Full text of DOI:10.1186/BF03351660

Earth Planets Space, 52, 527–538, 2000
Tectonic significance of magnetic susceptibility fabrics in Plio-Quaternary
mudstones of southwestern foothills, Taiwan
Teh-Quei Lee1 and Jacques Angelier2
1Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, Taiwan, 115, R.O.C.
2Universite´ P-M Curie, De´pt. de Ge´otectonique (ESA 7072), Tectonique Quantitative, 4 place Jussieu, T25-26, E-1, 75252 Paris Ce´dex, France
(Received February 15, 1999; Revised July 17, 2000; Accepted July 28, 2000)
Anisotropy of magnetic susceptibility (AMS) was studied in three Plio-Pleistocene turbiditic mudstone sequences
accumulated in the foreland basin of southwestern Taiwan. These formations were incorporated in the front units
of the collision belt and underwent folding and thrusting during the last 2 Ma. Five types of fabrics were identified
from more than 3,000 samples collected in 352 sites, with 251 sites allowing determination of a magnetic lineation.
NNE-SSW trends are predominant, minor N-S and NE-SW trends are present. Magnetic lineations are widespread
in the lower section where folds are tight, and scarce in the youngest sediments where folds are gentle. The strong
correlation between the structural features and the AMS orientations suggests a tectonic origin for most magnetic
lineations superimposed on the initial flattening that results from sediment compaction. This is confirmed by
tectonic studies based on structural analysis and paleostress tensor reconstructions. The tectonic studies reveal a
major WNW-ESE compression, which provide orientations of compressive tectonic regimes consistent for resulting
the magnetic lineations. In contrast, the hypothesis of a sedimentary origin can be ruled out in most cases, because
the orientations of magnetic lineations and those of depositional fabrics (paleocurrents, sediment supply directions
and even slumps) are oblique at a variety of angles. Furthermore, based on magnetostratigraphy, we conclude that
this compression culminated about 0.9–1 Ma ago. Earlier minor events, NW-SE and W-E compression, have also
been found and we propose that they have occurred in approximately 1 and 2 Ma ago, respectively. Thus, the main
cause of AMS trend is thought to be the WNW-ESE Quaternary compression responsible for major folding and
thrusting. In addition, the magnetic fabric of tectonic origin is absent, or poorly marked, in formations younger than
about 0.9 Ma to the north. However, it is still recognized but decreased after about 0.7 Ma ago to the south. This
indicates that the WNW-ESE compression propagated southward between 0.9 and 0.7 Ma ago, consistent with the
migration of folding and thrusting during the last Taiwan collision.
1. Introduction
velopment (Suppe, 1981) and tectono-sedimentary evolution
The analysis of the anisotropy of magnetic susceptibil- (Teng, 1990). Various models were presented to explain the
ity (AMS) has been applied to rock fabric investigation. The origin of the main units in the belt (Lu and Hsu, 1992).
most successful application of AMS analysis dealt with stud-
AMS studies have been carried out in the Plio-Pleistocene
ies of ductile deformation of rocks. Several papers showed successions at the Coastal Range of eastern Taiwan (Lee
that magnetic fabric provides a reliable strain indicator et al., 1990). Also paleomagnetic studies revealed that a
(Graham, 1966; Henry, 1973; Hrouda, 1979; Hrouda and diachronic clockwise rotation had affected the units of the
Janak, 1976; Hrouda et al., 1978; Kligfield et al., 1977, Coastal Range in eastern Taiwan, occurring successively
1982). During the last ten years, the AMS analysis was also fromnorthtosouthduringthelast3Ma(Lee, 1989; Leeetal.,
employed to study the fabric of deformed successions, espe- 1991a). This phenomenon is consistent with the oblique col-
cially in fold-and-thrust belts in compressional field. In Tai- lision of the Luzon Arc against the Central Range of Taiwan,
wan, it was found that despite limited strain the magnetic lin- alsomigratingfromnorthtosouthasrevealedbygeodynamic
eations formed in the late Cenozoic sedimentary rocks were studies (Suppe, 1981). Taking these rotations into account,
generally perpendicular to regional compression (Kissel et Kissel et al. (1986) and Lee et al. (1990) showed that the
al., 1986; Lee et al., 1990).
magnetic anisotropy acquired by the deformed marine suc-
Collision has occurred since the late Miocene in Taiwan, cessions was consistent with, and related to, the compression
at the boundary between the Philippine Sea plate and the oriented NW-SE to WNW-ESE in this region (Barrier and
Eurasian continental plate. The characteristics of the Taiwan Angelier, 1986). Late Cenozoic rotations were identified not
belt have been described by many authors in terms of regional only in the Coastal Range, but also in northern Taiwan (Lee
geology (Ho, 1986), geophysics (Tsai, 1986), structural de- et al., 1991b; Lue et al., 1995). There the rotation history
is consistent with the tectonic evolution of the sharply bent
segment of the collision belt (Lu et al., 1995).
Thick mudstone sequences accumulated during the Late
Cenozoic in the foreland basin of southwestern Taiwan (Ho,
c
Copy rightꢀ The Society of Geomagnetism and Earth, Planetary and Space Sciences
(SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan;
The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.
