Kurai Basin Research Articles

Large paleoearthquakes and Holocene faulting in the Southeastern Gorny Altai: implications for ongoing crustal shortening in Central Asia

ABSTRACT Shrinking of intermontane basins and expansion of their flanking ranges by reverse faulting and backthrusting in two counter-dipping systems is a typical mechanism of crustal shortening and mountain building in Central Asia. This mechanism is realized along the Kurai Fault Zone (southeastern Gorny Altai). Motions on two reverse fault systems maintained thrusting of the Kurai Range and the Kubadru Uplift on the Kurai Basin sediments and caused the growth of a foreberg before the mountain front. Forberg separates narrow Aktash Basin from the Kurai Basin. The paleoearthquakes were generated by reverse faults that delineate the foreberg. Analysis of the QuickBird satellite images, drone imagery, trenching, archaeoseismological research, radiocarbon dating, dendrochronology, and previous results show that eleven large (М W = 6.5–7.6) paleoearthquakes left traces as surface ruptures along the Kurai Fault Zone: twice before 7.5 ka BP, three events between 7.5 and 5.9 ka BP (7.0, 6.3, and 5.9–5.8 ka BP), one from 5.8 to 4.6 ka BP, four more at 4.6, 3.2, 1.5, and 1.3–1.2 ka BP, and the ultimate earthquake no older than 1450–1650 AD. The time difference between large earthquakes was from 200 to 1700 years. Surface faulting occurred mainly along the northern border of the foreberg where fault scarps are progressively younger northward and the Cenozoic sediments of the Aktash Basin thus become involved into uplift. GPR data to a depth of 12 m confirm the complex structure and slip geometry of the observed surface ruptures. The fault scarps are located < 1 km from the planned route of the gas pipeline from Russia to China, and the potential seismic hazard has to be taken into account in its design and construction.

The age and size of the last glacial-underground lake of the Chui basin (Gorny Altai)

Chronology and size of the last glacier-dammed lake in the Chuya Basin is still actively debated, because it is inextricably linked with the problem of the age and scale of the last glacial megaflood, which passed through the Chuya, Katun, and Ob valleys. For the first time, direct dating of lacustrine sediments of the last glacial-dammed lake of the Chuya Basin reaching up to 1980 m has been carried out. Obtained ages corresponds to the last glacial maximum (MIS-2) and confirms the glaciation of this age in the area within the valet connecting Chuya and Kurai basins.

Irrigation Systems of the Altai: Results and Prospects of Archaeological Studies

We present the fi ndings of studies concerning the irrigation systems of the Altai and outline the directions of their further exploration. Irrigation canals, widely distributed in alpine valleys and intermontane depressions, are streams of the drift type. Most are found in central Altai and in the Chulyshman River valley of eastern Altai. Complex irrigation systems were recorded in the Bilgebash and Sarduma river mouths in the Chuya valley, in the Chulcha River mouth in the Chulyshman valley, and in Tötö, the Kurai basin. Pilot excavations of the main canals showed that wooden troughs had been placed on their bottoms. Radiocarbon analysis of wood from those troughs (Cheba and Oroktoi) suggests that they date to the Late Middle Ages, and a soil sample from the bottom of the canal of the Tenga irrigation system indicates early medieval age. In the 1800s and early 1900s, canals were used by the natives mainly for watering small plots of barley, but also of wheat and rye. Agriculture has been practiced in the Altai at least since the Early Iron Age, having fl ourished, apparently, during the Early Middle Ages. The fi rst irrigation systems must have appeared together with the fi rst farmers; however, taking into account the prolonged use and modifi cations of the main canals, assessing the time of their initial construction is diffi cult.

