421

The International Conference on Stratigraphy and Tectonic Evolution of Southeast Asia and the South Pacific Bangkok, Thai/and, 19-24 August 1997

Tertiary tectonic denudation in Northwestern Thailand : Provisional results from Apatite Fission-Track Analysis

Upton, D.R., Bristow, C.S., Hurford, AJ. and Carter, A.

Research School of Geological & Geophysical Sciences, Birkbeck and University College London, Gower Street, London, WC I E 6BT, United Kingdom.

ABSTRACT

This paper docwnents the preliminary results of an apatite fission-track (AFT) study carried out in Northwestern Thailand, to constrain the response of the upper crust to Tertiary tectonics. AFT results are consistant with gentle inversion during the Late CretaceouslEarly Tertiary prior to a phase of rapid cooling in a discrete north-south belt of gneissic and plutonic rocks during the Late OligocenelEarly Miocene. Minimwn cooling rates during the unroofing of the north-south belt is estimated to be between

8.5°C Ma- I to 25°C Ma- I. Assuming a geothermal gradient of 30-35°C km- I , cooling rates equate to between 2.75 km and 3.5 km of section being denuded during the Late Oligocene over a -3±2 m.y. period. It is most likely that this later rapid cooling represents a phase of tectonic denudation possibly associated with the unroofmg of a metamorphic core complex.

INTRODUCTION

Based on the geological maps (Department of Mineral Resources, 1987), the oldest rocks in this region are shown to be Precambrian gneisses (Baum et aI., 1970), which have been documented recently as having their last high-grade metamorphic overprint at ~200 Ma. (Ahrendt et al., 1993). The Palaeozoic rocks contain a wide variety of rock types ranging from, sediments, to volcanics and limestones (Bunopas, 1994; Bunopas & Vella, 1983; Hutchison, 1989) into which voluminous S-type granites were emplaced during the collision of the Shan-Thai and Indochina blocks (Beckinsale et aI., 1979; Cobbing et aI., 1986; Darbyshire, 1988; Mahawat et al., 1990; Charusiri et aI., 1993) during the Late Permian to Triassic Indosinian Orogeny (Cooper et al., 1989; Hutchison, 1989; Mitchell 1981; Mouret, 1994). The Mesozoic deposits comprise a series of Triassic marine basin facies, overlain by the continental facies of the Khorat red-beds.

Further tectonic development during the Tertiary led to the basin and range morphology

seen at the present-day, with a series of broadly oriented north-south arcuate mountains separated by fault bounded, sigmoidal, isolated and interlinked intermontane Tertiary basins. Sinistral displacement along major strike-slip faults, including the Mae Ping Fault Zone, which dissects the southwestern margin of the study area, are believed to accommodate the south-eastward extrusion of Sibumasu and Indochina from in front of the advancing Indian craton (Tapponnier et aI., 1982, 1986). The nature and role of Tertiary tectonics in the region are still poorly understood. Several contrasting models have been invoked to explain the initiation and subsequent development of numerous Tertiary basins in the area: rifting (Pigott & Sattayarak, 1993), rifting associated with gravitational collapse (Dwming et aI., 1995), dextral shear (Polachan, 1988; Polachan & Sattarayak, 1989) and thrusting (Ahrendt et aI., 1994). The aim of this paper is to present data from Northwestern Thailand concerning the influence that Tertiary tectonism has had on the denudation (cooling) history of the region. Such informatio!1 is important in

422

understanding the distal effects of continental collision, especially with regard to sediment buget, mass balance studies and models of long­term Jandscape evolution.

The method used in this study is apatite fiss ion-track analysis (AFT), a low temperature « 110± 1 O° C) dat ing techn ique that is particularly sensitive to changes in temperature brought about by erosive and tectonically driven denudation in the uppermo t parts of the crust. By comb in ing AFT cooli ng data wit h palaeogeothermal gradient data it is possible to reconstruct long-term denudation rates, enabling estimates to be made concerning the amount and timing of overburden removal. The resul ts presented here are part of a larger ongoing study concerning the Tertiary tectonic influence on the denudation history of Thai land.

