Structural position of the Tolmachova active magmatic center in the south of Kamchatka and its origin

The article is of a review nature, where the results of deep geological and geophysical studies carried out in the south of Kamchatka and in the nearest water area of the Pacific Ocean are presented. A description of the volumetric density model and its analysis in combination with other data are given. Information on the structural position of the Tolmachоva active magmatic center (TAMC) and its origin is supplemented. As a result of the studies, a mantle protrusion was revealed, which has closed contours and was formed in the Nachikinsky zone of transverse dislocations (NZTD) no later than the Early Miocene. The sizes of the major and minor axes of the protrusion are ~ 123 and 84 km, respectively. In the lower part of the mantle protrusion, at a depth of 35–45 km, local areas of decompression are identified, which are associated with centers of melting. The formation of the ledge may be caused by the pressure of ultrabasic magma from the upper mantle and its subsequent intrusion into the lower layers of the Earth’s crust. The intrusion occurred along a weakened zone formed at the initial stage of shear dislocation that occurred in the Miocene-Pliocene time. Differentiation of magma entering the earth’s crust from melting centers, as well as heat flows from the same sources, form areas of focal melting and, as a consequence, lead to the formation of an intrusive massif of medium to medium acidic composition. Periodic movement of magma along a weakened zone in the TAMC area is accompanied by a swarm of weak earthquakes. TAMC is genetically related to the mantle ledge and is an integral part of it.

Inflection zones of the subducting oceanic lithosphere are areas of accumulation of tectonic stress and its periodic unloading in the form of earthquakes. The highest density of seismic events with magnitude M ≥ 5 is observed in the seismic lineament located closest to the coastline – in the zone of maximum slab bending in the depth range of ~ 30–50 km.

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