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TECTONOPHYSICS I I

ELSEVIER Tectonophysics 280 (1997) 239-256

Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau

Dennis Arne a,*, Brenton Worley b, Christopher Wilson b, She Fa Chen h, David Foster c, Zhi Li Luo d, Shu Gen Liu d, Paul Dirks e

a Department of Earth Sciences, Dalhousie University, Halifax B3H 3J5, Canada b School of Earth Sciences, Melbourne University, Parkville 3052, Australia

c Victorian Institute of Earth and Planetary Sciences, La Trobe University, Bundoora 3083, Australia d Chengdu Institute of Technology, Chengdu, Sichuan Province, Peoples Republic of China

e Department of Earth Sciences, University of Zimbabwe, Harare, Zimbabwe

Received 18 April 1996; accepted 24 February 1997

Abstract

Thermochronological data from the Songpan-Ganz~ Fold Belt and Longmen Mountains Thrust-Nappe Belt, on the eastern margin of the Tibetan Plateau in central China, reveal several phases of differential cooling across major listric thrust faults since Early Cretaceous times. Differential cooling, indicated by distinct breaks in age data across discrete compressional structures, was superimposed upon a regional cooling pattern following the Late Triassic Indosinian Orogeny. 4°Ar/39Ar data from muscovite from the central and southern Longmen Mountains Thrust-Nappe Belt suggest a phase of differential cooling across the Wenchuan-Maouwen Shear Zone during the Early Cretaceous. The zircon fission track data also indicate differential cooling across a zone of brittle re-activation on the eastern margin of the Wenchuan-Maouwen Shear Zone during the mid-Tertiary, between ~38 and 10 Ma. Apatite fission track data from the central and southern Longmen Mountains Thrust-Nappe Belt reveal differential cooling across the Yingxiu-Beichuan and Erwangmiao faults during the Miocene. Forward modelling of apatite fission track data from the northern Longmen Mountains Thrnst-Nappe Belt suggests relatively slow regional cooling through the Mesozoic and early Tertiary, followed by accelerated cooling during the Miocene, beginning at ca. 20 Ma, to present day.

Regional cooling is attributed to erosion during exhumation of the evolving Longmen Mountains Thrust-Nappe Belt (LMTNB) following the Indosinian Orogeny. Differential cooling across the Wenchuan-Maouwen Shear Zone and the Yingxiu-Beichuan and Erwangmiao faults is attributed to exhumation of the hanging walls of active listric thrust faults. Thermochronological data from the Longmen Mountains Thrust-Nappe Belt reveal a greater amount of differential exhumation across thrust faults from north to south. This observation is in accord with the prevalence of Proterozoic and Sinian basement in the hanging walls of thrust faults in the central and southern Longmen Mountains. The two most recent phases of reactivation occurred following the initial collision of India with Eurasia, suggesting that lateral extrusion of crustal material in response to this collision was focused along discrete structures in the LMTNB.

Keywords: Tibetan Plateau; Longmen Mountains; fission track dating

* Corresponding author. Present address: School of Science, University of Ballarat, P.O. Box 663, Ballarat, VIC 3352, Australia. Fax: +61 (03)5327-9144; E-mail: [email protected]

0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0040- 1 95 1 (97)00040- 1

240 D. Arne et al./Tectonophysics 280 (1997) 239-256

1. Introduction

The Tibetan Plateau forms a major physiographic feature on the planet, with a mean elevation of just over 5 km above sea level (Fielding et al., 1994; Fig. 1), and as such its development is considered to have had a significant impact on world climate (e.g., Burbank et al., 1993; Molnar et al., 1993). However, the timing and mechanism for uplift of the Tibetan Plateau remains controversial. In addition, recognition of the Himalayan Mountain chain and the Tibetan Plateau as resulting from continent-continent collision has ensured that the region has received consid- erable scientific attention. Most western researchers have focused on the Himalayas themselves, or on major left-lateral faults on or along the margins of the Tibetan Plateau. By comparison, relatively little effort has been expended on the eastern margin of the Tibetan Plateau, particularly in the Longmen Moun- tains where southeast-directed thrusting is well documented. The results of this study represent the out- come of a collaborative effort between Western and Chinese researchers to constrain the timing of thrust faulting in the Longmen Mountains and thus provide insight into the response of the region to continental collision and uplift of the Tibetan Plateau.

The present position of the eastern margin of the Tibetan Plateau is considered by many to have re- suited from the lateral extrusion of material from west to the east and southeast (Tapponier et al., 1986; Avouac and Tapponier, 1993), roughly per- pendicular to the trajectory followed by the Indian subcontinent in its final collision with Eurasia. In the Western Sichuan Province the eastern margin of the Tibetan Plateau roughly coincides with the boundary between the Songpan-Ganz6 Fold Belt (SGFB) and the Western Sichuan Foreland Basin (WSFB; Fig. 1). The SGFB underwent multiple deformation, greenschist- to amphibolite-facies metamorphism, and felsic magrnatism during the Late Triassic In- dosinian Orogeny (Dirks et al., 1994; Worley et al., 1996) in response to closure of the Palaeo-Tethys Ocean. Peak-metamorphic, kyanite zone conditions were reached within the Wenchuan-Maowen Shear Zone (WMSZ; Fig. 2), a 20-25 km wide zone of non-coaxial shear which developed along the eastern margin of the SGFB. The final stages of Indosinian deformation in the WMSZ are related to the initi-

ation of thrusting within the Longmen Mountains Thrust-Nappe Belt (LMTNB; Dirks et al., 1994; Chen et al., 1995) and the evolution of the Sichuan Basin from a stable platformal setting to a terrestrial foreland basin (Chen et al., 1994a).

