JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B12, 2344, doi:10.1029/2001JB000807, 2002
Asthenospheric flow and asymmetry of the East Pacific Rise,
MELT area
James A. Conder,1 Donald W. Forsyth, and E. M. Parmentier
Department of Geological Sciences, Brown University, Providence, Rhode Island, USA
Received 24 July 2001; revised 12 April 2002; accepted 6 September 2002; published 13 December 2002.
[1] Although the Pacific and Nazca plates share the East Pacific Rise (EPR) as a
boundary, they exhibit many differing characteristics. The Pacific plate subsides more
slowly and has more seamounts than the Nazca plate. Both the seismic and
magnetotelluric components of the Mantle ELectromagnetic and Tomography Experiment
(MELT) found pronounced asymmetry in mantle structure across the spreading axis near
17S. The Pacific (west) side has lower S-wave velocities, exhibits greater shear wave
splitting, and is more electrically conductive than the Nazca (east) side. These results
suggest asymmetric mantle flow and melt distribution beneath the EPR. To better
understand the causes for these asymmetric properties, we construct numerical models of
melting and mantle flow beneath a midocean ridge migrating to the west over a fixed
mantle. Although the ridge is migrating to the west, the migration has little effect on the
upwelling rates, requiring a separate mechanism to create the asymmetry. Models that
produce asymmetric melting with a temperature anomaly require large (>100C) excess
temperatures and may not be consistent with the observed subsidence and crustal
thickness. A possible mechanism for creating asymmetry without a temperature anomaly
is across-axis asthenospheric flow, possibly driven by pressures created by upwelling
beneath the Pacific Superswell to the west. Pressure-driven asthenospheric flow follows
the base of the lithosphere, extending the upwelling region to the west as it follows the
thinning lithosphere toward the axis, and shutting off melting as it crosses the axis and
INDEX TERMS: 8120 Tectonophysics:
encounters an increasingly thick lithosphere to the east.
Dynamics of lithosphere and mantle—general; 8121 Tectonophysics: Dynamics, convection currents and
mantle plumes; 8150 Tectonophysics: Evolution of the Earth: Plate boundary—general (3040); 8162
Tectonophysics: Evolution of the Earth: Rheology—mantle; 9355 Information Related to Geographic Region:
Pacific Ocean
Citation: Conder, J. A., D. W. Forsyth, and E. M. Parmentier, Asthenospheric flow and the asymmetry of the East Pacific Rise, MELT
area, J. Geophys. Res., 107(B12), 2344, doi:10.1029/2001JB000807, 2002.
1. Introduction
[2] Although considerable study has been devoted to
understanding midocean ridges and their central role in
plate tectonics, much is still not known about dynamic
processes within the oceanic mantle beneath midocean
ridges. The Mantle ELectromagnetic and Tomography
Experiment (MELT) [Forsyth and Chave, 1994] was
designed to image the melting structure beneath the fast
spreading (145 mm/yr) East Pacific Rise (EPR) near 17S
(Figure 1). The main objective was to distinguish between
models of broad and narrow zones of upwelling and melting
at midocean ridges. There were two primary components of
the experiment, a passive-source seismic deployment utilizing 52 ocean bottom seismometers (OBSs) distributed
across the axis in two linear arrays [MELT Seismic Team,
1
Now at Department of Earth and Planetary Sciences, Washington
University, St. Louis, Missouri, USA.
Copyright 2002 by the American Geophysical Union.
0148-0227/02/2001JB000807$09.00
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1998] and a magnetotelluric experiment utilizing 47 seafloor instruments, also deployed across the axis in two
linear arrays [Evans et al., 1999]. The results of the experiment are consistent with a broad zone of melting [MELT
Seismic Team, 1998; Forsyth et al., 1998; Hung et al.,
2000], but one of the most robust and pervasive observations in both the seismic and magnetotelluric data is a
pronounced asymmetry about the rise axis (Figure 2). The
experiment was successful in answering many questions
about mantle structure beneath midocean ridges, but the
unexpected degree of asymmetry raises many new questions
about midocean ridge and oceanic mantle structure.
[3] Asymmetry is manifest in nearly every geophysical
observation in the MELT region. For instance, the west
(Pacific) side subsides significantly slower than the east
(Nazca) side. Slower subsidence may be an isostatic
response to hotter mantle to the west [Cochran, 1986] or
could be dynamically supported by pressures within the
mantle [Phipps Morgan and Smith, 1992; Phipps Morgan et
al., 1995]. The Pacific side has more seamounts than the
Nazca side [Scheirer et al., 1996, 1998], suggesting that
8-1
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CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
Figure 1. Bathymetry of the MELT area of the East
Pacific Rise (EPR). Triangles and squares show the
geometry of the MELT experiment. Triangles are seismometer locations and squares are magnetotelluric instrument
locations. Although the crustal accretion rate is nearly
symmetric (black arrows), absolute plate motion is asymmetric (white arrows), with the Pacific plate moving 101
mm/yr and the Nazca plate moving 44 mm/yr. RRS
denotes the Rano Rahi seamount field, and PPR with
associated arrow shows direction of the Pukapuka ridges.
more melt is present beneath the Pacific plate or that melt
more easily penetrates the overlying lithosphere. Along the
west side of the axial high is a flanking low, possibly caused
by an asymmetry in crustal and mantle viscosity [Eberle
and Forsyth, 1998]. Shear wave splitting delays from OBSs
on the Pacific side are nearly twice those from OBSs on the
Nazca side [Wolfe and Solomon, 1998]. Larger shear wave
splitting values indicate a stronger lattice-preferred orientation (LPO) of anisotropic mantle minerals (e.g., olivine,
enstatite), and/or a thicker layer of those anisotropically
aligned mantle minerals. Both the anisotropic layer thickness and the degree of strain-induced LPO are expected to
depend on the mantle flow field. The asymmetry in shear
wave splitting suggests that mantle flow beneath the EPR is
not the symmetric corner flow usually assumed in midocean
ridge models [e.g., Phipps Morgan, 1987]. Rayleigh wave
phase velocities are lower [Forsyth et al., 1998] and P and S
wave arrivals are significantly later [Toomey et al., 1998] to
the west of the EPR. These results strongly suggest that
more melt is present beneath the Pacific plate than the
Nazca plate. In addition, the electrical conductivity has an
abrupt transition near the rise axis [Evans et al., 1999] with
relatively conductive mantle to the west, possibly indicating
the presence of melt, and highly resistive mantle to the east,
suggesting dry, depleted mantle containing very little interconnected melt beneath the Nazca plate.
