Rates of cooling and denudation of the Early Penglai Orogeny, Taiwan, as assessed by fission-track constraints

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Rates of cooling and denudation of the Early Penglai Orogeny,

Taiwan, as assessed by fission-track constraints

T.-K. Liu

a,*, Y.-G. Chen a, W.-S. Chen a, S.-H. Jiang b

a Department of Geology, National Taiwan University, 245 Choushan Road, Taipei 106 Taiwan

b Department of Engineering and System Science, National Tsiang Hua University, 101 Kuang-Fu Road, Sec.2, Hsinchu, Taiwan Received 15 June 1998; accepted for publication 14 January 2000

Abstract

As an attempt to define the timing of the peak temperature of the Penglai Orogeny and estimate the early cooling and denudation rates of the Taiwan mountain belt, fission-track dating of zircon and apatite was conducted on several key metamorphic rock samples. The zircon fission-track ages for the metasandstone clasts, collected from the central and northern parts of the Coastal Range, were determined as 4.0±0.5 and 3.6±0.3 Ma, respectively. Both of the clasts were identified as from the ancient Central Range which was metamorphosed at temperatures high enough to cause a complete reset of the zircon fission-track system. During 1–2 Ma, they were exhumed and deposited in the Coastal Range basin due to the later Penglai Orogeny of Taiwan. Obviously, they have not further been annealed since their deposition in the Coastal Range. The difference between the above fission-track ages and the stratigraphical age of the host sedimentary formation represent the ancient cooling ages when they were exposed on the early Central Range. These ancient cooling ages are comparable with the zircon fission-track age of a present-day outcrop of the Tananao Schist, 1.8±0.2 Ma. This accordance implies that at ca. 5 Ma the northern and central parts of the Central Range achieved the peak temperature of the Penglai Orogeny and then they began to emerge above sea level. Accepting this scenario, we calculate the rates of denudation and cooling of the Central Range to be ca. 2.5–4.6 mm yr−1 and ~120°C m.y.−1, respectively, for the last 4 Ma. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: arc–continent collision; cooling rate; fission-track dating; Taiwan

1. Introduction west to east are: the Coastal Plain; the Western Foothills; the western Central Range (including The mountain belt of Taiwan is one of the best the Hsueshan Range and the Slate belt); and the examples in the world of active arc–continent eastern Central Range (the Tananao Schist) to the collision (e.g. Biq, 1972, 1973; Chi et al., 1981; west of the Longitudinal Valley and the Coastal Suppe, 1981; Lo and Onstott, 1995). The Range to the east. The first three belts comprise Longitudinal Valley marks a portion of the suture the Cenozoic cover strata and the fourth one, the between the Eurasian plate and the Philippine Sea Tananao Schist, is the underlying pre-Tertiary plate ( Fig. 1). The tectonostratigraphic belts from metamorphic basement. These all lie on the Asiatic continental margin. The Coastal Range is a Neogene magmatic arc with overlying sediments

* Corresponding author. Tel.:_{+88-2-2365-7380;}

derived from the arc itself and the continental side.

fax:+886-2-2365-7380.

E-mail address: liutk@ccms.ntu.edu.tw (T.-K. Liu) The eastern Central Range is nearly 250 km in

0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 04 0 - 1 95 1 ( 0 0 ) 00 0 2 8- 7

length and from 10 to 30 km in width. The meta- ing to an uplift of 4.9–8.9 mm yr−1, have also been calculated.

morphic complex is made up mostly of schists,

metamorphosed limestone or marble, and scattered In spite of a large number of K–Ar, Rb–Sr and 40Ar/39Ar ages for the metamorphic rocks (Yen gneiss and amphibolite bodies exposed mainly in

the northern part. All these metamorphic rocks and Rosenblum, 1964; Juan et al., 1972; Juang and Bellon, 1986; Jahn et al., 1986; Lan et al., are grouped under the general stratigraphic term

