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Skew of mantle upwelling beneath the East Pacific Rise governs segmentation



Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is generally assumed that such upwelling is symmetric beneath active ridges. Here, however, we use seismic imaging to show that the isotropic and anisotropic structure of the mantle is rotated beneath the East Pacific Rise. The isotropic structure defines the pattern of magma delivery from the mantle to the crust. We find that the segmentation of the rise crest between transform faults correlates well with the distribution of mantle melt. The azimuth of seismic anisotropy constrains the direction of mantle flow, which is rotated nearly 10 degrees anticlockwise from the plate-spreading direction. The mismatch between the locus of mantle melt delivery and the morphologic ridge axis results in systematic differences between areas of on-axis and off-axis melt supply. We conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise and variations in the intensity of ridge crest processes.
Skew of mantle upwelling beneath the East
Pacific Rise governs segmentation
Douglas R. Toomey
, David Jousselin
, Robert A. Dunn
, William S. D. Wilcock
& R. S. Detrick
Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is gener ally assumed that
such upwelling is symmetric beneath active ridges. Here, however, we use seismic imaging to show that the isotropic and
anisotropic structure of the mantle is rotated beneath the East Pacific Rise. The isotropic structure defines the pattern of
magma delivery from the mantle to the cru st. We find that the segmentation of the rise crest between transform faults
correlates well with the distribution of mantle melt. The azimuth of seismic anisotropy constrains the direction of mantle
flow, which is rotated nearly 10u anticlockwise from the plate-spreading direction. The mismatch between the locus of mantle
melt delivery and the morphologic ridge axis results in systematic differences between areas of on-axis and off-axis melt
supply. We conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise
and variations in the intensity of ridge crest processes.
The origin of segmentation of oceanic spreading centres is contro-
versial. According to one point of view, along-axis differences in
ridge crest processes result directly from three-dimensional mantle
. Sites of vigorous volcanic and hydrothermal activity
are thus thought to overlie regions of greater magma supply.
Limited knowledge of mantle structure, however, has given rise to
diverging opinions on the scale of three-dimensional upwellings
Alternatively, segmentation of ridge crest processes may be regulated
by the tectonic rifting of young lithosphere
, and thus not directly
linked to the form of mantle upwelling. In this case mantle flow could
be either two-dimensional
or three-dimensional and less obvious
because of efficient along-axis transport of magma by viscous flow
Along the global ridge system, the fast-spreading East Pacific Rise
(EPR) between the Siqueiros and Clipperton transforms (Fig. 1) cur-
rently offers our best opportunity for understanding the relations
between mantle upwelling and ridge crest processes. There are several
reasons for this. This section of the EPR encompasses a full spectrum
of rise axis discontinuities, including: two large-offset transform
faults; large, long-lived (9u 039 N) and small, short-lived (9u 379 N)
overlapping spreading centres (OSCs)
; and smaller-scale mor-
, petrologic
and seismic
discontinuities that are
typical of fast-spreading ridge segments. Accompanying these axial
discontinuities are well known along-axis variations in seafloor
depth, axial high morphology
, crustal structure and thickness
lava chemistry
, and seafloor hydrothermal
and biological
. These characteristics of the EPR, including the origin of
ridge crest segmentation, have been hypothesized to result from the
supply of magma from the mantle.
We conducted the UNDERSHOOT experiment (our data-gathering
cruise) to seismically image the crustal and mantle structure between
the Clipperton and Siqueiros transforms to determine the pattern of
magma delivery from the mantle to the crust. Here we present the
first images of mantle structure beneath an entire ridge segment
bounded by long-lived tectonic discontinuities (Fig. 1). Good image
resolution allows direct comparison between the scales of segmenta-
tion observed along this section of the EPR with the physical structure
of the topmost mantle. Our results allow conclusions to be drawn
about the driving and controlling processes for segmentation of fast-
spreading ridges.
Experiment geometry and tomographic imaging
The distribution of seismic receivers and sources used to image
crustal and mantle structure is shown in Fig. 1. The experiment
constrains the structure of the uppermost mantle within 4 km of
the Mohorovic
discontinuity and within an area extending 15 km
to either side of the rise axis and 230 km along the spreading centre. A
three-dimensional model of off-axis crustal structure and thickness
is used to analyse the mantle refraction data (see Supplementary
The P
data (from the wave refracted below the Moho) provide
good spatial sampling of mantle structure throughout the image
volume (Fig. 2a). P
travel-time residuals plotted by azimuth reveal
a cos2H pattern (Fig. 2b), a signal indicative of azimuthal seismic
anisotropy. The azimuth of anisotropy (that is, the fast direction for
propagation) is N73uE 6 1u (see Supplementary Information).
Tomographic inversions, discussed below, confirm this result. The
azimuth of anisotropy is rotated 9u anticlockwise with respect to the
predicted spreading direction
(N82uE). Plotted by rise crossing
point, P
travel-time residuals show evidence for anomalously low
and variable upper-mantle velocities (Fig. 2c). The average isotropic
velocity that best fits the P
data (7.6 km s
) is less than typical
upper-mantle velocities, whereas delays are greater towards the
centre of the transform-bounded segment and less within 20 km of
the transforms.
Tomographic inversion (see Supplementary Information) of P
travel-time data reveals a mantle low-velocity zone (MLVZ) that is
segmented on a scale comparable to tectonic offsets of the EPR
(Fig. 1). The MLVZ decreases in amplitude towards each transform,
in agreement with the decrease in mean P
delays (Fig. 2c). Between
transforms, the MLVZ follows two en echelon trends that are ortho-
gonal to the azimuth of seismic anisotropy (Fig. 1b, green lines). The
en echelon trends are offset in a right lateral sense and rotated
Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA.
, CRPG, 54501 Vandoeuvre les Nancy, France.
Department of Geology and
Geophysics, University of Hawaii-SOEST, Honolulu, Hawaii, 96822, USA.
School of Oceanography, University of Washington, Seattle, Washington 98195, USA.
Department of
Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA.
Vol 446
22 March 2007
anticlockwise with respect to the axis of plate spreading. Owing to
this azimuthal rotation, much of the MLVZ is not centred beneath
the plate boundary. Beneath and immediately north of the OSC, the
MLVZ steps rightward, defining a geometrically complex transitional
region that connects the en echelon trends of the MLVZ. The trans-
itional region, located between 9u 009 N and 9u 189 N, is shifted
northward of the OSC itself and coincides with a region of thicker
North of the OSC, local minima of the MLVZ occur at intervals of
approximately 25 km. Pronounced sub-axial anomalies are located
near 9u 569 N and 9u 449 N. Equally pronounced anomalies are
centred more than 10 km off-axis at 9u 329 N and a few kilometres
off-axis at 9u 159 N. Between 9u 159 Nto9u 359 N the MLVZ is located
5–15 km east of the rise axis, in a region where magnetotelluric stud-
ies detect anomalously low mantle conductivities
. South of the OSC
there is a local minima of the MLVZ near 8u 439 N. Within our study
area, the most pronounced mantle anomaly occurs beneath the east-
ern limb of the OSC, where several compliance measurements sug-
gest near-Moho sills
and seismic imaging detects substantial
amounts of melt in the lower crust
. The transition of the MLVZ
between opposing limbs of the OSC also coincides well with a similar
transition of the crustal-level axial magma chamber reflector
(Fig. 1).