[4] Possible mechanisms for creating the observed asymmetry about the EPR include ridge migration to the west,
hot mantle material fed to the axis from the west, and
across-axis, pressure-driven asthenospheric flow [Toomey et
al., 2002]. Crustal accretion in the MELT area is fairly
symmetric (Pacific 69, Nazca 76 mm/yr), but absolute
plate motions are asymmetric (Pacific 101 mm/yr, Nazca
44 mm/yr), requiring 32 mm/yr of westward ridge
migration [Scheirer et al., 1998]. The asymmetric motion
may be directly responsible for the asymmetry in shear
wave splitting [Wolfe and Solomon, 1998]. Potentially, as
the ridge migrates to the west, the less depleted material
beneath the leading (Pacific) plate could be preferentially
melted compared to material beneath the trailing (Nazca)
plate, which may already be somewhat depleted. Hotter
mantle beneath the Pacific plate has been suggested to
explain the asymmetric subsidence. The hotter material,
possibly fed from the Pacific Superswell 1500 km to the
west of the axis [McNutt, 1998], would also generate a
greater amount of melting beneath the Pacific plate than
beneath the Nazca plate. In this paper, we demonstrate that
across-axis asthenospheric flow can create an asymmetry in
the melt produced on either side of the axis by enhancing
the upwelling on the ‘‘upwind’’ side and inhibiting upwelling on the ‘‘downwind’’ side. A pressure gradient strong
enough to drive across-axis flow may be created by the
superswell. We use numerical models to investigate these
possible mechanisms and the conditions required to create a
strong asymmetry about the EPR. In particular, we make the
case that pressure-driven asthenospheric flow is likely an
important factor in creating asymmetric melting and subsequent asymmetric geophysical anomalies.
2. Numerical Models
[5] To investigate the possible causes of asymmetry in the
melting region, we construct 2-D finite element-method
numerical models of mantle flow beneath a midocean ridge.
We solve the equations for viscous flow using a standard
penalty function method [Reddy, 1993] with rectangular
elements. Solutions are calculated on an 81 41 node mesh
with nonuniform rectangular elements (Figure 3). The mesh
is centered about the ridge axis, and extends 300 km to each
side and to 600 km depth. Node spacing decreases toward
the ridge axis and with decreasing depth, providing the best
resolution near the spreading center. For models including a
temperature anomaly advected into the area from the west,
10 columns of grid cells are added to either side of the
mesh, extending the box dimensions to ±1000 km from the
Figure 2. Cross-section of S-wave velocities beneath the
EPR in the MELT area [from Forsyth et al., 2000]. The
region of velocities lower than 4.1 km/s, probably indicating
the presence of melt, extends hundreds of kilometers to the
west, but only several tens of kilometers to the east of the
axis. Cross-section location is midway between the two
main seismic lines shown in Figure 1. See color version of
this figure at back of this issue.
CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
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Figure 3. Mesh and boundary conditions used for numerical flow models. The models are in the ridgefixed reference frame. To account for ridge migration to the west, nodes along the bottom are assigned an
eastward velocity corresponding to the ridge migration rate. To investigate the influence of a pressure
gradient, different pressures (P1 and P2) are assigned along the left and right edges. For models
investigating a pressure gradient only, the smaller (81 41 node) mesh outlined by the dashed line is
used. For models including a thermal anomaly introduced on the left edge, the larger (101 41 node)
mesh is used.
axis. Resolution near the edges is not critical, but the
introduced temperature anomaly needs to begin far enough
away that edge effects are not important. The model
methods and governing equations for conservation of mass,
conservation of energy, and mechanical equilibrium with
variable viscosity are given by Braun et al. [2000] and Jha
et al. [1994]. In addition to the temperature and the 2-D
velocity field, calculated variables are viscosity, melt production, and mantle depletion from melting. Advection of
temperature and depletion is calculated by an upwind finite
differencing method with higher order corrections to reduce
artificial diffusion [Smolarkiewicz, 1983].
[6] Boundary conditions along the top edge are velocity
conditions. The models are in the ridge-fixed reference
frame. Along the top, nodes left of the axis move to the
left at 72 mm/yr and those right of the axis move to the right
at 72 mm/yr, corresponding to the average half-spreading
rate [Scheirer et al., 1998]. To account for ridge migration in
the ridge-fixed reference frame, we apply a horizontal
velocity along the bottom boundary of 30 mm/yr to the
right, equivalent to the ridge migration rate to the left in the
hot spot coordinate frame. We ignore the minor asymmetric
spreading from rapid propagation of small-offset propagating rifts [Cormier and Macdonald, 1994] that adds to the
net migration rate. In most of our models, no flow is
allowed across the bottom boundary. The right and left
edges have shear stresses set to zero and are assigned
normal stresses to create the desired pressure gradient.
Temperatures along the bottom are set to 1350C (plus
the adiabatic gradient, 0.3C/km), while those across the top
are set to 0C. Temperature anomalies introduced from the
side are applied as step functions, with material entering the
box along the left edge above a certain-depth assigned a
temperature in excess of 1350C.
[7] We do not explicitly include buoyancy forces. We
approximate the effects of a far-off-axis, buoyant plume
with a pressure gradient driving flow away from the hot
spots toward the ridge axis. Buoyancy could lead to small-
scale convection that would tend to homogenize temperatures within the low-viscosity asthenosphere and thin the
lithosphere, but with our 2-D model geometry, Richter rolls
[Richter and Parsons, 1975] are not allowed to develop
even if buoyancy terms are included. Any off-axis convective component with a symmetry axis parallel to the
spreading center would tend to be suppressed by shear in
the asthenosphere. Buoyancy could create a narrow zone of
upwelling at the ridge axis [Buck and Su, 1989], but the
MELT found no indication of the predicted narrowing
beneath the EPR [Hung et al., 2000]. Thus, we believe
that the dominant effects for the purposes of this study can
be represented with a passive, plate-driven flow model
with the addition of pressure gradients driving flow in the
asthenosphere. The absence of buoyancy forces does
require that we use a no flow (vertical velocity = 0)
boundary condition along the bottom to balance the pressures in pressure-driven flow (or flow exits through the
bottom), and to permit passive temperature anomalies to
advect to the spreading center (or a temperature anomaly
introduced from the side is almost immediately caught in
the corner flow and carried back outside the box).
[8] Melting in the models occurs when the upwelling
mantle exceeds the solidus temperature at a given pressure.
We include solidi for both wet and dry melting. The wet and
dry solidi are defined as
Twet ¼ 900 C þ rm gzðdT=dPÞwet
ð1Þ
Tdry ¼ 1100 C þ rm gzðdT=dPÞdry
ð2Þ
and
where rm is density of the mantle, z is depth, g is
acceleration due to gravity, and dT/dP is the slope of the
solidus with pressure. The total amount of melt generated
in the wet melting regime is expected to be small [Hirth
and Kohlstedt, 1996], so we only allow 2% wet melting
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CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
(Fwet = 0.02), after which no more melting occurs until the
dry solidus is exceeded. We allow up to 20% total melting,
assuming the exhaustion of clinopyroxene at this value
[Hirschmann et al., 1998]. Melt production for a given
amount of decompression is given by
F ¼ Fwet ðT Twet Þ= Tdry Twet
ð3Þ
F ¼ ð@F=@TÞp T Tdry þ Fwet
ð4Þ
and
in the wet and dry melting regimes respectively. The
amount of melt produced is adjusted by the isobaric melt
productivity, (@F/@T)P, that takes into account the amount
of depletion each parcel of mantle has already undergone.
