doi: 10.1111/j.1365-3121.2005.00636.x
Fossil fluid reservoir beneath a duplex fault structure within the
Central Range of Taiwan: implications for fluid leakage and
lubrication during earthquake rupturing process
Yu-Chang Chan,1 Kazuaki Okamoto,1 Tzen-Fu Yui,1 Yoshiyuki Iizuka1 and Hao-Tsu Chu2
1
Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan; 2Central Geological Survey, Taipei, Taiwan
ABSTRACT
In order to understand the kinematics which likely facilitated
the speedy rupturing process of the 1999 Mw 7.6 Taiwan ChiChi earthquake, we examined exposed rocks in the Taiwan Slate
belt, where the pressure and temperature conditions most
resembled the hypocentre of the Chi-Chi earthquake, i.e. subgreenschist facies. Field observations and composition analyses
of the silicified vein-rich zones beneath the duplex structure
suggest that impermeable slate layers may serve as cap rocks
for confining deep-seated fluids. These fluids most likely come
Introduction
The 1999 Mw 7.6 Taiwan Chi-Chi
earthquake occurred at a shallow
depth of around 10 km (Chung and
Shin, 1999; Kao and Chen, 2000; Ma
et al., 2000) within the fold and thrust
belt of Taiwan (Fig. 1). The damaging
earthquake ruptured the geologically
well-defined Chelungpu fault and the
faulting propagated more than 80 km
in length from south to north (e.g.
Shin and Teng, 2001). The whole
rupturing process consisted of five
major subevents propagating from
south to north and the amazing speed
of the earthquake rupturing was
measured c. 2.2–2.6 km s)1 (Chen
et al., 2001; Kao and Angelier, 2001).
After rupturing the largest vertical
displacement occurred along the Chelungpu thrust fault was measured to be
approximately 9 m (e.g. Lee et al.,
2002).
The unusual rupturing speed has led
to a common postulation that fault
lubrication occurred during the earthquake (Ma et al., 2003). Actually,
many studies on exhumed fault rocks
indicated that faulting at seismogenic
depths (c. 5–20 km) occurred as fluid
Correspondence: Dr Kazuaki Okamoto,
Institute of Earth Sciences, Academia
Sinica, PO Box 1-55, Nankang, Taipei,
Taiwan 115. Tel/: 00 886 2 2783 9910 (ext.
502); fax: 00 886 2 2788 3493; e-mail:
[email protected] or kokamoyo@
hotmail.com
2005 Blackwell Publishing Ltd
from the Taiwan metamorphic complex at deeper depths by the
dehydration and decarbonation reactions (or partial melting). In
addition, the gouge zone of a link fault above the detachment
also indicates the presence of overpressured fluids during
faulting. It is probable that episodic leakage of the confined
fluid reservoirs may provide essential fluids for fault lubrication
during earthquake ruptures.
Terra Nova, 17, 493–499, 2005
pressure exceeds hydrostatic (e.g. Kerrich, 1986; Parry and Bruhn, 1990;
Cox, 1995; Robert et al., 1995; Nguyen et al., 1998; Crampin et al., 2002)
or even lithostatic levels (e.g. Boullier
and Robert, 1992; Cox, 1995; Nguyen
et al., 1998) as well as the existence of
overpressured fluids during overthrusting (Hubbert and Rubey,
1959). Accumulated geological evidence has shown that within the
middle to upper crust, active fault
zones can play a crucial role in controlling the architecture of crustal
fluid migration (e.g. Kerrich, 1986;
Sibson et al., 1988; Hickman et al.,
1995; Yamamoto et al., 2004). Fluids
migrating through faults also have a
substantial influence on fault mechanics (e.g. Sibson, 1989, 1992, 1996;
Rice, 1992). However, little research
has been performed to understand
possible fluid chambers and to evaluate the role of fluids contributing to
large earthquakes in Taiwan, such as
the recent Taiwan Chi-Chi earthquake.
Here we assume the sub-greenschist
facies Taiwan slate belt closely resemble the geological conditions now
occurring at the same depths in the
Taiwan fold and thrust belt. This
assumption may be justified by the
observations that the accretion process of the Taiwan Mountain belt
forward from east to west (Fig. 1).
