Tectonics and geodynamics of Gorny Altai and adjacent structures of the Altai–Sayan folded area

Packages of Late Paleozoic tectonic nappes and associated major NE-trending strike-slip faults are widely developed in the Altai–Sayan folded area. Fragments of early deformational phases are preserved within the Late Paleozoic allochthons and autochthons. Caledonian fold-nappe and strike-slip structures, as well as accompanying metamorphism and granitization in the region, are typical of the EW-trending suture-shear zone separating the composite Kazakhstan–Baikal continent and Siberia. In the Gorny Altai region, the Late Paleozoic nappes envelop the autochthon, which contains a fragment of the Vendian–Cambrian Kuznetsk–Altai island arc with accretionary wedges of the Biya–Katun’ and Kurai zones. The fold-nappe deformations within the latter zones occurred during the Late Cambrian (Salairian) and can thus be considered Salairian orogenic phases. The Salairian fold-nappe structure is stratigraphically overlain by a thick (up to 15 km) well-stratified rock unit of the Anyui–Chuya zone, which is composed of Middle Cambrian–Early Ordovician fore-arc basin rocks unconformably overlain by Ordovician–Early Devonian carbonate-terrigenous passive-margin sequences. These rocks are crosscut by intrusions and overlain by a volcanosedimentary unit of the Devonian active margin. The top of the section is marked by Famennian–Visean molasse deposits onlapping onto Devonian rocks. The molasse deposits accumulated above a major unconformity reflects a major Late Paleozoic phase of folding, which is most pronounced in deformations at the edges of the autochthon, nearby the Kaim, Charysh–Terekta, and Teletskoe–Kurai fault nappe zones. Upper Carboniferous coal-bearing molasse deposits are preserved as tectonic wedges within the Charysh–Terekta and Teletskoe–Kurai fault nappe zones.Detrital zircon ages from Middle Cambrian–Early Ordovician rocks of the Anyui–Chuya fore-arc zone indicate that they were primarily derived from Upper Neoproterozoic–Cambrian igneous rocks of the Kuznetsk–Altai island arc or, to a lesser extent, from an Ordovician–Early Devonian passive margin. A minor age population is represented by Paleoproterozoic grains, which was probably sourced from the Siberian craton. Zircons from the Late Carboniferous molasse deposits have much wider age spectra, ranging from Middle Devonian–Early Carboniferous to Late Ordovician–Early Silurian, Cambrian–Early Ordovician, Mesoproterozoic, Early–Middle Proterozoic, and early Paleoproterozoic. These ages are consistent with the ages of igneous and metamorphic rocks of the composite Kazakhstan–Baikal continent, which includes the Tuva-Mongolian island arc with accreted Gondwanan blocks, and a Caledonian suture-shear zone in the north. Our results suggest that the Altai–Sayan region is represented by a complex aggregate of units of different geodynamic affinity. On the one hand, these are continental margin rocks of western Siberia, containing only remnants of oceanic crust embedded in accretionary structures. On the other hand, they are represented by the Kazakhstan–Baikal continent composed of fragments of Gondwanan continental blocks. In the Early–Middle Paleozoic, they were separated by the Ob’–Zaisan oceanic basin, whose fragments are preserved in the Caledonian suture-shear zone. The movements during the Late Paleozoic occurred along older, reactivated structures and produced the large intracontinental Central Asian orogen, which is interpreted to be a far-field effect of the colliding East European, Siberian, and Kazakhstan–Baikal continents.     

2D model of the crust and uppermost mantle along rift profile, Siberian craton

A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km.     

Early evolution of the Paleoasian ocean: LA-ICP-MS dating of detrital zircon from Late Precambrian sequences of the southern margin of the Siberian craton

We present U–Pb (LA-ICP-MS) data on detrital zircon from the Late Precambrian terrigenous rocks of the Baikal Group and Ushakovka Formation, western Cisbaikalia (southern flank of the Siberian craton). The sources of clastic material for the studied sediments are interpreted. The youngest group of detrital zircon grains from the upper Baikal Group and Ushakovka Formation permits assigning these sediments to the Vendian. The lack of Mesoproterozoic detrital zircon in most of the analyzed samples confirms the hypothesis of a global (~ 1 Gyr) break in endogenic activity within the southern flank of the Siberian craton through the Precambrian. The abundance of Neoproterozoic zircon in sandstones from the upper horizons of the Baikal Group and the Ushakovka Formation might be due to the shrinkage of the ocean basin as a result of the convergence of the craton with the microcontinents and island arcs within the Paleoasian ocean.     