The melt production rate, qm, as a function of depth is
qm ¼ uz ðdF=dzÞ
ð5Þ
where uz is the upwelling rate and dF/dz is the amount of
melt produced per unit depth.
[9] Depletion, dp (total amount of melting a particle has
experienced), boundary conditions for material entering the
box below the solidus are set to zero (no prior melting).
Material advected out of the box simply retains its prior
depletion value (ddp/dxjedge = 0). Material that enters the
box above the solidus is assigned a value greater than zero,
depending on its depth and temperature, to reflect prior
melting it must have undergone. This latter condition is
only important with pressure-driven flow. Passive-flow
cases mostly tend to draw material into the box from below
the solidus.
[ 10 ] The asymmetry produced in the flow models
depends on the viscosity structure. Dislocation and diffusion
creep are the two primary deformation mechanisms in the
upper mantle [e.g., Karato and Wu, 1993]. The effective
viscosity, h, for each creep mechanism is described by
h ¼ Asðn1Þ exp½ðE* þ Pv*Þ=RT
ð6Þ
where E* is the activation energy, P is the pressure, v* is the
activation volume, R is the gas constant, T is the temperature,
s is differential stress, n is the stress exponent (n = 1 for
diffusion creep, n 3 for dislocation creep), and A is a
preexponential constant. The activation and preexponential
values for the different mechanisms are not necessarily the
same. The uppermost part of the mantle that we are most
interested in is expected to be dominated by dislocation
creep, which we adopt for the entire region. The temperature
dependence of mantle viscosity is reasonably well known, as
E* for olivine undergoing dislocation creep is well
constrained by laboratory experiments (515 ± 25 kJ/mol
[Hirth and Kohlstedt, 1996]), but the activation volume
(pressure dependence) is not as well constrained. Possible
values range from 10 to 20 106 m3/mol [Karato and Wu,
1993; Hirth and Kohlstedt, 1996]. To remove the nonlinear
dependence on stress for dislocation creep, we use a linear
approximation for viscosity by reducing the activation
values by a factor of two (E* = 250 kJ/mol, v* = 5 – 10 106 m3/mol) and setting the stress exponent equal to 1
[Christensen, 1984]. This approximation greatly simplifies
the flow calculations, and because activation values for
diffusion creep are roughly half those of dislocation creep
[Karato and Wu, 1993], no significant change to the
viscosity in the model is required deeper in the mantle
where the transition to diffusion creep is likely to occur.
Activation values hereafter described in this paper are those
we are trying to approximate, i.e., before dividing by two
and changing the stress exponent, and thus can be directly
comparable to laboratory constraints. We evaluate A by
assigning the viscosity at the base of the model to 1021 Pas.
Different values within the possible range of v* can lead to
different degrees of asymmetry. The larger the value of v*,
the more pressure-dependent the viscosity, and the more
plate- or pressure-driven flow will be confined within a
shallow, low-viscosity layer, which can more directly affect
the region of melting beneath the axis.
3. Model Results
[11] To provide a quantitative description of the degree of
asymmetry about the axis for a given model, we determine a
value y given by
y ¼ qmwest =qmtot
ð7Þ
where qmtot is the integrated melt production rate over the
entire box and y is the fraction of the total west of the axis.
This measure is nonunique in that it does not weight melt
production values further from the axis relative to values
near the axis, but in practice it works well for relating
models to each other in all but a few cases. Visual
examination of maps of calculated melt production shows
that an unambiguous asymmetry exists when two-thirds or
more of the melt production occurs to the west of the axis.
Therefore we choose y 0.66 as a threshold for
‘‘significant’’ asymmetry.
3.1. Ridge Migration
[12] Rapid ridge migration is caused by the more rapid
motion of the Pacific plate in the hot spot reference frame
[Scheirer et al., 1998]. The rapid migration of the ridge (32
mm/yr) has been widely cited as a possible cause of melting
asymmetry [e.g., MELT, 1998; Scheirer et al., 1998; Buck,
1999; Evans et al., 1999]. As the ridge migrates, it may tap
less depleted material beneath the leading (Pacific) side,
preferentially creating more melt to that side [e.g., Davis and
Karsten, 1986]. Calculations for this case show that ridge
migration by itself does not create any significant asymmetry
in the melting region (Figure 4). The deep mantle (>200 km)
moves from west to east, but the shallow mantle (<100 km),
where melting occurs, has a predominantly symmetric
upwelling structure. To a first approximation, the flow is a
superposition of that driven by plate separation, which
generates upwelling, and simple shear driven by migration
of the entire system relative to the deeper mantle. Because
the latter involves no upwelling except that caused by
changes in plate thickness, it has little effect on the upwelling
rates and subsequent melt production.
[13] The symmetric upwelling structure creates a symmetric layer of depleted mantle about the axis. Because flow
into the melt production region comes from below the
CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
Figure 4. Depletion from melting, streamlines, and melt
production for ridge migration only case. Melt production
contours used throughout the manuscript. Although mantle
flow deeper than 200 km is dominantly west to east, flow in
the upper 150 km is dominated by symmetric corner flow,
resulting in symmetric depletion (grayshades) and melt
production (contours) about the axis. Units are dimensionless, so only relative magnitudes are important. Melting
within the wet region is far less than that in the dry melting
region and does not show up at these contour levels.
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8-5
our models, the incoming temperature anomaly must be at
least 100C (Figure 5) and extend over the depth range in
which flow lines turn back westward for the anomaly to
reach the axis, but no deeper, or hot material is fed to the
east side of the axis as well (Figure 6). This depth range
depends somewhat on v*, which controls the depth distribution of the return flow. If the anomaly extends more than
200– 250 km deep (base of return flow) the model advects
high temperature material to both sides of the ridge,
reducing the asymmetry by increasing melting on both sides
of the axis (Figure 6c). If the extent of the anomaly is much
shallower than the base of the return corner flow (<150 km),
depleted layer (Figure 4), the geometry of melting is not
affected by influx or recycling of mantle that has been
previously partially melted at the ridge. A greater pressure
dependence on viscosity (larger v*) confines the return flow
to a narrower shallow channel, potentially increasing the
incorporation of previously melted material into the axial
melting region. Because the depleted layer tends to be
symmetric, however, even with the highest experimental
estimate of v*, no asymmetry develops in the melting region
with ridge migration alone. Faster ridge migration may also
increase asymmetry, but even a migration rate equal to the
full spreading rate does not produce significant melting
asymmetry. With reasonable parameters, ridge migration in
our models never produces y > 0.53, well below the threshold of any detectable asymmetry. Melting of enriched
heterogeneities, as envisioned by Davis and Karsten
[1986], does not necessarily produce asymmetry of the
correct sense because all mantle material entering the
melting region comes from below. If heterogeneities are
randomly distributed with depth, just as much excess
melting would occur on the east side as the west side,
because the material on both sides of the axis in the melting
region has come from the west, below the recently depleted
mantle (Figure 4).
