The system of neotectonic faults in southeastern Altai: orientations and geometry of motion

Klein F.W.

Farr T.G., Kobrick M.

On February 22, 2000, the Space Shuttle Endeavour landed at Kennedy Space Center, completing the highly successful 11-day flight of the Shuttle Radar Topography Mission (SRTM). Onboard were over 300 high-density tapes containing data for the highest resolution digital topographic map of Earth ever made. SRTM is a cooperative project between the National Aeronautics and Space Administration (NASA) and the National Imagery and Mapping Agency (NIMA) of the U.S. Department of Defense. The mission was designed to use a single-pass radar interferometer to produce a digital elevation model (DEM) of the Earth's land surface between about 60 north and 56 south latitude. When completed, the DEM will have 30-m pixel spacing and about 15-m vertical accuracy. Two ortho-rectified image mosaics, one from the ascending passes with illumination from the southeast, and one from descending passes with illumination from the southwest, will also be produced (Figure 1).

Rautian T.G., Khalturin V.I., Fujita K., Mackey K.G., Kendall A.D.

The size of local and regional earthquakes in the former Soviet Union (USSR) has been given by the energy class ( K -class) system since the late 1950s. K -class was originally developed as a rapid and simple means of estimating the radiated energy ( E ) from an earthquake and was defined as K = log10 E (in joules). The nature, origin, and methodology of this system are poorly known to Western seismologists studying Soviet and Russian seismological data, and yet are of great interest and importance to those conducting detailed research on the seismicity of the former USSR. Since its inception, K -class has been the primary means of quantifying the size of small events in the former USSR and continues to be used for that purpose today. In most of this region, scientists employed the method of Rautian (1960), using the maximum horizontal (for the S wave) and vertical (for the P wave) amplitudes, which became the standard for local and regional networks in the early 1960s. In this paper, we describe the origins and basic principles of the energy class system, as well as the methodology generally used today by the regional networks (figure 1) of the states of the former USSR. Shortly after World War II, between 1946 and 1949, three large earthquakes occurred in Soviet Central Asia and triggered an intense study of seismicity. After the magnitude 7.4 Khait earthquake of 10 July 1949, the Geophysical Institute (now the Institute of Physics of the Earth) of the USSR dispatched an expedition to Garm, Tajikistan (figure 1), to deploy a temporary network around the epicentral region. This Complex Seismological Expedition (CSE), which included the senior authors of this paper (Khalturin and Rautian) at its inception, became permanent in 1954. The study of regional seismicity was not well-developed …

Zhang H.

We have developed a double-difference (DD) seismic tomography method that makes use of both absolute and relative arrival times. By reducing systematic errors using the more accurate relative arrival times, the method produces an improved velocity model. Simultaneously, it yields event locations of a quality equivalent to those of the DD location method, while avoiding simplifying assumptions of that method. We test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography. We also test this method on a Hayward fault, California, earthquake dataset spanning 1984–1998. The earthquakes relocated by this method collapse to a thin line along the fault trace, consistent with previous results. The DD velocity model has sharper velocity contrasts near the source region than the standard tomography model.

Cunningham W.D.

The Altai is one of the great Cenozoic intracontinental and intraplate mountain ranges of central Asia, but it is one of the less studied mountain belts on Earth from a modern structural geology standpoint, and few western scientists are familiar with its tectonic evolution. The range is located dominantly in Mongolia with important sectors in Russia, China, and Kazakhstan and structurally links with the Chinese Tien Shan and Russian Sayan ranges. The Altai is tectonically active and is best understood as two kinematically distinct mountain belts that intersect at 46°N, 96°E: the right-lateral transpressive western Altai and the left-lateral transpressive Gobi Altai. Transpressional deformation dominates the late Cenozoic deformation of the Altai and is manifested by throughgoing strike-slip faults, restraining bends, thrust fault-bounded ranges linked by strike-slip faults, and possibly inverted Mesozoic graben. Regionally, there is strong correlation between Cenozoic fault trends and older basement strike trends. Cenozoic deformation regimes appear to be dictated by the angular relationship between preexisting basement structural trends and the prevailing NE directed maximum compressive stress. The Altai is constructed on Paleozoic terranes dominantly consisting of arc and subduction complex assemblages and passive margin sedimentary rocks juxtaposed to the west, southwest and south of the Precambrian block that underlies the Hangay Dome area of central Mongolia. Because the Hangay Dome is characterized by diffuse extension and not transpression, regionally upwarped topography, late Cenozoic volcanism, and elevated heat flow, it is kinematically separate from the Altai region and has previously been interpreted to overlie a mantle plume or asthenospheric diapir. It is proposed that anomalously hot mantle beneath the Hangay Dome coupled with the regionally domed surface topography creates a radial horizontal stress field that acts against the regional NE directed maximum horizontal stress within the Altai. These conditions may deflect postulated NE flowing lithospheric mantle/lower crust that is believed to be driving the shallow upper crustal deformation in the Altai.

