Content uploaded by Spahr Webb
Author content
All content in this area was uploaded by Spahr Webb on May 27, 2014
Content may be subject to copyright.
13. H. Rabitz et al.,Science 288, 824 (2000).
14. H. Rabitz, W. S. Zhu, Acc. Chem. Res. 33, 572 (2000).
15. R. J. Levis, M. J. DeWitt, J. Phys. Chem. A 103, 6493
(1999).
16. J. M. Geremia, W. S. Zhu, H. Rabitz, J. Chem. Phys.
113, 10841 (2000).
17. M. M. Wefers, K. A. Nelson, J. Opt. Soc. Am. B 12,
1343 (1995).
18. D. Goldberg, Genetic Algorithms in Search Optimiza-
tion and Machine Learning (Addison-Wesley, Reading,
MA, 1989).
19. M. Demiralp, H. Rabitz, Phys. Rev. A 47, 809 (1993).
20. K. Sundermann, H. Rabitz, R. de Vivie-Riedle, Phys.
Rev. A 62, 013409 (2000) (available at http://link.
aps.org/abstract/PRA/v62/e013409).
21. In the reference experiments, the intensity of the
laser pulse was decreased by inserting glass coverslips
into the beam before focusing, which decreased the
intensity by ⬃7% per slide. The pulse duration was
increased by placing a linear chirp on the pulse by
either increasing or decreasing the position of the
second grating in the compressor. The mass spectra
were recorded and analyzed as a function of these
two variables to provide a reference for the shaped-
pulse control experiments.
22. M. J. DeWitt, R. J. Levis, J. Chem. Phys. 110, 11368
(1999).
23. R. J. Levis, G. M. Menkir, H. Rabitz, data not shown.
24. Drawing unambiguous mechanistic conclusions from
analysis of the detailed kinematic field structures
alone is difficult at this time. Furthermore, before
analyzing the field, suitable cost functions need to be
introduced in the algorithm guiding the experiments
to assure that only the essential structure is retained
(16).
25. J. Berkowitz, G. B. Ellison, D. Gutman, J. Chem. Phys.
98, 2744 (1994).
26. The authors acknowledge the support of the Office of
Naval Research, the Army Research Office, the NSF,
and the Sloan and Dreyfus foundations for the sup-
port of this research. R.J.L. acknowledges fruitful dis-
cussions with A. Markevitch, N. P. Moore, and P.
Graham.
18 January 2001; accepted 15 March 2001
Published online 29 March 2001;
10.1126/science.1059133
Include this information when citing this paper.
A Complex Pattern of Mantle
Flow in the Lau Backarc
Gideon P. Smith,
1
Douglas A. Wiens,
1
Karen M. Fischer,
2
Leroy M. Dorman,
3
Spahr C. Webb,
4
John A. Hildebrand
3
Shear-wave splitting analysis of local events recorded on land and on the ocean
floor in the Tonga arc and Lau backarc indicate a complex pattern of azimuthal
anisotropy that cannot be explained by mantle flow coupled to the downgoing
plate. These observations suggest that the direction of mantle flow rotates from
convergence-parallel in the Fiji plateau to north-south beneath the Lau basin
and arc-parallel beneath the Tonga arc. These results correlate with helium
isotopes that map mantle flow of the Samoan plume into the Lau basin through
an opening tear in the Pacific plate.
Seismic anisotropy (1) is usually attributed to
the alignment of crystal orientations, which
in turn can be related to the strain history of
the rock (2–5). Strain can also be inferred
from modeling of mantle flow (6), and thus
observation of seismic anisotropy can be used
to map mantle flow at length scales related to
the wavelength of the seismic waves. Many
observations of anisotropy have been made in
the region of subduction zones (7). However,
an unambiguous interpretation of these re-
sults is often difficult because of poor station
coverage or nonuniform source distribution.
Here we use a unique data set, which spans an
active backarc basin and spreading center, to
map out the mantle flow in a backarc system
and compare the seismic measurements to
geochemical studies and model predictions.
