Seismological evidence for variable growth of crust along the Izu intraoceanic arc

Abstract

The processes that create continental crust in an intraoceanic arc setting are a matter of debate. To address this issue, we conducted an active source wide‐angle seismic study to examine along‐arc structural variations of the Izu intraoceanic arc. The data used were acquired over a 550‐km‐long profile along the volcanic front from Sagami Bay to Tori‐shima. The obtained structural model showed the existence of felsic to intermediate composition middle crust with a P wave velocity (Vp) of 6.0–6.5 km s−1 in its upper part and 6.5–6.8 km s−1 in its lower part. The thickness of the middle crust varied markedly from 3 to 13 km. The underlying lower crust also consisted of two layers (Vp of 6.8–7.2 km s−1 in the upper part and Vp of 7.2–7.6 km s−1 in the lower part). The upper of these layers was interpreted to consist of plutonic gabbro, and the lower layer was interpreted to be mafic to ultramafic cumulates. Average crustal velocities calculated from our model showed remarkable lateral variation, which correlated well with arc volcanism. Low average crustal seismic velocities (∼6.7 km s−1), due to thick middle crust, were obtained beneath basaltic volcanoes (e.g., O‐shima, Miyake‐jima, Hachijo‐jima, Aoga‐shima), while higher average velocities (∼7.1 km s−1) were obtained beneath rhyolitic volcanoes (e.g., Nii‐jima, Kurose, South Hachijo caldera, Myoji knoll, and South Sumisu caldera). We concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the Izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution, or remelting of preexisting continental crust, or both.