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In the North Atlantic Ocean, a mid-ocean ridge bisects the Icelandic mantle plume, and provides a window into its temporal evolution. V-shaped ridges of thick oceanic crust observed south of Iceland are thought to record pulses of upwelling within the plume. Specifically, excess crust is thought to form during the quasi-periodic generation of hot solitary waves triggered by thermal instabilities in the mantle. Here we use seismic reflection data to show that V-shaped ridges have formed over the past 55 million years—providing the longest record of plume periodicity of its kind. We find evidence for minor, but systematic, asymmetric formation of crust, due to migration of the mid-ocean ridge with respect to the underlying plume. We also find changes in periodicity: from 55 to 35 million years ago, the V-shaped ridges form every 3 million years or so and reflect small fluctuations in plume temperature of about 5–10 °C. From 35 million years ago, the periodicity changes to about 8 million years and reflects changes in mantle temperature of 25–30 °C. We suggest that this change in periodicity is probably caused by perturbations in the thermal state at the plume source, either at the mantle-transition zone or core–mantle boundary.

Chirp sub-bottom profilers are marine sonar systems which use a highly repeatable source signature to facilitate the acquisition of correlated data with decimetre vertical resolution in the top 20–30 m of sediments. Source signatures can be readily developed and implemented, but an applicable methodology for assessing resolution and attenuation characteristics of these wide-band systems did not exist. Methodologies are developed and applied to seven contrasting source signatures which occupy the same frequency band, but differ in their Envelope and Instantaneous Frequency functions. For the Chirp source signatures tested, a Sine-Squared envelope function is shown to produce seismic data with the optimum resolution and penetration characteristics.

We present a high‐resolution 2‐D P‐wave velocity model from a 225‐km‐long active seismic profile, collected over ~60–75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three nontransform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two‐layer velocity structure with a high gradient Layer 2 (~0.9 s−1) above a lower gradient Layer 3 (0.2 s−1). Here, PmP coincides with the 7.5 km s−1contour, and velocity increases to >7.8 km s−1within 1 km below. We interpret these segments as magmatically robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between the upper and lower crust and PmP shallower than the 7.5 km s−1contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. While velocity‐depth profiles fit within previous envelopes for slow‐spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments, we make the first wide‐angle seismic identifications of buried oceanic core complexes in mature (>20 Ma) Atlantic Ocean crust. They have a ~20‐km‐wide “domal” morphology with shallow basement and increased upper crustal velocities. We interpret their midcrustal seismic velocity inversions as alteration and rock‐type assemblage contrasts across crustal‐scale detachment faults. We identify that magmatic and tectonic modes of slow Atlantic crustal accretion are distinct and equal, based on seismic characteristicsThe structure of crustal discontinuities is controlled by the accretion mode of neighboring segments, independent of ridge offset and orderTwo buried oceanic core complexes in mature Atlantic Ocean crust are identified, based on their dimensions and seismic structure