Denudation and cooling of the Lake Teletskoye Region in the Altai Mountains (South Siberia) as revealed by apatite fission-track thermochronology

Tectonophysics 349 (2002) 145 – 159 www.elsevier.com/locate/tecto

Denudation and cooling of the Lake Teletskoye Region in the Altai Mountains (South Siberia) as revealed by apatite fission-track thermochronology Johan De Grave*, Peter Van den haute Geological Institute, University of Gent, B-9000 Gent, Belgium Received 17 May 2000; accepted 27 July 2001

Abstract Lake Teletskoye occupies a narrow graben located in the northwestern sector of the Altai fold belt in South Siberia. The lake basin is thought to have formed during the Pleistocene as a distant result of the Cenozoic collision of India and Eurasia that caused a tectonic reactivation of the Palaeozoic Gorny – Altai (GA) and West Sayan (WS) blocks. The present work reports of a pilot fission-track study performed on 13 apatite separates collected from rocks that were sampled along two profiles in close proximity of the lake. The age – length data and AFT thermochronological modelling reveal two important phases of cooling in the Altai Mountains, a first one during the Late Jurassic – Early Cretaceous and a second one that started in the Miocene – Pliocene and that persists until today. The first event is interpreted to result from uplift-induced denudation probably related to the closure of the Mongol – Okhotsk Ocean; the second event can be linked to the young Cenozoic movements that lie at the origin of the formation of the Lake Teletskoye basin. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Apatite fission-track thermochronology; Altai Mountains; Meso/Cenozoic cooling; Lake Teletskoye

1. Introduction The Altai Mountains form part of the intracontinental Central Asian Mountain Belt, which runs over a distance of more than 5000 km, from the Pamir –Tien Shan region northwest of the Himalayan orogenic front to the Stanovoy ranges near the Okhotsk Sea. It sutures the Archean and Early Proterozoic Siberian cratons with assorted North Chinese and other Central Asian terrains (Fig. 1). Lake Teletskoye is located in the northwest of the mountain range, in the Autonomous

*

Corresponding author. Tel.: +32-9-264-45-64; fax: +32-9264-49-84. E-mail address: [email protected] (J. De Grave).

Republic of Gorny – Altai, South Siberia. The lake basin exhibits a narrow graben morphology, being 77 km long by 4 km wide on average. The main body of the lake has an almost north – south orientation (Figs. 1 and 2). Although the general tectonic evolution of the Altai Mountains is well constrained, the timing and intensity of the different phases of vertical movements that affected its separate units are not well known. It is thought that the most important movements in the study area were limited to the Palaeozoic, while the Mesozoic is seen as a period of tectonic quiescence (Dobretsov et al., 1995b). The remnants of a Late Mesozoic erosional peneplain in the region bears witness to this period of crustal stability, which assumingly was only disturbed during the Late Cen-

0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 0 2 ) 0 0 0 5 1 - 3

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ozoic by a tectonic reactivation that caused uplift of the Altai Mountains and the formation of the Teletskoye basin. The timing of the different movements in the study area essentially relies on stratigraphic work, the number of radiometric ages is limited to a set of K – Ar and Ar – Ar ages discussed further (Buslov and Sintubin, 1995; Dehandschutter et al., 1997). In the summer of 1999, we started a comprehensive fission-track study of the Russian Altai region, which involves an analysis of more than 50 apatite samples. This paper reports on the first important results obtained on two elevation profiles in the Teletskoye area.

2. Geological setting In the following paragraphs, a brief overview is presented of the main phases of the tectonic evolution of the study area since the Early Palaeozoic. This is desirable because the significance of the apatite fission-track data cannot be evaluated correctly if the entire structural history of the region is not properly considered. Although the history of the Altai Mountains (and the Central Asian Mountain belt, in general) can be traced back to the Late Precambrian, the main orogenic phases that formed and shaped the area are constrained to the Palaeozoic and Early Mesozoic, a time interval during which major crustal growth took place in the whole of Eurasia. Different phases of crustal accretion and intrusion created a complex assemblage of various crustal units that are separated primarily by strike – slip fault zones (e.g. Delvaux et al., 1995b; Dobretsov et al., 1996; Sßengo¨r et al., 1993; Han et al., 1997). In the Teletskoye region, the Altai Mountains are underlain by Palaeozoic rocks of two such units, the West Sayan (WS)

