PULL-APART STRUCTURE EVOLVING AT NORTHERN EXPLORER RIDGE, NORTHEAST PACIFIC

The Explorer Spreading Center (ESC) bounding the western edge of the Explorer microplate has undergone substantial reorganization over the past few million years as the microplate has rotated in response to increasing resistance to the subduction of its young crust. The northern Explorer ridge (NER) has evolved into a complex compound structure consisting of several rift basins bounded by half-graben and arcuate shaped faults with a superimposed pattern of rhombohedral grabens and horsts. This pattern is similar to structures on rift zones and pull-apart basins formed along major continental transforms and rift zones and contrasts with the ridge-parallel faults formed at seafloor spreading centers. However, initiation of faulting in the NER appears to have occurred along inactive off-axis faults generated by seafloor spreading.

The area appears to be evolving into a larger pull-apart-like structure. As extension continues, individual basins have widened with some of the larger arcuate boundary faults linking up to form accommodation zones between adjacent depressions. If diffuse rifting continues in the NER the nascent pull-apart structure will grow longer as it accommodates the changing regional strain field induced by the diminishment of the Explorer plate. Alternatively, a strike-slip fault could propagate through the area and join the two master faults. Although there’s no well-defined though-going fault with the strike of the Pacific-North American vector (~340°), there are a series of smaller faults that could be linking together.