NEW RESULTS OF MULTIDISCIPLINARY STUDIES OF MAJOR PLEISTOCENE CLIMATIC EVENTS: GLACIATIONS, FORMATION OF ICE-DAMMED LAKES AND THEIR CATACLYSMIC DRAINING IN ALTAI (MOUNTAINS OF SOUTHERN SIBERIA)

Application of geological geomorphological and geochronological (14C, OSL) methods allowed to establish that the bordering mountains of the Chuya basin (Russian Altai) were affected by glaciation already in the Middle Pleistocene – about 160–180 ka ago (MIS 6). However, the maximal ice-dammed lake with the highest (up to 2100 m a.s.l.) strandlines both in the Chuya and Kurai basins existed later than 90–80 ka ago, in the Late Pleistocene. This chronological benchmark limits the influence of the cataclysmic outburst floods from the largest ice-dammed lakes on the accumulation of the Inya formation in the Chuya and Katun river valleys. It is confirmed that a large ice-dammed lake existed and had a water level of at least 1730 m a.s.l. in MIS 2 in the Kurai basin. It was drained no later than 16 ka ago. Synchronously (about 17–16 ka ago) the accumulation of diluvial-lacustrine cyclites began in the Inya mouth. The Saldzhar formation had been depositing as a result of not only flood events, but also gradual hydrological processes in the Katun river valley between the mouths of the Chuya and Sema rivers during the entire MIS 2. It is necessary to revise the concept adopted by SibRISC in 2018. This concept assumes i) absence of ice-dammed lakes in intermountain basins of the Russian Altai in MIS 2 and ii) accumulation of the Inya and Saldzhar formations deposited by glacial megafloods in the valleys of the Chuya and Katun rivers before 90 ka ago.

Recent Data of Multidisciplinary Studies of the Major Pleistocene Climatic Events: Glaciations, Formation of Ice-Dammed Lakes, and Their Catastrophic Drainage in Altai (Mountains of Southern Siberia)

The use of geological–geomorphological and geochronological (14C, OSL) methods allowed establishing that the bordering mountains of the Chuya basin (Russian Altai) were affected by glaciation as early as the Middle Pleistocene, about 160–180 ka (MIS 6). However, the ice-dammed lake with the highest (up to 2100 m a.s.l.) strandlines in the Chuya and Kurai basins existed later than 90–80 ka, i.e., in the Late Pleistocene. This chronological benchmark limits the influence of the catastrophic outburst floods of the largest lakes on the accumulation of the Inya formation in the Chuya and Katun river valleys. It is confirmed that a large ice-dammed lake existed and had a water level of at least 1730 m a.s.l. in MIS 2 in the Kurai basin. The lake drained no later than 16 ka. Synchronously (about 17–16 ka) the accumulation of diluvial–lacustrine cyclites began in the Inya mouth. It was found that the accumulation of the Saldzhar formation in the Katun River valley between the mouths of the Chuya and Sema rivers was long-lasting during the entire MIS 2 not only as a result of flood events, but also due to gradual hydrological processes. It is necessary to revise the concept adopted by SibRISC in 2018. This concept assumes (i) the absence of ice-dammed lakes in intermont basins of Russian Altai in MIS 2 and (ii) the accumulation of the Inya and Saldzhar catafluvial formations, as well as the high and medium terraces they deposited in the valleys of the Chuya and Katun rivers before 90 ka.

Dating of megaflood deposits in the Russian Altai using rock surface luminescence

Catastrophic drainage of ice-dammed lakes in the Altai Mountains has been inferred from geomorphological evidence in the Katun Valley (Russia), and is presumed to have occurred during the Pleistocene. The sedimentary features have been difficult to date directly, due to the absence of organic carbon, and the improbability that luminescence signals in sand grains would be reset during transport. However, the development of rock-surface luminescence dating provides a new opportunity to date the features: clasts have a different transport history to sand grains, and their luminescence depth profiles can be inspected for evidence of bleaching before burial. Here we investigate two sites in the Altai Mountains, and use rock-surface luminescence burial dating to constrain the age of the megaflood deposits. In the Katun Valley, we sampled granite cobbles from a frozen sediment clast emplaced as a dropstone within a massive megaflood gravel terrace. Burial ages for the clasts range from 16.7 to 21.4 ka, with a mean age of 19.8 ± 1.5 ka. This represents the depositional age of the fluvial sediments that preceded the lake outburst flood, (and hence places a maximum age on the catastrophic flood). Clasts sampled from mega-ripples in the Kurai Basin are shown to have a mid-to-late Holocene burial age, which is not consistent with the possible origin of these features during a catastrophic drainage of a glacier-dammed lake. Instead, the burial age of the Kurai Basin sediments may reflect local-scale periglacial or seismic processes along the Kurai Fault Zone.

