Rheological control on the segmentation of the mid-ocean ridges: Laboratory experiments with extension initially perpendicular to the axis - Université de Bretagne Occidentale Access content directly
Journal Articles Earth and Planetary Science Letters Year : 2021

Rheological control on the segmentation of the mid-ocean ridges: Laboratory experiments with extension initially perpendicular to the axis

Abstract

Mid-ocean ridges (MOR) axes are not straight, but segmented over scales of 10s to 100s of kilometers by several types of offsets including transform faults (TF), overlapping spreading centers (OSC) and nontransform, non-overlapping offsets (NTNOO). Variations in axial morphology and segmentation have been attributed to changes in magma supply, axial thermal structure (which depends on mantle temperature and spreading rate), and axial mechanical properties. To isolate the effect of each of these processes is difficult with field data alone. We therefore present a series of analogue experiments using colloidal silica dispersions as an Earth analogue. Diffusion of salt from saline solutions placed in contact with these fluids, cause formation of a skin, whose rheology evolves from viscous to elastic and brittle with increasing salinity. Applying a fixed spreading rate to this pre-formed, brittle plate results in cracks, faults, and ridge segments. Lithospheric thickness is varied independently by changing the surface water layer salinity. Experimental results depend on the axial failure parameter Π F , the ratio of a mechanical length scale (Z m) and the axial elastic thickness (Z axis), which depends on mantle temperature and spreading velocity. Slow-spreading faultdominated, and fast-spreading fluid intrusion-dominated, ridges on Earth and in the laboratory are separated by the same critical value Π F c ±0.024, suggesting that the axial failure mode governs ridge geometry. Here, we examine ridge axis segmentation. Measurements of >4000 experimental ridge segments and offsets yield an average segment length L m that is quasi-constant at all spreading velocities. Scaled to the Earth, L m ∼55 km, in agreement with the natural data. Experiments with low Π F show offset size varying as d l = cste √ L m Z axis regardless of offset type, a correlation well explained by fracture mechanics. Finally, as on Earth, experimental ridge segments are separated by transform and non-transform discontinuities, and their nature and occurrence vary with Π F. NTNOOs develop when Π F < Π F c , while OSCs develop when Π F > Π F c. In contrast, TF may form at any Π F , but the proportion of TFs relative to OSCs or NTNOOs decreases when Π F /Π F c >> 1 or << 1, in agreement with natural MOR.
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Dates and versions

hal-03453730 , version 1 (28-11-2021)

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A.L.R. L R Sibrant, A. Davaille, E. Mittelstaedt. Rheological control on the segmentation of the mid-ocean ridges: Laboratory experiments with extension initially perpendicular to the axis. Earth and Planetary Science Letters, 2021, 557, pp.116706. ⟨10.1016/j.epsl.2020.116706⟩. ⟨hal-03453730⟩
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