Geochemistry of metamorphic rocks of the Kurai block (Gorny Altai)

We consider the primary nature and sources of the protoliths of metamorphic rocks of the Kurai block located in the large Teletskoe-Kurai system of deep faults separating the Gorny Altai and West Sayan structures. It has been established that the protoliths of the Kurai block metapelites were deposits of transitional crust: They lack typical rocks of mature continental crust (arkoses, litharenites) and have reduced (relative to the upper continental crust) contents of lithophile elements and elevated contents of transition elements. The average Nd model age of the protoliths of the metapelites of the Kurai complex corresponds to the Middle Riphean (1.4-1.6 Ga). The metabasites of the Kurai complex are similar in petrologic composition to metamorphic products of oceanic basalts. It is shown that the formation of metamorphic associations of the Kurai block was caused by tectonometamorphic transformations of a compositionally heterogeneous rock unit (basalts, aluminous and volcanomict sediments), which were, most likely, a fragment of Middle Cambrian-Early Ordovician turbidite basin with an oceanic basement.     

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

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.     

Stratotypes of Quaternary deposits of the Yaloman-Katun’ zone (Gorny Altai)

We revised geological data substantiating the unified 1983 Regional Stratigraphic Chart of Gorny Altai Quaternary deposits. Based on our own and literature data, we showed that Lower and Middle Quaternary glacial horizons are erroneously distinguished in the Yaloman-Katun’ zone of southeastern Altai. A new correlation is proposed, according to which the glacial complex of the maximum glaciation (MIS-6) corresponds to the Inya catafluvial series and the glacial complex of the first postmaximum glaciation (MIS-4 unit), to the Sal’dzhar catafluvial series. The lectostratotypes of both series are described. The event history of the second half of the Late Neopleistocene in Gorny Altai (MIS-3 and MIS-2) was less catastrophic for ancient biota and Paleolithic man than it was believed earlier.     

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.     

Paleoearthquakes in the Uimon basin (Gorny Altai)

Paleoseismological studies confirm that the Uimon basin is thrust by its northern mountain border along the active South Terekta fault. The latest motion along the fault in the 7-8th centuries AD induced an earthquake with a magnitude of M_w= 7.4-7.7 and a shaking intensity of I = 9-11 on the MSK-64 scale. The same fault generated another event (M > 7, I = 9-10), possibly, about 16 kyr ago, which triggered gravity sliding. The rockslide dammed the Uimon valley and produced a lake, where lacustrine deposition began about 14 ± 1 kyr ago, and a later M > 7 (I = 9-10) earthquake at ~ 6 ka caused the dam collapse and the lake drainage. Traces of much older earthquakes that occurred within the Uimon basin are detectable from secondary deformation structures (seismites) in soft sediments deposited during the drainage of a Late Pleistocene ice-dammed lake between 100 and 90 ka and in ~ 77 ka alluvium. The magnitude and intensity of these paleoearthquakes were at least M > 5.0-5.5 and I > 6-7.     

Traces of paleoearthquakes in the Quaternary deposits of intermontane basins in central Gorny Altai

Study of the sections of Neopleistocene–Holocene deposits filling the basins in central Gorny Altai has revealed earthquake-induced soft-sediment deformation (seismites). They formed as a result of the brittle deformation of deposits and liquefaction of loose water-saturated sediments under vibration seismic impact. The paleoearthquakes resulting in such seismites had the minimum intensity I = 6 and magnitude M = 5–6. Hence, the study region underwent strong earthquakes in the Neopleistocene–Holocene.     

Geoelectrical studies of the Chuya basin sedimentary fill (Gorny Altai)

The VES method was used to determine the geoelectric parameters of Cenozoic rocks in the Chuya intermontane basin. It was shown that the polyfacies and polychromous sediments filling the basin are well differentiated by their electrical parameters. A combination of methods used in data acquisition and processing, due to their high resolution capacity at shallow depths, provided information on the structure and physical properties of the Chuya rocks from surface to depths of 250–300 m. Despite a reasonable consistence among these methods to the above depths, the VES method is a more effective tool for detecting shallower layers. Interpretation of the VES data proved the existence of a large outburst channel as new geological evidence for an ice-dammed origin of the Middle Neopleistocene lake. A more detailed layering of the topmost part of the geoelectrical section across the Irbistu–Kokozek interfluve using the VES data revealed a number of fine-grained lenticular structures of the ancient lakes in the Chuya depression, which are hidden beneath a cover of fluvioglacial and lacustrine bouldery pebbles and moraine diamictons.     

