Yu.A. Zorin

GEODYNAMICS OF THE WESTERN PART OF THE MONGOLIA-OKHOTSK COLLISIONAL BELT, TRANS-BAIKAL REGION (RUSSIA) AND MONGOLIA

Abstract After the western edge of the Mongolian microcontinent joined the Siberian continent in the region of Central Mongolia in the earliest Permian, these two continental blocks remained turned at an angle of about 120° with respect to each other and separated (on greater extent of their present-day boundary) by an enormous gulf of the Paleopacific called the Mongolia–Okhotsk ocean. Closure of this ocean at the Early/Middle Jurassic boundary led to the complete collision of Siberia and Mongolia, which by then had already become part of the Mongolia–North China continent. This main collisional episode, which lasted through the Middle and Late Jurassic, involved thrusting, folding and magmatism and produced the Mongolia–Okhotsk belt. The Onon island-arc, which was located in the Mongolia–Okhotsk ocean, was squeezed between the two major continents. Inasmuch as the third element (the island arc) was involved in the collision it is reasonable to distinguish two branches of the Mongolia–Okhotsk suture. These branches control the spatial distribution of gold mineralization in the Trans-Baikal region. On the southeastern periphery of Siberia the crust thickened considerably after the collision and a plateau-like uplift formed. In the Early Cretaceous, when compression ceased, the collisional uplift collapsed and the thrusts were transformed into low-angle normal faults, the motions on which were responsible for the formation of rift basins and exhumation of metamorphic core complexes.

Baikal rift zone: structure and geodynamics

Abstract The Cenozoic Baikal rift zone is superimposed on the Caledonian Baikal fold belt, representing the suture between the Precambrian Siberian craton and several microcontinents. The Baikal rift zone consists of a system of disconnected fault-bounded basins and extensional and wrench faults that straddles a major arch, having a topographic relief of 2–3 km. Rifting activity commenced during the Oligocene and is still active as evident from high seismicity of the Baikal rift zone. Across the Baikal rift zone, upper crustal extension is considerably smaller than would be expected from its crustal thickness which decreases by 8–11 km across the Baikal basins as compared to the adjacent unextended areas. The discrepancy between upper and lower crustal attenuation could be attributed to intrusion of a major diapir. Indeed, geophysical data indicate that the Sayan-Baikal dome coincides with such a diapir, the top of which is located at the crust/mantle boundary. This intrusion is held responsible for Oligocene and Quaternary doming of the rift zone. Development of the intra-continental Baikal rift system is thought to be related to asthenospheric diapirism (active rifting); however, intra-plate stresses in conjunction with the Himalayan collision may have played some role.

The baikal rift: An example of the intrusion of asthenospheric material into the lithosphere as the cause of disruption of lithospheric plates

Abstract The development of the Baikal rift zone is an initial stage in the break-up of the Eurasian plate. In this zone the anomalous mantle is found under the crust. It is characterized by decreased seismic velocity, decreased density and high temperature. These physical properties suggest that this anomalous mantle is an asthenospheric intrusion some 250–300 km wide. As the Baikal rift represents an extension of the first tens of kilometres, the appearance of the anomalous mantle is not regarded as the result of extension. It is more likely that the asthenospheric intrusion into the lithosphere was the cause of extension and faulting. If intrusion began about 30 m.y. ago and reached the base of the crust about 3 m.y. ago, the calculated temperature field is consistent with the present geophysically determined temperature distribution. Similar anomalous mantle occurs under other structurally similar crustal zones.

Thickness of the lithosphere beneath the Baikal rift zone and adjacent regions

Abstract Methods of separation of gravity anomalies related to mantle density inhomogeneity are developed. Based on the inversion of these anomalies with regard to some constraints obtained from seismic data, the map of lithospheric thickness beneath the Baikal rift zone and adjacent regions was made. The lithospheric thickness beneath the Baikal rift zone is estimated to be 40–50 km, i.e. the lithosphere is thinned here to a crustal thickness. Beneath the Siberian platform the lithospheric thickness increases to 200 km, and underneath the Trans-Baikal region of moderate Cenozoic tectonic activity it ranges from 75 to 160–175 km. Therefore, a wide asthenospheric upwelling was revealed beneath the rift zone. The data available on the configuration of the Baikal depression and that of the crust and the lithosphere as a whole made it possible to estimate the magnitudes of extension at different lithospheric levels. The significant increase in extension with depth implies that rifting in the Baikal zone was caused by asthenospheric diapirism.

