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Tectonophysics 324 (2000) 189–201
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Thermal modeling of continental subduction and exhumation
constrained by heat flow and seismicity in Taiwan
Cheng-Horng Lin *
Institute of Earth Sciences, Academia Sinica, PO Box 1-55, Nankang, Taipei, Taiwan, ROC
Received 17 December 1998; accepted for publication 12 May 2000
Abstract
The thermal evolution of crustal exhumation subsequent to the subduction of the continental margin is modeled
through numerical solutions to the two-dimensional heat conduction equation. The boundary conditions used in the
modeling are basically constrained by available geophysical and geological observations in the Taiwan area.
Temperature distributions are calculated at one-million-year intervals over a 5 Ma duration. Although this study
presents a tentative estimation based on some reasonable assumptions, the preliminary results not only explain the
sharp change in seismicity within the crust but also agree with the general heat flow pattern on the surface of Taiwan.
More specifically, both the high heat flow and the aseismic belt beneath the Eastern Central Range, which are
probably attributed to higher temperatures within the crust, are modeled by crustal exhumation after the subduction
of the continental margin. However, the occurrence of deeper earthquakes east of the Central Range is a result of the
lower thermal gradient, which is due to the subduction of the cold continental crust into the uppermost mantle. In
summary, preliminary thermal modeling supports the possibility of a tectonic model of active continental subduction
and crustal exhumation for the Taiwan Orogen. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: exhumation; heat flow; seismicity; subduction; thermal modeling
1. Introduction
The island of Taiwan is one of the youngest
and most active orogenic belts on the earth. Crustal
deformation ( Yu and Chen, 1996) and seismicity
( Tsai et al., 1977) are still actively going on due
to the convergence of the Eurasian and Philippine
Sea Plates (Fig. 1). East of Taiwan, the Philippine
Sea Plate subducts toward the north beneath the
Eurasian Plate along the Ryukyu Trench, while
south of Taiwan, the Eurasian Plate underthrusts
the Philippine Sea Plate along the Manila Trench.
Both subduction features are directly identified
* Tel.: +886-2-2783-9910 x521; fax: +886-2-2783-9871.
E-mail address: [email protected] (C.-H. Lin)
from seismicity in and around the island. To the
northeast of Taiwan, a clear Wadati-Benioff zone
is located in the A–A∞ cross-section, while in the
south, another seismic zone is found in the B–B∞
cross-section (Fig. 2).
In the major part of the island, the convergent
characteristics are more complicated than those in
either of the subduction zones. In the past decade,
different tectonic models have been proposed to
characterize the Taiwan Orogen based on different
constraints and approaches (e.g. Suppe, 1981;
Davies et al., 1983; Angelier et al., 1986; Dahlen
and Barr, 1989; Huang and Wang, 1993; Wu et al.,
1997), but the actual mechanism for the Taiwan
Orogen remains a topic of wide debate. None of
the proposed models has clearly explained all of
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C.-H. Lin / Tectonophysics 324 (2000) 189–201
discussion of tectonic implications of the exhumation model by comparing the calculated thermal
structure to both observed abundant seismicity
and the heat flow pattern in the Taiwan area.
Finally, the merits of this model over the previous
models used are discussed.
2. Exhumation model
Fig. 1. General tectonics in the Taiwan area. Bold lines indicate
the major convergent boundary between the Philippine Sea and
Eurasian Plates. Triangles marked along either side of the
boundary represent the direction of subduction. The large arrow
shows the movement of the Philippine Sea Plate toward the
northwest.
the fundamental observations. In particular, the
unusual features of both high heat flow and seismicity, which are usually dependent on the thermal
structures in an orogenic belt, are still not fully
understood in the case of Taiwan.
The purpose of this paper is to construct a
thermal model based on an evolution model of
active crustal exhumation subsequent to the subduction of the continental margin in Taiwan (Lin
et al., 1998). This model not only provides a
possible explanation for the unique seismicity and
heat flow across the island in general (Lin, 1998),
but also agrees a variety of geophysical and geological observations, including seismic tomography,
focal mechanisms, GPS, leveling and isotope
analyses (Lin and Roecker, 2000).