Petroleum Habitat of East Siberia,Russia

East Siberia comprises three petroleum provinces—Lena-Tunguska, Lena-Vilyuy, and Yenisey-Anabar—that occupy the area of the Siberian craton. Petroleum has been generated and has accumulated in Precambrian rifts beneath the sedimentary basins and, more importantly, within the section of the basin itself. The platformal deposits of the basins extend beneath overthrusts on the east and south and are covered by sedimentary rocks of the West Siberian overthrusts on the east and south and are covered by sedimentary rocks of the West Siberian province on the west. Permafrost and gas hydrate deposits are present throughout most of East Siberia.

In the Lena-Tunguska province, rifts that developed during Riphean time are filled by thick sedimentary rocks, in which petroleum deposits have formed. In Early Cambrian time a barrier reef extended across the East Siberian craton from southeast to northwest. A lagoon to the west of this reef was the site of thick rhythmic salt deposits, which are the main seal for petroleum in the province. The sedimentary section of the platform cover ranges in age from Late Proterozoic to Permian. More than 25 oil and gas fields have been discovered in the province, all in Riphean through Lower Cambrian rocks.

The Lena-Vilyuy province includes the Vilyuy basin and the Cis-Verkhoyansk foredeep. During Middle Devonian time, a rift formed along the axis of what was to become the Vilyuy basin. This rift is filled by Upper Devonian and Lower Carboniferous basalt, elastics, carbonates, and evaporites. During this rift stage the region that was to become the Cis-Verkhoyansk foredeep was an open geosynclinal sea. The sedimentary cover consists of Permian, coal-bearing sedimentary rocks as well as elastics from the Lower Triassic, Lower Jurassic, Lower Cretaceous, and Upper Cretaceous, the latter only in the Vilyuy basin. In the Lena-Vilyuy petroleum province as many as nine gas and gas-condensate fields have been discovered.

The Yenisey-Anabar province is largely an extension of the West Siberian petroleum province. Permian sedimentary rocks are present only in the east, where they consist of elastics and some salt. The Triassic, Jurassic, and Cretaceous each are represented by thick clastic deposits. Total thickness of the sedimentary cover is up to 15 km on the west and 8 km on the east. Twelve gas and gas-condensate fields have been discovered in the western part of the province.     

Lithogeochemistry of rocks of the Upper Neopleistocene Ust-Oda reference section in the Irkutsk Amphitheater of the Siberian Platform

The results of study of rocks of the Ust-Oda section, one of the key outcrops of Upper Neopleistocene sediments in the Irkutsk Amphitheater of the Siberian Platform, are presented. It is shown that primary sediments were deposited in the Neopleistocene under subarid conditions and relatively high sedimentation rates. Similar contents of trace elements in all rock lithologies suggest that the provenance was unaltered during sedimentation. It is shown that variation in the sediment transport mechanism is one of the signs of changes in the paleoclimatic setting. Climatic cyclicity of the Ust-Oda section formation and succession of sedimentation setting, which were reconstructed on the basis of grain size data, correspond to variations in the petrochemical correlations and geochemical coefficients.     

Latest erosional incision in river valleys of southern East Siberia

Examples of the geological and geomorphic framework of river valleys in the Tunka rift basin (Baikal rift system) and in the Irkutsk amphitheater (Siberian craton) have been used to show that horizontal and vertical motions of tectonic units in southern East Siberia are superposed with periodic movements. In the latter, the waves of slow uplift are attendant with erosional incision events, whereas during the subsidence cycles the incised valleys become filled with mainly alluvial sediments. The latest incision events in the area occurred in the past 70 kyr.     

