Collaborative Research: Paleomagnetism and Geochronology of Mafic Dikes in Morocco, Reconstructing West Africa in Proterozoic Supercontinents

在地球几十亿年的历史中，大陆在地球上漂移，周期性地组合成超级大陆。盘古大陆是其中最近的一个：在大西洋扩张之前，它是连接美洲、欧洲和非洲的单一大陆。盘古大陆之前可能有更古老的超级大陆，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。虽然这些超级大陆有名字，但它们的大陆碎片的确切排列仍然不确定。不管怎样，地质学家们开始推测，超级大陆的转变是否通常有一个手风琴式的运动模式（想象一下，美洲大陆未来会逆转方向，向东漂移，靠近大西洋，与欧洲和非洲再次碰撞），或者大陆是否环绕地球(想象一下，美洲大陆继续向西漂移，靠近太平洋，与东亚和澳大利亚碰撞，从而将古老的超大陆“从内到外”翻转过来)。这些想法有助于塑造我们对数十亿年来地球内部深处演变的看法，也为矿床的形成、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。这个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——由岩石记录的地磁场的古代记录；用铀对铅的放射性衰变来测定岩石的年代。该项目的重点是努纳和罗迪尼亚超大陆板块中最不为人所知的大陆碎片：西非克拉通。在西非，特别是摩洛哥的反阿特拉斯山脉，最近地质认识的进展提供了一个机会，可以将这两种方法应用于各种年龄的古代火山岩系统（“基性岩脉”），以产生更准确的努纳和罗迪尼亚大陆块体重建，并发现地球超大陆过渡的模式。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，并在同行评议的期刊上发表结果。除了项目的科学目标之外，与该项目相关的重要社会成果还包括在重要的科学、技术、工程和数学（STEM）学科中培养下一代地球科学家；将项目成果纳入公共博物馆展示；与K-12公立学校教师一起开发教育模块。目前，Nuna和Rodinia超大陆构造的不确定性允许对长期全球地球动力学的两种末端成员观点中的一种：超大陆是否倾向于恢复先前的构造，还是“由内到外”。也有可能它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最不受限制的，它之前的假设位置要么在前泛古大陆的中间，要么沿着它们的边缘，这对前面的争论至关重要。本项目旨在对摩洛哥反阿特拉斯山脉前寒武纪内部的基性岩脉群进行古地磁和地质年代学的综合研究。本研究获得的高质量、精确定年的古地磁极将填补西非及其附近Nuna和Rodinia地块构造和运动史上的重要空白，并为超大陆过渡样式的争论提供重要的基础事实。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.