This paper begins by briefly reviewing the evolution model of active crustal exhumation. A preliminary two-dimensional thermal structure across the
convergent boundary between the Eurasian and
Philippine Sea Plates is then constructed through
a numerical solution. This is followed by a detailed
The model of continental subduction and exhumation as proposed by Lin et al. (1998) and Lin
and Roecker, 2000 explains the evolution of the
Central Range of Taiwan. It is briefly summarized
here. The current arc–continent convergence initiated in the northern part of Taiwan and propagated southward (e.g. Ernst, 1983; Pelletier and
Stephan, 1986). Accordingly, the step of evolution
can also now be observed in the south of Taiwan.
The Taiwan Orogen began following the subduction of the Oligocene–Miocene South China Sea
along the Manila Trench (Profile 1 in Fig. 3).
Analogous to the simulations conducted by
Chemenda et al. (1995, 1996), the continental shelf
of the Eurasian Plate was dragged down into the
mantle (Profile 2 in Fig. 3), carrying with it some
of the thickness of the continental crust. Although
the continental crust is commonly considered
difficult to subduct, there is widespread evidence
in the orogens that the continental crust may, in
some circumstances, be at least partially subducted
(Chopin, 1984; Wang et al., 1992; Dewey et al.,
1993). As more crust was consumed in this fashion
(Profile 3 in Fig. 3), the resistive buoyancy of the
lighter material eventually increased to the point
where decoupling occurred near the base of the
crust, forming a crustal slice which, through the
combined effects of buoyancy and erosion, made
its way back up to the surface, even while the
lithospheric mantle continued to subduct. The
exhumed crustal slice proposed in the model
(Profile 4 in Fig. 3) is now exposed at the surface
in the Eastern Central Range where it is covered
with metamorphic rocks.
Overall, this evolution model of continental
subduction and crustal exhumation agrees well
with a variety of geophysical and geological observations, including those for seismic tomography,
C.-H. Lin / Tectonophysics 324 (2000) 189–201
191
Fig. 2. Epicenters with magnitudes greater than 3.0 and three hypocenter projections (A–A∞, B–B∞ and C–C∞) recorded by the Central
Weather Bureau Seismic Network (CWBSN ) in the Taiwan area from 1992 to 1996. Dotted lines at the A–A∞ and B–B∞ cross-sections
mark the Benioff zone in the northeast and in the south of Taiwan, respectively. Dashed lines delineate a seismicity boundary between
seismic and aseismic regions in the epicenter map and the C–C∞ cross-section. The location of unusual seismic belts is marked by a
question mark.
focal mechanisms, GPS, leveling and isotope
analyses, seismicity and heat flow pattern. First,
the existence of the exhumed crust, simply by the
ascension of hot, ductile crustal material to shallow
depth, might explain the observed high temperatures and an aseismic belt beneath the Eastern
Central Range (Lin, 1998). Thus, the exhumation
of a slice of the crust accounts for the prevalence
of normal faulting (Cheng, 1995; Lin et al., 1998)
and the extension observed in the Eastern Central
Range (Lee, 1995; Crespi et al., 1996; Yu and
Chen, 1996). Because the development of the
crustal slice would occur only after an extended
period of continental subduction, the evolution of
the range would not be a steady-state process, but
instead would be characterized by an acceleration
of uplift following decoupling. Thus, this type of
orogenic evolution is consistent with the observed
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C.-H. Lin / Tectonophysics 324 (2000) 189–201
Fig. 3. (a) General geological regions in Taiwan (after Ho, 1988) including the Coastal Plain (CP), Western Foothills ( WF ), Western
Central Range ( WCR), Eastern Central Range ( ECR), Longitudinal Valley (LV ) and Coastal Range (COR). The bathymetry is
shown by contour on a gray scale. (b) Schematic diagrams summarizing the evolution model of active continental subduction and
crustal exhumation. The model is also presented in Profiles 1–4 from south to north. Profile 1 cutting through the Manila Trench is
a typical subduction of the oceanic crust. Profile 2 just cutting through the southern area of the Hengchung Peninsula shows the
subduction of the continental margin and failure in the front of the subducting crust. Also, the overlying oceanic crust beneath the
forearc basin is initially deformed. Profile 3 cutting through southern Taiwan shows exhumation in the Eastern Central Range
originally due to buoyancy, and the strong deformation of both the exhuming and overlying crusts. Profile 4 cutting through central
Taiwan shows the acceleration of exhumation in the Eastern Central Range due to the combination of both rapid erosion and
buoyancy.
acceleration of uplift over the past 1 Ma (Lan
et al., 1990; Tsao et al., 1992; Tsao, 1996). Being
a body with thermal and lithological characteristics
distinctively different from the surrounding rocks
at the same depth, an exhumed slice of the crust
might very well explain the lateral variations in
seismicity and the connection of the Eastern
Central Range to the deeper structure, as observed
in tomographic images (Lin et al., 1998).