527

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T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
ing and foldings was carried out independently in the same
sections. As for syndepositional features, this analysis of
tectonic structures was indispensable in order to confirm, or
rule out, the influence of tectonism in the AMS. Finally,
the results of our AMS analyses are examined in light of
the tectonic evolution of the foreland basin of southwestern
Taiwan.
2. Geological Framework
The Plio-Pleistocene mudstone succession is dominated in
the southwestern Taiwan. Studies of clay minerals in these
mudstone successions tell us highlighted the rapid denuda-
tion and sedimentation processes during the mountain build-
ing of Taiwan (Chamley et al., 1993). Sandstones and shales
are common in the older formations. Continuous exposures
are present along the valleys of the Tsengwenchi (Fig. 2),
Hokouchi (Fig. 3) and Erhjenchi (Fig. 4), which allow the
extensive sampling to be carried out.
The Tsengwenchi section (Fig. 2) is approximately 4 km
long. It cuts perpendicular to the NNE-SSW structural trend,
on the footwall of the west vergent Wushantou thrust fault. In
this section, the beds dip westward and decreases in dip angle
to the west: it averages 60◦–70◦ in the eastern half of the sec-
tion, and 30◦–50◦ in the western half. Two high angle thrust
faults cut this sequence: the west-vergent Wushantou fault it-
self, and a small east-vergent fault, located about 200 m west
of the Wushantou fault. Nearly vertical beds crop out locally
near thrusts. The lithostratigraphic units can be divided into
Fig. 1. The southwestern Foothills: location in Taiwan and simplified map
of the area studied. Small rectangles indicate location of the three sections
and refer to maps of Figs. 2, 3 and 4. Numbers shown in the index map
are: (1) Coastal Range which is thought to be the northern part of Luzon
arc; (2) Central Range; (3) Hsuehshan Range and (4) western Foothills.
1988). They were incorporated in the front units of the colli- five formations: the Yunshuichi, Liuchungchi, Kanhsialiao,
sion belt during the Plio-Pleistocene time and they underwent Erchungchi and Liushuang formations in ascending order
folding and reverse faulting mainly related to the Pleistocene- (Stach, 1957; Chang, 1962). These units comprise mainly
Holocene compressional event. The previous AMS results dark gray sandy shale, shale and muddy sandstone; the total
showed that magnetic lineation trends have different orienta- thickness is about 3,500 m. Based on nannofossil analy-
tions at different horizons in the Tsengwenchi and Hokouchi sis, this sequence is late Pliocene (NN16) to late Pleistocene
sections (Lee and Horng, 1990, 1993). What the significance (NN20) in age (Chi, 1978). Magnetostratigraphic analysis
of it needs to be determined. Paleomagnetic results from the indicated that sediment ages in this section vary continuously
Tsengwenchi section suggested that a rotation of about 20◦ from the late Gauss to the Brunhes normal epochs (Horng,
clockwise may have affected the Pleistocene sediments older unpubl. Ph.D. dissertation, 1991).
than 0.98 Ma (Horng, unpubl. Ph.D. dissertation, 1991). Un-
The Hokouchi section (Fig. 3) is located approximately 8
fortunately, these determinations are questionable because of km east of the Tsengwenchi section. It comprises the older
the nature of the magnetic minerals used (greigite). Deter- succession of late Miocene to Pliocene in age. The beds
minations made with magnetite in the Pliocene sediments in generally strike NE-SW and dip 30◦–45◦ eastward. The
age (which should have experienced this rotation) did not whole sequence studied is located on the eastern side of the
reveal any significant rotation according to Horng (1991). Choutouchi fault. Lithostratigraphic units of this sequence
Thus, it seems that significant rotations does not take place are, in ascending order, the Tangenshan Sandstone, the Yen-
during the Quaternary.
shuikeng Shale, the Ailiaochiao Formation, the Choutouchi
In this paper, we principally analyze the distribution and Formation and the Peiliao Shale. The Tangenshan sand-
tectonic significance of the AMS in the Pliocene and the stone and the Choutouchi Formation are mainly constituted
Pleistocene thick mudstones, which underwent folding and by fine-grained sandstone and muddy sandstone interbed-
thrusting during the last 2 Ma in the Western Foothills of ded with thin layered sandy shale. In the other formations,
the southern Taiwan belt. Our AMS analysis aims first to dark-gray shale interbedded with sand shale, yellow-brown
describe patterns of magnetic lineation with better age con- sandstone and muddy sandstone are predominant. A bios-
trol through combining magnetostratigraphic and biostrati- tratigraphical study in the adjacent Kueitanchi section sug-
graphic data. Second, our AMS analysis aims at better un- gested that the Miocene/Pliocene boundary is located within
derstanding the origin of magnetic fabrics. This required the Tangenshan Sandstone, whereas the Pliocene/Pleistocene
consideration of different possible factors: deposition, com- boundary is located at the upper part of the Peiliao Shale (Chi,
paction, and tectonic deformation. Particular attention was 1978). Thus, the ages in the Hokouchi section range from
paid to syndepositional sedimentary structures in the mud- late Miocene to early Pleistocene.
stone succession, because they are numerous and may have
The Erhjenchi section (Fig. 4) is located about 20 km
influenced the AMS. On the other hand, a study for fault- south of the other two sections (Fig. 1). It was possible

T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
529
Fig. 2. Location of sampling sites (small dots), magnetic lineation trends and proposed paleostress directions in the Tsengwenchi section (located in Fig. 1).