FISSION-TRACK ANALYSIS

Apatite fissio n-track (AFT) analysis is

based upon the spontaneous fission of 238U atoms present in the host crystal. During uraniwn decay, the process of spontaneous fiss ion creates a linear zone (a spontaneous fission-track) of damage in the crystal lattice. The number of spontaneous tracks formed in a host crystal is dependent on the amount of uranium present and the time span over which tracks have been allowed to accumulate. A unique feature of fission-tracks is the way in which they react to temperature . All tracks start wi th the same initial length (~ 16±1 I..lm long in apatite- Gleadow et aI. , 1986), however, they ar semi-stable features, and exposure to elevated temp rature leads to progressive track short ni ng (annealing) . Above a critical temperature, fiss ion-tracks dissapear and a sample is reset. The temperature window, or partial annealing zone (PAZ- Gleadow & Fitzgerald, 1987) within which tracks are semi-

stable is a function of time, for periods of 106-

107 yrs it is equivalent to ~60- 1 1O°C. At temperature s <60°C track shortening is c nsidered to be insignificant and tracks become effectively stable. Since tracks form continuous ly indiv idual tracks wi ll have experienced di ffering proportions of a samples history an d th e re for e the track len gth

distribution is a record of the thermal history. Using measured age and track length data together with kinetic annealing models (e. g. Laslett et aI., 1987) and a data driven modelling program (Gallagher, 1995) it is possible to extract probable thermal histories in the form of time temperature paths. Such data can be then used to calculate cooling rates through time. Since in most geological settings the recorded cooling is due to movement of rock towards the earth's surface (denudation), rather than elevation of the earth's surface with respect to the geoid (crustal uplift) most cooling data can be converted via a value for the geothermal gradient, to a denudation rate and estimation of lost section.

ANALYTICALPROCEEDURE

Apati te crystals were extracted from 5 kg rock samples using standard heavy mineral separation techniques; jaw crushed, milled, heavy mineral concentration using a WilfeyTM table pr ior to heavy liquid and magnetic separation. Apatite grains were mounted in epoxy resin, polished and etched in 5N HN03

for 20 seconds at 20oe. Irradiation at the RIS0 reactor (cadmium ratio for Au >400) Denmark was monitored using standard uranium glass CN5. Age and length measurements were made on an internal crystal surfaces under an optical microscope with a total magnification of 1250x using a 100x dry objective. Central ages were calculated using the lUGS-recommended zeta calibration approach (Hurford, 1990) usmg personal value of z315± 10 for CN -5.

RESULTS

Of the 30 samples collected from Northwestern Thailand (Fig. 1 ), only 16 samples yielded suffi cient apatite grains for fi ssion-track analysis . The results are detailed in Table 1. Sample localities have been accurately recorded using six-figure UTM grid co-ordinates (±50m) acquired by a Trimble™ global positioning system (OPS), whilst elevation (±100m) was obtained us ing a barometric wrist altimeter.

.A..

16°N .A..

-"'-

: Mae Hong Son

~ .00 o

•

. ', . ;;. ,": .. --.1. _ I ,

• - - - - - - - - - - -(:'<t;_ .. - --.. ..;, lang . : .

• 0:

0:

• Chiang Mai i 0

o o

o o

o o

Nan •

:0 9-. -1-. 0 • Lamp;ang

~ , .

" ' I I ~ __ ___ ~ _ _ __ _ _ _ _ __ __ 0 .

. . : . --- ----- -- ; ----- ----- --

.A..

, ' , ' . '

• • •• Tak []

• •

Sukhothai . ' +

o 150 10[J(m

• I:e+----wj

1000 E "

.... ....

...

....

" "

.... .... ....

.... ....

.... .... ....

.... ....

.... ....

....

Northwestern Thailand

Area Enlarged

... .... ... .... ... ... ... ". .... ... ....

" " " " " " " " " " " " " " "

....

"

.... .... ....

.... ....

"" \

+ .

