Multiscale structure of magma feeding system between the Klyuchevskoy volcano group in Kamchatka

The Klyuchevskoy group of volcanoes (KGV) is a unique complex, which includes extremely productive volcanoes with variable composition and eruption regimes. During the past ten years, a considerable progress in understanding the deep processes beneath KGV was achieved owing to a number of seismic tomography studies based on data of permanent and temporary seismic networks. The purpose of this review consists in summarizing and systematizing these results and in building an integral model of volcano feeding systems beneath KGV.

The regional scale mantle tomography model shows the presence of high-velocity slabs beneath the Kamchatka and Aleutian arcs and a clearly pronounced gap between them. On a crustal scale, seismic velocity structures and seismicity highlight different types of feeding systems beneath separate volcanoes. Beneath Klyuchevskoy, the seismicity traces a "vertical pipe" that delivers magmatic material from a mantle source to the surface. A prominent low-velocity anomaly beneath Bezymianny represents an area of accumulation and fractioning of magma in the middle crust. Linear velocity anomalies and earthquake lineaments beneath the Tolbachinsky complex mark fault zones serving as pathways for rapid ascent of basaltic magma.

The detailed structure of the mantle wedge beneath the Klyuchevskoy group and surroundings was studied based on the data of a large temporary seismic network with more than a hundred seismic stations installed within the KISS Project. Beneath the Klyuchevskoy volcano, the Vp/Vs distribution reveals three flows of melts and volatiles coming out from the slab at depths of 100, 120, and 150 km. These flows unite at shallower depths and form a large reservoir at the base of the crust that feeds the Klyuchevskoy volcano. The low-velocity anomalies of the P and S waves in the mantle wedge indicate the hot asthenospheric flow vertically ascending through the slab window below Shiveluch volcano, and then spreading horizontally toward the volcanoes of the Klyuchevskoy Group. The presence of this flow together with active release of fluids from the slab are the main causes of the extremely high activity of the volcanoes of the Klyuchevskoy group.

The detailed structure of the magmatic system in the upper crust beneath Bezymianny was studied based on the data of a local seismic network, installed a few months before a strong explosive eruption occurred on December 20, 2017. The derived 3D seismic velocity distribution beneath Bezymianny illuminates its eruptive state days before the eruption. It infers the coexistence of magma and gas reservoirs revealed as anomalies of low (1.5) and high (2.0) Vp/Vs ratios, respectively, located at depths of 2-3 km and only 2 km apart. The reservoirs both control the current eruptive activity: while the magma reservoir is responsible for episodic dome growth and lava flow emplacements, the spatially separated gas reservoir may control short but powerful explosive eruptions of Bezymianny.

This research was supported by the Russian Science Foundation Grant #20-17-00075.