In addition, the exposed slates and
meta-sandstones within the Central
Range (the internal fold-and-thrust
belt) consist of largely Eocene–Miocene, continental slope and rise sediments. These passive continental
margin sediments do not differ greatly
with the fold and thrust belt of the
Taiwan foothills where the Chi-Chi
earthquake had occurred.
In this study, we described and
analysed outcrops from central Taiwan showing that slate belt can be
effective for confining fluids from
great depths and that the vein materials contain high-temperature minerals. We proposed that a leakage of
fluids and subsequent volatilization of
gases (mainly H2O and CO2) can
readily facilitate and lubricate the
movement of the faults above the
reservoirs. This explanation is consistent with the outcrop observation that
the fossil fluid reservoirs situated
underneath a well-developed duplex
fault structure and that overpressured
gouge zones developed along link
faults above detachment.
Outcrop description
An outcrop of highly silicified zone
containing abundant quartz veins
appears along the Central Cross Island highway among the Backbone
Range slate belt of Taiwan (Fig. 1).
We have checked several tens of outcrops and excluded the outcrops obviously suffered polyphase deformation
during exhumational stage. The lithology of the outcrop consists mainly of
meta-sandstone and slate interlayers.
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Fossil fluid reservoir beneath a duplex fault structure • Y-C. Chan et al.
Terra Nova, Vol 17, No. 6, 493–499
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Fig. 1 Simplified geological map of Taiwan showing the sampling site of the study.
Cross-section along the central Taiwan showing a conceptual fluid source and path of
migration along the Chelungpu thrust fault. Stars indicate outcrop locations in the
present study. Figure modified from Lee et al. (2002).
These rocks have undergone the latest
Plio-Pleistocene Taiwan orogeny and
were later brought up to the surface.
The major sub-horizontal silicified
zone exhibits abundant quartz veining, fracturing and brecciation
(Fig. 2). On the borders of the silicified zone, fault slickensides are commonly observed. The silicified zone
was approximately 1 m thick and
above the zone sits a well-organized
duplex fault system indicating a topto-the-northwest thrust faulting. In
494
addition, the silicified zone is noticeably capped by layered slates which
contain very few veining structures.
Sub-horizontal zones of possible
fluid-filled veins are interconnected
with vertical veins (Fig. 3a) suggesting
interconnections of fluids in horizontal silicified zones through vertical
cracks. A large, lenticular quartz mass
is located within a dilatant zone
(Fig. 3b) of the silicified zone. The
quartz mass exhibits that the pressurized fluid is concentrated in lensoidal
compartments rather that distributed
over large domains (Bruhn et al.,
1994). The quartz mass also suffered
brittle deformation characterized by
the intracrystalline fractures and
cracking. The fractured quartz grains
are commonly surrounded by matrix
cataclasites.
Minor thin quartz veins are recognized in the meta-sandstone layers
alternating with the slate layers above
the silicified zone. These veins are
largely perpendicular to the sedimentary layers. One important observation is that V-shaped veins typically
abutted the less permeable slate layers
suggesting that fluids were trapped
and contained mostly within the
highly
fractured
meta-sandstone
layers (Fig. 4). These minor thin
quartz veins are interconnected with
other fluid sources, especially, the
silicified zone. The V-shaped vein
features in the meta-sandstone beneath the slate layer, are recognized
as Ôarrested hydrofracturesÕ in the
layer beneath soft layer (Gudmundsson and Brenner, 2001; Brenner and
Gudmundsson, 2004). The hydrofracture features depend on the local
stresses controlled by abrupt changes
in the rock’s Young’s moduli (Gudmundsson and Brenner, 2001).
Another outcrop examined is close
to the Lishan lineament, a major
tectonic boundary in Taiwan near
the sampling site (Fig. 1). The Lishan
lineament appears to be one of the
most obvious geomorphic features in
Taiwan. The lineament was interpreted as a tectonic fault, the Lishan
fault, which separated the two domains of the Taiwan slate belt, the
Hsuehshan Range and the Backbone
Range, and is considered as the link
thrust to the detachment (Lee et al.,
1997; Ho, 1998). Although the Lishan
fault remain largely covered by river
terraces and sediments, a fault outcrop with unusual veining activities
was discovered along a river along the
Lishan fault.
Most veins at the outcrop are subvertical and concentrated within a
zone where sandy slates dominate.
