Plate-boundary strain partitioning along the sinistral collision suture of the Philippine and Eurasian plates: Analysis of geodetic data and geological observation in southeastern Taiwan

Abstract

Crustal deformation and strain partitioning of oblique convergence between the Philippine Sea plate and the Eurasian plate in the southern Longitudinal Valley of eastern Taiwan were characterized, based on geodetic analysis of trilateration network and geological field investigation. The Longitudinal Valley fault, one of the most active faults on Taiwan, branches into two individual faults in the southern Longitudinal Valley. These two active faults bound the Plio-Pleistocene P inanshan conglomerate massif between the Coastal Range (the Luzon island arc belonging to the Philippine Sea plate) and the Central Range (the metamorphic basement of the Eurasian plate). A geodetic trilateration network near the southern end of the valley shows a stable rate of the annual length changes during 1983-1990. The strain tensors for polygonal regions (including triangular regions) of the Taitung trilateration network reveal that there are two distinct zones of deformation: a zone of shortening (thrusting) between the Pinanshan massif and the Central Range on the west, and a strike-slip movement between the Pinanshan massif and the Coastal Range on the east. The analysis of a discontinuity model consisting of three-rigid-blocks separated by two discontinuities has been carried out. The results show that the deformation in this region can be characterized by two major faults. A reverse fault is located between the Plio-Pleistocene Pinanshan massif and the metamorphic basement of the Central Range, with a shortening rate of about 12 mm/yr in the direction N280 ° E. A strike-slip fault is located principally along the river between the Pinanshan massif and island arc system of the Coastal Range with an purely strike -slip component of about 22 mm/yr in the direction N353°E. The analysis of the geodetic data analysis further suggests that substantial deformation (probably strike-slip in type) occurs within the Pinanshan massif. Geological evidence of deformation in the Plio-Pliestocene Pinanshan conglomerate includes regional folding, conjugate set of strike -slip fractures at the outcrop scale, and morphological lineaments related to fracturing, all indicating that the Pinanshan massif is being deformed within a transpressive stress regime. Regional kinematic data indicate that a significant portion of the 82 mm/yr of motion between the Eurasian plate and the Philippine Sea plate is absorbed in the southern Longitudinal Valley by the decoupling of two distinct major faults. The geometry of the oblique convergence and the rheology of the rock units (the well-consolidated Plio-Pleistocene conglomerate and the sheared mélange formation) play the two important factors in the partitioning of crust deformation.

Full text

TECTONICS, VOL. 17, NO. 6, PAGE 859-871, DECEMBER, 1998
859
Plate-boundary strain partitioning along the sinistral collision
suture of the Philippine and Eurasian plates: Analysis of geodetic
data and geological observation in southeastern Taiwan
Jian-Cheng Lee,1 Jacques Angelier,2 Hao-Tsu Chu,3 Shui-Beih Yu,1 and Jyr-Ching Hu1
Abstract. Crustal deformation and strain partitioning of oblique
convergence between the Philippine Sea plate and the Eurasian plate
in the southern Longitudinal Valley of eastern Taiwan were
characterized, based on geodetic analysis of trilateration network and
geological field investigation. The Longitudinal Valley fault, one of
the most active faults on Taiwan, branches into two individual faults
in the southern Longitudinal Valley. These two active faults bound
the Plio-Pleistocene Pinanshan conglomerate massif between the
Coastal Range (the Luzon island arc belonging to the Philippine Sea
plate) and the Central Range (the metamorphic basement of the
Eurasian plate). A geodetic trilateration network near the southern
end of the valley shows a stable rate of the annual length changes
during 1983-1990. The strain tensors for polygonal regions
(including triangular regions) of the Taitung trilateration network
reveal that there are two distinct zones of deformation: a zone of
shortening (thrusting) between the Pinanshan massif and the Central
Range on the west, and a strike-slip movement between the
Pinanshan massif and the Coastal Range on the east. The analysis of
a discontinuity model consisting of three-rigid-blocks separated by
two discontinuities has been carried out. The results show that the
deformation in this region can be characterized by two major faults.
A reverse fault is located between the Plio-Pleistocene Pinanshan
massif and the metamorphic basement of the Central Range, with a
shortening rate of about 12 mm/yr in the direction N280°E. A strike-
slip fault is located principally along the river between the Pinanshan
massif and island arc system of the Coastal Range with an purely
strike-slip component of about 22 mm/yr in the direction N353°E.
The analysis of the geodetic data analysis further suggests that
substantial deformation (probably strike-slip in type) occurs within
the Pinanshan massif. Geological evidence of deformation in the
Plio-Pliestocene Pinanshan conglomerate includes regional folding,
conjugate set of strike-slip fractures at the outcrop scale, and
morphological lineaments related to fracturing, all indicating that the
Pinanshan massif is being deformed within a transpressive stress
regime. Regional kinematic data indicate that a significant portion of
the 82 mm/yr of motion between the Eurasian plate and the
Philippine Sea plate is absorbed in the southern Longitudinal Valley
by the decoupling of two distinct major faults. The geometry of the
1Institute of Earth Sciences, Academia Sinica, P.O. Box 1-
55, Nankang, Taipei, Taiwan, R.O.C.
2Géotectonique (URA 1759), Univ. P. & M. Curie, 4 pl.
Jussieu, T26-25-E1, 75252 Paris, France.
3Central Geological Survey, P.O. Box 968, Taipei, Taiwan,
R.O.C.
Copyright 1998 by the American Geophysical Union.
Paper number 98TC02205.
0278-7407/98/98TC-02205$12.00
oblique convergence and the rheology of the rock units (the well-
consolidated Plio-Pleistocene conglomerate and the sheared mélange
formation) play the two important factors in the partitioning of crust
deformation.