‘Tananao Schist’. The overall metamorphic grade 1990; Lo and Onstott, 1987, 1995), the timing of peak temperature and the early cooling rate of the of the rocks is mainly greenschist facies and partly

amphibolite facies. Some glaucophane schist Penglai Orogeny remained poorly known. The major reason is that these isotopic systems com-bodies have been found on the southeastern part

of this belt. Only a few deformed Permian fusuli- monly exhibit complicated ages due to mixing or partial resetting by the late Tertiary–Quaternary nids have been found in the limestone ( Yen et al.,

1951). The Tananao Schist have been subjected to Penglai Orogeny which followed the main meta-morphic event during the late Mesozoic Nanao several major or minor phases of deformation or

metamorphism (Liou and Ernst, 1984). Two major Orogeny (Lo and Onstott, 1995). For example, in the lower greenschist facies area, hornblendes and episodes of metamorphism have been recognized.

They are: (1) formation of late Mesozoic paired coarse-grained muscovites still retain most of their radiogenic argon and display fairly flat 40Ar/39Ar metamorphic belts (approximately 90 Ma); and

(2) Plio-Pleistocene greenschist to amphibolite age spectra with plateau dates of 82–95 Ma which record a cooling during the late Mesozoic tecto-facies metamorphism. The metamorphic grade

decreases westward from the Tananao Schist. It is nothermal event of the Nanao Orogeny. In con-trast, all microclines which has 40Ar/39Ar closure the second and most important orogenic episode,

called Penglai Orogeny, which affected the whole temperature at ca. 240–250°C are completely reset and yield young plateau dates of 1.6–1.7 Ma. Most island and is responsible for the deformation,

metamorphism and uplift of the Taiwan Island. of the partially reset minerals (e.g. muscovite from the upper greenschist facies area and biotite from The sediments that were eroded in the early

stages of the Penglai orogeny from the eastern the lower greenschist facies area) yield geologically meaningless 40Ar/39Ar integrated dates that fall flank of the uplifting Central Range were deposited

in basins to the east. They were deformed and between the ages of the two tectonothermal events. On the other hand, the fission tracks in the squeezed up along with the underlying volcanic

basement by the collision to form the Coastal zircons separated from submetamorphosed Paleogene sandstones of the western Central Range. Previous apatite and zircon fission-track

studies on the samples collected from the present- Range (i.e. the Hsueshan Range) have undergone varying degrees of annealing during the Penglai day outcrops of the eastern Central Range have

shown completely reset apatite ages ranging from Orogeny as was indicated by fission-track grain age distribution relative to their respective strati-0.3 to 0.6 Ma and zircon ages from 0.9 to 2.0 Ma

(Liu, 1982, 1986). According to the above litera- graphic ages (Liu, 1988). Zircon grains from each completely reset sample show a restricted range of ture, the effective track retention temperatures can

be estimated as 135±20 and 235±20°C for apatite ages, the mean of which can be used to define a cooling age for each individual sample. The cooling and zircon, respectively. Further, assuming a

geo-thermal gradient of 30°C km−1 for the last 2 Ma, ages obtained fell in the range of 4–5 Ma, implying much lower cooling (~50°C m.y.−1) and erosion cooling rates of~130–260°C m.y.−1,

correspond-Fig. 1. General tectonic and geological map of Taiwan showing geologic provinces (after Ho, 1975), stratigraphic column of the Coastal Range (after Teng, 1982 lower left inset) and sample localities. The line connecting the star symbols, which represent the main summits, denotes the major divide of the island.

Fig. 2. Schematic diagram of the deposition of the Shuilien Conglomerate and the geologic configuration of its source terrain (modified from Teng, 1982).

rates (~1.7 mm yr−1) than the eastern Central of the Tananao Schist (Fig. 1), and from meta-sandstone boulders in the Shuilien Conglomerate. Range. For the samples collected from the lower

temperature margin of the completely reset terrain, The boulders themselves were well recrystallized as indicated by quartz veinlets, interlocking grain their cooling ages can also be regarded as the

timing for the peak metamorphic temperature boundaries, and the preferred orientation of min-eral grains observed under the microscope. In attained.

