Wolf–Darwin lineament and plume–ridge interaction in northern Galápagos

Abstract

The Wolf–Darwin Lineament (WDL), located in the northwestern sector of the Galápagos Archipelago, lies between the focus of the Galápagos hot spot and the Galápagos Spreading Center. Consequently, most researchers have attributed its origin to the interaction between the plume and the adjacent ridge. We propose that the WDL is caused only partially by the plume–ridge interaction, and instead that it is primarily the result of tensional stresses emanating from the inside corner of the transform fault at 91°W. An additional factor that amplifies the tension in this region is the oblique orientation of the major transform fault with respect to the Nazca plate's spreading direction. This setting creates a transtensional zone whereby strain is partitioned into strike‐slip motion along the transform and extension throughout the inside corner of the ridge–transform system. The area under tension is magmatic owing to the overlapping effects of the ridge and the Galápagos plume. The extensional model predicts no age‐progressive volcanism, which is supported by observed age relationships. The WDL volcanoes define two distinct chemical groups: lavas erupted south of Wolf Island have compositions similar to those produced along the GSC west of 93°W, while those from the northern WDL resemble GSC lavas from the segment directly north of the lineament. This geographic distribution implies that the WDL is supplied by the same type of plume‐affected mantle as the segment of the GSC that produced the lithosphere underlying the volcanoes. The observed WDL geochemical gradients are consistent with the extension model; the region under tension simply taps hybrid products of mixing at the margins of the subridge convection system and the periphery of the plume. Essentially, the stress field around the transform fault, normally not observable in a typical midocean ridge setting, is illuminated by the presence of melt from the adjacent hot spot.