3.2. Temperature Anomaly
[14] The explanation of hotter mantle temperatures to the
west will produce more melt beneath the Pacific plate. A
temperature gradient in the mantle was proposed by
Cochran [1986] to explain the slow subsidence of the
Pacific plate. In his model, a linear gradient was superimposed on a cooling half-space model, but the mechanism
for maintaining this gradient was not discussed. The MELT
Seismic Team [1998] and Toomey et al. [2002] suggest that
hot asthenospheric material is fed to the ridge from the
Pacific Superswell, 1500 km to the west. The explanation
of incoming hotter material from the west as a cause for
asymmetric melting about the axis is more complicated than
it may initially seem. To exceed the 0.66 threshold for y in
Figure 5. Dependence of asymmetry (y = 0.5, symmetric,
1, completely asymmetric) on viscosity structure (v*),
temperature anomaly magnitude (Ta), and anomaly depth
extent (zta). Higher v* corresponds to more pressure
dependent viscosity. Weaker pressure dependence creates
deeper corner flow, requiring a deeper anomaly to affect
melting at the axis. The dotted contour is y = 0.66, where
significant asymmetry begins to develop. All models require
a temperature anomaly 100C to create significant
asymmetry. Temperature anomaly dimensions <150 km
and >250 km deep do not create much asymmetry for any of
the allowable viscosity structures.
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CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
to the west and constricts the melting region to the east is
consistent with the observed asymmetry about the EPR.
[16] A global convection model, based on density variations inferred from seismic tomography, predicts pressures in
the Pacific Superswell region strong enough to create 2 km
of dynamic topography [Forte and Perry, 2000]. Dynamic
pressures of this magnitude could create a pressure gradient
in the mantle as large as a few 10s of kPa/km, large enough
to create significant across-axis flow for reasonable mantle
viscosities. Application of a pressure gradient of a few kPa/
km in the models induces significant across-axis asthenospheric flow, which is necessarily faster than simple platedriven flow. The resultant structure of the velocity field
depends mainly on the relative viscosity structure, while the
magnitude of the velocities depends on the absolute viscosity structure. A strong pressure dependence on viscosity
(large v*) confines pressure-driven flow to a shallow asthenospheric channel. Because a shallower high velocity channel more directly affects the melting region beneath a
spreading center, higher values of v* require smaller horizontal velocities within the asthenosphere to produce the
same degree of asymmetry. A midrange v* value of 16 106 m3/mol and a pressure gradient of 1500 Pa/km (assuming a normalization viscosity of 1021 Pas) creates highly
asymmetric melting (y 0.75), extending 100s of km to the
west and 10s of km to the east (Figure 7a), and implies a
peak horizontal asthenospheric velocity of 300 mm/yr near
Figure 6. Temperature (T), streamlines, and melt production for models with a 100C thermal anomaly (Ta)
introduced from the west at (a) 150 km, (b) 200 km, and
(c) 250 km depth. Asymmetry values for each are 0.58,
0.66, and 0.62, respectively. Melt production asymmetry in
these models comes from a larger increase the amount of
melt on the west side than on the east side. If the anomaly is
confined above the 150 km deep streamline, there is little
effect on the melting region. If the anomaly extends down to
the 250 km or deeper streamlines, the anomaly increases
melt production on both sides of the axis. See color version
of this figure at back of this issue.
the excess temperatures will not reach the axis and no
excess melting will occur, producing y values comparable
to ridge migration alone (Figure 6a).
3.3. Pressure-Driven Flow
[15] Pressure-driven asthenospheric flow may be an
important factor in creating asymmetry about the EPR.
Pressure-driven flow tends to follow the base of the overlying lid (lithosphere). Since the lithosphere thins toward
the axis, flow from the west has more upwelling on the
western (‘‘upwind’’) side, enhancing the melt production. In
contrast, flow across the axis to the east encounters an
increasingly thick lithosphere, forcing a downward component to the flow, restricting the amount of upwelling and
melt production on the eastern (‘‘downwind’’) side of the
axis. Flow that simultaneously expands the melting region
Figure 7. Melt production in models including a pressure
gradient. The degree of asymmetry increases with an
increasing pressure gradient and with increasing pressure
dependence on viscosity. (a) With intermediate pressure
dependence of viscosity and a modest pressure gradient,
asthenospheric flow rates reach 300 mm/yr and produce
asymmetry, y, equal to 0.75 close to that suggested by
seismic results of the MELT experiment (see text). (b) with
a slightly larger pressure gradient and stronger pressure
dependence on viscosity, asthenospheric flow reaches rates
near 1 m/yr and produces y equal to 0.93, close to that
suggested by the electrical conductivity results of the MELT
experiment.
CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
Figure 8. Dependence of asymmetry, y, on viscosity
structure (v*) and asthenospheric flow velocities (ux) for
pressure-driven flow. Asymmetry increases with faster
asthenospheric flow and increasing v*. The dotted contour
is y = 0.66, where notable asymmetry begins to develop
(same as Figure 5), and the dashed is y = 0.75, a value
consistent with the seismic results from the melt experiment. The white line corresponds to asthenospheric velocity
of 300 mm/yr, consistent with the Pukapuka ridge
propagation rate towards the EPR from the superswell.
Our preferred model is found where the white line and
dashed contour intersect.
100 km depth. Stronger pressure gradients and larger activation volumes for a given pressure gradient increase both
the degree of asymmetry and the horizontal velocities within
the asthenosphere. A pressure gradient of 5000 Pa/km and
v* = 18 106 m3/mol creates an extremely asymmetric
melting region (y 0.93) (Figure 7b), but also implies a
peak asthenospheric velocity near 1 m/yr.
[17] To explore the range of possible states of the mantle,
assuming the known asymmetries are caused by a pressure
gradient, we ran a suite of models over a range of pressure
gradients and possible v* values. Velocities in the asthenosphere depend on both the magnitude of the pressure
gradient and the absolute mantle viscosity. Because mantle
pressure gradients and viscosity are constrained only by
order of magnitude estimates, we report our results here in
terms of peak asthenospheric velocity (ux) rather than
absolute pressure gradient magnitudes. Peak asthenospheric
velocity as reported in this paper is the maximum horizontal
velocity within the mantle column 300 km west of the axis.
The dependence of the asymmetry, y, on ux and v* are
shown in Figure 8. y increases with increasing ux and with
increasing v*. Upper values of v* (17 – 20 106 m3/mol)
begin to exhibit significant asymmetry (y 0.66) with
asthenospheric velocities 200 mm/yr. The same degree of
asymmetry at lower values of v* requires asthenospheric
velocities near 400 mm/yr.
