Nontransform offsets of the Pacific-Cocos plate boundary and their traces on the rise flank
A multi-beam bathymetric reconnaissance of the fast-spreading crest of the East Pacific Rise between 7°N and 2.6°N found the axial ridge to be interrupted by four 4–13 km non-transform offsets. They have probably persisted since initiation by a small change in spreading direction 3.5 Ma. At each offset, the volcanic rift zones veer 15° toward each other and overlap for 5–25 km around deep basins. Intervening lateral displacements of the rift zones are too short (<2.5 km) to break the linear axial volcanoes but cause local bends in their plan shapes; some examples have a morphology that mimics the larger intervolcano offsets. Interpretation of a thorough bathymetric and magnetic survey around the 9 km, 5.5°N offset indicates that the mainly one-sided spreading from the overlapped sections of rift zone occurs by a combination of highly asymmetric dike accretion and episodic small-scale jumps of the eruptive zone. The northern rift zone is elongating (propagating) at the expense of the southern one. The resulting along-axis migration of this and other examples, at speeds comparable to spreading rates, causes the rise-flank scars of the nontransform offsets (equivalent to the fracture zones of transform faults) to be oblique to the spreading direction. These traces are recognizable within the highly lineated fault-block terrain of the rise flanks because of the distinctive morphology of the 5–25-km-wide bands that accreted at overlapped sections of the rift zones. Here, lineation is less marked and oblique to the spreading direction, and volcanic relief is more important than relief caused by faulting. The west-flank trace of the 5.5°N offset is occupied by a line of volcanoes and shallow volcanic ridges. Several other volcanic chains, previously identified as hot-spot traces, seem to have grown in similar tectonic settings.