t13b …t13b-1866canridgeaxialdeformationsconstrainmantleviscosity?t13b-1866 eunseo choi and w....

T13B-1866 Canridgeaxialdeformationsconstrainmantleviscosity? T13B-1866Eunseo Choi∗ and W. Roger Buckechoi@ldeo.columbia.edu

Lamont-Doherty Earth Observatory, P.O. Box 1000, 61 Rt. 9W, Palisades, NY 10964, USA

IntroductionCan ridge axial deformation and morphology providea constraint on the upper mantle viscosity?

• If dike opening accommodates the entire spread-ing motion, an axial high as well as little tec-tonic (i.e. distributed) deformations would be ex-pected.

• Otherwise, a rift valley would be created withmore distributed tectonic deformations.

• Mantle viscosity is likely to have a strong influ-ence on the behaviors of dikes since it determinesthe stress state of mantle.

• Therefore, the distribution of tectonic deformationzone as well as the axial topography might be ableto provide some constraint on mantle viscosity.

• This idea can also be used to test the idea ofhighly viscous upper mantle due to partial melt-ing and dehydration (e.g., Hirth and Kohlstedt(1996)

• Focusing on the fast-spreading (full rate of 10cm/yr) environment, we simulate these two ob-servables in an internally consistent model andinvestigate if they can provide constraints onmantle viscosity.

Model setup

600 0 C isotherm

Mantle

Gabbro

DikeDike

LavaLava

DikingFaulting

Mush Zone

(adapted from Qin (2008))

(Qin and Buck, 2008)

Model setup (cont’d)

(Qin, 2008)

• dV/dP = −0.06(m2/Pa). • Dike interval: 10 yrs • Magma head (hmt0) is 400 m.

Results

• Axial highs formed and maintained for low man-tle viscosity (≤ 1019 Pa·s).

• Axial highs formed but not stable for high mantleviscosity (≥ 1020 Pa·s).

• Dikes can propagate deeper when mantle viscos-ity is high: 9-10 km for low viscoity; 15-18 km for

high viscosity.

• Shallow dikes can better accommodate spreadingfor a given magma productivity while larger vol-umes of deeper dikes make it more difficult.

• Distributed plastic deformation observed whenmantle viscosity is high.

Conclusions• Mantle viscosity has strong influences on the axial topography as well as the distribution of tectonic defor-

mation.

• Weak mantle: Shallow dikes fully accommodate plate spreading. Axial highs are stable. Only near-axisregion deforms significantly.

• Strong mantle: Dikes propogate deeper and tend to run out of magma to hold up sufficient opening. Axialtopography is not stable. Plastic deformation is distributed over the entire lithosphere.

ReferencesHirth, G., Kohlstedt, D.L. (1996), Water in the oceanic upper mantle: implica-

tion for rheology, melt extraction and the evolution of the lithosphere, EarthPlanet. Sci. Lett. 144, 93-108.

Qin, R. (2008), Mid-ocean ridge morphology and tectonics: Insights from nu-

mercal modeling of faults and dikes, Ph.D. thesis, Columbia University.Qin, R., and W. R. Buck (2008), Why meter-wide dikes at oceanic

spreading centers?, Earth Planet. Sci. Lett., 265(3-4), 466–474, doi:10.1016/j.epsl.2007.10.044.

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