Seismotectonic deformations and stress fields in the fault zone of the 2003 Chuya earthquake, Ms = 7.5, Gorny Altai

The seismotectonic deformations related to the Chuya earthquake September 27, 2003 in the Gorny Altai (M_s = 7.5) are studied in detail. These deformations developed as advanced systems of R-and R’-shears, gash fractures, and compression structural features in loose sediments. In bedrocks, the older shear zones were reactivated, the previously existing fractures were renewed and propagated further, and new faults and crush zones were formed. The system of seismic dislocations is a fault zone no less than 4 km wide that extends in the northwestern direction. As follows from the structural elements that reveal a systematic mutual orientation, the internal structure of this zone is typical of a right-lateral strike-slip fault. The initial stress field that led to the development of the entire assemblage of seismotectonic deformations related to the Chuya earthquake corresponds to the strike-slip type with the NNW, almost meridional direction of compression axis (σ₁) and the ENE, almost latitudinal direction of the tension axis (σ₃). The local variations of the stress state were expressed in an insignificant shift of σ₁ to the northwest or northeast, in the short-term change of relative stress values with retention of their spatial orientation, and in the increasing inclination of σ₁ in front of the previously existing fault. The comparison of the internal structure of the seismotectonic fault zone with a tectonophysical model of faulting in large continental systems with a right-lateral offset indicates that the distribution of the advanced faults corresponds to the late stage of faulting, when the main fault is still not formed completely, but its particular segments are already developed distinctly. It is shown that at high rates of displacement the structural features in markedly different rocks develop according to the general laws of solids’ deformation even near the day surface.     

Features of magmatim-related metallogeny of Gorny Altai and Rudny Altai (Russia)

The Rudny Altai and Gorny Altai regions had different geologic histories and differ in metallogenic patterns. The Vendian-Early Cambrian to Permian-Triassic multistage evolution of Gorny Altai included subduction, accretion-collision, and rifting events accompanied by magmatism and related mineralization. Metallogeny evolved in discrete pulses, with especially abundant Late Paleozoic-earliest Mesozoic mineralization. The Devonian-Carboniferous pulse produced diverse mineral deposits (iron, mercury, gold, silver, molybdenum, tungsten, cobalt, polymetallic ores, and rare earths), some of considerable economic value. The territory of Gorny Altai includes several large ore districts that belong to different zones. They are the Beloretsk-Kholzun iron district in the west, the Kayancha-Sinyukha fluorine-gold district in the northeast, the Kurai gold-mercury and Yustyd rare-metal-silver districts in the southeast, and the Kalguty rare-metal-tungsten and Ulandryk U-REE-Cu districts in the south. The largest mineral deposits are Kholzun (Fe, P₂O₅), Karakul (Co, Bi), Sinyukha (Au), Aktash and Chagan-Uzun (Hg), Ozernoe and Pogranichnoe (Ag), Kalguty (Mo, W), Alakha (Li, Ta), Rudnyi Log (Y,Fe-specularite), and Urzarsai (W-scheelite). Mineralization in Rudny Altai is mainly pyritic: copper-pyrite, pyrite-polymelallic ore, and barite-polymelallic ore. It resides in suprasubduction basalts and rhyolites and in Emsian to Frasnian island-arc volcanics at different stratigraphic levels of Devonian volcanosedimentary sequences in six ore districts. The Kurchum high-grade metamorphic block hosts copper-pyrite and gold-quartz mineralization related to Hercynian volcanism.     

Geometry of the fault zone of the 2003 Ms = 7.5 Chuya earthquake and associated stress fields, Gorny Altai

The co-seismic deformations produced during the September 27, 2003 Chuya earthquake (Ms = 7.5) that affected the Gorny Altai, Russia, are described and discussed along a 30 km long segment. The co-seismic deformations have manifested themselves both in unconsolidated sediments as R- and R′-shears, extension fractures and contraction structures, and in bedrock as the reactivation of preexisting schistosity zones and individual fractures, as well as development of new ruptures and coarse crushing zones. It has been established that the pattern of earthquake ruptures represents a typical fault zone trending NW–SE with a width reaching 4–5 km and a dextral strike–slip kinematics. The initial stress field that produced the whole structural pattern of co-seismic deformations during the Chuya earthquake, is associated with a transcurrent regime with a NNW–SSE, almost N–S, trending of compressional stress axis (σ₁), and a ENE–WSW, almost E–W, trending of tensional stress axis (σ₃). The state of stress in the newly-formed fault zone is relatively uniform. The local stress variations are expressed in insignificant deviation of σ₁ from N–S to NW–SE or NE–SW, in short-term fluctuations of relative stress values in keeping their spatial orientations, or in a local increase of the plunge angle of the σ₁. The geometry of the fault zone associated with the Chuya earthquake has been compared with the mechanical model of fracturing in large continental fault zones with dextral strike–slip kinematics. It is apparent that the observed fracture pattern corresponds to the late disjunctive stage of faulting when the master fault is not fully developed but its segments are already clearly defined. It has been shown that fracturing in widely different rocks follows the common laws of the deformation of solid bodies, even close to the Earth surface, and with high rates of movements.     

The First Data on Late Pleistocene Ostracods from the Kurai Depression (Gornyi Altai)

Doklady Earth Sciences - In the sections of Late Pleistocene deposits of the Kurai Depression of Gornyi Altai studied, an ostracod assemblage, including Leucocythere sp. 1, L. sp. 2, Leucocythere...     