1. Introduction
Deformation adjacent to large crustal-scale strike-slip faults
within a region of transpressive stress has been interpreted in two
different manners [Mount and Suppe, 1992]. The first interpretation
involves wrench tectonics with relatively strong coupling along fault
systems [Wilcox et al., 1973] leading to oblique slip. In contrast, the
second model involves decoupling of oblique convergence into
components of thrust and strike-slip faulting [Fitch, 1972], which
results in partitioning of stress and strain. Regions of decoupling of
active transpressive faulting are often characterized by the major
strike-slip fault and associated compressional structures which
generally show a high angle between the principal stress direction
and the major fault orientation. Analyses of stress fields [Mount and
Suppe, 1987, 1992; Zoback et al., 1987] effectively revealed the
common presence of decoupling across active transpressive plate
boundaries, such as observed along the San Andreas fault, Great
Sumatran fault, Alpine fault, and Philippine fault. In this paper, we
aim at presenting the characteristics of strain partitioning and
decoupling of strike-slip faulting and thrusting in an active suture
zone in eastern Taiwan. This partitioning is caused by the
convergence between the Luzon island-arc system of the Philippine
Sea plate and the Chinese continental margin of the Eurasian plate
(Figure 1), which is oblique to the plate boundary. This study
presents geophysical and geological evidence, which includes
analysis of geodetic trilateration measurement, field structural
analysis, and morphological study based on remote sensing data.
The Taiwan orogenic belt is the product of the convergence
between the Philippine Sea plate and the Eurasian plate. The relative
plate motion is 70 km/My in the direction N310°E according to Seno
and others [1987]. Recent estimates of the displacement between the
islands of the Philippine Sea plate (Lutao and Lanhsu) and those of
the Taiwan Strait (Penghu) has revealed a velocity of 8.2 cm/yr in
the N309°E direction [Yu et al., 1997] (Figure 1a).
The recent mountain building process resulted from this plate
convergence in the Taiwan area began about 5 Ma and is still active.
The Longitudinal Valley (Figure 1b), trends NNE-SSW in eastern
Taiwan; it has commonly been interpreted as the active suture zone
[Ho, 1986], which links the Ryukyu trench-arc system to the north
with the Luzon arc-Manila trench system to the south. To the west of
the Longitudinal Valley, Pre-Neogene (mostly late
Paleozoic/Mesozoic basement) metamorphic rocks belong to the
Central Range. To the east, the Miocene volcanic arc and overlying
Plio-Pleistocene sediments form the Coastal Range. The
Longitudinal Valley forms a narrow zone from one to several

LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
Figure 1. (a) Convergence between the Philippine Sea plate and the Eurasian plate in the Taiwan area. Arrow
indicates the relative vector across Taiwan island with 8.2 cm/yr in the direction N309°E [Yu et al., 1997]. (b)
General geological map of the Coastal Range in eastern Taiwan [Ho, 1988]. Thick solid and dashed lines represent
the traces of the active Longitudinal Valley Fault.
kilometers wide that is the site horizontal crustal shortening of about
20 mm/yr [Yu et al., 1990; Lee and Angelier, 1993]. The
deformation zone along the Longitudinal Valley corresponds to the
area of greatest crustal shortening throughout the Taiwan mountain
belt and represents approximate 25-30% of the total plate
convergence between the Luzon island arc of the Philippine Sea
plate and the Chinese continental margin of Eurasia.
The large amount of crustal shortening in the active suture zone
of the Longitudinal Valley is principally accommodated by active
faulting of the Longitudinal Valley Fault, which reveals
substantially different slip vectors along the length of the
Longitudinal Valley based on the analysis of repeated trilateration
data [Yu et al., 1990]. At the northern tip of the Longitudinal Valley
near Hualien (Figure 1b), the Longitudinal Valley Fault consists of a
strike-slip fault with left-lateral slip of about 23 mm/yr [Yu et al.,
1990]. In the middle part of the valley from Juisui to Chihshang
(Figure 1b), the Longitudinal Valley Fault acts as a left-lateral
reverse fault (2/3 of transverse component and 1/3 of strike-slip
component) with about 21 mm/yr of oblique horizontal shortening
[Yu and Liu, 1989; Lee and Angelier, 1993]. In the southern part of
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LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
the valley, the Longitudinal Valley Fault shows a total horizontal
shortening of 34 mm/yr (Yu et al., 1990). Oblique shortening in this
region is apparently accommodated along two branches of the fault
(a reverse fault and a strike-slip one, Figure 1b), located on both
sides of the Plio-Pleistocene molasse deposits (Pinanshan
Conglomerate) between the Central Range and the Coastal Range.
Our study examined the southern part of the Longitudinal Valley
near Taitung (Figure 2) where crustal deformation is decoupled into
thrust and strike-slip component. This region provides a good
opportunity for understanding decoupling and strain-stress
partitioning within an active oblique collision. The purpose of this
paper is to present new observations, based on field geological
investigations and photo-image interpretation. Modeling geodetic
data also supports these observations. A decoupling model is
proposed, in order to elucidate the partitioning of strain within the
crustal shear zone between the converging Philippine Sea plate and
Eurasian plate.
2. General geology
The Longitudinal Valley separates the Neogene island arc of
Luzon exposed in the Coastal Range and the Pre-Tertiary
metamorphic basement and overlying Paleogene slate of the
Eurasian plate in the Central Range. In the southern Longitudinal
Valley, near Taitung (Figure 2a), the Pinanshan Conglomerate
Figure 2. (a) Geological map and trilateration network in the southern Longitudinal Valley. Annual average
length changes in the network are shown for each segment, with standard errors in the parentheses (value in mm).
Values are negative for shortening, positive for lengthening. (b) Geological cross-section in the southern
Longitudinal Valley. The vergence of the thrust on the western side of the Pinanshan Conglomerate is still unknown
(question mark in section), although westward thrusting seems more likely (half-arrow).
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LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
forms a molasse deposit between the Coastal Range and the Central
Range.