China Sea

~ o 500 l000~ , , I

Figure J Map showing the location of samples collected in Northwestern Thailand as part of a regional apatite fission-track study

t!3 w

Table I Fiss ion track apatite and zircon analytical data

Field Number

Lithology No. Dosimeter of

crystals pd Nd

NORTHWESTERN THAll..AND Apatite-Group 1 THI9577 granite 7 1.332 3691 THI9581 grani te 14 1.332 3691 THI2258 t orthogneiss 20 1.385 7681 THI2259t paragneiss 20 1.385 7681 THI2260t granodiorite 20 1.385 7681 THI 2261 t gneiss 20 1.385 7681 THI2262t gneiss 20 1.385 768 1 THI2263 t dyke into gneiss 20 1.385 7681 THI2265 t gneiss 20 1.385 7681 THI226M Bi gneiss 20 1.385 7681 Apatite-Group 2 THI9564 granite 20 1.332 3691 THI9565 granite 20 1.332 3691 THI9566 granite 1 I 1.332 3691 THI9578 granite 13 1.332 3691 THI9580 granite 20 1.218 8442 THI9584 granite 20 1.332 3691

Zircon THI2266t Bi gneiss 16 0.456 3163

Spontaneous Induced

ps Ns pi Ni

0.993 117 9.065 1068 0.149 205 1.343 1850 0.752 704 7.903 7394 0.502 310 5.788 3578 0.459 349 5.315 4037 0.564 388 5.677 3903 0.2% 327 3.701 4083 0.352 244 3.532 2433 1.019 729 6.175 4416 0.905 775 11.43 9793

0.190 956 6.853 3448 0.232 1946 8.029 6744 0.706 191 6.327 1710 0.156 592 3.847 1461 0.956 371 4.929 1913 1.294 771 8.042 4793

6.821 1644 10.31 2514

Table 1: Fission track apatite and zircon analytical data Notes:

Age Dispe~ion

X2 %

2 0 11 0 15 10.9 97 0.0 60 0.1 40 8.1 45 3.3 13 17. 1 <1 13.4 4 11.0

19 <1 46 13 5 0 24 17 12 0 26 8

7 8.8

(i). Track densities are (x 106 tr cm-2) numbers of tracks counted (N) shown in brackets;

Age (Ma)

± lcr

23±2 23±2 22±1 20±1 20.±1 23±1 19±1 24±2 40±2 18±1

58±2 60±3 23±2 80±6 37±2 34±2

19±1

(ii). analyses by external detector method using 0.5 for the 41t/21t geometry correction factor;

Mean Track

length (J.un)

14.41±O.1 13.6±O.1 14.19±O.09 13.65±O.17 14.11±O.12 13.79±O.11 12.72±O.19 13.56±O.l2 13.38±O.14 13.61±O.11

12.37±O.21 12.73±O.2 12.63±O.17 12.35±0.25 12.64±O. 17 I I.87±O.22

S.d.

1.05 1.11 0.88 1.62 1.16 1.10 1.90 1.20 1.49 1.12

2.19 2.03 1.72 2.49 1.73 2.24

No. of

tracks

102 119 100 94 100 100 101 100 108 100

112 100 100 103 103 102

(iii). ages calculated using dosimeter glass CN-5 (apatite); analyst 1990 Cartert 1;,cN5 =339±5; CN-2 (zircon) 1;,cN2 =127±6; 1994 Upton 1;,cN5 =315±1O calibrated by multiple analyses of IUGS apatite age standards (see Hurford 1990);

(iv) . Px2 is probability for obtaining X2 value for v degrees of freedom, where v = no. crystals - 1; (v). Central age is a modal age, weighted for different precisions of individual crystals (see Galbraith 1992);

~ N ~

425

From the results listed in Table 1. it is clear that samples can be subdivived into two groups on the basis of their FT age and length characterstics. Discussion of the FT results will be made on this subdivision.

Group 1 Thi~ subset includes Triassic granites

and gneissic rocks. The gneisses which were once thought to represent the Precambrian of Thailand (Baum et aI. , 1970), extend from Mae Hong Son province in the north, along the western side of Thailand into the peninsula. Eight of the samples situated within this zone have been kindly provided by Dr. Hans Ahrendt and co-workers at the University of Gottingen, Germany. he samples were collected as part of a regional isotopic dating program with the aim of obtaining more information on the age of granitoid intrusion and the tectono-metamorphic development of northern Thailand.

