Collaborative Research: Testing the Hypothesis of Late Cretaceous True Polar Wobble (TPw)

合作研究：检验晚白垩世真极摆（TPw）假说

• 批准号: 1114432
• 负责人: Joseph Kirschvink
• 金额: $ 25.74万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-09-01 至 2015-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1114432&HistoricalAwards=false
• 关键词: Collaborative; Research; Testing; Hypothesis; Late

Like a Frisbee, all planets moving freely in space want to spin around an axis called their 'principal moment of inertia'. Any large 'wart' on the planet, or inside it, that is not balanced gravitationally will migrate towards the Equator, forcing the spin axis (the 'poles' of the planet) to 'wander?' relative to its solid mass. This is called 'True Polar Wander' (TPW), and is part of the well-understood physics of many planetary bodies. The entire solid part of the planet wanders at the same time relative to the spin axis, and as such is quite different from the normal motion of tectonic plates here on Earth. A vigorous debate has been raging within the geophysical community for the past decade about the possible existence of a short burst of TPW associated with the end of a long period of geomagnetic Normal polarity, called the Cretaceous Long Nomal Chron, around 84 million years ago. Similarly, small wiggles in the pattern of sea-floor magnetic anomalies at about the same time have been interpreted as a result of an anomalously weak, fluctuating geomagnetic field. Recently, the investigators have obtained higher-resolution fossil magnetic (paleomagnetic) data from the classic Scaglia Rossa lime-stones in Italy which confirm the presence of a major (~20 degree) shallowing of inclination in Chron 33R, whereas coeval data from South Dakota display both persistent declination and inclination anomalies as predicted by the TPW hypothesis. The Italian data also provide provocative hints that there might be a series of short, ~million-year scale oscillations superimposed on this long-term trend. The PIs call these 'True Polar wobbles' (TPw) to distinguish them from longer time-scale TPW motions which have been the focus of most previous geophysical investigations. The entire solid Earth may have been doing something akin to a geological 'Hula dance', according to the PIs. However, distinguishing these hypotheses requires a globally distributed set of high-quality, high-resolution magnetic data that can be compared accurately from area to area. This project will support field and laboratory work aimed at increasing the density of such observations in this focused interval by expanding the geographic extent of the sampling sites, and to refine the stratigraphic correlation using biostratigraphy and high-resolution Sr isotope variations. Ths plan is to launch a shallow scientific drilling program with portable (Winkie(TM)) diamond-bit coring systems to collect continuous oriented core from critical sections for these studies.Intellectual Merit. If either form of these TPW motions exist, they would be fundamentally important for at least two important reasons: First, they would represent a previously-unrecognized class of TPW, with an as-yet unknown driving mechanism. Second, they seem to be associated with the end of one of the few time intervals in which the frequency of geomagnetic reversals drops essentially to zero. These anomalous directions have longer durations than can be explained plausibly by the normal dynamo processes thought to operate in Earth's outer core. On the other hand, the motions and changes implied by the data seem too rapid to be explained by mantle dynamics if some form of TPW is responsible. For the counter-hypothesis, there are similar problems in understanding how an anomalous state of the geodynamo would have a memory persisting for the ~5 Myr span of the Italian data set. In either case the new results should increase our understanding of terrestrial geophysics, and could possibly yield clues to the underlying cause of the end of the Cretaceous Long Normal Chron. Broader Impacts. Either anomalous field configurations or TPW could help sub-divide geological time in and around the Cretaceous Long Normal Chron, providing a basis for higher-resolution magnetostratigraphy at a time of major global hydrocarbon sequestration. If TPW is the cause, it has the additional implication that the 3rd order sea-level variations would be globally asynchronous, contradicting a fundamental assumption in the field of sequence stratigraphy. TPW predicts a quadrature pattern of sea-level variation, with geographic areas moving towards the Equator experiencing relative transgressions, and those moving away, regressions. In turn, this implies that the global sequence stratigraphic framework that petroleum geologists have devised over the past 30 years would be 180 degrees out of phase over half of the planet. Results of this work could also have bearing on the great 'Baja-BC' debate, as comparisons with displaced terranes would need to be made between units closely matched in age. This research will involve the training of under graduate and graduate students and an early career PI.