4. Discussion
4.1. Degree of Asymmetry
[18] Figures 5 and 8 show the model parameters required
for various degrees of melt production asymmetry. The
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degree of asymmetry required to match the observations is
less clear. None of the observations are directly sensitive to
the melt production rate. Densities, seismic velocities, and
electrical properties all depend on the amount of melt
actually present and how it is distributed, rather than the
rate it is produced. We do not know the pathways and
efficiency of melt extraction from the mantle, but we note
that none of the observations reported to date require that
more than 1– 2% melt be present [Forsyth et al., 1998;
MELT Seismic Team, 1998; Toomey et al., 1998; Webb and
Forsyth, 1998; Evans et al., 1999]. Consequently, we
assume that some small melt fraction is retained wherever
melt is produced and that melt above that fraction is
efficiently extracted. In the exploration of models presented
in this paper, we do not attempt to match the observations
precisely. Instead, we compare our models to qualitative
indications of the presence of melt inferred from the seismic
and electromagnetic components of the MELT. The Rayleigh wave tomography (Figure 2) shows that low velocities
at 30– 60 km depth in the primary melt production range
extend 200 – 300 km off-axis to the west, but only tens of
km to the east. Resistivities [Evans et al., 1999] within 10–
30 km of the axis to the east give no indication of the
presence of melt or water, while a more conductive structure
to the west indicates the possible presence of 1 – 2%
interconnected melt. The horizontal extent of the more
conductive region to the west is not well constrained.
[19] There are several possible explanations for the apparent difference in the abruptness of the change in structure at
the axis. At first glance, the electrical conductivity suggests
essentially no melt production east of the axis, or y 1
(e.g., Figure 7b). The Rayleigh wave data seems compatible
with lower degrees of asymmetry, closer to y 0.75 (e.g.,
Figure 7a). One possibility is just differences in horizontal
resolution. The Rayleigh wave tomography, for example,
involves horizontal averaging over scales on the order of
100 km [Forsyth et al., 1998]. There also may be geological
reasons for the discrepancy. For instance, the difference
could be in the connectivity of the melt present. The
velocity structure is sensitive to melt fraction and the shape
of the melt pockets, whereas conductivity depends critically
on whether the melt pockets are interconnected [Schmeling,
1986]. Thus, if some small melt fraction is retained outside
the melt production region and is no longer being extracted
through interconnected channels, it could have both low
conductivity and low seismic velocities. Another possible
explanation is an anisotropic distribution of melt pockets or
crystalline fabric. The Te and Tm modes of the electrical
conductivity measurements are sensitive to conductivity in
the horizontal directions, but relatively insensitive to conductivity in the vertical direction [Evans and Everett, 1992].
If water is present in olivine, it most strongly lowers
resistivity along the a-axis of the crystals [Mackwell and
Kohlstedt, 1990]. If the a-axes were oriented upward in a
vertically upwelling region, the apparent resistivity in a
magnetotelluric experiment would resemble that of dry
olivine. Similarly, if melt on the east side of the axis is
aligned in vertical channels, it may give slow seismic
velocities and a resistive conductivity structure. Why melt
would align differently on one side of the axis than the other
is speculative, but it may occur if melt is focused to the axis
through a ‘‘fractal tree’’ of melt channels, as some geo-
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CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
chemical and field mapping studies suggest [Hart, 1993;
Kelemen et al., 1995]. A fractal tree that drains a region
with a strong asymmetry in melt production may comprise
more horizontal ‘‘branches’’ to the west, and more vertical
‘‘branches’’ to the east. Given the uncertainties in the
interpretation of the electrical conductivity measurements,
we prefer a model with a y value 0.75.
4.2. Temperature Anomaly
[20] As noted above, melting asymmetry 0.66 is created
by an incoming temperature anomaly from the west only if
the anomaly is >100C and extends from the shallowest
mantle to 200 km depth (Figure 8). The anomaly required
to match the observed asymmetry (y 0.7– 0.75) must be
150C– 200C. Even 100C is much larger than the 40C
temperature anomaly beneath the superswell inferred from
the seismic structure [McNutt and Fischer, 1987]. A 100C
anomaly extending to 200 km depth also overpredicts the
asymmetric seafloor subsidence. The appropriate depth of
compensation for calculating seafloor topography depends
on the viscosity structure, as a shallow, low viscosity layer
isolates deeper structure from creating a topographic
response [Robinson et al., 1988]. Depending on the viscosity contrasts in the mantle, density anomalies can have
different contributions to seafloor topography. For a large
contrast between a low-viscosity asthenosphere and a
higher-viscosity mesosphere, density anomalies within the
asthenosphere have a decreasing effect on seafloor topography with increasing depth, becoming effectively zero
for anomalies at the base of the asthenosphere. For smaller
viscosity contrasts, the seafloor has a greater response to
deep density anomalies, particularly at longer wavelengths
[Robinson et al., 1987].
[21] To calculate the seafloor topography for a given
model we assume a 20 km thick, high-viscosity lithosphere,
which contributes fully to seafloor topography, overlying a
200 km thick asthenosphere that has a linearly decreasing
contribution with depth, above a semiinfinite mesosphere
that does not contribute to seafloor topography. By assuming that the viscosity contrast is large between the asthenosphere and the mesosphere so that the seafloor response
goes to zero at the base of the asthenosphere, we obtain a
minimum estimate of the subsidence asymmetry created by
different thermal anomalies within the asthenosphere. If the
viscosity contrast is not large, any additional response from
deep anomalies will only increase subsidence asymmetry.
Assuming this response kernel, a 200 km deep, 100C
anomaly makes the subsidence too asymmetric by shallowing the western side of the axis by 100 – 200 meters relative
to the eastern side (Figure 9). Another observation inconsistent with the temperature anomaly model is that the
crustal thickness in the MELT area (5.5 – 6.0 km) is near
or possibly thinner than the global average [Canales et al.,
1998]. Higher temperatures should result in more melting
and generate thicker crust unless the melt does not make it
to the axis or the east side is anomalously cold. A final
caveat to the incoming temperature anomaly model is that
introducing anomalously warm material to one side of the
axis can increase the melt production on that side, but
cannot decrease the amount of melting on the other side,
as many of the observations suggest (delay times, velocity
structure, electrical conductivity, etc.). A small incoming
Figure 9. EPR bathymetry (solid line) with predicted
subsidence from models creating significant melting
asymmetry solely with a thermal anomaly axis from the
west. The dashed line is the predicted subsidence with 200
km deep, 100C anomaly (Figure 6b). The model overpredicts the asymmetric subsidence by more than 100
meters. If a viscosity increase with depletion from melting is
assumed, a 100C anomaly 300 km deep (Figure 13) is
required to create significant melting asymmetry, producing
an even poorer fit to the subsidence (dotted line).
temperature anomaly may likely be present but is not
sufficient to explain the observed asymmetry, with the
simple, passive flow models considered here.
4.3. Subsidence
[22] The southern EPR exhibits the largest subsidence
asymmetry of any oceanic spreading center, subsiding at
rates of 200– 225 m/Myr1/2 to the west and 350– 400 m/
Myr1/2 to the east [Cochran, 1986]. The asymmetry has
usually been attributed to thermal variations within the
mantle [Rea, 1978; Cochran, 1986; Marty and Cazenave,
1989; Lago et al., 1990; Perrot et al., 1998]. As an
alternative to thermal models, the asymmetry could be
caused by dynamic topography from a pressure gradient
superimposed on a symmetrically subsiding lithosphere.
