THE ROLE OF NEOTECTONICS IN PERMAFROST ORIGIN AND FEATURES OF THE BAIKAL-AMUR MAINLINE REGION, RUSSIA

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1 THE ROLE OF NEOTECTONICS IN PERMAFROST ORIGIN AND FEATURES OF THE BAIKAL-AMUR MAINLINE REGION, RUSSIA S.M. Fotiev, M.O. Leibman Earth Cryosphere Institute SB RAS, Vavilov str.,30/6-74a, Moscow , Russia Abstract The Baikal-Amur Mainline (BAM) of Siberia was characterized by differential tectonic movements during the Neogene and Holocene (Nikolaev and Naimark, 1979). This mountainous region can be subdivided into: the Riftogen zone (with alpine relief and narrow intermontane depressions); Platform zone (plateaus and lowlands) and Orogen zone (combining features of the first two zones). Different levels of heat exchange between the atmosphere and the earth surface in each zone, due to the interaction of relief and climate, lead to variability in permafrost conditions. Two extremes are: alpine permafrost of the Riftogen type with altitudinal zonality prevailing, and plateau permafrost of the Platform type with the greatest difference between the "cold" valleys and "warm" interfluve areas. Patterns of permafrost distribution are generalized in a classification of permafrost features and in a geocryological regionalization map of the BAM. Various permafrost species were characterized by both generalization of borehole data and modeling of ground temperature. Introduction The Baikal-Amur Mainline (BAM) region covers the territory from the Lena River in the west to the Tatar Strait in the east, being km wide and about 2,500,000 km 2 in area (Figure 1A). Geocryological conditions of this region are widely discussed in the literature (Nekrasov and Klimovsky, 1978; Fotiev et al., 1978; Romanovskii, 1981; Kozlovsky, 1988; Ershov, 1989). Ground temperature, permafrost thickness, continuity, and structure are known to vary in a wide range. In some areas, permafrost occupies almost 100% of the territory, while in the other ones only isolated islands of permafrost exist. Such a drastic difference in permafrost distribution south of 60û latitude, can be explained only by specific conditions of heat exchange for various landforms. Neogene-Quaternary differential tectonic movements formed three main Earth surface forms: Riftogen, Orogens and Platforms. These forms differ greatly from each other in topographic, morphologic, geologic, and climatic conditions: conditions that affect heat exchange, and permafrost characteristics. Based on relations between tectonic movements and permafrost characteristics, a regional classification of permafrost species is suggested, and geocryological characterization of the BAM region is reported. Principles and conceptual sketch of geocryological regionalization Many publications deal with the principles of regionalization (e.g., Popov, 1958; Baranov, 1965; Shpolyanskaya, 1966; Fotiev, 1978). Relationships between the permafrost features and various natural parameters are used in these publications to classify permafrost. In the present paper, the geocryological regionalization of the BAM region is based on a three-step classification (Table 1) for heat exchange between the surface and atmosphere. The highest taxons are Types of Permafrost, differentiated by general heat exchange patterns determined by the amplitude and diversity of neotectonic movements. The next step is subdividing the taxons into permafrost classes by heat exchange specific features connected with the amplitude and degree of relief dissection. Finally, species are ranked by permafrost parameters. Their geocryological characteristics are shown in Table 2. Based on the suggested classification a new conceptual sketch of geocryological regionalization is considered. On the map (Figure 1B), various types, classes and species of permafrost are shown. Analysis of the map is helpful in establishing the common features of large areas of the BAM region and the irregularity in geocryological conditions of certain parts. S.M. Fotiev, M.O. Leibman 283

