Peter A. Chekhovich

Analysis is presented for the history of crustal subsidence in three areas of the West Siberian sedimentary basin in the Mesozoic and Early Cenozoic. In the Koltogor-Urengoy Rift, the South Kara Depression, and the southern Gydan Peninsula, the most significant changes in the rate of crustal subsidence occurred in the Cretaceous. During that time, there were repeated periods of rapid acceleration and deceleration of subsidence lasting 20–30 Ma. Variations in the rate of crustal subsidence were quite different in the above regions. In the absence of stretching and with the preservation of isostatic equilibrium, variations in the subsidence rate were associated with changes in the rate of rock contraction in the lower crust due to prograde metamorphism. To sustain metamorphic reactions over 220 Ma, an influx of fluids into the Earth’s crust from the sublithospheric mantle was required. Over several tens of millions of years, these fluids reached the Earth’s crust by passing through the mantle part of the lithosphere ~100 km thick. This indicates that the fluids had surface-active properties and could wet the rock grains in this layer. The influx of hydrogen from the Earth’s outer core could have played a major role in the formation of such fluids.

Most researchers relate the formation of large continental sedimentary basins to strong lithospheric stretching (rifting). The formation of the world’s largest West Siberian Basin is explained by post-rift crustal subsidence after the Permian–Triassic rifting. This subsidence is typically described with a classical pure shear model. The analysis of seismic profiles of a total length of 2500 km showed that this model is inapplicable to the West Siberian Basin. The rate of post-rift crustal subsidence should rapidly and smoothly decrease with time. In the West Siberian Basin, the most rapid subsidence occurred, however, in the Jurassic and Cretaceous, when the post-rift subsidence should almost have been completed, rather than in the Triassic just after the suggested stretching. Moreover, the crustal subsidence in the Mesozoic and Cenozoic was very complex and highly heterogeneous both in time and in space. Far from active plate boundaries, this subsidence with an accumulation of up to 6–7 km of sediments could have been caused only by significant contraction of crustal rocks as a result of prograde metamorphism, which was catalyzed by an influx of surface-active mantle fluids into the crust. The high heterogeneity of subsidence indicates the temporal and, possibly, compositional heterogeneity of this fluid influx.

Based on analysis of taxonomic identifications of previously collected fossils combined with the results of a lithological study, some traditional models are revisited, and the nature of cyclic units in the sections of the western megazone of the Urals is re-interpreted. Lists of ichthyofauna from the Kuba River Formation stratotype (Middle Ural, Silurian, Ludlow) suggest a gap in the transgressive sequence of shelf carbonates. It is expressed as a blended unconformity corresponding to most of the Gorstian Stage. The sedimentary nature of the cyclic units of the Ordovician–Silurian mega-sequence are due to the irregular rate of tectonic subsidence and not related to eustatic fluctuations.

The history of the crustal subsidence in the Mesozoic and Cenozoic in the West Siberian Basin – the largest sedimentary basin in the world – is considered. Most researchers associate its formation with post-rift subsidence of the crust, which followed an episode of strong lithospheric stretching about 250 million years ago near to the Permian to the Triassic transition. A characteristic feature of post-rift subsidence is a decrease in its rate in time. During the Mesozoic-Cenozoic history, in Western Siberia the rate of crustal subsidence should have slow down by an order of magnitude. However, the analysis of long (700–900 km) seismic profiles in the north of Western Siberia and in the Southern Kara Sea shows that since the beginning of the Mesozoic in these regions, on average, there has been an acceleration of the crustal subsidence. Under such circumstances, lithospheric stretching in them could be responsible for only a small part of the total subsidence of the crust of 6–7 km. In Western Siberia, the Earth’s crust is close to the isostatic equilibrium. Then, in the absence of strong stretching, the accumulation of thick sedimentary sequences in the basin could only have been caused by rock contraction in the lower crust due to prograde metamorphic reactions. To obtain the above results, we used, for the first time, some simple methods to analyze the structure of the sedimentary cover in the West Siberian Basin. Detailed seismic profiles have been published for many other deep basins on all the continents. The methods of their interpretation implemented in this paper can be easily applied to determine the role of lithospheric stretching in the formation of deep sedimentary basins on the global scale.

The history of the crustal subsidence in the Mesozoic and Cenozoic in the West Siberian Basin – the largest sedimentary basin in the world is considered. Most researchers associate its formation with post-rift crustal subsidence, which followed an episode of strong lithospheric stretching about 250 Ma ago at the Permian–Triassic transition. A characteristic feature of post-rift subsidence is a decrease in its rate in time. During the Mesozoic–Cenozoic history, the crustal subsidence rate in Western Siberia should have decreased by an order of magnitude. However, the analysis of long (700–900 km) seismic profiles in the north of Western Siberia and in the Southern Kara Sea shows that, on average, there has been an acceleration of the crustal subsidence since the beginning of the Mesozoic in these regions. Under such conditions, lithospheric stretching in them could be responsible for only a small fraction of the total crustal subsidence by 6–7 km. In Western Siberia, the crust is close to the isostatic equilibrium. Due to this, in the absence of strong stretching, the accumulation of thick sedimentary sequences in the basin could only have been caused by rock contraction in the lower crust due to prograde metamorphic reactions. To obtain the above results, we used some simple methods for the first time to analyze the structure of the sedimentary sequences in the West Siberian Basin. Detailed seismic profiles for many other deep basins on all the continents have been published. The methods of their interpretation implemented in the present paper can be easily applied to determine the role of lithospheric stretching in the formation of deep sedimentary basins on the global scale.