Cunningham W.D., Windley B.F., Dorjnamjaa D., Badamgarov J., Saandar M.

The Gobi Altai region of southwestern Mongolia is a natural laboratory for studying processes of active, transpressional, intracontinental mountain building at different stages of development. The region is structurally dominated by several major E—W left-lateral strike-slip fault systems. The North Gobi Altai fault system is a seismically active, right-stepping, left-lateral, strike-slip fault system that can be traced along the surface for over 350 km. The eastern two-thirds of the fault system ruptured during a major earthquake (M = 8.3) in 1957, whereas degraded fault scarps cutting alluvial deposits along the western third of the system indicate that this segment did not rupture during the 1957 event but has been active during the Quaternary. The highest mountains in the Gobi Altai are restraining bend uplifts along the length of the fault system. Detailed transects across two of the restraining bends indicate that they have asymmetric flower structure cross-sectional geometries, with thrust faults rooting into oblique-slip and strike-slip master faults. Continued NE-directed convergence across the fault system, coupled with left-lateral strike-slip displacements, will lead to growth and coalescence of the restraining bends into a continuous sublinear range, possibly obscuring the original strike-slip fault system; this may be a common mountain building process. The largely unknown Gobi-Tien Shan fault system is a major left-lateral strike-slip fault system (1200 km + long) that links the southern ranges of the Gobi Altai with the Barkol Tagh and Bogda Shan of the easternmost Tien Shan in China. Active scarps cutting alluvial deposits are visible on satellite imagery along much of its central section, indicating Quaternary activity. The total displacement is unknown, but small parallel splays have apparent offsets of 20 + km, suggesting that the main fault zone has experienced significantly more displacement. Field investigations conducted at two locations in southwestern Mongolia indicate that late Cenozoic transpressional uplift is still active along the fault system. The spatial relationship between topography and active faults in the Barkol Tagh and Bogda Shan strongly suggests that these ranges are large, coalescing, restraining bends that have accommodated the fault's left-lateral motion by thrusting, oblique-slip displacement and uplift. Thus, from a Mongolian perspective, the easternmost Tien Shan formed where it is because it lies at the western termination zone of the Gobi-Tien Shan fault system. The Gobi-Tien Shan fault system is one of the longest fault systems in central Asia and, together with the North Gobi Altai and other, smaller, subparallel fault systems, is accommodating the eastward translation of south Mongolia relative to the Hangay Dome and Siberia. These displacements are interpreted to be due to eastward viscous flow of uppermost mantle material in the topographically low, E–W trending corridor between the northern edge of the Tibetan Plateau and the Hangay Dome, presumably in response to the Indo-Eurasian collision 2500 km to the south.

Dobretsov N.L., Buslov M.M., Delvaux D., Berzin N.A., Ermikov V.D.

This paper reviews and integrates new results on: (1) the Late Paleozoic and Mesozoic evolution of Central Asia; (2) Cenozoic mountain building and intramontane basin formation in the Altay-Sayan area; (3) comparison of the tectonic evolutionary paths of the Altay, Baikal, and Tien Shan regions; (4) Cenozoic tectonics and mantle-plume magmatic activity; and (5) the geodynamics and tectonic evolution of Central Asia as a function of the India-Himalaya collision. It provides a new and more complete scenario for the formation of the Central Asian intracontinental mountain belt, compared with the generally accepted model of the “indentation” of the Indian plate into the Eurasian plate. The new model is based on the hypothesis of a complex interaction of lithospheric plates and mantle-plume magmatism. Compilation and comparison of new and published structural, geomorphological, paleomagnetic, isotopic, fission-track, and plume magmatism data from the Baikal area, the Altay, Mongolia, Tien Shan, Pamir, and Tibe...