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and the Gorny– Altai (GA) tectonic blocks (Fig. 1; Buslov and Sintubin, 1995; Dobretsov et al., 1995b; Sintubin et al., 1995). Both blocks are separated by the vast West Sayan (WS) fault zone, which is one of the most important structural elements in the study area and forms one of the main lineaments in the Altai tectonic collage as a whole (Sß engo¨r et al., 1993). The fault zone stretches out over a distance of more than 1000 km and has an average width of 6 to 8 km. It sutures the GA with the WS unit and acted as a major strike –slip zone during the Permo – Triassic, displacing blocks over distances as much as 3000 km. At present, it accommodates active left lateral movements. Running SW –NE in the northeastern part of the study area, the fault zone branches off in a set of N –S trending Permo – Triassic thrusts and oblique strike– slip faults to the west of the lake. These structures form the boundary between the GA and WS in the southern part of the area depicted in Fig. 1. The basement rocks of the GA and WS units reflect the complex accretionary history of the entire Central Asian orogenic system and of the Altai, in particular. The Late Proterozoic and Early Palaeozoic rocks that compose the GA unit were deposited in an island arc and accretionary prism environment (including ophiolites, blueschists, eclogites, metasediments, lavas, tuffs and carbonates). The WS unit evolved independently from the GA in a marginal sea and forearc setting as indicated by the Late Riphean to Ordovician metasediments, predominantly turbidites, which are abundantly exposed to the east of Lake Teletskoye. These rocks were intruded by granitoids in the Early Devonian. The granites of the Altyntauss Massif, exposed along the southwestern margin of the lake, date from this period (Figs. 1 and 2). During the Late Devonian – Early Carboniferous, closure of the central part of the Paleoasian Ocean (Dobretsov et al., 1995a) resulted in a major phase of

Fig. 1. Simplified structural map of Gorny Altai (South Siberia). Lake Teletskoye is located in the border zone of the two main tectonic units in the area, the West Sayan (WS) and the Gorny Altai (GA) blocks. Main basins, structures and intrusive bodies are indicated (after Buslov, personal communication). Symbols: Rf3 = Late Riphean, V – C = Vendian – Cambrian, Pz1 = Early Palaeozoic, Pz1 – 2 = Early to Middle Palaeozoic, Pz2 = Middle Palaeozoic, D = Devonian, and T – J = Triassic – Jurassic. The GA block is shaded, it is separated from the WS unit by the West Sayan fault zone in the north and by Palaeozoic thrustzones in the south. Both blocks are part of the Altai – Mongolian microplate. The study area is situated on a simplified tectonic map of Central Asia. Our study area (shaded box) is located within the Altai mountain range, which is part of the extensive Central Asian fold belt. ISZ = Irtysh Shear Zone, ATF = Altyn Tagh Fault system and MFT = Main Frontal Thrust.

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collision, causing the Altai – Mongolian microplate with its GA and WS terranes, on one side, to come into frontal contact with the Siberian active margin on the other. In the WS unit, this collision gave rise to metamorphism, emplacement of several granites and granite –gneiss domes, and the development of largescale NW – SE trending strike –slip faults and thrusts such as the Teletsk – Bashkaus (TB) and Shapshal faults (Figs. 1 and 2). To the best of our knowledge, the only radiochronometric information in the study area consists of K – Ar and Ar – Ar studies carried out on micas and amphiboles from WS rocks near Lake Teletskoye. These yield ages between 320 and 380 Ma and are interpreted as post-metamorphic (gneiss and schist samples) and post-intrusion cooling ages (granitic samples) directly related to this collision (Buslov and Sintubin, 1995; Dehandschutter et al., 1997). The final closure of the western Paleoasian Ocean due to the collision of the combined Kazakhstan/Tien Shan/Tarim block with the orogenic rim of Siberia occurred in Permo – Triassic times (Berzin et al., 1994; Pecherskii et al., 1994; Allen et al., 1995; Carroll et al., 1995; Chen et al., 1999). Strike– slip movements and thrusting along the WS fault zone during this period were responsible for the contact between the GA and WS units as it appears nowadays. During the Triassic, the Altai region became an extensive uplifted, mountainous area, bordered by the young West Siberian Basin to the west and by a vast Paleopacific gulf, the Mongol – Okhotsk Ocean, to the east. A number of intramontane depressions were formed and filled with volcanics, continental molasse, lacustrine and coal-bearing deposits during the Jurassic and Cretaceous. These depressions mainly occur in the Eastern Altai. They are scarce in the study area (Ermikov, 1994; Dobretsov et al., 1996). In general, the region is thought to have experienced a period of tectonic quiescence that resulted in the development of extensive regional peneplains during the Cretaceous to Early Paleogene (Deev et al., 1995; Delvaux