就像飞盘一样，所有在太空中自由移动的行星都希望绕着一个称为“主惯性矩”的轴旋转。 行星上或行星内部任何重力不平衡的大“疣”都会向赤道迁移，迫使自转轴（行星的“两极”）“徘徊”？相对于其固体质量。 这被称为“真正的极地漫游”（TPW），是许多行星体众所周知的物理学的一部分。 地球的整个固体部分同时相对于自转轴漂移，因此与地球上构造板块的正常运动有很大不同。 过去十年来，地球物理学界一直在激烈争论，关于是否可能存在短暂的 TPW 爆发，该爆发与大约 8400 万年前的长期地磁正常极性（称为白垩纪长正常年代）的结束有关。 同样，大约在同一时间海底磁异常模式的小幅波动被解释为异常微弱、波动的地磁场的结果。 最近，研究人员从意大利经典的 Scaglia Rossa 石灰岩中获得了更高分辨率的化石磁（古磁）数据，证实了 Chron 33R 中存在重大（约 20 度）的倾角变浅，而来自南达科他州的同期数据显示了 TPW 假说所预测的持续磁偏角和倾角异常。 意大利的数据还提供了令人兴奋的暗示，即在这一长期趋势之上可能会出现一系列短期的、约百万年规模的振荡。 PI 将这些称为“真极地摆动”(TPw)，以将其与较长时间尺度的 TPW 运动区分开来，后者是大多数先前地球物理研究的焦点。 PI 表示，整个固体地球可能一直在进行类似于地质学“草裙舞”的活动。 然而，区分这些假设需要一组全球分布的高质量、高分辨率的磁数据，这些数据可以在不同区域之间进行准确比较。 该项目将支持现场和实验室工作，旨在通过扩大采样点的地理范围来增加这一重点区间内此类观测的密度，并利用生物地层学和高分辨率锶同位素变化来完善地层相关性。该计划是利用便携式 (Winkie(TM)) 金刚石钻头取芯系统启动浅层科学钻井计划，从这些研究的关键部分收集连续定向的岩心。智力优点。如果这些 TPW 运动中的任何一种形式存在，它们都将具有根本性的重要意义，至少有两个重要原因：首先，它们将代表一类以前未被识别的 TPW，其驱动机制尚不清楚。 其次，它们似乎与地磁反转频率基本上降至零的少数时间间隔之一的结束有关。这些异常方向的持续时间比被认为在地球外核运行的正常发电机过程所能合理解释的持续时间要长。 另一方面，如果某种形式的 TPW 负责的话，数据所暗示的运动和变化似乎太快而无法用地幔动力学来解释。 对于反假设，在理解地球发电机的异常状态如何具有意大利数据集~5 Myr跨度的记忆时也存在类似的问题。 无论哪种情况，新的结果都应该增加我们对陆地地球物理学的理解，并可能为白垩纪长正常年代结束的根本原因提供线索。 更广泛的影响。无论是异常场配置还是 TPW 都可以帮助细分白垩纪长正时年及其周围的地质时间，为全球碳氢化合物主要封存时期的更高分辨率磁地层学提供基础。如果 TPW 是原因，则还意味着三阶海平面变化将是全球异步的，这与层序地层学领域的基本假设相矛盾。 TPW 预测海平面变化呈正交模式，向赤道移动的地理区域会经历相对海侵，而远离赤道的地理区域则会经历回归。 反过来，这意味着石油地质学家在过去 30 年里设计的全球层序地层框架将在地球的一半区域出现 180 度的异相。这项工作的结果也可能对伟大的“Baja-BC”辩论产生影响，因为需要在年龄密切匹配的单位之间进行与位移地体的比较。 这项研究将涉及对本科生和研究生以及早期职业 PI 的培训。