raided by fluid pressure to explain the thin-skinned or
decollement-style deformation m western Taiwan. He evaluated the role of fluid pressure in the thrust
faulting of northwestern Taiwan (Suppe and Wittke, 1977), using actual fluid pressure data obtained from
wells drilled by the Chinese Petroleum Corporation on many of the major structures in northwestern Taiwan.
Suppe and others have found that fluid pressure in excess of the hydrostatic gradient is commonly recorded
in these drill holes. Active decollements within the western fold-and-thrust belt lie mainly within the
over- pressured sections, an observation in agreement with the fluid-pressure hypothesis. Their calculation
shows that approximately half as much work is required to push a thrust sheet along the top of the
overpressured zone as along the base of the hydrostatic zone. They concluded that abnormally high fluid
pressures played an important role in the tectonic development of actively deforming regions such as western
Taiwan. Furthermore, the surface slope of the mountains of western Taiwan and dip of the decollement are in
agreement with pushing the wedge of imbricated thrust sheets above the basal detachment (Suppe, 1981).
A common characteristic of shallow fold-and-thrust belts is the development of strike-slip faults oriented
at a high angle to the length of the belt, often in conjugate sets. Strike-slip faults transverse to the
fold belt occur in the western foothills, but are restricted to several particular areas. The well studied
areas of strike-slip faulting are the Alishan area in southern Taiwan (Tsan and Keng, 1962) and the
Shantzechiao-Chingshuikeng area in northern Taiwan (Ho, 1967b).
Two sets of strike-slip faults are known in western Taiwan. Both sets are recognized by the displacement of
conspicuous topographic and stratigraphic units. One set is dextral, striking chiefly west-northwest or
east-west in general. The other set is sinistral, trending mainly north-northeast or north-south in general.
As a rule, the right-handed set is much better developed than the left-handed set. These two sets of
strike-slip faults may be two complementary shear fractures running in two systematic directions. The acute
dihedral angle between them is bisected by the axis of maximum principal stress axis, oriented northwest or
northwest-by- west. The strike-slip faults usually originate in a late phase of the thrusting processes.
In the imbricate thrust system, these strike-slip faults are probably restricted to the upper thrust sheets
only; that is, they are tear faults. They usually disappear at the margin of the thrust sheet to which they
are related and do not extend into lower sheets. This is additional evidence that the thrust faults have
shallow dips. They are manifested by sheared and crushed zones, transverse to the structural trends, and are
caused by sliding of the upper plate by lateral pushing. Displacements along the strike-slip faults vary in
different parts of the fault blocks. This shows that the folding that pre-dated the strike-slip faulting
still may be active during and/or after the faulting. This is called transcurrent buckling.
The major structural features of the western foothills can be divided into two zones: an inner foothills
zone on the east and an outer foothills zone on the west. The inner foothills extend along the western front
of the argillite-slate belt of the Central Range system, separated from the latter by a thrust fault named
the Chuchih fault in northern Taiwan and the Laonungchi fault in southern Taiwan. The characteristic
structural deformation in the inner foothills comprises imbricate thrusting and asymmetric folding as stated
previously. Most folds are concentric folds of flexural-slip type. The anticlinal structures mostly have
cores of Miocene strata. These folds are usually bounded by major thrust faults. A series of south-
east-dipping imbricate thrust sheets is known, each succeeding fault overriding the one northwest of it
(Ho, 1967a and b; and 1974). Fig. 7 is a series of structure cross sections from northern Taiwan to southern
Taiwan to illustrate this imbricate fault system in the western foothills.
The outer foothills structural zone is separated from the inner zone of imbricate thrusts and tight folds by
a great sole thrust or zone of thrusting. This outer zone covers the western part of the foothills belt and
the tablelands or terraces in north- western and central Taiwan. Some structures characteristic of the inner
zone extend into the area immediately west of the boundary between these two zones. To the west, however,
are fairly broad and gentle folds with fold axes subparallel to those in the inner zone. Faulting is less
prevalent and on a smaller scale than in the inner foothills zone. The oldest rocks exposed at the crests of
anticlines are generally Pliocene or Pleistocene, Biq (1972a) suggested that the thrusts in the outer
foothills are parautochthonous, and are secondary structural features produced in response to the impetus of
the allochthonous glide blocks that have come to rest on the inner side.
The outer foothills grade westward into the coastal plain in southwestern Taiwan. Close to the margin of the
foothills, several gentle anticlines have been found beneath the alluvial fill of the western coastal plain
by geophysical surveys and exploratory drilling. These folds clearly represent a westerly extension of the
gently folded zone of the other foothills. Further west of the coastal plain area, an apparent wedging of
the Neogene sediments toward the basin margin or a basement high is coupled with regional tilting, with the
west side up. A number of small tensional faults have been recorded. The Taiwan Strait, which bounds the
western edge of Taiwan, overlies low-dipping to nearly flat Neogene sediments with intercalated basalt lying
on possibly Mesozoic basement.
|