et al., 1995b; Dobretsov et al., 1996). Recent studies, however, revealed that to the east of the study area, the closure of the Mongol –Okhotsk Ocean during the Jurassic and Early Cretaceous involved the collision of the active margin of Siberia with an amalgamated East Mongolia/North Chinese (Sino – Korean) continent and created an orogeny thought to be similar in style and dimensions to the modern Himalayas (Zorin et al., 1993; Gordienko, 1994; Xu et al., 1997; Zorin, 1999). From the Eocene – Oligocene onwards, so-called neotectonic movements are recorded in the entire Central Asian region. They are related to the India – Eurasia collision and indentation. Due to a northward propagation of stress, away from the Himalayan front, deformation and epeiorogenic movements occurred throughout the Central Asian orogenic belt (e.g. Molnar and Tapponnier, 1977; Tapponnier and Molnar, 1979; Tapponnier et al., 1982; Cobbold and Davy, 1988). The tectonic forces that are currently still active affected the various parts of Central Asia in different ways. They installed a transpressional regime (Cunningham et al., 1996) causing a sinistral reactivation of the WS and TB faults and a dextral reactivation along the Shapshal fault zone in the study area. During the Late Pliocene and Quaternary, tectonic activity intensified in the Teletskoye area, in particular along the WS, TB and Shapshal faults, eventually leading to the opening of the Teletskoye basin (e.g. Lukina, 1996; Delvaux et al., 1995a; Dehandschutter et al., submitted for publication). The major part of the lake is located within the active zone of the TB fault. Structural, morphological and sedimentological evidence constrain the onset of basin formation to the Mid-Pleistocene. Data on the amount of vertical displacement were obtained by Deev et al. (1995). These authors assess the total vertical displacement of the Late Cretaceous –Early Paleogene peneplain to be 600 m in the north (cf. Katayatsk) and as much as 3000 m in the south (cf. Altyntauss) of the Teletskoye region. Sedi-

Fig. 2. Digital elevation model (after Trefois and Dehandschutter, 1997) with a geological and structural sketch map of the Lake Teletskoye basement. The West Sayan unit is located to the east of the Teletsk – Bashkauss and West Sayan fault zones, while the Gorny Altai unit is situated to the west of these fault systems. The vast Shapshal strike – slip fault zone, which runs parallel to the Teletsk – Bashkauss shearzone, is also indicated. The AFT sample locations from the Altyntauss profile (circles) and the Katayatsk profile (squares) are shown. The length distributions of confined tracks observed in the apatite samples are also presented.

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Table 1 Location and description of AFT samples from the Teletskoye region, Gorny Altai, South Siberia AFT samples from Gorny Altai Sample

Latitude

Longitude

Altitude (m)

Locality

Lithology

SH1 SH2 SH3 SH4 SH5 TEL TEL TEL TEL TEL TEL TEL TEL

51j45V13WN 51j44V42WN 51j44V32WN 51j44V58WN 51j45V31WN 51j23V49WN 51j26V10WN 51j27V37WN 51j27V39WN 51j28V19WN 51j28V22WN 51j28V27WN 51j28V39WN

087j55V40WE 087j55V44WE 087j55V44WE 087j55V55WE 087j55V11WE 087j41V31WE 087j41V22WE 087j41V23WE 087j41V23WE 087j41V29WE 087j41V41WE 087j41V52WE 087j42V00WE

1850 1950 2010 2100 1720 2210 2000 1585 1390 1153 937 820 610

Katayatsk River Katayatsk River Katayatsk River Katayatsk River Katayatsk River Korumbu Hill Korumbu Hill Korumbu Hill Korumbu Hill Korumbu Hill Korumbu Hill Bolchoy-Chulu Valley Bolchoy-Chulu Valley

Diorite Diorite Diorite Diorite Granodiorite Granite Granite Granite Granite Granite Granite Granite Granite

101 105 107 108 109 110 111 112

SH samples are from the Katayatsk profile, TEL samples from the Altyntauss profile.

ment infill in Lake Teletskoye reaches thicknesses of up to 800 m (Seleznev et al., 1995; Dehandschutter et al., 1997). Other Cenozoic basins in the region, e.g. the Dzhulukul and Chuya depressions, are covered by thick Cenozoic, mainly Quaternary, sediments. Other geomorphological indications such as recent massive landslides and active fault scarps are also plentiful in the studied region and the Altai Mountains as a whole (Deev et al., 1995; Cunningham et al., 1996; Novikov,

1996). All these observations clearly indicate young (mainly since the Pliocene) active tectonics in the study area.

3. Apatite fission-track ages and lengths Samples for apatite fission-track (AFT) analysis were collected along two profiles in the Lake Tele-

Table 2 AFT data for samples around the Teletskoye Lake (Katayatsk profile and Altyntauss profile) Sample

n

qs (Ns)

qi (Ni)

qs /qi

P(v2)

t( Q)

t(f)

lm (Am)

S.D.