We conducted a series of tomographic inversions for fixed models
of mantle anisotropy to explore the tradeoffs between isotropic and
anisotropic structure. An isotropic mantle results in a large x-squared
travel-time misfit (,6). Figure 3 shows that the data misfit is smallest
–104° 30 -104° 00
2,500 3,000 3,500
Depth (m)
–104° 30 –104° 00
7.2 7.6 8.0 8.4
P-wave velocit
(km s
9° 03 N
8° 30
9° 00
9° 30
10° 00
Figure 1
Location and geometry of the seismic experiment and
tomographic image of the mantle low-velocity zone (MLVZ) and
orientation of mantle anisotropy. a
, The Clipperton and Siqueiros
transform faults bound the study area. Dashed lines show the location of the
plate boundary. Seismic data were collected on 37 ocean-bottom receivers
from the Woods Hole Oceanographic Institution. Twenty of these were
Office of Naval Research, three-component seismometers (open squares)
equipped with 1-Hz geophones and a hydrophone; the remaining units were
ocean-bottom hydrophones (open circles). The seismic source was the RV
Maurice Ewing’s 20-gun, 8,500-cubic-inch (139 litre) air gun array, fired at
intervals of 210 s (shot spacing of 500 m) along the tracks indicated by solid
black lines. Locations of axial magma chamber reflector from multichannel
seismics shown in red
. Black box indicates the location of the three-
dimensional multichannel seismics experiment.
b, Tomographic image of
mantle P-wave velocity; contour interval is 0.1 km s
and depth of section is
9 km beneath the sea floor. Green lines with arrowheads indicate azimuth of
seismic anisotropy (see Figs 2b and 3); black lines with arrowheads indicate
plate-spreading direction
. Green lines without arrowheads are
perpendicular to seismic anisotropy and indicate locations of en echelon
segments of the MLVZ. Seafloor compliance measurements are indicated by
white symbols
; larger white squares are locations where near-Moho melt
sills were detected.
–104.5 –104
Longitude (º)
Latitude (º)
0 50 100 150
Mean delay (ms)
–100° 100°
Mean delay (ms)
Figure 2
Map of the distribution of seismic ray paths and mean P
times versus azimuth and latitude. a
, Map of the distribution of the 4,892
ray paths used to image mantle structure. P
data sample the mantle
within about 615 km of the rise axis.
b, Mean P
delay plotted by azimuth.
Delays are calculated relative to an isotropic model, corrected to 40 km range
and binned at intervals of 5u. Vertical bars indicate uncertainty in mean
delay as determined by a Student’s t-test (95% confidence interval). Solid
line is the best-fit cos2H curve. The azimuth of seismic anisotropy is
N73uE 6 1u.
c, Mean P
delay plotted by rise-axis crossing point. Delays are
calculated relative to 7.8 km s
, corrected to 40 km range and have been
corrected for seismic anisotropy (6% with fast axis azimuth of N73uE). Data
are binned at intervals of 10 km and bars indicate uncertainty in mean delay
time as determined from a Student’s t-test (95% confidence interval).
Vol 446
22 March 2007
when the azimuth of anisotropy imposed on the tomographic
procedure is identical to that estimated directly from the P
(Fig. 2b). The data misfit is significantly larger when azimuthal aniso-
tropy matches the predicted spreading direction. A robust result of
our analysis is that both the azimuth of seismic anisotropy relative to
the spreading direction and the en echelon trends of the MLVZ with
respect to the plate boundary are rotated anticlockwise by similar
The Supplementary Information summarizes our analysis of
model sensitivity and resolution and details how our current results
differ from previously published studies. Therein we show that a ray-
tracing error in a previous study of the 9u 039 N OSC
resulted in an
incorrect image.
Skew of mantle upwelling and asthenospheric flow
The isotropic component of our tomographic image constrains the
distribution of melt in the topmost mantle. Regions of anomalously
low seismic velocities are consistent with higher melt fractions. If
melt resides in film-like geometries
, then our results can be
explained by the presence of 1–3% melt (see Supplementary
Information); melt fractions would be greater if distributed aniso-
tropically or if melt pockets are more spherically shaped. We attribute
the MLVZ to a region of melt accumulation that lies beneath the base
of newly formed oceanic crust; given the seismic wavelength, the
vertical extent of this region must be several kilometres. We infer
that the locus of sub-crustal melt accumulation overlies the melt
production region located at depths of several tens of kilometres.
The alternative, that large volumes of partially molten mantle have
been transported off-axis, seems unlikely. According to this view, the
axis of mantle upwelling that gives rise to decompression melting is
skewed beneath the plate boundary.
The anisotropic component of our model constrains the direction
of shallow mantle flow. This is because the maximum compressional
wave speed for single-crystal olivine parallels the crystallographic a
axis and deformation of mantle peridotites preferentially aligns the
a axis in the direction of maximum shear
. Immediately beneath a
spreading centre, the overturn accompanying mantle divergence
generates shear strains that are many times larger than the deforma-
tion resulting from the movement of a plate over the asthenosphere
We thus infer that the azimuth of seismic anisotropy beneath the
spreading axis is related to the azimuth of mantle divergence. We
conclude that in relation to the ridge, mantle flow is skewed by 9u
with respect to the plate-spreading direction. The transport of asth-
enosphere away from the EPR and the axis of decompression upwel-
ling are thus rotated anticlockwise in a coherent manner (Fig. 1).
The azimuthal rotation of mantle flow beneath the EPR is in the
same direction as recent changes in the Euler pole for Pacific–Cocos
plate motion, which has been progressing anticlockwise for the past
several million years
. Ongoing changes in plate kinematics
, how-
ever, are lagging behind the current direction of asthenospheric
flow. We propose that the skew of mantle flow beneath the EPR is
one of the driving forces for changes in plate boundary kinematics.
Specifically, basal tractions imposed by mantle flow are contributing
to anticlockwise changes in the spreading direction. This flow may
also contribute to the transpressional and transtensional tectonics of
the Clipperton and Siqueiros transforms
, respectively (Fig. 4). The
lag of the rigid plate system relative to the viscous asthenosphere may
indicate that transpressive transform faults, such as the Clipperton,
limit the rate at which plates adjust to changes in plate driving
Ridge segmentation and mantle structure
Our study shows that the tectonic segmentation of the EPR correlates
well with the pattern of melt delivery from the mantle to the crust.
