Overlapping rift zones at the 5.5°S offset of the East Pacific Rise

Abstract

A Seabeam and magnetometer survey of the Pacific‐Nazca plate boundary around 5.5°S mapped a “nontransform offset” whose geology and kinematics seem typical of a whole class of structures formed where fast‐spreading rises are laterally offset for distances less than the width of a subaxial magma chamber. Though the spreading center is abruptly displaced by 15 km right laterally, there is no trace of strike‐slip faulting. Instead the ends of two 100 km‐long axial volcanic ridges veer 15° toward each other and overlap for more than 20 km. In this overlapping region, between 5°22′S and 5°32′S, accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one‐sided spreading. A continuous but dog‐legged magma chamber is inferred to underlie both rift zones and an intervening 9‐km wide volcano‐studded basin. A strong magnetic signature from the crust, which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank; in the past 0.5×106 years the southern axial ridge has propagated north at 40 mm/yr, 55% of the spreading half‐rate.