In Group 1. nine of the samples analysed fall into a cluster ranging from l 8± 1 Ma (THI 2266) to 24±2Ma (THI 2263) (see Fig.2) , whilst sample THI2265 has a slightly older central age (40±2Ma). Confined MTL distributions vary from 14. 1 9±0.09~m (THI 2258) to 12 .72±0 .1 9~ m (THI 2262). Long MTL distributions, greater than - 14~m (see Fig.2) are indicative of fairly rapid cooling through the partial annealing zone. Samples THI 2258 (22± 1 Ma) and TH12266 (18± 1 Ma) also exhibit long MTL disributions (l4 .l9±0 . 09~m and 13 .61±0.1 I ~m respecti ve ly) with narrow (0 . 88-1.12~m) unimodal distributions characteristic of rapid cooling. For these samples the central age approximates to the time of onset of cooling. This quantitative interpretation i confirmed by sampl THI 2266 (l 8± 1 Ma) which has a concordant (within errors ±2s) zircon fission­track age of 19± 1 Ma. The zircon FT system which has a higher effective closure temperature of - 250±50°C and indicates that cooling has been in excess of 200°C.

Granitic sample THI 9577 has been cho en to ill ustra te the C ol ing history xperienced for it group of samples that lie in a

north- outh belt. The MTL of ample THI 9577 is 14.41±0.1 j.lm with a narrow distribution (s .d. 1.05f.lm), typical of a sample that has rapidly cooled through the partial annealing zone. The central age (23±2 Ma) approximates to the

timing of accelerated cooling. The modelled best fitting time-temperature path for sample THI 9577 (see Fig.3) confirms the qualitative interpretation and shows that at a 95 % confidence level, entry into and exit from the partial annealing zone, began and ended during the Late Oligocene (24-30 Ma). Because the data cannot constrain the earlier section of the thermal history a Late Oligocene date must be taken as a minimum age for the onset of cooling. Within the 95% confidence bounds the cooling path suggests a cooling rate withid the

range 8.5°C - 25°C Ma- 1. Assuming a

geothermal gradient of 30-35°C km- l, this would equate to between 2.75 km and 3.5 km of section denuded since the Late Oligocene. The qualitative and quantitative interpretations of the dataset for Group 1. suggest that the majority of section loss occurred over a short period (3 ±2 m.y. ) of rapid cooling, at

denudation rates between 550 to 3500 m Ma- 1.

GROUP 2 The fi ssion-track data from the Group 2.

have fission-track age and length characteristics indicative of a different syle of thermal history than samples from the Group 1. Samples, Till 9564, 9565 and 9578 collected from Triassic granites, contain the oldest central ages in th region and also exhibit broad (2.03 j.lm to 2 .49~ m), negatively skewed track length distributions (see Fig.3 , THI 9565 & THI 9566) indicative of protracted cooling through the partial annealing zone. Till 9578 exhibits a bimodal MTL (s.d. 2. 49~m) which suggests a two phase (cooling and reheating) thermal history. It may therefore contain valuable information on an earlier thermal history which has not been totally overprinted. Sample THI 9584 collected from a Triassic granite in the Fang Province, exhibits the shortest MTL (11.87±0.22Ilm) of all samples. Such a short MTL implies recent cooling from the PAZ (insufficient time for the accumulation of a significant proportion of full length tracks). The sample comes from a site close to the Fang Tertiary basin and cooling may have been influenced by localised fault displacement related to basin development.

Modelling data from the Triassic granite samples THI 9564 and THI 9565 (Fig.3) from the Fang Province reveals that some regi ns of

426

• Sampl. locality M.P.F-Z. Ma. Ping Fault lon. , r.P.F-Z. Th,.. Pagodas Fa u/U lone

- Suture lone 8 Permian

o QuatornaryfLat. r.rtiary D Una i ffsrentia~a' Palaeoloic

O undiffar.ntiaud MesOlO ic and Early rortiary o Mainly Hypo granltos

Figure 2 Apatite fission-track central age and mean track length distributions for Northwestern Thailand (simplified geological map after Cooper et aI., 1989).

