Joyjeet Sen, Shamik Sarkar, Nibir Mandal

Mid-ocean ridge (MOR) systems evolve in strongly heterogeneous stress environments. However, the origin of such stress complexity still awaits a comprehensive explanation, which is the central theme of the present article. This study develops a thermo-mechanical model to demonstrate the multi-ordered 3D convective circulations, produced by decompression melting in the mushy region, as the key factor to modulate the MOR dynamics. The model mechanically couples the sub-ridge mushy regions with the elastic crustal layer within a mathematical framework of fluid-structure interaction (FSI) mechanics. FSI model simulations show that the heterogeneous stress field of a MOR forms characteristic segmented patterns on a time scale of million years, resembling axial as well as off-axis topographic morphologies observed in MORs. This article provides a model calculated estimate of the total stress tensor, focusing on the following components: across- and along-axis horizontal tension / compression (σ⟂ and σ∥) and across-axis horizontal shear stress (σ#) that dominantly control the ridge-axis morphologies. The stress mapping reveals a distinct 30 km wide axial zone of tensile σ⟂ localization (median < 250 MPa), whereas compressive σ⟂ localization (median < 100 MPa) in off-axis ridge-parallel linear belts on either side of the MOR axis. This finding leads to an alternative explanation for the off-axis ridge-parallel second-order hill topography, located at a distance of 20 km to 50 km, as a consequence of compressional σ⟂ localization. Along-axis compressional σ∥ concentrates in a row of ridge-normal narrow, 10 to 30 km wide stripes, giving rise to segmentation of the stress field on a wavelength of 40-150 km, which conforms to the second-order magmatic segmentation patterns of MORs. From σ# mapping, it is also shown that ridge-transverse discontinuities, including transform offsets and transpression zones originate spontaneously from the FSI interactions during the MOR evolution.