Modeling of the strain resulting from flow
coupled to the subducting plate (6,8) predicts
a fairly uniform pattern of anisotropy, with a
fast direction parallel to the absolute plate
motion of the downgoing plate. A variety of
shear-wave (S-wave) splitting measurements
at island stations in backarc areas are consis-
tent with this pattern (9–14 ) or with flow
coupled to both subducting and overlying
plates. However, closer to the trench and slab,
the pattern of mantle flow may become more
complex. Large-scale deviation of mantle
flow due to retrograde motion of the sub-
ducted slab has been postulated (15) and was
reported by S-wave splitting studies in South
America (16 ). Similar observations in New
Zealand (17 ) and Kamchatka (18) may also
result from such a flow pattern. Physical
modeling of subduction zone flow also indi-
cates strong variations in mineral alignment
with slab dip (19). Numerical modeling of the
likely induced lattice preferred orientation of
olivine and orthopyroxene produces results
that are non-unique and may only be fully
tested with a more detailed mapping of the
backarc system (13,20). It is often difficult to
infer the exact location of the anisotropy and
thus to determine whether observations result
from propagation within an anisotropic man-
tle wedge or within the slab.
In the Lau backarc, there is also the ques-
tion of the effect of the small-scale processes
associated with the spreading center. Al-
though modeling predicts vertical preferential
alignment of the olivine aaxis due to the
upwelling flow (21), a variety of fast direc-
tions have been noted in other spreading re-
gions (22–26 ).
In this study, we present splitting mea-
surements from the Lau backarc. These ob-
servations provide strong constraints on lat-
eral variations in the fast axis and thus allow
us to distinguish geographic variations in an-
isotropy that may occur across the backarc
basin. The region of the Lau basin and Tonga
arc contains both an active backarc spreading
center and a rapidly subducting slab (at a rate
of 240 mm/year), so there should be a strong
and variable signature of mantle flow. The
high rate of seismic activity in this region
also provides numerous high-energy sources
for S-wave studies.
We analyzed S-wave splitting in arrivals
from local earthquakes occurring beneath the
Lau backarc. Data were obtained from the
southwest Pacific seismic experiment
(SPASE) and from the Lau basin ocean-bot-
tom seismograph (OBS) survey (LaBatts).
The SPASE array was deployed for 2 years
and consisted of 12 broadband stations in
Fiji, Tonga, and Niue Island. LaBatts was a
concurrent 3-month deployment of 29 OBSs
in the Lau backarc and Tonga forearc.
The OBS instrument orientations (27 ) were
determined by comparing the polarization an-
gles (28)ofthePwaves and Rayleigh waves
from large, well-located, distant events, with
known back azimuth. Splitting observations
(29) were obtained using a cross-correlation of
the two Swaves calculated for a range of
rotation angles, , and time offsets, ␦t(9). The
␦tand providing the maximum cross-corre-
lation are the splitting time and fast anisotropy
azimuth (Fig. 1). Some of the land station ob-
servations are taken from the analysis of Fi-
scher and Wiens (10). Reanalysis of a subset of
the Fischer and Wiens (10) data set using this
method produced identical results, indicating
that there is no bias between the results from the
two studies. In order to avoid interference from
the free surface or crustal phase conversions,
we restricted our analysis to arrivals inside the
S-wave “window” (incidence angles ⬍35°).
Well-constrained splitting parameters
were obtained for 77 arrivals at the OBS
stations and were combined with the existing
53 observations at land stations (10). Seven-
teen new land observations were also ob-
tained at Kadavu Island and at land stations at
1
Department of Earth and Planetary Sciences, Wash-
ington University in St. Louis, 1 Brookings Drive,
CB1169, St. Louis, MO 63130, USA.
2
Department of
Geological Sciences, Box 1846, Brown University,
Providence, RI 02912, USA.
3
Scripps Institution of
Oceanography, University of California, San Diego, La
Jolla, CA 92093–0215, USA.
4
Lamont-Doherty Earth
Observatory, Post Office Box 1000, 61 Route 9W,
Palisades, NY 10964, USA.