Names of formations and ages in Ma (based on magnetostratigraphic and biostratigraphic analysis) indicated. Origin of stratigraphical level is set at the
Wushantou Fault.
to take samples almost continuously from older than 1.7 Ma SSW trending Hsiaokunshui anticline. To the east, the lower
(in the eastern part of the area) to younger than 0.7 Ma (to subsection is separated by the Hsiaokunshui Fault. The Er-
the west). The bedding commonly strikes N-S to NNE-SSW, hjenchi section (Fig. 4) corresponds to the Pleistocene lower
with some NE-SW deviations in the eastern subsections, and and upper Gutingkeng Formations. The lower Gutingkeng
dips 50◦–70◦ to the west. The bedding dip decreases from Formation is mainly constituted by very fine-grained mud-
east to west. The oldest layers crop out near the axis of NNE- stone, whereas in the upper Gutingkeng Formation mudstone

530
T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
Fig. 3. Location of sampling sites (small dots), magnetic lineation trends and proposed paleostress directions in the Hokouchi section (located in Fig. 1).
Names of formations and major stratigraphical boundaries (based on biostratigraphic analysis) indicated. Origin of stratigraphical level is set at the
boundary of Tangenshan Sandstone and Yenshuikeng Shale.
and fine-grained sandstone are interbedded. Magnetostrati- are principally made of magnetite, pyrrhotite and greigite.
graphic analyses showed that the lower Gutingkeng Forma- This magnetic mineralogy was identified based on X-ray
tion ranges from the Olduvai normal event to the Brunhes- diffraction patterns, acquisition of saturation isothermal re-
Matuyama boundary, whereas the upper Gutingkeng For- manent magnetization (SIRM), and the semi-destructive field
mation deposited during the Brunhes normal epoch (Horng, of SIRM. Magnetite and pyrrhotite are present throughout
unpubl. Ph.D. dissertation, 1991).
the sections, but in variable proportions. Magnetite is abun-
Magnetic minerals contained in the Tsengwenchi and Er- dant in the succession older than 1.6 Ma and pyrrhotite dom-
hjenchi sections have been studied. Horng et al. (1992) in- inates in the younger formations. Greigite is very rare in
dicated that the magnetic grains in the formations studied the older succession of the sections studied, and dominates

T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
531
Fig. 4. Location of sampling sites (small dots), magnetic lineation trends and proposed paleostress directions in the Erhjenchi section (located in Fig. 1).
Names of formations and ages in Ma (based on magnetostratigraphic and biostratigraphic analysis) indicated. Origin of stratigraphical level is set at the
thrust fault shown in the map.
in the formations having the age interval of 1.6–0.7 Ma in 3. Analysis of Magnetic Susceptibility
age. Magnetite, greigite and pyrrhotite were detrital, mainly 3.1 Methods and results
provided by rivers from the Central Range, undergoing uplift
More than 3,000 samples from 352 sites collected from the
and erosion, to the marine mudstone basin where a reducing three major sections (Fig. 1) were subjected to analyze the
environment prevailed. The magnetic minerals were rapidly anisotropy of magnetic susceptibility (AMS). Each sample
deposited in accumulating sediments and chemical transfor- was cut into several specimens of 22 mm in length and 25.4
mation was limited. It is known that the present-day sedi- mm in diameter. At each sampling site, at least five speci-
ments of the Taiwan Strait revealed similar situations (Horng, mens were chosen and 3 to 23 successful AMS determina-
pers. comm., 2000).
tions were obtained. The bulk susceptibility and anisotropy

532
T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
erage bedding plane of the corresponding site. This geo-
metrical relationship indicates that the fabric resulting from
syn-depositional and diagenetic flattening plays a significant
role. It also suggests that a pronounced magnetic fabric of
tectonic origin, if present, should have been acquired before
or during the initial stages of folding: at least two axes of the
AMS would have been found oblique to the bedding plane
otherwise. 70% of these 352 sites allowed determination
of magnetic lineation orientations. To quickly describe the
shape of the AMS ellipsoid regardless of the absolute mag-
nitudes of principal values, we made P’ vs. T plots, as Fig. 5
shows.
3.2 Types of magnetic fabrics
Analyzing both the P’ vs. T plots mentioned above and
the orientations of the AMS axes, five different AMS types
could be distinguished in our samples (Fig. 5). A typical de-
positional origin referred to as a-type for the AMS (Fig. 5a),
the axes of minimum susceptibility, Kmin, are tightly grouped
near the vertical (the normal to the backtilted bedding plane)
while the other two axes have rather scattered orientations
in the bedding plane. Another typical situation (b-type), is
characterized by the probable influence of paleo-current or
paleostress resulting in the presence of a magnetic lineation
(Fig. 5b): each of the three sets of principal axes displays
a consistent orientation, the Kmin axes being again nearly
normal to the bedding plane.