Near transforms or first-order offsets of the ridge crest
, the increase
in seismic velocities is consistent with a decrease in the amount of
65° 70° 75° 80° 85°
Normalized misfit
Inferred mantle
flow direction
Figure 3
Normalized data misfit following tomographic inversion versus
azimuth of seismic anisotropy imposed on starting model.
For each
inversion, percentage (6%) and azimuth of anisotropy are held fixed. Data
misfit is a minimum for an azimuth of N73uE. Misfit is significantly larger
when the azimuth of anisotropy parallels the spreading direction predicted
by the NUVEL-1A model
9° 03 N
Melt flow
Mantle flow
Axis-centred delivery
Off-axis delivery
~10 km
Figure 4
Proposed model of segmentation beneath the East Pacific Rise.
, Map view showing plate boundary and tectonic discontinuities (solid
black lines), regions of melt accumulation beneath the crust (orange area
labelled ‘Mush’), orientation of mantle flow (green lines with arrowheads)
and trend of the en echelon segments of melt accumulation (green lines
without arrowheads; perpendicular to mantle flow). Thin black line with
arrowheads indicates NUVEL-1A spreading direction
. Large, paired arrows
indicate regions of transpression (solid) and transtension (open) along the
Clipperton and Siqueiros transform faults, respectively
. Circles with
roman numerals indicate location of cross-sections shown in
b. b, Two rise-
perpendicular sections depicting magma plumbing beneath axis-centred
(top) and off-axis (bottom) sites of mantle melt delivery. Above centres of
melt delivery, magma accumulates at near-Moho depths, presumably within
a mush-like compaction layer (orange region), with substantially lower melt
concentrations in surrounding regions of the mantle. Melt migration paths
indicated by red arrows; near Moho melt sills indicated by red ellipses. Small
filled circles depict the exsolution of magmatic volatiles that occurs during
differentiation. Sites of axis-centred delivery of mantle melt are more likely
to be hydrothermally active and resurfaced frequently by extrusive
volcanism (plume and smooth sea floor in top panel). Sites of off-axis
delivery of mantle melt sustain greater amounts of tectonic extension, erupt
less frequently and host more intermittent high-temperature hydrothermal
activity (rougher sea floor in bottom panel).
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22 March 2007 ARTICLES
melt at near-Moho depths and possibly a decrease in temperature.
Towards the Siqueiros transform the increase in seismic velocity is
most pronounced, consistent with the observed decrease in crustal
Between transform offsets, EPR segments are separated by OSCs or
second-order ridge crest discontinuities
. The right-stepping 9u 039 N
OSC occurs near the ends of en echelon segments of the MLVZ, or
where the centre of mantle upwelling is sheared in a right-lateral
sense. We suggest that the large-scale azimuthal rotation of mantle
upwelling, with respect to the current Euler pole of Pacific–Cocos
plate motion, governs the formation and evolution of the OSC
(Fig. 4). According to this view, mantle upwellings organize into en
echelon segments and large-offset OSCs provide an accommodation
zone that moves the lithospheric plate boundary from one segment of
mantle upwelling to another. A previous hypothesis for the origin of
OSCs attributes them to variations in the duration, timing or intens-
ity of magmatic pulses that inflate crustal magma chambers and that
propagate along the plate boundary, in which case a reduced magma
supply is expected near second-order discontinuities
. An alternative
model infers that OSCs are entirely lithospheric features that form in
response to changes in plate motion
. We conclude that the seg-
mentation of the EPR by OSCs is a direct result of en echelon upwel-
ling at mantle depths. OSCs thus do not form in response to reduced
magma supply, nor is their evolution related to along-axis migration
of magmatic pulses away from centres of replenishment towards
magma-starved discontinuities. We propose that OSCs form because
the geometry of the rigid plate system is inconsistent with the under-
lying orientation and geometry of asthenospheric upwelling. As the
pattern of mantle upwelling evolves, OSCs will respond to maintain
their positions where en echelon segments of upwelling are sheared in
the cross-axis direction. We predict that both the size and the sense of
offset of an OSC, as well as its propagation history, will be related to
the underlying and evolving pattern of mantle upwelling.
Between transforms and OSCs the EPR is subtly segmented at a
scale of ,25 km, referred to as third-order or volcanic segmenta-
. Multidisciplinary studies reveal that volcanic segments are
chemically, structurally and geologically distinct. Petrologic data
show that lavas erupted within volcanic segments are composition-
ally similar, yet differences in lava chemistry between volcanic seg-
ments can be pronounced
. Geophysical studies indicate that the
centres of volcanic segments are associated with a more pronounced
magmatic system at mid- to lower-crustal depths, and increased
temperatures within a thermal boundary layer that connects the
magmatic and hydrothermal systems
. Near the boundaries of
volcanic segments, seismic reflection and tomographic imaging
detect abrupt changes in mid- to upper-crustal structure
Lastly, seafloor mapping shows that volcanic and tectonic features
are also segmented at this scale
. The structure of the MLVZ corre-
lates well with previous indicators of third-order segmentation, sup-
porting the hypothesis that volcanic segmentation of the EPR is
inherited from variations in mantle melt delivery
. We note that
the spacing of volcanic segments is similar to that of diapirs mapped
in the Oman ophiolite
. Additional studies will be necessary to
determine whether this is a coincidence or the result of dynamic
Ridge activity and skew of mantle upwelling
We find that the state of volcanic and tectonic activity along the ridge
correlates with the cross-axis distance to a centre of mantle melt
delivery. Above axis-centred delivery of mantle melt (for example,
9u 509 N), lavas are young and fissure density is decreased
Where mantle melt arrives off-axis (for example, 9u 309 N), older
lavas and increased fissuring characterize the axial high. Within
our study area, the oldest lavas and the highest density of fissured
sea floor
is found where the MLVZ is farthest from the rise crest
(9u 209 N). Axial eruptive styles also differ, with larger volume flows
inferred where off-axis delivery of mantle melt occurs
. These
relations suggest that shorter repose times and smaller eruption
volumes may typify volcanoes fed by axis-centred delivery of mantle
The vigour of hydrothermal venting also correlates with the
pattern of magma delivery. In regions of axis-centred delivery, high-
temperature hydrothermal venting is associated with recent volcan-
ism and vent-supported biologic communities are abundant
Either more mature vents or a lack of high-temperature vents and
a general decrease in the abundance of vent-supported biota
are observed where mantle melt arrives off-axis. A transition from
on-axis to off-axis delivery of mantle melt occurs near 9u 379 N, where
a ridge discontinuity separates a volcanic section of the axial high
from a tectonic one and demarcates a significant hydrothermal
. North of 9u 379 N, lavas are less evolved (higher MgO
content), eruption temperatures are .1,190uC, hydrothermal vent-
ing is usually .350 uC and vent fluids are consistent with derivation
from the vapour phase of phase-separated fluids
. In contrast, south
of 9u 379 N, where mantle melt is delivered off-axis, basalts are more
evolved (lower MgO), eruption temperatures are ,1,190 uC, hydro-
thermal venting temperatures are ,325 uC and vent fluids are derived
from the brine phase of phase-separated fluids
Magma plumbing and the segmentation of the EPR
Three significant new findings of our study are that: (1) mantle
upwelling and asthenospheric flow are skewed beneath the plate
boundary, and thus the delivery of mantle melt is in many places
not centred beneath the rise axis; (2) the 9u 039 N OSC, a second-
order discontinuity of the EPR, occurs where en echelon segments of
mantle upwelling are offset in the cross-axis direction; and (3)
between first- and second-order discontinuities the cross-axis offset
between a centre of mantle melt delivery and the rise axis correlates
with the intensity of rise crest volcanic, hydrothermal and tectonic
activity. Building on these results, we propose a new model of mag-
matic segmentation beneath fast-spreading ridges, illustrated in
Fig. 4. In our model the skew of mantle upwelling beneath the plate
boundary governs second-order segmentation of the EPR and the
intensity of geologic processes occurring within third-order or vol-
canic segments. Above we discussed the implications of our results
for transforms and OSCs.
