Collaborative Research: Characterizing Quaternary Fault Behavior and Surface Processes of an Active Rift: The Lake Malawi (Nyasa) Rift, East Africa

海洋盆地形成的最早阶段包括大陆的分裂。对大陆裂谷的研究对于了解自然资源的危害和潜力至关重要。裂谷的过程涉及到活跃的断层、火山活动和相关的地震，并与重大的灾害联系在一起。裂谷作用还导致盆地的形成，这些盆地可以捕获沉积物并容纳石油和天然气矿床。大陆裂谷系统的一个基本问题是裂谷如何分布在不同的断层系统中，以及地表过程（包括沉积物供应、侵蚀和湖泊水位的变化）如何影响断层行为。这些早期地质构造作为不同过程的记录者，但经过多年的海底扩张，通常被埋在厚厚的海洋沉积物之下，因此成熟的大陆边缘和海洋盆地是研究早期裂陷过程的困难地点。另一方面，活跃的大陆裂谷，在那里大陆处于分裂的早期阶段，揭示出与裂谷有关的裸露和/或浅层地质构造和矿床。这项研究考虑了东非裂谷的一部分，几乎完全被淡水淹没（马拉维湖裂谷），在那里，非常高质量和相对低成本的海洋地球物理方法可以用来成像裂谷的底部及其地下。通过收集非常高分辨率的地球物理图像，结合以前的学术钻探和数值模拟的信息，我们将重建过去数十万年的断层几何形状和滑动速率，评估特定断层活跃的控制因素，并估计过去的地震活动。这项研究的结果将使人们更好地理解裂谷断层是如何保存的，以及它们如何在被动的大陆边缘被重新激活，例如在北美东海岸，它是由北美从非洲裂谷形成的。此外，这项研究还将提供有关裂谷断层如何将沉积物流重定向到裂谷的信息，并有助于了解裂谷系统中的石油和天然气发生情况，以及气候驱动的地表过程可能影响断层和地震活动的作用。项目期间将培养3名博士生和1名博士后。我们的国际团队由美国居民和东非国民组成，其中包括两名在锡拉丘兹大学（Syracuse University）入学的东非人，我们预计实地团队中男女研究人员的比例大致相等。我们将在马拉维的高等教育机构和中学开展广泛的外联项目，并在马拉维发展博物馆展览。我们将与马拉维的科学同事进行深入互动，并与马拉维渔业和古物部门合作，向主要利益攸关方分享新的数据集。我们将对东非裂谷系西部分支马拉维湖裂谷活动断裂带的第四纪变形和地表过程进行综合观测、分析和数值模拟研究。该系统是世界上最大的活动裂谷之一，是弱岩浆系统中早期低应变速率裂谷的象征。我们将使用强大的海洋型高分辨率地震和回声测深工具，在几个不同的结构设置中进行高保真的观测。利用多波束回声测深数据和高分辨率CHIRP地震反射数据，我们将确定最近（过去25-100 ka）内断层的变形是如何分布在整个裂谷上的，并评估内断层是如何沿着它们的长度传播的，然后随着时间的推移联系在一起的。我们将生成不同构造背景下的一系列断裂带的时间位移剖面（例如，弯曲断层边缘与边界断层边缘）；生成不同断层系统的断层长度-位移关系库；并使用经典的标度定律来估计过去100万年的地震记录。断层位移记录将受到来自马拉维湖钻井项目的超定年、高分辨率同裂谷地层学的时间限制，从钻井岩心扩展到CHIRP地震反射数据的新网格。新的断层位移历史将与已知的高分辨率水文气候记录进行比较，以确定气候制度的变化是否会调节裂谷断层的活动。我们的数值模拟工作将评估与裂缝上的水和沉积物负荷以及侵蚀变化相关的应力状态，并将与观测结果进行比较。新的湖底测深数据将揭示马拉维裂谷中构造控制的沉积物扩散路径是如何发展的，从而更好地理解裂谷-湖泊系统中的盆地填充过程。我们的研究结果将为过去的地震历史提供独特的新约束，并为概率地震危害评估提供基础。本项目资金由NSF EAR构造和地球物理项目提供。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。

The earliest phases of the formation of ocean basins involve the break-up of the continents. The study of continental rifts is essential for understanding hazards and potential for natural resources. The process of rifting involves active faulting, volcanic activity and associated earthquakes, and are linked with significant hazards. Rifting also leads to the formation of basins that trap sediment and can host oil and gas deposits. A fundamental question in continental rift systems is how rifting is distributed across different fault systems and how surface processes, including changes in sediment supply, erosion and lake levels may impact fault behavior. These early phase geological structures act as recorders of the different processes, but after many years of seafloor spreading, are usually buried beneath thick packages of ocean sediment, and thus mature continental margins, and ocean basins, are difficult places to study early-rifting processes. On the other hand, active continental rifts, where continents are in the early stages of break-up, reveal exposed and/or shallow geological structures and deposits related to rifting. This study considers a part of the East African Rift Valley that is nearly fully flooded with freshwater (the Lake Malawi Rift), where very high quality and relatively low cost marine geophysical methods can be used to image the floor of the rift and its subsurface. By collecting very high resolution geophysical images and integrating with information from previous academic drilling and with numerical modeling, we will reconstruct fault geometries and slip rates over the last hundreds of thousands of years, evaluate controls on why particular faults are active, and estimate past earthquake activity. Results of the study will allow for a better understanding of how rift faults may be preserved and