The Shuilien Conglomerate (Chi et al., 1981; order to detect the thermal influence induced by the Penglai Orogeny for the Coastal Range, a Teng, 1982) is a lithologically distinct stratigraphic

unit exposed in the northern Coastal Range. Its sample rich in apatite was also collected from a large exotic amphibolite block in the Lichi me´lange framework clasts are mainly composed of

meta-sandstone, slate, and vein quartz, with minor ophi- of the Coastal Range.

Zircons were separated in the usual manner. olitic rocks. Some clasts reach>1 m in diameter,

implying that the adjacent early Central Range Euhedral to subhedral grains were selected and aligned with one of their flat prismatic external (eastern flank) was the principal source terrain

( Fig. 2). The Conglomerate started to deposit at surfaces of each individual sitting on glass slides under a binocular reflective microscope. They were about the base of NN16 (Chi et al., 1981) which

is chronologically equivalent to 3.0±0.5 Ma as then mounted parallel to their c-axis in PFA (perfluoralkoxyethylene) teflon sheets, which in inferred by paleomagnetic results (Lee et al., 1991).

This paper will demonstrate that this formation turn were carefully ground with fine calcite pow-ders until a natural external prismatic surface of has not been heated to a high temperature and the

zircon fission-track ages of its constituent meta- each crystal was exposed and the two pyramid ends of crystals were still embedded in the PFA sandstone clasts provide constraints on the early

history of the orogeny. sheets. Because the hardness of calcite (Mohs scale=3) is greater than that of PFA teflon but much less than that of zircon (Mohs scale=7.5), this method can render a flush contact between

2. Samples and experimental methods

zircon prismatic surfaces and mica detectors and give a 2p geometry for fossil track densities. There For the purpose of understanding recent

tec-tonic activity of the Central Range, rock sample are two additional advantages as compared with the conventional ‘4p’ method. (1) The ‘2p’ method were collected from the Yuantoshan gneiss body

is better in preventing zircon grains from falling _{from the Poisson uncertainties in Ns, Ni, Nd and f} value in the manner described by Green (1981). off the PFA sheets; and (2) the spontaneous and

The final fractional uncertainty in grain age T is: the induced track densities were measured in the

same 2p geometry and for the same uranium

contents. Common rock-forming minerals are very s(T )

T =

GC

s(Ns) N s

D

2 +

C

s(Ni) N i

D

2 low in uranium content as compared with zircon.

Hence the number of tracks registered on the

external surfaces of zircon due to neighboring ₊

C

s(Nd)

N d

D

2 +

C

s(f) f

D

2

H

12 common rock-forming minerals is negligible.

Zircons were etched with a mixture of NaOH, _{where N}

s and Nidenote the number of counts of KOH, and LiOH · CH

2O (6:14:1 mol%; Zaun and spontaneous and induced tracks, respectively, in Wagner, 1985) at 200±5°C for 30±5 h until the _{each crystal and N}

d is the count induced by the weakest tracks, usually those parallel to the c-axis _{standard glass dosimeter and registered on a mica} (Gleadow, 1981), were clearly revealed. Track _detector.

counting was conducted using a magnification of 2500 under oil immersion. ‘Grain-by-grain’ and

mica external detector techniques were adopted to _{3. Results and tectonic implications} obtain individual grain ages.

