E.V. Deev

Unknown large ancient earthquakes along the Kurai fault zone (Gorny Altai): new results of palaeoseismological and archaeoseismological studies

ABSTRACT Palaeoseismological and archaeoseismological studies in the Kurai fault zone, along which the Kurai Range is thrust onto Cenozoic deposits of the Chuya intramontane basin, led to the identification of a long reverse fault scarp 8.0 m high. The scarp segments are primary seismic deformations of large ancient earthquakes. The scarp’s morphology, results of trenching investigations, and deformations of Neogene deposits indicate a thrusting of the piedmont plain onto the Kurai Range, which is unique for the Gorny Altai. Similarly for Northern Tien Shan, we explain this by the formation of both a thrust transporting the mountain range onto the depression and a branching thrust dislocation that forms the detected fault scarp. In a trench made in one of the scarp segments, we identified the parameters of the seismogenic fault – a thrust with a 30° dipping plane. The reconstructed displacement along the fault plane is 4.8 m and the vertical displacement is 2.4 m, which indicates a 7.2–7.6 magnitude of the ancient earthquake. The 14C age of the humus-rich loamy sand from the lower part of the colluvial wedge constrains the age of the earthquake at 3403–3059 years BP. Younger than 2500 years seismogenic displacements along the fault scarp are indicated by deformations of cairn structures of the Turalu–Dzhyurt-III burial mound, which was previously dated as iron age between the second half of I BC and I AD.

Quaternary travertine of the Kurai fault zone (Gorny Altai)

In the Kurai fault zone, travertine forms a matrix cementing clastic material of colluvial and glacial deposits or rarely forming a stockwork in a system of fractures in Palaeozoic rocks. The regular change of composition of solutions in the process of travertine formation has resulted in change of stable Mg–calcite by Sr–aragonite. According to the carbon isotopic composition, the travertine has intermediate genesis between thermal and meteogene. The light oxygen isotopic composition of CaCO3 indicates formational water input. The carbonates inherited Y, Sr, U, and Ni and in some areas, V, As, and Zn from the endogeneous water sources. Given that the Kurai zone travertine cements the Late Pleistocene–Holocene sediments and 14C dating of the carbonates gives a range of >40 000–3475 ± 35 years, the faults serving as routes of migration of the solutions forming the travertine should be considered as active structures.

Evidence of ancient earthquakes in the alluvial deposits of Katun River (Uimon depression, Gorny Altai)

NEOTECTONIC STRUCTURE AND SEISMICITY OF THE STUDY AREAInstrumental observations show that the GornyAltai is a seismoactive territory. For solving problemsconcerning the evolution of its seismicity, analysis ofdifferent type deformations, which disturb the primarystructures of sedimentary complexes and can be foundin natural outcrops and artificial sections, have gainedincreasing importance recently. Examples of suchstudies are for the Neopleistocene interval are publications [1, 2] and [7] for the upper Holocene. Paleoseismogeological studies have been mostly concentratedin the southeastern part of the Gorny Altai, where thetwo largest basins (Chuya and Kurai) are located andwithin the limits of major river valleys (Chuya andKatun). However, specialized paleoseismogeologicalstudies in the central part of the Gorny Altai, wherethe Uimon basin, which is third in size, is located(Fig. 1), may also be promising.To the north, the Uimon basin is framed by theTerekta and Kamza ridges (see Fig. 1) of 2000 to 2900m high. Their block structure and general decrease inaltitude towards the depression is determined by acombination of three fault systems of different orientations: northwestern, northeastern, and sublatitudinal [3]. The geological and geophysical data indicatethe thrust of the northern mountain framing onto theCenozoic deposits of the Uimon depression along theSouth Terekta fault zone; the deposits are involved inthe tectonic motion and a series of tectonic scarps isformed [3, 10]. To the south, the intermontane is confined by the branches of Katun Ridge of 2000 to3000 m high and higher. Their neotectonic structure isdetermined by a combination of submeridional andarcshaped faults [3].In the mountain framing of the basin and in theadjacent areas (including Rudny Altai), earthquakesof up to the 12th energy class are recorded by a regularseismic network (see Fig. 1). An example of a relativelystrong events is the Tyngur earthquake of September18, 1997, whose magnitude was 4.0 to 5.2 based on thedata of various agencies. The regional value of magnitude was