OSL Dating of the Sukor Earthquake-induced Rockslide in Gorny Altai: Paleoseismological and Paleogeographic Implications

Abstract —Lacustrine sediments, related subaerial aeolian sands, as well as fluvioglacial sands enclosed in ablation moraines in the Chuya River valley between the Chuya and Kurai basins in Gorny Altai, have been dated by optically stimulated (OSL) and infrared stimulated (IRSL) luminescence. The OSL and IRSL ages indicate that the formation of the lake, which spread into the Chuya Basin, was caused by the Sukor rockslide that dammed the Chuya Valley. The rockslide was triggered by a large earthquake (shaking intensity at least IX-X) generated by an active fault in the Kurai Fault Zone about 16 ka, which is the oldest known date for its activity. Thus, large landslides or rockslides can remain geomorphically expressed for at least 16 ka in the climate and landscape conditions as in the Chuya and Kurai basins. The lake existed till ~10 ka BP and then was gradually drained. The Kuyuktanar glacier reached its greatest extent in the early marine isotope stage (MIS) 2, and its ablation moraines formed about 25 ka. Judging by the age and good preservation of moraines left by the glacier, the hilly terrain in the area of the Sukor rockslide, and the presence of lacustrine sands in the vicinity of the lake dam, no catastrophic flooding events in the Chuya and Katun valleys have occurred since the Last Glacial Maximum (LGM; ~20 kyr BP). The water table of the glacier-dammed lake that occupied the Kurai Basin around the LGM could not have exceeded 1750 m above sea level (a.s.l.). Higher lake terraces traceable till elevations of 2100–2200 m a.s.l. should be attributed to more ancient lake stages. Archaeological sites in the area of the junction between the Chuya and Kurai basins were formed in the post-lake period, the oldest dating back to the latest Late Paleolithic time.

Numerical modeling of fault structures in the Kurai basin of Gorny Altai based on data from direct current methods

The southern site is located in the junction zone of the Southwestern and Eshtykel bench, where vertical electrical soundings were performed, and a preliminary fault-block depth model was built based on the results of field data interpretation using a horizontally layered model. Comparison of geoelectric and seismological data showed that the epicenters of significant earthquakes (M> 4) are concentrated in the identified faults. In the central site, three profiles of electrotomography were made through a bench, well expressed in the relief. Three-dimensional modeling was used to verify and clarify the structural features of both sections. Modeling is performed using programs EMF_DC3Dmod (GPU) - an accelerated version of the program EMF_DC3Dmod for vertical electrical sounding and SCALA-48 (GPU) for the method of electrotomography. The study is relevant for the tasks of geodynamics, seismic zoning and seismic hazard assessment.

Surface-rupturing paleoearthquakes in the Kurai Fault Zone (Gorny Altai, Russia): Trenching and geophysical evidence

The Kurai Fault Zone (KFZ) is one of the most hazardous seismogenic structures in the Gorny Altai (Russia). The KFZ have generated five paleoearthquakes with Mw = 6.6–7.6 and ESI 2007 shaking intensities of VIII to XI. The ultimate event, presumably in the second half of the 18th century, was preceded by four paleoearthquakes approximately 1.3, 3.2, 5.8, and 6.3 ka ago. The morphology of fault scarps, trenching results, deformation features in Neogene sediments, as well as electric resistivity surveys, reveal reverse slip mechanisms. All earthquakes were sourced by south-dipping backthrusts with respect to the main north-dipping reverse and thrust faults along which the Kurai Range is stepping over the two basins. The backthrusting maintains the growth of forebergs in front of the Kurai Range. Narrow basins limited by reverse faults separate forebergs from the Chuya and Kurai basins. Joint interpretation of geological (including trenching) and geophysical (ERT and GPR) data reveals the resistivity structure of the area with seismogenic faults traceable to depths of 40–50 m. The KFZ model, with the reconstructed relationships between the intramontane basins and the flanking mountains, as well as the structural control of large earthquakes, makes good reference for other worldwide regions of current and past seismicity in compressional settings. Assessment of probable magnitudes and recurrence of earthquakes within the KFZ is of special importance in view of the future deployment of the gas pipeline from Russia to China.