Apatites were dated by the commonly used _{The fission track data are presented in Tables} ‘population method’ (Naeser, 1967) whereby apa- _{1–4. The radial plots as described by Galbraith} tite concentrates were divided into two parts, one _{(1990) and histograms for the zircon grain ages} was mounted in epoxy disc and polished to expose _{were displayed in Fig. 3. The positive correlation} internal grain surfaces (i.e. 4p geometry) for meas- _{between fossil track densities (rs) and induced} uring fossil track density (rs). The other was track densities (r

i) implies that variations in the annealed at 550°C for 2 h (Fleisher et al., 1975), _{grain-age population for each rock sample are} mounted, irradiated, polished, and etched for _{mainly due to the variations in the uranium content} induced track density (r

i). At least two pieces of between different grains. All sample ages are calcu-standard glass NBS SRM-610 or SRM-612, which _{lated using the ZA program v. 4.5 (Brandon,} have been calibrated against the proposed fission- _{1996). Both the Yuantoshan gneiss from the} track age standard — Fish Canyon Tuff (Naeser _{Tananao Schist Complex and the metasandstone} et al., 1981) were wrapped tightly and irradiated _{clast from Shuilien pass the x2-test at 5%and each} with samples. The zeta (f) values (Hurford and is judged to define a single population. The mean Green, 1983) for the standard glass SRM-612 ages (1.8±0.2 and 4.2±0.5 Ma, respectively) were and SRM-610 were evaluated to be 340±12 (1s) assigned as representative ages for the two samples. and 27.5±1.0 (1s), respectively. The former is On the other hand, the statistical parameters of comparable to the value of 339±10 (2s) by the metasandstone clast from Chimei slightly fail Hurford and Green (1983) and 342.1±6.2 (2s) in the x2-test at 5%. In this case, the central age by Tagami (1987). was calculated from the logarithmic mean of the Fission-track grain ages were calculated using individual fission-track density log _(rs/ri) the equation: (Galbraith and Laslett, 1993; Andriessen and Zeck, 1996), and 3.6±0.3 Ma is assigned instead of the mean age (3.8±0.5 Ma). Actually, all the mean

Tunk=1

ld ln

C

1+

A

rs

ri

B

ldrdf

D

ages of the three samples are not that di_{from their respective central ages.} fferent where l

d is the total decay constant of uranium The stratigraphic ages of the low-grade meta-(1.551×10−10 yr−1), and r

d is the detector track morphosed sedimentary rocks in the Central density from the standard glass dosimeter (tracks Range are Paleocene–Miocene, that is, 65 to 7 Ma. Their constituent detrital sediments were derived per cm2). Uncertainties in ages were calculated

Table 1

Results of fission track dating for detrital zircon grains extracted from a metasandstone boulder of the Shuilien Conglomerate at the Shuilien villagea

Crystal _rs/106 N_s G_s(=G_i) _ri/106 N_{i rd/105} N_d Age±1s U±1s

(tracks cm₋₂₎ (tracks cm₋₂₎ (tracks cm₋₂₎ (Ma) (ppm)

1 0.044 7 360 1.42 227 1.32 3741 1.4±0.5 132±9 2 0.282 10 80 3.04 108 1.32 3741 4.1±1.4 283±28 3 0.386 12 70 5.18 161 1.32 3741 3.3±1.0 482±39 4 0.809 14 39 7.39 128 1.32 3741 4.9±1.4 688±62 5 0.196 6 69 2.64 81 1.32 3741 3.3±1.4 246±27 6 0.788 21 60 6.12 163 1.32 3741 5.8±1.3 570±46 7 0.631 14 50 4.46 99 1.32 3741 6.3±1.8 415±43 8 0.601 8 30 5.86 78 1.32 3741 4.6_±1.7 545_±63 9 0.135 3 50 3.02 67 1.32 3741 2.0_±1.2 281_±35 10 0.451 10 50 5.59 124 1.32 3741 3.6_±1.2 520_±48 11 0.413 11 60 6.19 165 1.32 3741 3.1±0.9 577±46 12 0.135 3 50 1.13 25 1.32 3741 5.4±3.3 105±21 13 0.225 5 50 1.35 30 1.32 3741 7.5±3.6 126±23 Mean age 4.2±0.4 Central age 4.0±0.5 Age dispersion 24 Pooled age 3.8_±0.4 x2 18 P (x2) 11 Degree of freedom (df ) 12