Isotopic-geochemical features of rocks of the Middle-Upper Cambrian Verkhnyaya Lena formation in the Siberian Craton

Isotopic compositions of carbon and oxygen in carbonates and sulfur in sulfates of the Verkhnyaya Lena Formation (ε₂–ε₃), which terminates the Cambrian section of the Irkutsk Amphitheater of the Siberian Craton, are studied. Sulfates of the Verkhnyaya Lena Formation are marked by unusually low δ³⁴S values (4.6–12.0‰) relative to sulfates of the underlying Angara Formation. This is likely caused by variations in the facies-paleogeographic sedimentation at the transition of the Angara and Verkhnyaya Lena formations, as well as associated variations in the water and salt alimentation budget in sedimentation basins, due to their isolation from open sea and intensification of the continental and underground discharge. The δ¹⁸O(PDB) value in carbonates decreases from ?4.4‰ at bottom to ?10.4‰ at top, reflecting variation in postsedimentary transformations and probable continuous freshening of sedimentation basin. Isotopic composition of carbon in most samples shows normal marine δ¹³C values (0 ± 1‰). Only in some samples, does the δ¹³C value increase up to ?3.8 and 2.2‰ due to specific features of postsedimentary processes. The Rb-Sr systems of the clayey component of marls from the 500-m-thick section of the Angara Formation and bottom of the Verkhnyaya Lena Formation record an age of 512 ± 10 Ma, which is close to the assumed stratigraphic age of the Verkhnyaya Lena Formation. The ⁸⁷Sr/⁸⁶Sr initial ratio is 0.7082 ± 0.0004.     

Kinematic reconstruction of the Early Caledonian accretion in the southwest of the Siberian paleocontinent based on paleomagnetic results

The kinematics of the Early Caledonian accretion process in the southwest (in modern coordinate) of the Siberian paleocontinent, and the structure of its active continental margin are debatable subjects. This paper contains a generalization of paleomagnetic data on island-arc terranes of the territories of the Altai–Sayan and Baikal–Vitim folded areas for the Late Vendian/Cambrian–Early Ordovician time interval, obtained mostly with the author’s participation during the last two decades. The large accumulated database finally allows one to find unambiguously interpretable patterns in the distribution of paleomagnetic poles for the analyzed terrane system and to justify numerically the kinematics of the Early Caledonian accretion. In particular, the analysis of paleomagnetic data proves our idea stating that the transformation of the active continental margin in the Cambrian consisted in its breakup and segmentation as well as in the detachment of fragments of the initially whole island arc along a system of sinistral strike-slips during the clockwise rotation of the craton and conform drift of the continental and oceanic lithospheric plates. It also validates the mostly oblique conditions of the subduction and subsequent accretion, which means a subduction-transform mode on the ocean–continent margin. We propose a complemented version of the paleotectonic reconstruction for the Cambrian evolution of the Siberian continent western margin, based on the kinematic scheme constructed from paleomagnetic data.     

Quartz-albite-microcline metasomatic rocks in the Main Sayan Fault Zone: Evolution of metasomatism and composition of accessory minerals

Quartz-albite-microcline metasomatic rocks (qualmites) localized in the Main Sayan Fault Zone at the boundary of Irkutsk oblast and Buryatia (Eastern Sayan, middle reaches of the Kitoi River) were formed after dynamometamorphic biotite and biotite-amphibole granite gneisses. The zone of alkaline metasomatism up to 250–300 m thick extends for about 12 km along the Main Sayan Fault. Riebeckite-aegirine and hornblende-clinopyroxene qualmites were formed during the early alkaline stage of metasomatism. Biotite-magnetite qualmites, which are occasionally superimposed on riebeckite-aegirine rocks, are products of the following stage of increasing acidity. The ⁴⁰Ar/³⁹Ar age of amphibole from metasomatic rocks is 321 ± 5 Ma. The metasomatic rocks are enriched by 2–4 times in Zr, Nb, Y, REE, Be, Th, and U relative to unaltered granite gneisses and contain rare-metal mineralization (fergusonite-(Y), betafite, Nb- and Y- bearing titanites, gadolinite-(Y), zircon, thorite, allanite-(Ce), chevkinite-(Ce), etc.). The composition of accessory minerals was studied on a LEO-1430VP SEM. Their composition and mineral assemblages show that meta-somatic alteration and Ti-Nb-Zr-REE mineralization were formed synchronously. The stage of acid solution neutralization was characterized by crystallization of epidote and andradite and by replacement of chevkinite with allanite and titanite in metasomatic rocks. Hydro- and fluorcarbonates of LREE, phosphates (monazite-(Ce)), and fluorides (F-bearing thorianite?) were formed during the final low-temperature stage.     