REPORTS
www.sciencemag.org SCIENCE VOL 292 27 APRIL 2001 713
the eastern end of the basin (Fig. 2). Results
at the land-based stations on the Fiji platform
to the west are consistent with the direction of
subducting plate motion (10). However, the
pattern of fast azimuths paralleling the sub-
duction direction for stations on the Fiji plat-
form is not apparent for stations in the Lau
basin and Tonga arc. Indeed, many of the fast
vectors are almost trench parallel, perpendic-
ular to the azimuth of Pacific plate motion,
and the magnitude of the splitting varies with
smaller splitting times near the spreading
center. This observation is not predicted by
two-dimensional (2D) flow modeling, which
instead predicts an almost uniform splitting
time across the basin (20).
The splitting observations made for sta-
tion LKBA indicate a variation in the ob-
served splitting with source region (Fig. 3).
Events in the northern part of the basin indi-
cate a distinct north-south anisotropic fast
direction (Fig. 3). This contrasts with the
plate-motion-parallel directions in the south-
ern and western part of the basin and is
indicative of raypath dependence for the an-
isotropy. Neither variations at this length
scale nor the fast direction rotation are pre-
dicted by mantle flow that is driven by simple
coupling to the overlying or subducted plates,
assuming uniform viscosity and an infinite
planar slab (13).
For station NUKU (Fig. 4), most of the
splitting observations are from intermediate-
depth events (at depths of 150 to 300 km).
Several of these are within 200 km of the
station, and because of their relative location,
they are unlikely to have significant path
lengths within the slab. This indicates that the
along-strike azimuthal observations should
be explained by processes occurring within
the mantle wedge and cannot be attributed to
anisotropy within the subducted plate.
Fig. 1. (A) The original horizontal component seismograms recorded at OBS10. (B) Result of the
cross-correlation. The correlation coefficient at different azimuths and delay times has been
contoured, and a maximum was found at 120°, ⫺0.5 s. (C) The final rotated and time-shifted
seismograms.
Fig. 2. Stations used in the current study (triangles). Splitting observations are plotted at the
stations as vectors. The azimuth of each vector is the fast splitting direction, and its length is
proportional to the splitting time. Stations LKBA, NUKU, and OBS10 are marked. Land stations
where no well-constrained measurements were possible are marked with diamonds. The ocean-
bottom station where null measurements were made is marked with a circle. The bold single-
headed arrows indicate absolute plate motion vectors (39,40).
Fig. 3. Splitting observations made for station
LKBA plotted as vectors at the midpoint be-
tween station and event locations. Stations are
plotted as triangles.
REPORTS
27 APRIL 2001 VOL 292 SCIENCE www.sciencemag.org714
The observation of splitting fast axes par-
allel to the convergence direction at the west-
ern land stations (Fig. 2) can be explained by
the large-scale deformation of the mantle
driven by local coupling to the overlying and
subducting plates. Fischer et al.(20) demon-
strated that, given reasonable assumptions re-
lating the strength of lattice preferred orien-
tation to strain, the magnitude of the observed
splitting times could also be predicted by
plate-driven mantle flow models (13,20).
Although this type of modeling works
well for the stations at the western end of the
basin, the large-scale mantle flow cannot re-
produce either the source-region– dependent
anisotropic variations (Fig. 3) or the direction
and splitting times across the whole basin. To
explain the trends in our data, we need to
account for the structures in the backarc.
Splitting times in the center of the basin are
reduced and are almost orthogonal to the
plate motion direction. Interpretation of this
phenomenon as being related to the spreading
center, which is geographically close, would
fit qualitatively with the modeling of Black-
man et al. (21). They used a finite element
flow model to predict deformation in the
vicinity of a spreading ridge. Elsewhere at
mid-ocean ridges, surface waves have shown
a similar pattern (26 ), although the rotation
was not observed in SKS measurements,
which possibly integrated this signature with
the signature of deeper mantle flow (24 ).
However, although the direction of anisotro-
py is consistently north to south, the magni-
tudes are highly variable from station to sta-
tion, suggesting that this effect may not be
due to processes on the length scale of the
spreading center.
