40Ar/39Ar age of rock deformations across the Bazhenov suture zone (eastern border of the Middle Urals)

Abstract

Research subject . This article presents data on the nature of rock deformations in the Bazhenov suture zone. The data was obtained by 40Ar/39Ar dating of mica samples from schists and blastomylonites, which were collected acrossan area extending approximately for 100 km along the Bazhenov suture. This suture separates the Eastern zone of the Middle Urals dipped under the cover of the West Siberian plate from exposed geologic structures of the Urals. Methods .The character of the deformations was studied by means of direct geological observation. The age of the deformations was determined by\xa0mica dating using the 40Ar/39Ar method. Results. The deformations were found to have occurred in several phases. The initial phase, which led to the formation of a band of blastomylonites and rocks showing a varying degree of schistosity with a width of over 10 km in particular areas, is likely to have had the character of left-lateral fault. The upper age limit of this deformation phase is determined by the intrusion of the subalkaline rocks of the Petukhovskii complex (280 Ma), which had not been affected by any deformation processes. The subsequent type of deformations had been left-lateral strike-slip faults of submeridianal strike and subvertical dip, which were represented by low-thickness (usually about 10 cm, rarely up to 0.5 m) zones of intensely deformed rocks cutting the schistosity of the previous phase at an acute angle. The final phase of the deformations is shown to be a sloping fault. The dislocation planes of this type were represented by slickensides and chlorite incrustations. Despite the apparent polychronicity of the deformation processes, which were established within the Bazhenov suture zone by our geological observations, 40Ar/39Ar age dating identified only one event. The average value of five conducted tests was about 251 Ma. Apparently, this age should be associated with the most recent geological processes having occurred at temperatures sufficient to fully overload the K-Ar isotopic system of the studied rocks, such as the deformations caused by strike-slip dislocations. Conclusions. The strike-slip fault dislocations dated in this work occurred approximately 25 Ma after the completion of the tectonic activity in the Serov-Mauk fault zone, which is located to the west of the Bazhenov zone. In addition, the time of these dislocations very nearly coincides with that of the formation of the grabens of meridional strike at the base of the West Siberian plate, which took place about 250 Ma ago. This supports apreviously made conclusionon the similar character of the geological development of the eastern border part of the Urals and the adjoining basement of the West Siberian plate.