Stratigraphic age of the clast in the Coastal Range 2.5–3.4 Inferred ancient cooling age when the sample was exposed on the Central Range 1.8–0.9

a r_s, spontaneous track density; N_s, N_iand N_d, number of tracks actually counted to determine the reported track density (2p geometry); ri, induced track density from sample zircon grain; rd, detector track density from the standard glass dosimeter NBS SRM-612; G

sand Gi, number of grids counted for fossil and induced tracks, respectively; 1 grid=4.44×10−7 cm2.

from the pre-Tertiary (>65 Ma) source rocks in Range and deposited as the Shuilien Conglomerate.

southeastern China (Jahn et al., 1976). All the

fission-track ages for detrital zircons from the In contrast, the clastic sedimentary rocks of the Fanshuliao Formation, Shuilien Conglomerate and unmetamorphosed stratigraphically equivalent

strata in the Western Foothills are also greater Paliwan Formation overlying the volcanic base-ment of the Tuluanshan Formation ( Fig. 1) of the than 65 Ma (Liu, 1988). Fig. 3 shows the measured

temporal relationship among the fission-track Coastal Range terrain were not subjected to a temperature high enough to cause partial annea-grain ages, the possible depositional ages and the

minimum cooling ages of pre-metamorphic source ling of the fission tracks in zircon after deposition. This conclusion is supported by fission-track rocks for the detrital zircon grains. All the zircon

fission-track ages for the Yuantoshan gneiss and thermochronological studies on apatites from the Coastal Range. The oldest core of the Coastal the two clast samples from the Shuilien

Conglomerate are very young and have a narrow Range is formed of Miocene andesitic rock plex (the Tuluanshan Formation) which is com-range in grain ages as compared with their

pre-metamorphic provenence rock ages. Evidently, the posed of lava flows, agglomerates, tuffs and associated volcanogenic sediments. Apatite fission-zircon fission-track clock for the metasandstones

was completely reset by the Penglai Orogeny before track ages for the hydrothermally altered andesitic complex at Chimei fall in the range from 15 to the metasandstones were eroded from the Central

Table 2

Results of fission track dating for detrital zircon grains extracted from a metasandstone sample of the Shuilien Conglomerate at Chimei villagea

Crystal _rs/106 N_s G_s(=G_i) _ri/106 N_{i rd/105} N_d Age±1s U±1s

(tracks cm₋₂₎ (tracks cm₋₂₎ (tracks cm₋₂₎ (Ma) (ppm)

1 0.48 38 178 5.88 465 0.132 3741 3.7±0.6 548±27 2 0.48 25 118 5.67 297 0.132 3741 3.7±0.8 528±33 3 0.29 36 280 4.84 602 0.132 3741 2.7±0.5 451±20 4 0.42 26 138 4.06 249 0.132 3741 4.7±1.0 378±25 5 0.05 1 50 1.31 29 0.132 3741 1.5±1.6 122±23 6 0.55 19 78 7.57 262 0.132 3741 3.2±0.8 704±45 7 0.71 22 70 8.11 252 0.132 3741 3.9±0.9 755±50 8 0.20 9 100 1.62 72 0.132 3741 2.8_±0.9 151_±18 9 0.39 7 40 2.76 49 0.132 3741 6.4_±2.6 257_±37 10 0.38 10 60 5.63 150 0.132 3741 3.0_±0.9 524_±44 11 0.23 3 30 1.73 23 0.132 3741 5.8±3.6 161±33 12 0.23 5 49 5.61 122 0.132 3741 1.8±0.8 522±49 13 0.09 2 52 2.56 59 0.132 3741 1.5±1.1 238±31 14 0.49 13 60 7.39 197 0.132 3741 3.0±0.8 688±51 15 0.47 5 24 5.16 55 0.132 3741 4.1±1.9 481±65 16 0.23 5 50 5.86 130 0.132 3741 1.7±0.8 545±49 17 0.23 4 40 1.18 21 0.132 3741 8.5±4.6 110±24 18 0.45 14 70 4.31 134 0.132 3741 4.7±1.3 401±35 19 0.83 7 19 7.11 60 0.132 3741 5.2±2.1 562±86 Mean age 3.8±0.5 Central age 3.4±0.3 Age dispersion 21 Pooled age 3.3±0.3 x2 30 P (x2) 3.8 Degree of freedom (df ) 18

Stratigraphic age of the clast in the Coastal Range 1.0–1.7 Inferred ancient cooling age when the sample was exposed on the Central Range 2.8–2.1

a See footnote to Table 1.