Closer to the trench, we measured large
trench-parallel splitting. To explain these obser-
vations, we need to consider the presence of the
subducting slab, because this is likely to be the
dominant factor in determining the anisotropy
close to the trench. Several explanations for the
along-arc fast direction orientations must be
considered. Water content may affect the defor-
mation (30) and lattice preferred orientation of
olivine (31) and thus may provide an explana-
tion for the observations near the slab. Howev-
er, the exact relationship between strain and
anisotropy, and how the observations in the
laboratory translate into observations in the real
Earth, are still poorly understood, and prelimi-
nary laboratory results indicate that this mech-
anism would not produce the observed rotation
(31). A second alternative is the anisotropic
effect of thin melt sheets or pockets (32). 2D
models of mantle flow predict melt sheet ori-
entations that would produce a trench-parallel
fast direction (20,33). However, this interpre-
tation is not entirely supported by the observa-
tions made of a progressively rotating anisotro-
pic fast direction. In addition, neither explana-
tion in terms of melt anisotropy nor the effect of
water content can explain the difference be-
tween our results in Tonga and those in the
Marianas and Izu-Bonin. Previous studies show
that both the Marianas (34 ) and Izu-Bonin (11)
instead exhibit strong anisotropy perpendicular
to the strike of the trench. If a generalized
explanation were possible for the rotation of the
fast direction in Tonga, such as either melt
anisotropy or water content, it should also be
observed in these other regions. Instead, we
must appeal to the individual tectonics of the
different regions to explain their measurements.
One such possible hypothesis is that the abso-
lute plate motions contain trench-parallel com-
ponents that result in the lithosphere between
the Lau spreading center and arc moving south-
ward relative to the slab and the Australian
plate. However, in uniform viscosity models for
Tonga with an infinite planar slab, this produces
an insufficient rotation of the fast directions
(13). We must also consider transpressional
deformation in the overlying plate. This mech-
anism should produce compression parallel to
convergence and thus aligment of olivine along
the arc. However, this explanation is inconsis-
tent with observations in various subduction
zones that the anisotropy increases with depth.
In addition, in Tonga we have rapid trench
rollback, implying extension within the arc.
One possible explanation is the effect of
slab rollback on mantle flow. The Tonga trench
axis is moving eastward at an absolute velocity
of ⬃10 cm/year, and the dip of the Tonga slab
has become progressively shallower over the
past few million years. Buttles and Olson (19)
examined the alignment of the olivine aaxis
using a laboratory analogy. They showed that
the rollback component of plate motion can
produce variations in mineral alignments. Their
results indicated trench-parallel aligment in the
Fig. 4. Splitting ob-
servations for station
NUKU. The splitting
parameters are plot-
ted as vectors at the
event location. The
station is shown as a
triangle. The depths
of the events (in
kilometers) are also
annotated.
Fig. 5. After (38). Helium isotope data suggest southward flow of shallow mantle into the Lau basin
through a tear in the subducted plate.
REPORTS
www.sciencemag.org SCIENCE VOL 292 27 APRIL 2001 715
forearc and subvertical realignment in the
wedge. However, their modeling did not in-
clude both slab dip and plate rollback, thus
preventing direct comparison to the Tonga
backarc. There is, however, geochemical evi-
dence to suggest that along-arc mantle flow is
occurring in this area. A change in Fiji magma-
tism from arc-like to ocean island basalt was
attributed to influx of the Samoan Plume
around 3 million years ago (35). A similar
explanation for high Nb relative to other high-
field-strength elements in lavas at the islands
Tafehi and Niuatoputapu at the northern end of
the Tonga-Kermadec subduction zone was also
proposed (36 ). In addition, helium isotope data
suggest flow of the Samoan Plume magma
toward the Peggy Ridge at the northern end of
the Lau basin (37 ). Later mapping by Turner
and Hawkesworth (38) mapped the presence of
these high
3
He:
4
He further south into the Lau
backarc. Such isotope signatures, which are
characteristic of the Samoan Plume, may be
evidence of the flow of shallow mantle (38)
from the Samoan Plume into the Lau basin,
parallel to the trench, through a tear in the
subducting Pacific plate (Fig. 5). These results
match both the geographical locations of our
stations and the azimuth of mantle flow we
would infer from our anisotropy observations.
