A Composite Steens Mountain Reversal Record and the Very-Rapid-Field-Change Hypothesis

点击翻译按钮获取中文摘要

How the geomagnetic field behaves as it reverses polarity is an intrinsically interesting question. It also informs modeling of the geodynamo, gives information about fluid velocity in Earth's molten core, and puts limits on the electrical conductivity of Earth's mantle. The coincidence of intense eruptive activity during the earliest stage of extensive basaltic volcanism in SE Oregon with a geomagnetic reversed-to-normal polarity transition 16.6 million years ago provides the opportunity to study a polarity transition at extraordinarily high resolution. Thick stacks of lava flows that recorded this same reversal occur at and around Steens Mountain over an area spanning at least 300 km, and possibly twice that much. Flows in different stacks did not cool and become magnetized at exactly the same moments of time, so that a more complete record can be pieced together from several of them. A preliminary composite from four such sections displays a much longer transition path than previously known, with multiple large swings in direction and additional evidence that renews the case for an episode of rapid field change during the cooling of a single lava flow. If this hypothesis is correct, the implied rate of change is extraordinary, about 3 degrees per day. Thus, one focus of this proposal is to examine evidence pertaining to this hypothesis very closely. The other focus concerns the propagation and extent of early Yellowstone hotspot volcanism, to which the Steens Basalt flows are attributed. This information is relevant to the controversy about whether the enormous Yellowstone hotspot flood basalt flows were produced by a deep-seated plume or by a shallow instability in the mantle localized by overlying zones of weakness. The reversal shortly after hotspot inception provides a unique stratigraphic marker to establish the area and relative timing of early Yellowstone hotspot eruptions. Paleomagnetism coupled with radiometric dating has pinpointed this reversal as the top of chron C5Cr of the geomagnetic polarity time scale. Application of this method shows that this chron boundary occurs 200 km to the northeast of Steens Mountain, near the base of the oldest Columbia River Basalt flows, but so far it is traced only 70 km to the south of Steens. Suggestive reversed-to-normal sequences of flows occur much further to the southeast in the Northern Nevada Rift and to the southwest in northeastern California, but current thought is that they are younger than C5Cr. Dating these polarity boundaries and others is establishing which eruptions occurred at the same time as the Steens reversal, when Yellowstone hotspot volcanism began, and which occurred a little later during the big outburst of Columbia River Basalt Group flows 700 km to the north. Broader impacts of this study involve the large-scale environmental effects of geomagnetic reversals and flood basalt eruptions, which include increased electromagnetic disturbances and radiation at the earth's surface, climate change, and possible species extinctions. In addition, the genesis of the economically important ore deposits in this region is best understood in terms of the extent and timing of Yellowstone hotspot rifting and magmatism. There are also broader educational impacts. This project is the subject of a graduate student's PhD research. It will provide support for undergraduate research assistants in the field and laboratory and the basis for a senior thesis for one of them. The scientific results will be deposited in pertinent databases, disseminated in journal articles, and made available to broader audiences, who have demonstrated considerable interest in learning what happens during a reversal.