COLLABORATIVE RESEARCH: Paleomagnetism and Geochronology of Mafic Dikes in Morocco, Reconstructing West Africa in Proterozoic Supercontinents

在地球数十亿年的历史中，大陆漂移到全球各地，周期性地聚集成超级大陆。盘古大陆是其中最新的一个：在大西洋扩张之前，它是连接美洲与欧洲和非洲的单一大陆。在盘古大陆之前，可能还有更古老的超级大陆，分别名为罗迪尼亚(约9亿年前)和努纳(约15亿年前)，两者都存在于原始生命的“元古代”时间间隔内。尽管这些超级大陆有名字，但它们大陆碎片的确切排列仍然不确定。无论如何，地质学家开始推测，超大陆的过渡是否普遍具有手风琴式的运动模式(想象一下，未来美洲逆转航向，向东漂移，关闭大西洋，重新与欧洲和非洲相撞)，或者大陆是否绕着地球漂移(想象一下，美洲继续向西漂移，关闭太平洋，与东亚和澳大利亚相撞，从而将古老的超大陆“由内而外”地颠覆)。这些想法有助于塑造我们对地球内部数十亿年来演化的看法，也为矿藏的形成、古代气候记录和最长时间尺度的生物进化提供了地理背景。该项目使用两种互补的实验室测量方法来确定大陆是如何在地球表面移动的：古地磁--由岩石记录的古老的地磁场记录；以及使用铀到铅的放射性衰变来测定岩石的年龄。该项目的重点是努纳和罗迪尼亚超大陆大陆中最不为人所知的大陆碎片：西非克拉通。在西非，特别是摩洛哥的安阿特拉斯山脉，地质认识的最新进展为将这两种方法应用于不同时代的古代火山岩系统(“基性岩脉”)提供了机会，以更准确地重建努纳和罗迪尼亚陆块，并发现地球超级大陆过渡的模式。该项目将包括在实地收集岩石样本、对这些样本进行实验室分析以及在同行评议的期刊上发表结果。除了该项目的科学目标外，与该项目相关的重要社会成果包括在一个重要的科学、技术、工程和数学(STEM)学科方面培训下一代地球科学家；将该项目的成果纳入公共博物馆展示；以及与K-12公立学校教师一起开发教育模块。目前，超大陆努纳和罗迪尼亚的结构不确定，允许终端成员对长期全球地球动力学提出两种观点中的任何一种：超大陆是否倾向于恢复到以前的结构，或由内向外。也有可能他们在时间上在这两种模式之间交替。西非的古地理是所有主要的前寒武纪克拉通中限制最少的，它之前的假设位置要么位于前盘古超级大陆的中间，要么沿着它们的外围，这对上述辩论至关重要。该项目旨在对摩洛哥安阿特拉斯山脉前寒武纪内的镁铁质岩墙群进行综合的古地磁和地质年代学研究。从这项研究中获得的高质量和精确年代的古地磁极将填补西非以及努纳和罗迪尼亚邻近地块的构造和运动学历史上的一个显著空白，并为关于超级大陆过渡样式的辩论提供基本的地面事实。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Over billions of years of Earth history, continents have drifted across the globe, periodically assembling into supercontinents. Pangea was the most recent of these: a single landmass that joined the Americas to Europe and Africa, prior to spreading of the Atlantic Ocean. Pangea was likely preceded by more ancient supercontinents named Rodinia (about 900 million years ago) and Nuna (about 1.5 billion years ago), both existing within the “Proterozoic” time interval of primordial life. Although these supercontinents have names, their exact arrangements of continental fragments remain uncertain. Regardless, geologists are beginning to speculate on whether the supercontinental transitions generally have an accordion-like pattern of motion (imagine a future for the Americas reversing their course and drifting eastward to close the Atlantic Ocean and re-collide with Europe and Africa), or whether continents circumnavigate the globe (imagine the Americas continuing to drift westward to close the Pacific Ocean and collide with eastern Asia and Australia, thus turning the old supercontinent “inside-out”). These ideas help shape our view of Earth’s evolving deep interior over billions of years, and also give geographical context to the formation of mineral deposits, ancient climate records, and biological evolution at the longest timescales. This project uses two complementary methods of laboratory measurement to determine how the continents have moved across the Earth’s surface: paleomagnetism—the ancient record of the geomagnetic field that is recorded by rocks; and the isotopic dating of the age of rocks using the radioactive decay of uranium to lead. The project focuses on the least well-understood continental fragment in the Nuna and Rodinia supercontinental landmasses: the West African craton. Within West Africa, particularly the Anti-Atlas Mountains of Morocco, recent advances of geological understanding provide an opportunity to apply the two methods to ancient volcanic rock systems (“mafic dikes”) of a variety of ages, to produce more accurate reconstructions of the Nuna and Rodinia landmasses and to discover the patterns of Earth’s supercontinental transitions. The project will involve collection of rock samples in the field, laboratory analyses on those specimens, and publication of results in peer-reviewed journals. In addition to the scientific goals of the project, important societal outcomes associated with this project include training the next generation of Earth scientists in an important science, technology, engineering and mathematics (STEM) discipline; incorporation of the project’s results into public museum displays; and development of educational modules with K-12 public school teachers.Uncertainties in the configurations of supercontinents Nuna and Rodinia currently permit either of two end-member views on long-term global geodynamics: whether supercontinents tend to revert to prior configurations, or turn “inside-out.” It is also possible that they have alternated between those two patterns in time. West Africa's paleogeography is the least constrained of all major Precambrian cratons, and its previously hypothesized placement either within the middle of pre-Pangean supercontinents or along their periphery is crucial to the aforementioned debate. This project seeks to conduct an integrated paleomagnetic and geochronological study of mafic dike swarms in Precambrian inliers of the Anti-Atlas Mountains, Morocco. High-quality and precisely dated paleomagnetic poles obtained from this study will fill a notable gap in the tectonic and kinematic history of West Africa and neighboring blocks in Nuna and Rodinia, and provide essential ground-truth to the debate on supercontinental transitional styles.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.