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Geology
New seismological constraints on growth of continental crust in the Izu-Bonin
intra-oceanic arc
Shuichi Kodaira, Takeshi Sato, Narumi Takahashi, Seiichi Miura, Yoshihiko Tamura, Yoshiyuki
Tatsumi and Yoshiyuki Kaneda
Geology 2007;35;1031-1034
doi: 10.1130/G23901A.1
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New seismological constraints on growth of continental crust
in the Izu-Bonin intra-oceanic arc
Shuichi Kodaira
Takeshi Sato
Narumi Takahashi
Seiichi Miura
Yoshihiko Tamura
Yoshiyuki Tatsumi
Yoshiyuki Kaneda
Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology,
Showa-machi 3173-25, Kanazawa-ku, Yokohama, Japan
Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology,
Natsushima 2-15, Yokosuka, Japan
ABSTRACT
The process by which continental crust has formed is not well
understood, though such crust mostly forms at convergent plate margins today. It is thus imperative to study modern intra-oceanic arcs,
such as those common in the western Pacific Ocean. New seismic
studies along the representative Izu-Bonin intra-oceanic arc provide
unique along-strike images of arc crust and uppermost mantle to
complement earlier, cross-arc lithospheric profiles. These reveal two
scales (1000–10 km scale) of variations, one at the scale of the Izu versus Bonin (thick versus thin) arc crust, the other at the intervolcano
(~50 km) scale. These images show that: (1) the bulk composition of
the Izu-Bonin arc crust is more mafic than typical continental crust,
(2) the middle crust with seismic velocities similar to continental
crust is predominantly beneath basaltic arc volcanoes, (3) the bulk
composition beneath basaltic volcanoes changes little at thick and thin
arc segments, and (4) a process to return lower crustal components to
the mantle, such as delamination, is required for an arc crust to evolve
into continental crust. Continued thickening of the Izu-Bonin crust,
accompanied by delamination of lowermost crust, can yield velocity
structure of typical continental crust.
Keywords: intra-oceanic arc, continental crust, crustal evolution, seismic
imaging, Izu-Bonin arc.
INTRODUCTION
The growth of continents is an important geoscientific issue. It
appears that crust of this composition was accomplished by the episodic generation of intermediate or andesitic magmas, perhaps at intraoceanic arcs (Taylor, 1967; Rudnick, 1995; Rudnick and Fountain, 1995;
Suyehiro et al., 1996), but that is questionable. The most problematic
issue in understanding the evolution of continental crust is that presentday intra-oceanic arcs are dominated by basaltic magmatism, but the bulk
composition of continental crust is andesitic (Arculus, 1981; Pearcy et al.,
1990). Several petrological models have been proposed to address this
paradox (Rudnick, 1995; Tatsumi, 2005), for example: (1) direct generation of andestic magma in the mantle by the reaction of slab-derived melts
with mantle peridotites (e.g., Kelemen, 1995; Rapp et al., 1999) or by
hydrous melting of mantle peridotite (e.g., Kushiro, 1974), and (2) fractional crystallization of basaltic magmas and/or remelting of basaltic (or
mafic) initial arc crust and subsequent delamination of mafic lower crustal
material (e.g., Kay and Kay, 1993; Jull and Kelemen, 2001; Tatsumi and
Kogiso, 2003). The first model predicts the direct production of crust with
both andesitic and basaltic components throughout the crustal evolution
process (e.g., Tamura and Tatsumi, 2002), while the second model predicts a mafic bulk composition of crust before delamination, and that the
composition is probably even more mafic in the initial stage of crustal
evolution. Delamination is also required in the second model because even
the mantle-derived magnesian andesites are more mafic than continental
crust. Until arcs are deeply drilled, only geophysical investigations can
constrain the composition and structure of arc crust.