3. Thermal modeling
3.1. Methodology
A finite-difference method is employed in this
study to compute two-dimensional geothermal
models. Minear and Toksoz (1970a,b) initially
reported the computational method, and its computational code, first written by N.H. Sleep, was
C.-H. Lin / Tectonophysics 324 (2000) 189–201
later modified by K. Creager et al. Temperatures
are computed from the conservation of energy
equation:
∂T
=V(KVT )+H,
C r
v ∂t
where C is the specific heat at constant volume,
v
r is the density, T is the temperature, t is the time,
K is conductivity, and the H is the heat source.
The prime concern in this study is the temperature variation caused by the subducting slab and
exhuming crust. Here, it is necessary to use a
simple model, in which temperature is internally
consistent, although the heat source considered for
the down-going slab might include radioactivity,
phase change, shear strain and adiabatic compression. It should be noted that no convection term
is included in the equation because of the basic
quasi-dynamic approach. The computational
scheme involves translating temperatures following
the motion field and then allowing thermal diffusion over the time interval until the next movement. In other words, temperature in the
subducted crust translates downward and then
warms up during each time interval.
3.2. Boundary conditions and parameters
The boundary conditions and parameters in the
estimation of the thermal structure across the
island are largely constrained by a variety of
observations or reasonable assumptions ( Table 1).
The temperatures on the surface and below 100 km
are assumed to be 0 and 1000°C respectively, and
are in a steady-state condition. This assumption
Table 1
Parameters for thermal modeling in this study
Parameters
Values
Plate thickness
Surface temperature
Basal temperature
Thermal conductivity ( K )
Specific heat (C )
v
Subduction dip (w)
Convergence rate
Density
100 km
0°C
1000°C
4.7×105 erg/cm/°
1.25×107 erg/g/°
35°
7 cm/yr
3.3 g/cm3
193
implies that the initial heat flow is ca 95 mW/m2,
a rate in good agreement with the averaged observation on the surface of the Coastal Plain, which
is not in close proximity to the convergent boundary. The subduction process ( Fig. 4a) is described
by a convergent rate of 7 cm/yr (Seno, 1977) with
a subduction angle of 35°. The exhumation of the
subducted crust ( Fig. 4b) within a trapezoid 30 km
wide and 30 km thick took place after five steps,
with each step taken as one million years. An
exhumation rate of 3 cm/yr during the last one
million years agrees with the uplifting rate of about
3–4 cm/yr observed across the Central Range
during the past 20 years (Liu, 1995).
3.3. Results
The thermal structures through six steps of
evolution during the past 5 Ma are shown in
Fig. 4c and d. Upon the evolution of continental
subduction and exhumation, geothermal structures
change considerably from west to east across the
profile. Major thermal depression along the subducted slab is gradually enhanced through the
subduction processes during the past 4 Ma
( Fig. 4c), while a small area of higher temperature
is clearly caused by the exhumation in the last step
( Fig. 4d ). For a detailed discussion of the temperature perturbation, a small box around the convergent boundary profile (Fig. 4d ) is divided into
four sections based on the horizontal distances,
namely (I ) 0–70 km, (II ) 70–120 km, (III ) 120–
150 km, and (IV ) 150–200 km.
Basically, the geothermal gradient in Section I
( Fig. 4d ) has not been disturbed by the effects of
continental subduction and crustal exhumation. It
maintains a gradient of about 20°C/km. The thermal gradient in Section II increases considerably
within the upper crust due to the effect of crustal
exhumation. The thermal gradient near the surface
is over 50°C/km, and thus, the temperature reaches
up to a few hundred degrees in the uppermost
crust. The geothermal gradient in Section III
decreases again due to the subduction of the
continental crust. The thermal structure is clearly
depressed along the top boundary of the subducted
slab. On average, the gradient is less than 10°C/km.