Cunningham W.D., Windley B.F., Dorjnamjaa D., Badamgarov G., Saandar M.

We present results from the first detailed geological transect across the Mongolian Western Altai using modern methods of structural geology and fault kinematic analysis. Our purpose was to document the structures responsible for Cenozoic uplift of the range in order to better understand processes of intracontinental mountain building. Historical right-lateral strike-slip and oblique-slip earthquakes have previously been documented from the Western Altai, and many mountain fronts are marked by active fault scarps indicating current tectonic activity and uplift. The dominant structures in the range are long (>200 km) NNW trending right-lateral strike-slip faults. Our transect can be divided into three separate domains that contain active, right-lateral strike-slip master faults and thrust faults with opposing vergence. The current deformation regime is thus transpressional. Each domain has an asymmetric flower structure cross-sectional geometry, and the transect as a whole is interpreted as three separate large flower structures. The mechanism of uplift along the transect appears to be horizontal and vertical growth of flower structures rooted into the dominant right-lateral strike-slip faults. The major Bulgan Fault forms the southern structural boundary to the range and is a 3.5-km-wide brittle-ductile zone that has accommodated reverse and left-lateral strike-slip displacements. It appears to be linked to the North Gobi Fault Zone to the east and Irtysh Fault zone to the west and thus may be over 900 km in length. Two major ductile left-lateral extensional shear zones were identified in the interior of the range that appear to be preserved structures related to a regional Paleozoic or Mesozoic extensional event. Basement rocks along the transect are dominantly metavolcanic, metasedimentary, or intrusive units probably representing a Paleozoic accretionary prism and arc complex. The extent to which Cenozoic uplift has been accommodated by reactivation of older structures and inversion of older basins is unknown and will require further study. As previously suggested by others, Cenozoic uplift of the Altai is interpreted to be due to NE-SW directed compressional stress resulting from the Indo-Eurasian collision 2500 km to the south.

Li S., Yuan W., Zhao Z., Zhang A., Dong G., Li X., Sun W.

This study presents new fission track data from 40 apatite and 40 zircon samples in the Southern Altai Mountains (SAMs), revealing apatite fission track (AFT) ages of 110 ± 8 Ma to 54 ± 4 Ma and zircon fission track (ZFT) ages of 234 ± 24 Ma to 86 ± 7 Ma. The exhumation rates derived from three thermochronological methods range from 0.01 to 0.1 km/Ma (Age-Elevation method), 0.01 to 0.14 km/Ma (Half-Space thermal model), and 0.027 to 0.075 km/Ma (Age2exhume model). Thermal history modeling using HeFTy software reveals similar thermal histories on both sides of the Kangbutiebao Fault, with a notable cooling event and higher exhumation rates to the northeast. The Late Cretaceous (100–75 Ma) rapid cooling is associated with tectonic reactivation, likely linked to the collapse of the Mongol–Okhotsk Orogen and slab rollback in the southern Tethys Ocean. In the Late Cenozoic (10–0 Ma), cooling and uplift reflect the influence of tectonic stresses from the India–Eurasia collision, which also drove the reactivation of the Kangbutiebao Fault. These findings suggest a complex interplay of tectonic processes driving exhumation in the SAMs from the Late Jurassic to the Early Paleogene.

Emanov A.F., Emanov A.A., Novikov I.S., Gladyshev E.A., Fateev A.V., Polyansky P.O., Shevkunova E.V., Ershov R.A., Arapov V.V., Krivov A.A.

