spreading ridges is increasingly controlled by the uprising
asthenosphere. This is visible in their morphology and
their large-scale tectonics. There is some evidence from
ophiolites (Nicolas et al., 1999) and slow spreading ridges,
based on seismic anisotropy (Li and Detrick, 2003;
Nowacki et al., 2012), suggesting that mantle flow is ridge
parallel and not more ridge perpendicular. As discussed
above, ultraslow ridge morphology and crustal composition respond to variations in mantle upwelling and associated melt delivery.
The combined study of ophiolites and geophysical data
at oceanic ridges has allowed to comprehend the inner
working of ocean spreading processes. The ophiolites
allowed us to image deep accreted lithosphere and, indirectly, to investigate accretion processes, whereas the geophysics provide direct images of the accreting ridge
systems. These observations showed that these accreting
systems are different depending firstly on spreading rates
and sometimes displaying significant variations along
the same segment of a ridge axis. For example, along the
most studied 9
N segment of the EPR, the width of the
AML changes significantly and so does the melt-mush
distribution along the ridge axis. Along slow spreading
ridges, the crust is normally thickest at the center of a segment and thinnest at segment ends. The crustal accretion
process at slow spreading centers is further complicated
by the presence of ocean core complexes, which are not
just present at segment ends, but could be present along
a significant part of a segment (Escartin et al., 2008).
The ultraslow spreading ridges, a special class of ridges,
show extreme variations in seafloor morphology, crustal
thickness, and geochemistry; more geophysical studies
of ultraslow spreading ridges are required to fully appreciate the diversity of ridge processes on earth.
Acknowledgments
We would like to thank F. Boudier and M. Cannat for fruitful discussions and M.R. Perfit for a very comprehensive
and thoughtful review.
Bibliography
Boudier, F., and Nicolas, A., 2011. Discovery of oceanic ridge axial
melt lenses in the Oman ophiolite. Earth and Planetary Science
Letters, 304, 313–325.
Canales, J. P., Detrick, R. S., Lin, J., and Collines, J. A., 2000.
Crustal and upper mantle seismic structure beneath the rift
mountains and across nontransform offset at the Mid-Atlantic
Ridge (35
N). Journal of Geophysical Research, 105,
2699–2719.
Canales, J. P., et al., 2011. Network of off-axis melt bodies at the
East Pacific Rise. Nature Geoscience, 5, 279–283.
Cannat, M., 1996. How thick is the magmatic crust at slow spreading oceanic ridges? Journal of Geophysical Research, 101,
2847–2857.
Cannat, M., Sauter, D., Bezos, A., Meyzen, C., Humler, E., and Le
Rigoleur, M., 2008. Spreading rate, spreading obliquity, and
melt supply at ultraslow spreading Southwest Indian Ridge.
Geochemistry, Geophysics, Geosystems, 9, Q04002.
Cathles, L. M., 1993. A capless 350
C flow zone model to explain
megaplumes, salinity variations and high-temperature veins in ridge
axis hydrothermal systems. Economic Geology, 88, 1977–1988.
Collier, J., and Singh, S. C., 1998. Poisson’s ratio in young oceanic
crust from two-ship wide-angle data using waveform inversion.
Journal of Geophysical Research, 103, 20981–20996.
Collier, J., and Sinha, M. C., 1990. Seismic images of a magma
chamber beneath the Lau Basin back-arc spreading centre.
Nature, 346, 646–648.
Crawford, W. C., and Webb, S. C., 2002. Variations in the distribution of magma in the lower crust and at the Moho beneath the
East Pacific Rise at 9
-10
N. Earth and Planetary Science Letters, 203, 117–130.
Crawford, W. C., Singh, S. C., Seher, T., Combier, V., Dusunur, D.,
and Cannat, M., 2010. Crustal structure, magma chamber faults
beneath the Lucky Strike hydrothermal fields. AGU Geophysical
Monograph Series, 188, 440.
DeMartin, B. J., Sohn, R. A., Canales, J. P., and Humphris, S. E.,
2007. Kinematics and geometry of active detachment faulting
beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal
field on the Mid-Atlantic Ridge. Geology, 35(8), 711–714,
doi:10.1130/G23718A.1.
Detrick, R. S., Buhl, P., Vera, E., Mutter, J., Orcutt, J., Madsen, J.,
and Brocher, T., 1987. Multi-channel seismic imaging of a
crustal magma chamber along the East Pacific Rise. Nature,
326, 35–41.