2.1. Coastal Range
The Coastal Range can generally be divided into three rock units:
(1) the Tuluanshan Formation, with Miocene andesitic volcanic
rocks [Hsu, 1956; Yang et al., 1995], constituting the basement of
the Coastal Range; (2) the Takangkou Formation, with Plio-
Pleistocene flysch-type deposits, overlying the Tuluanshan volcanic
basement [Teng and Wang, 1981; Huang et al., 1995]; (3) the Lichi
Formation, a Pliocene mélange formation composed of numerous
exotic blocks of continental and ophiolitic origins within a sheared
muddy matrix, situated principally on the western edge of the
Coastal Range near to the Longitudinal Valley [Hsu, 1976; Page and
Suppe, 1981].
The Coastal Range exhibits numerous west-vergent thrusts,
generally striking NNE-SSW, parallel or sub-parallel to the trend of
the Longitudinal Valley. Within these thrust blocks, the overlying
Takangkou sedimentary units are deformed and folded.
In the study area, the Lichi mélange crops out to the east of the
Pinantachi River whereas the Pinanshan Conglomerate is exposed to
the west (Figure 2). Numerous striated micro-faults within the Lichi
mélange suggest strong shearing during the convergence between the
Luzon island arc (the Coastal Range) and the Central Range [Barrier
and Muller, 1984]. However, another interpretation considers the
Lichi mélange to be a large olistostrome [Page and Suppe, 1981],
suggesting that shearing is related to gravity sliding rather than to
plate convergence. These interpretations are not mutually exclusive.
2.2. Central Range
The pre-Neogene metamorphic rocks of the Central Range are
situated to the west of the Longitudinal Valley. In the Taitung area,
the east edge of the Central Range principally is composed of highly
deformed low grade Paleogene slate (or phyllite) and quartz-
feldspars metasandstone, Hsinkao Formation [Stanley et al., 1981].
The Paleogene slate is underlain by the Pre-Tertiary high grade
Tananao complex is composed of marble, green schist, quartz-mica
schist, and granitoid rocks, which are believed to be Permian to
Cretaceous in age [Yen, 1951; Chen, 1989] as the basement of the
Eurasian plate in Taiwan. The metamorphism of the Central Range
is generally in two major events of late Mesozoic and Plio-
Pleistocene [Jahn et al., 1986]. The general orientation of the
metamorphic foliation along the eastern part of the Central Range is
NNE-SSW and dipping moderately to the west [Stanley et al., 1981]
in the study area.
2.3. Pinanshan Conglomerate
The Pinanshan Conglomerate lies in the southern end of the
Longitudinal Valley (Figure 2a) along the western side of the
Pinantachi River, which separates the Coastal Range (to the east)
and the Pinanshan Conglomerate (to the west). Much smaller,
discontinuous valleys, on the other hand, exists to the west, between
the Pinanshan Conglomerate and the Central Range. As a
consequence, the Pinanshan Conglomerate constitutes a topographic
massif within the Longitudinal Valley.
The Pinanshan Conglomerate is composed of an accumulation of
coarse fluvial sediments [Hsu, 1956; Teng and Wang, 1981; Page
and Suppe, 1981; Barrier et al., 1982], which were deposited at the
foot of the Central Range. The total thickness of the Pinanshan
Conglomerate is more than 2000 meters, though the complete
sedimentary sequence is not entirely exposed. The majority of clasts
within the Pinanshan Conglomerate derived from the metamorphic
units of the Central Range, whereas clasts from the Coastal Range
are present but few, implying that the depocenter of the Pinanshan
Conglomerate was closer to the Central Range (to the west) than to
the Luzon arc. The proto-Coastal Range, which was at the beginning
stage of uplift during collision process, was for most part under the
sea level during the deposition of the Pinanshan Conglomerate.
Lack of paleontological data, the accurate age of the Pinanshan
Conglomerate remains uncertain, though reworked late Miocene
nannofossils have been found in shale layers of the unit [Chi et al.,
1983]. In the adjacent area, the Takangkou flysch Formation of the
Coastal Range reveals several periods of rapid deposition with
coarse sediments [Horng and Shea, 1996; Horng et al., 1997],
indicating probable major tectonic events. The Pinanshan
Conglomerate, with coarse terrestrial deposits, could thus be
compared to and be related to one of these stratigraphic/tectonic
events.
The Pinanshan Conglomerate has deformed into an asymmetrical
syncline (Figure 2b), with a slightly dipping western limb and
steeply to vertically dipping layers in the eastern limb. The western
side of the Pinanshan Conglomerate is in contact with schistose
rocks of the Central Range (Figure 2a). The eastern limb of the fold
is exposed in a cliff along bank of Pinantachi River and shows
vertical and sometimes overturned conglomerate beds.
3. Geodetic data analysis
Our work of geodetic data analysis including strain tensor
determination and discontinuous model analyses is based on data
collected by Yu et al. [1992], using the Taitung trilateration network
(Figure 2) situated on the southern extremity of the Longitudinal
Valley. The network extends across three major tectonic units: from
east to west, the Coastal Range, the Longitudinal Valley (including
the Pinanshan Conglomerate), and the Central Range.
Each line of the trilateration network has been measured annually
between 1983 and 1990 with a medium-range electronic distance
meter [Yu et al., 1992]. The precision of the measurements over
distances of 1-12 kilometers is represented by a standard deviation
σ=(a2+b2L2)1/2, where a=3mm, b=0.7ppm, and L is the line length.
Details of the survey procedures and precision of the trilateration
network have been described before [Lee and Yu, 1985; Yu et al.,
1992]. The results of the Taitung trilateration network have been
summarized in terms of average annual length changes for 48
measured lines in the network [Yu et al., 1992]. Annual data showed
relatively stable rates of length changes within the Taitung network
between 1983 and 1990 [Yu et al., 1992].