The Baikal rift zone: the effect of mantle plumes on older structure

Abstract The main chain of SW–NE-striking Cenozoic half-grabens of the Baikal rift zone (BRZ) follows the frontal parts of Early Paleozoic thrusts, which have northwestern and northern vergency. Most of the large rift half-grabens are bounded by normal faults at the northwestern and northern sides. We suggest that the rift basins were formed as a result of transformation of ancient thrusts into normal listric faults during Cenozoic extension. Seismic velocities in the uppermost mantle beneath the whole rift zone are less than those in the mantle beneath the platform. This suggests thinning of the lithosphere under the rift zone by asthenosphere upwarp. The geometry of this upwarp and the southeastward spread of its material control the crustal extension in the rift zone. This NW–SE extension cannot be blocked by SW–NE compression generated by pressure from the Indian lithospheric block against Central Asia. The geochemical and isotopic data from Late Cenozoic volcanics suggest that the hot material in the asthenospheric upwarp is probably provided by mantle plumes. To distinguish and locate these plumes, we use regional isostatic gravity anomalies, calculated under the assumption that topography is only partially compensated by Moho depth variations. Variations of the lithosphere–asthenosphere discontinuity depth play a significant role in isostatic compensation. We construct three-dimensional gravity models of the plume tails. The results of this analysis of the gravity field are in agreement with the seismic data: the group velocities of long-period Rayleigh waves are reduced in the areas where most of the recognized plumes are located, and azimuthal seismic anisotropy shows that these plumes influence the flow directions in the mantle above their tails. The Baikal rift formation, like the Kenya, Rio Grande, and Rhine continental rifts [Achauer, U., Granet, M., 1997. Complexity of continental rifts as revealed by seismic tomography and gravity modeling. In: Jacob, A.W.B., Delvaux, D., Khan, M.A. (Eds.), Lithosphere Structure, Evolution and Sedimentation in Continental Rifts. Proceedings of the IGCP 400 Meeting, Dublin, March 20–22, 1997. Institute of Advanced Studies, Dublin, pp. 161–171], is controlled by the three following factors: (i) mantle plumes, (ii) older (prerift) linear lithosphere structures favorably positioned relative to the plumes, and (iii) favorable orientation of the far-field forces.

Baikal rift: Active or passive? — comparison of the Baikal and Kenya rift zones

Abstract The comparison of geological and geophysical data from the Kenya and Baikal rift zones permits us to consider the formation of these zones to be associated with the development of wide asthenospheric upwellings beneath them. In this sense, both rifts are active. Differences between them in amounts of volcanics and geological features may be explained by a higher degree of partial melting of asthenospheric material beneath the Kenya rift.

Structure of the lithosphere of the Mongolian-Siberian mountainous province

Abstract The Mongolian-Siberian mountainous province distinguished by Florensov (1978) includes the Sayan-Baikal, the Altai-Sayan regions and East Mongolian high ranges, and those of Mongolian Altai, Goby Altai and Khangai. In the Late Cenozoic all this province was involved in an intensive orogeny, the latter occurring both in the area under extension and in that under compression (the Sayan-Baikal domal uplift and the Altai uplift system, respectively). The study of the deep structure of the mountainous province and of relatively stable adjacent regions must contribute to a more complete understanding of the causes of intracontinental orogeny. The main subject of this study was to map the Moho depth and the thickness of the lithosphere as a whole, based on the interpretation of geophysical data covering the Mongolian-Siberian mountainous province, the Southern Siberian platform and the East Mongolian high plains.

Geodynamic setting of gold deposits in Eastern and Central Trans-Baikal (Chita Region, Russia)

Abstract It is proposed that there are three types of gold deposits in Eastern and Central Transbaikalia (Trans-Baikal province), namely: (i) high-sulphide intrusion-related deposits with some signs of porphyry deposits, (ii) low-sulphide intrusion-related deposits, and (iii) low-sulphide epithermal Au–Ag deposits. Most of the gold deposits belong to the first two types, and their ages are Middle–Late Jurassic. Deposits of the third type are not numerous, and their age is Early Cretaceous. The majority of the gold mineralization is spatially related to the two branches of the Mongolia–Okhotsk suture, along which Siberia collided, at the Early/Middle Jurassic boundary, with the Mongolia–North China continent and the Onon island-arc terrane located between the two continents. Collision-related thrusting, folding and magmatism lasted until the latest Jurassic, when they gave way to post-collisional rifting that continued until the end of Early Cretaceous. According to their age, relation to magmatism and tectonic framework, the intrusion-related deposits (high- and low-sulphide) were formed in a regional collisional setting. Extensional environments at that time existed only in local areas in the roofs of great magmatic chambers. Low-sulphide epithermal deposits were formed during Early Cretaceous post-collisional rifting.

The East Siberia Transect

The new East Siberia transect, constructed by synthesizing recently analyzed geological and geophysical data, runs in a broken line through the vicinities of the towns of Nizhneangarsk, Chita, and Borzya, traversing the Siberian Platform margin and the Baikal and Mongolia-Okhotsk fold areas. The transected region encompasses a number of terranes, most of which once were arc-trench systems with fore-arc and back-arc basins. The island arcs involved Early Precambrian blocks that were sialic cores of some of their islands. The terranes have complex spatial relations in the present tectonic framework, with most of the rocks belonging to magmatic arcs, whereas the associated basins comprise allochthons thrust over continental margins. Accretion of the terranes to the Siberian continent occurred in two stages, the Early Proterozoic and the Early Paleozoic. Middle Paleozoic and Late Paleozoic–Early Mesozoic subduction on the periphery of the Mongolia-Okhotsk ocean, which actually was an enormous guK of the Pacif...

Crustal extension in the Baikal rift zone

Analysis of the gravity field along four profiles crossing the Baikal rift zone permits an estimate of the amount of anomalous mass produced by 1. (1) graben-fill sediments, 2. (2) Moho uplift and intrusion of mantle sills and dikes, 3. (3) an asthenospheric bulge. Crustal extension is evaluated based on the idea of mass and volume balance of material introduced into and removed from the initial volume of the crust. Extension in the Baikal rift increases southwestward from 0.9 km in the Chara depression to 19.3 km in the South Baikal depression. These values generally agree with the position of the Euler pole determined from seismic data (fault plane solutions). Average rotation velocity for the lithospheric plates separated by the rift zone is estimated to be 5.93 × 10-4 rad/m.y. over about 30 m.y.