Periodicities in volcanism and longitudinal magma flow on the East Pacific Rise at 23°N

Reflection profiling data across the East Pacific Rise at the mouth of the Gulf of California show an axial high north and an axial valley south of the Tamayo Fracture Zone which converts to a high as one proceeds southwards. On the south side of the fracture zone there is a highly symmetrical distribution of ridges and valleys across the rise axis. Rocks dredged from the ridges are pillow basalts whereas rocks drilled in the valleys on DSDP Leg 65 are massive basalts. It is shown that this axial valley is probably not a steady‐state featurebut rather a periodic feature associated with variations in magma supply rate; the ridges are interpreted as ancient axial highs; and the valleys as ancient axial valleys in which massive basalt flows were ponded. The most recent volcanic ridge appears to be time transgressive suggesting that the volcanic periodicities are associated with longitudinal flow. A consequence of longitudinal flow is that at the fracture zone the flow will be impeded resulting in the observed differences between the rise axis on the north and the south sides of the Tamayo.