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Crust-Mantle Connections in the Kermadec Arc Yoshihiko Tamura, Takeshi Sato, Toshiya Fujiwara, Shuichi Kodaira and Alexander Nichols Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan Telephone: +81-46-867-9761. Tax: +81-46-867-9625. E-mail: [email protected] A new study presents the straightforward but unexpected relationship, which relates crustal thickness to magma type in the Izu-Ogasawara (Bonin) and Aleutian oceanic arcs (Tamura et al., 2016). Volcanoes along the southern segment of the Izu-Ogasawara arc and the western Aleutian arc (west of Adak) are underlain by thin crust (10-20 km). In contrast those along the northern segment of the Izu-Ogasawara arc and eastern Aleutian arc are underlain by crust ~35 km thick (Figs. 1 and 2). Interestingly, andesite magmas dominate eruptive products from the former volcanoes and mostly basaltic lavas erupt from the latter. The Kermadec arc (Fig. 3) is a typical oceanic arc produced by subduction of the Pacific Plate beneath the Australian Plate. The water depths between volcanoes change drastically from the northern segment (~1,000 m) to the southern segment (~3,000 m) of the Kermadec arc (Fig. 4). The crustal thickness of the Kermadec arc is not independently known. However, Fig. 5 suggests, based on the water depth between arc front volcanoes, that the crustal thickness of the northern and southern segments of the Kermadec arc cold be ~30 km and ~15 km thick, respectively. The Kermadec arc is the perfect place to see the relationships between crustal thickness and magma type and to examine the hypothesis presented through the study of the Izu-Ogasawara and Aleutian arcs. According to the hypothesis presented by Tamura et al. (2016), rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, melting occurs at relatively low pressures in the mantle wedge producing andesitic magmas. Where the crust is thick, melting pressures are higher and only basaltic magmas tend to be produced. The implications of this hypothesis are: (1) shallow mantle melting in hydrous conditions is crucial for the genesis of andesitic magmas, and andesite is produced only when the crust is thin, thus only in oceanic arcs; (2) the rate of continental crust accumulation, which is andesitic in composition, would have been greatest soon after subduction initiated on Earth, when most crust was thin; and (3) most andesite magmas erupted on continental crust could be recycled from “primary” andesite originally produced in oceanic arcs. Tamura, Y. et al. (2016). Advent of Continents: A New Hypothesis. Scientific Reports 6, 33517; doi: 10.1038/srep33517. Figure 1. Bathymetric features of the Izu-Ogasawara (Bonin) arcs. Old seafloor (135-180 Ma) of the western Pacific Plate subducts beneath the active Izu-Ogasawara arcs at the Izu-Ogasawara Trench. Figure 2. The water depth and crustal thickness along the seismic profile. The profile is along the volcanic front, but is just off the summits of the volcanoes. Figure 3. Bathymetric features of the Kermadec arc system. A-B shows bathymetry of the Kermadec arc through the arc volcanoes. Figure 4. The profile of water depth along the volcanic front of the Kermadec arc. Figure 5. Depth of water between arc-front volcanoes versus crustal thickness along the Izu-Ogasawara arcs based on Fig. 2. The northern segment of the Izu-Ogasawara arcs is in shallower water and has thicker crust than the southern segment. Torishima is located and plots between them. In the Kermadec arc, the water depths between volcanoes change drastically from the northern segment (~1,000 m) to the southern segment (~3,000 m) of the Kermadec arc (Fig. 4). It is suggested, based on the water depth between arc front volcanoes, that the crustal thickness of the northern and southern segments of the Kermadec arc cold be ~30 km and ~15 km thick, respectively.