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.
This article exploits the interaction dynamics of the elastic oceanic crust with the underlying mush complexes (MC) to constrain the axial topography of mid-ocean ridges (MORs). The effective viscosity ($μ_{eff}$) of MC beneath MORs is recognized as the crucial factor in modulating their axial high versus flat topography. Based on a two-step viscosity calculation (suspension and solid-melt mixture rheology), we provide a theoretical estimate of $μ_{eff}$ as a function of melt suspension characteristics (crystal content, polymodality, polydispersity and strain-rate), and its volume fraction in the MC region. We then develop a numerical model to show the control of $μ_{eff}$ on the axial topography. Using an enthalpy-porosity-based fluid-formulation of uppermost mantle the model implements a one-way fluid-structure interaction (FSI) that transmits viscous forces of the MC region to the overlying upper crust. The limiting non-rifted topographic elevations (-0.06 km to 1.27 km) of model MORs are found to occur in the viscosity range: $μ_{eff}$ = $10^{12}$ to $10^{14}$ Pa s. The higher-end ($10^{13}$ to $10^{14}$) Pa s of this spectrum produce axial highs, which are replaced by flat or slightly negative topography as $μ_{eff} \leq 5\times 10^{12}$ Pa s. We discuss a number of major natural MORs to validate the model findings.
This article exploits the interaction dynamics of the elastic oceanic crust with the underlying mush complexes (MC) to constrain the axial topography of mid-ocean ridges (MORs). The effective viscosity (μeff) of MC beneath MORs is recognized as the crucial factor in modulating their axial high vs flat topography. Based on a two-step viscosity calculation (suspension and solid-melt mixture rheology), we provide a theoretical estimate of μeff as a function of melt suspension characteristics (crystal content, polymodality, polydispersity, and strain rate) and its volume fraction in the MC region. We then develop a numerical model to show the control of μeff on the axial topography. Using an enthalpy-porosity-based fluid formulation of uppermost mantle, the model implements a one-way fluid–structure interaction that transmits viscous forces of the MC region to the overlying upper crust. The limiting non-rifted topographic elevations (−0.06–1.27 km) of model MORs are found to occur in the viscosity range of μeff = 1012–1014 Pa s. The higher end (1013–1014) Pa s of this spectrum produces axial highs, which are replaced by flat or slightly negative topography as μeff≤5×1012 Pa s. We discuss a number of major natural MORs to validate the model findings.
Abstract This study investigates the first-order Himalayan mountain topography from the perspective of deep-crustal flow patterns in the Indo-Asia collision zone. Using a thin-viscous-sheet model we theoretically predict that flat hinterland topography with a stable elevation (Type I) can develop only when the lithospheric slab underthrusts with a threshold velocity ( V s *). For V s > V s *, the hinterland continuously gains in elevation, leading to Type II topography. This type is characterized by varying first-order surface slopes, but always facing the mountain front. Conversely, the elevated hinterland masses undergo gravity-driven subsidence, forming a topography (Type III) with characteristic backward surface slopes when V s < V s *. We evaluate V s * as a function of: (i) the regional slope of the initial first-order surface topography (α); (ii) the angle of underthrusting (β); and (iii) the relative width of foreland plain (λ), assuming little effects of surface erosion. Our model shows two characteristic deep-crustal flow patterns: corner flow and vortex flow. The corner flow pattern, described by upwardly pointed hyperbolic streamlines, is responsible for Type II topography. Conversely, the vortex flow leads to Type III, whereas the transition between the two gives rise to Type I. This corner-to-vortex type flow transition commences on decreasing V s .
'%3e%3ccircle r='20' fill='%23008'/%3e%3ccircle r='17.5' fill='%23fff'/%3e%3ccircle r='3.5' fill='%23008'/%3e%3cg id='d'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a' fill='%23008'%3e%3ccircle r='.875' transform='rotate(7.5 -8.75 133.5)'/%3e%3cpath d='M0 17.5.6 7 0 2l-.6 5L0 17.5z'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(15)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(30)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(60)'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='rotate(120)'/%3e%3cuse xlink:href='%23d' transform='rotate(-120)'/%3e%3c/g%3e%3c/svg%3e)
