| Summary: | International audience Ultra-slow spreading ridges such as the South West Indian ridge orthe Arctic ridge system are oddities amongst oceanic ridges. Indeed,conversely to faster oceanic ridges, petrographic and seafloor studieshave shown that they characterized by low melt supply and present lowcrustal thicknesses and heat flow; these features are interpreted as anevidence for a cooler sublithospheric mantle. In cartoonish sketches ofplate tectonics, ridges open above upwellings, subduction zones occurover downwellings, and plates are riding over the mantle convectioncells. In this study, we designed a simple yet dynamically consistentthermal convection model to test the impact of far-field forces on spreadingridges and show that this pattern is disrupted by plate tectonics. Inparticular, continental collisions modulate the spreading rates becauseresisting forces build up at plate boundaries. As a consequence, thismodifies the surface boundary conditions and therefore the underlyingmantle flow. We show that the ideal convection cell pattern quicklybreaks down when plate motion is impeded by continental collisions inthe far field. Not only the decreasing spreading rates are diagnostic, butin the same time, (i) the heat flow is decreasing at the ridge, (ii) the thermalstructure of the cooling lithosphere no longer matches the coolinghalf-space model, and (iii) the mantle temperature beneath the ridgedrops by more than 100 degrees. We compare our model predictions toavailable observables and show that this simple mechanism explains theatypical thermo-mechanical evolution of the South West Indian ridgeand Arctic ridge system. Last, the recent S wave seismic tomographymodel of Debayle and Ricard (2012) reveals that only away from thosetwo ridges does lithospheric thickening departs from the half-space coolingmodel, in accord with our model predictions.
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