Evolution of the Kurile-Kamchatkan Volcanic Arcs and Dynamics of the Kamchatka-Aleutian junction

The Cenozoic tectonic evolution of the Kurile-Kamchatkan arc system has been reconstructed based on the spatial-tectonic setting of the volcanic-rock formations and their petrologic-geochemical characteristics, using gravity and seismic data. Three volcanic arc trench systems of different ages that become successively younger toward the Pacific have been recognized in the region: the West Kamchatka (Eocene), Mid-Kamchatka-Kurile (Late Oligocene–Quaternary), and Recent Kurile-Kamchatka systems. The Kamchatka volcanic belts are viewed as the products of these systems, which originated above the subduction zones. The geometry of the present-day Kurile-Kamchatka subduction zone and dynamics of contemporary volcanism can be defined from seismic data. The contemporary Kurile-Kamchatka arc can be subdivided into individual segments in accord with its tectonic evolution and geodynamics. The East Kamchatka segment represents the initial subduction stage (7–10 Ma ago) of the Pacific Plate. The Petropavlovsk segment (the Malka-Petropavlovsk zone of transverse faults) is a zone of discordant superposition of the contemporary Kurile-Kamchatka arc over the older Mid-Kamchatka arc. Within the South Kamchatka segment subduction remained practically unchanged since the Late Oligocene, i.e., since the origin of the Mid-Kamchatka-Kurile arc system, as well as within the three Kurile segments. Geodynamics controlled magma genera tion and is imprinted in the petrochemical properties of the volcanic rocks. Typical arc magmas are generated at the steady-state geodynamic regime of subduction. Lavas of an intraplate geochemical type are generated at initial and final stages of subduction, and also at the Kamchatka-Aleutian junction

Transition from arc to oceanic magmatism at the Kamchatka-Aleutian junction

Spatial geochemical variations of Quaternary lavas erupted along the northern segment of the Kamchatka arc are used to trace changes in magma generation across the subducting Pacific slab edge. The late Pleistocene–Holocene lavas of the northern end of the Central Kamchatka depression north of the Pacific slab edge show strong enrichment in high field strength elements and light rare earth elements, relatively unradiogenic strontium and lead but radiogenic neodymium isotope ratios, and oxygen isotope compositions similar to those of mid-oceanic-ridge basalts. These geochemical characteristics are distinct from the southern Central Kamchatka depression volcanoes located above the subducting Pacific slab. Extensive fluid-triggered mantle melting dominates magma genesis beneath the largest Kamchatka volcanoes in the south, whereas low-degree decompression melting of the Pacific asthenospheric mantle is the major magma generation process north of the Pacific slab edge. Quaternary detachment of the subducted Pacific plate fragment resulted in the influx of fertile mantle beneath Kamchatka. We propose that upwelling and southward flow of this hotter, more fertile mantle is the main reason for recent magmatism in northern Kamchatka and for the exceptional productivity of the Central Kamchatka depression volcanoes (Klyuchevskoy and Sheveluch), the most active arc volcanoes on Earth