The subvertical veining zone is also
truncated by several sharp and thin
faults. A gouge zone contains unusual
fabrics: fractured quartz and carbonate grains floating within the clay
matrix (Fig. 5). These fractured mineral grains possibly came from crush 2005 Blackwell Publishing Ltd
Y-C. Chan et al. • Fossil fluid reservoir beneath a duplex fault structure
Terra Nova, Vol 17, No. 6, 493–499
.............................................................................................................................................................
West
East
silic
ifie
d
zon
e
sili
cifi
ed
zo
n
e
Fig. 2 A sub-horizontal silicified zone which contains abundant quartz veining,
fracturing and brecciation within the zone. Above the silicified zone sits a wellorganized duplex fault system indicating top-to-the-northwest thrust faulting. The
silicified zone is capped by layered slates, which contain only few veining structures.
ing of the surrounding quartz/carbonate veins. These veins are strongly
sheared and show wavy extinction
suggesting the deformation did occur
at great depths. There are no particular shear fabrics or foliation within
the grain-filled gouge zone. The random arrangement of the crushed mineral grains within the clay gouge
matrix suggests that gouge zone may
have been once filled with high-pressure fluid leading to very low shear
strength of the fault.
The veining zone of the Lishan fault
outcrop also shows slickensides along
its borders. The fault striations
indicate variable sliding directions
including down-dip, oblique and
strike-slip. Away from the narrow
veining zone, there is little occurrence
of veins indicating that these veins are
quite localized along the observed
fault zone.
Petrochemical characteristics
In order to understand the fluids
origin and interaction with the country rocks, petrochemical analyses
(XRF scanning, SEM-EDS and
EPMA observation) were performed
on rock samples collected from the
concentrated vein zone of the sampling site (Fig. 1) in the Backbone
Range slate belt (Figs 1 and 2). A
detailed description and the petrological significance are summarized as
follows.
Veins have complex cross-cutting
relationships but can be distinguished
primarily as vertical and horizontal
ones. Observation of polished slabs
and thin sections revealed that veins
can also be classified into two types in
terms of mineralogy: a calcite vein and
quartz vein. Calcite veins contain
calcite minor apatite and sulphide
minerals. Quartz veins consist of
quartz,
albite,
apatite,
calcite
(Fig. 6), as well as minor rutile,
K-feldspar and monazite. One observation shows that a quartz vein was
cut by a calcite vein, which in turn was
cut by a quartz–feldspar vein. This
cross-cutting relationship suggests almost simultaneous and/or repeated
vein formation of the observed vein
structures.
Mineral group in the observed
quartz veins indicates that the fluids
came from a high-temperature source,
likely higher than the metamorphic
peak temperature of the outcrop
because rutile is quite insoluble in an
aqueous fluid at upper crustal conditions (e.g. Pearce and Cann, 1973).
The metamorphic grade of the host
rock is sub-greenschist to lower greenschist facies characterized by chlorite + K-feldspar mineral paragenesis
without biotite. Thus, based on the
observed mineral group in the veins,
the source fluids of veins are not of
in situ but of exotic origin from depths.
In order to better characterize the
P, T conditions of the silicified zone,
we also carried out a fluid inclusion
Fig. 3 Complex mesh veins in the silicified zone. (a) Sub-horizontal fluids-filled veins are interconnected with vertical veins
suggesting interconnections of fluids in horizontal silicified zones through vertical veins. (b) A large, irregularly shaped quartz mass
is located within a dilatant zone.
2005 Blackwell Publishing Ltd
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Fossil fluid reservoir beneath a duplex fault structure • Y-C. Chan et al.
Terra Nova, Vol 17, No. 6, 493–499
Permeable
Impermeable
Permeable
Impermeable
.............................................................................................................................................................
Fig. 4 Two field photographs showing typical V-shaped veins abutted to the less
permeable slate layers. The phenomenon suggests that fluids reside mostly within the
fractured-prone meta-sandstone layers.
study of the sampled quartz veins.
Table 1 lists the fluid inclusion data.
To sum up, the total homogenization
temperatures (Th) are in the range of
208–255 C. Fluid pressure (Pf) is
estimated to be higher than 228 Mpa
at temperatures above 250 C. These
data are consistent with the trapping
of an H2O–CO2–NaCl fluid in the
single-phase region.