Detrick, R. S., White, R. S., and Purdy, G. M., 1993. Crustal structure of North Atlantic fracture zone. Reviews of Geophysics, 31,
439–458.
Dick, H., Lin, J., and Schouten, H., 2003. An ultraslow spreading
class of ocean ridge. Nature, 426, 405–412.
Dunn, R. A., and Toomey, D. R., 1997. Seismological evidence for
three-dimensional melt migration beneath the East Pacific Rise.
Nature, 388, 259–262.
Dunn, R. A., Toomey, D. R., and Solomon, S. C., 2000.
Three-dimensional seismic structure and physical properties
of the crust and shallow mantle beneath the East Pacific
Rise 9
30
0 N. Journal of Geophysical Research, 105,
23537–23555.
Escartin, J., et al., 2008. Central role of detachment faults in accretion of slow-spreading oceanic lithosphere. Nature, 455,
790–794.
Fontaine, F. J., and Wilcock, W. S. D., 2007. Two-dimensional
models of hydrothermal convection at high Rayleigh and
Nusselt numbers: implications for mid-ocean ridges. Geochemistry, Geophysics, Geosystems, 8, Q07010, doi:10.1029/
2007GC001601.
Hansen, L. N., et al., 2013. Mylonitic deformation at the Kane oceanic core complex: implication for the rheological behavior of
oceanic detachment faults. Geochemistry, Geophysics,
Geosystems, 14(8), doi:10.1002/ggge.20184.
Harding, A. J., et al., 1989. Structure of young oceanic crust at 13 N
on the East Pacific Rise from expanding spread profiles.
Journal of Geophysical Research, 94, 12163–12196.
Harding, A. J., Kent, G. M., and Orcutt, J. A., 1993. A multichannel seismic investigation of upper crustal structure at 9
N at the East Pacific Rise: implications of crustal accretion. Journal of Geophysical Research, 98, 13925–13944.
Henig, J. M., Blackman, D. K., Harding, A. J., Canales, J. P., and
Kent, G. M., 2012. Downward continued multichannel seismic
refraction analysis of Atlantis Massif oceanic core complex,
30
N, Mid-Atlantic Ridge. Geochemistry, Geophysics,
Geosystems, 13(1), doi:10:1029/2012GC004059
OCEANIC SPREADING CENTERS
585
asthenosphere. This is visible in their morphology and
their large-scale tectonics. There is some evidence from
ophiolites (Nicolas et al., 1999) and slow spreading ridges,
based on seismic anisotropy (Li and Detrick, 2003;
Nowacki et al., 2012), suggesting that mantle flow is ridge
parallel and not more ridge perpendicular. As discussed
above, ultraslow ridge morphology and crustal composition respond to variations in mantle upwelling and associated melt delivery.
The combined study of ophiolites and geophysical data
at oceanic ridges has allowed to comprehend the inner
working of ocean spreading processes. The ophiolites
allowed us to image deep accreted lithosphere and, indirectly, to investigate accretion processes, whereas the geophysics provide direct images of the accreting ridge
systems. These observations showed that these accreting
systems are different depending firstly on spreading rates
and sometimes displaying significant variations along
the same segment of a ridge axis. For example, along the
most studied 9
N segment of the EPR, the width of the
AML changes significantly and so does the melt-mush
distribution along the ridge axis. Along slow spreading
ridges, the crust is normally thickest at the center of a segment and thinnest at segment ends. The crustal accretion
process at slow spreading centers is further complicated
by the presence of ocean core complexes, which are not
just present at segment ends, but could be present along
a significant part of a segment (Escartin et al., 2008).
The ultraslow spreading ridges, a special class of ridges,
show extreme variations in seafloor morphology, crustal
thickness, and geochemistry; more geophysical studies
of ultraslow spreading ridges are required to fully appreciate the diversity of ridge processes on earth.
Acknowledgments
We would like to thank F. Boudier and M. Cannat for fruitful discussions and M.R. Perfit for a very comprehensive
and thoughtful review.
Bibliography
Boudier, F., and Nicolas, A., 2011. Discovery of oceanic ridge axial
melt lenses in the Oman ophiolite. Earth and Planetary Science
Letters, 304, 313–325.
Canales, J. P., Detrick, R. S., Lin, J., and Collines, J. A., 2000.
Crustal and upper mantle seismic structure beneath the rift
mountains and across nontransform offset at the Mid-Atlantic
Ridge (35
N). Journal of Geophysical Research, 105,
2699–2719.
