The role of volcanism in the transformation of the earth's crust at continental margins with reference to Kamchatka

Abstract

The composition of the earth's crust in Kamchatka beneath the volcanoes was modelled using the evidence available from geological and petrological investigations, deep seismic sounding (DSS), seismological investigations, gravimagnetic surveys, magnetotelluric sounding, measurements of geomagnetic variation, and experimental studies on melting and elastic properties ( V p , ρ) of rocks at variable pressure. Independent magma sources (foci) were discovered in the upper mantle and in the crust, the former being indicated by the manifestations of basic magmatism, and the latter by those of intermediate, andesitic and more acidic magmatism. Magmatic rocks are grouped and classified into rock associations characterized by specific petrological and structural-tectonic features. Typical basic rock associations (group I) are controlled by longitudinal deep faults of volcanic rift zones and are actually indications of these faults. A group of differentiated intermediate-acidic rock associations (group II) are related to structures similar to intermontane depressions, of crustal origin, 15 ×30 to 30–240 ×70 km in size. The continental crust was converted into oceanic crust in the course of rift formation. The accumulation of basic rocks along deep crustal faults is considered as a factor of “mechanical” basification of the crust. A low-velocity layer between the crust and mantle was replaced by a high-velocity mantle restite. In areas of rock associations of group II at the depth of the basalt layer, the thickness of the crust was reduced due to its physicochemical transformation. The transformation consisted in the first place in the alteration (e.g. water saturation) of the substance of the basalt layer which resulted in the decrease of the velocity V p from 7.1–7.2 km/sec to 6.6 km/sec. Later melting occurred. After the layer was fractionally differentiated into constituents and andesite eruption took place, the basification of the layer occurred, and it became, in part or in whole, part of the mantle. As a result, the crust decreased in thickness. For instance, beneath the Avacha volcano, the velocity V p in the basalt layer thus regenerated is 7.9 km/sec, and the crustal thickness is 22 km. The results of this study support V.V. Belousov's hypothesis on the basification and oceanization of the earth's crust in Kamchatka and on the great role of magmatism (volcanism) in this process. Basification was most intense in the frontal part of the peninsula facing the ocean at a depth of 20 or 30 km. The reported evidence reveals that the petrogenetic concepts that follow from the hypothesis of plate tectonics are unsatisfactory.