Away from OSCs and transforms, volcanic segments receive
mantle-derived melt from approximately equally spaced centres. In
contrast to previous hypotheses, rise-parallel variations in ridge pro-
cesses are not simply a function of magma supply or the along-axis
redistribution of magma away from a mantle source
. Instead, the
cross-axis offset between a centre of mantle melt delivery and the
associated axial volcano causes axis-parallel changes in ridge crest
processes. We illustrate our magma-plumbing model with two
cross-sections (Fig. 4). One is through an axis-centred site of mantle
melt delivery and a volcano characterized by frequent extrusive vol-
canism and vigorous seafloor hydrothermal and biologic systems;
this is representative of the rise crest near 9u 509 N (refs 22, 43)
(Figs 1 and 4). In contrast, the second cross-section is through an
off-axis centre of melt delivery that feeds melt to an axial volcano
where seafloor eruptions occur less frequently, surface tectonic
extension is greater and seafloor hydrothermal and biologic activity
are less intense; this is representative of the rise crest near 9u 309 N
(refs 22, 43, 44).
The contrasting processes at these two sites cannot be attributed to
differences in the size and shape of upper crustal magma chambers,
which are similar
. Nor is it likely that the observed differences in
hydrothermal activity can be attributed to near-surface permeability,
which is expected to be greater where seafloor fissuring and faulting is
. We further rule out magma supply as the controlling factor
because crustal thickness, and by association the long-term magma
supply, is similar at each location
. We propose that as the cross-
axis offset between a centre of mantle melt delivery and the rise crest
increases, so does the differentiation of magma that is delivered to the
Vol 446
22 March 2007
volcano. With cooling and differentiation, magma changes its com-
position, increases its density and enhances the exsolution of mag-
matic volatiles. We infer that such fundamental changes in the
qualities of magma will shape near-surface processes driven by crus-
tal magma chambers.
Our model predicts that magma entering the crustal system from
an axis-centred site of mantle melt delivery will have undergone
relatively less differentiation. Such magma will be higher in temper-
ature, more buoyant (that is, higher MgO) and retain more of its
primary volatiles, thus increasing the breaching of the reaction zone
above the axial magma chamber, the exchange of energy between the
magmatic and hydrothermal systems and the recurrence of extrusive
volcanism that localizes vigorous, high-temperature venting and bio-
logical activity. Sites of axis-centred delivery of mantle melt are most
likely to be characterized as volcanically and hydrothermally robust
on the basis of surface geology. In the case of off-axis delivery of
mantle melt our model predicts that lateral migration of melt at
sub-crustal depths will promote igneous differentiation. Crustal
magma reservoirs will thus receive melt that is both cooler and denser
(higher FeO) than in reservoirs above axis-centred upwellings.
Depending on the degree of differentiation, considerable open-
system fractionation of magmatic volatiles could occur off-axis, in
which case the rate of pressurization of a ridge-crest magma chamber
by volatiles would be lower. A decrease in the volatile content of an
axial magma chamber should decrease the breaching of the hydro-
thermal reaction zone and reduce the frequency of volcanic erup-
tions. In this setting, localized high-temperature hydrothermal
venting may be intermittent and lower-temperature, diffuse-flow
venting could prevail. Such characteristics are commonly attributed
to magma starvation. In our magma plumbing model, two axial
volcanoes (or third-order segments) can receive similar volumes of
magma, but behave differently (extrusive versus intrusive) because of
sub-crustal, differentiation-induced changes in magma quality.
Previous models of mid-ocean ridges have usually assumed that
magma supply controls segmentation, that asthenospheric transport
parallels the spreading direction and that mantle upwelling and melt
delivery is symmetric about the rise axis. Ours is the first study to
show large-scale skew of mantle upwelling beneath mid-ocean ridges
and as such it renews the debate over the origin and significance of
spreading-centre segmentation. One implication of our results is that
local plate motions alone are not the sole cause of sub-ridge mantle
flow. On the contrary, the skew of mantle upwelling and transport
can act as a driving force for the tectonic reorganization of the EPR
and cause along-axis variations in the intensity of ridge-crest pro-
cesses. We speculate that the skew of the sub-ridge asthenosphere
owes its origin to global patterns of mantle flow, which are strongly
influenced by the viscous coupling between subducting oceanic slabs
and the surrounding mantle. If this speculation holds, it implies that
the flux of slabs into the mantle may be linked to the segmentation of
mid-ocean ridges.
Received 30 August 2006; accepted 8 February 2007.
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Supplementary Information is linked to the online version of the paper at
Acknowledgements We thank the officers and crew of the RV Maurice Ewing and
members of the scientific party for their assistance. D.R.T. thanks E. Hooft and
E. Humphreys for numerous discussions and J. Karson, T. Durant and D. Villagomez
for comments. Supported by the RIDGE and RIDGE 2000 Programs, Ocean
Sciences Division, NSF.
Author Contributions All authors participated in the experimental design, the
collection of the data and in several stages of data reduction and analysis. D.R.T.
conducted the tomographic analysis and wrote the manuscript with comments
from co-authors.
Author Information Reprints and permissions information is available at The authors declare no competing financial interests.
Correspondence and requests for materials should be addressed to D.R.T.
Vol 446
22 March 2007
... However, as this melt rises from below, melt distribution near the Moho may be the real controlling factor of the tectonic and magmatic segmentation (Carbotte et al., 2013). Seismic imaging (Barth et al., 1991), compliance studies (Crawford and Webb, 2002), and tomographic imaging (Toomey et al., 2007) point to melt accumulation in the upper mantle, which appears to be segmented, and not always centered beneath the ridge. Despite recent progress in multichannel seismic techniques (Aghaei et al., 2014), it remains difficult to image the Moho structure and melt distribution at this depth, in as much detail as the upper-crust axial melt lens. ...