The LMTNB lies between the SGFB and West- ern Sichuan Foreland Basin (WSFB; Fig. 2), and is characterised by a series of north-northeast-trending listric thrust faults that represent a zone of thick- to thin-skinned deformation (Burchfiel et al., 1995). An abrupt drop in metamorphic grade from kyanite to chlorite zone (6-8 kbar to 3-4 kbar; Worley et al., 1996) across the Wenchuan-Maowen Fault, which marks the transition from the SGFB to the LMTNB, indicates that approximately 10-12 km of differential exhumation has occurred across this structure since the Indosinian Orogeny. The restriction of upper greenschist- to amphibolite-facies rocks to the WMSZ suggests that much of the initial exhumation, during the later stages of the Indosinian Orogeny, was related to ductile reverse shear along the high- strain shear zone (Worley et al., 1996).

Episodic terrestrial deposition in the WSFB from the end of the Indosinian Orogeny to the present day indicates that the LMTNB continued to be tectonically active following the Indosinian Orogeny (Chen et al., 1994a, 1995; Burchfiel et al., 1995). This ac- tivity was presumably in response to the progressive accretion of terranes onto the southern margin of Eurasia prior to its final collision with the Indian sub-continent (Dewey et al., 1989). Stratigraphic and structural evidence can be used to constrain the initiation of movement on several of the major bounding thrust faults, but how long the faults remained active and the magnitude of displacement remains specula- tive. Burchfiel et al. (1995) argue that deformation in the central and northeastern Longmen Mountains was dominated by right-lateral movement during the middle to late Cenozoic, whereas these structures were subsequently overprinted by left-lateral motion on NW-trending structures in the southern LMTNB. Fault-plane solutions indicate that present-day movement along major northeast-trending listric faults is directed towards the southeast, whereas a north- trending zone of seismicity oblique to the main structural grain of the region may reflect the termina- tion of left-lateral movement along the Kunlun Fault to the north (Fig. 1; Chen et al., 1994b).

D. Arne et al. / Tectonophysics 280 (1997) 239-256 241

140°1

40°N

30°N

20ON.

Fig. 1. Tectonic setting of the Songpan-Ganz6 Fold Belt, Longmen Mountains Thrust-Nappe Belt, and Western Sichuan Basin, China with respect to the Tibetan Plateau and southeast Asia. ATF = Atlyn Tagh Fault; HYF = Hai Yuan Fault; KF = Kunlun Fault; XHF = Xianshui Fault; JJF = Jinsha Jiang (Red River) Fault.

This paper represents the culmination of a fission track thermochronological study in conjunction with some preliminary 4°Ar/39Ar results from across major structures within the LMTNB between the eastern margin of the SGFB and the WSFB. The study provides new constraints on the prolonged history of deformation associated with the accretion of terranes onto the southern margin of Eurasia, the differential exhumation history across several major thrust faults, and the complex geological history of the LMTNB.

2. Methodology

2.1. Fission track analysis

Apatite and zircon concentrates obtained using conventional heavy mineral separation techniques were analyzed using the external detector method following the procedures described by Green (1986) for apatite and Green (1985) for zircon. Thermal neutron irradiations for apatite were mostly per- formed in the X-7 facility of the HIFAR reactor, Lucas Heights, Australia, which has a Cd ratio for gold of ~125 (FT- and CW-series samples). Zircon grain mounts and FT93- series apatite samples were irradiated at the Dalhousie University Slowpoke-2

reactor, which has a Cd ratio for Au of 4.3 in the outer site used for irradiations. Fission track ages were determined using the zeta calibration method as recommended by Hurford (1990). A pooled age was calculated for those samples in which individual grains do not show a significant variation in fission track age, as determined by the X 2 statistical test (Galbraith, 1981), and uncertainties were calculated using the conventional approach of Green (1981). A central age was calculated for those samples in which individual grains do show a significant spread in fission track age (Galbraith and Laslett, 1993). Horizontal confined track lengths were measured using the procedure described in Gleadow et al. (1986).

Fission track data are presented in Table 1 and shown in Figs. 2 and 3 with respect to the major structures in the LMTNB. Apatite fission track (AFT) and zircon fission track (ZFT) data from the central transect are plotted against sample elevation in Figs. 4 and 5. The AFT data from all three transects are also summarized in Fig. 6, a plot of mean track length against fission track age. All uncertainties are quoted at the 2or level. The AFT ages of the samples show consid- erable variation, from less than ~ 10 Ma to ~ 180 Ma. Mean track lengths for these samples vary between 11.4 dz 0.5 # m and 13.7 -4- 0.8/zm. Those samples

242 D. Arne et al./ Tectonophysics 280 (1997) 239-256

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Fig. 2. Regional geological map of the study area (modified from Dirks et al., 1994) showing the positions of major thrust faults and the

locations of samples described in Tables 1 and 2.

