Vol 52, No 5 (2018)

The paper considers geological and geophysical data on the crustal structure of the volcanic Kerguelen Plateau and nearby Indian Ocean, as well as evolution of the Kerguelen Plateau during the Gondwana breakup. It is assumed that three isolated continental blocks (microcontinents) evolved within the Kerguelen Plateau. In order to provide insight into the mechanisms for the microcontinent formation, physical modeling under three different conditions has been carried out. The first experiment simulates a homogeneous lithosphere with two rift fractures propagating toward each one other; in the second experiment, the propagating rift fractures collide with a structural “barrier” having a stronger lithosphere; the third experiment simulated the effect of a local heat source (a hot spot). Based on these experiments and taking into account the available structural and geophysical data, it is suggested that isolation of continental block in the southern Kerguelen Plateau could have begun due to counterpropagation of rifting branches: oceanic from east to west and continental from west to east. The isolation of the southern block with attachment to the Antarctic Plate took place 120 Ma ago after extinction of the spreading ridge in the Princess Elizabeth Trough. Because of the structural inhomogeneity in the prebreakup lithosphere, which is represented by a zone of intracontinental Lambert–Mahanadi rifts, opening of the ocean could have developed in a more complex regime. Under the effect of a hot spot, a jump of the spreading axis occurred at the moment of its emplacement into the lithosphere.

The studied deep-seated plutonic rocks of the Malyi Ural Paleozoic island arc include Sob’ gabbroid, diorite, and plagiogranitoid and Kongor gabbroid, diorite, and monzonitoid, which formed under similar P–T conditions. U–Pb LA-ICP-MS zircon dating established similar concordant age values: 406 ± 2 Ma for hornblende gabbrodiorite of the major intrusive phase in the Sob’ complex and 396 ± 1 and 393 ± 2 Ma for bipyroxene gabbrodiorite of the early and major phases in the Kongor complex. Our age data have made it possible to determine the formation time of the Kongor complex as Late Emsian–Early Eifelian (399–393 Ma). The largest volumes of island-arc igneous rocks belonging to the calc-alkali gabbro–diorite–tonalite–plagiogranite series formed in the Praghian–Early Eifelian (410–393 Ma). The Late Emsian–Early Eifelian (399–393 Ma) was characterized by the development of much smaller bodies consisting of Kongor rocks pertaining to the calc-alkali and high-K calk-alkali range, gradually transitioning into shoshonite–latite. High-K rocks formed upon completion of calc-alkali magmatism, likely due to the gradual decay of Devonian suprasubduction magmatism and partial melting in magma generation or due to the involvement of a second magma source.

This paper reports the results of numerical modeling of the stress–strain state of the epicentral area before and after the August 24, 2014 earthquake in Napa, California, USA, compared to calculated data obtained in instrumental studies in the dilatation areas based on GPS observation results. Numerical modeling has made it possible to calculate the stress–strain state of the epicentral area affected by the tectonic fault system. GPS observation data on the epicentral area of the earthquake and the results of numerical modeling of the stress–strain state before and after the considered earthquake have been analyzed. A trend toward localization of earthquake epicenters in the region of high stress intensity concentration has been confirmed. It has been proved that aftershock development is due to the drop in stress caused by a new fracture and that the aftershock cluster that occurred was localized in the area of decreased stress intensity.