Download Crust-Mantle Connections in the Kermadec Arc

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.