Abstract
We acquired 27 wide-angle seismic profiles to investigate variation in crustal structure along the Kyushu-Palau Ridge (KPR), a 2600-km-long remnant island arc in the center of the Philippine Sea plate; 26 lines were shot across the strike of the KPR at 13°–31°N, and one was shot along the northernmost KPR. The derived P-wave velocity (Vp) models show that the KPR has a crustal thickness of 8–23 km, which is thicker than the neighboring backarc basin oceanic crusts of the West Philippine Basin to the west and the Shikoku and Parece Vela Basins to the east. While the KPR crust consists mainly of lower crusts with a Vp of 6.8–7.2 km/s, the thicker crust contains a thick middle crust with Vp of 6.0–6.8 km/s. In general, the KPR crust is thicker in the north than in the south. The uppermost mantle velocities just below the KPR bathymetric highs are lower than 8.0 km/s and are commonly associated with a slightly high Vp of 7.2 km/s at the base of the crust. Large amplitude reflection signals are sometimes observed at far offsets on several lines suggesting the existence of several reflectors at depths of 23–40 km in the mantle beneath the KPR. The characteristics of these reflections are similar to these observed beneath the Izu–Ogasawara (Bonin) island arc, the tectonically conjugate arc of the KPR before backarc basin spreading. Very thin crust and high Pn velocities characterize the transition between the KPR and the eastern basins, which is probably a relic of the initial stage of the rifting. West of the KPR, the crust varies in structure from north to south as a result of the different tectonic settings in which it evolved.
Highlights
The Kyushu-Palau Ridge (KPR) is a 2600-km-long bathymetric high extending north–south at the center of the Philippine Sea plate in the northwestern Pacific (Fig. 1)
The KPR is regarded as a remnant of the proto Izu–Ogasawara (Bonin)–Mariana (IBM) island arc that was separated by backarc spreading of the Shikoku and Parece Vela Basins in the late Eocene (e.g., Mrozowski and Hayes 1979; Seno and Maruyama 1984; Okino et al 1994)
Our results show the following: 1. the crustal thickness below the KPR bathymetric high varies along the ridge axis, the northern KPR is generally thicker than the southern KPR
Summary
The Kyushu-Palau Ridge (KPR) is a 2600-km-long bathymetric high extending north–south at the center of the Philippine Sea plate in the northwestern Pacific (Fig. 1). Ishihara and Koda (2007) deduced the crustal thickness of the KPR by threedimensional gravity modeling and showed that the KPR crust has a continuous crustal “root” along the ridge and that the crustal thickness exceeds 15 km in most of its northern part and 10 km in most of its southern part. They detected a belt of thin crust with thickness of approximately 5 km along the transition zone between the Shikoku Basin and KPR. The magnetic anomaly map (Fig. 2 right) shows many dipolar anomalies are distributed along the KPR, and their intensity is larger in the Nishizawa et al Earth, Planets and Space (2016) 68:30
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