Discussion
For the development of fluid overpressure in the sub-greenschist facies
at depths, there must be low-per496
meability capping layers formed in
either stratigraphic process (e.g. shale
formation) or hydrothermal cementation (e.g. Bruhn et al., 1990; Hunt,
1990; Blanpied et al., 1992; Byerlee,
1993; Sibson, 2000). Our field observations suggest that the slate layer
above the detachment plays a critical
role as impermeable capping layer
because the observed vein textures
clearly indicate that hydrofractures
become arrested and poorly interconnected upward, as explained by Gudmundsson and Brenner (2001) and
by Brenner and Gudmundsson (2002,
2003).
Taiwan is a typical continent-arc
collision zone, and continental slope
and rise deposits were overthrust in
the western half of the Taiwan upper
crust. The stratigraphic cross-section
indicates that shale and slate are the
major rock types above the fold-andthrust belt detachment (Fig. 1).
According to direct fluid pressure
measurements taken in the western
foothills and coastal plain of Taiwan,
a permeable zone of hydrostatic fluid
pressure gradients (with depths greater than 2–4 km) overlies less permeable sediments exhibiting an
overpressured fluid gradient (Suppe
and Wittke, 1977; Suppe et al., 1981;
Davis et al., 1983). That is, the fluid
pressure increases from hydrostatic
towards nearly lithostatic at depths
greater than about 4 km. This overpressured fluid gradient is rather constant throughout the western Taiwan.
However, zones with overpressured
fluid gradients are now exposed in the
region where the less permeable PlioPleistocene shale sediments had already eroded (Davis et al., 1983). This
indicates that low permeability and
arrested hydrofracture in the region is
controlled by the collisional type stratigraphy, in this case, the slightly metamorphosed slate belt.
The petrochemistry of the veins
stacked beneath the detachment suggests that stored fluid was of high
temperature in origin and from great
depths because of the presence of
albite, K-feldspar and REE minerals.
Existence of calcite veins and presence
of calcite in quartz veins suggest that
the migrated fluid contains CO2.
Underlying basement lithology beneath the slate layer is considered as
the pre-Tertiary Tananao schist metamorphic complex, which suffered
greenschist and amphibolite facies
metamorphism. The Tananao schist
complex consists of four major lithological types: marble, granodioritic
ortho- and banded para-gneisses, pelitic and psammitic schists and chlorite schists (e.g. Ernst, 1983).
Therefore, carbon dioxide rich fluid
could have arisen from the pre-Tertiary basement because of dehydration/decarbonation reactions.
Alternatively, fluid may also be
derived from partial melting of the
pre-Tertiary basement complex underlying the Taiwan slate belt. Recent
study of shear-wave seismic attenu 2005 Blackwell Publishing Ltd
Y-C. Chan et al. • Fossil fluid reservoir beneath a duplex fault structure
Terra Nova, Vol 17, No. 6, 493–499
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qz
Gouge
qz
Sandstone
qz
2 cm
Fig. 5 Outcrop vein feature adjacent to Lishan fault. A gouge zone contains fractured
quartz and carbonate grains floating within the mud matrix. The quartz (qz) veins
beneath the gouge are strongly sheared and show wavy extinction under the
microscope. Origin of porphyroclasts in the gouge are interpreted to be from the
crushed adjacent veins (some shown in outlines).
Fig. 6 BSE image of the fibrous vein and element concentration images for Si, Ca,
and P of the vein. Note that albite, calcite and apatite fibres were grown with fibrous
quartz.
ation has revealed low Qs zone
beneath the slate belt of the Central
Range at depths of 30–40 km (Wang
et al., 2003). This low Qs zone correlates with a low velocity zone in both
2005 Blackwell Publishing Ltd
P- and S-waves, as determined from
seismic tomography studies. These
seismic features imply that there is a
fluid or a melt dominant zone within
the subducted continental crust or
basement rock. The temperature conditions of the zone are roughly estimated to be higher than 600 C from
the presumed geotherm of 15 C per
km for the collisional type metamorphism (e.g. Spear, 1993). It is consistent with the max P, T conditions
deciphered from the garnet submicron
inclusion from the Tananao Schist
(P > 8.3–8.8 kbar and T < 660–
690 C) (Hwang et al., 2001). The
temperature conditions exceed the
solidus temperature for the continental basement, implying that the low Qs
zone is possibly a partially melting
zone rather than a dehydration zone.
