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beneath the volcanic front is the thinnest in the entire Izu-Ogasawara arc, as shown
by previous studies. The thinner crust in this region reflects greater extension here
relative to the arc to the north.
Keywords
Crustal rifting • Crustal production rate • MCS • OBS
1
Introduction
The Izu-Ogasawara (Bonin) arc, the northern part of the Izu-Bonin-Mariana arc
(IBM) arc, is an intra-oceanic arc formed by interaction between two oceanic
crusts without involvement of continental crust (e.g., Taylor
1992
; Stern
2010
). It
is thus an excellent location to study how juvenile arcs create mature continental
crust with an intermediate and andesitic composition (Tatsumi et al.
2008
).
Because the middle crust in the Izu-Ogasawara arc has a P-wave velocity (
Vp
) of
6 km/s, similar to the average
Vp
of continental crust (Suyehiro et al.
1996
;
Takahashi et al.
1998
), it is commonly inferred that subduction has produced
andesitic proto-continental crust, and that this early stage of continental crustal
growth can be studied here.
According to Tatsumi et al. (
2008
), crustal growth in this setting occurs as
follows: (1) Subduction causes melting in the mantle wedge, producing basaltic
magmas that rise and are underplated at the bottom of the crust beneath the volcanic
front. (2) Basaltic melts are repeatedly differentiated and remelted producing dense
olivine-rich cumulates, tonalitic middle crust, and complementary restites. (3) The
differentiated basalts and restites make up the lower arc crust and uppermost
mantle, respectively. (4) Dense materials of crustal origin must be recycled back
into the mantle, as shown by comparisons of the petrologic model and volumes of
the crustal layers identified from seismic studies (Takahashi et al.
2007a
; Tatsumi
et al.
2008
).
The amount of crustal growth depends on the volume of the underplated basal-
tic magmas and the heat supply. Kodaira et al. (
2007a
) showed that rhyolitic
magmas are found where the crust is characterized by low average
Vp
, and Tatsumi
et al. (
2008
) suggested that the andesitic middle crust is consumed to produce
rhyolitic magmas and complementary restites. However, continuous crustal dif-
ferentiation to produce continental middle crust requires a continuous input of
heat. Production of felsic middle crust likely stops without heat brought by melts
of basaltic material.
This simple petrologic scenario should be modified because the Izu-Ogasawara
arc has experienced not only underplating of basaltic melts, as shown by many
papers, but also crustal rifting and spreading since construction of the Eocene arc
at 50 Ma (e.g., Okino et al.
1994
; Taylor
1992
). It is well known that the Shikoku
and Parece Vela Basins were produced by back-arc opening between the current
IBM arc and the Kyushu Palau Ridge (Okino et al.
1994, 1998
). It is also well
known that crustal rifting occurred at the volcanic front and within both rear-arc
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