New chronological (14C, OSL), paleontological and geochemical data on developing of ice-dammed lakes in Kurai basin (south-eastern part of the Russian Altai) during the MIS-2

One of the highly debatable issue of the Altai Pleistocene paleogeography is the chronology and extension of the last ice-dammed lakes. For the first time paleontological, mineralogical characteristics, and optically stimulated luminescence (OSL) ages for lacustrine deposits confirm accumulation of ice-dammed lake with depth at least 170 m in Kurai basin at the end of the Late Pleistocene - 16-19 ka. New data provide information about the ecology and evolution of this Sartan reservoir. Draining of this lake less affected the topography in comparison with earlier cataclysmic glacier-lakes outburst floods. Nevertheless, the lake controlled human settlement of the basin in the Late Paleolithic. Drying of this reservoir defined formation of the modern hydrological network, including flowing of the Chuya River along the steep gorge between the Maashey and Belgibash mouths. New data evidence for significant difference in evolution of the Kurai-Chuya system of intermountain depressions and Uimon basin at the end of the Late Pleistocene.

New Chronological, Paleontological, and Geochemical Data on the Formation of Glacier-Dammed Lakes in the Kurai Depression (Southeastern Russian Altai) at the End of the Late Pleistocene

One of the debated problems in Pleistocene paleogeography of the Altai region is the extent and time of existence of the last glacier-dammed lakes. For the first time paleontological, mineralogical characteristics, and optically stimulated luminescence (OSL) ages of lacustrine deposits confirm accumulation of ice-dammed lake with depth at least 170 m in Kurai basin during the second marine isotopic stage (MIS-2 according to SPECMAP scale), 16‒19 ka BP. In addition, new information about the ecology and evolution of this Sartanian reservoir is obtained. The consequences of its draining had been less catastrophic than the previous glacial lake outburst floods; however, the lake limited human colonization of the depression in the Late Paleolithic. Drying of the lake determined the formation of the modern river network, including the outlet of the Chuya River along the gorge between the mouths of the Maashei and Belgibash rivers. These new data indicate that the Kurai–Chuya and Uimon depressions had different evolutions at the end of the Late Pleistocene.

BLOCK STRUCTURE OF THE SOUTHERN KURAI BASIN OF GORNY ALTAI ACCORDING TO GEOELECTRIC DATA COMPARED TO THE DISTRIBUTION OF EARTHQUAKE EPICENTERS

The Kurai Basin of Gorny Altai is located in the area of high seismic activity, which is involved in the focal zone of the 2003 M7.3 Chuya earthquake. Its aftershock process has not ceased yet, and shows the likelihood of major seismic events. The seismic monitoring records of the last 15 years after the devastating earthquake show the state of stress in the basin. A comprehensive field database has been consolidated from the studies of direct and alternating currents by electromagnetic methods, including transient electromagnetic sounding, vertical electrical sounding, and electric field tomography. Using a combination of the research techniques and the method of interpretation based on data inversion, it becomes possible to select optimal models, ensure higher reliability, and improve the contents of the study. The available seismological data are used to identify the zones of concentration of seismic events in the southern mountain frame of the Kurai Basin. Our study aims to determine and clarify the geoelectric structure of the southern troughs. The first section of the Southwestern trough is constructed, and the latitudinal fault boundaries of the Eshtykel graben are confirmed. A comparison of electromagnetic and seismic data for 2015 shows that the earthquake epicenters were mainly confined to the submeridional fault zones. In our study of the Southwestern trough, it is established that most of the earthquakes concentrated in active fault structures separating the blocks of varying depths, which are identified from the geoelectric data. The southern piedmont troughs of the Kurai Basin are cut by numerous faults of latitudinal and submeridional strikes.

