Abstract
The Kultuk volcano erupted at the axial South Baikal basin of the Baikal rift zone (BRZ). Now it exhibits facies of subvolcanic bodies, land-lava eruptions and subaqueous pillow lavas and hyaloclastites. The volcano was controlled by the Obruchev fault that is currently a border of the basin which amplitude of vertical movements is rapidly decreasing in the westward direction. It is found that the Kultuk volcano was active at the beginning and end of the volcanic activity period of the Kamar, Stanovaya, and Bystrinskaya volcanic zones, which took place 18–12 Ma ago. In previous papers, it was assumed that dominant structures in the area under study were major Cenozoic shear displacements along the Main Sayan fault and/or along the Tunka rift valley; however, at the current stage of our study, linear configurations of the volcanic zones do not reveal any of such displacements. Based on analyses of distribution of volcanic rocks in the relief at the western coast of Lake Baikal, distinct vertical crustal movements are revealed; such movements started in the Early Miocene and continue to the present time. It is concluded that volcanism was controlled by the trans-tensional system of volcanic zones. Sources are identified for the shallow lithospheric mantle melt with the substantial admixture of the low-crust component and deeper asthenospheric mantle melts in the Kamar and Stanovaya volcanic zones; for the Bystrinskaya volcanic zone, only components from the deeper source are revealed. The local shallow mantle magmatism occurred only within the lithosphere extension zone beneath the South Baikal basin. The lithosphere thinning is reflected in the change of activity from the sub-lithospheric to lithospheric sources under the Kamar zone. Rifting of the axial structure is recorded at the root of the Slyudyanka lithospheric block that was subjected to the collision-related Early Paleozoic metamorphism. Geochemical characteristics of the collision-type components were inherited by the Miocene basaltic melts.
Highlights
Аннотация: На западном окончании осевой ЮжноБайкальской впадины Байкальской рифтовой зоны, в постройке Култукского вулкана, представлены фации субвулканических тел, наземных лавовых извержений и подводных из вержений подушечных лав и гиалокластитов
Cотношения вулканизма с развитием кайнозойских континентальных рифтовых зон рассматривались в многочисленных работах [Milanovsky, 1976; Logatchev, 1977; Kiselev et al, 1979; Rasskazov, 1985, 1993]
3, Россия Snopkov, Sergei V., Candidate of Geology and Mineralogy, Associate Professor Irkutsk State University, Geological Faculty, Geology and Geophysics Chair 3 Lenin street, Irkutsk 664003, Russia
Summary
Cотношения вулканизма с развитием кайнозойских континентальных рифтовых зон рассматривались в многочисленных работах [Milanovsky, 1976; Logatchev, 1977; Kiselev et al, 1979; Rasskazov, 1985, 1993]. 1–4 – породы вулкана: 1 – долериты, 2 – подушечные лавы и гиалокластиты, 3 – микродолериты, 4 – базальты лавового потока; 5 – две ветви Обручевского разлома: северная, затухающая в субаквальной части Култукского вулкана, и южная, продолжающаяся в широкую долину р. 1–4 – rocks of the volcano: 1 – dolerites, 2 – pillow lavas and hyaloclastites, 3 – microdolerites, 4 – basalts from the lava flow; 5 – two branches of the Obruchev fault (the northern brand terminates in the subaqueous part of the Kultuk volcano; the southern branch goes into the broad Kul tuchnaya river valley to be replaced along the strike with the reactivated fragment of the Main Sayan fault (hatched lines are directed toward the downthrown block): a – the segment manifested in the relief, б – the segment buried under the modern slope sediments; 6 – sampling sites (for simplification of the diagram, the numbers of samples from 1 to 12 are given, corresponding to field numbers from KL–12–1 to KL–12–12; sam ple р659 was taken in the field study in 1982 [Rasskazov, 1993]); 7 – tectonic tilt of the volcanic edifice’s block (direction of two hatched lines); 8 – the Kultuk settlement area; 9 – roads (a – highway, б – railway); 10 – horizontals (in 10 m) and steep cliffs. Такое же линейное распределение без признаков боковых смещений име ют вулканы и лавовые покровы Быстринской зоны (см. рис. 2, 3)
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