Provenance change in the Irkutsk coal basin during the early and middle Jurassic: Geochemical and Sm-Nd isotope evidence

On the basis of the recently accepted chronostratigraphic chart, Jurassic sedimentation in the Irkutsk coal basin took place over a short time interval (~23 My), from the Pliensbachian (~191 Ma) to the Aalenian (~170 Ma). In this study, we present geochemical and Sm-Nd isotope data for sedimentary rocks of the Prisayan and Kuda formations and those in the upper course of the Angara River, which were deposited over an even shorter time interval (from ~174 to 170 Ma), as indicated by new data on biostratigraphy. Our results suggest that a greater contribution from Transbaikalia compared to that from the Siberian Platform during sediment deposition in the Irkutsk coal basin may reflect the onset of mountain building in Transbaikalia and reorganization of the river drainage network during the Middle Jurassic.     

大巴山地区水系沉积物地球化学特征及找矿方向

大巴山地区跨秦岭、扬子两个一级构造单元,基于1:5万水系沉积物测量成果,统计计算了成矿元素地球化学参数,分析研究了元素分布特征、元素相关性及异常元素组合特征,认为研究区Pb、Zn、Ba、Cu、Mn找矿潜力较大,推断研究区所在的秦岭构造单元成矿类型主要为与震旦系—下寒武统黑色岩系有关的低温热液锌铜矿和沉积型钡矿;研究区所在的扬子构造单元成矿类型主要为与寒武系碳酸盐地层有关的低温热液铅锌矿和与震旦系碳酸盐岩有关的沉积型锰矿;在此基础上,划分出杨家坪—葛洞钡锌成矿远景区、镇坪钡锌成矿远景区、黄安坝—迷魂阵锌铜成矿远景区、百羊坪铅锌成矿远景区和修齐镇—石坊乡锰成矿远景区。     

Belt zoning of sedimentation in the Angara cascade reservoirs

Analysis of genetic and compositional types of bottom deposits, rates of their sedimentation and accumulation, and thickness of sedimentary bed in the Angara cascade reservoirs made it possible to reveal for the first time the vertical belt zoning in one of the largest cascades of the artificial reservoirs. The belt zoning is determined by sedimentation conditions, including petrographic and lithological-geochemical composition of the rocks of the water drainage area, hydrodynamic parameters, and bottom topography of the cascade reservoirs. The vertical zonal distribution in the Angara cascade reservoirs consists in the relation of grain-size sedimentary fractions, their constituent minerals, and geochemical fields to definite elements of bottom topography: morphodynamic belts, main of which are coastal bank, underwater slope of coastal bank, flooded terraces, and flooded Angara River bed.     

Glacier-dammed lake in Southeastern Tibetan Plateau during the Last Glacial Maximum

Lacustrine, alluvial terraces and sediments record at least one Holocene glacially dammed lake in Songzong Basin immediately upstream of the Purlung TsangpoRiver, a main tributary of the downstream of Yarlung Tsangpo River, at the northeastern syntaxis of the Himalaya. The lacustrine deposit is more than 88 meters thick at the SongzongLandslide. There is an 18.33 meters thick layer of lacustrine silt within the lacustrine terrace. The Optical Simulated Luminescence (OSL) ages at the bottom and top of the lacustrine silt layerare 22.5±3.3ka B.P., and 16.1±1.7ka B.P., respectively, which indicates that the lacustrine deposits were formed during the Last Glacial Maximum ranging from 25kaB.P. to 15kaB.P.The ancient shorelines and the lake erosion zone confirm the preexistence of Songzong Lake. There are also terraces formed by moraines in the Songzong Basin. The high and large moraineterrace seen near the mouth of the Dongqu Valley is very prominent. The special characteristics of thelacustrine sediment and the ancient lake line in the Songzong Basin indicate that the lacustrinesediments are related to the blocking of the Purlung Tsangpo River by a glacier from DongquValley during Last Glacial Maximum.     