Abstract —Years after the Chuya earthquake of 2003, geological structures adjacent to the focal area of the Chuya earthquake are still seismically active. The Aigulak focal area is one of them, but energetically the most pronounced. Detailed studies have been carried out with the network of stations of the Altai seismological testing site, supplemented by temporary stations. The region activated in the form of a local and compact structure measuring 10 × 10 km with focal depths from the first 100 m to 20 km. The focal area is not a subsequent activation along the same fault with the Chuya earthquake, but is located on a subparallel fault in the nodal region with its branching into three faults. The seismic activation of the Aigulak focal area is not an aftershock process after a major earthquake, but is an activated structure with a dynamically changing seismic process. An intensive process has formed since the earthquake in 2012 with ML = 6.1 with a gradual decrease in the number of earthquakes, and in 2019 the Aigulak earthquake with ML = 5.5 occurred with a very strong aftershock process after it. Our results of an area study of earthquake density in the focal zone indicate a change in the regime over time: from chaotic to self-organizing along short faults. We conclude that the focal area has not reached the maximum level of seismic energy release.

Shao Y., He J., Wang X., Zhao Y.

The 1931 MW7.8 Fuyun earthquake occurred around the Altai mountains, an intracontinental deformation belt with limited active strain-rate accumulation. To explore whether seismic activity in this deformation belt was affected by stress interaction among different active faults, we calculate the Coulomb failure stress change (ΔCFS) induced by the Fuyun earthquake due to coseismic deformation of the elastic crust and postseismic viscoelastic relaxation of the lower crust and upper mantle. Numerical results show that the total ΔCFS at a 10-km depth produced by the Fuyun earthquake attains approximately 0.015–0.134 bar near the epicenter, and just before the occurrence of the 2003 MW7.2 Chuya earthquake, which distances about 400 km away from the Fuyun earthquake. Among the increased ΔCFS, viscoelastic relaxation from 1931 to 2003 contributes to approximately 0.014–0.131 bar, accounting for >90% of the total ΔCFS. More importantly, we find that for the recorded seismicity in the region with a radius of about 270 km to the Fuyun earthquake from 1970 to 2018, the percentage of earthquakes that fall in positive lobes of ΔCFS resolved on the NNW-SSE Fuyun strike-slip fault, on the NWW-SEE Irtysh strike-slip fault, and on the NW-SE Kurti reverse fault is up to 67.22%–91.36%. Therefore, the predicted ΔCFS suggests that the impact of the 1931 MW7.8 Fuyun earthquake on seismic activity around the Altai mountains is still significant as to hasten occurrence of the 2003 MW7.2 Chuya earthquake at a relatively far distance and to trigger its aftershocks in the near-field even after several decades of the mainshock.

Shitov A.V., Pulinets S.A., Budnikov P.A.

In this paper, we analyze the effect of the preparation of the Chuya earthquake on September 27, 2003, the strongest event in the Altai-Sayan folded region for the entire instrumental period of seismological observations, on meteorological characteristics (temperature, relative humidity, atmospheric chemical potential correction, latent heat of evaporation, and the mean and dispersion values of these data) and their spectral characteristics in the Gorny Altai region. The meteorological and spectral characteristics are shown to change for a certain time before the main shock. The spectral characteristics distinctly change both during the preparation of the main shock and during the aftershock process.

Tveritinova T.Y., Marinin A.V., Deev E.V.

In the Katun Fault Zone, the structural-paragenetic analysis of stress indicators and the method of cataclastic analysis of discontinuous faults have been used to determine the features of the structure of this fault and the conditions of its formation at the Late Alpine stage. It is proved that the Katun Fault Zone is the strike-slip fault that develops at different sites in transpressive or transtensive conditions. The newest grabens along the zone were formed under strike-slip fault displacements and locally manifested conditions of horizontal extension or horizontal extension with shear.

Tveritinova T.Y., Marinin A.V., Deev E.V.

The kinematic characteristics of faults included in the structure of the active Katun Fault in Gorny Altai have been determined by methods of structural–paragenetic and cataclastic analysis of disjunctives (faults). It is proven that the Katun Fault is a strike-slip fault developing in different areas under transpressive or transtensional conditions. The most recent grabens along the Katun fault were formed under conditions of strike-slip displacements and locally manifested horizontal extension or extension with shear.

