How Complex are Geomagnetic Reversals? Tracking the Steens Reversal in Yellowstone Hotspot Lavas to Construct a Composite Record

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EAR-0310316Robert CoeSome evidence suggests that geomagnetic reversals may be far more complex than currently believed. This proposal to conduct a paleomagnetic and geochronologic study of the widespread Yellowstone hotspot (YSHS) flood-basalt volcanism, which occurred between 17 and 15 Ma in Oregon and its four bordering states, will enable a critical examination and reconstruction of what happens during a reversal. There are numerous reversed-to-normal (R-N) polarity transition zones in sections of YSHS lava flows, including the well-known, unusually detailed one recorded at Steens Mountain in southeastern Oregon, others that are known only at the reconnaissance level, and still others yet to be discovered. Because no N-R zones have been found, it seems probable that most of these sections record this same Steens reversal. The few such sections that have been studied at the reconnaissance level indicate transition records that are similar overall, but they also exhibit some directions that are distinctly different from the Steens record. Working outward from Steens Mountain with detailed paleomagnetic determinations of direction and Ar/Ar dating will allow positive identification of the sections that record the same reversal yet include some different lava flows that carry new transitional directions. In this way a more comprehensive record of what happens during a reversal can be pieced together. Furthermore, recent studies show that eruption of the Steens Basalt marks the beginning of YSHS volcanism. Thus, identifying the sections that contain the R-N Steens reversal will also delineate the spatial extent over which YSHS eruptions began almost simultaneously and indicate how rapidly the onset of volcanism propagated. This will improve understanding of how plumes and hotspots interact with the lithosphere to emplace their magmas.Even though neither geomagnetic reversals nor flood basalt eruptions are likely to occur in the near future, they are societally relevant because of their large-scale environmental effects. These include increased electromagnetic disturbances and radiation at the earth's surface during the former and climate change and possible species extinctions during the latter. Therefore, it behooves us to learn as much as possible about them. Moreover, the genesis of the economically important epithermal ore deposits in this region is best understood in terms of the extent and timing of YSHS rifting and magmatism.