northern Thailand have not undergone rapid cooling during the Late Oligocene. In contrast to the rapid cooling histories for samples from Group 1., THI 9564 and THI 9565 reveal a more complex cooling history extending back into the Cretaceous. The oldest preserved tracks occur during the Cretaceous, however the sparcity of old tracks means there is poor resolution of this part of the samples thermal history. Following a period of residence in the PAZ maximum temperatures (21100 C) and onset of cooling take place during the Late Cretaceous/Early Tertiary (50-70 Ma). Sample

THI 9564 shows evidence of reheating ( by - 35±15°C) during the Cretaceous, equivalent to an increase in burial of -1 ±OA km assuming a 35°Clkm geothermal gradient. A possible cause for reheating is the deposition of Khorat sediments (youngest formations). The rate of cooling defined by the 'best fit' is much slower than the Group 1. samples discussed above. The rate of cooling through the partial annealing zone during the early Tertiary is ~ 1.3-2 . 5°C

Ma- 1.

T(°C) TH19564 O-r-- --------.---- --.

20

40

~ ~~~~~~-~r_~~~~~

1: ·1··········· 120 ~~~~~~-~~~~~~~

140 112 84 56 28 o Time (Ma)

427

40~-------------~~----~ 30 PIK~) = 0.479

P(Chl) = 0.673

N 20

10

o Track Length (microns)

20

0.,.. Age: or.al Moo

Pre<!. Age : 57.87 Ma

Oldeel tnck (Ma) : Hili 1011

Obs. M .. n length: 12.386

Prad. M .. n length : 12.344

ot... S.D. : 2.189

Pred. S .D. : 2.031

T(OC) TH1 9565 0

20

40

~

80

100

120 •• ••• • • •• •• • • )0 ••••

180 144 108 72

Time (Ma)

0.,.. Age : 80.00 Moo

Prad. Age : 80.42 Ma

OIdee ltrack (Ma) : eo eo

T("C) THI 9577 o

20

40

60

80

100

120

120 96

Oba. Age: 22.i4 Ma Prad. Age : 23.74111a

OldMt track (Ma): 26 25

I 72 48

Time (Ma)

p(K·S) = 0.1155 30 PIChi) = 0.995

36 o

N 20

10

Qt.. iii..., length : 12.732

Pred. M .. n length: 12.880

5 10 15 Track Length (microns)

Oba. S.D. : 2.030

Pred. S.D. : I.lI8Ii

20

50T-------------~

/ 40

N 30

20

10 . . ,.

24 o

Qt.. M .. n length: 14.414

Pred. Mean length : 14.330

PIK~) = 0.1112 PIChi) = 1.000

5 10

Track Length (microns)

Ob • . S.D. : 1.060

Pred. S.D. : 1.072

Figure 3 Results of three representative numerica//y modelled datasets using the Monte Trax program developed by Gallagher (/995). The thick black line is the "best fit " thermal history constrained by the AFT data. Shaded regions define the 95% confidence level contour Dashed boxes are time-temperature constrains for the

datasets, these allow the user to input other known geological information into the model iterations, e.g. , dated unconformities or other radiometric data, such as K-Ar, U-Pb and 40 Ar_39 Ar.

DISCUSSION AND CONCLUSIONS

The separation of samples into two groups based on their FT characteristics is supported by qunatitative modelling which

reveals two styles of thermal history. In Group 1. samples from gneissic and granitic bodies generally exhibit rapid cooling through the apatite fission-track partial annealing zone (60-110D C) during the Late Oligocene to Early Miocene (~28-22 Ma). Estimated denudation

428

rates lie between 550-3500 m Ma-1. In contrast, samples from the Group 2. display significantly slower and more complex cooling histories such a the bimodal track length distribution found in granite THI 9578. Cooling for most samples began during the Late CretaceouslEarly Tertiary (70-50 Ma). Modelled cooling rates typically lie

between ~1.3-2 . 5°C Ma- 1 equivalent to

denudation rates of ~43 - 83 m Ma-1. Examination of the spatial distribution

of the two groups reveal that Group 1. samples lie in a di stinct north-south belt whilst Goup 2. samples are marginal to this. The rapid cooling recorded by the Group 1. samples, initiated

during the Late Oligocene to Early Miocene (~2 8 -22 Ma), has probably involved cooling through a substantial section of crust. A zircon FT age (effective closure temperature ~250±50° C) from sample TID 2266 is concordant with the apatite FT age and indicates that 3200°C of cooling occurred at this tinle. Thus the apatite FT data in Group 1. do not necessarily record the total amount of cooling during the Oligo­Miocene, but instead provide a lower limit .