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C.-H. Lin / Tectonophysics 324 (2000) 189–201
Fig. 4. Kinematics of (a) subduction and (b) exhumation for computing the thermal models on the right. Thermal structures calculated
after (c) the five-step subduction and (d ) the last step subduction with exhumation. Isotherms from 0 to 1000°C are separated by an
interval of 100°C. Triangles mark the beginning of subduction on the surface.
The thermal structure in Section IV is back to
normal, with a gradient of 20°C/km.
4. Discussion
Owing to limitations caused by uncertainties in
the heat flow observations and the material properties of the rocks, the model derived in this study
is not intended to reproduce exactly the thermal
and mechanical conditions for the Taiwan Orogen.
Instead, the relatively accessible and tectonically
simple Taiwan Orogen is discussed as an example
of general thermal and mechanical behavior in
active collision zones undergoing rapid exhumation after the subduction of the continental margin.
Therefore, a comparison of the major features of
the calculated thermal structure with the overall
characteristics of surface geology, heat-flow and
seismicity is primarily focused upon.
4.1. Surface geology
The major geological units in Taiwan from west
to east include the Coastal Plain, the Western
Foothills, the Central Range, the Longitudinal
Valley and the Coastal Range ( Fig. 3a). Those
regions, roughly striking NNE–SSW, are often
C.-H. Lin / Tectonophysics 324 (2000) 189–201
bounded by faults and other discontinuities (Ho,
1988). The Coastal Plain, the Western Foothills
and the Central Range belong to the Eurasian
continental passive margin and shelf, while the
Coastal Range is an island arc usually interpreted
as the leading edge of the Philippine Sea Plate.
The Longitudinal Valley, commonly considered
the suture, represents the convergent boundary
between the two plates.
The general features in the upper crust of the
calculated thermal structure ( Fig. 4d ) are comparable to the geological units of Taiwan. To
illustrate, the undisturbed temperatures in Section
I (0 – 70 km) in the model largely correspond with
the Coastal Plain and Western Foothills in the
western part of Taiwan. Both geological units are
far away (greater than 50 km) enough from the
Longitudinal Valley, which is a clear suture
between the two plates. The Western Foothills are
composed of Oligocene to Pleistocene clastic sediments that have been stacked up by a combination
of northwest vergent folds and low-angle thrust
faults dipping to the southeast. The Coastal Plain
consists of Quaternary alluvial deposits derived
from the Central Range and the Western Foothills.
These features indicate that the western part of
Taiwan could never have been disturbed by any
major tectonic process, such as subduction or
exhumation.
The higher thermal gradient affected by the
exhumed crust in Section II (70–120 km) correlates
reasonably well with the uplifting mountains in
the Central Range where metamorphic rocks are
exposed on the surface. It has been estimated that
metamorphism in much of the Central Range
occurred at temperatures of 250–450°C and pressures of 300–400 MPa (e.g. Liou, 1981a,b; WangLee et al., 1982; Ernst, 1983, 1984; Ernst and
Harnish, 1983; Warneke and Ernst, 1983). A recent
study of the metasedimentary rocks by ‘illite crystallinity’, fission-track dating and K–Ar dating
( Tsao et al., 1992; Tsao, 1996) reported that the
metamorphic basement has been uplifted at rapid
rates of 7.5–15 mm/yr during the last 1.5 Ma.
Uplifting in the Eastern Central Range has also
been confirmed by leveling and GPS surveys. On
the basis of the leveling results, Liu (1995) reported
that during the past decade, the Central Range
195
has been uplifting rapidly (32–40 mm/yr) relative
to the Longitudinal Valley. These observations
indicate that metamorphic rocks in the Eastern
Central Range can be explained by a wide slice of
the exhumed crust.
The temperature depression caused by subduction in Section III (120–150 km) is roughly associated with the geological units of the Longitudinal
Valley and Coastal Range in eastern Taiwan. The
Coastal Range, composed of Neogene volcanogenic and flyschoid rocks, was folded into a series
of NNE-trending anticlines and synclines and cut
by several thrust faults dipping mainly to the east
(Barrier and Angelier, 1986). There is no evidence
that metamorphism occurred in the Coastal Range.