3.1. Strain tensor analysis
We first undertook a ‘classical’ analysis in terms of strain tensor,
using the data of the annual average rate of length changes in the
Taitung trilateration network. Here we do not enter into the detail for
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LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
the algorithms of the calculation of strain tensor, which has been
documented before [Lee and Angelier, 1993] and is described here
in the appendix. Instead of reconstructing the average strain tensor
for the whole network [e.g., Yu et al., 1992], we first calculated the
local strain tensors by dividing the network into several triangles
(Figure 3). Each triangle is assigned a strain tensor which represents
average local deformation within the triangle area. Note that we did
not consider the network deformation to be plane-strain; we
considered it as the expression in the horizontal plane of a 3-
dimensional strain (in which one principal axis is vertical). This is
the reason why the maximum and minimum principal strains may
have the same sign in the calculation.
Table 1 shows that some strain tensors estimated in triangles have
large uncertainties whereas other ones are tightly constrained. For
instance, the triangle 7-10-13 reveals large uncertainty in terms of ε,
the triangle 7-10-11 reveals large uncertainty in terms of ε and
θ. The reason for obtaining large uncertainties lies in the original
geodetic data, especially along line 7-10 (Figure 2a) where a poorly
constrained datum was obtained.
On the other hand, the determinations in triangles use only three
length changes for solving the three unknowns of the strain tensor.
For this reason, we also determined the strain tensors in polygon
areas (Figure 3b), which provide a smaller number of determination
in a larger areas, hence generally smaller uncertainties of the results
(Tables 1 and 2; see Figure 3 for comparison).
The results of the strain tensor analysis (Figure 3 and Table 1)
show that the crustal deformation in the area of the Taitung
trilateration network generally illustrates two major different styles
prevailing along the eastern and western sides of the Pinanshan
massif. The western part of the survey area exhibits a distinctive
zone of E-W to ESE-WNW horizontal shortening between stations
in the Pinanshan Conglomerate and stations in the Central Range
(Figure 3). In contrast, the triangles on the eastern part of the
network show a variety of strain tensors, with horizontal deformation
being generally represented by a combination of NW-SE shortening
and NE-SW elongation, except on the southeastern part where
extension prevails (Figure 3, Tables 1 and 2). The stations in the
southeast are located within mélange of Lichi Formation, which is
composed of large exotic blocks within the sheared muddy matrix,
probably leading to locally heterogeneous deformation.
The E-W to ESE-WNW shortening between the Pinanshan
Conglomerate and the Central Range is generally perpendicular to
the boundary between these two units (trending NNE-SSW; Figure
3). This indicates possible thrusting along an E-W to ESE-WNW
direction between the Pinanshan Conglomerate and the Central
Range. On the other hand, strain with NW-SE shortening and NE-
SW elongation in the eastern part corresponds to a stress regime of
strike-slip fault zone which trends N-S along the boundary between
the Coastal Range and the Pinanshan Conglomerate. Taking into
account geological observations (see a later section), which show
that a major discontinuity existed between the Coastal Range and the
Pinanshan Conglomerate, one observes that the deformation in the
eastern part of the study area concentrates in this N-S to NNE-SSW
trending zone. The N-S to NNE-SSW fault zone under a
transpressive strain with NW-SE shortening and NE-SW elongation
hence results in a left-lateral strike-slip. As a consequence, the strain
tensors in the southern Longitudinal Valley are mechanically
consistent with the presence of two major fault systems, (1) thrusting
between the Pinanshan Conglomerate and the Central Range, and (2)
left-lateral strike-slip faulting between the Pinanshan Conglomerate
and the Coastal Range.
Figure 3. Strain tensor analysis of the Taitung trilateration network. (a) Strain tensors for triangular regions. (b)
Strain tensors for polygonal regions. Explanation in text. The deformation for each triangle or polygon is represented
by a 2-D strain tensor. Arrows indicate the principal components of the strain tensor (divergent for extension,
convergent for shortening). Scales of strain rate in lower left corner. See also Tables 1 and 2.
863

LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
Table 1. Results of Analysis of Strain Tensors for Triangular Areas in the Taitung Trilateration
Network.
Triangle
ε
1 (×10-6) standard error
of
ε
1 (×10-6)
ε
2 (×10-6) standard error
of
ε
2 (×10-6)
θ
(°)
standard error
of
θ
(°)
1-2-3 2.061 0.788 -0.055 0.279 166.6 0.4
1-2-5 0.713 0.71 -4.118 0.133 88.8 0.2
2-5-6 0.014 0.202 -3.561 0.258 99.5 0.7
2-3-6 -1.292 0.372 -3.298 0.28 94.4 1.8
3-6-7 0.196 0.227 -3.337 0.318 98.4 0.9
3-4-7 -1.099 0.179 -3.132 0.311 116.5 0.9
5-6-8 1.534 0.193 0.029 0.335 109.5 0.8
6-8-9 1.490 0.241 -1.249 0.2 131.1 1.0
6-7-9 1.059 0.178 -1.588 0.165 117.4 0.5
7-9-13 -0.035 0.223 -1.098 0.15 114.8 2.0
7-10-13 -0.826 1.71 -1.259 0.92 77.5 2.6
7-10-11 0.748 0.91 0.547 1.827 78.2 24.2
7-11-12 0.550 0.19 0.749 0.147 125.5 5.4
4-7-12 -0.112 0.128 -1.548 0.119 61.0 0.1
10-11-13 1.905 0.343 -0.329 0.476 129.3 1.8
11-12-13 4.134 0.161 1.614 0.133 127.4 0.4
12-13-14 3.247 0.136 1.555 0.169 99.7 0.3
9-13-14 -0.11 0.268 -0.225 0.201 89.7 2.3
8-9-14 0.944 0.287 0.012 0.23 123.3 0.9
ε
1: the maximum principal strain axis,
ε
2: the minimum principal strain axis.
θ
: the azimuth of the minimum
principal strain axis.
