V. D. Chekhovich

Cenozoic geodynamics of the Bering Sea region

In the Early Cenozoic before origination of the Aleutian subduction zone 50–47 Ma ago, the northwestern (Asian) and northeastern (North American) parts of the continental framework of the Pacific Ocean were active continental margins. In the northwestern part, the island-arc situation, which arose in the Coniacian, remained with retention of the normal lateral series: continent-marginal sea-island arc-ocean. In the northeastern part, consumption of the oceanic crust beneath the southern margin of the continental Bering shelf also continued from the Late Cretaceous with the formation of the suprasubduction volcanic belt. The northwestern and northeastern parts of the Paleopacific were probably separated by a continuation of the Kula-Pacific Transform Fracture Zone. Change of the movement of the Pacific oceanic plates from the NNW to NW in the middle Eocene (50–47 Ma ago) was a cause of the origin of the Aleutian subduction zone and related Aleutian island arc. In the captured part of the Paleopacific (proto-Bering Sea), the ongoing displacement of North America relative to Eurasia in the middle-late Eocene gave rise to the formation of internal structural elements of the marginal sea: the imbricate nappe structure of the Shirshov Ridge and the island arc of the Bowers Ridge. The Late Cenozoic evolution was controlled by subduction beneath the Kamchatka margin and its convergence with the Kronotsky Terrane in the south. A similar convergence of the Koryak margin with the Goven Terrane occurred in the north. The Komandorsky minor oceanic basin opened in the back zone of this terrane. Paleotectonic reconstructions for 68–60, 56–52, 50–38, 30–15, and 15–6 Ma are presented.

Late Cretaceous-Paleogene transform zone between the Eurasian and North American lithospheric plates

Within the limits of the Chukchi Sea and the Amerasian Basin of the Arctic Ocean, study have been carried out to calculate D function anomalies. The result was the discovery of elongated faults that cut, according to the positions of the upper and lower extents of the disturbing masses, both the upper crust and the upper mantle. It is shown that these faults are right-lateral strike-slips continuing the Late Cretaceous-Paleogene structures of the same type in the Bering Sea. This suggests that the en echelon strike-slip system of the Bering Sea, Chukchi Sea, and Amerasian Basin is a relic of the Late Cretaceous-Paleogene transform fault zone between the Eurasian and North American lithospheric plates.

Age of the Shirshov submarine ridge basement (Bering Sea) based on the results of investigation of zircons using the U-Pb SHRIMP method

The Shirshov Ridge holds an important position in the structure of the Bering Sea Basin. Stretching from north to south for over 500 km, it divides the Bering Sea into two deep water basins, the Aleutian and Komandorsky basins. The age of oceanic crust of the Aleutian basin based on linear magnetic anomalies is conventionally considered Early Cretaceous, of the Komandorsky basin — Miocene, according to the K-Ar dating of the basalts (9.8 Ma) exposed in 191 deep water drill borehole. Rocks belonging to the basement of the Shirshov Ridge were dredged during the 29th cruise of research vessel Dmitrii Mendeleev and are represented by amphibolitic gabbro whose composition is similar to that of gabbroids of mid-oceanic ridges. The age of metamorphism based on the results of K-Ar dating of amphibole is 47 ± 5 Ma. The U-Pb zircon dating method was used to determine the age of gabbro. Zircons were extracted from a ∼5 kg combined amphibolitic gabbroid sample, and the age of zircons was determined using a SHRIMP-II sensitive high resolution secondary ion microprobe (Center of Isotopic Studies, A.P. Karpinskii Russian Geological Research Institute, St. Petersburg). The average concordant age value for the 25 determinations performed based on 20 points for 18 grains is 72 ± 1.4 Ma (Late Campanian). For 5 grains, the measured age values are within the range of 88 ± 3.5 Ma to 126.5 ± 4.5 Ma. Given the western vergence of the thrust structure of the Shirshov Ridge, the acoustic basement of the Shirshov Ridge is most probably a complexly deformed oceanic crust of the Aleutian Trench, which most likely dates from the Early to Late Cretaceous.

Deep neogene structure and modern seismicity of the southern part of the Koryak Highland academician

The Northern Kamchatka and southern part of the Koryak Highland is considered to be an accretion-collision system in the Late Cretaceous and Cenozoic the development of which was caused by the subsequent accretion of various large terranes to the Asian continental margin. The Paleogene Goven Terrane accreted in the Miocene closes this system. Its boundary with the Olyutor Terrane is hidden under the Cenozoic sediments of the Il’pino-Pakhachino interarc trough. The destructive Khaily (March 8, 1991) and Olyutor (April 20, 2006) earthquakes are characterized by an aftershock area extended in the northeastern direction along the axial part of the Il’pino-Pakhachino trough. The aftershock area was intersected by a profile of the earthquake converted-wave method (ECWM) the interpretation of which reveals a correlation loss of the deep reflecting horizons under this area and three faults dipping to the southeast on seismograms.

Geological events of the Late Cretaceous–Early Paleogene in Western Kamchatka

The age ranges of Upper Cretaceous lithotectonic complexes of Western Kamchatka—terrigenous Kikhchik, volcanic Irunei, and terrigenous Omgon—are analyzed to reveal their almost simultaneous deposition. The pre-Cenozoic settings of these complexes are reconstructed. Based on analysis of the composition and structural features of Late Cretaceous lithotectonic complexes and on correlation of events, the Late Cretaceous paleogeography is reconstructed. It is found that the formation of the contemporary structure of the studied region would have required significant displacement of the volcanic Irunei complex from west to east and the terrigenous Omgon complex from north to south. It is concluded that the Western Kamchatka continental block (minor lithospheric plate) was independent in the Late Cretaceous.