17 Ma ( Yang et al., 1988). Overlying the volcanic during the Penglai Orogeny. Two of the exotic amphibolite blocks of ophiolitic clan yield apatite rocks is a series of clastic sedimentary units

includ-ing the Lichi Formation. This formation is a fission-track ages of ca. 11 Ma ( Table 4). All the above-mentioned apatite ages are much older than chaotic and non-stratified, muddy to clayey

forma-tion containing many exotic blocks of different the timing (~5 Ma) of the peak temperature induced by the Penglai Orogeny. This implies that sizes, ages and lithologies (Hsu, 1976) emplaced

Table 3

Results of fission track dating for apatite extracted from two large exotic amphibolite blocks of the Lichi Melange at Tonglia Sample No. _{rs(tracks cm−2)} N_s G_{s ri (tracks cm−2)} N_i G_{i rd (tracks cm−2)} N_d Age±1s(Ma) CLW-12 4.56×103 126 1079 2.46×105 501 154 1.64×106 5556 11.2±1.5 CLT-50 1.31×104 190 594 7.40×105 1391 66 1.64×106 5556 10.8±1.3

Table 4

Zircon fission track analytical data for Yuantoshan geniss from the Tananao Schist Complex of the Central Rangea

Grain No. _rs/105 N_s G_s(=G_i) _ri/106 N_{i rd/106} Age±1s U±1s

(tracks cm−2) (tracks cm−2) (tracks cm−2) (Ma) (ppm)

1 3.2 58 408 9.93 1801 2.47 2.2±0.3 605±18 2 3.51 49 315 11.4 1602 2.47 2.1±0.3 697±24 3 2.97 37 280 11.7 1450 2.47 1.7±0.3 710±23 4 7.45 129 390 20.9 3620 2.47 2.4±0.3 1272±33 5 2.48 16 146 14.1 313 2.47 1.2±0.3 146±33 6 1.98 17 194 8.53 379 2.47 1.6±0.4 194±22 7 2.25 33 329 9.38 417 2.47 1.6±0.4 571±19 8 2.25 33 329 9.05 893 2.47 1.7±0.3 551±19 9 1.98 15 140 11.3 504 2.47 1.2_±0.3 170_±27 10 1.98 15 140 10.9 871 2.47 1.2_±0.3 170_±26 11 3.08 39 285 9.77 781 2.47 2.2_±0.4 595_±20 12 1.17 17 326 6.43 571 2.47 1.3±0.4 326±15 13 6.98 62 200 24.3 270 2.47 2.0±0.3 1480±43 14 1.94 31 360 13.5 567 2.47 1.8±0.3 823±29 15 2.09 28 300 7.23 546 2.47 2.0±0.4 300±16 16 3.67 18 110 14.8 329 2.47 1.7±0.5 110±38 17 2.34 27 260 8.66 462 2.47 1.8±0.4 527±20 18 5.47 101 415 17.9 915 2.47 2.1±0.2 1090±29 Mean age 1.8_±0.2 Central age 1.8_±0.2 Age dispersion (%) 15.9 x2 age 1.8±0.2 Pooled age 1.9±0.2 x2 5.0 Degree of freedom (df ) 17

Stratigraphic age of the pre-metamorphic rock >90

a See footnote to Table 1; standard glass dosimeter used was NBS SRM-610.

the clastic sediments of the Coastal Range have metasandstones from the Central Range can be estimated by substracting their depositional ages not experienced a temperature which can

com-pletely reset the apatite fission-track system, not mentioned above in the Coastal Range from respective zircon fission track ages, that is, 4.2±0.5 to say a temperature for the resetting of zircon.