3.2 Discontinuous model analysis
A discontinuous model analysis was used to interpret the results
of the trilateration network and to illustrate quantitatively the
characteristics of the surface crustal deformation along highly
localized active faulting. In the case of this discontinuous model, the
deformation is simply represented by the relative movement of rigid
blocks separated by rectilinear fault [Lee and Angelier, 1993]. The
horizontal movement is defined by two series of variables: amount
and direction of the relative movement. These variables, together
with the location of the discontinuity (fault), are considered through
an inversion technique, in order to reconstruct a regional
deformation model. The vectors of displacement, *
k
L
v
, for each line
in the trilateration network, are compared with the calculated vectors,
k
L
v
, produced by the discontinuous model. The sum of the difference,
S, between these two vectors, for all the line segments in the network,
can be minimized through a least square-root method. The details of
the method are shown in Appendix.
Two major active faults have been identified in the study area
based on field investigation. These faults compose branches of the
Longitudinal Valley Fault in the southernmost extent of the
Longitudinal Valley. The two faults divide the area into three blocks:
from west to east, the Central Range, the Pinanshan Conglomerate,
and the Coastal Range. In addition to these two major discontinuities,
an analysis of SPOT imagery reveals that two distinct linear
structures cut across the Pinanshan Conglomerate (Figure 4). Yü
[1996] identified two active conjugate strike-slip faults (a right-
lateral fault trending approximately N95°E in the north and a left-
lateral fault trending about N140°E in the south), which may
correspond to the two major lineaments observed in SPOT image
(Figure 4).
Three problems of discontinuous behavior deserve consideration
in our model: (1) the location of the main western discontinuity, (2)
the location of the main eastern discontinuity, and (3) the possible
presence of additional discontinuities within the Pinanshan massif.
As a first approximation, we started our analysis with a model of
three rigid blocks with two major discontinuities, ignoring the
additional effects related to deformation within the Pinanshan
Table 2. Results of Analysis of Strain Tensors of Polygons in the Taitung Trilateration Network
Polygon
ε
1 (×10-6) standard error
of
ε
1 (×10-6)
ε
2 (×10-6) standard error
of
ε
2 (×10-6)
θ
(°)
standard error
of
θ
(°)
1-2-3-5-6 -0.734 0.246 -3.796 0.207 94.6 0.5
3-4-6-7 -0.496 0.185 -2.232 0.271 109.8 1.7
5-6-8-9 1.373 0.255 -0.904 0.287 120.6 0.0
6-7-9-13 0.822 0.203 -1.511 0.156 114.2 0.6
7-13-10-11-12 2.657 0.199 -0.037 0.621 119.9 0.1
8-9-13-14 0.644 0.239 -0.040 0.220 107.4 0.6
ε
1: the maximum principal strain axis,
ε
2: the minimum principal strain axis.
θ
: the azimuth of the
minimum principal strain axis.
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LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
Figure 4. SPOT image of the area of the southern Longitudinal Valley. Note the linear structures along the eastern edge
of the Pinanshan Conglomerate and the western edge in the southern part. Two main drainage alignments in the Pinanshan
Conglomerate (trending N95°E in the north and N140°E in the south) possibly reveal major strike-slip faults.
Conglomerate.
For the location of the main western discontinuity, we adopted
the boundary between the Central Range and the Pinanshan
Conglomerate, as indicated by the geological evidence and by the
above strain tensor analysis. On the other hand, on the eastern part of
the area, although there exists substantial deformation generally
between the Pinanshan Conglomerate and the Coastal Range (Figure
3), the location of the discontinuity remained unclear in the
southeastern part of the network. As a result, we introduced a
particular computer process in order to search automatically for
active discontinuities within the network by minimizing the sum of
the differences between the measured data and the length changes
deduced from the model. The uncertainty of the solutions has been
estimated by introducing the standard errors of the measured data
(Figure 2).
The results of the analysis of this discontinuous model indicate
that the Pinanshan Conglomerate moved westward toward the
Central Range with a rate of 10±3 mm/yr in the direction of
N280±20°E and the Coastal Range moved northward compared to
the Pinanshan Conglomerate (Figure 5) with a rate of 22±5 mm/yr in
the direction of N353±7°E. The discontinuity, which trends N-S and
is found to be a left-lateral strike-slip fault, generally lies along the
Pinantachi River, between the Pinanshan Conglomerate and the
Coastal Range, and passes between station 10 and station 13
(Figures 2 and 5). The two major displacements produce a total
vector of 28 mm/yr in the direction N329°E (Figure 6), which
represents the total displacement across the Longitudinal Valley
between the Central Range and the Coastal Range.
The displacements calculated with the discontinuous model have
then been applied to the trilateration network system to determine
theoretical length changes for each line of the geodetic network
(Figure 5). Comparing these calculated changes with the real data
(Figure 5a) revealed by the histograms (Figure 5b), one observes that
the induced length changes are generally in a good agreement with
the measured ones (the differences between the measured data and
the calculated ones are basically restricted within ±5 mm/yr, see
Figure 5c).
The misfits lie essentially on lines related to the stations within
the Pinanshan Conglomerate (Figure 5). This indicates that
substantial deformation exists within the Pinanshan Conglomerate.
We take into account the major alignments in the Pinanshan massif,
as revealed by the morpho-geological data mentioned before (Figure
4, with N95°E trends in the north and N140°E trends in the south).
Therefore, the N140°E trending alignment in the southern Pinanshan
Conglomerate probably acts as a locally significant left-lateral strike-
slip fault. Displacements and strain tensors consistently show a N-S
to NNW-SSE elongation and E-W to ESE-WNW shortening around
the Pinanshan Conglomerate, implying left-lateral strike-slip fault(s)
between the northernmost station and the middle one. Another
865

LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
Figure 5. Geodetic analysis of the discontinuous model in the Taitung trilateration network. (a) Data of length
changes obtained from trilateration measurements [Yu et al., 1992]. (b) Results of the geodetic analysis from the
discontinuous model. Different patterns of gray indicate the three rigid blocks (inferred boundaries as dashed lines).