SH1 SH2 SH3 SH4 SH5 TEL TEL TEL TEL TEL TEL TEL TEL

50 50 52 50 51 49 50 50 19 50 50 50 49

2.551 (2718) 4.013 (4148) 1.618 (2961) 6.291 (3304) 1.442 (2746) 3.047 (1732) 2.799 (2676) 4.336 (2444) 4.319 (549) 2.663 (2189) 6.084 (2956) 5.296 (2879) 4.115 (2480)

2.033 3.089 1.293 4.542 1.160 2.199 2.167 2.999 3.901 2.599 5.958 4.978 4.942

1.269 F 0.036 1.324 F 0.031 1.287 F 0.036 1.429 F 0.039 1.296 F 0.037 1.377 F 0.050 1.312 F 0.039 1.507 F 0.047 1.104 F 0.068 1.066 F 0.032 1.027 F 0.027 1.067 F 0.028 0.889 F 0.024

95 39 72 80 81 <1 31 26 88 9 75 97 1

103.0 F 4.5 107.4 F 4.3 104.5 F 4.5 115.9 F 4.9 105.2 F 4.6 111.7 F 5.5 106.5 F 4.7 122.1 F 5.5 89.7 F 6.2 86.6 F 3.9 83.5 F 3.5 86.7 F 3.7 72.3 F 3.1

102.8 F 3.7 107.3 F 3.4 104.3 F 3.7 115.7 F 4.0 105.0 F 3.8 111.5 F 4.6 106.3 F 3.9 122.0 F 4.7 89.6 F 5.8 86.5 F 3.2 83.4 F 2.8 86.6 F 3.0 72.2 F 2.5

12.84 13.29 13.21 12.84 13.07 12.19 12.35 12.47 – 12.29 12.32 12.43 11.22

1.78 1.46 1.32 1.76 1.62 1.90 1.69 1.73 – 1.67 1.79 1.54 1.56

101 105 107 108 109 110 111 112

(2177) (3167) (2350) (2324) (2200) (1332) (2039) (1721) (520) (2130) (2905) (2758) (2899)

Track densities (q) are given as 106 tracks cm 2, n is the number of crystals counted, Ns,i the number of spontaneous/induced tracks, P(v2) chisquare probability in %, t( Q) is the age determined by means of the procedure factor method, t(f) the age determined by means of the zeta method, lm is the mean track length and S.D. is the standard deviation on the mean track length. Calculations are based on a thermal neutron fluence value / = 2.789.1015 cm 2 and an average qd (glass dosimeter track density) value of 5.58.105 tracks cm 2.

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tskoye area (Fig. 2, Table 1). The first sample profile was taken in the granitoids of the Devonian Altyntauss massif along the steep southwestern margin of the lake (eight samples, TEL 101– TEL 112, between an altitude of 600 and 2200 m). The second sample profile (five samples, SH1 – SH5, altitude: 1700 – 2100 m) was taken in a small diorite and granodiorite complex near the Katayatsk river at about 15 km to the northeast of the lake. The first profile is located to the west of the TB fault zone, while the second is situated to the east of the Shapshal fault zone, close to its junction with the WS fault zone. The AFT ages presented in this paper were determined as conventional f ages and also as Q ages involving an absolute determination of the thermal neutron fluence (Wagner and Van den haute, 1992; Van den haute et al., 1998). Information on the analytical procedures and on the procedures of sample preparation is given in Appendix A, which also includes a table that summarizes the analytical data on age calibration. The results of the AFT analysis of the Teletskoye samples are given in Table 2. Apparent AFT ages vary between 122 and 72 Ma, i.e. from Early to Late Cretaceous. Younger ages are found for samples that were taken at lower elevation and except for TEL 107, all ages define a single trend in the age– elevation plot without a break in slope (Fig. 3a). The track-length distributions are rather broad (f1.4 Am < r < 1.9 Am), negatively skewed and show a relatively small amount of tracks with lengths above 14 Am (Fig. 2). Although the ages are similar, the average track lengths in the apatites from the Katayatsk profile are slightly higher than those in the apatites from the Altyntauss profile. They range from 12.2 to 13.3 Am in the Katayatsk samples and from 11.2 to 12.5 Am in the Altyntauss samples. No clear correlation can be observed in the age versus mean track-length plot (Fig. 3b). In the Altyntauss profile, the youngest sample exhibits the shortest average track length, but the age variation between the other samples is not accompanied by a significant variation in average track length.

4. AFT modelling and discussion While the analyses presented here represent only the initial stages of a more detailed study, the results

Fig. 3. Plots of elevation, mean track length and standard deviation vs. AFT age. The age – elevation plot (a) shows a fairly linear trend, without break in slope. No obvious trend is displayed in the age – mean track length plot (b) or the age – length standard deviation plot (c).