Samples in Group 2. including Till 9564 and THI 9565 from the Fang Province, and THI 9578 from Mae Hong Son Province, share a common Late CretaceouslEarly Tertiary (50-70 Ma) timing for the onset of cooling, suggesting that both regions were inverted by the same mechanism. This probably took the form of a smaJi component of tectonic uplift that started a cycle of erosional denudation driven by isostatic rebound. Initial calculations (Brown, 1991) suggest a ~ 600 ±200 m component of tectonic uplift would be enough to generate the observed levels of denudation. The earlier reheating seen in samples THI 9564 and THI 9565 during the Late Cretaceous may be the result of reburial by Khorat sediments. Isolated outcrops of the Khorat Group are found throughout the Nan and Chiang Rai provinces of No rthwestern Thailand. Timing of inversion and subsequent rates of cooiing in this group compare well with FT data from the Khorat of Eastern Thailand (Lovatt-Smith et aI. , 1996; Upton et aI., 1995, 1996; Upton 1997) where post inversion denudation has removed up to 3.6 km of cover rocks.

Examination of the various published models proposed for the Tertiary tectonic

evolution of northern Thailand (rifting- Pigott and Sattayarak, 1993; rifting associated with gravitational collapse- Dunning et aI. , 1995; dextral shear- Polachan, 1988; Polachan and Sattayarak, 1989; and thrusting- Ahrendt et aI., 1994) suggest that the fi ssion-track data analysed so far , are most consistent with the model advocating unroofmg of the metamorphic core complex theough gravitational collapse (Dwming et aI. , 1995, MacDonald et ai., 1993). Granitoid samples from Group 2. (THI 9564. TID 9565 and THI 9578) are equivalent to the cover sequence and therefore recorded and earlier thermal history. Samples recording rapid, geologically instataneous cooling (Group I. ) correlate with the unroofing of the Doi Inthanon metamorphic complex. An extensional setting, rather than a compressional tectonic regime would also explain the presence of widespread Late T e rt ia r y basalti c lav a er upt io ns documented throughout Thai land. Coeval with the umoofmg of the Doi Inthanon core complex is the emplacement of the Mae Klang granite dated by U-Pb on monazite and zircon at 26.8 ± 0.5 Ma (Dunning et ai., 1995). The pluton post­dates high grade metamorphism (MacDonal et aI. , 1993) and contains foli ations wi th mylonitisation locally, whilst intruding both the orthogn iss and paragneiss of the Doi Inthanon complex .

Timin g o f t e c t on i c upl i ft in Northwestern Thailand has inlplications fo r the supply of sediment to contemporary Tertiary basins in Northern Thailand and the Gulf of Thailand. The intermontaine basin of Northern and Central Thailand show two phases of clastic fill. The early basin infilI is dominated by lacustrine deposits which represent underfi lled basins where sediment supply was insufficient to keep pace with subsidence. The basin fill later switched to fluvial clastic infill due to a relative i ncr e as e i n se d im e n t supp ly . Conventionally, these basin fill switches are explained by changes in the rates of subsidence with a constant sediment supply. (see Fig.4a), where initial rates of subsidence exceeded sediment supply leading to the development of underfilled lacustrine basins. However, it is tentatively suggested that the style of basin fill could be linked more strongly to changes in sediment supply (see Fig.4b). The early phase of extension, where cover material is being

Subsidence controlled

a re~

~ ... "m.".,""" leading to underl1l1ed lacustrine basin.

\ ~~ ~\~0. ~;r

\; ;; Sub,iden"" ,ab! dec",a,.,s • while sediment supply remains ... constant, leading to flu\lial

deposition.