... Conversely, no reflectors deeper than the Moho should be found directly right beneath the axis. The mantle tomography study of Toomey et al. (2007) provides an image of the last 4 km layer beneath the Moho; it cannot resolve the presence of a MTZ, but reveals the presence of several enhanced melt upwelling centers, approximately at 9 • 57 ′ N, 9 • 43 ′ N, 9 • 30 ′ N, 9 • 15 ′ N, 9 • 05 ′ N, a more diffuse one at 8 • 57 ′ N, and another at 8 • 30 ′ N not shown in Fig. 7. Modeling shows that these anomalies could be consistent with the presence of Oman-like diapirs (Jousselin et al., 2003). The anomalies have a near 20-25 km spacing that matches the Oman diapir spacing. ...
... The Oman ophiolite has preserved accretion centers that fed a fast spreading center; it provides a useful window into processes that occur at the crust-mantle boundary, which are complementary to marine geophysical data collected at currently active ridges. The Oman paleoridge is underlain by mantle diapirs, which are similar in size and spacing to those of velocity minima found in the mantle, 0-4 km beneath the Moho at the EPR (Toomey et al., 2007). The diapirs are capped by a Moho transition zone, mainly made of dunite (>95%), which sometimes contains "melt" (plagioclase and clinopyroxene) impregnations, and layered gabbro lenses (less than 5% vol.). ...
Our knowledge of melt distribution in the lower crust and upper mantle at oceanic fast spreading centers is very limited. Evidence of melt accumulation, sometimes away from the axis, has been imaged and interpreted at the Moho of the East Pacific Rise; but the detailed structures of these deep magma lenses remains much more difficult to unveil than that of the shallow axial melt lens at the top of the plutonic crust. Ophiolites offer on-land sections of oceanic lithosphere that can complement marine geological and geophysical observations. We present results of a geological survey of the Moho transition zone at a paleospreading center in the Oman ophiolite. We find that the thickness of this dunite-rich horizontal layer increases from a few meters at the axis to hundreds of meters 6 km away from the axis, and is reduced to a few meters 2–3 km further away. The base of the Moho transition zone contains dunite and very depleted harzburgite with isotropic plagioclase and clinopyroxene impregnations, and stockwork-like magmatic breccia, indicative of episodic high melt fractions. We conclude that the melt-free dunite horizontal layer may stop the progression of ascending melt; this lead to melt accumulation within the uppermost harzburgite beneath the Moho transition zone and forms the isotropic impregnations. As the melt dissolves the harzburgite orhtopyroxene, and is flushed to the top of the MTZ though the breccia, it leaves new dunite at the base of the Moho transition zone. Repetition of this process renders the Moho transition zone thicker as it moves away from the ridge axis, until it leaves the main area of mantle melt delivery. Then, tectonic thinning and intrusion of parts of the MTZ into the lower crust reduce the MTZ thickness. These processes seem coherent with several marine geophysical observations.
... Seismic and compliance studies at the EPR indicate the presence of a broad subcrustal melt reservoir 10-20 km wide and centered about the ridge axis with 3-11% melt fraction within and beneath the MTZ, possibly distributed in sills ( Figure 9) [Garmany, 1989;Dunn et al., 2000;Crawford and Webb, 2002;Toomey et al., 2007]. This subcrustal melt reservoir is likely a general feature of intermediate-to-fast spreading centers, with its melt content being a function of magma supply to the ridge . ...
... Although the early discovery of volcanism on mid-ocean ridges (MORs) made a major contribution to the development of modern plate tectonic theory, debate has continued about the spatial and temporal relationships between near-and off-axis volcanism associated with MOR magmatism (e.g., Ballmer et al., 2013;Niu & Batiza, 1997;Perfit et al., 1994;Reynolds & Langmuir, 2000;Sohn & Sims, 2005;Waters et al., 2013). The distribution and shape of seamounts provide important information on the timing of intraplate and spreading-center related volcanism and crustal and mantle melting processes, as seamounts have the potential to sample both the on-and off-axis melt regimes (Batiza, 1982;Smith, 1988;Toomey et al., 2007). At MOR, seamounts proximal to the axis are often small, ~<1 km in relief, and are located in a variety of settings. ...
... Fine-scale tectonic segmentation is commonly observed at fast-spreading MORs like the EPR, and seismic reflection imaging of the AML shows segmentation of the seafloor fissure system (i.e., AST) is strongly controlled by the stress field imparted by partitioning in the magma reservoir located ∼1.4-1.6 km below the seafloor (e.g., Carbotte et al., 2013;Fornari et al., 1998;Toomey et al., 2007;White, 2002). At the smallest scale, fourth-order discontinuities are defined by ∼50-500 m lateral offsets, or small changes in the width or trend of the AST (e.g., . ...
... This observation, in addition to plunging mantle lineations (Fig. 1b), led Nicolas et al. (1988) to interpret the area as a former mantle diapir feeding the former ridge axis with magma. However, while normal diapirs are normally located below the ridge axis, like the ones observed along the East Pacific Rise (Toomey et al., 2007) and most of the other mantle diapirs identified in Oman (Fig. 1a), the Batin diapir is an off-axis diapir located ca. 50 km from the ridge axis. ...
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Serpentinization of mantle peridotites has first order effects on the rheology and tectonic behavior of the oceanic lithosphere, on the global water cycle, and on the biosphere at mid-oceanic ridges. Investigating serpentinization of abyssal peridotites is limited by the scarce occurrences of peridotites at or close to the ocean floor at slow and ultra-slow ridge environments where peridotite is exposed by long-lived detachments. The processes controlling hydration of the upper mantle below a thick magmatic crust at fast spreading ridges are poorly constrained. Here we present results based on samples from cores drilled in peridotites from the Samail ophiolite obtained during the Oman Drilling Project. We describe an early generation of highly localized brittle faults ubiquitous through all the peridotite cores and investigate their relation to the main serpentinization event represented by mesh-textured serpentinites. We combine microstructural observations with mineral and bulk chemical analyses as well as oxygen isotope microanalyses obtained by secondary ion mass spectrometry (SIMS). Asymmetric wall rock damage, weakening of crystal preferred orientation (CPO) in small fault clasts, and intense fragmentation within the fault zones even in association with very small displacements suggest that the early stage faults represent seismic events and predate mesh formation. Hydration and mesh texture formation follows in the wake of this faulting. Serpentinization is associated with moderate enrichment of fluid mobile elements including B, Li, Rb and U, indicative of fluid rock interaction characterized by relatively low fluid/rock ratios. This is consistent with a scenario where serpentinization took place below a thick magmatic crust following an earthquake-induced permeability increase. The oxygen isotope compositions of mesh serpentine are consistent with off-axis serpentinization at temperatures in the range 200-250 • C
... In regions of upwelling (e.g. East Pacific Rise), V SV >V SH is observed (Toomey et al., 2007) and interpreted as vertical flow of melt, similarly to what we observe within the EAMS. Strong crustal radial anisotropy has been observed beneath large calderas in arc settings, continental hotspots (e.g. ...