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have clear implications for the thermal and tectonic evolution of the area and are worthy of discussion in their own right. It seems clear that our measured FT ages can only reflect a cooling history as we are dealing with crystalline basement that has never been buried by a sedimentary cover after the major Palaeozoic orogenies. In one of the pioneering applications of FT dating of basement apatites including confined track-length analyses, Gleadow and Fitzgerald (1987) re-corded two-stage age –elevation profiles in the Wright Valley (southern Victoria Land, Transantarctic Mountains). These profiles were composed of a lower, steeper part starting around 50 Ma, characterized by narrow tracklength distributions and average track lengths over 14 Am and an upper, more gently sloping part characterized by broader length distributions and average lengths between 12.3 and 13.3 Am. The lower part was interpreted to reflect a phase of rapid (Cenozoic) uplift of the basement rocks, while the samples from the upper part were interpreted to have been residing in the apatite partial annealing zone (PAZ; e.g. Wagner, 1972; Gleadow et al., 1986; Wagner and Van den haute, 1992) for considerable time prior to the uplift movement. The age variation along the elevation profiles in the Telestkoye region and the accompanying track-length distributions with average lengths between 11.2 and 13.3 Am give evidence of a considerable PAZ signature, comparable to the apatite samples from the upper part of the Wright Valley profiles. To get a more precise insight into the cooling history of the Teletskoye region, a model cooling path was constructed for each sample by reverse modelling, using the program AFTSolve version 1.1.3 (Carlson et al., 1999; Donelick et al., 1999; Ketcham et al., 1999, 2000) in which the annealing model of Laslett et al. (1987) was applied. Each thermal history model was constrained to surface temperatures at present time and to higher than apatite PAZ temperatures ( > 120 jC) in the Triassic (230 Ma), i.e. the period when major Palaeozoic plate collision and tectonic exhumation and cooling had halted. Fig. 4

shows the best fits that have been obtained for the individual samples of both the Altyntauss (TEL samples) and Katayatsk (SH samples) profiles. The figure also contains a panel showing the good and statistically acceptable fit envelopes for an individual sample (SH1) as an indication of the uncertainty of the thermal history models. If the relevant parts of the curves (i.e. below 120 jC) are considered, they all exhibit a fairly similar threestage cooling history, starting with a Late Mesozoic cooling event which brought the rocks fairly rapidly into the upper part of the apatite PAZ, followed by a period of relative thermal stability keeping the rocks at upper apatite PAZ temperatures, and ending with a new phase of rapid cooling during the Late Neogene – Pleistogene. The youngest cooling phase can also be inferred from the track-length distributions (Fig. 2), which, because of the deficiency in long tracks, indicate that substantial thermal track shortening must have persisted until relatively recent times. If only the cooling path delimited by the acceptable fit envelope in a single sample such as SH1 would be considered (Fig. 4), one would rather derive a monotonous cooling history at a more or less constant rate from the Early Cretaceous onwards up until present times. The three-phase history is indeed only expressed in the curves delineating the good and best fit. However, if it is accepted that the samples from each profile experienced a common thermotectonic history, the fact that the three-stage cooling history with its intermittent phase of stability is observed in the best (and good) fitting solutions for practically all investigated samples from both profiles, provides sufficient statistical proof for the validity of this interpretation. Especially, the cooling curves of the Katayatsk samples are all quite similar, except for the highest sample (SH4) which crosses the 120 jC isotherm earlier, in accordance with its present elevation, but which paradoxically appears to end at a somewhat higher temperature and, hence, lower in the crust than the other samples at the end of the Cretaceous cooling event.

Fig. 4. Thermal history model for the basement rocks outcropping in the Lake Teletskoye area. The AFTSolve 1.1.3 modelling program (Ketcham et al., 2000) with the Laslett et al. (1987) annealing kinetics was used. Best fits are shown for all separate samples from the Altyntauss profile (TEL samples) and the Katayatsk profile (SH samples). The last panel in the figure shows the uncertainty envelope (good and acceptable fits) for a single sample (SH1) as an indication. See text for discussion of modelling and interpretation.

