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

复合斯廷斯山反转记录和极快速场变化假说

• 批准号: 0711418
• 负责人: Robert Coe
• 金额: --
• 依托单位: University of California-Santa Cruz
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-07-01 至 2011-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0711418&HistoricalAwards=false
• 关键词: Composite; Steens; Mountain; Reversal; Record

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.

地磁场在极性反转时的行为是一个本质上很有趣的问题。它还为地球发电机的建模提供了信息，提供了有关地球熔融核心中流体速度的信息，并限制了地球地幔的电导率。俄勒冈州东南部大规模玄武岩火山活动的最早阶段的强烈喷发活动与1660万年前地磁极性反转到正常极性转变的巧合，为以极高的分辨率研究极性转变提供了机会。记录了同样逆转的厚厚的熔岩流堆积发生在Steens山及其周围，覆盖范围至少300公里，可能是两倍。在不同的堆栈中的流动不会在完全相同的时刻冷却和磁化，因此可以将其中几个更完整的记录拼凑在一起。从四个这样的部分初步合成显示一个更长的过渡路径比以前已知的，多个大的摆动方向和额外的证据，更新的情况下，一个插曲的快速场变化在冷却的单一熔岩流。如果这个假设是正确的，那么隐含的变化率是非常大的，大约每天3度。因此，本提案的一个重点是非常密切地审查与这一假设有关的证据。另一个焦点是早期黄石热点火山活动的传播和范围，Steens玄武岩流归因于此。这一信息是有关的争议是否巨大的黄石热点洪水玄武岩流产生的深层次的羽或浅不稳定性在地幔局部覆盖区的弱点。热点开始后不久的逆转提供了一个独特的地层标志，以建立早期黄石热点爆发的区域和相对时间。古地磁学与放射性测年相结合，已经确定了这一反转是地磁极性时间尺度的C5Cr年代的顶部。该方法的应用表明，这一年代界线出现在Steens山东北200 km处，靠近最古老的哥伦比亚河玄武岩流的底部，但迄今为止，它只追踪到Steens以南70 km处。在北方内华达州裂谷的东南部和加州东北部的西南部，出现了暗示性的反向流动序列，但目前认为它们比C5 Cr年轻。确定这些极性边界和其他边界的年代，是为了确定哪些喷发发生在与斯廷斯反转同时，也就是黄石热点火山活动开始的时候，哪些喷发发生在稍晚一些的哥伦比亚河玄武岩群向北流动700公里的大爆发期间。这项研究的更广泛的影响涉及地磁逆转和洪水玄武岩喷发的大规模环境影响，其中包括地球表面电磁干扰和辐射的增加，气候变化和可能的物种灭绝。此外，该地区具有重要经济意义的矿床的成因最好从黄石热点断裂和岩浆活动的程度和时间来理解。还有更广泛的教育影响。该项目是一名研究生的博士研究课题。它将为本科生研究助理在现场和实验室提供支持，并为其中之一的高级论文的基础。科学成果将存入相关数据库，在期刊文章中传播，并提供给更广泛的受众，他们对了解逆转期间发生的情况表现出相当大的兴趣。