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The Izu-Bonin-Mariana Subduction Factory
Robert J. Stern, Geosciences Dept., U. Texas at Dallas, Box 830688, Richardson TX
75083-0688, [email protected]
The IBM arc system lies in the Western Pacific and defines the eastern margin of
the Philippine Sea Plate (Fig. 1). Because IBM is an endmember arc system in a number
of ways, it is an outstanding natural laboratory for studying earth’s largest geodynamic
system, the Subduction Factory. IBM is the largest intra-oceanic convergent margin,
being constructed entirely within and upon oceanic lithosphere. Lithosphere produced
during the ~48 million years that this Subduction Factory has operated makes up a region
about the size of India, but the presently active part extends for about 300 km west of the
IBM trench. IBM manifests subduction of Pacific lithosphere, with the Pacific moving
NW relative to IBM, at rates that vary from about 20 mm/a south of Guam to almost
60mm/a near Japan [1]. The Philippine Sea Plate itself is moving rapidly northwestward.
Subducted lithosphere varies in age from mid-Jurassic (~170 Ma) outboard of the
Marianas to early Cretaceous (~130Ma) adjacent to Japan (Fig. 2). The combination of a
retreating upper plate and an extremely old (hence dense) subducting plate results in a
strongly extensional convergent plate boundary, particularly in the southern IBM where
these effects are maximized.
Off-ridge volcanism was common during mid- and Late Cretaceous time on the
Pacific Plate now outboard of southern IBM. As a result, there are significant differences
in the bulk composition of subducted sedimentary columns in the north and south,
although the thickness of sediments is relatively constant at about 500m [2]. This modest
sediment thickness is well below the ~1km thickness required for development of an
accretionary prism [3] and is completely subducted. Plate convergence is oblique over
most of the IBM arc, approaching pure sinistral strike-slip motion in the northern
Mariana arc (Fig. 3). The Wadati-Benioff zone is well-defined beneath IBM, dipping
about 45° in the north and nearly vertically in the south (Fig. 4); some of the deepest
seismic activity in earth – down to 700 km - is found beneath the central Mariana arc.
These and other physical constraints need to be identified and incorporated in the
construction of realistic, 4-D models of mantle and fluid flow, thermal evolution, and
fluid/melt generation, migration, and storage.
Earth’s only T-T-T triple junction defines the northern end of IBM, where
collision continues with southern Honshu at a rate of about 40mm/a. This provides an
opportunity to study terrane accretion in action [4] as well as exhuming IBM arc middle
crust [5]. These exposures of arc crust are correlatable with crustal structure inferred for
in situ IBM crust (Fig. 5). Not only does northern IBM provide an unparalleled
opportunity to examine the formation of juvenile continental crust, societal
considerations compel it: continuing convergence between buoyant IBM crust and
Honshu presents an imposing earthquake hazard to the greater Tokyo metropolitan area,
with far-reaching implications for the global economy. An analogous opportunity to
study deeper arc crust and upper mantle lies on the wall of the 11km-deep Challenger
Deep at the south end of the IBM arc system.
Three combined forces – retreat of the Philippine Sea Plate, subduction of
unusually dense lithosphere, and oblique convergence – combine to make IBM the most
strongly extended convergent margin on the planet. This evisceration provides unique
opportunities to monitor the Subduction Factory. Extension in the southern IBM is
oriented trench-parallel in the forearc and trench-normal in the back-arc [6]. This
strongly extensional regime provides three opportunities - forearc, active arc, and backarc basin - to sample Subduction Factory fluids and melts, more than any other
convergent margin on the planet. The Mariana forearc contains the only known
occurrences of subduction-related serpentinite mud volcanoes (SMV) [7] on this planet.
Flows from Mariana SMV contain fragments of blueschist from the subducted slab [8] as
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well as abundant mantle fragments [9]; several SMV are actively venting slab-derived
fluids, some of which support active chemosynthetic communities.
Magmatic activity is expressed differently within and along the IBM arc. A true
back-arc basin (BAB), with seafloor spreading, is developed only in the Mariana Trough
[10], although well-developed inter-arc basins are developed in the Bonin Arc Rifted
Zone farther north (Fig. 6) [11, 12]. The active magmatic arc is largely submarine but
with abundant subaerial volcanoes [13, 14]; strong variations in magmatic compositions
affect both submarine and subaerial edifices. BAB lavas are often aphyric or have pillow
rim glass that can readily be separated, and a substantial proportion are primitive basalts;
these characteristics suit BAB lavas for analysis using approaches perfected for MORB.