We can infer the bulk composition of crust if we have good information about its seismic velocity structure (e.g., Smithson et al., 1981;
Kelemen and Holbrook, 1995). The temperature and composition of
crust affect seismic velocities and thus calculations of average velocity
of the crust. For example, seismic imaging, laboratory experiments, and
thermodynamic modeling have shown that the seismic velocities at constant temperature depend primarily on the MgO and SiO2 content. The
seismic velocity of bulk crust increases with increasing MgO content
and decreasing SiO2 content (Kelemen and Holbrook, 1995; Behn and
Kelemen, 2006), but addition of other oxides, such as FeO, CaO, and
Al2O3, did not substantially affect seismic velocity, Vp, (Kelemen and
Holbrook, 1995). This means that crust of higher bulk seismic velocity
than typical continental crust has a more mafic or basaltic bulk composition (i.e., higher MgO content) than typical continental crust, and
that crust of slower bulk seismic velocity is more silicic or andestic
(i.e., higher SiO2 content). Temperature also influences seismic velocities in island arcs. However, we concluded that for the purpose of this
study, the effect of temperature on velocity (i.e., ~0.2 km/s) is insignificant. Temperature (T ) variations of several hundreds of degrees
Celsius are required to obtain a velocity variation of ~0.2 km/s if an
average value of the temperature derivative for crustal rocks is used
(dVp/dT = –4 × 10−4 km/s/°C) (Rudnick, 1995). However, even larger
temperature variations (>1 × 103 °C) are required to explain the velocity variations discussed in this study if we apply a temperature derivative measured directly from rocks sampled at the Izu collision zone
[dVp/dT = (–2.7 to –5.3) × 10−5 km/s/°C] (Kitamura et al., 2003). Temperature variations of this magnitude are unprecedented in crustal rocks.
In this paper, we show new seismologically derived constraints on
the evolution process of continental crust. For our study, we selected the
Izu-Bonin arc, where the Pacific plate subducts beneath the Philippine Sea
plate (Fig. 1). An advantage of the Izu-Bonin arc is that the effect of preexisting continental crust has been minimal since subduction of the Pacific
plate initiated ca. 48 Ma (Stern et al., 2003). Moreover, different stages of
a crustal evolution have been proposed for the Izu and Bonin parts of the
arc system, on the basis of submarine topography, rock chemistry, distribution of hypocenters of deep earthquakes, and tectonic history (Stern et al.,
2003; Yuasa, 1991). If we accept a tectonic scenario where the spreading
center of the Parece Vela Basin once propagated to the south of the Izu
arc ca. 25 Ma, an earlier stage of crustal evolution may be expected at
the Bonin arc than at the Izu arc. This scenario proposes the generation
of a wedge-shaped oceanic crust to the south of the Izu arc in the area of
the present-day Bonin arc (Yuasa, 1991). In addition to the difference of the
tectonic history between the Izu and the Bonin arc, a difference of magma
production rate might be another factor controlling variation of crustal
volume along the arc.
DATA ACQUISITION AND PROCESSING
We conducted two active-source wide-angle seismic experiments
(Fig. 1) to image the structure immediately beneath the volcanic fronts of
the Izu arc (in 2004) (Kodaira et al., 2007) and the Bonin arc (in 2005).
© 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
GEOLOGY,
November
2007
Geology,
November
2007;
v. 35; no. 11; p. 1031–1034; doi: 10.1130/G23901A.1; 4 figures; Data Repository item 2007252.
1031
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ren
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Downloaded from geology.gsapubs.org on February 2, 2012
35°
Myk
Mkr
c
Izu ar
Krs
Hcj
Shc
Ags
Ssc
Tsm
Sfg
G
Nsi
Bonin
Kkt
Ktk
arc
Kij
Pacific
Plate
ar
Parece Vela Basin
na
ia
ar
M
Philippine
Sea Plate
c
135°
140°
145°
Figure 1. Location map and topographic map of Izu-Bonin arc. Black
and blue lines indicate wide-angle seismic profiles in 2004 and 2005,
respectively. Os—Oh-shima; Nij—Nii-jima; Myk—Miyake-jima; Mkr—
Mikura-jima; Krs—Kurose hole; Hcj—Hachijo-jima; Shc—South
Hachijo caldera; Ags—Aoga-shima; Myn—Myojin knoll; Sms—
South Sumisu; Ssc—South Sumisu caldera; Tsm—Torishima; Sfg—
Sofugan; G—Getsuyo seamount; Ka—Kayo seamount; S—Suiyo
seamount; Kn—Kinyo seamount; D—Doyo seamount; Nsi—Nishinoshima; Kkt—Kaikata seamount; Ktk—Kaitoku seamount; Kij—Kita
Iou-jima; SFG-TL—Sofugan tectonic line.