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As a whole, the Katayatsk profile clearly reveals a first cooling event that brought the rocks from 120 down to about 70 jC in the period 125 – 100 Ma and a second still active event that started around 5– 10 Ma ago. The average cooling rate of the Cretaceous event amounts to ~ 2 jC/Ma and that of the Neo-Pleistogene event up to ~ 4 to 5 jC/Ma (when considering the final and most intensive phase of this event). Between these two major cooling events, the crust appears to be thermally stable at least till about 50 Ma when a gentle increase in cooling rate preceding the Neo-Pleistogene event seems to have occurred. This mild cooling brought the rocks to temperatures of 45 – 50 jC between 10 and 30 Ma ago. The t –T paths for the Altyntauss profile samples exhibit the same general shape, but they are more scattered. The difference in elevation between the highest and the lowermost sample is much larger (1600 m) along this profile and the higher samples tend to cross the 120 jC isotherm earlier (~160 Ma, TEL 101) than the lower ones (~100 Ma, TEL 112). This together with the observed trend in the age – elevation plot (Fig. 3a) suggests that the samples belong to a single vertical rock column. On the other hand, it can be observed (Fig. 4) that for samples that start to cool later, cooling also seems to have halted later (up to 40 Ma), and this cannot be reconciled with such an interpretation. However, a detailed comparison of the individual t–T paths seems hazardous here. It requires precise information on the faults that occur in the area, on the geothermal gradient and on the chemical composition of the apatites, none of which is available yet. In addition, the effect of the statistical variability in our AFT data on the shape of the modelled cooling curves needs to be considered as well. If we confine ourselves to the average trend of the cooling curves, it appears from Fig. 4 that at the end of the Early Cretaceous cooling phase, the Altyntauss samples seem to have halted at temperatures that were slightly higher (75 –80 jC) than those of the Katayatsk area and that they remained at that temperature until the onset of the Pliocene– Pleistogene cooling phase, without any evidence for a gradual increase in cooling rate during the Tertiary (except for the divergent sample TEL 107). This is also reflected in the slightly shorter average track lengths of the Altyntauss samples compared to those of the Katayatsk area with similar ages.

The young Cenozoic event cooled the samples down from this higher temperature to ambient temperatures within a few Ma. Hence, this event must have been stronger here than in the Katayatsk area and attains cooling rates of more than 10 jC/Ma. Taking into account the regional geological context, the Early Cretaceous cooling event registered by the AFT data, can best be described as an upliftinduced denudation which, assuming a normal geothermal gradient of 25– 30 jC, must have amounted to at least 1500– 2000 m in the Katayatsk area and to about 500 m less in the Altyntauss massif. This denudation phase may be related to the closure of the Mongol – Okhotsk Ocean connected to the collision of Siberia with a composite of Mongolian and North Chinese terrains (e.g. Zorin et al., 1993; Delvaux et al., 1995b; Xu et al., 1997; Zorin, 1999). Several major orogenic phases related to this collision (between 180 and 120 Ma) have been recognized in Central Asia. Effects of these orogenies have been mentioned in the literature for western Mongolia and the eastern Altai Mountains (Ermikov, 1994; Buslov and Sintubin, 1995; Dobretsov et al., 1996). Van der Beek et al. (1996) arrived at similar conclusions based on their AFT observations in the Baikal area. Apparently, our AFT data suggest that the range of impact of the Mongol – Okhotsk closure has even reached the northwestern sector of the Altai. Whatever the cause of the uplift may be, it seems clear that the Mesozoic tectonic history of the Teletskoye area was not as tranquil as is classically thought. The period of relative quiescence during the Late Cretaceous –Early Paleogene as indicated by the model can be related to the formation of the extensive erosional peneplain, remnants of which are found throughout the region (Deev et al., 1995; Delvaux et al., 1995b; Dobretsov et al., 1996). The gradual increase in cooling rate that is shown by part of the samples (Katayatsk area) around 50 Ma corresponds in time with the onset of the India/Eurasia collision. Sediments were deposited around that time in newly forming tectonic basins, e.g. in the Chuya an Dzhulukul basins, about 300 and 100 km south of Lake Teletskoye, respectively (Dobretsov et al., 1996). However, it remains problematic why such a megatectonic event is not equally detected in both profiles and, hence, in our opinion, it remains questionable whether this slow cooling phase around 50 Ma is substantial or not. The analysis of

