Mantle defomation during slow seafloor spreading constrained bv observations of seismic anisotropy in the western Atlantic

LDEO Publication: 
Publication Type  Journal Article
Year of Publication  2004
Authors  Gaherty, J. B.; Lizarralde, D.; Collins, J. A.; Hirth, G.; Kim, S.
Journal Title  Earth and Planetary Science Letters
Volume  228
Issue  3-4
Pages  255-265
Journal Date  Dec 15
ISBN Number  0012-821X
Accession Number  ISI:000225936100004
LDEO Publication Number  6689
Key Words  seismic anisotropy; upper mantle; refraction; mantle flow; east pacific rise; oceanic lithosphere; midocean ridges; refraction experiment; uppermost mantle; strain; beneath; dependence; olivine; crust

During mantle flow at mid-ocean ridges, viscous shear deformation imparts a structural fabric that remains in the lithos here as it cools, preserving a record of ridge processes over time. This fabric can be detected by seismic imaging, as it produces an azimuthal anisotropy in the velocity of seismic waves that propagate in the shallow mantle. Using data from a novel refraction survey in the western Atlantic, we found that the maximum P-wave velocity in the upper 10 km of mantle lithosphere formed at the slow-spreading Mid-Atlantic Ridge is parallel to the paleo-spreading direction, consistent with viscous shear deformation dominated by corner flow. The magnitude of the P-wave anisotropy is 3.4+/-0.3%, approximately one-half that found in lithosphere formed at faster spreading rates in the Pacific. Weaker anisotropy in the Atlantic suggests that more pervasive conductive cooling at slow spreading ridges increases the proportion of localized (brittle) deformation in the mantle lithosphere, thereby limiting the degree of viscous deformation. By scaling our field observations to laboratory experiments, we estimate that total viscous strain during slow seafloor spreading is of order 0.5. as opposed to similar to1-2 for the fast-spreading case. Finally, our travel-time observations display ail azimuthal asymmetry that can be interpreted in one of two ways: either the average elastic tensor in this region is not orthorhombic as is commonly assumed, or the underlying shear fabric is rotated similar to15degrees relative to fossil spreading, (C) 2004 Elsevier B.V. All rights reserved.


882JVTimes Cited:3Cited References Count:40

URL  <Go to ISI>://000225936100004
DOI  DOI 10.1016/j.epsl.2004.10.026