Sources of clastic material of the Neoproterozoic metasedimentary rocks of the Baikal–Patom belt,Northern Transbaikalia: Evidence from Sm-Nd isotope data

First systematic data on the variations of ε_Nd(T) in the Neoproterozoic sedimentary sequence of the Baikal–Patom fold belt (Northern Transbaikalia) are reported. The range of obtained εNd(T) is–19.4…–2.0. It was established that the rocks of the Ballaganach and most part of the Dal’nyaya Taiga groups are characterized by ε_Nd(T) from–19.4 to–16.3. Upsection, beginning from the Khomolkha Formation, the rocks show a sharp change in the initial Nd isotope composition (ε_Nd(T)–8.3…–2.0). The results of Sm-Nd study of metasedimentary rocks of the BPB, as well as the comparison of their Sm-Nd characteristics with those of the inferred source areas suggest that the input of terrigenous material at the early stage of sedimentation in the Baikal–Patom belt was mainly related to the destruction of the Early Proterozoic crust of the Siberian craton. Owing to a change of sedimentation setting from passive continental margin to the “foreland” basin at the late stage, the terrigenous material of the Siberian craton was diluted by clastics from juvenile Neoproterozoic crust of the Baikal–Muya belt.     

Moho depths and three-dimensional velocity structure of the crust and upper mantle beneath the Baikal region,from local tomography

We studied the 3D velocity structure of the crust and uppermost mantle beneath the Baikal region using tomographic inversion of ∼25,000 P and S arrivals from more than 1200 events recorded by 86 stations of three local seismological networks. Simultaneous iterative inversion with a new source location algorithm yielded 3D images of P and S velocity anomalies in the crust and upper mantle, a 2D model of Moho depths, and corrections to source coordinates and origin times. The resolving power of the algorithm, its stability against variations in the starting model, and the reliability of the final results were checked in several tests. The 3D velocity structure shows a well-pronounced low-velocity zone in the crust and uppermost mantle beneath the southwestern flank of the Baikal rift which matches the area of Cenozoic volcanism and a high velocity zone beneath the Siberian craton. The Moho depth pattern fits the surface tectonic elements with thinner crust along Lake Baikal and under the Busiyngol and Tunka basins and thicker crust beneath the East Sayan and Transbaikalian mountains and under the Primorsky ridge on the southern craton border.     

Geochemistry,Tectonic Settings,and Age of Metavolcanic Rocks of the Isakovskii Terrane,Yenisei Range: Indicators of the Early Evolution of the Paleo-Asian Ocean

The geodynamic nature of the Late Neoproterozoic island-arc dacites (691 ± 8.8 Ma) and rift basalts (572 ± 6.5 Ma) of the Kiselikhinskaya Formation, Kutukasskaya Group, in the Isakovskii terrane is inferred from geochemical data and U–Pb zircon (SHRIMP-II) dates. The volcanic rocks were produced during the late evolutionary history of the Yenisei Range, starting at the origin of oceanic crustal fragments and their accretion to the Siberian craton to the postaccretionary crustal extension and the onset of the Caledonian orogenesis. The reproduced sequence of geological processes marks the early evolution of the Paleo- Asian Ocean in its junction zone with the Siberian craton. The data refine the composition and age of volcanic rocks in the trans-Angara part of the Yenisei Range and specifics of the Neoproterozoic evolution of the Sayan–Yenisei accretionary belt.     

扬子陆块西缘寒武系砂岩的物源分析：对古地理位置重建和构造背景的指示

扬子陆块西缘寒武系主要为一套碎屑岩- 碳酸盐岩的岩石组合,前人研究多认为形成于相对稳定的克拉通盆地。但同时期出现的大陆岩浆作用显然与前期认定的克拉通盆地性质不符,需要借助扬子西缘的物质来源探讨构造背景。基于野外露头等资料,本文通过对扬子陆块西缘会泽和会东附近寒武系3件砂岩样品进行重矿物分析、电气石电子探针和碎屑锆石U- Pb测年分析,确定扬子西缘寒武纪沉积物的源区;并结合沉积序列等综合探讨扬子陆块西缘寒武纪的构造背景。沉积序列表明,扬子西缘寒武系沧浪铺组、西王庙组和二道水组主要由砂岩和白云岩等组成,沉积环境为滨岸—潮坪。细—粗砂岩碎屑颗粒为次棱角状—次圆状,分选较差;碎屑组分主要为石英,岩屑几乎全部为燧石,长石含量较少。测试分析结果表明：重矿物分析指示扬子西缘寒武系砂岩重矿物主要由锆石、赤—褐铁矿、电气石、钛铁矿、金红石、磷灰石等组成,重矿物组合指示岩浆岩为其主要母岩;电气石电子探针分析结果表明,物源主要来自于贫锂花岗岩和变砂岩、变泥岩;碎屑锆石测年分析表明物源区母岩主要为983~540 Ma岩浆岩。碎屑锆石年龄对比等综合分析表明,寒武系沉积物部分源自康滇古陆983~708 Ma的岩浆岩和变沉积岩,部分源自冈瓦纳大陆东非造山带663~540 Ma的岩石,物源区岩石经历再旋回产物作用。扬子西缘寒武系的沉积序列、碎屑锆石年龄谱图和碎屑组成等特征综合分析表明,扬子陆块西缘寒武系形成于前陆盆地。     