2 Figure 1. (A) Study area and (B) Map of geocryological regionalization, BAM region: 1-4, permafrost classes; 1 - Lowland, Depression (l, d), 2 - Plateau (p); 3 - Highland (h); 4 - Alpine (a); 5-9, permafrost boundaries; 5 - of the Platform and Orogen types, 6 - of the Platform and Riftogen types, 7 - of Orogen and Riftogen types; 8 - of permafrost classes; 9 - of permafrost species; 10 - codes for permafrost species (according to Tables 1 and 2). Platforms permafrost type The Amur-Zeya and Siberian platforms are located in the BAM region. Low differentiation of neotectonic movements, low relief amplitude and altitude, vast plateau-like hilltops and wide U-shaped valleys in sedimentary rocks determine the heat exchange conditions: of the Platform permafrost type: mainly Upper Holocene age; discontinuous and sporadic extent; warm (up to -3 C) ground temperature; thin permafrost (up to 150 m); and sections containing mainly icebonded frozen ground. Differentiation of permafrost characteristics occurs in the lateral direction. The Platform type is subdivided into Lowland and Plateau permafrost classes (Table 1). (1) the Earth surface interacts with a homogeneous atmospheric layer of limited thickness; (2) the amount of solar radiation arriving at the surface is rather uniform across vast areas, and differs mainly in lateral direction; (3) altitudinal zonality is expressed in the form of differentiation of heat-exchange conditions between valley bottoms and interfluve areas (slopes and tops); (4) heat exchange conditions varied dramatically during the Quaternary when periods of warm discontinuous permafrost gave way to periods of entire permafrost degradation. Heat exchange conditions determine specific features Lowland Permafrost Class (Amur-Zeya and Zeya- Bureya lowlands, Figure 1A) is found at the far southern part of the region. Valley bottoms and low flat-top hills represent landforms with altitudes of 125 to 500 m a.s.l. They are composed of sandy and clayey deposits. Latitudinal zonality is clearly defined, and there is no differentiation between the valley bottoms and interfluve areas. Islands of warm permafrost (up to -2 C, and up to 100 m thick) are found in both valleys and on divides. Open taliks develop on well-drained and exposed locations. Permafrost features are more severe at the shaded boggy sites. Plateau Permafrost Class is characteristic of territories with highly dissected surfaces in plateaus, tablelands and low mountains (up to 1000 m a.s.l.), where broad divides occupy the bulk of the territory (Angara-Lena and Olekma-Chara tablelands, Figure 1A). The main 284 The 7th International Permafrost Conference

3 Table 1. Classification of permafrost for the BAM region regularity of heat exchange here is its distinct differentiation between the narrow valleys and broad flat interfluve areas. This differentiation results from both orographic inversion of air temperature, and more shaded valleys compared with divides that receive high insolation. The diversity of permafrost characteristics is determined by topographic position and the permeability of rocks and soils. The warmest temperatures characterise wide flat watershed areas built of permeable rocks and soils through which rain water percolates and thus prevents rocks from freezing. Permafrost, if formed on divides, has ground temperatures of 0 to -2 C and thicknesses up to 100 m, while in the valleys permafrost exhibits ground temperatures down to -3 C and thicknesses up to 300 m. Open infiltration and insolation taliks are found on hilltops and southern slopes, and only locally in river channels along tectonic faults. Riftogen permafrost type Riftogen permafrost is the most severe in the BAM region (Stanovoe Highland, Figure 1A). Active neotectonic uplifts have formed high alpine mountains occupying about 70% of the territory. Narrow graben-like depressions have flat bottoms. Pronounced relief, with amplitudes of up to 3000 m high, narrow ridges constructed mainly of igneous and metamorphic rocks, and narrow depressions, determine specific features of contemporary heat-exchange conditions and their fluctuations during the Quaternary: (1) the Earth surface interacts with much of the lower atmosphere, which has different properties changing vertically depending on elevation; (2) the amount of solar radiation arriving at the surface is irregular and differs in the vertical direction; (3) variations in the heat-exchange conditions during the Quaternary changed the severity of permafrost, but did not lead to thaw during the warm periods. Heat exchange conditions determine specific features of the Riftogen permafrost type: mainly Pleistocene age; continuous extent; cold (-3 to -12 C) ground temperatures; thick (up to 1200 m) permafrost; mainly nonice-bonded cryotic rocks. Differentiation of permafrost characteristics is found in the vertical direction. Open taliks are very rare and are linked to tectonic faults. The area of taliks is negligibly small compared with the permafrost area. The Riftogen type is subdivided into Alpine and Depression permafrost classes (Table 1). Depression Permafrost Class is found within the Baikal-type depressions filled in by sediments of S.M. Fotiev, M.O. Leibman 285

4 Table 2. Permafrost species features various origins. The permafrost history was different in each depression and even among parts of one depression. Thick Pleistocene permafrost at some sites degraded entirely or in part, whereas at other sites it did not thaw. Freezing during the Holocene varied as well, leading to essentially diverse geocryological conditions in each of the Baikal-type depressions. The most severe geocryological conditions are characteristic of Upper- 286 The 7th International Permafrost Conference