The partial melt likely contains H2O
and CO2, i.e. a volatile-rich fluid,
which may separate from the melt
during upward migration to the
detachment fluid reservoirs at much
shallower depths.
The deformation textures from the
Lishan fault suggest that it may result
from leaking of the stored fluid
beneath the detachment leading to
faulting at suprahydrostatic conditions. When ruptures in the link thrust
transect suprahydrostatic gradients in
fluid pressure, they have the capacity
to react as valves promoting upwards
discharge from overpressured portions of the detachment and a local
reversion towards a hydrostatic
gradient (Sibson, 1992). Such a
Ôfault-valve-behaviourÕ has also been
postulated in large earthquake zones.
The 1995 Kobe earthquake (M7.2) is
one of the best examples. Zhao et al.
(1996) and Zhao and Negishi (1998)
determined 3-D P- and S-velocity and
Poisson’s ratio structures in the Kobe
source area. They found that the
Kobe main-shock was located in a
distinctive zone characterized by a low
velocity and a high Poisson’s ratio,
which was interpreted to be a fluidfilled fractured rock matrix that contributed to the initiation of the Kobe
earthquake.
Leaked fluid from the detachment is
considered to contain a certain
amount of CO2 because calcite is
present in the silicified zone and
gouge. Phase separation in H2O–
CO2–NaCl crustal fluids is thought
to occur in response to decompression
during fault movements in the faultvalve model (e.g. Sibson et al., 1988).
The phase separation process is
dependent on the fluid chemical composition. Our fluid inclusion data as
497
Fossil fluid reservoir beneath a duplex fault structure • Y-C. Chan et al.
Terra Nova, Vol 17, No. 6, 493–499
.............................................................................................................................................................
Table 1 Summary of microthermometric data of the study
Two-phase inclusions (n ¼ 22)
Degree of fill (%)
CO2 melting (C)
Ice melting (C)
Clathrate melting (C)
Salinity (wt% NaCl)
Total homogenization (C)
CO2 wt%
CO2 mol%
shown in Table 1 are consistent with
the trapping of an H2O–CO2–NaCl
fluid in the single-phase region. The
phase separation of this leaking fluid
along link faults can result from
transient decompression during earthquake fault rupturing propagation. In
addition, the phase separation can be
greatly enhanced in zones of maximum dilation (Wilkinson and Johnston, 1996). The decompression
triggered by earthquake was also well
described in the 1997 Umbria-Marche
seismic sequence, northern Italy (Miller et al., 2004). Although they considered that high-pressure CO2 was
from the mantle (see also Chiodini
and Cioni, 1989), there is a possibility
as suggested in our study that the CO2
was quickly generated because of the
phase separation of H2O–CO2 rich
fluids, which were within the fluid
reservoirs around and beneath a
major detachment fault.
Concluding remarks
Although the overpressured fluid
scenario has been employed to explain
the conditions under which overthrusting may have taken place, the
fluid source, fluid chemical characteristics and fluid storage remain largely
uncertain. Thus, many efforts have
been made to validate the scenario
especially applying field data from
petroleum companies (e.g. Suppe and
Wittke, 1977; Suppe et al., 1981). The
present study also lends supports to
this scenario, although based on limited outcrops. Here we found investigations of surface outcrops with
comparable metamorphic P/T conditions similar to those of the hypocentre of the Chi-Chi earthquake to be
valuable for testing the presence of
fluids and characteristics of fluids at
such depths.
498
60 to 70
)60 to )56
)3.5 to )5.8
5.9–9.3
1.7–7.6 (4.9)
208–225
11.9
5.2
Acknowledgements
We thank Jian-Cheng Lee and Chia-Yu Lu
for their helpful discussion on the Central
Range geology in this study. We very much
appreciate the thoughtful and constructive
comments by Agust Gudmundsson and an
anonymous reviewer. This study was partly
supported by the Academia Sinica and the
National Science Council in Taiwan under
grant NSC92-2116-M001-004.
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Received 21 July 2004; revised version
accepted 3 May 2005
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