Arrows: relative movements of blocks (for eastern block, motion relative to central block; for central block, motion
relative to western block). Values along lines: calculated length changes from inversion with the discontinuous model
(in mm). (c) Histogram: number of lines in network (ordinate) as a function of difference between observed and
computed length changes (abscissa, mm/yr).
possible interpretation is that numerous small conjugate strike-slip
faults exist, which are mechanically compatible with the regional
strain pattern within the Pinanshan Conglomerate. In fact, the results
of earlier photo-interpretation as well as the analysis of fault-slip
data collected in the field [Barrier et al., 1982] showed numerous
strike-slip sets affecting in the Pinanshan Conglomerate. This aspect
will be discussed in more detail in the next section.
4. Geological evidence of deformation in the
Pinanshan Conglomerate
The Pinanshan Conglomerate constitutes an asymmetric synclinal
fold (Figure 1b) with a gently inclined western limb and a steeply
inclined (sometimes overturned) eastern limb. Along the western
limb of the syncline, the Pinanshan Conglomerate is in contact with
metamorphic rocks of the Central Range, while on the eastern limb
the conglomerate is in contact with the Lichi mélange of the Coastal
Range. The strain tensor analysis discussed before indicates a zone
of thrusting on the western side of the Pinanshan Conglomerate and
a zone of strike-slip faulting on the eastern side. These major faults
exposed poorly in the field because of coverage of recent sediments
and erosion of river. On the eastern side of Pinanshan Conglomerate,
the adjacent Lichi mélange reveals numerous left-lateral fault
striations trending NNE-SSW, consistent with strike-slip movement
between the Pinanshan Conglomerate and the Coastal Range.
However, one outcrop of fault in the eastern edge of the Pinanshan
Conglomerate near the Pinantachi shows dip-slip striated
slickensides, indicating thrusting of the Coastal Range over the
Pinanshan Conglomerate [Barrier et al., 1982].
Numerous strike-slip faults, observable at the outcrop scale, have
been found in the Pinanshan Conglomerate. The strike-slip faults are
commonly associated with conjugate joint sets which are usually
marked by streams across steep cliffs and can be easily identified in
the field and on the air-photo imagery. Striations on strike-slip fault
planes allow to reconstruction of deformation in terms of paleostress
[Angelier, 1984]. Barrier et al. [1982] first identified strike -slip
conjugate joints occurred prior to tilting (folding) of the Pinanshan
Conglomerate. After tilting, strike-slip movements continued,
including reactivation of the initial conjugate fractures. The
paleostress analysis consistently showed that strike-slip faulting
occurred under a regime of WNW-ESE (N100°E to N105°E), in a
good agreement with the compression that can be expected
considering the folding of the Pinanshan syncline (trending
approximately N-S to NNE-SSW). The chronology of the strike-slip
structures in the Pinanshan Conglomerate implies an extended period
of strike-slip stress regime during deformation. Syncline folding and
reverse faulting, which undoubtedly occurred under compressional
stress regime, associated with prevailed strike-slip faults of outcrop
scale suggest a permutation of principal stress axes σ2/σ3 occurred
within the Pinanshan Conglomerate during compression.
The morphological lineations represent the surface features,
which generally reflect recent movements on major structures. We
analyzed the morphological lineations and subsequently compared
them with the present-day surface deformation revealed by the
geodetic data analysis. Two available morphological images have
866

LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
Figure 6. Crustal deformation and displacement deduced from the results of the geodetic data analysis (discontinuous
model) and geological observation in the southern Longitudinal Valley, eastern Taiwan. (a) Crustal deformation involves
two major faults on both sides of the Pinanshan Conglomerate massif, with additional deformation within the Pinanshan
Conglomerate. (b) Application of model of discontinuous deformation along the Longitudinal Valley [Lee and Angelier,
1993]. Arrows indicate displacements of the upthrust, eastern block of the Longitudinal Valley Fault relative to the Central
Range. Computed azimuths of displacement in degrees, computed velocity in mm/yr.
been used, the SPOT panchromatic image (Figure 4) which has
ground resolution of 10m×10m, and the aerial photographs with a
higher resolution of less than 1m×1m. As a consequence, the SPOT
image shows regional structural features while the aerial
photographs are useful in detailed fracture analysis.
The photo-interpretation of features in the Pinanshan
Conglomerate (Figure 7) [Barrier et al., 1982] shows two dominant
sets of lineations at NW-SE (N140°E) and WNW-ESE (N70°E). The
trends of these sets coincide with trends of conjugate strike-slip fault
sets observed in the field. This relationship indicates that similar
lineations in the Pinanshan Conglomerate likely represent numerous
strike-slip faults, which indirectly supports the results of the geodetic
data analysis, suggesting substantial deformation within the
conglomerate.
South of the Pinanshan Conglomerate, the faults disappear
beneath the Taitung plain and offshore. North of the Pinanshan
Conglomerate, two branches of the Longitudinal Valley Fault merge
into a single major fault towards the Chihshang area (Figure 1)
where deformation is concentrated along a narrow zone generally
less than 50 m wide [Angelier et al., 1997]. The transition of the
Longitudinal Valley Fault from a single fault to two fault systems
should occur between the Pinanshan Conglomerate and Chihshang.
The evolution of deformation of the Pinanshan Conglomerate can
thus be illustrated, based on the structural relationships between the
regional folding, thrusting, strike-slip faulting, and the consideration
of morphological features: (1) development of conjugate sets of
strike-slip joints before tilting (folding) of the Pinanshan
Conglomerate; (2) regional asymmetric synclinal folding possibly
synchronous with overthrusting by the Lichi mélange of the Coastal
Range; (3) conjugate strike-slip faulting following folding including
reactivation of the initial conjugate sets of fractures.