Canales, J. P., et al., 2011. Network of off-axis melt bodies at the
East Pacific Rise. Nature Geoscience, 5, 279–283.
Cannat, M., 1996. How thick is the magmatic crust at slow spreading oceanic ridges? Journal of Geophysical Research, 101,
2847–2857.
Cannat, M., Sauter, D., Bezos, A., Meyzen, C., Humler, E., and Le
Rigoleur, M., 2008. Spreading rate, spreading obliquity, and
melt supply at ultraslow spreading Southwest Indian Ridge.
Geochemistry, Geophysics, Geosystems, 9, Q04002.
Cathles, L. M., 1993. A capless 350
C flow zone model to explain
megaplumes, salinity variations and high-temperature veins in ridge
axis hydrothermal systems. Economic Geology, 88, 1977–1988.
Collier, J., and Singh, S. C., 1998. Poisson’s ratio in young oceanic
crust from two-ship wide-angle data using waveform inversion.
Journal of Geophysical Research, 103, 20981–20996.
Collier, J., and Sinha, M. C., 1990. Seismic images of a magma
chamber beneath the Lau Basin back-arc spreading centre.
Nature, 346, 646–648.
Crawford, W. C., and Webb, S. C., 2002. Variations in the distribution of magma in the lower crust and at the Moho beneath the
East Pacific Rise at 9
-10
N. Earth and Planetary Science Letters, 203, 117–130.
Crawford, W. C., Singh, S. C., Seher, T., Combier, V., Dusunur, D.,
and Cannat, M., 2010. Crustal structure, magma chamber faults
beneath the Lucky Strike hydrothermal fields. AGU Geophysical
Monograph Series, 188, 440.
DeMartin, B. J., Sohn, R. A., Canales, J. P., and Humphris, S. E.,
2007. Kinematics and geometry of active detachment faulting
beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal
field on the Mid-Atlantic Ridge. Geology, 35(8), 711–714,
doi:10.1130/G23718A.1.
Detrick, R. S., Buhl, P., Vera, E., Mutter, J., Orcutt, J., Madsen, J.,
and Brocher, T., 1987. Multi-channel seismic imaging of a
crustal magma chamber along the East Pacific Rise. Nature,
326, 35–41.
Detrick, R. S., White, R. S., and Purdy, G. M., 1993. Crustal structure of North Atlantic fracture zone. Reviews of Geophysics, 31,
439–458.
Dick, H., Lin, J., and Schouten, H., 2003. An ultraslow spreading
class of ocean ridge. Nature, 426, 405–412.
Dunn, R. A., and Toomey, D. R., 1997. Seismological evidence for
three-dimensional melt migration beneath the East Pacific Rise.
Nature, 388, 259–262.
Dunn, R. A., Toomey, D. R., and Solomon, S. C., 2000.
Three-dimensional seismic structure and physical properties
of the crust and shallow mantle beneath the East Pacific
Rise 9
30
0 N. Journal of Geophysical Research, 105,
23537–23555.
Escartin, J., et al., 2008. Central role of detachment faults in accretion of slow-spreading oceanic lithosphere. Nature, 455,
790–794.
Fontaine, F. J., and Wilcock, W. S. D., 2007. Two-dimensional
models of hydrothermal convection at high Rayleigh and
Nusselt numbers: implications for mid-ocean ridges. Geochemistry, Geophysics, Geosystems, 8, Q07010, doi:10.1029/
2007GC001601.
Hansen, L. N., et al., 2013. Mylonitic deformation at the Kane oceanic core complex: implication for the rheological behavior of
oceanic detachment faults. Geochemistry, Geophysics,
Geosystems, 14(8), doi:10.1002/ggge.20184.
Harding, A. J., et al., 1989. Structure of young oceanic crust at 13 N
on the East Pacific Rise from expanding spread profiles.
Journal of Geophysical Research, 94, 12163–12196.
Harding, A. J., Kent, G. M., and Orcutt, J. A., 1993. A multichannel seismic investigation of upper crustal structure at 9
N at the East Pacific Rise: implications of crustal accretion. Journal of Geophysical Research, 98, 13925–13944.
Henig, J. M., Blackman, D. K., Harding, A. J., Canales, J. P., and
Kent, G. M., 2012. Downward continued multichannel seismic
refraction analysis of Atlantis Massif oceanic core complex,
30
N, Mid-Atlantic Ridge. Geochemistry, Geophysics,
Geosystems, 13(1), doi:10:1029/2012GC004059
OCEANIC SPREADING CENTERS
585