D. Arne et al./Tectonophysics 280 (1997) 239-256 243

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244 D. Arneet al. / Tectonophysics 280 (1997) 239-256

Yingxiu Guanxian Beichuan Tangwanzhai Anxian

33_+8 Fault Syncline Fault 162_-+23 189+_20 ~ J K

3o ~r"r=r=r"r" 1 ~ I

Northern Transect

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,ir~T Devonian - Permian 10+~? Apatite Fission Track 5 km Age (Ma -+ 2(~)

Fig. 3. Idealized cross sections for each of the three sample transects showing the positions of data points described in the text. The locations of these transects are shown in Fig. 2. Upper Triassic to Recent strata have been further subdivided into Upper Triassic (Tr3), Jurassic (J), Cretaceous (K) and Eocene (Eoc) units.

giving the oldest AFT ages also generally show a significant spread in age between grains, as do individual zircon grains from the four samples in which they were analyzed (Fig. 3). Central ZFT ages for these samples range from 110 + 14 Ma to 38 4- 4 Ma.

40 9 2.2. Ar/~ Ar thermochronology

Three mica schist samples were disaggregated and muscovite separates prepared using standard mag- netic and heavy-liquid separating techniques prior to 4°Ar/39Ar analysis at La Trobe University. Mus- covite mineral separates were irradiated for 30 h at the Soreq Nuclear Research Center IRR-1 reactor, Israel (Heimann et al., 1992) along with the flux monitor GA1550 biotite (97.9 Ma; McDougall and Roksandic, 1974). Single grains or small numbers of grains (2 to ~8, depending on grain size) were loaded into a gas extraction line fitted with a quartz glass view port. Gas was extracted using a defo- cused 6 W argon ion laser with stepwise heating

accomplished by varying the power output of the laser. The gas was purified with Zr-Ti SAES getters and argon isotopes were measured using a VG3600 mass spectrometer with a Daly photomultiplier. Sys- tem blanks for the extraction system were typically < 2 x 10 -16 mol 4°Ar and of atmospheric composition. Correction factors for interfering isotopes were measured using K2SO4 and CaF2 salts irradiated with the samples, and the following values were used: (4°Ar/39Ar)K = 1.6 × 10-2; (39Ar/37Ar)ca =

9.4 × 10-4; and (36Ar /37Ar)ca = 2.4 x 10 -4. The sample locations and 4°Ar/39Ar age spectra of

the three analysed samples are shown in Fig. 2. Data for these three samples are also presented in Table 2.

3. Interpretation of thermal history

3.1. Late-Triassic to Cretaceous cooling history,

Concordant 4°mr/39Ar age spectra, such as those yielded by samples C93/244 and C93/246B from


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Fig. 4. ZFT ages plotted against sample elevation. Also shown are radial plots of fission track ages from individual zircon grains for each sample. The relative precision of ZFT age increases from left to right in each radial plot. Standard error scales on left of radial plots apply to each individual grain age. See Galbraith (1988) for details of plot construction.

within the high-strain central portion of the WMSZ, are typical of samples that have remained thermally undisturbed since cooling rapidly below ca. 350°C (McDougall and Harrison, 1988). However, it is possible that the fiat spectra could be due to the homogenization of Ar isotopes as a result of de- hydrogenation during heating under vacuum (e.g., Gaber et al., 1988). This is probably not the domi- nant cause for two reasons; firstly, muscovite appears to be relatively stable under vacuum (Wijbrans and McDougall, 1986) compared to biotite; and secondly, an analytical artifact might be expected to affect all samples (cf. sample C93/82).

Discordant spectra, such as that observed for sample C93/82 from the western edge of the shear zone, result from diffusive loss of argon related to slow

cooling, a thermal event subsequent to cooling of the sample through the argon retention temperature (Harrison and McDougall, 1980), or to argon loss during post-cooling mylonitisation (Costa and Maluski, 1988; West and Lux, 1993). In each of these scenarios the oldest age from the spectrum is a minimum age for the original cooling, whilst the youngest release age gives a maximum estimate of the age of the overprinting thermal or tectonic event (e.g., McDougall and Harrison, 1988). There is no evidence of any Tertiary magmatism in the eastern SGFB, and sample C93/82 has not undergone any post-metamorphic mylonitisation. In fact muscovite in this schist defines both an early S] slaty cleavage and lies in $2 crenulation cleavage domains which predate or are coincident with the peak of


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Fig. 5. AFT ages plotted against sample elevation. Also shown are radial plots of fission track ages from individual apatite grains and horizontal confined track length distributions in representative samples. The relative precision of apafite ages increases from left to right in each radial plot. Standard error bars on left of radial plots apply to each individual grain age. See Galbraith (1988) for details of plot construction.

metamorphism (Worley et al., 1996). Formation of the crenulation cleavage resulted in minor muscovite deformation, substantial crystallisation and hence a range of grain sizes, prior to cooling of the sample. Most importantly, even weak intracrystalline deformation of the mica structure would lead to a range of effective diffusion domain sizes (Lovera et al., 1989). This effect coupled with the observable optical grain size variation means that sample C93/82 could be recording a range of closure temperatures, possibly as low as 270°C for a cooling rate of 10°C/m.y. (Lister and Baldwin, 1996). Hence, based on both microstructural observations of the dated sample and geological constraints at both a regional and local level, a model involving slow cooling over a closure temperature range is preferred over one in which argon is diffusively lost following cooling.