We infer, therefore, that the observations of
along-arc fast anisotropy axes reflect this geo-
chemical mapping of along-arc mantle flow and
are probably resulting from slab rollback and
the along-strike component of the absolute plate
motions.
References and Notes
1. Seismic anisotropy is the phenomenon in which the
velocity of a seismic wave through a medium de-
pends on the wave’s polarization or propagation
direction.
2. N. I. Christensen, Geophys. J. R. Astron. Soc. 76,89
(1984).
3. S. Zhang, S. Karato, Nature 375, 774 (1995).
4. A. Tommasi, Earth Planet. Sci. Lett. 160, 1 (1998).
5. M. Bystricky, K. Kunze, L. Burlini, J. P. Burg, Science
290, 1564 (2000).
6. N. M. Ribe, J. Geophys. Res. 94, 4213 (1989).
7. M. K. Savage, Rev. Geophys. 37, 65 (1999).
8. D. McKenzie, Geophys. J. R. Astron. Soc. 58, 689
(1979).
9. J. R. Bowman, M. Ando, Geophys. J. R. Astron. Soc. 88,
25 (1987).
10. K. M. Fischer, D. A. Wiens, Earth Planet. Sci. Lett. 142,
253 (1996).
11. M. J. Fouch, K. M. Fischer, J. Geophys. Res. 101, 15987
(1996).
12. K. M. Fischer, M. J. Fouch, D. A. Wiens, M. S. Boettcher,
Pure Appl. Geophys. 151, 463 (1998).
13. C. E. Hall, K. M. Fischer, E. M. Parmentier, J. Geophys.
Res. 105, 28009 (2000).
14. E. a. J. N. Sandvol, J. Geophys. Res. 102, 9911 (1997).
15. W. Alvarez, J. Geophys. Res. 87, 6697 (1982).
16. R. M. Russo, P. G. Silver, Nature 263, 1105 (1994).
17. K. Gledhill, D. Gubbins, Phys. Earth Planet. Int. 95,
227 (1996).
18. V. Peyton et al.,Geophys. Res. Lett. 28, 379 (2001).
19. J. Buttles, P. Olson, Earth Planet. Sci. Lett. 164, 245
(1998).
20. K. M. Fischer, M. Parmentier, A. R. Stine, E. R. Wolf, in
preparation.
21. D. K. Blackman et al.,Geophys. J. Int. 127, 415
(1996).
22. H. H. Hess, Nature 203, 629 (1964).
23. C. J. Wolfe, S. C. Solomon, D. R. Toomey, D. W.
Forsyth, Eos 77, F653 (1996).
24. C. J. Wolfe, P. G. Silver, J. Geophys. Res. 103, 749
(1998).
25. I. T. Bjarnason, C. J. Wolfe, S. C. Solomon, G. Gud-
mundson, Geophys. Res. Lett. 23, 459 (1996).
26. D. W. Forsyth, S. C. Webb, L. M. Dorman, Y. Shen,
Science 280, 1235 (1998).
27. Seismic instruments are composed of three orthog-
onal components: one vertical and two horizontal.
Land stations are normally oriented so that one
horizontal component is east-west and the other is
north-south. OBS instruments are not emplaced by
hand, and so the orientations of the horizontal com-
ponents are not known a priori.
28. E. A. Flinn, Proc. IEEE 53, 1874 (1965).
29. S-wave splitting is the observation of two S-wave
arrivals with orthogonal polarization due to aniso-
tropy. The splitting time is the time between the
arrivals. The fast azimuth corresponds to the polar-
ization azimuth of the first arriving Swave.
30. P. N. Chopra, M. S. Paterson, J. Geophys. Res. 89,
7861 (1984).
31. H. Jung, K.-H. Lee, S.-i. Karato, Eos 81, S53 (2000).
32. J.-M. Kendall, Geophys. Res. Lett. 21, 301 (1994).
33. M. E. Zimmerman, S. Zhang, D. L. Kohlstedt, S. Karato,
Geophys. Res. Lett. 26, 1505 (1999).