5. Discussion
The contact between the Luzon island arc system (the Coastal
Range) and the Plio-Pleistocene Pinanshan Conglomerate is
presently characterized by a fault zone of left-lateral slip as
suggested by the above analyses. However, the existence of the
asymmetric Pinanshan Conglomerate syncline with a NNE-SSW
fold axis and the steeply inclined eastern limb with nearly vertical
layers imply that the unit has been shortened under regional
compression trending approximately WNW-ESE (nearly
perpendicular to fold axis). Overthrusting by the Coastal Range with
NW-SE compression along the eastern boundary of the
conglomerate has been documented [Barrier et al., 1982]. The
867

LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
Figure 7. Extraction of alignments from aerial photographs in the
Plio-Pleistocene Pinanshan Conglomerate, from Barrier et al. [1982],
with rose diagram representing the frequency of trends. Two major
preferred orientations (N70°E and N140°E) show conjugate set of
fractures, which correspond to the strike-slip faults identified in the
field.
orientation of the young fold, the direction of thrusting, and the
direction of compression deduced from the paleostress analysis of
strike-slip faults all indicate compression stress at a high angle to the
active strike-slip suture zone in the southern Longitudinal Valley.
5.1. Kinematics at local and regional scales
The relative motion between the Philippine Sea plate and the
Eurasian plate is about 70 mm/yr in the direction of N310°E [Seno et
al., 1987]. Recent GPS measurements show a shortening rate of 82
mm/yr across the Taiwan mountain belt between the islands Lanhsu
east off Taiwan and the island Penghu in the Taiwan strait west of
Taiwan [Yu et al., 1997]. Around southeastern Taiwan, GPS data
show shortening of more than 40 mm/yr between the island Lanhsu
and the eastern margin of the Central Range [Yu et al., 1997]. Our
analysis of discontinuous model from the trilateration networks
around the southern Longitudinal Valley reveals a displacement rate
of 28 mm/yr in the direction N329°E between the Coastal Range and
the Central Range. We conclude that the amount of crustal
deformation in the southern Longitudinal Valley accounts for a
significant portion of the total convergence between the Philippine
Sea plate and the Eurasian plate and is similar to that in the central
portion of the valley with shortening rate 21mm/yr in the direction
N320°E [Lee, 1994; Angelier et al., 1997].
Paleostress analysis of fault -slip striation in the Pinanshan
Conglomerate area [Barrier et al., 1982] revealed a N279°E (N81°W)
trending compression, which differs from the direction of total
relative displacement between the Coastal Range and the Central
Range in the southern Longitudinal Valley (N329°E; Figure 5).
However, the N279°E compression is similar to the direction of
shortening between the Central Range and the Pinanshan
Conglomerate (N280°E), and it is almost perpendicular to the major
trend of the suture zone (approximate in N20°E). Thus, it may
principally represent the transverse component of the oblique
collision along the Longitudinal Valley Fault, because of the
partitioning in stress/strain.
5.2. Tectonic evolution of the Pinanshan Conglomerate
The tectonic evolution of the Pinanshan Conglomerate can be
summarized as follows. The Pinanshan Conglomerate deposited at
the foot of the Central Range when the Luzon island arc system of
the Coastal Range approached during the Plio-Pleistocene (Figure
8a). The coarse clastic nature and contents of the Pinanshan
Conglomerate resemble that of some youngest layers of the adjacent
Takangkou flysch deposits of the Coastal Range [Horng and Shea,
1996], although there is no geological continuity between the two
formations,. When the Coastal Range continued to move westward
and collided with the Central Range, the Coastal Range was thrusted
above the in-between Pinanshan Conglomerate, whic h began to
deform as a faulted syncline (Figure 8b). As these three units (the
Coastal Range, the Pinanshan Conglomerate, and the Central Range)
were colliding under increasing compression, strain partitioning
developed (Figure 8c), the convergence oblique to the boundary
being decoupled into strike-slip faulting (on the eastern side of the
Pinanshan Conglomerate) and thrusting (on the western side).
6. Conclusions
1. Based on the analysis of geodetic data combined with
geological field investigations, we recognize the role of decoupling
and partitioning in crustal deformation within an active suture zone
of oblique convergence: the southern Longitudinal Valley of eastern
Taiwan. Crustal deformation is characterized by the splitting of the
Longitudinal Valley fault into two major active branches, resulting
in a suture zone composed of three blocks: from east to west, the
Coastal Range (the island arc system of Luzon), the Pinanshan
Conglomerate (syn-collision molasse deposits), and the Central
Range (the metamorphic basement of Eurasia). In contrast, further to
the north, the active Longitudinal Valley Fault consists of a single
fault with shear concentrating in a very narrow zone [Angelier et al.,
1997]. In the southern Longitudinal Valley this fault zone splits into
two distinct faults on both sides of the Pinanshan Conglomerate, a
thrust fault to the west and a strike-slip fault to the east, exhibiting
characteristics of strain partitioning at a convergent plate boundary.
868

LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
Figure 8. Tectonic evolution of the Pinanshan Conglomerate. (a) Sedimentation of the Pinanshan Conglomerate at the foot
of the Central Range during the convergence between the Coastal Range (Luzon island) and the Central Range, during the
middle Pleistocene approximately. (b) Late Pleistocene: folding of the Pinanshan conglomeratic layers associated with
thrusting of the Coastal Range. (c) Present: the fault across the Longitudinal Valley combines two types of faults: a reverse
fault, probably west-vergent, on the western side of the Pinanshan Conglomerate, and a strike-slip fault, left-lateral, on the
eastern side.
2. Deformation in the active suture zone of the southern
Longitudinal Valley is characterized by a displacement of
approximate 28 mm/yr in the direction of about N329°E. The total
displacement is decoupled into strike-slip faulting (displacement of
22 mm/yr in the direction N353°E) on the east and thrusting
(shortening of 12 mm/yr in the direction N280°E) on the west.
3. Detailed geodetic data analysis indicates that secondary
deformation locally occurs within the Plio-Pleistocene Pinanshan
Conglomerate. This additional deformation is probably
accommodated by numerous sets of conjugate strike-slip faulting
(dextral faults trending about N70°E and sinistral ones trending
approximately N140°E).