The concordant 4°Ar/39Ar spectra from the gar- net zone schists indicate that the high-grade portion of the Wenchuan-Maowen Shear Zone underwent a

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I [ ] Central Transect T O Southern Transect

T

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o s0 16o 2so

Apatite Fission Track Age (Ma)

Fig. 6. Relationship between mean track length and AFT age for all samples. Data from the three different transects are distin- guished on the diagram.

period of cooling through the closure temperature of muscovite at approximately 130-120 Ma. Consider- ing that an appropriate closure temperature for mus-


covite is on the order of 350 ° to 400°C for a cooling rate of 10°C/m.y. (Lister and Baldwin, 1996), these ages are unlikely to represent exhumation related to the ductile reverse shear observed within the shear zone. A more probable explanation is that by 130- 120 Ma reverse shear was already being accommo- dated by the localised retrograde high-strain zones. While these central samples were experiencing relatively rapid localised exhumation, the lower-grade rocks to the west (e.g., C93/82) appeared to have cooled much more slowly through a closure temperature range of "-~350-270°C up to at least 50 Ma ago.

3.2. Tertia~ reactivation of faults

Since all fission track ages from samples of the Indosinian granite or Precambrian basement are significantly less than the respective emplacement ages, these data clearly indicate post-Indosinian cooling of the SGFB and the LMTNB. The ZFT data show a progressive decrease in age from 68 4- 8 Ma at 3950 m to 384-4 Ma at 2400 m (Fig. 4) that could either be indicative of slow cooling through the temperature interval of ~240-180°C (Brandon and Vance, 1992), or may reflect progressive partial annealing of fission tracks in zircon prior to a discrete cooling event (Kamp et al., 1989). The spread in fission track age for individual zircon grains in these samples (Fig. 4) is similar to that characteristic of partially annealed AFT profiles (e.g., Green, 1989) and, by analogy, may therefore reflect differential annealing of fission tracks in different zircon grains from the sample.

Sample FT-7 from east of the Wenchuan-Maowen Fault at an elevation of 950 m also gives a significantly older ZFT age when compared to the three samples from higher elevations, indicating differential cooling across this structure (Figs. 2 and 4). The time of movement across this fault therefore post- dates the youngest ZFT age offset by the structure (i.e., < 38 4- 4 Ma), but is older than the AFT ages, which show no displacement across the Wenchuan- Maowen Fault (i.e., >~10 Ma). An interpretation involving slow regional cooling followed by rapid cooling northwest of the Wenchuan-Maowen Fault between ~38 and 10 Ma is also consistent with the discordant 4°Ar/39Ar data from sample C93/82.

The AFT data from the central and southern transects also reveal a dramatic break in age across the

Yingxiu-Beichuan and Erwangmiao faults. Samples from Proterozoic-Sinian basement complexes and Indosinian intrusions northwest of these faults con- sistently give AFT ages less than 10 Ma and show little variation relative to sample elevation, which ranges from 3950 m to 950 m (Fig. 5). These data are interpreted to reflect Miocene cooling of the basement complexes and southeastern SGFB to temperatures at which fission tracks in apatite are stable (i.e., below --~120°C, the precise value depending on cooling rate and apatite composition). Mean track lengths from these samples vary from 12.4 4- 1.2 to 13.7 4- 0.8 #m, indicating that subsequent cooling was prolonged and that track retention commenced slightly before the time indicated by the observed fission track ages. This grouping of data is repre- sented by those samples with relatively long mean track lengths and young AFI" ages in Fig. 6.

By contrast, Upper Triassic to Upper Cretaceous sedimentary rock samples from southeast of the Yingxiu-Beichuan and Erwangmiao faults give fission track ages between 394-10 Ma and 1734-18 Ma. Mean track lengths for these samples are generally

1 to 2/zm shorter than the samples northwest of the Yingxiu-Beichuan Fault which give consistent Late Miocene fission track ages (Fig. 6). The spread in fission track age for individual apatite grains in some samples (e.g., CW-14 and FF93-154), with ages significantly less than the stratigraphic age (Fig. 5), and the degree of observed track length reduction indicates that they have generally undergone partial track annealing in the temperature range 60 ° to 120°C at some time following deposition. Statistical analysis of the single-grain age data from sample CW-14 using a binomial fitting routine (Brandon, 1992), places an age of 71 4- 15 Ma on the youngest apatite grains in this sample. Without knowing whether fission tracks in these young grains were totally or only partially annealed prior to post-depositional cooling, it can only be inferred that final cooling in the West- ern Sichuan Foreland Basin occurred during the Late Cretaceous, if tracks in these grains were totally annealed prior to cooling, or some time afterwards if they were only partially annealed.