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more samples distributed over the area is undoubtedly required here. The youngest phase of rapid denudation and cooling is mainly confined to the last 5 Ma. As outlined in Section 2, there is abundant terrain evidence for young tectonic activity in the study area, directly related to effects of the India/Eurasia collision (Delvaux et al., 1995a,b; Dobretsov et al., 1995b, 1996). The vertical movements caused the basement rocks in the Teletskoye area to cool down from about 70 jC (TEL samples) and 45 jC (SH samples) to surface temperatures. Considering a normal geothermal gradient, denudation amounted to about 1000 m for the SH and 2500 to 3000 m for the TEL samples. We interpret this faster cooling of the Altyntauss massif by faster denudation and exhumation as the massif forms a direct border of the Quaternary Teletskoye basin, giving rise to a dramatic local relief and, hence, faster erosion. The mean elevation of this part of the study area is more than 1000 m higher than in the Katayatsk area. The TB and Shapshal fault zones which show clear indications of recently intensified reactivation (Lukina, 1996) may well be the sites along which major differential movements occurred. At least seven major earthquakes have been recorded in the area in historical times and in places unconsolidated Quaternary tectonic breccias are found along the TB fault zone. Ongoing FT analysis of key profiles crossing these structures will shed more light on this particular question. Other AFT data in the Central Asian orogenic system reveal an Early Miocene cooling, for example, in Tibet (e.g. Copeland et al., 1987; Arne et al., 1997) and the Tien Shan (e.g. Hendrix et al., 1994; Sobel and Dumitru, 1997; Bullen, 1999). Although some model cur-ves seem to exhibit some accelerated cooling around 20 Ma, a distinct Miocene event is not detected here. We are well aware of the possibility that iterative models based on the apatite annealing kinetics established by Laslett et al. (1987) may lead to an artefactual Late Cenozoic cooling event. Among others, this matter was a hot topic during the 9th International Fission Track Conference at Lorne (February 2000). As stated earlier, our thermal history model is based on these kinetics and it does not escape this possibility. However, in our curves, an event is registered that is greatly confined to the very last 5 Ma and that is in

155

accordance with other geological and geophysical information obtained in the study area including active tectonic indicators. Hence, in our opinion, if the annealing kinetics effectively underestimate track instability at low temperatures, the effect of this underestimation may cause an overestimation of the intensity of the event registered by our model curves, but it did not induce the event as such.

5. Conclusions The AFT analysis carried out on two profiles in the basement rocks of the Lake Teletskoye area has to be regarded as a reconnaissance study for a larger ongoing research project focussing on the low temperature cooling history of the Altai fold belt. The data obtained so far allow detection of a three-stage thermal history for the area. A first phase of rapid cooling occurred during the Early Cretaceous and may already have started during the Late Jurassic. It brought the basement rocks into the upper part of the apatite PAZ. It is followed by a period of relative thermal stability until the Late Cenozoic when a new phase of rapid cooling starts which lasts till present times. The first cooling phase is interpreted to reflect a large scale epeiorogenic uplift of the Altai. The movement is possibly related to the closure of the Mongol – Okhotsk Ocean and the frontal collision of the Mongolian and North Chinese (Sino –Korean) cratons with the Siberian continent. The second phase of rapid cooling reflects the young (Pliocene to Recent) vertical tectonics and denudation that occurred in the area as a result of the frontal India/Eurasia collision in southern Asia, causing the Himalayan orogeny and the uplift of the Tibetan Plateau. In detail, the cooling history of the areas represented by the two investigated profiles appears to be somewhat different. In the Katayatsk area, the Early Cretaceous cooling was relatively more important and cooling appears to gradually accelerate since the Eocene before passing into the final Pliocene– Quaternary event. In the Altyntauss profile which is located along the margin of the Teletskoye lake basin, the young Cenozoic phase is more important and the Tertiary acceleration is not observed.

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Acknowledgements The authors wish to thank the IWT (JDG) and FWO (PVdh) for financial support. Tony De Wispelaere and Prof. Dr. Frans De Corte are gratefully acknowledged for technical assistance with the irradiations and fluence measurements. The help Dr. Raymond Jonckheere provided with sample preparation is greatly appreciated. We are very grateful to Dr. Michael M. Buslov for support during the 1999 and 2000 fieldtrips in Siberia and for sharing his insights on the geology of the Altai region. A word of gratitude in this respect also to Prof. Dr. Jan Klerkx and Boris Dehandschutter from the KMMA in Tervuren, Belgium. Great appreciation also goes to Dr. Damien Delvaux from the same institute who supplied an important part of the investigated samples (TEL) and whose first geological reconnaissance of the area provided us with keydata. Mr. Xu Changai is acknowledged for help with the graphics.