We note that there is no identifiable long-wavelength trend
in the free-air gravity anomaly to suggest dynamically
supported topography. Therefore, dynamic topography
must either be small or compensated by a downward
deflection of a deeper density contrast, such as the
‘‘410’’ or ‘‘660’’ mantle discontinuities or even an asthenosphere-mesosphere boundary [Phipps Morgan et al.,
1995]. To investigate the different mechanisms for subsidence asymmetry, we extracted from the predicted topography of Smith and Sandwell [1997] a profile normal to
the EPR spreading axis to compare with various models
(Figures 9, 10, and 11). The profile location was chosen to
avoid seamounts; it is centered on 113.15W, 17.1S, and
extends 1000 km to each side of the axis.
[23] Cochran [1986] proposed a horizontal, linear temperature gradient of 0.05 – 0.10C/km within the asthenosphere beneath a symmetrically subsiding plate that could
explain the subsidence of the EPR. Because thermal anomalies rapidly advect through the near-ridge region of melting,
homogenizing mantle temperatures, such a gradient must
either be a transient feature approximating the current
temperature structure, or be maintained by vigorous,
CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
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[25] With the addition of a linear gradient, the subsidence
is described by
d ¼ d0 þ C1 t1=2 þ mx
Figure 10. Subsidence for models with imposed temperature gradients of 0.1C/km (dashed) and 0.2C/km (dotted)
applied across the bottom. The bottom boundary must be
open to flow so the gradient can be advected upward
without being smeared out. The 0.1C/km model fits the
topography well, but does not exhibit much asymmetric
melting (y = 0.59), as the total temperature difference
across the melting region is only on the order of 20C. A
gradient of 0.2C/km produces more melting asymmetry (y
= 0.64), but has a noticeably worse fit to the topography
(dotted line).
small-scale convective mixing, tapping a larger mantle
reservoir. A gradient as high as 0.1C/km fits the topography (Figure 10), but is not large enough to produce
major asymmetry in melting (y = 0.59). Larger gradients
yield more easily detectable asymmetry in melting, but
poorer fits to the topography (Figure 10). Thus, in addition
to requiring an unknown mechanism to create the thermal
gradient, no linear temperature gradient alone adequately
matches both the asymmetric subsidence and the asymmetric seismic and conductivity structure.
[24] More complex, time-dependent, thermal models for
subsidence have also been proposed. For example, Lago et
al. [1990] proposed a model for asymmetric subsidence in
which the temperatures beneath the ridge axis are different
than asthenospheric temperatures, but still could not find a
reasonable fit to subsidence in the MELT area. Perrot et
al. [1998] suggest that the initial ridge depth (d0) and
seafloor subsidence parameter of each side of the axis
(C1,2), given by
d ¼ d0 þ C1;2 t1=2
ð9Þ
where x is distance from the axis (positive to the east). The
best fitting ridge depth and subsidence parameter for this
model are 2790 m and 319 m/Myr1/2, respectively. The
linear gradient has a slope (m) of 0.17 m/km, which can be
explained by a pressure gradient of 7800 Pa/km, assuming
a compensating deflection on a deeper boundary (twice the
gradient required for topography alone, since half the
excess pressure is required to deflect a deeper density
interface to satisfy the gravity constraint). The twelve
parameter model of Perrot et al. [1998] for four age zones
fits slightly better (Figure 11) than this simpler model, but
if the subsidence parameters are not allowed to be less
than 100 m/Myr1/2 in any age interval, then an F test
demonstrates that the fit with twelve parameters is not
significantly better than our fit with three.
4.4. Pressure-Driven Mantle Flow
[26] Pressure-driven mantle flow from the Pacific superswell can create asymmetry in the melting region that
matches the observations. An important effect of pressuredriven flow is that a low-viscosity asthenosphere can flow
significantly faster than plate-driven flow. The velocity at
which the asthenosphere flows is unknown. Vogt [1971]
suggested that asthenosphere flows from Iceland along
the Reykjanes Ridge at a rate of 200 mm/yr. Modeling
results have suggested asthenospheric velocities in the
Indian mantle as high as 1 m/yr [Yale and Phipps
Morgan, 1998]. For the highest acceptable activation
volume (20 106 m3/mol), melt production asymmetry
in our models closely matching S-wave velocity asym-
ð8Þ
where d is the seafloor depth and t is age, are dependent on
the thermal structure of the ridge at the time of crustal
production, which can vary with time. To explain the
asymmetry in subsidence, they appeal to the same asthenospheric temperature mechanism invoked by Cochran,
because variations in axial conditions alone predict no
asymmetry. Their calculations suggest an increase in
asymmetry with increasing age of the seafloor. We believe
that the asymmetry and the apparent increase in asymmetry
when modeled as subsidence proportional to the square root
of age are better explained by the superposition of a linear,
west-to-east, topographic gradient on a symmetric subsidence profile.
Figure 11. Thermal and dynamic models of asymmetric
subsidence in the MELT area. Solid line is across-axis
bathymetry. Vertical, dotted lines show four separate time
intervals used to find best-fitting parameters (following
Perrot et al. [1998]). Data is fit fairly well (thick dashes) if
unrealistically slow (<100 m/Myr1/2) subsidence parameters
are allowed. Thin solid line shows the same data fit with a
linear topographic gradient (0.174 m/km) superimposed on
symmetric subsidence. If the 12 parameter model is required
to use subsidence parameters >100 m/Myr1/2, the resulting
fit is not statistically better than the three parameter model.
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CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
metry (y 0.75) develops with an asthenospheric
velocity near 170 mm/yr (Figure 8). In a preferred model
with a less extreme activation volume (16 106 m3/
mol), but exhibiting the same asymmetry (y 0.75), the
asthenospheric flow rates are near 300 mm/yr (Figure 7a).
Although these flow rates seem high, independent evidence may support this rate of transport toward the ridge
axis in this area. The Pukapuka intraplate volcanic ridge
propagated from French Polynesia to near the EPR at
about 17S at a rate of about 300 mm/yr [Sandwell et al.,
1995]. The lavas carry a distinct isotopic signature and
are derived from shallow melting of an enriched source
[Janney et al., 2000]. Transport of a compositional
heterogeneity toward the EPR in the asthenosphere from
the superswell could have triggered melting and local
small-scale convection that may have been responsible for
ridge formation. Assuming the pressure gradient determined by the asymmetric subsidence, this flow rate
suggests that the asthenospheric viscosity in this area
near the EPR reaches a minimum of about 2 1018
Pas, consistent with the 1018 – 1019 Pas asthenospheric
viscosity inferred at Iceland from glacial rebound studies
[Sigmundsson, 1991].
[27] Seismic results from an experiment across the Lau
back arc spreading center [Wiens et al., 1995] also suggest
the possibility of melting asymmetry created by across-axis
flow. P wave tomography exhibits a markedly asymmetric
slow region beneath the axis, extending 200 km to the
west and <50 km to the east [Zhao et al., 1997]. Attenuation
tomography from the same data exhibits a fairly symmetric
low Q region just beneath the axis [Roth et al., 1999].