Full-text available
Where and how melt is stored in the crust and uppermost mantle is important for understanding the dynamics of magmatic plumbing systems and the evolution of rifting. We determine shear velocity and radial anisotropy in the magmatically rifting northern East African Rift to determine the locus and orientation of melt, both on and off-rift. Love and Rayleigh fundamental modes are extracted from ambient noise data from 9-26 s period and then inverted for shear velocity. VSV is 0.15 ± 0.03 km/s lower than VSH from 5-30 km depth on average. VSH>VSV across most of the study region suggests the crust is inherently horizontally layered, with the largest anisotropy in the upper 5-15 km. Effective medium theory suggests thin compositional layering of felsic and mafic intrusions can account for anisotropy up to 4%. However, to reconcile the largest observed anisotropy (6.5%), and lowest velocities, we require 2-4% partial melt oriented in sills. Along the rift, horizontally aligned radial anisotropy gets weaker north-eastwards, suggesting sills become less dominant with progressive rifting. The Erta Ale magmatic segment is the only location where VSV>VSH, suggesting the crust contains vertical micro-cracks and dykes. Overall, the results suggest during early continental breakup when the rift is narrow, sill formation is the dominant storage mechanism. As a rift widens, vertical dyke intrusion becomes dominant and is likely controlled by variations in crustal thickness and stress state.
... The higher range might explain the appearance of a second trend in the segment length natural data at large spreading velocity ( fig.7D), unless this trend is again the result of a small number bias ( fig.2). On the other hand, seismic observations (Toomey et al. (2007); Vanderbeek et al. (2016)) suggest that reorientation of segments and formation of new ridge discontinuities could result from mantle flow oblique to the ridge axis. We do not observe any coherent, large-scale circulation in our experimental mantle. ...
Full-text available
Mid-ocean ridges (MOR) axes are not straight, but segmented over scales of 10s to 100s of kilometers by several types of offsets including transform faults (TF), overlapping spreading centers (OSC) and non-transform, non-overlapping offsets (NTNOO). Variations in axial morphology and segmentation have been attributed to changes in magma supply, axial thermal structure (which depends on mantle temperature and spreading rate), and axial mechanical properties. To isolate the effect of each of these processes is difficult with field data alone. We therefore present a series of analogue experiments using colloidal silica dispersions as an Earth analogue. Diffusion of salt from saline solutions placed in contact with these fluids, causes formation of a skin, whose rheology evolves from viscous to elastic and brittle with increasing salinity. Applying a fixed spreading rate to this pre-formed, brittle plate results in cracks, faults, and ridge segments. Lithospheric thickness is varied independently by changing the surface water layer salinity. Experimental results depend on the axial failure parameter ΠF, the ratio of a mechanical length scale (Zm) and the axial elastic thickness (Zaxis), which depends on mantle temperature and spreading velocity. Slow-spreading fault-dominated, and fast-spreading fluid intrusion-dominated, ridges on Earth and in the laboratory are separated by the same critical value ΠFc±0.024, suggesting that the axial failure mode governs ridge geometry. Here, we examine ridge axis segmentation. Measurements of >4000 experimental ridge segments and offsets yield an average segment length Lm that is quasi-constant at all spreading velocities. Scaled to the Earth, Lm∼55 km, in agreement with the natural data. Experiments with low ΠF show offset size varying as dl=csteLmZaxis regardless of offset type, a correlation well explained by fracture mechanics. Finally, as on Earth, experimental ridge segments are separated by transform and non-transform discontinuities, and their nature and occurrence vary with ΠF. NTNOOs develop when ΠF<ΠFc, while OSCs develop when ΠF>ΠFc. In contrast, TF may form at any ΠF, but the proportion of TFs relative to OSCs or NTNOOs decreases when ΠF/ΠFc>> 1 or << 1, in agreement with natural MOR.
... This could be explained by either by pressure gradient-driven flow or small-scale convection. In addition, many regional studies of oceanic lithosphere do not necessarily support simple fossil spreading orientations in the subcrustal lithosphere (Dunn et al., 2005;Eilon & Forsyth, 2020;Russell et al., 2019;Shintaku et al., 2014;Takeo et al., 2016Takeo et al., , 2018Toomey et al., 2007;VanderBeek & Toomey, 2017). If these results reflect frozen-in olivine alignment via dislocation, then it could instead reflect ancient APM (rather than fossil spreading) (Takeo et al., 2016), more complex near-ridge mantle flow than simple passive upwelling, and/or an alternate E-type fabric type Katayama et al., 2004;Russell et al., 2019). ...
Full-text available
Plate tectonic theory was developed 50 years ago and underpins most of our understanding of Earth’s evolution. The theory explains observations of seafloor spreading, linear volcanic island chains, large transform fault systems and deep earthquakes near deep sea trenches. These processes occur through a system of moving plates at the surface of the Earth, which are the surface expression of mantle convection. The plate consists of the chemically distinct crust and some amount of rigid mantle which move over a weaker mantle beneath. However, exactly where the transition between stronger and weaker mantle occurs and what determines and defines the plate are still debated. In the classic definition the plate is defined thermally, by the geotherm‐adiabat intersection, where the plate is the conductively cooling part of the mantle convection system. Many observations such as heat flow, seafloor bathymetry, seismic imaging, and magnetotelluric imaging (MT) are consistent with general lithospheric thickening with age which suggest temperature is an important factor in determining lithospheric thickness. However, while age averages give a good indication of overall properties, the range of lithospheric thicknesses reported is large for any given tectonic age interval, suggesting greater complexity. A number of observations including sharp discontinuities from teleseismic scattered waves and active source reflections and also strong anomalies from surface and body wave tomography and MT imaging cannot be explained by a purely thermal model. Another property or process is required to explain the anomalies and sharpen the boundary. Many subsolidus models have been proposed, although none can universally explain the variety of independent global observations. Alternatively, a small amount of partial melt can easily satisfy a range of observations. The presence of melt could also weaken the mantle over geologic timescales, and it would therefore define the lithosphere‐asthenosphere boundary (LAB). The location of melt is important to mantle dynamics and the LAB, although exactly where and exactly how much melt exists in the mantle are debated. Asthenospheric melt interpretations include a variety of forms: small or large melt triangles beneath spreading ridges, channels or multiple layers, along a permeability boundary leading to the ridge, at a depth of neutral buoyancy, punctuated, or pervasively over broad areas and either sharply or gradually falling off with depth. This variability in melt character or geometry may explain the previously described variability in LAB depths. The LAB is likely highly variable laterally as are the locations, forms, and amounts of melt, and the LAB is likely dynamic, dictated by small scale convection and the dynamics of melt generation and migration. A melt‐defined, dynamic LAB and a weak asthenosphere have broad implications for our understanding of Earth systems and planetary habitability. A weak asthenosphere caused by volatiles or melt could enable plate tectonic style convection, allow multiple scales of convection, and dictate the driving forces of the system. This has implications for mitigating many natural disasters caused by plate motions including volcanoes, earthquakes, and tsunamis. In addition, uplift and subsidence of the tectonic plates affects the sea level, impacting the level of the paleo‐oceans, and potentially affecting climate change estimates through geologic time. Finally, plate tectonics shapes the surface morphology of the planet, making continents that enable our existence on land and the ocean basins that hold our free‐surface water. Remarkably, despite large amounts of material transfer into and out of the mantle, and multiple scales of convection, plate tectonics has maintained a hydrosphere over billions of years that is favourable for life.<br/
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Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid-ocean ridge dynamics. Yet, interpreting anisotropy in the context of grain-scale deformation processes observed in laboratory and natural olivine samples has proven challenging due to the vast length scale differences. We bridge this observational gap by estimating an in situ elastic tensor for oceanic lithosphere using compressional- and shear-wavespeed anisotropy observations. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil-spreading direction consistent with corner-flow deformation fabric. This model is compared with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250–400% in the shallow mantle. We find evidence for D-type olivine lattice-preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes. D-type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during mid-ocean ridge spreading; we hypothesize that grain boundary sliding during dislocation creep relaxes strain compatibility, allowing D-type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation-accommodated grain-boundary sliding (disGBS) has implications for in situ stress and grain size during mid-ocean ridge spreading and implies grain-size dependent deformation, in contrast to pure dislocation creep.
