Pacific seafloor in the 40–52 Myr old portion of the Molokai to Murray corridor

A detailed study of the character of 40–52 Myr old Northeast Pacific seafloor illustrates how volcanism that occurs outside a spreading center axial zone contributes to the morphology of a region. A compilation of new and pre-existing multibeam sonar data forms the basis of our study, which lies within the spreading corridor bounded by Molokai and Murray fracture zones and does not include a major volcanic chain. The broad structure is consistent with constant crustal thickness and lithospheric cooling with age, and our analyses focus on the deviations from this 'reference' model. We find three types of volcanic features where typical abyssal hill fabric is generally not observed: 1) volcanic ridges that have a length of 30–120 km and a height of 1–2 km; 2) moderate seamounts that have a diameter of 8–15 km and a height of several 100's m; and 3) fields of very small seamounts, which extend several 10's km and are more common at ~25% coverage than in other Pacific regions away from major volcanic chains. Gravity analysis suggests that the volcanic ridges and a few of the moderate seamounts are associated with local crustal thickening whereas the fields of very small seamounts do not display a distinct mass anomaly. Quantifying the distribution of past volcanism in this section of the Pacific plate during its evolution after initial accretion helps illustrate the degree of off-axis magmatism. Although we interpret the volcanic ridges as presently inactive, their formation on lithosphere older than at least a few Myr likely had an impact on the local seismic structure.