Emanov A.F., Emanov A.A., Fateev A.V.

Abstract —The 2003 Chuya earthquake aftershocks are studied using the data obtained during experiments with dense networks of stations. Density maps of the foci of more than 50,000 aftershocks are compared with the day surface faults and the block structure and tectonics of the focal area. The large shearing strain caused by the Chuya earthquake is accompanied by a spatially intermittent aftershock structure stretching along it. The density maps of long-lasted aftershocks differ in structure from the maps of seismic activity in the initial aftershock area. The study has revealed a relationship between the block structure of the epicentral area and the structure of the aftershock process. The nodes of the intersection of faults with the aftershock area are characterized by reduced aftershock activity. The aftershock process is only partly confined to the block-separating faults. In many cases, the aftershock process is shifted relative to these faults or wanders from them.

Zhimulev F.I., Pospeeva E.V., Novikov I.S., Potapov V.V.

The Salair fold-nappe terrane (a.k.a. Salair orogen, Salair) is the northwestern part of the Altai-Sayan folded area of the Central Asian Orogenic Belt. It is composed of Cambrian – Early Ordovician volcanic rocks and island-arc sedimentary deposits. In plan, Salair is a horseshoe-shaped structure with the northeast-facing convex side, which is formed by the outcrops of the Early Paleozoic folded basement. Its inner part is the Khmelev basin composed of Upper Devonian – Lower Carboniferous sandstones and siltstones. The Early Paleozoic volcanic rocks and sediments of Salair are overthrusted onto the Devonian-Permian sediments of the Kuznetsk basin. The Paleozoic thrusts, that were reactivated at the neotectonic stage, are observed in the modern relief as tectonic steps. Our study of the Salair deep structure was based on the data from two profiles of magnetotelluric sounding. These 175-km and 125-km long profiles go across the strike of the Salair structure and the western part of the Kuznetsk basin. Profile 1 detects a subhorizontal zone of increased conductivity (100–500 Ohm·m) at the depths of 8–15 km. At the eastern part of Profile 1, this zone gently continues upward, towards a shallow conducting zone that corresponds to the sediments of the Kuznetsk basin. Two high-resistance bodies (1000–7000 Ohm⋅m) are detected at the depths of 0–6 km in the middle of the section. They are separated by a subvertical conducting zone corresponding to the Kinterep thrust. The main features are the subhorizontal positions and the flattened forms of crustal conductivity anomalies. At the central part of Profile 2, there is a high-resistance block (above 150000 Ohm⋅m) over the entire depth range of the section, from the surface to the depths of about 20 km. In the eastern part of Profile 2, a shallow zone of increased conductivity corresponds to the sediments of the Kuznetsk basin. The subhorizontal mid-crust layer of increased conductivity, which is detected in the Salair crust, is typical of intracontinental orogens. The distribution pattern of electrical conductivity anomalies confirms the Salair thrust onto the Kuznetsk basin. The northern part of the Khmelev basin is characterized by high resistivity, which can be explained by abundant covered Late Permian granite massifs in that part of the Khmelev basin. The Kinterep thrust located in the northeastern part of the Khmelev basin is manifested in the deep geoelectric crust structure as a conducting zone, which can be considered as an evidence of the activity of this fault.

Vysotsky E.M., Novikov I.S., Lunina O.V., Agatova A.R., Nepop R.K.

A 48 km long zone of surface deformation produced by the Ms = 7.3 intracontinental earthquake of 2003 in Gorny Altai is studied in its five segments between the Aktru and Irbistu rivers, where ruptures show the greatest offsets and distinct structural patterns. A total of 554 coseismic ruptures of five slip geometry types are analyzed in terms of length, orientation, and relative percentage. The rupture patterns are discussed with reference to previously published evidence and compared with other strike-slip zones worldwide.

Eremin M.

