Kinematic evolution of the Reykjanes Ridge: Influence of mantle melting gradients on divergent plate boundary evolution

The Reykjanes Ridge has experienced systematic changes in configuration due to changes in plate opening direction and the influence of the Iceland hotspot. The ridge formed at about anomaly 24 as a linear axis spreading orthogonally. At about anomaly 17 an abrupt change in opening direction led to the nearly synchronous fragmentation of the ridge into short offset segments spreading orthogonally. In the subsequent evolution, the offset segments progressively and diachronously migrated to eliminate the offsets and reestablish the original axis, even though this required oblique spreading as the axis became linear again. The fragmentation appears to have occurred by rapid propagation of short segments from the previous linear ridge as indicated by asymmetry of conjugate magnetic anomalies. Segment lengths and offsets become smaller towards Iceland and in the northernmost part of the Reykjanes Ridge offsets are not evident and spreading appears to have simply switched from orthogonal to oblique within the same linear plate boundary. Modeling of the pattern of plate-driven mantle advection for the different plate boundary configurations indicates that the pattern of upwelling near the solidus depth and deeper remained linear throughout this process. This deep linear zone of mantle advection appears to have guided the reassembly of the axis back to its original linear geometry. Along-axis propagation of small-scale mantle upwelling centers, driven by the gradient in melting away from the Iceland hotspot, maintained the linear deep melting regime and prevented the formation of stable buoyant upwelling cells as typically form at slow-spreading ridge segments. At the Reykjanes Ridge, the regional Iceland mantle melting gradient appears to have caused buoyant mantle upwelling instabilities to rapidly propagate along-axis helping to maintain the linearity of the deep upwelling zone and forming the acute V-shaped crustal ridges during the linear axis phases. At the Reykjanes Ridge the large and systematic melting gradient associated with the Iceland hotspot produces a strong driving mechanism leading to a systematic reassembly of a linear axis even though this requires oblique spreading. Our observations suggest that mantle melting heterogeneities are a strong driving mechanism for divergent plate boundary evolution. Where such heterogeneities are small and irregular, ridge segment evolution is similarly irregular; where such heterogeneities form large, monotonic and persistent gradients, plate boundary evolution is organized.