In the c-type (Fig. 5c), the axes of maximum susceptibil-
ity, Kmax, are well-grouped and lie in the bedding plane to
form the magnetic lineation, while the other two axes have
scattered positions within a girdle. This distribution is not
easily explained by depositional process and/or compaction
phenomena taken separately. It rather suggests a tectonic
origin, with compression perpendicular to the Kmax axis. In
the d-type (Fig. 5d), the Kmin axes are grouped, while the
other two axes form a girdle; the Kmin axes are parallel to, or
make a low angle with, the bedding plane. This distribution
also suggests that the sediments may have undergone some
horizontal compression, along the trend of the Kmin axes (i.e.,
NW-SE in Fig. 5d). Finally, the e-type is characterized by
well grouped Kmax axes nearly perpendicular to bedding, the
other axes being rather scattered in the bedding plane. This
may indicate that the sediments have undergone compres-
sion, but the direction of compression is poorly constrained.
However, such a pattern may also result from the inversion of
fabric caused by the predominance of single domain grains.
Further investigation is needed to solve this ambiguity. Note
that because our results were based on consideration of mag-
netic lineation, the d-type and e-type fabrics were not used
(only b- and c-types were used).
Fig. 5. Types of magnetic fabric identified in this study. a, b, c, d, e refer
to the five types described in text, compatible with increasing tectonic
strain. On left: P’ vs. F plots. On right: stereo plots (Schmidt projection
of lower hemisphere) of corresponding AMS principal axes (squares:
Kmax, triangles: Kint and circles: Kmin), shown after untilting. Further
details in text.
of magnetic susceptibility of these samples were measured
on the low-field (about 7 Gauss) Molspin Minisep system.
For each specimen, three principal axes and their magnitudes
(referred to as Kmax, Kint and Kmin) describe the ellipsoid
of AMS. Bedding attitudes (strike and dip) were accurately
measured in the field at each sample location. Orientations of
the three principal axes of AMS and its corresponding mag-
nitudes is calculated before and after bedding corrections.
No particular difficulty arose when making these geometri-
cal corrections, because most folds in the area are cylindrical
in type without any significant plunge. The diagrams and
the numerical results presented in this paper are given after
bedding correction.
3.3 Orientation of magnetic lineations
ThedistributionofthemagneticfabricpatternsintheTsen-
gwenchi section changes depending on the depositional age
of the samples. Major of the samples younger than 0.9 Ma
clearly show a-type fabric whereas the samples older than
0.9 Ma commonly show b-type and c-type fabrics. Mag-
netic lineations trending N5◦E to N30◦E prevail (Lee et al.,
1990). They form a single group of trends, N15◦E ( 10◦)
on average (Fig. 2).
In the Hokouchi section, most samples show magnetic
fabric of types b or c. Types d or e appeared in few sites,
Most fabrics revealed Kmin axes perpendicular to the av-

T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
533
and less than 10% of sites showed an a-type fabric (Lee and
Horng, 1993). Magnetic lineations in the Hokouchi section
can be divided into two groups in trends (Fig. 3). The most
common trend, N15◦E ( 10◦), is distributed in the entire
section. About20%ofthesiteshaveN5◦WtoN15◦Wtrends;
these sites are found in the western portion of the section,
where the age of sediments is older than the middle Pliocene.
Only a, b, and c types of magnetic fabrics were found in the
Erhjenchi section. They are randomly distributed, although
the lineation is less pronounced in the upper section (a-type
dominates in sediments younger than 0.9 Ma). About 50% of
sites show b or c types. The NNE-SSW magnetic lineation is
well represented (Fig. 4), but some deviation occurs relative
to the other two sections, resulting in an average trend N30◦E
( 10◦). Smaller subsets are also present, with some N-S
trends and some NE-SW trends.
3.4 Origin of magnetic anisotropy
The magnetic fabrics of type c to type e (Fig. 5) are gen-
erally resulted from tectonic deformation. However, type b
may be generated due to tectonic deformation and/or depo-
sitional process. To investigate the possible origin of type b,
we thus aimed at comparing the orientation data of both the
sedimentary structure and the tectonic structure with those
obtained from AMS analyses in order to determine what fac-
tor played the major role on the development of the AMS.
Field observation effectively seemed to bring some support to
the hypothesis of sedimentary origin, because paleochannels
and sedimentary structure clearly related to paleocurrents
were identified at several horizons in the sedimentary suc-
cession. In principle, depositional process may be responsi-
ble for the development of magnetic lineations because they
imply preferential orientation of mineral grains. To evaluate
their possible influence on the development of the observed
magnetic lineations, the orientations of all these sedimentary
features should be analyzed and then compares with the ori-
entations of the magnetic lineations (the original trends and
dips prior to folding being of course restored to stratigraphic
coordinates). De´ffontaines et al. (1994) analyzed the geom-
etry of the late Pleistocene-Holocene drainage pattern in this
large mudstone area of southwestern Taiwan, including our
studied sections, and indicated no systematic flow directions.