Fission tracks in sample FI'93-152, from the central transect southeast of the Yingxiu-Beichuan Fault, are inferred to have been totally annealed following deposition and this is supported by an Ro max


value of 1.49% for vitrinite from the same outcrop, as fission tracks in apatite are totally annealed at values greater than 0.8 4-0.1% Ro in sedimentary basins (Arne and Zentilli, 1994). The low bireflectance of telovitrinite from this sample suggests anoma- lous heating related to a localized heat source (i.e., contact heating; A. Cook, pers. commun., 1993), possibly due to the restricted migration of hot fluids.

The interpretation of AFT data from a sample of Upper Cretaceous sandstone from the southern transect (CW-52) is also less than straightforward. Taken at face value, fission tracks in this sample must have been totally annealed following deposition and then cooled at some time in the Miocene to give a fission track age of 14 -4- 3 Ma. Since nearby samples give much older fission track ages, heating must have been localized, as inferred for the interpretation of data from sample FT93-152. Again, the localized migration of hot fluids could be a possible explanation for this unusually young age.

Samples from the northern transect show a significant change in fission track age across the LMTNB (Fig. 2), although it is not clear from the available sample distribution whether a dramatic change oc- curs in the vicinity of the Yingxiu-Beichuan Fault, as it does further south, or whether a gradual transition exists. Apatite ages from Jurassic and Creta- ceous sedimentary rock samples from the WSFB are either indistinguishable from, or significantly older than their respective stratigraphic ages and these samples are inferred to have undergone partial track annealing following deposition. By contrast, samples of Palaeozoic to Triassic sandstone and Indosinian intrusives from the LMTNB and the SGFB give AFT ages in the range 33 4- 8 Ma to 122 4- 12 Ma that generally increase with increasing elevation. The fission track ages of these samples are significantly younger than their respective stratigraphic ages, indicating that track retention began during regional cooling at various times in the Mesozoic and early Tertiary.

Two samples, FT93-149 and FT93-150, give relatively young AFT ages from an elevation around 3000 m that do not conform to the overall trend of increasing fission track age with increasing elevation for the northern transect. These samples are located on the flanks of a small graben along the Min Jiang Fault which forms the western boundary of the Min Shan Uplift Zone (Chen et al., 1994b) and thus may

have undergone a relatively greater degree of Ter- tiary cooling in response to erosion along the flank of the Min Jiang Uplift Zone (Burchfiel et al., 1995).

4. Modelling of thermal histories

All samples west of the Yingxiu-Beichuan Fault along the northern transect give significantly older AFT ages than similarly positioned samples further to the south, and thus show a lesser degree of late Tertiary cooling. AFT data from these samples therefore retain a record of their early to mid-Ter- tiary cooling histories, but this information can only be inferred through forward modelling of the data. Therefore cooling histories have been modelled for three samples, FT93-151, FT93-144 and FT93-143, from the northern SGFB using the forward ran- dom search approach described by Ravenhurst et al. (1994). These three samples are located to the west of the Yingxiu-Beichuan Fault and span elevations between 4050 m and 750 m. They thus represent the greatest observed spread in fission track age over the largest vertical elevation difference of samples from the western end of the northern transect.

The modelling approach is based on the annealing data of Laslett et al. (1987) for Durango apatite and uses the algorithm described by Willett (1992) and Issler (1996) modified to use the mathematical fit of Crowley et al. (1991). In the case of either mixed provenance or differential annealing due to compositional differences, use of modelling programs based on a single apatite composition with allowance for only a single source area thermal history is inap- propriate and would produce misleading output. For this reason modelling of AFF data from partially annealed samples to the southeast of the Yingxiu- Beichuan Fault has not been attempted as most samples show a significant spread in fission track age from individual apatite grains within the samples. Such a spread may either indicate variations in cooling history from different source areas or differential annealing of apatite grains having different chlorine contents during partial annealing after deposition (Green et al., 1986; O'Sullivan and Parrish, 1995). Thus, all samples selected for modelling give pooled fission track ages and show no obvious evidence for differential track annealing due to compositional variations. Fission tracks in the modelled samples are

250 D. Arneet al./Tectonophysics 280 (1997) 239-256

also interpreted to have been totally annealed prior to the onset of cooling, and so potential complications due to a mixed provenance of apatite grains are also avoided.

The summary diagrams in Fig. 7 show the ex- ponential mean (or preferred) solution for 250 ac-

ceptable thermal history solutions that are each compatible with both the measured fission track age at 2tr uncertainties and the observed track length distribution using the Kolmogorov-Smirnov statistic. The preferred thermal history solution in each fig- ure is bounded by an upper and lower temperature

0.5~

~ 15o

~ lO0 ¸

E I.- 50

~ + , , ~ =, =1, ........ I ......... I ......... p

0.4 t Average age = 147.9 ± 8.8 Ma

0.3

~ o.al if_ +il+ ~o.

. . . . . . . i . l+J. i . . . . . , . : . ; .