429

b Sediment supply controlled

Sub,idence>sediment supply leading to underl1l1ed lacusb-in< basin.

\ ~ '<-\ .,--,----:~f"71

Y Change in basin fill caused by r increase in sediment supply following tEctonic denudation and isontic uplift:. :

Figure 4 Schematic evolution ofTertairy basin infill in Northwestern Thailand. It is tenatatively suggested that sediment supply rather than subsidence rates may have dominated the style of basin infill.

effectively transported in the horizontal plane, results in minor localised sediment supply from surrounding basin margins, thus creating lacustrine deposits typical of a sediment starved environment. As the cover sequence is removed, the metamorphic complex rebounds and initiates a phase of rapid erosion and enhanced sediment supply, thus switching the basin infill to clastic dominated deposits.

CONCLUSIONS

• Substantial and rapid cooling took place during the Late Oligocene/Early Miocene in what appears to be a discrete north-south belt of gneissic and plutonic rocks.

• Minimwn cooling rates during the unroofing of the north-south belt is estimated to be between 8.5°C Ma- I to 25°C Ma- I

. Assuming a geothermal gradient of 30-35°C km- I

, this equates to the loss of between 2.75 km - 3.5 km of section during the Late Oligocene over a ~3± 2 m.y. time span at a denudation rate of 550-

3500m Ma- 1. • It is most likely that this cooling

represents a phase of tectonic denudation and unroofing of a metamorphic core complex.

• It is tentatively suggested that sediment supply linked to the Middle Tertiary unroofing epiosde controlled the infill of local Tertairy basins.

• Evidence is found in some samples for an earlier phase of inversion which has a timing comparable with that of the Khorat basin in Eastern Thailand.

ACKNOWLEDGEMENTS

The findings documented in this paper fonn part of a regional apatite fission-track study funded by the Natural Environmental Reasearch Council, United Kingdom (GT4/214/95/E), with additional support and fmacial assistance being provided by the Southeast Asia Research Consortium, University of London. Samples (THI 2258-66) kindly provided by Dr. Hans Ahrendt and co-workers, Georg-August Universillit, Gottingen. The authors would like to thank colleagues at the Department of Mineral Resources and the Geological Sciences Department, University of Chiang Mai for their assitance and infonnative discussions . In particalar, special thanks must go to Dr. Thanis Wongwanich and Dr. Pongpor Asnachinda for their hospitality and invaluable advice throughout the study.

REFERENCES

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Ahrendt, H., Hanson, B. and Wemmer, K., 1994, Tertiary nappe and thrust tectonics in Western Thailand deduced from U /Pb, Rb/Sr and K/Ar age detenninations, AAPG International Conference, Kuala Lumpur, Malaysia.

430

Baum, von Braun and Hahn., 1970, On the geology of Northern Thailand, Beiheft Geologisches Jahrbuch, 102p.

Beckinsale, R. D., Suensilpong, S., Nakapadungrat, S. and Walsh, 1. N., 1979, Geochronology and geochemistry of granite magmatism in T:lailand in relation to a plate tectonic model, Journal of the Geological Society, London, 136,529-540.

Brown, R. W., 1991, Backstacking apatite fission-track 'stratigraphy' : a method for resolving the erosional and isostatic rebound components of tectonic uplift histories, Geology, v.19, 74-77.

Bunopas, S., I 994 , Reg ion a 1St rat i g rap h y , Palaeogeographic and Tectonic Events of Thailand and Continental Southeast Asia, Proceedings of the International Symposium on Stratigraphic Correlation of SE Asia. IGCP Project 306, Bangkok, Thailand., 132-158.

Bunopas, S. & Vella, P., 1983, Tectonic and geologic evolution of Thailand, Proceedings of a workshop on stratigraphic correlations of Thailand and Malaysia, Geo!. Soc. Thailand, Bangkok/Geo!. Soc. Malaysia, Kuala Lampur, v.l, 307.

Cobbing, E. 1., Mallick, D. I. 1., Pitfield, P. E. 1. and Teoh, L. H., 1986, The granites of the Southeast Asian tin belt, Journal of the Geological SOCiety, London, 143,537-550.

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40 39

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