Abstract
A previous study of a longitudinal profile along the Bonin ridge concluded that remarkably thin forearc crust (< 10 km thick) along the northern half of the ridge indicates that the crust there was formed by forearc spreading during the initial stage of subduction along the Izu-Bonin intra-oceanic arc. However, a profile across the Bonin ridge shows a thicker crust. In this study, we construct a model that takes into account seismic and gravity data from both profiles. Re-modeling of the seismic data showed a north-south aligned area of thin crust (∼10 km thick) at the center of the Bonin ridge; this structure was confirmed by gravity data. The seismic data at the eastern end of the across-arc profile suggests that the crust thickens beneath the trenchward slope of the Bonin ridge. However, a petrological model suggests a trenchward extension of forearc oceanic crust that formed during the initial stage of subduction. Although further detailed investigation is required, we suggest that this contradiction can be explained either by the subduction of buoyant crust immediately beneath the forearc oceanic crust, or by the presence of a serpentinized mantle wedge beneath the forearc oceanic crust.
Highlights
The Izu-Bonin-Mariana (IBM) intra-oceanic arc, where the Pacific plate is subducting beneath the Philippine Sea plate, extends over 2800 km from central Japan in the north to Guam in the south (Fig. 1) and has long been studied to examine the evolution of arc crust
From our analyses of gravity data and seismic profiles along and across the Bonin ridge, we conclude that there is a north-south aligned zone of thin crust (∼10 km thick) in the center of the Bonin ridge, and that the width of the zone of thin crust is as little as ∼20 km
Our checkerboard test indicated that our modeling did not wellresolve the eastern end of the SPr2 profile, it did suggest that the crust thickens beneath the trenchward slope of the Bonin ridge
Summary
The Izu-Bonin-Mariana (IBM) intra-oceanic arc, where the Pacific plate is subducting beneath the Philippine Sea plate, extends over 2800 km from central Japan in the north to Guam in the south (Fig. 1) and has long been studied to examine the evolution of arc crust. The back-arc spreading process formed the Kyushu-Palau Ridge as a remnant arc at the western edge of the Parece Vela and Shikoku basins. Backarc rifting along the Mariana trough started at 10 Ma, and the consequent seafloor spreading began at 3–4 Ma (Bibee et al, 1980; Yamazaki and Yuasa, 1998). In the Izu arc, intra-arc rifting started to the west of the present-day volcanic front at 2 Ma (e.g., Ishizuka et al, 2002)
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