In the both experiments, a linear array of densely deployed (~5 km
spacing) ocean-bottom seismographs (OBSs) and a large air-gun array
(~197 L) were used. Our previous study (Kodaira et al., 2007) only
described a seismic model obtained from the 2004 experiment, while this
study processed the data from the 2005 experiment (Fig. 2; GSA Data
Repository Appendix, Fig. DR11) and compiled structural models imaged
from both data sets. Consequently, compiling the two data sets provides a
deep seismic transect along the total of a 1050-km-long profile.
We modeled the wide-angle seismic data in two ways: to image seismic velocity we used seismic refraction arrival tomography (Zhang and
Toksöz, 1998), and to image seismic reflectivity we used a diffractionstack migration approach using picked reflection traveltimes (Fujie et al.,
2006). Details of the modeling procedure are provided in the Appendix
(Figs. DR1–DR3). In order to estimate a resolution of the model, we performed a checkerboard resolution test following a procedure described
by Zelt and Barton (1998). The results of the test (Kodaira et al., 2007;
Fig. DR4) show that the structure shallower than the dashed line in Figure 3A is well resolved by our data set.
1
GSA Data Repository item 2007252, Appendix, (modeling procedure,
including reflectivity imaging, traveltime fitting, and resolution of the model),
is available online at www.geosociety.org/pubs/ft2007.htm, or on request from
editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder,
CO 80301, USA.
1032
S
8
4
Crus
tal r
efra
ction
160
120
80
40
Offset (km)
00
ion
act
refr
stal
u
r
C
40
Mantle refraction
refraction
Mantle
80
120
Offset (km)
160
Ridge
e
25°
SF
G-T
L
idg
North
American
Plate
Kn
D
Bonin
uR
ala
S
OBS50 Bonin arc
S N
Figure 2. Examples of observed record sections from OBS31 (oceanbottom seismometer) in the Izu profile and OBS50 in the Bonin profile. Because apparent velocities of the first arrivals were strongly
affected by seafloor topography, we applied static corrections for
the water layer in plotting data. Locations of OBSs are shown in Figure 3A. Reduction velocity used was 8 km/s. Horizontal axis indicates offset distance from OBS. A 5–15 Hz band-pass filter and 2 s
automatic gain control were applied. See Fig. DR1 (see footnote 1)
for more examples of data.
nch
-P
Ka
Eurasian
Plate
OBS31 Izu arc
N
6
0
Myn
Sms
nin Tre
hu
us
Ky
30°
10
2
Izu-Bo
Shikoku Basin
2005
Jap
a
Os
Nij
2004
T - D/8 (s)
Japan
SEISMIC IMAGE AND DISCUSSION
The seismic data revealed marked structural differences between
the Izu and Bonin arcs (Fig. 2). In the Izu section (left panel in Fig. 2), the
observed range of the crustal refraction phase, which reflects the thickness of crust, is more than three times as long as that of the Bonin section
(right panel in Fig. 2) (Fig. DR5). This difference indicates a considerable
increase of crustal volume toward the Izu arc. The final seismic image
using those data shows that the largest volume of crust (~32 km thick) is
under the Izu arc beneath Hachijo-jima, and that the smallest volume of
crust (~10 km thickness) is between the Kayo and Suiyo seamounts in the
Bonin arc, where the major tectonic line (the Sofugan tectonic line) meets
the volcanic front (Fig. 3). The 10-km-thick crust represents the thinnest
arc crust among reported crustal structure of arcs on the Earth.
The most important finding is the variation of the volume of the
middle crust (Vp = 6.0–6.8 km/s). We interpreted the middle crust to be
composed of plutonic rocks of felsic (Vp = 6.0–6.5 km/s) to intermediate
(Vp = 6.5–6.8 km/s) composition on the basis of comparison to sonic-wave
velocities of exposed rocks sampled at the collision zone between the Izu
arc and central Japan (Kitamura et al., 2003). The felsic to intermediate
composition is believed to be evidence of the continental composition of
crust (Rudnick, 1995). Comparing the chemical composition of volcanic
rocks (Bloomer et al., 1989; Yuasa and Nohara, 1992; Kodaira, et al., 2007)
with the variation in volume of the middle crust (Figs. 3B, 3C) clearly demonstrates that the volume of the middle crust increases toward the centers of
each of the basaltic volcanoes (red dots in Fig. 3B). Although the volume
of middle crust in the Bonin arc is one-third of that in the Izu arc, the above
structural characteristics (i.e., thickening the middle crust toward the centers of basalt volcanoes) are observed along the entire arc system (Fig. 3B).