The zircon fission-track ages obtained thus and 3.6±0.3 Ma measured today. The resulting time is ca. 1.2–2.6 m.y., both of which are close to undoubtedly represent the total time elapsed since

they passed effective closure temperature during the zircon fission-track ages 1.0~2.0 Ma of the present Tananao Schist in the eastern flank of the their exhumation.

The depositional age of the Shuilien Central Range. The consistence of the ancient and the present zircon fission-track dates implies steady Conglomerate has been well documented by

pale-ontological and paleomagnetic studies (Chang, exhumation and cooling rates since ~4Ma, sug-gesting that the Central Range achieved its steady-1968; Chi et al., 1980, 1981; Lee and Chi, 1990;

Lee et al., 1991). Nannofossils indicative of NN state form shortly after initial emergence above sea level ca. 5 Ma. This inferrence is in good 16–18 and formanifera of N21 from Shuilien and

of middle NN19 from Chimei correspond to depo- agreement with the conclusion obtained from the analysis of the mechanics of mountain building in sitional ages of 3.0±0.1 and 1.0±0.1 Ma,

Fig. 3. Graphic presentations of the analytical results for the zircon samples from the Shuilien Conglomerate at (A) Shuilien village and (B) Chimei village, and from (C ) Yuantoshan gneiss at Nanao. (a) rs versus ri plot, (b) radial plot, (c) single-grain age histogram, (d ) relationship between fission-track zircon grain ages (bold dots with 1s bars), stratigraphic age range (shaded area) of the pre-metamorphic parental sandstones, and the lower age limit (dotted line) of provenance rocks for the detrital zircons.

oblique arc–continent collision, the mountain belt ca. 160°C m.y.−1 Accordingly, the closure temper-ature of~240°C can be adopted for zircons from grows steadily wider and higher until the central

part (i.e. the segment of the Tananao Schist) has the Tananao Schist as compared with the values given by Wagner and Reimen (1972), Haack been a region of constant and steady state

topogra-phy. The rate of the growth of the Taiwan moun- (1977), Gleadow and Brooks (1979), Zaun and Wagner (1985), Hurford (1986) and Brandon and tain belt due to plate-boundary compression equal

the rate of erosion. It has been shown by Suppe Vance (1992).

Brandon et al. (1998) pointed out that a rapid (1981) that the Tananao Schist attained

steady-state topographic form in ca. 1 and 1/3 m.y. after erosion rate causes isotherms to move towards the surface but also induces a faster rate of cooling so initial emergency of the Taiwan mountains. In

other words, the erosion and cooling rates of the that closure occurs at a higher temperature. The average exhumation rate is determined by dividing Tananao Schist had been the same as present at

ca. 3 and 2/3 Ma. This inference is in good the closure depth by the fission-track age. Here we take a simplified way to estimate average exhuma-agreement with that obtained from zircon

fission-track ages stated above. tion rate of the Tananao Schist. Present-day local thermal gradients measured in geothermal wells In the northern Tananao Schist, the cooling

rate estimated from the pair of zircon and apatite in the northern Tananao Schist area are ca. 55°C km−1 (Lee and Cheng, 1986), which can be fission-track ages (0.9 and 0.3 Ma, respectively;

Liu, 1982) and their general closure temperatures taken as the upper limit of the regional thermal gradients for the Tananao Schist. On the other (~220–235 and~110–135°C, respectively) was

Fig. 4. Schematic thermal history of metasandstone boulders in the Shuilien Conglomerate. The temperature axis is divided into two zones according to the stability of the fission-tracks in zircon. The lightly shaded area show the transitional temperature range between the zones. The heavily shaded area represent the T–t paths for the present metasandstone samples in the Shuilien Conglomerate.

hand, the thermal gradient of 30°C km−1 for the culated by the method described by Brandon et al. (1998).