The lack of geometrical relationships between complex
depositional patterns and rather homogeneous trends of the
AMS suggests that syn-depositional phenomena contributed
little to the development of the magnetic anisotropy fabric,
whereas the certainly influenced vertical minimum axis as
mentioned before. The main result of our analysis is that
the part of magnetic fabrics which cannot be explained only
by the flattening resulting from compaction is consistent with
developmentindeformation(Fig. 5b–d). Thatthesyndeposi-
tional origin fails to account for most magnetic lineations ob-
served certainly does not imply that these lineations are tec-
tonic origin. Our conclusion is most magnetic lineations to
be tectonic origin in the study area. It is supported by the ge-
ometrical consistency between the trends of these lineations
and the orientations of the compressive tectonic regimes re-
constructed based on independent structure analyses. For
rocks with identifiable magnetic lineations, consistent trends
are thus obtained for the inferred compression axes. These
trends are summarized in the rose diagram of Fig. 6. The in-
Fig. 6. Rose diagram of the directions of compression inferred from the
complete set of reliable magnetic fabric determinations with probable
tectonic significance (1,891 AMS determinations in 251 sites).
ferred trend of compression is N105◦–120◦E for more than
60% of data, and 90% of data fall in the range N95◦–135◦E.
In detail, 5% of data indicate a minor isolated group of N80◦–
N85◦E trends, whereas the N120◦–140◦E trends merge into
the main azimuthal group. Although they are statistically
significant, the two minor trends are difficult to detect in
Fig. 6 because they are adjacent to the strongly dominating
N100◦–120◦E trends.
It is shown, in a later section, that the orientations of mag-
netic lineations are generally consistent with the paleostress
trends. A model involving tectonic deformation added to
compaction explains the magnetic lineations. In summary,
the NNE-SSW predominant trend of the magnetic lineations
did not fit the depositional patterns well, but generally co-
incided with the structural grain of the region investigated,
where major folds and thrusts strike NNE-SSW (Fig. 1). This
suggested that the AMS had been influenced by tectonism.
An independent tectonic analysis was thus undertaken, in
order to better characterize the compressional deformation.
4. Tectonic Analyses
In the Tsengwenchi, Hokouchi and Erhjenchi sections,
faults with observable slickenside lineations were measured
at numerous sites. We determined paleostress tensors from
fault slip data sets, based on the use of inversion methods
thoroughly described elsewhere (Angelier, 1984, 1990). We
thus reconstructed the principal paleostress axes σ1 (maxi-
mum compressional stress), σ2 (intermediate stress) and σ3
(minimumstress), aswellastheratioꢀ = (σ2−σ3)/(σ1−σ3)
between principal stress differences. Other data, such as for
the orientations of tension fractures, were also used in this
tectonic study.
Particular attention was paid to the geometrical relation of
fault-fracture systems to bedding attitudes and fold shapes.
This was because the geometrical analysis allows distinc-
tion between pre-folding, syn-folding and post-folding brit-

534
T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
Fig. 7. Examples of paleostress reconstructions based on fault slip analyses. Schmidt projections, lower hemisphere. For a, b, c and d, present-day attitude
shown. For e, f, g and h, present-day (on left) and backtilted (on right) attitudes shown. Fault-fractures planes as continuous lines; slikenside lineations
shown as small dots with arrows indicating the sense of motion (double arrows for strike-slip, outward directed arrows for normal slip, inward directed
arrows for reverse slip). Bedding planes shown as dashed lines, with poles to bedding as open dots. Inversion of fault slip data according to Angelier
methods (1984 and 1990): direct inversion for b, c, d and e, 4-D search for f, g and h. Axes of maximum compressive stress (σ1), intermediate stress
(σ2) and minimum stress (σ3) shown as 5, 4 and 3-branch stars respectively, with symbol size varing as a function of the ratio between principal stress
differences, ꢀ = (σ2 − σ3)/(σ1 − σ3). Large black arrows indicate trends of compression and/or extension. For each stress determination, value of
F, number of fault slips used, trends and plunges of axes and misfit parameter listed in Table. (a) late WNW-ESE compression: tension joints, west of
Erhjenchi section, Pleistocene Liushuang Formation (0.6–0.3 Ma); open triangles are poles to joints. (b) late WNW-ESE compression: strike-slip faults,
same section and formation. (c) main WNW-ESE compression, post-folding stage: reverse and oblique slip faults, Erhjenchi section (sediments approx.
0.6–0.7 Ma old). (d) WNW-ESE compression, post-folding stage: strike-slip and oblique slip faults, Erhjenchi section (sediments approx. 1.7 Ma old).
(e) NW-SE compression, syn-folding stage: tilted strike-slip faults, Erhjenchi section (Lower Gutingkeng Formation). (f) main WNW-ESE compression,
syn-folding stage: tilted strike-slip faults, Erhjenchi section (sediments approx. 0.8–0.9 Ma old). (g) E-W compression, pre-folding stage: tilted reverse
faults, eastern Tsengwenchi section (sediments approx. 3 Ma old). (h) NW-SE compression, pre-folding stage: Erhjenchi section (sediments approx. 0.6
Ma old).
tle tectonic events. By considering the attitudes of com- occurred, this axis is not vertical, but approximately perpen-
puted principal stress axes relative to bedding planes at sites dicular to tilted bedding. It was even possible to identify
where strata dips are steeper than 30◦, it was possible to fault systems which rotated during folding, as revealed by
distinguish brittle systems that predate or postdate folding. intermediate attitudes of computed axes. This approach had
In post-folding fracturing, one axis is usually nearly verti- been applied by Angelier et al. (1986, 1990) to the Western
cal (σ2 or σ3, depending on whether strike-slip or reverse Foothills and the Hsuehshan Range of Taiwan, in order to
faulting mode dominated). Where pre-folding fracturing has decipher the succession of events and to relate minor fault-

T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
535
fracture patterns with larger fold-and-thrust structures.