From these observations we conclude that the felsic to intermediate component of the crust was created beneath the basaltic volcanoes, and that this
process took place even for the thinner arc crust. The model shows seismic velocities in the range 6.8–7.6 km/s underlying the middle crust. The
seismic velocity of the upper half of this component (Vp = 6.8–7.2 km/s)
corresponds to the velocity of gabbroic plutons exposed in the collision
zone (Kitamura et al., 2003). The lower part of this component (Vp = 7.2–
7.6 km/s) is interpreted to represent mafic to ultramafic cumulates (Miller
and Christensen, 1994; Kodaira et al., 2007), but exposures of these rocks
have not been identified in the collision zone (Kitamura et al., 2003).
To qualitatively evaluate the variation of the structure and composition of the crust, we calculated the average crustal velocity (Fig. 3B) using
the seismic velocity between the top of the middle crust and the bottom of
the mafic to ultramafic cumulate component. The crustal material with a
velocity slower than 6.0 km/s (the upper crust) was excluded because upper
GEOLOGY, November 2007
Downloaded from geology.gsapubs.org on February 2, 2012
Figure 3. A: Seismic velocity image along volcanic front from Izu to Bonin arc obtained by seismic refraction tomography. Seismic image
of Izu section was reported on by Kodaira et al. (2007). Modeling procedure, including reflectivity imaging, traveltime fitting, and resolution
of the model, is shown in the Appendix, Figures DR2–DR4 (see footnote 1). Checkerboard test (Fig. DR3) shows that the structure shallower than the dashed lines is well resolved. Layers A–E indicate geological interpretations of seismic image: A—Upper crust consisting
of sediment, volcaniclastics, and volcanic rocks. B—Felsic composition plutons. C—Intermediate composition plutons. D—Mafic plutons.
E—Mafic to ultramafic cumulates. F—Upper mantle. See also the Appendix. B: Average crustal seismic velocity (black line) and thickness
of the middle crust (Vp = 6.0–6.8 km/s) (red line), which is interpreted to be plutonic rocks of felsic to intermediate composition. Black and
red dots indicate average seismic velocities and thicknesses of the middle crust, respectively, beneath basaltic volcanoes. Blue dots show
average crustal seismic velocities beneath basaltic volcanoes, but excluding the Vp = 7.2–7.6 km/s component. Orange shading shows the
velocity range of typical continental crust (Christensen and Mooney, 1995). C: Average wt % SiO2 of volcanic rocks sampled and dredged
from Quaternary volcanoes (Bloomer et al., 1989; Yuasa and Nohara, 1992; Kodaira, et al., 2007). Abbreviations as in Figure 1.
crustal velocities are considered to be strongly affected by several parameters other than crustal composition, such as variable fracture distribution
and porosity (e.g., Carlson and Gangi, 1985; Kelemen and Holbrook,
1995). We recognized the variation of average seismic velocity along the
arc, which correlates well with the volume variation of the middle crust.
However, the average seismic velocities beneath each basaltic volcano do
not vary (Vp = ~6.8 km/s) from the thick Izu arc to the thin Bonin arc
(black dots in Fig. 3B). This means that the volume ratios of each crustal
component are equivalent in the both thick Izu arc and the thin Bonin arc
beneath the basaltic volcanoes (Fig. 4A). It is important to note that those
velocities are remarkably higher than the average velocity of the typical
continental crust (Vp = 6.4 ± 0.21 km/s) (Christensen and Mooney, 1995).
These observations provide two strong constraints on the growth
process of continental crust: (1) the bulk chemical composition of the
crust beneath the basaltic volcanoes is the same for thick and thin arc
crust, and (2) even though felsic to intermediate crust has been formed
beneath the Izu-Bonin arc, the bulk chemical composition of the crust
beneath the basaltic volcanoes is still more mafic than that of typical continental crust. This latter observation suggests that to transform arc crust
into continental crust, there must be a process to return the mafic to ultramafic cumulates to the mantle, such as delamination (Kay and Kay, 1993),
foundering (Oliver et al., 2003), or transformation (Takahashi et al., 2007).
