Mathematics – Logic
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.125..221j&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 125, no. 1-4, p. 221-234
Mathematics
Logic
43
Earth Mantle, Geodynamics, Geological Faults, Mathematical Models, Solids Flow, Tectonics, Terradynamics, Viscosity, Viscous Flow, Flow Distribution, Three Dimensional Models, Two Dimensional Models
Scientific paper
Numerical experiments are used to examine the structure of mantle flow beneath the axis of a spreading center. Buoyancy results from the depletion of residual mantle in iron relative to magnesium as melt is extracted (mantle-depletion buoyancy) and from the presence of low-density melt (melt-retention buoyancy). 3-D buoyant mantle flow arises spontaneously from an initially 2-D solution for low mantle viscosity and low spreading rate. At high viscosity and high spreading rate, initially 2-D solutions remain 2-D. This may explain the fundamentally different structure of fast and slow spreading centers. In a uniform viscosity halfspace, the along-axis wavelength of 3-D buoyant upwelling scales with the maximum depth of melting, the only length scale in this system. For reasonable maximum depths of melting (60-90 km), along-axis wavelengths of 200-300 km are preferred, longer than the 50-100 km segmentation length of slow spreading centers. In a viscosity-layered halfspace, the thickness of the asthenopshere introduces another length scale. A wavelength of segmentation comparable to the asthenosphere thickness also develops in our numerical experiments. This suggests the possibility that a wavelength corresponding to the spacing between gravity lows may be controlled by the asthenosphere thickness, while the spacing of major fracture zones corresponds to the longer wavelength (approximately equals 3 times the maximum depth of melting) intrinsic to the melting region. The along-axis structure of 3-D flow varies from narrow, focused upwellings, at low spreading rates and mantle viscosities, to broad regions of upwelling at high spreading rates and mantle viscosity. To allow an along-axis crustal thickness variation no larger than that which is observed (approximately equals 3-4 km), the highly focused upwelling and crustal production predicted at slow spreading rates requires appreciable along-axis transport of melt.
Jha Kopal
Parmentier Marc E.
Phipps Morgan Jason
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