Arc lavas are predominantly porphyritic so that bulk compositions generally do not
correspond to magmatic liquids, and should not be studied using the techniques
appropriate for studying aphyric or glassy samples. Accumulation of plagioclase
phenocrysts in particular has led to a misperception that mafic members are dominantly
high-Al basalts when in fact aphyric samples or glass inclusions in phenocrysts are
tholeiites. Primitive compositions (Mg#>65) are uncommon among IBM arc lavas, so
fractionation conditions and history need to be resolved. Old techniques for studying
petrography should be revived and new techniques will have to be perfected if we are to
understand the magmatic evolution of porphyritic arc lavas. In future studies,
petrographic descriptions should be reported with major and trace element data. The
need to find new ways to study evolved, porphyritic samples promises to revive
traditional petrography as well as stimulate developments in quantitative
petrography/image analysis and microbeam analytical techniques. These techniques will
also aid investigations of abundant cumulate xenoliths, found in the lavas of several IBM
volcanoes. One issue that awaits resolution is the abundance of felsic material in the
IBM arc. IBM has traditionally been thought to have a basaltic bulk composition, but
recent evidence from geophysics [15], exposures in the collision zone [5], glass
inclusions in phenocrysts [16], and the abundance of felsic tephra in DSDP cores [17]
indicates that felsic rocks comprise an important part of the IBM arc.
In spite of the different eruptive styles and extent of fractionation for arc and
BAB, there are strong compositional affinities between arc and BAB suites, which
provide different perspectives on important controversies and enigmas. The trace element
signatures of these lavas strongly manifest the ‘subduction component’: enrichments in
large-ion lithophile elements and depletion in high-field strength elements, both
compatible and incompatible (Fig. 7). All arc and most BAB lavas have elevated water
contents [12, 18] such that it is controversial the extent to which melts are generated by
decompression [19] or fluxing by hydrous fluids [20]. One abiding mystery concerns how
water gets into the source of Mariana BAB magmas when the subducted slab does not lie
beneath the spreading ridge? In contrast to widespread recognition that water in IBM
melts is recycled from subducted materials, the source of other elements in IBM melts is
less clear. In particular, controversy continues regarding the extent to which the IBM
‘subduction component’ manifests fractionations imposed when elements are transferred
from the subducted plate to the overlying mantle wedge as opposed to being developed
during re-equilibration of hydrous fluids and melts with convecting mantle. U-Th
disequilibria studies indicate that strong fractionation of radionuclides occurred within
the last 30 kyr [21], providing timescales for fluid-mediated fractionation but where and
how this occurs are unresolved. There is abundant evidence that some subducted
components
are recycled and
can be found
in young207lavas,
especially elevated water
7
11
34
204
contents, ∂ Li [22], B/Be, ∂ B [23], ∂ S [24], and Pb/ Pb [25]. These studies
differently emphasize roles of subducted sediments and altered oceanic crust. Rare gas
data is lacking
for
arc lavas but BAB lavas contain recycled atmospheric Ar in spite of
3
4
mantle-like He/ He
[26]; similar datasets for arc lavas promise to provide important
10
constraints. The Be signal is muted probably because subducted sediment are much
older than the half-life of this isotope [27]. Other isotopic and trace element data sets do
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not readily allow recycled components to be identified: O, Sr, Nd, and Pb isotopic
compositions as well as K/Rb and K/Ba are remarkably constant in arc lavas despite
being extremely heterogeneous in subducted components. This homogeneity requires
efficient mixing of subducted components or effective re-equilibration of ascending
fluids with convecting mantle. It is critical to resolve the nature of the mantle source
beneath IBM; there are strong indications from Nd, Hf, Pb, and Os data that this mantle
has affinities to that beneath the Indian Ocean [28-30].
Disparate geochemical and isotopic data sets and controversial conclusions
provide important constraints for understanding how the IBM Subduction Factory
operates. It will be an exciting challenge for scientists with different talents and
perspectives to synthesize these observations and use these to develop and test
hypotheses to better understand the operation and budget of this outstanding example of
the Subduction Factory. A critical aspet will be to collect and distribute representative
samples to analysts and to communicate these results to modelers, and for modellers to
tell analysts what kinds of information they need. Another critical effort will be the
seismic imaging of the IBM Subduction Factory, with as high a resolution as possible.
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14. Yuasa, M., et al., Submarine topography of seamounts on the volcanic front of the
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Figure Captions:
Figure 1: IBM arc system, showing extent of crust generated over the 48 Ma life of the
arc (fossil plus active) as well as those components which comprise the presently active
IBM Subduction Factory. Also shown is the location of the deepest place on the face of
the earth, the Challenger Deep (~11km deep) and the IBM collision zone.
