Formation and Elimination of Segmentation on the Reykjanes Ridge

A Paleogene change in plate motion led to the abrupt segmentation of the linear Reykjanes Ridge into a stair-step configuration of ridge segments offset by transform and non-transform discontinuities. In its subsequent evolution the ridge diachronously eliminated the just-formed segmentation and re-established its original linear geometry over the mantle, although now spreading obliquely. During segmented stages accreted crust was thinner and during unsegmented stages southward pointing V-shaped crustal ridges formed. Although mantle plume effects have been invoked to explain the changes in segmentation and crustal features, here we propose that plate boundary processes are more likely. Reassembly of a fragmented axis to its original linear configuration suggests a strong organizing mechanism. We propose that a deep and weak damp melting regime supporting buoyant mantle upwelling persisted in a linear configuration following the abrupt change in opening direction. In contrast, the shallower and stronger mantle of the dry melting regime followed the plate boundary closely, leading to crustal segmentation. The persistent deep linear damp melting regime guided the reassembly of the ridge axis back to its original configuration by asymmetric spreading of the individual ridge segments. Plate boundary segmentation controls mantle upwelling in the high viscosity dry melting interval. A long linear divergent plate boundary induces greater total mantle upwelling than the same length axis offset into multiple segments due to suppression of mantle upwelling approaching segment ends. Thus a segmented ridge will produce regionally thinner crust than an unsegmented one without a need for mantle temperature variations. Buoyant upwelling instabilities propagate along the long linear deep melting regime driven by regional gradients in mantle properties away from Iceland. Once segmentation is eliminated, these propagating upwelling instabilities lead to crustal thickness variations evident as V-shaped ridges on the Reykjanes Ridge flanks, without requiring actual rapid radial mantle plume flow. Our study shows that the Reykjanes Ridge can be used to study how changes in plate motion over a gradient in mantle properties lead to a range of effects on lithospheric tectonics, melt production and crustal accretion.