No abstract

Consolidated crust in the North Barents basin with sediments 16–18 km thick is attenuated approximately by two times. The normal faults in the basin basement ensure only 10–15% stretching, which caused the deposition of 2–3 km sediments during the early evolution of the basin. The overlying 16 km of sediments have accumulated since the Late Devonian. Judging by the undisturbed reflectors to a depth of 8 s, crustal subsidence was not accompanied by any significant stretching throughout that time. Dramatic subsidence under such conditions required considerable contraction of lithospheric rocks. The contraction was mainly due to high-grade metamorphism in mafic rocks in the lower crust. The metamorphism was favored by increasing pressure and temperature in the lower crust with the accumulation of a thick layer of sediments. According to gravity data, the Moho in the basin is underlain by large masses of high-velocity eclogites, which are denser than mantle peridotites. The same is typical of some other ultradeep basins: North Caspian, South Caspian, North Chukchi, and Gulf of Mexico basins. From Late Devonian to Late Jurassic, several episodes of rapid crustal subsidence took place in the North Barents basin, which is typical of large petroleum basins. The subsidence was due to metamorphism in the lower crust, when it was infiltrated by mantle-source fluids in several episodes. The metamorphic contraction in the lower crust gave rise to deep-water basins with sediments with a high content of unoxidized organic matter. Along with numerous structural and nonstructural traps in the cover of the North Barents basin, this is strong evidence that the North Barents basin is a large hydrocarbon basin.

Sea-level change has been commonly interpreted to be of eustatic origin, and many eustatic events were hypothesized for the Phanerozoic, including several 1–3 Myr long cycles in the Ordovician with magnitudes up to 100 or 200 m. However, sea-level change modeling using stratigraphic data from Northern Estonia, which was an area of slow shallow-marine (<10 m) deposition through most of the Ordovician, indicates fluctuations of no more than 20 m. In the Late Ordovician the sea level fell only twice for ∼100 m within 1 Myr during the Gondwanian glaciation. Although the sea level remained relatively stable, there were frequent 100–200 m changes of sea depths we inferred with reference to the time spans of stratigraphic units and intervals between tectonic events estimated reliably against stable durations of East Siberian chronozones (biochrons) of the Ordovician. In the absence of eustatic events, the sea-depth changes most likely resulted from rapid crustal uplift and subsidence. According to correlated well-documented Ordovician sections from East Siberia, the rate of crustal subsidence changed rapidly in different periods and in different places of the area, thus being of a regional scale. The controversy between the sea-level stability and the regional-scale variations in sea depths controlled by rates of crustal uplift and subsidence can be resolved assuming a model of variable eclogitization rates in the lower crust caused by lithospheric stress change. Our inferences undermine the traditional petroleum prediction approach implying formation of depositional traps due to rapid eustatic sea-level change.

Precambrian cratons cover about 70% of the total continental area. According to a large volume of geomorphological, geological, paleontological, and other data for the Pliocene and Pleistocene, these cratons have experienced a crustal uplift from 100–200 m to 1000–1500 m, commonly called the recent or Neotectonic uplift. Shortening of the Precambrian crust terminated half a billion years ago or earlier, and its uplift could not have been produced by this mechanism. According to the main models of dynamic topography in the mantle, the distribution of displacements at the surface is quite different from that of the Neotectonic movements. According to seismic data, there is no magmatic underplating beneath most of the Precambrian cratons. In most of cratonic areas, the mantle lithosphere is very thick, which makes its recent delamination unlikely. Asthenospheric replacement of the lower part of the mantle lithosphere beneath the Precambrian cratons might have produced only a minor part of their Neotectonic uplifts. Since the above mechanisms cannot explain this phenomenon, the rock expansion in the crustal layer is supposed to be the main cause of the recent uplift of Precambrian cratons. This is supported by the strong lateral nonuniformity of the uplift, which indicates that expansion of rocks took place at a shallow depth. Expansion might have occurred in crustal rocks that emerged from the lower crust into the middle crust with lower pressure and temperature after the denudation of a thick layer of surface rocks. In the dry state, these rocks can remain metastable for a long time. However, rapid metamorphism accompanied by expansion of rocks can be caused by infiltration of hydrous fluids from the mantle. Analysis of phase diagrams for common crustal rocks demonstrates that this mechanism can explain the recent crustal uplift of Precambrian cratons.