The West Siberian System of Triassic Rifts: Was There an Ocean in West Siberia? (Results of Deep Structure Modeling)
摘要
The system of Triassic rifts plays an important role in the structure of the West Siberian platform. We have made geological maps of the pre-Jurassic basement within two rifts (the Koltogorsky rift, located in the center of the platform, and the Danilovsky rift, located in its extreme western part), and then performed computer 3-D modeling of the deep structure (down to a depth of 12 km). The created models consist of 289 800 parallelepipeds for the Koltogorsky area (in the horizon plane the cell size is 1802 × 1802 m) and 222 301 for the Danilovsky area (the cell size is 1510 × 1510 m). The basement maps were used in modeling. According to the results of modeling, at depths of 5 to 7 km, under the Triassic basalts, there appear (and continue down to the lower limit of the models) heavy, probably mantle-like rocks with excess densities of about 0.3 g/cm3. These rocks may be gabbro or weakly serpentinized peridotites. Therefore, there is no continental-type Earth crust under the West Siberian rifts. Consequently, these structures cannot be called grabens; they are full-fledged rifts. However, no significant spreading was detected in the region at that time, and the previously proposed so-called “Obsky Ocean” did not appear to exist.