The temperature distribution in Section 4 (150–
200 km) in the model is comparable to the geological units east of the Coastal Range in the Philippine
Sea Plate. The temperature distribution with a
normal thermal gradient of 20°C/km indicates that
no major tectonic process has occurred east of the
Coastal Range.
4.2. Heat flow
The estimated heat flow based on the thermal
structures obtained in this study generally agrees
with the measured heat flow near the surface (Lee
and Cheng, 1986). The heat flow map in Taiwan
( Fig. 5) is basically constructed according to the
data reported by Lee and Cheng (1986). Heatflow measurements are dense in the Coastal Plain
and the Western Foothills, but sparse in the Central
Range and the Coastal Range. In general, the heat
flow pattern is roughly associated with topographic
relief ( Fig. 5); a higher flow is often detected in
higher elevations. The overall contours of the
surface heat flow strike in a NNE–SSW direction
approximately, which is parallel to the strike direction of the main geological structures of Taiwan.
The surface heat flow changes significantly across
the main structures. A heat flow of less than
100 mW/m2 is observed in the Coastal Plain and
the Western Foothills, but it increases from west
to east, reaching its peak over 200 mW/m2 in the
Central Range, particularly between 23 and 24°N.
Further east, heat flow decreases to 160 mW/m2 in
the Coastal Range. In short, the strike parallel to
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C.-H. Lin / Tectonophysics 324 (2000) 189–201
and 250 mW/m2 are basically in good agreement
with the observed heat flow in Taiwan. In addition,
the highest heat flow is about twice as large beneath
the Central Range as that obtained in the Western
Foothills and Coastal Plain. This is consistently
the case in both the calculated and observed
results.
At the same time, however, some differences do
exist between the calculated and observed heat
flows. The major difference is in the Western
Central Range, where the heat flows in Fig. 6a
differ significantly from those reported by Barr
and Dahlen (1989) due to the absence of measurements. This means that the detailed comparisons
between the observed and calculated results lose
their physical meaningfulness due to the strong
uncertainty concerning the observed heat flows in
the Western Central Range. In addition, east of
120 km in Fig. 6a, the depression of calculated
thermal gradients, caused by the subduction of the
continental crust, cannot be examined from observations due to the absence of any constraint east
of the Coastal Range.
4.3. Seismicity
Fig. 5. Surface heat flow distribution in Taiwan (after Lee and
Cheng, 1986). Circles show the locations of heat flow measurements. The values marked at the contours represent the heat
flow in mW/m2 units.
the convergent boundary indicates that the geothermal structure is largely associated with tectonic
events produced along the convergence between
the Eurasian and Philippine Sea Plates.
A representative comparison of observed and
calculated heat flows is presented in Fig. 6a. The
calculated heat flow on the surface is derived from
the thermal structure after the six-step evolution,
including crustal subduction and exhumation in
Figs. 4d and 6b. These results are similar to the
observed heat flow pattern shown in Fig. 5.
Variations in the calculated heat flow between 100
The thermal model computed above can be
used to explain seismicity across the major part of
Taiwan. The seismicity of shallow earthquakes
with focal depths of less than 40 km shows strong
lateral variations in the major part of island. One
of the most significant features is an unusually
aseismic belt just along the west of the suture. This
belt, shown by a question mark in Fig. 2, follows
the main structures striking NNE–SSW and
roughly covers an area of 20×150 km2 beneath
the Eastern Central Range. Clearly viewed in the
C–C∞ cross-section, an aseismic zone with widths
of more than 20 km is identified in the crust. To
the west of this aseismic zone, the focal depths of
most earthquakes are less than 30 km. To the east,
earthquakes show an east-dipping seismic zone
distributed from the surface to depths of over
50 km. Such unusual seismicity features might be
directly associated with the thermal structures of
the Taiwan Orogen. This is because earthquakes
within the continental crust are generally limited
by the 350°C isotherm, which is a general indicator
C.-H. Lin / Tectonophysics 324 (2000) 189–201
197
Fig. 6. (a) Comparison of the calculated and observed heat flows across Taiwan. The thick dashed line marks the calculated heat
flow in this study. Circles and pluses show the observations obtained and observations reported by Dahlen and Barr (1989),
respectively. (b) Parts of the thermal structure as marked by the dashed box in Fig. 4d. White lines at intervals of 100°C show
isotherms, and the dotted line along the temperature of 350°C delineates the brittle–ductile transition boundary across the profile.
