Vol 53, No 4 (2019)

The structure and evolution of the eastern part of the Southwest Indian Ridge are discussed. Based on geological-geophysical data and cartographic materials, analysis of spatial and temporal changes of ridge morphostructure was performed. Analysis of the data made it possible to recognize the stages of the ridge evolution and delineate its sections differing in tectonics, structure, and morphostructural segmentation of the rift zone and the ridge flanks. It is shown that crust in the axial zone of the ridge formed under repeatedly changing spreading kinematics, degree of hotspot activity, and along-axis changes of underlying mantle temperatures. The temporal variations in these characteristics along the rift zone are established, with consideration of their influence on its segmentation and tectonic structural features.

Detrital zircons of Ordovician terrigenous sequences are studied in various Southern Uralian tectonic units. The age of detrital zircons of the West Uralian and Transuralian megazones, Taganai–Beloretsk Zone, and Kraka allochthons spans from the Late Archean to the end of the Vendian– beginning of the Cambrian; Early Precambrian and Early–Middle Riphean zircons are the most abundant. Vendian–Cambrian detrital zircons are strongly dominant in the Uraltau Zone, Sakmara allochthons, and East Uralian Megazone; the zircons of other ages are absent or extremely rare. The Vendian–Cambrian detrital zircons of all Southern Urals zones probably derive from volcanic and granitic rocks of the marginal continental belt, which are part of the Uraltau Zone, Sakmara allochthons, and East Uralian Megazone. The Lu–Hf isotopic characteristics of Vendian–Cambrian detrital zircons indicate that their parental rocks formed on a heterogeneous basement that includes blocks of juvenile and ancient continental crust. According to a model of the pre-Ordovician tectonic evolution of the Southern Urals, at the end of the Late Riphean, the passive margin of the East European Platform collided with a block on a heterogeneous basement. The formation of the block terminated with the Grenville Orogeny. After collision, a volcano-plutonic belt originated in the Vendian–Cambrian at the actively evolved margin of the East European Platform.

The recent evolution of the marginal segment of the Northwestern Caucasus orogen at its junction with the Kerch–Taman periclinal trough has been studied. Geomorphological analysis included geological and tectonophysical data and digital elevation models. The Northwestern Caucasus fault–fold morphostructures extend westward to the Crimean mountain structure, in the intermediate Kerch–Taman region, developing in a uniform regional compression setting. The prevailing direction of horizontal compression changed from NE in the Northwestern Caucasus to N–S in the Taman region, resulting in reorientation of the axes of fold morphostructures and geophysical anomalies from NW to EW. The compression vectors separated in the Pliocene–Quaternary. The NE-trending pre-Pliocene fault structures in this area were replaced by the N–S-trending Anapa–Dzhiginka and Abrau active fault zones. As follows from comparison of the Abrau and Anapa–Dzhiginka zones, faulting in Abrau zone became more dynamic in the Pliocene–Quaternary; we revealed clear signs of normal faulting with a vertical offset of 500‒600 m in the Abrau zone during this stage. The Abrau zone is the western boundary zone of the Greater Caucasus. The possible reasons for the restructuring of the regional structural geometry and lateral transition activity of faults transverse to the Northwestern Caucasus orogen are considered.