Fig. 2: Seafloor feeding into the IBM Subduction Factory, modified after [31]. Arrows
are relative velocities of the Pacific Plate with respect to the Philippine Sea Plate, in
mm/a, after [1]. DSDP and ODP sites sampling units being subducted beneath IBM are
shown as well. Note that the sedimentary section being subducted beneath northern IBM
has fewer volcanics and volcaniclastics than that being subducted beneath the southern
IBM.
Fig. 3: Obliquity of convergence between the Pacific and Philippine Sea plates, as
inferred from earthquake slip vectors and modified after [32]. Note that convergence is
highly oblique over much of the IBM arc system.
Fig. 4: Generalized topology of IBM Wadati-Benioff Zone, modified after [33]. Two
perspectives are shown, with contours colored at every 100 km depth.
Figure 5: Structure of IBM arc crust at 32°15’N, modified after [15]. Vertical
exaggeration is about 10x. Note the crustal thickness of 22km is nearly four times that of
oceanic crust but only about half that of normal continental crust.
Figure 6: Along-strike profiles of the IBM arc system, from Japan (left) to Guam (right).
The thick solid line shows the bathymetry and topography along the volcanic axis of the
active arc, with the thin dashed horizontal line marking sea level. The approximate
locations of the principal island groups (Izu, Bonin-Volcano, and Mariana) are shown.
Submarine volcanoes (and the Sofugan Tectonic Line, STL) are given as italicized
abbreviations: Ku, Kurose;Ms, Myojin-sho; Do, Doyo; Kk, Kaikata; Kt, Kaitoku;F,
Fukutoku-oka-no-ba; HC, Hiyoshi Volcanic Complex, Nk, Nikko; Fj, Fukujin, Ch,
Chamorro, D, Diamante; R, Ruby, E, Esmeralda; T; Tracy. Subaerial volcanoes are given
as normal abbreviations: O, Oshima; My, Miyakejima; Mi, Mikurajima; H, Hachijojima;
A, Aogashima; Su, Sumisujima, T, Torishima; Sg, Sofugan; Nishinoshima; KIJ, Kita Iwo
Jima; IJ, Iwo Jima; MIJ, Minami Iwo Jima; U, Uracas; M, Maug; As, Asuncion; Ag,
Agrigan; P, Pagan; Al, Alamagan; G; Guguan; S, Sarigan; An, Anatahan. Dominant
compositions of arc segments are also indicated. Locations of important zones of intraarc and back-arc extension in the north (Bonin Arc Rifted Zone) and south (Mariana
Trough Back-Arc Basin) are marked. The thick dashed line shows the maximum depth
in the trench along its strike. Frontal arc elements are not shown, but consist of the Bonin
or Ogasawara Islands between 26° and 28°N and the Mariana frontal arc islands between
13° and 16°N. ICZ = IBM collision zone.
Fig. 7: ‘Spider’ diagram for Mariana arc lavas. Elements are listed in order of increasing
compatibility in mantle minerals; data for typical Maraian arc lavas is from [34]. Notice
strong enrichments in LIL and depletions in HFSC, including Nb and Ta.
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IBM
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180°
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South
Izu Bonin
Mariana
60
30
orthogonal convergence
0
-30
-60
sinistral
strike-slip
34.1°
0
25.2°
16.9°N
1000
2000
plate convergence obliquity
(from earthquake slip vectors)
142°E
Obliquity (°) or rate (mm/y)
North
-90
3000 km
arc-parallel fore-arc slip rate
128°E
138°
148°
Izu
Trench
28°N
N
18°
Mariana
Trench
8°
500 km
N
Volcanic
Arc
0
Fore-arc
8.5
Depth (km)
2
5
Trench
10
Crust
15
Mantle
20
M
Crust
o
oh
Moho
Mantle
139
140
141
Longitude ( ¡E)
142
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
100 km
2.0
25
138
8.0
P-wave Seismic Velocity (km/sec)
Pacific
Plate
143
1.5
Shoshonite
ICZ
Low-K Tholeiite
Izu Islands
+1 O
SL
-2
My Ku
Ms
Mi H A
Su
Medium-K Basalt
Bonin &Volcano Islands
STL
T Sg
Do N KkKt KIJ
Mariana Islands
IJ F HC
MIJ Nk
Fj
U
Al
M Ch
An R
As Ag P G S D E
T
Active Arc
Bonin Arc Rifted Zone
-4
(Km)
-6
Mariana Trough Back-Arc Basin
500 km
Trench
-8
-10
34
32
30
28
26
24
22
Degrees N latitude
20
18
16
14
Sample/MORB
100
Mariana Shoshonites
10
Mariana Trough
1
Mariana Islands
Cs Ba U Ta La Pb Sr Nd Hf Eu Ti HREE
Rb Th K Nb Ce Pr P Zr Sm HREE