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Recent multi‐channel seismic studies of fast spreading and hot‐spot influenced mid‐ocean ridges reveal magma bodies located beneath the mid‐crustal Axial Magma Lens (AML), embedded within the underlying crustal mush zone. We here present new seismic images from the Juan de Fuca Ridge that show reflections interpreted to be from vertically stacked magma lenses in a number of locations beneath this intermediate‐spreading ridge. The brightest reflections are beneath Northern Symmetric segment, from ∼46°42′‐52′N and Split Seamount, where a small magma body at local Moho depths is also detected, inferred to be a source reservoir for the stacked magma lenses in the crust above. The imaged magma bodies are sub‐horizontal, extend continuously for along‐axis lengths of ∼1–8 km, with the shallowest located at depths of ∼100–1,200 m below the AML, and are similar to sub‐AML bodies found at the East Pacific Rise. At both ridges, stacked sill‐like lenses are detected beneath only a small fraction of the ridge length examined and are inferred to mark local sites of higher melt flux and active replenishment from depth. The imaged magma lenses are focused in the upper part of the lower crust, which coincides with the most melt rich part of the crystal mush zone detected in other geophysical studies and where sub‐vertical fabrics are observed in geologic exposures of oceanic crust. We infer that the multi‐level magma accumulations are ephemeral and may result from porous flow and mush compaction, and that they can be tapped and drained during dike intrusion and eruption events.
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Combined analyses of volcanic features in DSL-120 sonar data and Argo I images along the ridge crest of the East Pacific Rise, 9°09′-54′N reveal a consistent decrease in inferred lava effusion rate toward the ends of third-order segments. The correlation of tectonic segmentation and volcanic style suggests that third-order segmentation corresponds to the volcanic segmentation of the ridge. Along-axis changes in volcanic structures (from collapse troughs to basaltic lava domes) and lava morphology (from sheet to pillow flows) coincide with the boundaries of morphologically defined third-order tectonic segments of the ridge crest visible in shipboard multibeam bathymetry. Pillow lava flows cover 25% of the surveyed area of the ridge crest and are closely associated with small lava domes that occur primarily at third-order segment ends. An additional 25% of the surveyed area of the ridge crest is covered by sheet lava flows found in close association with an axial collapse trough. The remaining terrain consists of lobate lava flows. We interpret the spatial correlations of morphologic, structural, seismic, and petrologic data as evidence that individual volcanic plumbing systems are organized at ∼20 km spacing along the ridge axis (third-order segment scale) in agreement with the hypothesis that volcanic and tectonic segmentations are correlated. For fast spreading ridges, we estimate that the longevity of volcanic segments is ∼104-105 years, 1-3 orders of magnitude longer than fourth-order segments (∼102-103 years). This implies the present pattern of hydrothermal activity may reorganize tens or hundreds of times while volcanic segmentation remains fairly stable.
Full-text available
Submarine lava flows are the building blocks of young oceanic crust. Lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava, the characteristics of the eruption, and the topography it encounters. The resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50-500 m of the oceanic crust. We have mapped and measured numerous submarine channelized lava flows at the East Pacific Rise (EPR) crest 9°-10°N that reflect the high-effusion-rate and high-flow-velocity end-member of lava eruption and transport at mid-ocean ridges. Channel systems composed of identifiable segments 50-1000 m in length extend up to 3 km from the axial summit trough (AST) and have widths of 10-50 m and depths of 2-3 m. Samples collected within the channels are N-MORB with Mg# indicating eruption from the AST. We produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins. Modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0.4 to 4.7 × 103 m3/s, and modeled shear rates help explain the surface morphology variation. We suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off-axis region (1-3 km) and produces the layer 2A thickening that is observed at fast and superfast spreading ridges. In addition, the rapid, high-volume-flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the EPR. We find that high concentrations of channelized lava flows correlate with local, across-axis ridge morphology indicative of an elevated magma budget. Additionally, in locations where channelized flows are located dominantly to the east or west of the AST, the ridge crest is asymmetric, and layer 2A appears to thicken over a greater distance from the AST toward the side of the ridge crest where the channels are located.
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The extent to which crustal processes along mid-ocean ridges are controlled by either the pattern of mantle upwelling or the mode of magma injection into the crust is not known. Models of mantle upwelling vary from two-dimensional, passive flow to three-dimensional, diapiric flow. Similarly, beneath a ridge segment bounded by tectonic offsets, crustal magma chambers may be replenished continuously along the ridge or at a central injection zone from which magma migrates towards the segment's ends. Here we present tomographic images that reveal the seismic structure and anisotropy of the uppermost mantle beneath the East Pacific Rise. The anisotropy is consistent with two-dimensional mantle flow diverging from the rise, whereas the anomalous isotropic structure requires a three-dimensional but continuous distribution of melt near the crust-mantle interface. Our results indicate that crustal magma chambers are replenished at closely spaced intervals along-axis and that crustal systems inherit characteristics of scale from melt transport processes originating in the mantle.