0.3-

t- 0 2 -

U_ <33 > ~ 0.1

rr

Mean Track Length = 12.9 + 0.31~m

J i i i

200 ,

o¢~ 150

~ 100

~.- 50

o

0 .3-

•c>'0.4 o" 0.3 =... LL

.-> 0.2

rr 0.1

200

Average age = 102.7+_8.1 Ma

Mean Track Length = 12.2 :t: 0.4btm

0 c

"~ 02 O"

U_

._~

"~ 0,1 n. -

~" 15o o v

loo cl E

t-- 50

2 0 0 -

Cambrian sandstone

0 . . . . . . . . . [ . . . . . . . . . i . . . . . . . . . i . . . . . . . . . i

200 150 100 50 0

Geologic Time (Ma)

1.0

0.8 >,.

I1) 0 . 6

u. .

>m 0 . 4

~r 0.2

. i , i , . , l , ,

5O

Average age = 0.3 - Mean Track Length 58.1 _+ 3.3 Ma = 12,6 +- 0.4 l.tm

i 100 150 200 0 5 10 15 20

Track Leng~ (In'n) Onset of Track Retention (Ma)

O t -

02 0 "

U_

> "-=. 0.1

n -

Fig. 7. Cooling histories modelled from AFT data for three samples from the northern transect. Shown are preferred thermal history solutions with model boundaries on the left, the distribution of ages for the onset of track retention for 250 acceptable solutions in the centre, and measured track length distributions compared to the predicted distribution for the preferred thermal history on the right.

D. Arne et al./ Tectonophysics 280 (1997) 239-256 251

limit to define an envelope through which the 250 acceptable solutions pass. These temperature bounds are not acceptable thermal history solutions in themselves. Shown beside each thermal history diagram is a histogram which illustrates the distribution of times at which the onset of track retention began for the 250 acceptable thermal history solutions. Models were run for 200 Ma and permitted cooling only from a maximum temperature of 200°C. These limits were chosen to allow for the onset of track retention during cooling up to 10°C/m.y. (Willett, 1992). Also shown are the measured track length distributions for the modelled samples, as well as the distribution predicted for the preferred thermal history.

Thermal histories for all three samples are charac- terized by an initial phase of gradual cooling during the Mesozoic to early Tertiary followed by. more rapid cooling within the last 20 Ma. Note that constraints on the thermal histories are provided by the fission track data only for times younger than the average time for the onset of track retention, at which point the upper and lower temperature bounds define a relatively narrow zone in temperature-time space. The average age for the onset of track retention decreases with decreasing elevation, whereas the magnitude of late Tertiary cooling increases with decreasing sample elevation. These trends are consistent with gradual regional cooling of the northern SGFB during the Mesozoic and early Tertiary, followed by accelerated cooling during the Miocene.

The annealing data of Laslett et al. (1987) are now known to underestimate the degree of track annealing at low temperatures by some tens of de- grees (Vrolijk et al., 1992), although this remains the most widely accepted published annealing model currently in use. Therefore a component of the late Tertiary cooling indicated by the model output is probably an artifact of deficiencies in the Laslett et al. (1987) annealing data. However, since the amount of late Tertiary cooling for the three samples ranges from 50 ° to 100°C, inaccuracies in the annealing model are insufficient to explain the degree of late Tertiary cooling and could also not account for the relative differences in the magnitude of late Tertiary cooling indicated for the three samples:

Fission track data from a sample at the highest elevation along the central transect, PT-1, have also been modelled using a simple cooling history in

order to place a lower limit on the initiation of cooling in the hanging wall of the Yingxiu-Beichuan Fault. The modelling results indicate an average age for the onset of track retention in this sample of 10.2 4- 1.5 Ma, which indicates that cooling of the Yingxiu-Beichuan hanging wall began prior to this time. This constraint would therefore be compatible with evidence for rapid cooling beginning at "-~20 Ma inferred from modelling fission track data from the northern transect.

5. Discussion

Peak upper-greenschist to amphibolite-facies metamorphic conditions are restricted to a narrow 10-20 km wide zone within the Wenchuan-Maowen Shear Zone. The eastern margin of the shear zone is marked by intense mylonitic/phyllonitic textures and pervasive retrogression of the peak-metamorphic as- semblages as the east-bounding Wenchuan-Maowen Fault approached. To the east of, the Wenchuan- Maowen Fault the Palaeozoic sediments have undergone only minor ductile deformation, but extensive brittle faulting and fracturing, with no evidence that metamorphic grades ever exceeded chlorite zone conditions. Even before considering the significance of the 4°Ar]39Ar data it is important to realise the basic constraints that these structural and metamorphic observations place on the exhumation history of the eastern SGFB. Firstly, although regional isostatic rebound, possibly with accompanying erosion are logical responses to the substantial thickening which occurred within the SGFB, the restriction of 'high-grade' metamorphic rocks along the easternmost margin of the fold belt indicates that exhumation was localised along this boundary. Secondly, the juxtaposition of rocks of clearly different metamorphic grades across the Wenchuan- Maowen Fault (and easternmost portion of the shear zone) necessitates differential exhumation across this structure. Finally, structural kinematic observations indicate that the final ductile deformation within the shear zone was reverse in nature and was subsequently overprinted by progressively localised coplanar and co-linear semi-ductile to brittle shear (Worley et al., 1996). Hence the differential exhumation across the Wenchuan-Maowen Fault, although obviously erosion dependent, was tecton-


ically driven, and may have been episodic through time.

