Tecto-Magmatic Cycles, Faulting, and Morphology of Mid-Ocean Ridges
Abstract
Numerical models of brittle-ductile deformation of the crust and mantle are used to study the dynamical causes of the range of axial morphology observed at mid-ocean ridges including faulted rift valleys, relatively unfaulted axial highs, and transitional morphologies with highly faulted topography but no distinct axial valley or high. Tecto- magmatic cycles are simulated by varying the rheology and stress in a 2-km-wide magma accretion zone centered between two diverging lithospheric plates. During tectonic episodes, the whole region near the ridge axis deforms elasto-plastically (i.e., Mohr-Coulomb yielding) so that faults form and grow, and the ridge axis deepens below the level of isostatic equilibrium. During magmatic times, the center of the magmatic zone is allowed to open freely (i.e., stress is lithostatic) to simulate dike injections. Also, the magmatic zone deforms viscously with viscosity not exceeding an imposed maximum, ηM. This viscous deformation causes the axis to rise toward the level of isostatic equilibrium thus simulating topographic accretion by magmatism with a timescale τ.
Varying the fraction, FM, of the magmatic time period relative to the duration of the whole tecto- magmatic (magmatic+amagmatic) cycle primarily affects fault characteristics, with faults spacing and heave decreasing with increasing FM. The major factor controlling whether an axial valley or axial high forms is the timescale τ of axial topographic accretion by magmatism. The full range of morphologies from deep (>1 km) axial valleys, to faulted flat topography, to faulted highs, to smooth highs is predicted, simply by varying τ from much less than, to ~5 times greater than the length of magmatic periods. FM and lithospheric thickness effect τ but the dominant effect is the limiting viscosity ηM. Thus, in contrast to previous studies, which have argued that lithospheric thickness controls faulting and axial morphology, we show that it is the timescale of topographic growth by magmatism that is dominant factor. It is differences in the mechanics of magma storage and transport in the lithosphere that cause the wide range of morphologies at mid-ocean ridge as well as the abyssal hill fabric on the seafloor created.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.T22E..07I
- Keywords:
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- 3035 Midocean ridge processes;
- 3045 Seafloor morphology;
- geology;
- and geophysics;
- 8118 Dynamics and mechanics of faulting (8004);
- 8120 Dynamics of lithosphere and mantle: general (1213);
- 8145 Physics of magma and magma bodies