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Combined analyses of volcanic features in DSL-120 sonar data and Argo I images along the ridge crest of the East Pacific Rise, 9°09'-54'N reveal a consistent decrease in inferred lava effusion rate toward the ends of third-order segments. The correlation of tectonic segmentation and volcanic style suggests that third-order segmentation corresponds to the volcanic segmentation of the ridge. Along-axis changes in volcanic structures (from collapse troughs to basaltic lava domes) and lava morphology (from sheet to pillow flows) coincide with the boundaries of morphologically defined third-order tectonic segments of the ridge crest visible in shipboard multibeam bathymetry. Pillow lava flows cover 25% of the surveyed area of the ridge crest and are closely associated with small lava domes that occur primarily at third-order segment ends. An additional 25% of the surveyed area of the ridge crest is covered by sheet lava flows found in close association with an axial collapse trough. The remaining terrain consists of lobate lava flows. We interpret the spatial correlations of morphologic, structural, seismic, and petrologic data as evidence that individual volcanic plumbing systems are organized at ~20 km spacing along the ridge axis (third-order segment scale) in agreement with the hypothesis that volcanic and tectonic segmentations are correlated. For fast spreading ridges, we estimate that the longevity of volcanic segments is ~104-105 years, 1-3 orders of magnitude longer than fourth-order segments (~102-103 years). This implies the present pattern of hydrothermal activity may reorganize tens or hundreds of times while volcanic segmentation remains fairly stable.
We analyze four expanded spread profiles acquired at distances of 0, 2.1, 3.1, and 10 km (0-0.2 m.y.) from the axis of the East Pacific Rise between 9° and 10°N. At the seafloor we find very low VP and VS/VP values around 2.2 km/s and ≤ 0.43. In the topmost 100-200 m of the crust, VP remains low (≤ 2.5 km/s) then rapidly increases to 5 km/s at ~ 500 m below the seafloor. High attenuation values (QP < 100) are suggested in the topmost ~ 500 m of the crust. The layer 2-3 transition probably occurs within the dike unit, a few hundred meters above the dike-gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The on-axis profile shows arrivals that correspond to the bright AMC event seen in reflection lines within 2 km of the rise axis. The top of the AMC lies 1.6 km below the seafloor and consists of molten material where VP ≃ 3 km/s and VS = 0. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. -from Authors
Along the fast and ultrafast spreading East Pacific Rise, the cross-sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation. The cross-sectional area variation mimics the undulation of the ridge crest depth; local area maxima occur toward the middle of segments, and the axial area decreases by 40% or more at first- and second-order discontinuities. The correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross-sectional area represents a longer term average of the ridge's magmatic state than axial depth. A correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low-density crust and mantle. Also, a correlation between larger area and higher MgO content of axial basalts suggests that inflated areas generally erupt hotter magmas. The cross-sectional area of the axial ridge appears to correlate with the width of the axis-centered low-velocity zone in the crust. -from Authors
Plate motions relative to the hotspots over the past 4 to 7 Myr are investigated with a goal of determining the shortest time interval over which reliable volcanic propagation rates and segment trends can be estimated. The rate and trend uncertainties are objectively determined from the dispersion of volcano age and of volcano location and are used to test the mutual consistency of the trends and rates. Ten hotspot data sets are constructed from overlapping time intervals with various durations and starting times. Our preferred hotspot data set, HS3, consists of two volcanic propagation rates and eleven segment trends from four plates. It averages plate motion over the past ≈5.8 Myr, which is almost twice the length of time (3.2 Myr) over which the NUVEL-1A global set of relative plate angular velocities is estimated. HS3-NUVEL1A, our preferred set of angular velocities of 15 plates relative to the hotspots, was constructed from the HS3 data set while constraining the relative plate angular velocities to consistency with NUVEL-1A. No hotspots are in significant relative motion, but the 95 per cent confidence limit on motion is typically ±20 to ±40 km Myr−1 and ranges up to ±145 km Myr−1. The uncertainties of the new angular velocities of plates relative to the hotspots are smaller than those of previously published HS2-NUVEL1 (Gripp & Gordon 1990), while being averaged over a shorter and much more uniform time interval. Nine of the fourteen HS2-NUVEL1 angular velocities lie outside the 95 per cent confidence region of the corresponding HS3-NUVEL1A angular velocity, while all fourteen of the HS3-NUVEL1A angular velocities lie inside the 95 per cent confidence region of the corresponding HS2-NUVEL1 angular velocity. The HS2-NUVEL1 Pacific Plate angular velocity lies inside the 95 per cent confidence region of the HS3-NUVEL1A Pacific Plate angular velocity, but the 0 to 3 Ma Pacific Plate angular velocity of Wessel & Kroenke (1997) lies far outside the confidence region. We show that the change in trend of the Hawaiian hotspot over the past 2 to 3 Myr has no counterpart on other chains and therefore provides no basis for inferring a change in Pacific Plate motion relative to global hotspots. The current angular velocity of the Pacific Plate can be shown to differ from the average over the past 47 Myr in rate but not in orientation, with the current rotation being about 50 per cent faster (1.06 ± 0.10 deg Myr−1) than the average (0.70 deg Myr−1) since the 47-Myr-old bend in the Hawaiian–Emperor chain.
In February 2004 we carried out a combined broadband magnetotelluric (MT) and controlled source electromagnetic (CSEM) study of the mid-ocean ridge in the Pacific Ocean at 9-10° North latitude. This paper presents results from MT data collected at 9°30' on the East Pacific Rise (EPR). A 200~km aperture array of 40 sites deployed perpendicular to the ridge axis recorded data for 5-14~days duration. MT impedance responses were obtained in the period band of 20 to 4000~s, which is a higher frequency band than measured by previous ridge MT experiments and thus allows for stronger constraints on the structure of the shallow mantle. Impedance skew estimates, an indicator of 3D conductivity structure, are generally low for sites located west of the ridge axis, while skews as high as 0.7 are observed for sites east of the ridge. Two dimensional inversion of the MT data produces a model with a broad region of conductive mantle that is beneath the ridge axis. The highest conductivities are around 1 ohm-m and are observed 20-40~km east of the ridge axis and at 30-60~km depth. The conductivities constrain most of the mantle melt supply to only a few percent (1-6%) in a region about 150-200~km wide and about 40-60~km thick. However, the anomalously high conductivity region in the east lies atop the broader melting region and requires a nearly fully molten mantle and implies either a locally zone of dynamic upwelling, or else ponding of melt at a permeability barrier. While our modeling has concentrated on the mantle conductivity, the high conductivity region in the east may be related to recent seismic tomographic studies indicating the presence of off-axis crustal melt east of the ridge axis. We can explain our model with mantle melting of dry olivine if we assume a mantle temperature of 1450°C; if we assume a temperature of 1350°C then our model requires mantle hydration to get melting as deep as 80~km. We have generated a thermal model for the mantle by mapping conductivity to temperature, which provides a lithospheric cooling profile for the first 2 Ma of plate formation.