Late Pleistocene-Holocene coseismic deformations in the Malyi Yaloman River Valley (Gorny Altai)

Palaeoseismological studies were performed within the Yaloman graben (Gorny Altai). Five Quaternary sections with coseismic deformation structures (seismites) have been recognized in the lower course of the Malyi Yaloman River. Traces of ancient earthquakes are localized at two levels (Late Pleistocene-Holocene). The most likely mechanisms of the seismite formation are brittle failure, liquefaction, and fluidization. The types of coseismic deformations and their sizes suggest that the Yaloman graben was the locus of prehistoric earthquakes with M > 5–7, although modern-day seismic activity consists of smaller-magnitude earthquakes. This should be taken into account in assessing the seismic hazards during construction of gas pipeline to China and tourism infrastructure facilities.     

Fault structures and their geoelectric parameters in the epicentral zone of the 27 September 2003 Chuya earthquake (Gorny Altai) from resistivity data

This paper presents an integrated measurement technique based on DC methods (vertical electrical sounding, electrical resistivity tomography) which was used to identify faults and determine their geoelectric parameters in the western part of the Chuya basin. New information on the structure of the Chagan River valley located in the zone of the disastrous 27 September 2003 Chuya earthquake has been obtained from the results of these methods. Geoelectric cross-sections of the sedimentary sequence and the upper part of the basement were obtained from VES data, showing the block structure of the study area. Electrical resistivity tomography sections confirm the presence of a major fault between basement blocks of different heights and indicate the presence of faults bounding the valley on its right side and in the southwestern part.     

Chronology (SHRIMP II) of magmatism in the Kalguty rare-metal-tungsten-molybdenum ore-magmatic system,Gorny Altai,Russia

The Kalguty ore-magmatic system (OMS) contains economic greisen and vein rare-metal-tungsten-molybdenum mineralization. The data on U-Pb zircon (SHRIMP II) age of nine samples of igneous rocks from the Kalguty OMS are accompanied by chemical, ICP-MS, and ICP-AES analyses of the main rock varieties. Porphyritic biotite granite of the main phase of the Kalguty pluton is characterized by the concordant age of 207.5 ± 1.7 Ma (MSWD = 0.034). The concordant age of the leucogranitic dikes pertaining to the East Kalguty Complex is 204 ± 2 Ma (MSWD = 0.65) for elvan and 200.8 ± 1.1 Ma (MSWD = 0.72) for ultrapotassic rhyolite porphyry. The two-mica and muscovite leucogranite of the Eastern stock is significantly younger: 195 ± 2.7 Ma (MSWD = 0.076) and 193.1 ± 2.1 Ma (MSWD = 0.0009). Thus, the Chingadatui Complex (the main phase of the Kalguty pluton) and dikes of the East Kalguty Complex are Late Triassic in age. The two-mica leucogranites of the Eastern, Zhumaly, and other stocks are most likely correlated to the Early Jurassic Alacha Complex of rare-metal granites. The superposition of greisen mineralization on elvan and ongonite dikes may be related to the emplacement of younger, Early Jurassic ore-forming two-mica and muscovite granites. Judging from zircon xenocrysts in granites, the Mesoproterozoic igneous rocks dated at 1.5 Ga and products of erosion of the rocks dated at 1.7 and 2.5 Ga occur in the basement of the Kalguty volcanic-tectonic structure. This is sound evidence for the occurrence of ancient continental crustal blocks in the southwestern part of the Altai-Sayan region.     

A Vendian-Cambrian Island Arc System of the Siberian Continent in Gorny Altai (Russia, Central Asia)

An extended Vendian-Cambrian island-arc system similar to the Izu-Bonin-Mariana type is described in the Gorny Altai terrane at the margin of the Siberian continent.

Three different tectonic stages in the terrane are recognized. (1) A set of ensimatic active margins including subducted oceanic crust of the Paleo-Asian ocean, the Uimen-Lebed primitive island arc, oceanic islands and seamounts: the set of rocks is assumed to be formed in the Vendian. (2) A more evolved island arc comprising calc-alkaline volcanics and granites: a fore-arc trough in Middle-late Cambrian time was filled with disrupted products of pre-Middle Cambrian accretionary wedges and island arcs. (3) Collision of the more evolved island arc with the Siberian continent: folding, metamorphism and intrusion of granites occurred in late Cambrian-early Ordovician time.

In the late Paleozoic, the above-mentioned Caledonian accretion-collision structure of the Siberian continent was broken by large-scale strike-slip faults into several segments. This resulted in the formation of a typical mosaic-block structure.     

Active Faults and Late Holocene Surface Rupturing Earthquakes in the Kokorya Basin (Gorny Altai,Russia)

Doklady Earth Sciences - A system of 22 km long surface rupture produced by paleoearthquakes has been mapped for the first time along the Kubadru Fault which delineates the Kokorya Basin in the...     

Middle Paleozoic Rhyolite of the Gorny and Rudny Altai: Geochronology and Composition

Doklady Earth Sciences - The paper reports the results of geological, geochemical, and isotope–geochronological studies of subvolcanic rhyolites of NW Gorny Altai and Rudny Altai, which...