The requirement for such a process is well demonstrated by the calcu-
GEOLOGY, November 2007
lated average seismic velocities excluding the mafic to ultramafic cumulates (i.e., layer E in Fig. 3B; Vp = 7.2–7.6 km/s). The average velocities
derived this way beneath the basaltic volcanoes (blue dots in Fig. 3B) are
very close to the range of velocities for typical continental crust, with the
exception of the two volcanoes (Nishino-shima and Kaikata seamount).
This implies that continued thickening of the Izu-Bonin arc crust, accompanied by delamination of lowermost crust, can yield typical continental
crust velocity structure. Vertical extension of the velocity-depth (V-D)
profile (which represents crustal growth while maintaining constant volume ratios for each crustal component) (Fig. 4) also supports the proposed
process. A 250% vertical extension of the V-D profile beneath the middle
crust at the Suiyo seamount of the Bonin arc shows a similar pattern to that
of the Izu arc at Aoga-shima. In addition, the 150% vertical extension of
the V-D profile at the Izu arc agrees well with a typical continental crust
(Christensen and Mooney, 1995; Rudnick and Fountain, 1995), except for
~10 km of the mafic to ultramafic cumulates layer.
Another important outcome of this study is the finding of a unique
structure beneath the rhyolite volcanoes that is predominantly observed
between the large basalt volcanoes in the Izu arc among the Izu-Bonin
arc. Peak to peak values of the variation curve of the average velocity in
the Izu arc are larger (e.g., 0.25 km/s between Aoga-shima and Myojinknoll) (Fig. 3B) than those of the Bonin arc (e.g., 0.12 km/s near Kayo
seamount). This difference is mainly attributed by the difference of vol-
1033
Downloaded from geology.gsapubs.org on February 2, 2012
Shields and
Platforms
B
Erosion
Izu arc
Depth (km)
0
10
250 %
30
20
30
(7.1±0.2)
50
Delamination
Continental crust
C&M R&F
10
(6.3±0.2)
20 (6.6±0.1)150 %
40
0
Bonin arc
Depth (km)
A
40
1.8-5.8
Sediment, volcaniclastics,
volcanic rocks
6.0-6.5 Felsic plutons
6.5-6.8
Intermediate
composition plutons
6.8-7.2 Gabbroic plutons
7.2-7.6
Mafic to ultramafic
cumulates layer
Continental Moho
50
4
8
6
7
5
Velocity (km/s)
Aoga-shiima
Suiyo-smt.
Vertical extension
150 %
250 %
Figure 4. A: Schematic one-dimensional (1-D) structure and seismic
velocities of Bonin arc, Izu arc, and predicted continental crust (left).
Structure in red frame of left column represents typical continental
structure for shields and platforms (Rudnick and Fountain, 1995). For
thick arc crust to evolve to continental crust, a process is required to
return the component interpreted as mafic to ultramafic cumulates
(Vp = 7.2–7.6) to the mantle. The upper crustal component consisting of sediment, volcaniclastics, and volcanic rocks may be eroded
during the evolution. B: 1-D velocity-depth profile at Aoga-shima
in the Izu arc and at the Suiyo seamount in the Bonin arc. Dashed
blue and red lines show 150% and 250% vertically extended profiles
beneath the middle crust at Aoga-shima and the Suiyo seamount,
respectively. Velocity-depth profiles of typical continental structures
compiled by Christensen and Mooney (C&M, 1995) and Rudnick and
Fountain (R&F, 1995) are superimposed.
ume ratios of the middle crust, i.e., smaller volume ratios of the middle
crust beneath the rhyolite volcanoes. Those observations imply that the
bulk crustal composition beneath the rhyolite volcanoes in the Izu arc is
even more mafic. Although seismic data do not constrain a cause of the
rhyolite volcanism, our seismic image may suggest that there is a process
for making more mafic crust beneath the rhyolite volcanoes due to reduction of volume ratio of the middle crust.
ACKNOWLEDGMENTS
This study was funded by the Institute for Research on Earth Evolution, Japan
Agency for Marine-Earth Science and Technology.
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Manuscript received 19 March 2007
Revised manuscript received 11 May 2007
Manuscript accepted 29 June 2007
Printed in USA
GEOLOGY, November 2007