34. M. J. Fouch, K. M. Fischer, Geophys. Res. Lett. 25,
1221 (1998).
35. J. Gill, P. Whelan, J. Geophys. Res. 94, 4579 (1989).
36. J. I. Wendt, M. Regelous, K. D. Collerson, A. Ewart,
Geology 25, 611 (1997).
37. R. J. Poreda, H. Craig, Earth Planet. Sci. Lett. 113, 487
(1992).
38. S. Turner, C. Hawkesworth, Geology 26, 1019 (1998).
39. M. Bevis et al.,Nature 374, 249 (1995).
40. A. E. Gripp, R. G. Gordon, Geophys. Res. Lett. 17, 1107
(1990).
3 January 2001; accepted 19 March 2001
Detection of Widespread Fluids
in the Tibetan Crust by
Magnetotelluric Studies
Wenbo Wei,
1
Martyn Unsworth,
2
* Alan Jones,
3
John Booker,
4
Handong Tan,
1
Doug Nelson,
5
Leshou Chen,
1
Shenghui Li,
4
Kurt Solon,
5
Paul Bedrosian,
4
Sheng Jin,
1
Ming Deng,
1
Juanjo Ledo,
3
David Kay,
4
Brian Roberts
3
Magnetotelluric exploration has shown that the middle and lower crust is
anomalously conductive across most of the north-to-south width of the Tibetan
plateau. The integrated conductivity (conductance) of the Tibetan crust ranges
from 3000 to greater than 20,000 siemens. In contrast, stable continental
regions typically exhibit conductances from 20 to 1000 siemens, averaging 100
siemens. Such pervasively high conductance suggests that partial melt and/or
aqueous fluids are widespread within the Tibetan crust. In southern Tibet, the
high-conductivity layer is at a depth of 15 to 20 kilometers and is probably due
to partial melt and aqueous fluids in the crust. In northern Tibet, the conductive
layer is at 30 to 40 kilometers and is due to partial melting. Zones of fluid may
represent weaker areas that could accommodate deformation and lower crustal
flow.
The Tibetan plateau is the largest area of
thickened and elevated continental crust on
Earth and a direct consequence of the ongo-
ing India-Asia collision (1). Knowledge of
the structure and evolution of the plateau has
advanced through modern geophysical stud-
ies that began in the 1980s with a Sino-
French collaboration. Magnetotelluric (MT)
data collected during this project detected
unusually high electrical conductivity in the
crust of southern Tibet (2). In combination
with elevated heat flow (3), this was attribut-
ed to the presence of partial melt at shallow
depths in the crust. In 1995, Project
INDEPTH (4) acquired MT data in southern
Tibet with the use of more advanced instru-
mentation and data-processing techniques
(Fig. 1) (5). These data confirmed the exis-
tence of a high-conductivity zone at a depth
of 15 to 20 km in southern Tibet that was
coincident with seismic bright spots and a
seismic low-velocity zone (6–8). These ob-
servations gave additional support to the idea
that the high-conductivity layer represents
partial melt in the Tibetan crust (9).
However, both of these MT surveys (2,5)
were located within the Yadong-Gulu rift,
one of the north-south–trending rifts that ac-
commodate the ongoing east-west extension
in southern Tibet (10). To determine if the
conductive crust was limited to the rifts, we
collected additional MT data in 1998 and
1999 (500 and 600 lines, Fig. 1). A charac-
1
Department of Applied Geophysics, China University
of Geosciences, Beijing, People’s Republic of China.
2
Institute of Geophysical Research, University of Al-
berta, Edmonton, Alberta T6G 2JI, Canada.
3
Geologi-
cal Survey of Canada, Ottawa, Canada.
4
Geophysics
Program, University of Washington, Seattle, WA
98195, USA.
5
Geological Sciences, Syracuse Universi-
ty, Syracuse, NY 13244, USA.
*To whom correspondence should be addressed.
REPORTS
27 APRIL 2001 VOL 292 SCIENCE www.sciencemag.org716
