LARGE-AMPLITUDE variations in topography and inferred crustal thickness along the axes of mid-ocean ridges, often referred to as segmentation 1, are mainly observed at slow-spreading ridges 2-4. This observation has led to the suggestion that mantle processes give rise to segmentation only when spreading rates are lows 5,6. Here we make the alternative proposal that the development of segmentation is controlled by the temperature of the crust: segmentation cannot develop when the lower crust is hot enough to undergo rapid ductile flow. Thermal models predict that thick crust at a slow-spreading ridge may be as hot as normal-thickness crust along fast-spreading ridges; we accordingly test our hypothesis at a slow-spreading ridge characterized by thick crust-the Reykjanes Ridge. Topography and gravity data along the Reykjanes Ridge axis indeed show an absence of segmentation, suggesting that the thermal state of the crust, rather than any mantle process, controls the development of this structure.
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