how they can be reactivated on passive continental margins, for instance on the East Coast of North America that was formed by the rifting of North America from Africa. In addition, this study will also provide information on how rift faults redirect flows of sediment into a rift valley and assist in understanding oil and gas occurrences in rift systems as well as the role of climate-driven surface processes that may influence faulting and earthquake activity. We will train 3 doctoral students and a post-doctoral scholar during the project. Our international team consists of both U.S. residents and East African nationals, including two of the latter matriculated at Syracuse University, and we anticipate roughly equal representation by female and male researchers on the field team. We will execute an extensive outreach program in Malawi, both at institutions of Higher Education and in secondary schools, and develop museum displays in Malawi. We will interact deeply with our Malawian scientific colleagues, and also engage with the Malawi Departments of Fisheries and Antiquities to share new data sets to key stakeholders.We will undertake a combined observational, analytical and numerical modeling study of Quaternary deformation and surface processes associated with a series of active fault zones in the active Lake Malawi Rift, in the western branch of the East African Rift System. This system is one of the largest active rifts in the world and is emblematic of an early-phase, low strain rate rift in a weakly magmatic system. We will use robust marine-type high-resolution seismic and echosounder tools to make high-fidelity observations across several different structural settings. Using new offshore observations from multibeam echosounder data and high-resolution CHIRP seismic reflection data, we will determine how recent (last 25-100 ka) deformation on intrarift faults is distributed across the rift and assess how intrarift faults have propagated along their length and then linked together over time. We will generate time-displacement profiles across a series of fault zones in different structural settings (e.g., flexural vs. border fault margin settings); generate a library of fault length-displacement relationships for the different fault systems; and use classical scaling laws to estimate records of past earthquakes over the past 100 ka. The fault displacement records will be chronologically constrained by the superbly-dated, high-resolution syn-rift stratigraphy from the Lake Malawi Drilling Program, extended from the drill cores into the new grids of CHIRP seismic reflection data. The new fault displacement histories will be compared to known high-resolution hydroclimate records to determine if changes in climate regimes modulate rift fault activity. Our numerical modeling efforts will assess the stress regimes associated with changes in water and sediment loading and erosion across the rift, which will be compared with observations. New lake floor bathymetric data will reveal how structurally-controlled sediment dispersal paths have developed in the Malawi Rift, leading to a better understanding basin-filling processes in rift-lake systems. Results from our study will provide unique new constraints on past earthquake histories and provide the basis for probabilistic seismic hazard assessments.Funding for this project is provided by NSF EAR Tectonics and Geophysics Programs.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.