Slow crustal subsidence nonuniform in time and space occurred in the sedimentary basin of the Moscow Syneclise during 20 Ma in the Late Devonian. On the cool Precambrian lithosphere of the East European Craton, subsidence of this kind could have occurred only due to rock contraction in the lower crust resulting from prograde metamorphism. Crustal subsidence in the Middle and Late Devonian occurred after a long period of stability. This required the influx of mantle fluids into the lithosphere and then into the crust, which catalyzed the metamorphic reaction. The distribution of crustal subsidence was very heterogeneous, and its pattern repeatedly and radically changed over time. This indicates that the influx of deep fluids into the lithosphere was also highly variable, both in time and in space. There is no evidence of any connection between large basement faults and the pattern of the Late Devonian crustal subsidence. Together with a high variability of the subsidence, this suggests that, over the course of several million years, fluids have penetrated into the crust through an undisturbed mantle lithosphere 150 km thick. This indicates that the fluids had specific properties: they were surface-active and wetted grains of crystalline rocks, spreading as thin films into the inter-grain space. Rapid and frequent changes in the influx of deep fluids into the lithosphere rules out their connection with slow large-scale convective currents in the mantle. The slow, but long-term, extended over tens of millions of years, influx of surface-active mantle fluids into the lithosphere is also quite different from the powerful short-term outbursts of fluids from large mantle plumes approaching the lithospheric layer. The occurrence of crustal subsidence and kimberlite magmatism in a number of cratonic areas remote from each other suggests a widespread influx of deep fluids into the lithospheric layer on the continents.

The sedimentary basin of the Moscow syneclise is located in the central part of the East European craton. It is characterized by detailed drilling coverage and an extremely high degree of geological knowledge. During the Devonian the basin was exposed to laterally non-uniform crustal subsidence after a long period of erosion. Considering the large thickness of the ancient lithosphere, this subsidence regime could only take place here in connection with rock contraction in the lower part of the crust as a result of prograde metamorphism catalyzed by the infiltration of mantle fluids. Their inflow into the lithosphere of the East European craton is evidenced by the wide occurrence of basite and kimberlite magmatism during the Devonian. The lateral distribution of subsidence throughout the syneclise changed significantly over periods of several million years. This indicates the corresponding changes in the rate of inflow of the deep fluids into the ancient lithosphere and their high surface activity. This property of the fluids enabled their rapid infiltration into the mantle lithosphere and inflow into the crust over the entire syneclise. The long-term inflow of the deep fluids into the continental lithosphere, which frequently changed in time and space, is a hitherto unknown phenomenon not related to large-scale convection in the mantle or to the rise of large plumes. It could have been the cause of the formation of many intraplate sedimentary basins on the continents.

A vast area of up to five million square kilometers is located in the Pacific Ocean to the east of Australia. The Earth’s crust up to 10–23 km thick is submerged here to a depth of 1–4 km. This contrasts with the surrounding Southwest Pacific where the crust is submerged up to 5–6 km and has a thickness of 7 km, which is more typical for oceans. As is known from dredging and deep-sea drilling, Zealandia is composed of the continental crust. For a long time, it was close to the sea level and then it subsided into the depths. The same thickness, water depths, and subsidence history are found for some domains in the Central Arctic, Lomonosov Ridge, Podvodnikov Basin, and Mendeleev Ridge. The continental nature of the crust has been proven here by drilling and is suggested by the results of study of the sea floor bedrock. Deep basins of the Central Arctic and Zealandia were formed without intense crustal stretching. Its origin can be explained by the increase in density of gabbroids in the lower crust due to prograde metamorphism.

Rapid glacio-isostatic rebound in Fennoscandia and Canada that is nonuniform in time and space indicates that there is a layer with strongly decreased viscosity at shallow crustal depths. The upper boundary of the layer is near the depth of 15 km, which corresponds to the maximum depth of earthquake hypocenters in the Precambrian cratons of the Kola Peninsula and Karelia. The position of the lower boundary is less distinct; however, most likely it is located near the base of the crust. The formation of such a layer in the Pliocene–Quaternary occurred due to infiltration of a large volume of mantle fluids into the crust. In many regions, this has led to retrograde metamorphism with rock expansion and a strong decrease in rocks viscosity.