5 Charskaya, Myjsko-Kuandinskaya, Tsipikanskaya and Bambujskaya depressions of Stanovoe Highland (Figure 1A). Continuous permafrost, ground temperatures as low as -5.5 C and thicknesses up to 500 m are characteristic of these depressions. Upper-Angarskaya and Barguzinskaya depressions are characterized by discontinuous and sporadic permafrost with warmer ground temperature (up to -2 C) and thicknesses up to 100 m. Relic Pleistocene permafrost was found at some depth in Baikal-type depressions. Alpine Permafrost Class is characteristic of the mountain ranges, with high and deeply dissected relief and slopes occupying most of the territory. The main pattern of heat exchange here is its altitudinal zonality. Ground temperature decreases and permafrost thickness increases upward, reaching extremes (-12 C and 1200 m, respectively) at the highest peaks. Continuous in space and time, permafrost of the alpine class can attain severe conditions, depending on the degree of dissection (the higher the dissection, the more shaded from insolation and cooled in winter the surface is). Orogen permafrost type Orogen permafrost is diverse in the BAM region. Active neotectonic movements have led to the formation of high alpine mountains along with low mountains, plateaus, highlands and lowlands. A diversity of landforms (watershed divides are expressed as narrow ridges along with wide flat tablelands, and lowlands vary from trough-like depressions to vast aggraded plains), and types of geological formations (igneous, metamorphic rocks, and terrigenous, calcareous deposits) determine the specific heat exchange in the Orogen zone: (1) Platform-type heat-exchange conditions are common for the aggraded plains, plateaus, tablelands and low mountains (up to about 1000 m high) and result in formation of fairly "warm" discontinuous permafrost species of the Plateau and Lowland classes; (2) Riftogen-type heat exchange conditions are found in the middle and high mountains and form fairly cold, thick continuous permafrost of the Highland and Alpine classes. The largest part of the BAM region is occupied by Orogen-type permafrost. It is characterized by a wide range of ground temperatures, thicknesses and extent. Four classes and 24 permafrost species are found here (Table 1). Lowland Permafrost Class is found within intermontane depressions, and the coldness of associated geocryological conditions increases northward. Ice-bonded frozen deposits are found in all landforms, mainly on shaded, poorly drained, peat-covered landscapes. Plateau Permafrost Class is characteristic of territories with highly dissected relief in plateaus, tablelands and low mountains, and mainly depends on the elevation. When the elevation is up to m a.s.l., permafrost is coldest in the valleys, while flat divides are mostly unfrozen. Beneath the higher (more than m a.s.l.) plateaus and tablelands, continuous permafrost is found, and open taliks do not exist under contemporary climatic and geocryological conditions. Highland Permafrost Class is characteristic of territories with a combination of mountain ranges of deeply dissected surfaces, plateaus, and tablelands with narrow valleys. There is a transition of different properties of heat exchange from one landform to another. On plateaus and tablelands, valleys are characterized by colder geocryological conditions compared with watershed areas. Permafrost is represented by frozen icebonded deposits of discontinuous extent with taliks on flat watershed areas. In the mountain ridges, altitudinal zonality is common, and permafrost temperatures decrease up slope. Permafrost is represented by both ice-bonded deposits and non-ice-bonded cryotic rocks, and is of continuous extent. Alpine Permafrost Class is the most common of the Orogen type. Mountain ranges are 500 to 2000 m high with essentially irregular dissection of the surface. In high mountains, altitudinal zonality is the main variable in heat exchange conditions. At the same time, due to vast territories occupied by the Alpine permafrost class, it can be seen that permafrost characteristics also depend on the latitude and distance from the Pacific Ocean. Latitudinal zonality appears in the position of lateral permafrost zones: continuous extent in the northern and northwestern parts of the Orogen zone; discontinuous in the central part; and sporadic in the southern and eastern parts. Alpine permafrost is represented by ice-bonded deposits in the valley bottoms, and mainly by non-ice-bonded cryotic rocks in the mountain ranges. More severe permafrost occurs where the ridges are higher and more dissected. Permafrost regionalization map of the BAM region Field data from the region is highly variable. Many boreholes with ground temperature and permafrost thickness information are concentrated at sites of industrial interest, like ore and coal fields, and railway construction. Vast areas, especially in the high mountains, are very poorly studied. It was necessary to devise a procedure to describe permafrost in the region. Classification and regionalization were helpful in inter- S.M. Fotiev, M.O. Leibman 287