Reference section of Neogene-Quarternary deposits in the Uimon Basin (Gorny Altai)

An extraordinary-thick (400 m) section of the Neogene-Quaternary deposits is for the first time exposed by well No. 1 in the central Uimon Basin. The Miocene-Pliocene lacustrine Tueryk Formation is recognized at the base of the continuous section, verified by new paleontological data (ostracods, spores, and pollen). As assumed, overlaying deposits are represented by the Lower Pleistocene lacustrine-alluvial Beken Formation, Middle Pleistocene alluvial-proluvial Bashkaus Formation, undifferentiated Middle Pleistocene glacial, fluvioglacial, and alluvial deposits, and Upper Pleistocene lacustrine-glacial deposits. The data obtained from the core of well No. 1 undisputably demonstrate that the Uimon Basin had been developed prior the beginning of the Miocene Epoch, when it was characterized by accumulation of the lacustrine Tueryk Formation, incompletely exposed within the studied section. The presence of thick unexposed lower-Ohm interval of sedimentary filling of the basin suggests that the Uimon Basin was developed as early as the Paleogene. Therefore, the tectonic evolution and sedimentation history of the basin are assumed to have features similar to those of the Chuya and Kurai Basins of Gorny Altai.

Археологические памятники как маркер перестройки неоплейстоцен-голоценовой гидросети Курайской и Чуйской впадин (Юго-Восточный Алтай): результаты геолого-геоморфологических и геоархеологических исследований

Археологические памятники как маркер перестройки неоплейстоцен-голоценовой гидросети Курайской и Чуйской впадин (Юго-Восточный Алтай): результаты геолого-геоморфологических и геоархеологических исследований

Quantitative Estimations of the Holocene Erosion due to Seismically Induced Landslides in the SE Altai (Russia) Applying Detailed Profiling and Statistical Approaches

Earthquakes are some of the most disastrous natural hazards. Coseismic slope failures often significantly contribute to the global damage and may cause most of the casualties related to strong earthquakes. Seismically induced landslides are widespread phenomena within tectonically active mountain terrain. Their abundant occurrence and the large volumes of displaced slope material reveal their great influence on topographic changes. This paper presents new correlations between the earthquake magnitude and the total volume of displaced slope material and between earthquake magnitude and the volume of the largest triggered landslide. These relationships allow us to quantify erosion due to seismically triggered landslides. Calculation of the total volume of displaced slope material is based on the parameters of the largest landslides which could be preserved in the topography for thousands years. This approach was tested in the most seismically active southeastern part of Russian Altai, which is evidenced by numerous giant earthquake-induced paleo-landslides. The total volume of slope material displaced during ancient earthquakes within the Chagan-Uzun river basin, calculated on the basis of statistical correlations, is (3.0-4.3)×10 -1 km 3 , and the Holocene erosion rate due to seismically induced landslides (1.1-3.0)×10 -5 m a -1 . The numerical estimates were verified by calculating the volumes of all detected earthquake-triggered landslides within the Chagan-Uzun river basin and the neighboring Kurai basin on the basis of detailed profiling approach, which are 1.33×10 -1 and 2.25×10 -1 km 3 , respectively. The Holocene erosion rate due to seismically induced landslides in these basins could be estimated at 1.1×10 -5 and 1.4×10 -5 m a -1 , respectively. Thus the Holocene erosion rate of the SE Altai due to earthquake-triggered landslides obtained by applying different techniques can be estimated at (1.1-3.0)×10 -5 m a -1 , which more precisely characterizes topography changes within the Chuya-Kurai system of intermountain depressions and framing ridges.

Archaeological sites as markers of Neopleistocene-Holocene hydrological system transformation in the Kurai and Chuya basins, Southeastern Altai: Results of geomorphological and geoarchaeological studies

Multidisciplinary studies using geomorphological, geoarchaeological, and geochronological approaches indicate contrasting environmental changes in southeastern Altai beginning from the Late Pleistocene. Twenty-eight new radiocarbon dates from the subaerial complex overlying Late Neopleistocene sediments in the high-altitude Kurai and Chuya basins confi rm the degradation of a single ice-dammed reservoir in that area during the Early Holocene. In the fi rst half of the Holocene, those basins were fi lled with isolated lakes. At the Baratal River mouth in the western Kurai basin, a reservoir with a water level of at least 1480 m a.s.l. emerged ca 10–6.5 ka BP, whereas in the Chuya depression, numerous residual lakes had existed at least 8 ka BP. Landslide- and morainedammed lakes between the depressions in the Chuya valley had existed until 7–3 ka BP, when they drained away. The preservation state of in situ archaeological sites, their cultural affi liation, and their location within the depressions and along the main Chuya valley attest to spatial and temporal changes of the hydrological system. This evolution occurred in the second half of the Holocene and did not entail major consequences for humans.

