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，所有在太空中自由运动的行星都想绕着一个轴自转，这一轴被称为“主惯性矩”。行星上或行星内部任何重力不平衡的大“瘤”都会向赤道迁移，迫使自转轴(行星的两极)“徘徊”？相对于它的固体质量。这被称为“真正的极地漫游”(TPW)，是众所周知的许多行星体物理学的一部分。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。在过去的十年里，地球物理学界一直在激烈地讨论是否可能存在与大约8400万年前被称为白垩纪长正常时间的长时间地磁正常极性结束相关的短时间地磁场爆发。同样，海底磁异常模式在大约同一时间的微小摆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性(古地磁)数据，证实了Chron 33R存在重大(~20度)浅倾角，而南达科他州的同时代数据显示了TPW假说所预测的持续的偏角和倾角异常。意大利的数据还提供了挑衅性的暗示，即在这种长期趋势的基础上，可能会叠加一系列短期的、约百万年规模的振荡。PI们将这些称为“真极摆动”(TPW)，以区别于更长时间尺度的TPW运动，后者一直是大多数地球物理研究的重点。根据PI的说法，整个固体地球可能一直在做类似于地质上的“草裙舞”的事情。然而，区分这些假设需要一组全球分布的高质量、高分辨率的磁性数据，这些数据可以从一个地区到另一个地区进行准确的比较。该项目将支持实地和实验室工作，旨在通过扩大采样点的地理范围来增加这一重点区间内的此类观测密度，并利用生物地层学和高分辨率锶同位素变化来完善地层对比。该计划将启动一项浅层科学钻探计划，使用便携式(Winkie(TM))钻石钻头取心系统，从关键部位收集连续定向岩心，用于这些研究。如果这些TPW运动的任何一种形式存在，它们将具有根本的重要意义，至少有两个重要原因：第一，它们将代表一类以前未被认识的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为数不多的时间间隔之一的结束有关。这些异常方向的持续时间比人们认为在地球外核运行的正常发电机过程所能解释的要长。另一方面，这些数据所暗示的运动和变化似乎太快了，如果是某种形式的TPW造成的话，就不能用地幔动力学来解释。对于相反的假设，在理解地球发电机的异常状态如何在意大利数据集的~5Myr范围内保持记忆方面也存在类似的问题。无论是哪种情况，新的结果都应该增加我们对陆地地球物理学的理解，并可能为白垩纪长时间正常时钟的终结提供线索。更广泛的影响。无论是异常场结构，还是TPW，都可以帮助细分白垩纪长标准时序及其周围的地质时代，为全球主要油气封存时期的高分辨率磁性地层学提供基础。如果TPW是原因，它还意味着三级海平面变化将是全球不同步的，这与层序地层学领域的一个基本假设相矛盾。TPW预测了海平面变化的一种正交模式，向赤道移动的地理区域经历了相对海侵，而那些远离赤道的地理区域则经历了倒退。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半个地球上180度异相。这项工作的结果也可能对“巴哈-公元前”的伟大辩论产生影响，因为需要与流离失所的地体进行比较，这些单位在年龄上非常匹配。这项研究将包括本科生和研究生的培养以及职业生涯早期的PI。