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Three detailed and carefully positioned seismic refraction experiments have recently been carried out along the Kane fracture zone near 24°N, 44°W in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large Atlantic fracture zones. These three experiments, when combined with earlier studies, result in reversed structural control along about a 300‐km‐long segment of the Kane fracture zone extending from the ridge‐transform intersection out onto crust about 25 m.y. old. An analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and/or velocity. However, along most of the eastern fracture zone trough, upper mantle‐type velocities are found at depths of only 2‐3 km below the seafloor, less than half the typical depth to Moho in the ocean basins. This anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities, relatively high‐velocity gradients throughout most of the thinned crustal section (about 1–2 s−1) and a distinct crust‐mantle boundary. The very thin crust appears to be confined to the deepest parts of the Kane fracture zone trough (<10 km wide), although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone. A specially designed delay time experiment at the intersection of the Kane fracture zone and the Mid‐Atlantic Ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness (about 3 km) except beneath the nodal basin where the crust may be less than 1 km thick. The anomalous crust in the intersection area appears to extend at least part way down the median valley. We interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge‐transform intersections, although tectonic processes may also play a part, at least locally, in thinning the crust. Our results support a “ribbon” model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous, so‐called normal crust 50–100 km wide extend away from spreading centers, each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough.

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Abstract. Summit troughs wider than 500 m and 30-110 m deep are present along 15-20% of the fast spreading East Pacific Rise (EPR). They occur only along ridge segments with large cross-sectional areas, indicative of a time-averaged robust magma supply. Where available, seismic data confirm that these troughs are underlain by an axial magma chamber (AMC) 1.2-1.6 km below the seafloor. Furthermore, detailed investigation of the large summit troughs which notch the EPR between 17ø56 ' and 18ø35'S indicates that the vertical relief of the troughs tends to be maximum where the AMC is shallowest. Both of these observations are inconsistent with the predictions from a model in which large summit troughs form by rifting of the brittle upper crust during phases of amagmatic extension. Rather, we propose that they represent elongated collapsed calderas that form when the melt supply to formerly inflated AMCs wanes or nearly ceases. Because seismic studies constrain the melt lens along the EPR to be only 30-80 m thick, the proposed existence of collapsed calderas 40-110 m deep implies that the entire magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply. In particular, we suggest that a voluminous crystal mush zone will stretch in response to steady state seafloor spreading when the magma supply temporarily decreases. The resulting caldera will further widen with each subsequent dike intrusion. When an abundant melt supply finally resumes, the associated tumescence of the neovolcanic zone and profuse lava flows will combine to smooth out the caldera.