Evidence for seismicity of the southeastern Altai region during the Quaternary

The present-day morphology of the Altai orogen, its southeastern segment included, started forming in the early Neogene. The main orogenic phase began in the Late Pliocene. It is clear that mountain forming processes particularly during the Quaternary should have been accompanied by high seismicity. Until recently, its consequences were evident only from Holocene seismogenic macrodislocations: landslides, rock falls, scarps, extended troughs. Older intervals of the Quaternary section remain uninformative in this respect. Only discovery of seismites representing peculiar deformation structures reflected in sedimentary sequences of the southeastern Altai region [1] offered the opportunity for defining consequences of pre-Holocene seismic events. The main purpose of this study is to reveal a correlation between seismites and stratigraphic succession of Quaternary sediments in the region for determining periods of its paleoseismicity. In addition, morphological features and formation mechanisms of seismites are considered. Quaternary sedimentary complexes. The Quaternary complexes are underlain by lacustrine, proluval‐ fluvial, and talus sediments of the Beken Formation, which is attributed to the Upper Pliocene. The Quaternary section proper begins with brown boulder gravel, pebble gravel, and sands of the Pliocene‐Quaternary Bashkaus Formation of fluvial and proluvial‐talus‐fluvial genesis in valleys of the main rivers and intermontane troughs, respectively. The formation represents an intermontane molasse accumulated against the background of intense orogenic processes. It is assumed that the first glaciations in the Altai region occurred in the Early Quaternary time [2]. Two paleogeographic zones were forming at that time: (1) paleoglalcial influenced by glaciers (upstream of the Chuya River from the Bel’gibash River mouth to the Chuya Depression); (2) paleoperiglacial, free of glaciers and with giant glacial floods being a leading factor of Quaternary morpholithogenesis (from the Bel’gibash River mouth downstream along the Chuya and Katun river valleys up to mountain piedmonts).

The upper mantle structure and Cenozoic volcanism in Central Mongolia

The study of formation mechanisms and origin of large structures in the continental lithosphere is one of the fundamental directions of recent tectonic and geodynamic investigations. Cenozoic structure-forming processes are reflected clearly in the continental lithosphere of Central Asia. It is accepted that the tectonic evolution of its lithosphere was determined by different factors: external stress fields at boundaries of large interacting lithospheric plates and internal stress fields produced by processes in the sublithospheric mantle. This work is dedicated to defining the role of these factors in the formation of structures in Central Mongolia based on petrological‐geochemical and seismotomographic materials combined with geostructural and morphotectonic data. Central Mongolia demonstrates features related to Cenozoic intracontinental deformation processes. Their influence is reflected in the neotectonic structure of the region, which includes positive morphostructures of the Gobi Altai, Khangai Range (Upland), intervenient Lake Valley (Fig. 1), and contrasting structures around Lake Khubsugul. In addition, the Cenozoic stage was marked by the formation of basaltic fields, which spatially coincide with fields of Late Mesozoic volcanics in the southern part of this region. The volcanics are largely represented by subalkali olivine basalts and alkali basaltoids. The volcanic complexes of different ages are discriminated largely based on their geomorphologic position, K‐Ar dating, paleomagnetic data, and geological relationships with paleontologically well-substantiated sedimentary sequences [2‐5]. Most Cenozoic basalts fields of different sizes and shapes are localized in the Khangai Upland and its spurs mainly east of 100 ° E. Volcanic bodies form usually small watershed plateaus, fill fault-line Cenozoic depressions, and make up elements of river valleys (high and low terraces). The northern Khangai Upland comprises three areas (Fig. 1) with volcanics of different stages: Late Pliocene (Taryat, Orkhon, and Khanuingol), Middle‐Late Pleistocene (Taryat and Orkhon), and Holocene (Taryat) [2]. The study of volcanic edifices in these fields shows that they represent volcanoes of the central type and occur in marginal parts of Cenozoic depressions. The Upper Pliocene basalt section includes five to seven flows 2‐70 m thick; the Middle‐Upper Pleistocene section, two to three flows 5‐15 m thick; and the Holocene section, only one flow (8‐10 m thick) or monogenic scoria cones. Volcanics are dominated by light to dark

Mud volcanism as an indicator of late to neopleistocene-holocene activity of the Chilik-Kemin fault, Yli depression, Northern Tien Shan

It is revealed that the Altyn-Emel mud volcanic field (43°52′56″ N, 79°06′31″ E) in the Yli depression (Dzharkent trough) is structurally linked to the northeastern end of the Chilik-Kemin deep fault. The mud volcano is related to hydrocarbon-rich gases (including methane) and pressure artesian mineralized thermal waters, which uplifted to the surface along the fault zone. It is suggested that the earthquakes with M≤ 5 related both to the Chilik-Kemin fault and other seismic generating structures intensified the mud volcanic activity. In some cases, the eruptions were accompanied by the short-lived ignition of hydrocarbon gases and formation of the Na-rich paralavas. The mud volcanism of the Altyn-Emel field has been manifested during the last 15–20 k.a., and, consequently, the northeastern part of the Chilik-Kemin fault is an active structure.