We conclude that a major compression trending N100◦–
Examples of paleostress analyses are shown in Fig. 7. The 110◦E affected the area of Fig. 1. To the north (Tsengwenchi
simplest patterns of brittle features are found in the youngest and Hokouchi sections), this compression trends N100◦E
formations. Two sites were located west of the Erhjenchi ( 15◦). It affects sediments which range in age from about
section, in the Pleistocene Liushuang Formation, of 0.3–0.6 3 Ma (and older) to 0.9–0.7 Ma (younger formations ab-
Ma old or even younger. Both the tension joints (Fig. 7a) and sent) and is related to the last stages of folding. Observation
the strike-slip faults (Fig. 7b) are consistent with a compres- of angular unconformities, and consideration of fold orienta-
sion axis trending N100◦–110◦E, perpendicular to the axis tions below and around these uncomformities, suggests that a
of gentle folding.
peak stage in this N100◦E compression occurred during 0.9–
A stronger compression, with the same direction (N105◦– 1.0 Ma (although the same compression had certainly begun
115◦E), is revealed by sets of faults where reverse slip before and has continued later). To the south (Erhjenchi sec-
(Fig. 7c) or strike-slip (Fig. 7d) dominates. These two sites tion), this major compression trends N105◦E ( 5◦), resulted
belong to the Erhjenchi section, in tightly folded layers west in most of the folding, and affects sediments which range in
of the Hsiaokunshui anticline; the sediments affected are ap- age from about 0.9 Ma (and older) to 0.6–0.3 Ma. The poor
proximately 0.6–0.7 Ma (Fig. 7c) and 1.7 Ma old (Fig. 7d). record of this compression in the upper formations indicate
Because the reconstructed compressional trends are nearly that most of the folding occurred before about 0.6–0.7 Ma .
perpendicular to fold axes, most of the folding is attributed
An older N120◦–140◦E trending compression (Fig. 7f and
to the WNW-ESE compression. In Figs. 7a and b, the calcu- h) was reconstructed in both the northern and southern sec-
lated compression axes are nearly horizontal while the bed- tions. To the north, it trends mainly N120◦–135◦E and af-
ding steeply dips in fold flanks; this indicates a final stage of fects formations approximately 1.9–2.0 Ma. To the south,
compression, with the folds already formed.
it trends N120◦–140◦E (mainly 130◦–135◦) and affects for-
The same N110◦E compression is revealed by the pattern mations ranging in age from 1.7 to 1.0 Ma (and older). In
of oblique-slip faults of Fig. 7e, in the lower Gutingkeng For- bothcases, it unambiguously predatesthemainfoldingevent.
mation of the Erhjenchi section. The computed compression However, intheErhjenchisection, thisNW-SEtrendingcom-
axis dips significantly to the west; the minimum stress axis is pression is probably related to part of folding; syn-folding
nearly horizontal and parallel to bedding strike in fold flank N120◦–140◦E compression could be identified in sediments
(Fig. 7e, left). These faults developed as a conjugate strike- older than about 1.5 Ma.
slip system during the fold development. A simple untilting
process restores their initial attitude (Fig. 7e, right).
The other compression found in these sections trends
N085◦E on average (Fig. 7g), and only affects formations
In a nearly vertical flank of the Hsiaokunshui anticline in older than 2.5 Ma. It predates the major folding and affects
the Erhjenchi section, a tilted strike-slip fault system affects the eastern area, which suggests an inner origin in the belt.
sediments approximately during 0.8–0.9 Ma. It developed at
an intermediate stage of folding (Fig. 7f). The original trend
ofcompressionisaboutN130◦E, acommondirectionofcom-
pression in the eastern section. It was necessary to determine
the age of this NW-SE compression relative to that of the ma-
jor WNW-ESE compression. We took into consideration the
fold geometry, the existence of angular unconformities in ad-
jacent localities and the relative chronology between faulting
and folding as indicated by geometrical and crosscutting re-
lationships. We thus concluded that in the study area the
NW-SE compression is older than the widespread N110◦E
compression.
In a vertical fold flank west of the Wushantou thrust, in
the eastern Tsengwenchi section, the sediment ages average
3 Ma. The present-day pattern resembles a system of conju-
gate normal faults (Fig. 7g, left). The faults in fact developed
prior to any folding, as reverse faults under E-W compres-
sion (Fig. 7g, right). This N85◦E compression only affects
the oldest formations, and occurred early in the structural
development of the area.
