THE DARWIN RISE DEMISE - THE WESTERN PACIFIC GUYOT HEIGHTS TRACE THE TRANS-PACIFIC MENDOCINO FRACTURE-ZONE

The Darwin Rise has been proposed so many times and in so many forms a nd places that the time has come to make a more comprehensive examinat ion of the region. Lying on the NW Pacific Plate between the Geisha Gu yots, the Mid-Pacific Mountains, the equator, and the trenches, the re gion is roughly bounded by magnetic anomaly M20 (147 Ma). It was subje cted to a massive outpouring of lava about 105 to 120 Ma, which create d the guyots and seamounts in that region. Guyots are excellent tools for studying events of long ago because they eroded in the same lowsta nd in the Cretaceous and guyot relief, therefore, is a surrogate for p aleo-sealevel. The relief is derived by subtracting the break depth of the summit plateau of a guyot from the regional depth. Guyot relief w ould necessarily be less in the center than to the periphery if the fe ature formed on a pre-existing rise, as has been postulated. The exist ence of a paleo-Darwin Rise would give concentric contours for the reg ion in question. Of the sixty guyots used in this study, thirty-seven of these guyots were surveyed using SASS multibeam in the Marcus-Wake seamount group. Twenty-three guyots were surveyed using random track s ingle-beam sonar surveys. An entirely different scenario is shown. Dat a revealed a major fracture passing through the area coevally or after the guyots formed. Because the depths to the summit are not the same now, vertical tectonics occurred after subaerial erosion. This means t he fracture formed during and after the erosion (roughly 105 Ma) and i nfluenced the normal sequence of events in guyot formation. Depending on how one deciphers trends through the Hess Rise morass, SASS bathyme try shows a continuation of the Surveyor/Mendocino fracture zone swarm inside the M20 region to the NE of these data. The fracture swarm con tinues to the western Pacific trench system. Based on this information , if the Darwin Rise ever existed, it had to have done so elsewhere.