(c) Seismicity and topography projected on to the C–C∞ cross-section of Fig. 2. The dotted line shows the possibly lowest boundary
of seismicity. The dashed lines mark the boundary of the subducted crust. A schematic plot of subduction and exhumation is also
shown here.
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C.-H. Lin / Tectonophysics 324 (2000) 189–201
of the base of the brittle–ductile transition under
the assumption of quartz-dominant lithologies for
the crust (Chen and Molnar, 1983). In the
following, a detailed comparison of the thermal
structures in light of seismicity is discussed for
each of the four sections in Fig. 4d.
In Section I ( Fig. 6b), the calculated thermal
structure within the crust shows a normal thermal
gradient of about 20°C/km that has not been
disturbed by the evolution processes assumed in
the model. As a result, the 350°C isotherm within
the continental crust is flat. This is generally consistent with the seismicity ( Fig. 6c) that the boundary
between seismic and aseismic areas, thought to be
related to the brittle–ductile boundary, is almost
flat.
In Section II, the higher temperature obtained
within the crust roughly reflects the aseismic zone
observed in the Eastern Central Range ( Figs. 2
and 6c). Because of the higher thermal gradient
(over 50°C/km) near the surface, almost no earthquake has been located at the depths of more than
10 km. In other words, a temperature of more
than 350°C, which is generally thought to represent
the transition between a brittle and ductile boundary in the continental crust, can be reached at a
depth of just a few kilometers. Thus, the deformation beneath the Central Range is probably a
result of an aseismic slip due to higher temperatures. This can also be confirmed by the strong
folding within the metamorphic rocks on the surface across the Central Range (Lee, 1995; Crespi
et al., 1996). It is suggested that those rocks were
initially deformed underground under higher temperatures and were rapidly exhumed to the surface.
In Section III, the results of calculated thermal
depression can be used to infer the broad distribution of earthquakes from the surface to a depth of
50 km beneath the Coastal Range and the westernmost Pacific Ocean. The occurrence of those
earthquakes, similar to those within the typical
Benioff zone along the subducted slab (such as the
A–A∞ cross-section in Fig. 2), is most likely attributed to a low thermal gradient caused by the
subduction of a colder crust into the hotter mantle.
In addition, those earthquakes with an east-dipping seismic zone are evidently limited by an eastdipping boundary of about 40°. Again, this pattern
is largely associated with the eastward subduction
of the continental margin.
In Section IV, the calculated thermal structure
within the crust again shows a normal thermal
gradient of about 20°C/km. This agrees with those
observations of seismicity mostly limited to depths
of less than 25 km. The pattern of seismicity is
somewhat like that in Section I because neither of
their geothermal gradients was disturbed by the
evolution of continental subduction or crustal
exhumation.
4.4. Previous studies
Owing to the more complete geometrical coverage and additional constraints in a revised geodynamic model, the geothermal structure obtained
from this study is considered more appropriate to
describe the Taiwan Orogen than those from previous studies (Barr and Dahlen, 1989; Huang and
Wang, 1993). First, the thermal structure here has
considered a wider coverage of the convergent
zone between the two plates, taking into consideration the possibility of interaction between the
Eurasian and Philippine Sea Plates. In contrast,
the previous studies are basically limited to the
upper crust of the western Taiwan Orogen, which
represents the continental part of the convergent
belt and only belongs to a portion of the thermal
structure. It must be kept in mind that Taiwan is
deformed under a strong convergence between the
Eurasian and Philippine Sea Plates, and thus,
interactively geodynamic processes between the
plates are of concern. Consequently, thermal structures must be constructed in the convergent zone
rather than in one plate alone.
Besides the wider coverage, the thermal structure in this study is deeper than that of both
previous studies, which, in spite of different
approaches, are based on the same assumption
that deformation is limited above a major decollement in the Taiwan Orogen. In reality, the deformation probably occurred within the upper crust
as well as the lower crust and/or the uppermost
mantle. In fact, the deformation probably did not
just occur within the upper crust; on the contrary,
the lower crust and/or uppermost mantle most
likely underwent deformation (Roecker et al.,
C.-H. Lin / Tectonophysics 324 (2000) 189–201
1987; Rau and Wu, 1995; Wu et al., 1997; Lin
et al., 1998). For example, the occurrence of some
earthquakes below depths of 20 km in the Coastal
Plain and Western Foothills ( Fig. 2) indicates that
the deformation is still at work below the assumed
decollement, the bottom of the critical wedge.
