No 1 (2023)

A comprehensive study of the Upper Cambrian and Lower Ordovician complexes of Northern Kazakhstan was carried out, their ages were substantiated, the structures and compositions of the rocks were investigated. It has been ascertained that the Upper Cambrian formations comprise coarse clastic strata, mafic alkaline effusive rocks and gabbro, while felsic volcanic rocks and granites are represented only by boulders in conglomerates. The Lower Ordovician rocks contain basalt-rhyolite series, felsic alkaline volcanic rocks, and granitoids. The lateral rows of structures of the active continental margin have been reconstructed for the Late Cambrian and Early Ordovician. In the Late Cambrian, the lateral series includes only the structures of the rear extension region, where complexes with the within-plate geochemical characteristics were formed. In the lateral series of the Early Ordovician structures, the frontal volcanic area with the island-arc volcanic rocks and the rear extension area with the intraplate felsic volcanic rocks and granites, were revealed. It is assumed that the differences in the lateral rows of structures may be associated with a change in the tectonic mode of the active continental margin at the Cambrian–Ordovician boundary, when the transform mode with no evidence of suprasubduction magmatism was replaced by convergent magmatism accompanied by the wide distribution of island-arc volcanic rocks.

Acoustic basement of the Knipovich Ridge southeastern flank was interpreted on the time-domain CDP seismic sections and calibration of Bouguer gravity anomalies to depth was done with construction of basement structural map for the area with an oceanic crust type. On this map, to the east from Knipovich Ridge, there is the longitudinal uplift, which is the northern continuation of the Senja fracture zone and interpreted as a transverse ridge on the transform fault board. This uplift is framed by linear clusters of the off-axis seismicity epicenters, indicating the activation of this area structures. The CDP seismic data above the identified uplift show deformations of the Pliocene‒Quaternary sedimentary cover with reverse fault and shear kinematics. Physical modeling of structure formation in the area of the Knipovich Ridge clearly demonstrated the fe-atures of the main tectonic elements during oblique spreading. The result, especially close to reality, was obtained by conducting combined experiments with bending the weakened zone to large angles between the direction of stretching and perpendicular to the axis of the weakened zone. At the same time, the a-ppearance of typical accretion swells and nontransform axis displacements simulating the structures of the southeastern flank of the Knipovich Ridge is close to reality. The series of experiments conducted to study the possible formation of the spreading axis jump in an easterly direction to the continuation of the Senja fracture zone showed the fundamental possibility of this structure activation, which we consider as one of the reasons for the formation of features observed in geophysical data. The current position of the active zones of the region, seismicity, the structure of the basement and the structure of the sedimentary cover indicate a shift in the activity of the main tectonic elements in the east direction relative to the current position of the extension axis. The likely scenario for further development of the region will be the transformation of the Knipovich Ridge into one or the series of transform faults parallel to the western edge of the Barents Sea shelf and the series of short spreading segments between them.