The age breaks defined by the fission track data from the LMTNB are not associated with a change in sample elevation and are opposite in sense to those often associated with cooling recorded in vertical fission track profiles (i.e., fission track age usually increases with sample elevation). Thus we interpret the age breaks in the fission track data across the Wenchuan-Maouwen, Yingxiu-Beichuan and Erwangmiao faults to indicate differential exhumation across these structures during mid-Tertiary and Miocene reactivation, respectively. Evidence for the rapid influx of fanglomerate deposits into the WSFB during the Cretaceous and Tertiary (Chen et al., 1994a) favours tectonically driven exhumation across discrete structures superimposed upon a regional pattern of erosion. We therefore interpret the AFT cooling ages to provide a minimum estimate of the time of thrusting.

A direct estimate of the amount of differential exhumation across the Yingxiu-Beichuan and Erwang- miao faults during the Miocene is complicated by the horizontal distances between the samples, the possi- bility that movement has occurred across other faults to the west and uncertainties in past geothermal gradients. In addition, individual fault blocks have likely rotated during exhumation controlled by reactivation of listric reverse faults. However, differential exhumation across these faults during the Miocene must have been on the order of at least 1 km. Given uncertainties in interpreting the trend in fission track ages defined by the three highest zircon samples (Fig. 4), an estimate of displacement across the Wenchuan-Maowen Fault recorded by these samples has also not been attempted, although it is also likely to be on the order of several kilometres. How- ever, based on contrasts in metamorphic grade, the overall differential exhumation across the localised retrograde high-strain zones within the Wenchuan- Maowen Shear Zone and the Wenchuan-Maowen Fault itself is likely to be as high as 10-12 km.

Evidence for regional cooling is provided by fission tracks in detrital apatite grains from outcrop samples in the WSFB that give AFF ages greater than 100 Ma. The discordant nature of the 4°Ar/39Ar spectra from sample C93/82 and modelling of fission track data from the northern SGFB supports the

suggestion that cooling of the region was gradual prior to rapid Miocene cooling, apart from localised exhumation in the hanging wall of the Wenchuan- Maowen Shear Zone. This gradual cooling probably reflects erosion of the LMTNB which also affected the adjacent WSFB through resultant isostatic uplift of the lithosphere.

AFF data from the three transects across the LMTNB also show a consistent pattern along strike that reflects lateral variations in the importance of thrust faults during exhumation. Basement complexes are exposed only in the cores of broad an- ticlines in the northern part of the LMTNB and it is in this area that evidence for late Mesozoic to late Tertiary regional exhumation is preserved by the AFT data. Modelling results indicate that these samples also underwent rapid cooling beginning in the Miocene at "-~20 Ma, but not from temperatures sufficient to totally anneal fission tracks in apatite, as inferred for samples from the central and southern transects. Proterozoic-Sinian basement is increas- ingly common in the hanging walls of thrust faults along strike to the southwest, reflecting the relative importance of differential exhumation during re-activation of thrust faults in the Miocene. This interpretation is also consistent with the first appearance of granitic basement cobbles in the late Tertiary Dayi Formation in the LMTNB and with the migration of the foreland depocentre to the southwest with time (Chen et al., 1994a).

Hendrix et al. (1994) have documented a phase of rapid exhumation in the Tian Shan on the northern margin of the Tarim Basin during the Late Oligocene and Early Miocene that they attributed to the effects of continental collision. Modelling of the apatite fission track data from the Songpan-Ganz6 Fold Belt suggests accelerated exhumation beginning at ",~20 Ma and this observation is broadly consistent with the findings of Hen&ix et al. (1994), as is the mid-Tertiary timing inferred for reactivation of the Wenchuan-Maowen Fault from the ZFF data. Accel- erated cooling in the Gangdese Belt of southern Tibet also occurred during the Early Miocene (Copeland et al., 1987) and AFr age data from this region indicate that cooling continued into the Late Miocene (D. Arne, unpubl, data). These studies suggest similari- ties in cooling histories along the northern, southern and eastern margins of the Tibetan Plateau following


suturing of India with Eurasia in the early Tertiary at ca. 55 Ma (Klootwijk et al., 1992).

The onset of cooling to the northwest of the Yingxiu-Beichuan and Erwangmiao faults is con- strained to have begun prior to "-~10 Ma and to have continued through the Late Miocene. Thus reactivation of these structures probably began prior to the transition from N-S compression to E-W extension of the Tibetan Plateau between 5 and 10 Ma (Mercier et al., 1987; Pan and Kidd, 1992). Up- lift of the Tibetan Plateau itself remains a topic of some debate (Molnar, 1989; Harrison et al., 1992; Burbank et al., 1993; England, 1993; Turner et al., 1993), as timing constraints are indirectly inferred from palaeobotanical studies, thermochronological data or lithospheric modelling. Molnar et al. (1993), England (1993), and Burbank et al. (1993) present arguments for a substantial period of uplift of the Tibetan Plateau at ---8 Ma, broadly coincident with the onset of E-W extension. If a Late Miocene timing for uplift of the Tibetan Plateau is accepted, then the fission track data from the eastem margin of the Tibetan Plateau would indicate that lateral extrusion of material along major strike-slip faults was mainly accomplished prior to final uplift of the Tibetan Plateau. Conversely, if an earlier timing for the beginning of uplift in the Early Miocene is favoured (e.g., Harrison et al., 1992), then the fission track data suggest that lateral extrusion of material along the LMTNB accompanied uplift of the Tibetan Plateau. Whatever the interpretation, late Cenozoic shortening across the LMTNB is likely to have been more significant than suggested by Burchfiel et al. (1995).