Numerical modeling of fault zone evolution and related seismicity can provide an insight into the process of large earthquake occurrences in a complicated fault system. In this paper, we develop a three-dimensional finite-difference numerical model of stress-strain evolution in and around the Chuya and Kurai depressions of Gorny Altai, Russia, to understand the fault zone evolution and the observed distribution of earthquakes in the region. Unlike in previous studies, the process of seismo-tectonic deformations of this region is addressed in a three-dimensional formulation with an improved model of rock massif behavior. A new geometrical model is designed, which is based on the seismotectonic, paleoseismological studies, and high-resolution Space-Radar-Topography-Mission data. The mathematical model applied here represents a set of partial differential equations, which are based on the fundamental conservation laws and constitutive equations for elastic and inelastic deformations. The initial stress state of the model is due to the action of gravity forces. The model is activated by assigning different displacement fields at the model boundaries – strike-slip and GPS-based displacements. The modeling results illustrate the stages of fault zone development and are in satisfactory agreement with the field observations in the case of GPS-based boundary conditions, which has not been modeled yet. Modeled seismic process is associated with the development of a fault zone resulting from the loss of strength in the subsurface points where the inelastic strain exceeds a certain threshold. The areas of high-degree of inelastic strain localization and the modeled fault zone represent an en-echelon system of dextral strike slips and various types of shear bands. The modeled seismic process obeys the Gutenberg-Richter (frequency-magnitude) law. The number of earthquakes in the attenuation part of the rock massif fracture process follows the Omori (aftershock decay) law.

Makarov P.V., Smolin I.Y., Peryshkin A.Y., Kulkov A.S., Bakeev R.A.

Doppler laser interferometry is used to measure the transient time between the slow quasi-stationary stage of damage accumulation in rock samples to the ultrafast catastrophic stage of failure as well as the duration of the autocatalytic stage of macroscopic fracture. Small rock samples are tested for compression and three-point bending, and the velocity of displacement of their lateral surfaces is measured up to macroscopic fracture. The surface velocity at the catastrophic stage proves to be three orders of magnitude higher than the average surface velocity at the quasi-stationary stage of damage accumulation. The transient time to catastrophic failure is estimated at 60–100 ms, and the duration of the ultrafast catastrophic failure stage is 15–20 ms for small marble samples. The transient stage is the process of self-organization of individual acts of fracture into the state of self-organized criticality. At this stage, the distribution of individual acts of fracture evolves into power-law distributions. A simple fracture model with power laws is proposed, which is in full agreement with the experimental data. The developed mathematical model is used to calculate fracture of small rock samples, reproducing uniaxial compression and three-point bending tests, as well as fracture in rock masses with mine openings. We also model the process of faulting and fracturing in the mountains of Central Altai, including the foreshock process, main event (the Chuya earthquake of September 27, 2003) and aftershock process. The calculated seismic process fully corresponds to the Gutenberg–Richter recurrence law, and the calculated aftershock process conforms to the Omori law.

Turova I., Deev E., Pozdnyakova N., Entin A., Nevedrova N., Shaparenko I., Bricheva S., Korzhenkov A., Kurbanov R., Panin A.

• Kurai Fault Zone has generated five M w = 6.6–7.6 earthquakes for the past 6.5 ka. • Kurai Range is stepping over the basins along the main north-dipping reverse faults. • Paleoseismic slip occurred as backthrusting on south-dipping pinnate reverse faults. • The backthrusting maintains the growth of forebergs in front of the Kurai Range. • Geoelectrical data made it possible to trace faults to a depth of 50 m. 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 M w = 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.

Nepop R.K., Agatova A.R., Uspenskaya O.N.