The first computer model of currents in the Kurai intermountane basin, Altai, under release of a glacial-dammed lake

The first 2-D computer model of the currents (including circulation currents) inside the glacial-dammed Kurai Lake under its release caused by the dam break is simulated in the RMA2 program of the SMS 9.2 modeling system. The hydraulic parameters are calculated for several given water discharges in the transit flow (from 10 to 0.3 million m3/s) of the basin. A consecutive change in the circulation currents during the release of the lake is identified. A comparison of the character of circulation and the calculated fields of the depths and the current velocities in the lake with the orientation of the gravel ranges on the bottom of the Kurai basin allows reconstruction of the hydraulic conditions of possible formation of the giant ripple fields.

Human occupation of South Eastern Altai highlands (Russia) in the context of environmental changes

Multidisciplinary investigations including geomorphological and geoarchaeological approaches, litho-stratigraphic and pedogenetic analysis accompanied by radiocarbon dating show the contrasting human environmental changes within the highlands of the Russian Altai since the late Paleolithic. Radiocarbon ages of fossil soils formed at 1,475–1,730 m a.s.l. indicate draining of the ice-dammed lake in the Kurai basin before the beginning of the Holocene and disintegration of the Chuya lake into several shallow but quite spacious reservoirs by 8,223 ± 181 cal. BP. Using archaeological sites as spatio-temporal markers allows reconstructions to be specified based on geological data. Archaeological site distribution in the Chuya valley between the Chuya and Kurai basins indicates that all cataclysmic flood events related to the Holocene hydrological changes occurred before the Scythian epoch. Due to the redeposition of most Paleolithic finds in the region, they should be carefully examined before they are utilized for any reconstructions. Using fossil and contemporary soils as an independent informative climatic proxy archive supports the conception of generally more humid and warmer climate conditions in the first half of the Holocene within the SE Altai and a more arid and cold climate in the second one. The repeated climate deteriorations that caused glacier expansion and the progressive aridity intensification in the region along with the sociopolitical reasons are the major factors that controlled the habitat of nomadic communities and cultures shifting within the SE Altai in the second half of the Holocene. Anthropogenic impact together with the progressive aridization led to the deforestation of the eastern part of the Chuya depression.

Deep structure and margins of the Kurai Basin (Gorny Altai), from controlled-source resistivity data

The deep structure of the Kurai basin and its junction with the flanking mountains has been studied by controlled-source resistivity surveys (vertical electric sounding and transient electromagnetic methods). According to the data processing results, the basin is the deepest along its northern, southern, and eastern margins. The sedimentary fill comprises two resistivity units corresponding to two sequences deposited at different stages of the basin history. The lower, less resistive unit consists of Paleogene–Neogene lacustrine clay and the higher-resistivity upper unit represents coarser Quaternary deposits. In Paleogene–Neogene time, the basin formed by the left-lateral pull-apart mechanism. The earliest Quaternary strike-slip faulting in the setting of overall compression produced the Central Kurai basin within the northern Kurai basin, while the flanking ranges and fault blocks thrust upon the basin transforming it into a ramp. Thus, piedmont steps rose along the basin margins, and the marginal grabens became ramps and half-ramps.

Modeling Cenozoic crustal deformation in Gorny Altai

The Altai lithospheric structure has been generally understood due to available high-resolution digital models. As a further step in modeling, we have simulated the structure of southeastern Altai as interaction of eight blocks which comprise or surround the Chuya and Kurai basins, proceeding from the basic configuration of blocks and earthquake mechanisms. Should the stresses in the system remain invariable, the western periphery of the Kurai basin will deform to let the Uimen-Sumulta fault join the Chuya (western end of Tolbonur) fault and evolve further as a single shear zone. The best fit model was one with slip along a single border fault in the middle of the area between two rheologically different terranes. This setting corresponds to a fault boundary between the more plastic Gorny Altai and more rigid Teletskoe-Chulyshman domains, which is consistent with current crustal movements from GPS data. In addition to scientific significance, models of this kind have practical applications as they highlight areas of stress buildup prone to release in large earthquakes. The new approach was applied to simulate the stress and strain patterns of central and southeastern Gorny Altai, and the models were tested against available geomorphological and seismotectonic data.