4. The Plio-Pleistocene folding and strike -slip faulting in the
Pinanshan Conglomerate indicate a consistent direction of
compressional stress, with a compressional stress axis (WNW-ESE)
nearly perpendicular to the trends of the major faults (NNE-SSW).
This direction of compression is quite consistent with the thrust
behavior of the western fault zone. This behavior is made possible
by the decoupling along the eastern strike-slip fault, thus illustrates
strain/stress partitioning in a transpressive, reverse and left-lateral,
stress regime.
Appendix
A1. Strain Tensor Analysis
Within a homogenous deformation domain, we introduce a strain
tensor, T, representing the crustal deformation. we simply consider
the 2-D application, because of the absence of information in the
vertical direction (see text). The displacement, k
L
v
, for a point (x, y)
situated at the end of a segment in the network can be obtained by
multiplying the strain tensor, T, and the vector k
D
v
of the
coordinate of the point:
869

LEE ET AL.: PLATE BOUNDARY STRAIN PARTITIONNING IN EASTERN TAIWAN
kk DL
v
v
⋅=T (1)
The strain tensor, T, is described by three unknowns ε1, ε2 and θ,
which represent respectively the maximum principal strain, the
minimum principal strain in the horizontal plane (positive sign
means elongation), and the angle between the ε1 axis and the E-W
direction.







−















−
=θθ θθ
ε
ε
θθ θθ cossin
sincos
0
0
cossin
sincos
2
1
T (2)
The calculated displacement, k
L
v
, is compared with the
measured data, *
k
L
v
. The best-fit strain tensor T is found when the
differences between the vectors k
L
v
and *
k
L
v
are as small as
possible. For a network with N lines, we use a least-square approach
and search for the average tensor T which provides the smallest sum
S:
∑
=
=−= Nk
k
kk LLS
1
2
)( *
vv (3)
In order to obtain the minimum of S, we set the derivatives 1
S
ε∂
∂,
2
S
ε∂∂ and θ∂
∂S to zero. Combining with the equations (1), (2) and
(3), it results in the following system of equations, where A to F are
polynoms depending on the data:





θθθθεε θθθθεθθθθε
cos2F -sin2E = sin2C-cos2B((
cos2F -sin2E - D = sin2B-cos2C - A(
cos2F +sin2E + D = sin2B+cos2C + A(
2
+
1
2
1
))
)
) (4)
The solution of these equations gives the values of ε1, ε2 and θ.
An additional calculation provides the uncertainties 21 εε ∆∆ , and
θ∆, as functions of the uncertainties of the data of the trilateration
network.
A2. Discontinuous Model Analysis
In the discontinuous model [Lee and Angelier, 1993], the
deformation is represented by displacements between rigid blocks
separated by geological discontinuities (or faults). In this case with
two discontinuities, the deformation can be described by ten
variables. Deformation for each discontinuity comprises five
variables: the vector of relative motion between rigid blocks,
including amount (d) and orientation (θ), and three variables giving
the location of the rectilinear discontinuity.
The vectors of displacement, *
k
L
v
, for each line in the trilateration
network are compared with the calculated vectors, k
L
v
, produced by
the discontinuous model. The sum of the difference, S, between
these two vectors, is minimized through a least square approach, for
all the segments of the network:
∑
Nk
1k
2
kk LLS=
=
*)-( = vv (5)
In the case of the southern Longitudinal Valley, where two faults
are present (i.e., two discontinuities), equation (5) has a more
complicated form:
∑∑∑ ++ Tt
1t
2
tt
Nk
1k
2
kk
Mj
1j
2
jj LLLLLLS=
=
*
=
=
*
=
=
*)-()-()-( = vvvvvv , (6)
where j
L
v
, k
L
v
, and t
L
v
represent the calculated vectors of
displacements along two different discontinuities and the calculated
vectors of total displacement (depending on locations of faults
relative to stations), while *
j
L
v
, *
k
L
v
, and *
t
L
v
represent the
corresponding vectors from measured data.
In order to fasten the process and to obtain the smallest sum S
without considering unrealistic solutions, we limited the possible
portions of the discontinuities based on geological observation (see
text), and we executed a search process for d1 (amount of
displacement along the first discontinuity) = 1, 2, ……, 40 (in
mm/yr), θ1 (orientation of displacement along the first discontinuity)
= 1, 2, ......, 360 (in degree), d2 (amount of displacement along the
second discontinuity) = 1, 2, ……, 40 (in mm/yr), θ2 (orientation of
displacement along the second discontinuity) = 1, 2, ......, 360 (in
degree). We finally obtain the amount (mm/yr) and orientation
(azimuth) of displacement along both the discontinuities, as
illustrated in Figure 6a.
Acknowledgments. This work was supported by Institute of
Earth Sciences, Academia Sinica and National Science Council grant
NSC87-2116-M047-002. Helpful suggestions and reviews by J.C.
Savage and D.L. Reed greatly improved the manuscript. This is a
contribution IESEP98-006 of the Institute of Earth Sciences,
Academia Sinica.
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__________
J. Angelier, Géotectonique (URA 1759), Univ. P. &
M. Curie, 4 pl. Jussieu, T26-25-E1, 75252 Paris,
France. (ja@lgs.jussieu.fr)
H.-T. Chu, Central Geological Survey, P.O. Box 968,
Taipei, Taiwan, R.O.C. (chuht@linx.moeacgs.gov.tw)
J.-C. Hu, J.-C. Lee, and S.-B. Yu , Inst. of Earth
Sciences, Academia Sinica, P. O. Box 1-55, Nankang,
Taipei, Taiwan 115, (e -mail: jchu@earth.sinica.edu.tw;
jclee@earth.sinica.edu.tw;
eayusb@ccvax.sinica.edu.tw)
(Received September 18, 1997;
revised June 11, 1998 ;
accepted July 1, 1998)
871