Seismic velocities and attenuation are both sensitive to
temperature and melt present, but velocities are more
sensitive to melt while attenuation is more sensitive to
temperature [Forsyth, 1992]. Back arc mantle dynamics
are likely dominated by corner flow induced from the
subduction zone [Davies and Stevenson, 1992]. Imposing
corner flow over a back arc region with a spreading center
may induce more upwelling on one side of the axis than the
other, but would have little direct affect on the temperatures
near the axis, accounting for both the asymmetric velocity
structure and the symmetric attenuation structure. More
investigation is required to confirm this model, but qualitatively it matches the observations well.
4.5. Dehydration Effect
[28] Some laboratory experiments suggest that the presence of water reduces the viscosity of olivine by a factor
of 300– 500 [Hirth and Kohlstedt, 1996]. Since water is
incompatible during melting, melting extracts water and
may consequently increase mantle viscosity. Several midocean ridge modeling studies have incorporated this effect
[e.g., Phipps Morgan, 1997; Ito et al., 1999; Braun et al.,
2000]. These studies assume seafloor spreading is a
symmetric process, using symmetry boundary conditions
beneath the axis. Following Braun et al. [2000], we use a
parameter, hdry, to examine the effects of a possible large
increase in viscosity with dehydration. Mantle that has
been depleted by more than 2% is assumed to be ‘‘dry’’,
so its viscosity from equation (6) is multiplied by hdry.
Viscosity increases for mantle with depletion values less
than 2% are assumed to be linearly proportional to hdry.
Figure 12. (a) Viscosity profile directly beneath the axis
assuming a 100 viscosity increase with depletion from
melting (solid line). The dotted line is a profile from an
identical model without a viscosity increase with depletion.
(b) Melt production in a model including both the pressure
gradient used in Figure 7a and a 100 viscosity increase
with depletion. The shallowest mantle where melting has
occurred is coupled to lithospheric plate spreading, driving
symmetric upwelling beneath the spreading center (y =
0.53). (c) Temperature, streamlines, and melt production for
a model with the temperature anomaly (100C) required to
make asymmetric melting (y = 0.66) when an increase in
mantle viscosity with depletion is assumed. Because the
uppermost mantle couples to the lithosphere with an
increase in viscosity, spreading is symmetric to greater
depths. A temperature anomaly originating far from the
axis, must be 300 km deep to reach the axis. The
subsidence predicted by this model is a poorer fit to the data
than a model with no viscosity increase (Figure 9). See
color version of this figure at back of this issue.
Figure 12 shows a viscosity profile and the melting regime
for the same model shown in Figure 7a but with hdry equal
to 100. The increase in viscosity in the shallow mantle
couples the shallow mantle flow with the lithospheric plate
spreading, driving symmetric upwelling in the melting
regime. A viscosity increase with dehydration implies that
the lowest viscosity mantle will always be beneath the
melting zone. Because the lowest viscosities are deeper
than the melting region, shear between the plates and deep
mantle is accommodated below the upwelling and melting
zone. Figure 13 shows the asymmetry dependence on ux
and v* for hdry = 10 and 100. For asymmetric melting to
develop with pressure-driven flow, asthenospheric velocities must be roughly doubled even for a modest viscosity
increase of one order of magnitude with depletion. A
viscosity increase by a factor of 100 requires asthenospheric velocities on the order of meters per year even for
large v* values. A viscosity increase in the shallow mantle
CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
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8 - 11
higher water solubility with increasing pressure at very
different strain rates. The functional form of the dependence
on water content is not well constrained by experiment. If
the effective viscosity at constant temperature and pressure
is of the form
heff ¼ hdry hð1 expðkCH2O =C0 ÞÞ
ð10Þ
rather than linear, retention of a minute amount of volatiles
remaining after melting could keep the viscosity relatively
low. A more recent laboratory study on the effect of water in
mantle materials [Mei and Kohlstedt, 2000] shows a
viscosity decrease of less than one order of magnitude
(5– 6) for saturated olivine aggregates relative to dry
aggregates at 300 MPa confining pressure, while acknowledging that olivine is significantly weakened by even a trace
of water.
[30] Another possibility is that the influx of enriched
material from the superswell is water-rich and continually
rehydrating the uppermost mantle beneath the ridge. However, the decrease in incompatible elements in the Pukapuka
ridges toward the EPR [Janney et al., 2000] suggests that
there may not be much excess water remaining by the time
this anomalous mantle material reaches the ridge axis. The
important note for this study is that to maintain the observed
asymmetry about the axis, any viscosity increase in the
shallow mantle must be subdued.
Figure 13. Dependence of asymmetry on ux and v*
assuming a viscosity increase with depletion from melting.
Symbols are same as in Figure 8. Even a modest increase of
10 requires significantly faster asthenospheric velocities
to produce reasonable asymmetry. An increase of 100
requires asthenospheric velocities on the order of several
meters per year to produce reasonable asymmetry.
also inhibits a shallow incoming temperature anomaly
from reaching the axis by forcing symmetric upwelling
in the shallow mantle. The depth extent of an incoming
temperature anomaly must increase about 100 km to
follow the base of the return flow, but the magnitude must
still be on the order of 100C or higher. A temperature
anomaly of this form mostly increases the depth of melting
on the west side rather than increasing its lateral extent
(Figure 12). In addition, the fit of the predicted subsidence
is considerably worse than the already poor fit of the
thermal model without a viscosity increase (Figure 9)
because there is a sharper temperature contrast beneath
the axis and the required anomaly has a greater depth
extent. No realistic model parameters are found that
simultaneously include a large viscosity increase with
depletion while maintaining a pronounced asymmetry in
the melting region.
[29] One possible way to reconcile the observed asymmetry with the laboratory experiments is to discard the
assumption that the viscosity decrease varies linearly with
water concentration. The experiments discussed by Hirth
and Kohlstedt [1996] can be divided into two groups,
‘‘wet’’ (water saturated) and ‘‘dry’’ (water thoroughly
removed), with dry olivine 100– 180 more viscous than
water-saturated olivine. The predicted 300– 500 viscosity
decrease in wet mantle comes from a linear extrapolation to
4.6. Seismic Anisotropy
[31] Seismic anisotropy may provide constraints on flow
models. Shear wave splitting delay times in the MELT area
are roughly 1.5– 2 s on the Pacific plate and 1 s on the
Nazca plate, with the smallest delays (0.8 s) found
immediately east of the axis [Wolfe and Solomon, 1998].
Rayleigh wave anisotropy, sensitive primarily to the structure in the upper 80 km of the mantle, is roughly
symmetric about the axis, with a minimum near the axis
[Forsyth et al., 1998]. Shear wave splitting delay times
depend on the degree of alignment of anisotropic mantle
minerals and on the thickness of the layer of those
minerals, both of which are expected to depend on the
flow induced strain. Anisotropy from lattice-preferred
orientation is expected to develop only with dislocation
creep, but the depth extent of the anisotropic layer marking
the predicted transition to diffusion creep [Karato, 1992] is
unknown. Toomey et al. [2002] and Blackman and Kendall
[2002] calculated the strain and subsequent shear wave
splitting delay times for vertically traveling S-waves
through the various models in this paper using uniform
integration depths (300 km depth in the work of Toomey
et al. and 200 km in the work of Blackman and Kendall).