6. Conclusions

Many orogenic belts which formed as a result of continental collision exhibit clockwise P - T - t

paths (England and Thompson, 1984; Brown, 1993), which have been attributed to extensive crustal thickening during the collision followed by post-tectonic erosion accompanying isostatic uplift (i.e., England and Richardson, 1977). In many cases, specifically the Tibetan Plateau, orogenic collapse and extensional thinning resulting from removal of an unstable thermal boundary layer are also thought to be important driving forces for post-collisional exhumation and uplift (e.g., Houseman et al., 1981; England, 1993; Molnar et al., 1993). On the other hand, some workers have suggested models incorporating large- scale thrusting during exhumation to explain differential and often rapid cooling patterns (Hurford et al., 1991; Cook and Crawford, 1994; Eide et al., 1994), and the exposure of young high-grade metamorphic rocks along present-day reverse faults such as the Alpine Fault (Koons, 1987, 1990; Norris et al., 1990) and the Main Mantle Thrust in the Nanga Parbat region (Butler and Prior, 1988; Zeitler et al., 1993).

The thermochronological data presented here along with structural and metamorphic observations from the WMSZ and stratigraphic constraints from the WSFB suggest that slow exhumation of the eastern SGFB in response to isostatic rebound was episodically punctuated by tectonically activated rapid erosion, as summarized in Table 3. The initiation of tectonically enhanced erosion may have been as early as the end of the Indosinian Orogeny, in response to extensive ductile reverse shear within the WMSZ; however, the temperatures at which this deformation occurred preclude constraint by the thermochronological techniques employed in this study. On the other hand, 4 ° m r / 3 9 A r and fission track data have enabled the recognition of several more recent episodes of semi-ductile to brittle reactivation of the Wenchuan-Maowen Fault and other pre-existing reverse structures in a locally compressive regime

Table 3 Summary of Creatceous to Miocene cooling events in the LMFNB

Timing Temperature Event Method (Ma) (°C)

120-130 ca. 350 38- 10 ca. 250 20- 10 ca. 150

Ductile deformation of the Wenchuan-Maouwen Shear Zone 4°Ar/39Ar Differential cooling across the Wenchuan-Maouwen Fault ZFT Differential cooling across the Yingxui-Beichuan and Erwangmiao faults AFT


throughout the Tertiary which were responsible for the emplacement of the LMTNB.

Dirks et al. (1994), Chen et al. (1994a, 1995), Burchfiel et al. (1995), and Worley et al. (1996) have demonstrated the prolonged and complex nature of deformation across the eastern SGFB and LMTNB, stretching from the Indosinian at --~220 Ma until the Quaternary. This complex deformation history over a period >200 Ma can be broadly correlated with the accretion of terranes to the southern margin of Eurasia and with the development of the WSFB. The results of the present study indicate that thrust faults/shear zones along the eastern margin of the Tibetan Plateau have been episodically reactivated during this time. Early episodes of thrusting can be related to southeast-directed compression within the WMSZ during Indosinian collision between the Eurasian and Cimmerian continents and southwest- directed subduction in response to opening of the Neo-Tethys during the Jurassic-Early Cretaceous (Chen et al., 1995). Tertiary exhumation is inferred to represent the eastward extrusion of crustal material as a result of the collision of India with Eurasia prior to uplift of the Tibetan Plateau during the Late Miocene. Therefore, although the tectonics of the eastern margin of the Tibetan Plateau have been controlled by the Himalayan Orogeny during the Ter- tiary, the uplift history of this region is more episodic and protracted than generally portrayed.

Acknowledgements

Fieldwork for this project was financially supported by the Seismological Research Fund of the State Seismological Bureau of China, the Chinese Scientific Research Fund and from funding provided by the Australia-China Council and the Australian Research Council. The first author was supported by a Dalhousie University Killam Fellowship and Research Development Grant during the course of this study. B.W. and S.C. acknowledge the support of Australian Postgraduate Research Awards and University of Melbourne Postgraduate Scholarships, respectively, during their Ph.D. candidatures. Neu- tron irradiations were provided by a grant from the Australian Nuclear Science and Technology Organi- sation. Asaf Raza, Guodong Li and Helen Gibson are thanked for their assistance with sample preparation.

Dale Issler is thanked for providing the modelling software (AFTINV32) used in this study. The coop- eration of Geotrack International during the initiation of this study is also gratefully acknowledged. Critical reviews for Tectonophysics by Marc Hen- drix and Kerry Gallagher contributed greatly to the manuscript.

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