Furthermore, a simple wedge model, proposed by
Suppe (1981), cannot fit the gravity data in Taiwan
( Ellwood et al., 1996; Yen et al., 1998).
The geodynamic process of active subduction
and exhumation in this study is totally different
from that in previous studies. On one hand, Barr
and Dahlen (1989) computed thermal structures
based on critical wedge kinematics. On the other
hand, Huang and Wang (1993) estimated thermal
models based on a discrete sequential thrusting.
For these reasons, the thermal structures obtained
in this study are significantly different.
Finally, the thermal structures obtained in this
study are generally consistent with both heat flow
and seismicity, while the earlier results were constrained by heat flow alone. It is, however, reasonable that both heat flow and seismicity are two of
the most important factors reflecting the geothermal structures in active orogen zones. Heat flow
observed near the surface largely depends on the
geothermal gradients in the shallowest part of the
earth, whereas seismicity provides geothermal
information within the crust because the occurrence of earthquakes reflects the behavior of thermal properties.
Each of the observations has advantages and
drawbacks. Heat flow provides direct information
on temperature distribution near the surface, but
it may be biased by extraneous factors, such as
measurement errors and a paucity of data due to
difficult survey procedures in mountainous regions.
Seismicity, however, allows enough data within
the crust of active orogens, but the projection of
the seismicity pattern into the geothermal structure
is somewhat more complicated. For these reasons,
it is more effective to construct geothermal structures constrained by both heat flow and seismicity
in active orogens. Thus, it is not surprising that
the thermal models obtained from previous studies
fall short in explaining the strong lateral variations
in seismicity pattern across the island, such as the
aseismic zone beneath the Eastern Central Range
199
and the east-dipping seismic zone from the surface
to a depth of 50 km beneath the leading edge of
the Philippine Sea Plate ( Fig. 2). In summary,
thermal models from the previous studies might
not be as appropriate for the Taiwan Orogen as
originally thought.
5. Conclusions
The first-order agreement between the calculated thermal structure and observations of both
seismicity as well as heat flow supports the tectonic
model of crustal exhumation subsequent to the
subduction of the continental margin in the Taiwan
area. In this model, the Eastern Central Range is
the locus of the exhumed crust that had been
dragged downward by the dense mantle lithosphere
of the Eurasian plate. The exhumation occurred
when the buoyancy forces on the light subducted
continental crust increased sufficiently to produce
decoupling at the base of the crust and then failure
in front of the subduction zone. The higher temperatures produced by the exhumed crust exposed on
the Eastern Central Range are consistent with the
higher heat flow observed on the surface and the
aseismic zone within the upper crust. This exhumation in the Eastern Central Range also agrees with
a variety of recent evidence, such as the vertical
principal stress investigated from focal mechanisms, the rapid uplifting obtained from illite crystallinity, fission-track and K–Ar dating and the
crustal extension from GPS surveys. The lower
temperatures (or thermal depression), generated
by the subduction of the continental margin at a
rate of 5.74 cm/yr during the past 5 Ma also successfully explain the broad distribution of earthquakes at depths of 50 km beneath the Coastal
Range. An east-dipping zone of those earthquakes
is very consistent with the top boundary of the
subducted continental margin. In addition, a
normal gradient of 30°C/km for the Coastal Plain,
reflecting a normal heat flow on the surface and a
limited number of earthquakes occurring in the
upper crust, is usually equated with not having
been disturbed by any significantly tectonic process
during the last few million years.
200
C.-H. Lin / Tectonophysics 324 (2000) 189–201
Acknowledgements
The author would like to thank Giorgio Ranalli
and an anonymous reviewer for their constructive
comments on the manuscript. Thanks are also
extended to the Central Weather Bureau for providing earthquake data. Discussions with S.W.
Roecker, J.C. Lee, R.J. Rau, and P.L. Wang have
also been very helpful.
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