Appendix A The 13 apatite samples were separated using the conventional techniques and dated by the external detector (ED) method (muscovite was used as external detector). Track counting and length measurements were done using an Olympus BH-2 microscope (100  dry objective). Transmitted light was used for counting, both transmitted and reflected light for length measurements, which were carried out by means of a drawing tube attachment and a high resolution digitizing tablet. Where possible, 100 confined tracks were measured. Sample TEL 108 yielded insufficient amounts of confined tracks and was not further considered for tracklength analysis. A batch of five apatite standards (three Durango and two Fish Canyon Tuff, FCT, mounts) was co-irradiated with the samples. Irradiation took place in the best thermalized channel (channel 8, /th//epi = 158) of the Thetis reactor facility at the Institute for Nuclear Sciences (University of Gent). Shards of the glass dosimeter IRMM-540 (De Corte et al., 1998) were embedded with the standards. The conventional f calibration (Hurford and Green, 1982, 1983) factor was determined as 293 F 6 (OWMZ, Hurford and

Green, 1983) for IRMM-540 using the Durango and FCT standards, corresponding to a fCN5 of 334 F 7 (Table 2). Three pairs of metal activation monitors (IRMM-530, 0.1 wt.% Al – Au foil and IRMM-528 a, 1.0 wt.% Al – Co foil) were also included in the irradiation package. An absolute thermal neutron fluence (/) was determined from these monitors applying calibrated c-spectrometry, according to the techniques used by Van den haute et al. (1988), yielding an average value of (2.789 F 0.040) 1015 neutrons cm 2. The average /Co//Au fluence ratio was 1.017 F 0.013 [2r], indicating a good consistency between measurements and guaranteeing a correct / calibration. On the basis of the FT age equation, the absolute / value and the apatite standard ages (31.4 F 0.5 Ma for Durango and 27.8 F 0.7 for FCT; Hurford, 1990) a socalled procedure factor Q = kf/kaetska[(qs/qi)SGIr/ + 1] (see Table 2 for symbols) was calculated (Wagner and Van den haute, 1992). This gave a Q factor of 1.16 F 0.03 for Durango and 1.23 F 0.04 for FCT apatite (Table 2). In analogy with the conventional f factor, an overall weighted mean Q factor (OWMQ) was established as 1.20 F 0.03. The Q factor is a quantification for track registration, etching and observation characteristics and eliminates some of the nuclear parameters incorporated in the f factor. The Q factor depends solely on the track registration characteristics of the investigated mineral and on etching-observation conditions, it allows a more direct assessment of systematic effects related to techniques of track analysis. For more information concerning this factor, the reader is referred to Wagner and Van den haute (1992). An important observation is that the OWMQ appears to deviate from unity by 20%, indicating that track revelation and counting efficiency is substantially higher in the mica detector than in apatite. This confirms earlier observations made by Iwano et al. (1993), Jonckheere et al. (1993) and Van den haute et al. (1998). Substitution of the OWMQ in the absolute FT age equation yields an absolute age in the same way for the unknown samples as is done in the f method. Next to the recommended and generally accepted manner of reporting (Hurford, 1990), the ages in this paper are also given based on the Q approach. Further investigation of this approach is undoubtfully interesting, but beyond the scope of this paper.

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157

Table A Results for the age calibration (f and Q method) based on analysis of apatite age standards (Durango and FCT). Spontaneous, induced and glass dosimeter (IRMM-540) track densities qs, qi and qd are given in 105 tracks cm 2, qb represents the background tracks in the Jahre ED mica (in tracks cm 2), n the number of counted grains and N the number of counted tracks. SWMZ/Q is the standard weighted mean zeta/Q factor and OWMZ/Q the overall weighted mean zeta/Q factor. The Q factor was calculated on the basis of Q = kf /kaetska[(qs/qi)SGIr/ + 1] with the 238U decay constant for spontaneous fission kf = 8.46  10 17a 1, ka the 238U decay constant for a decay (ka = 1.55125  10 10 a 1; Steiger and Ja¨ger, 1977), ts the apatite standard age, (qs/qi)S the spontaneous/induced track density ratio in the standard apatite grains, G the ratio of the geometry factors (0.5 for the ED method), I the natural 235U/238U ratio (7.2527  10 3; Steiger and Ja¨ger, 1977), r the cross-section of 235U for thermalneutron-induced fission (570.8 barn [1 barn = 10 24 cm 2] for the Thetis reactor; Van den haute et al., 1988; De Corte et al., 1991; Wagner and Van den haute, 1992) and / = 2.789  1015 cm 2. Differences in qd values are the result of the axial gradient in thermal neutron fluence; for further calculations, an average value of 5.58  105 tracks cm 2 was used. Sample

t (Ma)

ns

qs (Ns)

ni

qi (Ni)

qs/ qi

DUR5

31.4 F 0.5

100

40

31.4 F 0.5

100

DUR7

31.4 F 0.5

100

5.094 (1324) 5.324 (1419) 5.077 (1321)

0.401

DUR6

2.044 (1308) 2.102 (1345) 1.998 (1279)

SWMZ/Q FCT1

27.9 F 0.7

100

40

FCT2

27.9 F 0.7

80

2.300 (1472) 2.250 (1152)

6.791 (1761) 6.925 (2242)

40 40

50

SWMZ/Q OWMZ/Q

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