Western Foothills (Suppe and Witte, 1977) can be

considered as the lower limit. The average thermal Based on fluid inclusion studies of quartz from similar rock types, the peak temperature attending gradient for this area as a whole is thus estimated

at 42°C km−1 by interpolation. With the above- the metamorphism of the metasandstones was estimated to be 260±40°C (Tan and Wang-Lee, mentioned closure temperature (~240°C) and the

thermal gradient (~42°C km−1), the zircon ages 1977). The approximation of zircon fission-track closure temperature to the peak metamorphic tem-of 1.0-2.6 Ma for the Tananao Schist suggest

ero-sion rates of ca. 2.3~6.0 mm yr−1. The upper perature implies that the timing of peak temper-ature of the Penglai Orogen was only slightly older bound of this range is close to the modern erosion

rate (5.5 mm yr−1) determined by sediment yield than 4.3 Ma and agrees well with the inference of 5 Ma from fission track studies on the Hsuehshan data (Li, 1976), while the lower bound equals

approximately to the value (2.5–3.5 mm yr−1) cal- Range (Liu, 1988). If regional cooling could be

Fig. 5. Schematic geologic cross-section of present-day central Taiwan. The lines connecting the dots ($), which denote progressive equal-time positions of points initially aligned vertically are material trajectories (travelling paths) of rocks now occurring on the surface.

attributed to large-scale erosion, then the age of of the Coastal Range, they have remained at temperatures less than the annealing temperature ca. 5 Ma could also mark the timing when the

Taiwan Mountain belt began to emerge above sea of apatite. The zircon fission-track ages obtained at present are~4.2–3.6 Ma. Their zircon cooling level. Fig. 4 shows the entire thermal history of

the metasandstone clasts found in the Shuilien ages at the time of deposition of the Shuilien Conglomerate (1.2–2.6 m.y.) are basically the same Conglomerate.

It is worthy mentioning that the zircon fission- as that determined for the Tananao Schist (1– 2 Ma) presently exposed on the eastern flank of track ages obtained previously for the present

metasandstone outcrops of the Hsueshan Range the Central Range. Both results demonstrate rapid erosion associated with the Penglai Orogeny. The are ca. two to five times of the zircon cooling ages

of the early or present eastern Central Range. This Central Range, at least its central and northern parts, had already reached the peak metamorphic implies that the rates of uplift-exhumation-cooling

for the rear (eastern) and frontal (western) flanks temperature of the Penglai Orogeny at ca. 5 Ma. Subsequent cooling of the rear flank at an aver-of the Taiwan accretionary wedge have been

dis-tinctly asymmetrical since the beginning of the age rate of ~120 °C m.y.−1 for the last 4 m.y. is attributed to rapid large-scale erosional Penglai Orogeny. Fig. 5 shows schematic

illustr-ations of the present-day geological profile and exhumation. associated metamorphic grade and uplift rate. Two

points are worthy to mention. First, the suture of

the two converging plates, that is, the Longitudinal _{Acknowledgements} Valley, is bounded by two high- angle thrust faults.

Second, the metamorphic grade of surface rocks _{The authors are grateful to Professor Y. Wang} generally increases eastward but the highest grade _{for comments on early version of the manuscript.} rocks are exhumed in the orogen interior at high _{Critical reviews by Drs M.T. Brandon and P.A.M.} elevation of the rear flank (retro-wedge) instead of

Andriessen helped sharpen the presentation and the frontal flank (pro-wedge) as considered by

focus of this paper. This study was supported by Willett et al. (1993). Actually, there is no

remark-the National Science Council of remark-the Republic of able difference in modern annual mean

precipita-China under Grant No. NSC72-0202-M02-03. tion (2700–2900 mm yr−1) between both flanks of

the Taiwan Orogen ( WRPC, 1995). In the sense of orographically controlled precipitation and its

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