New Data on Ages of the Basement of the Central Kamchatka Depression and the Base of the Klyuchevskaya Group of Volcanoes

Abstract

A unique feature of the Kamchatka Peninsula is an extensional structure running for more than 400 km along the peninsula—the Central Kamchatka Depression (CKD). Some geodynamic models associate the formation of the CKD with the movements of East Kamchatka toward the Pacific Ocean due to retreat of the northern edge of the subduction zone. However, isotope–geochronological data on the time of the CKD formation have been lacking up to the present time. Therefore, we could not directly associate the CKD emergence with the stages of the Kamchatka tectonic evolution and the intensity of its recent deformations. This work presents the chemical compositions and K–Ar ages of effusive rocks from the KL-1 borehole drilled in the northern part of the CKD. Samples from the depth interval of 50–475 m, represented by medium-K basalts and high-K basaltic trachyandesites date back to 0.7 to 1.5 Ma. By their composition, these rocks are close to effusive rocks forming the base of the Klyuchevskaya group of volcanoes. Before this study, the age of the effusives had been estimated as much younger, up to 0.3 Ma. Below, in the interval of 475–505 m, a pack of sands and boulder conglomerates was described. They unconformably overlap the aphyric and olivine–pyroxene porphyritic andesibasalts with interlayers of volcanic ash and volcanic sands of the same composition. At the depth of 905–1513 m, the borehole recovers highly altered tuffs, mafic and intermediate in composition, interbedded with andesites and andesibasalts. The K–Ar estimation of 3.5 ± 0.4 Ma was obtained for the least altered sample of medium-K andesibasalt from the depth of 1255 m. We suggest that the altered effusives at 905 m and below are associated with the completion of accretion of the Kronotskii Island Arc and with the formation of the modern structure of the Kamchatka Peninsula. The CKD setting occurred after this event in the interval of 1.5–3.5 Ma.