This paper presents the results of geological, geomorphological, geochronological investigations and the analysis of the biological composition of deposits within the high-mountainous Boguty river basin. The main stages of landscape evolution during the last 14 ka have been investigated through 32 radiocarbon dates. The area is located in the most arid southeastern part of Russian Altai and constitutes the eastern periphery of the Chuya intermountain depression, and the western side of the Chikhachev range. Postglacial landscape development of the upper reaches of the Chuya River began from the degradation of its vast glaciation. By 14 ka cal BP the glaciers had either completely melted or retreated above 2500 m above sea level. As a result, numerous lakes occupied open places and five types of these basins can be seen in the modern topography. They are large moraine-dammed lakes connected with the Boguty River, while some basins have been dammed by uplifted tectonic blocks and numerous lakes are of thermokarst origin or are small cirque lakes and temporal lakes fed by seasonal meltwater from meteoric, ground water and permafrost degradation. Large moraine-dammed lakes within the Boguty basin were developed before the Holocene. The water filling these basins was controlled by the height of moraine dams and by the depth of the Boguty river incision. The moraine-dammed Low Boguty Lake formed earlier than 10 ka cal BP. Its partial draining and transformation of littoral zone into peat bog took place about 8.2-7.6 ka cal BP. As a result of the analysis of biological composition, four stages of water level fluctuations in the lacustrine-boggy system were recognized. The specific geomorphic and sedimentary features in the middle part of the Boguty basin determined the recurrence of debris flow events at about 8-7; 2.9-1; 0.65 and 0.3 ka cal BP, but the last event occurred in June 2017. The occurrence of fossil forest soils dating to about 11-8.5 ka cal BP and of charcoals (Larix sibirica Ledeb) of about 8.5-7.8 ka cal BP indicate a prolonged period of developing forest vegetation in the Boguty basin, which is currently treeless. In general, the data suggest that in SE Altai the climate in the first half of the Holocene was warmer and more humid in comparison with the modern one. The second half of the Holocene has been characterized by colder climate conditions accompanied by aridity intensification, which became especially pronounced during the last 2-1.5 ka.

Eremin M., Stefanov Y.

Numerical modeling of fault zone evolution can elucidate the process of formation of a complicated fault system. Here we develop a numerical model of stress-strain state evolution in and around the Chuya-Kuray fault zone of Gorni Altay, Russia, to understand the fault zone evolution. The model’s structure is constructed on the basis of seismotectonic and paleoseismological studies as well as high-resolution Space-Radar-Topography-Mission data. A mathematical model is described by a set of partial differential equations of solid mechanics. Constitutive equations for inelastic strains were derived earlier and are implemented in this work. Inelastic behavior is described by the modified Drucker-Prager plasticity model with non-associated plastic flow rule. An initial stress state of the model is a result of gravity forces, and the model is activated by a slip of a buried dextral strike-slip fault located in the basement of the model. The results of modelling illustrate the stages of fault development, the development of fault brunches and the structure of the modeled fault zone.

Deev E.V.

The conducted paleoseismological and archaeoseismological studies reveal three zones of concentration of the ancient and historical earthquakes in Gorny Altai which are related to the Kurai Fault zone, Katun, and South Terekta faults. The surface ruptures are detected within the Kurai Fault zone, which were formed in the epicentral zones of the paleoearthquakes that occurred 6500, 5800, 3200, and 1300 years ago and had magnitudes Mw = 6.7–7.6. The recurrence period of the paleoearthquakes is 700 to 2600 years. The detected secondary seismogenic deformations indicate that an epicentral zone of the paleoearthquake with an age of less than 12.5 ka (Mw = 7.2–7.6, intensity I = 10–11), the traces of earthquakes and their clusters with M ≥ 5–5.5 and I ≥ 6–7, which occurred about 150 and 90 ka ago, in the intervals of 38–19 ka ago (with a recurrence period of about 2 ka), and 19–12.5 ka ago are related to the southern part of the Katun Fault. The earthquake of I ≥ 5–6 which damaged the constructions of the Chultukov Log 1 burial mound in the period from IV century B.C. to the beginning of I century A.D. is associated with the northern part of the Katun Fault. In the zone of the South Terekhta Fault, the seismogenic displacements that occurred in VII–VIII centuries A.D. (Mw = 7.4–7.7, I = 9–11) and about 16 ka ago (M ≥ 7, I = 9–10) are revealed. The latter triggered the formation of a landslide-dammed lake which was destroyed by the earthquake about 6 ka ago (M ≥ 7, I = 9–10). Secondary paleoseismic deformations of the ancient earthquakes (M ≥ 5–5.5, I ≥ 6–7) are recorded in the sediments of the Uimon Basin with an age of 100–90 ka and about 77 ka. These results should be taken into account in designing a gas pipeline in the People’s Republic of China, building infrastructure for tourism, and elaborating the seismic zoning maps for the territory of the Russian Federation.