5. Discussion and Conclusion
To determine the tectonic significance of the magnetic fab-
rics recorded in the southwestern Foothills of Taiwan (Fig. 1),
we carried out systematic comparisons between the results
of AMS studies (Figs. 2–6) and the results directly provided
by structural analyses of fold and fault-fracture structure sys-
tems (Fig. 7 and Table 1). Similar geometrical comparisons
are shown that the depositional process were unable to ac-
count for the magnetic lineation trends. We then considered
a model of increasing compressional strain, compatible with
the types of magnetic fabrics illustrated in Fig. 5, and we
checked its capacity to account for both the AMS and tec-
tonic data. The compression inferred from AMS analyses
generally lies in the N100◦–120◦E direction (Fig. 6). This
is in close agreement with the results of both the new local
deformation analysis presented in this paper and the earlier
studies of the surrounding Foothills: a WNW-ESE compres-
sion dominated during the last Taiwan collision (Angelier et
al., 1986; Chu, 1990; Lee, 1994).
A situation of pre-folding compression is also illustrated in
Fig. 7h for the NW-SE compression: strike-slip faults devel-
oped in nearly horizontal or shallowly dipping layers prior
to the development of the main folding event with NNE-
SSW axes. This example provides further evidence that the
N130◦E compression predated the N110◦E one. The sedi-
ments affected in the case of Fig. 7h are approximately 0.6
Ma old, still younger than those of Fig. 7f. A long time span
cannot have taken place between these events.
A major conclusion of this study is the strong general con-
sistency between the trends of orientation in AMS analyses
(Figs. 2–4, Fig. 6) and the trends of compression revealed
by fault slip data analyses in the same sections (examples in
Fig. 7, and Table 1). In terms of magnetic fabrics, the inferred
compressional trends are perpendicular to the directions of
magnetic lineations for types b and c, or approximately par-
allel to the normal vector of flattening for type d (Fig. 5). The
directions of magnetic lineations (Kmax axes) have tectonic

536
T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
Table 1. Examples of results of fault slip data inversion (first column refers to Fig. 7). Trends and plunges of computed principal paleostress axes in
degrees. Ratio ꢀ defined in text. Average misfit angle given in degrees. N, number of fault slips. For location and stratigraphic ages, see Fig. 7 and text.
Attitudes of principal paleostress axes after untilting given on right, for sites where folding continued after faulting (compare with Fig. 7).
Ref.
In
trend and
plunge of
σ1 axis
trend and
plunge of
σ2 axis
trend and
plunge of
σ3 axis
Ratio
Misfit
N
tr. and pl.
of σ1 axis
backtilted
tr. and pl.
of σ2 axis
backtilted
tr. and pl.
of σ3 axis
backtilted
ꢀ
angle
Fig.
b
c
d
e
f
291
113
287
292
309
316
323
14
141
19
75
23
207
192
198
207
87
7
0.14
0.02
0.40
0.56
0.48
0.14
0.05
4
9
27
24
18
16
32
24
15
4
13
37
37
75
29
45
65
53
49
11
60
45
22
5
45
10
9
101
99
114
307
89
0
24
41
79
204
217
234
49
11
15
10
8
10
6
1
14
3
77
9
13
74
5
g
h
179
124
356
197
228
3
319
84
significance for the sites having magnetic fabrics of types b proximately 1 Ma ago in the area studied. It was recorded
and c, and the trend of compression is perpendicular to that in sediments older than 1.0 Ma in the southern section. This
of the magnetic lineation (after untilting). For type d, this compression only left a minor magnetic lineation in some of
trend is that of the average Kmin axis, perpendicular to the gir- our samples, because the total deformation remained limited
dle. Both the geometrical incompatibility with depositional and also because of the strong overprinting due to the later
patterns and the good fit with the results of independent tec- compression. To the north, the youngest formations affected
tonic analyses supported our tectonic interpretation of the are approximately 1.5 Ma old. This NW-SE compression
magnetic lineations.
generally predates most of folding. However, it is related
In terms of regional tectonic evolution, the timing between to part of folding in the Tsengwenchi section (with N130◦–
the WNW-ESE, W-E and NW-SE compressions were not 135◦E trends before this folding stage, and N120◦–135◦E
firmly established (Chu, 1990). This suggests complex stress during it). Distinguishing between this NW-SE compression
partitioning during the Quaternary rather than a definite suc- and the later WNW-ESE compression is locally difficult, es-
cession of tectonic events with typical trends (Lacombe et pecially in the areas where intermediate trends are present
al., 1993). In terms of local AMS distribution, and consid- (around N120◦E).
ering the stratigraphic ages of the formations (Figs. 2–4), it,
(3) The paroxysmal compression occurred at about 0.9–
however, appears that the different AMS directions are time 1.0 Ma. Its trends are rather homogeneous (N100◦–110◦E),
and space dependent. This is also the case for the local fault despite some local deviations as in the Erhjenchi section (lo-
slip analyses presented in this paper. Discrepancies remain cal N120◦E trends). This major event is responsible for the
minor, within the range of reasonable uncertainties for both main folding and thrusting, which strongly affects sediments
the trends and the age determinations.