The ridge migration case shows similar maximum splitting
times (2 –2.5 s) on either plate with a broad low (0 s)
near the axis. Depending on the depth of integration, the
incoming temperature anomaly model gives larger (1 s)
splitting times on the Pacific plate than the Nazca plate
and a low (<1 s) splitting time near the axis, producing a
reasonably good fit to the observations. The pressure
gradient model produces a roughly symmetric degree of
mineral alignment about the axis and therefore does not
exhibit a good fit to the asymmetry for any uniform
integration depth.
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CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
[32] The discrepancies between the splitting observations
and the predictions for the pressure gradient model may
simply imply that the pressure gradient model should be
ruled out in favor of the temperature anomaly model, but
given its success in explaining other geophysical observations, we believe there may be geological reasons for the
differences that were not considered in the calculations.
Possibly the largest discrepancy arises from the assumption
of a constant thickness LPO layer, requiring that the
asymmetry be entirely due to an asymmetry in degree of
mineral alignment in the uppermost mantle. As noted earlier, the Rayleigh wave anisotropy is fairly symmetric about
the axis, suggesting a similar degree of alignment within the
upper 80 km of the two plates; therefore the asymmetry in
splitting times may result from an asymmetry in anisotropic
layer thickness rather than degree of mineral alignment.
Dislocation creep, the deformation mechanism thought to
produce LPO, is highly stress dependent. Because return
flow in the pressure gradient model is almost entirely
confined beneath the Pacific plate, stresses may be higher
at depth leading to a thicker anisotropic layer than beneath
the Nazca plate. Another possible problem with directly
comparing the calculations to the splitting observations is
that all of the models presented in this paper assume that
flow is entirely two-dimensional, with no ridge-parallel
flow. The seismic velocity anomalies are most pronounced
in the vicinity of the Rano Rahi seamount field west of the
axis, which connects to the Pukapuka ridge system to the
west. If asthenospheric flow were more rapid in channels
beneath intraplate ridges, the channelized flow might spread
out along-axis near the spreading center, reducing the
amount of across-axis flow and shear beneath the Nazca
plate. Along-axis flow could also reduce the ridge-perpendicular anisotropy near the axis. Models of three-dimensional flow will be explored in later studies, but at present,
the underlying causes for the asymmetric shear wave splitting pattern remains unclear.
5. Summary
[33] The MELT region of the EPR (15 – 19S) exhibits
pronounced asymmetry in many geophysical characteristics
including P and S wave velocity, electrical conductivity,
subsidence, seamount distribution, and shear wave splitting.
The observations suggest more melt in the mantle beneath
the Pacific side of the axis than the Nazca side. Ridge
migration alone has minimal effect on the symmetry of the
melting region, therefore another mechanism is required.
The observed asymmetry is difficult to reconcile with just
an added temperature anomaly. To create asymmetry by a
ridge-fed temperature anomaly, the anomaly must be
>100C and within a limited depth range. A temperature
anomaly with dimensions that do predict substantial melting
asymmetry overpredicts the subsidence asymmetry and
predicts a thicker than normal crust, which is not observed.
[34] Pressure-driven mantle flow from the superswell
explains many of the observations. Asthenosphere driven
toward the axis from the west continually upwells as it
encounters increasingly thinner lithosphere. When the flow
crosses the axis, it has a component of downwelling as it
encounters a thickening lithosphere, effectively shutting off
any melting. Flow models with asthenospheric flow rates
near 300 mm/yr (based on the propagation rate of the
Pukapuka ridge toward the EPR) exhibit a strong asymmetry, consistent with the seismic observations, with melting
occurring 100s of km west of the axis, but only 10s of km
east of the axis. The asymmetric subsidence of the EPR can
be explained by a linear gradient superimposed on symmetric subsidence. A linear gradient entirely due to dynamic
topography implies a pressure gradient of 7500 –8000 Pa/
km, consistent with the few kPa/km gradients imposed in
the flow models. It is possible that a small temperature
anomaly (<50C) along with across-axis asthenospheric
flow from the superswell contribute to the asymmetry
observed about the EPR in the MELT area. Further study
of seismic anisotropy may be key to improving our understanding of dynamic mantle processes beneath the EPR.
[35] Acknowledgments. We thank Chad Hall, Dan Scheirer, Rob
Dunn, and Doug Toomey for insightful discussions. Donna Blackman
and Doug Toomey provided the predicted strain and shear wave splitting
calculations. We thank Roger Buck and Rob Evans whose constructive
reviews greatly improved the paper. This work was supported by the
National Science Foundation grant OCE-9812208.
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J. A. Conder, Department of Earth and Planetary Sciences, Washington
University, Campus Box 1169, One Brookings Drive, St. Louis, MO
63130, USA. ([email protected])
D. W. Forsyth and E. M. Parmentier, Department of Geological Sciences,
Brown University, Providence, RI 02912, USA.
CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
Figure 2. Cross-section of S-wave velocities beneath the EPR in the MELT area [from Forsyth et al.,
2000]. The region of velocities lower than 4.1 km/s, probably indicating the presence of melt, extends
hundreds of kilometers to the west, but only several tens of kilometers to the east of the axis. Crosssection location is midway between the two main seismic lines shown in Figure 1.
Figure 6. Temperature (T), streamlines, and melt production for models with a 100C thermal anomaly
(Ta) introduced from the west at (a) 150 km, (b) 200 km, and (c) 250 km depth. Asymmetry values for
each are 0.58, 0.66, and 0.62, respectively. Melt production asymmetry in these models comes from a
larger increase the amount of melt on the west side than on the east side. If the anomaly is confined above
the 150 km deep streamline, there is little effect on the melting region. If the anomaly extends down to the
250 km or deeper streamlines, the anomaly increases melt production on both sides of the axis.
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CONDER ET AL.: ASTHENOSPHERIC FLOW AND ASYMMETRY OF THE EPR
Figure 12. (a) Viscosity profile directly beneath the axis assuming a 100 viscosity increase with
depletion from melting (solid line). The dotted line is a profile from an identical model without a
viscosity increase with depletion. (b) Melt production in a model including both the pressure gradient
used in Figure 7a and a 100 viscosity increase with depletion. The shallowest mantle where melting has
occurred is coupled to lithospheric plate spreading, driving symmetric upwelling beneath the spreading
center (y = 0.53). (c) Temperature, streamlines, and melt production for a model with the temperature
anomaly (100C) required to make asymmetric melting (y = 0.66) when an increase in mantle viscosity
with depletion is assumed. Because the uppermost mantle couples to the lithosphere with an increase in
viscosity, spreading is symmetric to greater depths. A temperature anomaly originating far from the axis,
must be 300 km deep to reach the axis. The subsidence predicted by this model is a poorer fit to the data
than a model with no viscosity increase (Figure 9).
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