Westward migration of oceanic ridges and related asymmetric upper mantle differentiation

Abstract

Combining geophysical, petrological and structural data on oceanic mantle lithosphere, underlying asthenosphere and oceanic basalts, an alternative oceanic plate spreading model is proposed in the framework of the westward migration of oceanic spreading ridges relative to the underlying asthenosphere. This model suggests that evolution of both the composition and internal structure of oceanic plates and underlying upper mantle strongly depends at all scales on plate kinematics. We show that the asymmetric features of lithospheric plates and underlying upper asthenosphere on both sides of oceanic spreading ridges, as shown by geophysical data (seismic velocities, density, thickness, and plate geometry), reflect somewhat different mantle compositions, themselves related to various mantle differentiation processes (incipient to high partial melting degree, percolation/reaction and refertilisation) at different depths (down to 300 km) below and laterally to the ridge axis. The fundamental difference between western and eastern plates is linked to the westward ridge migration inducing continuing mantle refertilisation of the western plate by percolation-reaction with ascending melts, whereas the eastern plate preserves a barely refertilized harzburgitic residue. Plate thickness on both sides of the ridge is controlled both by cooling of the asthenospheric residue and by the instability of pargasitic amphibole producing a sharp depression of the water-undersaturated solidus, its intersection with the geotherm at ~90 km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the waterundersaturated lherzolite solidus explains the existence of a low-velocity zone (LVZ). As oceanic lithosphere is moving westward relative to asthenospheric mantle, this partially molten upper asthenosphere facilitates the decoupling between lower asthenosphere and lithosphere. Thereby the westward drift of the lithosphere is necessarily slowed down, top to down, inducing a progressive decoupling within the mantle lithosphere itself. This intra-mantle decoupling could be at the origin of asymmetric detachment faults allowing mantle exhumation along slow-spreading ridges. Taking into account the asymmetric features of the LVZ, migration of incipient melt fractions and upwelling paths from the lower asthenosphere through the upper asthenosphere are oblique, upward and eastward. MORB are sourced from an eastward and oblique, near-adiabatic mantle upwelling from the lower asthenosphere. This unidirectional mantle transfer is induced by isostatic suction of the migrating spreading ridge.

Distinctive Seafloor Fabric Produced Near Western Versus Eastern Ridge‐Transform Intersections of the Northern Mid‐Atlantic Ridge: Possible Influence of Ridge Migration

“…An additional factor may also contribute to the variability of melt supply at leading versus trailing RTIs. The lateral migration of a mid‐ocean ridge over the asthenosphere is thought to result in a compositional asymmetry in the upper mantle relative to the ridge axis, whereby the upper mantle beneath the trailing plate would be systematically depleted compared to the upper mantle beneath the leading plate (Chalot‐Prat et al, ; Cormier & Sloan, ; Doglioni et al, ). As a result, leading RTIs would be expected to sample a relatively undepleted mantle from the melting regime that underlies the adjacent leading plate.…”

“…ngdc.noaa.gov/mgg/bathymetry/ multibeam.html) and GeoMapapp (http://www.geomapapp.org). Bathymetric data have been processed with MB-System and figures have been generated with GMT, two freely available software packages (Caress et al, 2015;Wessel et al, 2013). In addition, we thank the following scientists for making additional bathymetric data available: Toshiya Fujiwara provided a bathymetric grid of the Kane Fracture zone compiled from data acquired during the 1994 expedition MODE'94 of the M/S Yokosuka; Mathilde Cannat provided a bathymetric grid of the Oceanographer fracture zone compiled from data acquired with the N/O L'Atalante during the 1998 expedition SUDACORES; Pascal Gente released a high quality bathymetric grid of the area south of the Kane fracture zone via GeoMapApp.…”

“…Small and Danyushevsky () proposed that ridge migration accounts for the prominent discontinuity in both axial depth and geochemistry that exists between the East Pacific Rise and the Pacific‐Antarctic Ridge. Doglioni et al () suggested that ridge migration could explain why the eastern flanks of mid‐ocean ridges tend to be shallower than their western flanks, and Chalot‐Prat et al () further proposed that ridge migration leads to asymmetric upper mantle differentiation.…”

A heterogeneous subcontinental mantle under the African-Arabian plate boundary revealed by boron and radiogenic isotopes

“…Since the mantle becomes depleted in Fe after melting, it moves "eastward" relative to the lithosphere: the higher topography of the eastern side of the rift is therefore interpreted in terms of isostatic adjustment (lower thermal subsidence). Due to the net rotation of the lithosphere, the depleted and lighter sub-ridge mantle may eventually transit 43 beneath continental rifts, uplifting its eastern shoulder. In this view, no heat anomalies are required to explain the high topography of the eastern margins of both the Red Sea and Dead Sea faults.…”

Publisher’s Note

“…consistent with the observation that the speed with which plates move is related to the length of the subducting slab to which they are attached. Considerable work has been done subsequently to investigate this relationship, including study of the apparent east-west asymmetry in the global subduction slab system (Doglioni and Anderson, 2015) and the systematic westward migration of spreading ridges which imparts east-west asymmetry to the composition of the mantle (Chalot-Prat et al, 2017).…”