older than 0.9–1 Ma in the area studied. Major folds and
The tectonic history of the area studied is summarized as thrusts trending N010◦–020◦E (Fig. 1) are perpendicular to
follows:
compression. The magnetic fabric was deeply influenced by
(1) A N080◦–090◦E compression probably occurred be- this event. The age of the peak compression is constrained
fore 2.5 Ma. It is revealed by local tectonic observations in by the magnetostratigraphic ages: in the southern section
the northern sections where it affects sediments older than (Erhjenchi), sediments younger than 0.7 Ma showed little or
2.5 Ma (Tsengwenchi). It is consistently revealed by AMS no magnetic lineation. In sediments younger than 0.9 Ma,
analyses in sediments older than the middle Pliocene of the the magnetic lineation is more pronounced to the south than
Hokouchi section. This compression affected flat-lying sedi- to the north, consistent with a stronger recent WNW-ESE
ments which subsequently suffered folding and faulting. It is compression to the south.
likely that this compression was stronger to the east, while the
In summary, the interpretation of the AMS data, supported
area studied was a simple foreland with almost undeformed by independent results of structural analysis, strongly sug-
formations. However, in southwestern Taiwan, analyses of gests that a widespread N100◦–110◦E compression, early
Quaternary brittle tectonics (Lacombe et al., 1993) and fo- Quaternary in age, affected this area until about 0.9 Ma and
cal mechanisms of earthquakes (Yeh et al., 1991) revealed decreased later. In detail, traces of an earlier NW-SE com-
a complex pattern of stress (including E-W compression). pression were found, and some E-W compression had oc-
Based on these tectonic observations, our conclusion is that curred before. Comparisons between the sections suggest
the N080◦–090◦E compression represents a relatively old that the major N100◦–110◦E compression occurred (or per-
event, may be of local value, and, in any case, should not be sisted) later to the south. This conclusion is not surprising
extrapolated to the whole western Foothills of Taiwan.
(2) A N130◦–140◦E compression (with few additional ev- along the Taiwan belt.
idences for a N150◦E compression) may have occurred ap-
In previous magnetic studies, it was pointed out that a
in light of the migration of the collision from north to south

T.-Q. LEE AND J. ANGELIER: MAGNETIC SUSCEPTIBILITY FABRIC IN SOUTHWESTERN TAIWAN
537
diachronic clockwise rotation had occurred in the Coastal in the study area between the orientations of the magnetic lin-
Range area of eastern Taiwan, on the opposite side of the eations revealed by our AMS analyses and the orientations
mountain belt (Lee et al., 1990, 1991a). This rotation propa- of paleocurrents, sediment supply directions and even slump
gated southward while the units of the Luzon Arc were suc- structures (which in addition display large scatter whereas
cessively colliding against Taiwan (resulting in the Coastal AMS trends show systematic grouping in few directions).
Range); it occurred in the northern part about 3 Ma ago, in Second, the predominant trends of magnetic lineations were
the middle part about 2 Ma ago and finally in the southern found geometrically consistent with the orientation of the
part about 1 Ma ago. Taking into account both the mag- compressional tectonic regimes reconstructed independently
netic data and the tectonic information, Lee et al. (1991a) based on structural analysis and fault slip data inversion.
proposed that just before the rotation of units, the compres-
The main tectonic source of anisotropy in the study area
sional paleostress had been trending E-W, and thus changed was the WNW-ESE regional compression, which also re-
to a N100◦–120◦E direction during and after rotation. Com- sulted in major Quaternary folds and thrusts. In addition,
paring the results of our present study to those the eastern twosecondarytrendsofcompression, E-WandNW-SE, were
Coastal Range, they display some similarity.
recognized, also consistent with tectonic evidence. Although
However, a difference between the situations east and west this is not the aim of this paper, it is important to observe that
of Taiwan lies in the fact that not only the paleomagnetism the regional distribution of these compressions in space and
failed to reveal large Plio-Quaternary rotations of units in time, as identified for the period of the last 2 Ma, is closely
the southwestern Foothills, but also the most reliable data related to the southward migration of the Taiwan collision.
suggest that little Plio-Pleistocene rotation has occurred. In
Acknowledgments. The magnetic study was supported by the Na-
tional Science Council of Taiwan (grants NSC82-0202-M-001-108,
NSC83-0202-M-001-059 and NSC84-2111-M-001-012). The
structural analysis was supported by the Institut Franc¸ais a` Taipei
the Coastal Range, the apparent counterclockwise rotation of
the compression results from both the clockwise rotation of
the thrust units and the counterclockwise deviation of stress
(increasing with mechanical coupling). In the southwestern
and the National Science Council within the framework of the Sino-
Foothills, the latter factor is predominant. In the absence of French cooperation in Earth Sciences. The help from Dr. Lee Jian-
large rotation, it explains the change from N130◦–140◦E to
Cheng and Dr. Hu Jyr-Ching for improving maps is acknowledged.
Very useful comments were done by the referees, Dr. K.-P. Kodama
and Dr. W. Soh.
N100◦–110◦E compressional trends in the youngest parts of
the Hokouchi and Tsengwenchi sections. This counterclock-
wise change may be related to the presence of the Peikang
High, a zone of uplifted foreland basement located northwest
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