Early Carboniferous paleogeography of the northern Verkhoyansk passive margin as derived from U–Pb dating of detrital zircons: role of erosion products of the Central Asian and Taimyr–Severnaya Zemlya fold belts

The first U–Pb dating of detrital zircons from the Lower Carboniferous sandstones in the frontal part of the northern Verkhoyansk fold-and-thrust belt showed that detrital zircon age spectra for the Lower Visean (Krestyakh Formation) and the Upper Visean–Serpukhovian (Tiksi Formation) rocks are quite different. The Early Visean sandstones contain up to 95% detrital zircons of Precambrian age, while those of Late Visean–Serpukhovian age, only 55%. The shape of age distribution plots of Precambrian zircons for both samples is similar, indicating that reworking of terrigenous sediments of the Krestyakh Formation or the same sources dominated in Early Visean time (crystalline basement of the craton, eroded Meso- and Neoproterozoic sedimentary complexes, and igneous rocks of Central Taimyr) contributed significantly to the accumulation of the Late Visean–Serpukhovian deposits. In the rocks of the Tiksi Formation, 45% of detrital zircons are of Paleozoic age, while 24% are Early Paleozoic, with prevailing Cambrian and Ordovician ages. Possible provenance areas with abundant igneous rocks of this age could be the Taimyr–Severnaya Zemlya and Central Asian fold belts extending along the northern, western or southwestern margins of the Siberia. The presence of Middle–Late Devonian zircons is thought to be related to the erosion of granitoids of the Yenisei Ridge and the Altai–Sayan region. Early Carboniferous detrital zircons probably had a provenance in igneous rocks of the Taimyr–Severnaya Zemlya fold belt, on the assumption that collision between the Kara block and the northern margin of the Siberian continent had already occurred by that time. In Early Visean time, sedimentation occurred in small deltaic fans, likely along steep fault scarps that formed as a result of Middle Paleozoic (Devonian–Carboniferous) rifting. The clastic material came from small rivers that eroded the nearby area. Late Visean–Serpukhovian time was marked by a sharp increase in the amount of clastic material and by the appearance of detrital zircons coming from new provenance regions, such as fold belts extending along the northern and southwestern margins of the Siberian continent. A large river system, which was able to transport clastic material over large distances to deposit it in submarine fans on the northern Verkhoyansk passive continental margin, had already existed by that time.     

The Vendian passive continental margin in the southern Siberian Craton: geochemical and isotopic (Sr,Sm–Nd) evidence and U–Pb dating of detrital zircons by the LA-ICP-MS method

Geochemical and isotopic (Sm–Nd and Sr) studies of deposits of the Baikal and Oselok Groups in the southern Siberian Craton and LA-ICP-MS U–Pb dating of detrital zircons show that they accumulated in passive continental-margin settings in the Vendian. The time limits of sedimentation were assessed on the basis of Sr chemostratigraphy of carbonate deposits of the Baikal Group and LA-ICP-MS U–Pb dating of detrital zircons in first-cycle terrigenous deposits of the Oselok Group. The main provenances for rocks of these groups were constant. These were rocks of the cover and basement of the Siberian Craton. Tuffite horizons in upper portions of the groups are the only sign of Late Vendian activation of this block, which is reflected in changes of geochemical indices of terrigenous rocks and their younger Sm–Nd model ages.     

Radon activity of faults (western Baikal and southern Angara areas)

We report results of transect radon surveys on twenty six large and small faults of different geometries in the western Baikal and southern Angara (southern Siberian craton) areas. The studied faults show up in the radon field as broad zones of high radon activity concentration, with their widths 1.4 times the widths of faulting-related deformation zones. Other things being equal, radon emanation depends on the size and slip geometry of faults and on their activity in the Cenozoic. Relative radon activity is higher in rift faults than in cratonic ones, in normal relative to strike-slip faults, and in larger and more active faults, the latter factor being the basic geodynamic control of radon emanation.