Crustal Structure and Magmatism of the Southern Kyushu-Palau Ridge

Abstract

The Kyushu–Palau Ridge (KPR) is the result of the subduction of the Pacific Plate, separated from the proto-Izu–Bonin–Mariana arc by the back-arc spreading, showing variations in arc structures. The volcanism related to the subducted slab at the remnant arc stops before the initial back-arc spreading. However, the KPR accretion did not stop right when the back-arc spreading started. The internal structure not only reveals the arc characteristics but also contains crisis information about how the arc formed. The less revealed velocity structure in the southern KPR might hinder an understanding. Herein we studied the crustal magmatism at the southern KPR based on the detailed velocity structure. Results show that the KPR’s crust to the west is 6.0–7.6-km-thick with one summit at the basement and a flat Moho boundary; to the east, the crust has a thickness of 7.0–11.0 km, and three summits are observed at the basement sharing one depressed Moho boundary. These structural diversities indicate the presence of two independent seamounts (west and east), with different periods of magmatism. By calculating the duration of volcanism, we hypothesized that the formation of the western seamount of the KPR requireed a magmatism duration of ~4 Myr and one stable volcanic source. In contrast, the eastern seamount needed two volcanic sources and a total magma loading time of ~10 Myr with ~19-Myr period gap in magmatism. By analyzing the KPR evolution, we conclude that multiple volcanic sources and episodic magmatism caused the diversities in arc structures at the KPR; the initiation of back-arc spreading at ~26 Ma might have inhibited the magmatism related to the subduction; however, the back-arc-related partial remelting of the lower island arc would have led to a new magma source and reconstructed the preexisting island arc, thus leaving multiple conduits inside it.