6 polating and extrapolating borehole data. They were also used to devise a model for calculating ground temperature for sites where permafrost species were never cored, nor their parameters measured. The ground temperature model was based on crossregioning (climatic and geomorphic) of the territory. Analyses of climatic parameters and patterns of their distribution (air temperature and precipitation characteristics, types of latitudinal and altitudinal zonality, differentiation between valleys and watershed areas) were used to prescribe boundary conditions for ground temperature, permafrost thickness and active-layer depth calculations. Generalized parameters of the surface and sub-surface components were prescribed according to publications and field data obtained by the authors. Results of calculations were compared with field measurements and rather high correlation was found. Permafrost thickness was calculated from ground temperature and generalized data on thermal flux, and evaluated by published field data (Leibman, 1983). A regionalization map of the the BAM region (Figure 1b) was compiled according to the patterns recognized, calculations performed, and classification presented in Tables 1 and 2. These tables are part of the map legend. The information content of the map is high. It is possible to interpret: the range of permafrost characteristics for an area, their variations in valleys compared with the divides; and the main patterns of permafrost distribution in relation to various landforms. Summary 1. A three-step classification of permafrost types, classes, and species was carried out on the basis of the types of heat exchange between the surface and the atmosphere, which in turn depend on the character, direction, and intensity of neotectonic movements, and on earth-surface forms resulting from these movements. 2. A regionalization map of the BAM region was compiled using the suggested classification and patterns of permafrost distribution as established by the authors. Altitudinal and latitudinal changes of permafrost characteristics, their variations in the valleys and interfluve areas, and the wide range of absolute values of these characteristics determine the information content of the map mosaic. 3. The regionalization map of the BAM region is useful in (a) collecting and generalizing borehole data; (b) calculation of permafrost parameters by specifying boundary conditions for subdivided areas; (c) setting real ranges of permafrost parameters for each permafrost species. Acknowledgments Authors appreciate valuable suggestions of Dr. J. Brown and anonymous referee that helped to improve the paper. References Baranov, I.Ya. (1965). Principles of geocryological regionalization of permafrost area. Nauka, Moscow (65 pp.). Ershov, E.D. (ed.) (1989). Geocryology of USSR. East Siberia and Far East. Nedra, Moscow (516 pp.). Fotiev, S.M. (1978). Hydrogeothermal peculiarities of the permafrost zone of the USSR. Nauka, Moscow (236 pp.). Fotiev, S.M. (ed.) (1988). The map of geocryological regionalization of the Baikal-Amur Mainline, 1:3,000,000. Ministry of Geology USSR (1p.). Fotiev, S.M., Koreisha, M.M., Leibman, M.O., Koldysheva, R.Ya., Lakhtina, O.V., Sheveleva, N.S. and Znamensky, E.N. (1978). Geocryological map of the BAM territory. In Methods of engineering-geological studies and mapping of permafrost areas. Permafrost Institute SB RAS, Yakutsk, pp Kozlovsky, E.A. (ed.) (1988). Geology of the BAM zone. Hydrogeology and engineering geology. Ministry of Geology USSR, Moscow, Vol. 2 (448 pp.). Leibman, M.O. (1983). Specific procedure to prescribe climatic parameters for ground temperature modeling in the mountainous areas. In The problems of regional and engineering geocryology. PNIIIS Transactions, Stroiizdat, Moscow, pp Nekrasov, I.A. (1976). Cryolithozone of the North-Eastern and Southern Siberia and Patterns of its development. Yakutsk (248 pp.). Nekrasov, I.A. and Klimovsky, I.V. (1978). Permafrost of the BAM zone. Nauka, Novosibirsk (120 pp.). Nikolaev, N.I. and Naimark, N.O. (1979). Neotectonic map of the USSR and adjacent areas, 1:5,000,000. Moscow State University (12 pp.). Popov, A.I. (1958). Permafrost-geological regionalization of permafrost areas of USSR. In Informational collected articles on the study during the International Geophysical Year, Faculty of Geography MSU. Moscow University, Moscow, 1, pp The 7th International Permafrost Conference

7 Romanovskii, N.N. (ed.) (1981). Natural conditions of the BAM development zone. Moscow University, Moscow (169 pp.). Shpolyanskaya, N.A. (1966). On geothermal regionalization of Transbaikal region. In Materials to the VIII All-Union inter-institutional permafrost conference. Yakutsk, issue 3, pp S.M. Fotiev, M.O. Leibman 289