Strength of the Oceanic Lower Crust: New Experimental and Microstructural Constraints

新的地壳在洋中脊形成。在缓慢扩张的山脊上，大型拆离断层充当地质传送带，将岩石从下地壳和上地幔向上移动到海底。关于海洋下地壳是强还是弱一直存在争论。了解下地壳的强度对于了解剥离断层的形成和演化是很重要的。该项目将使用来自西南印度脊分离断层的岩石样本来测量下地壳的强度。项目调查人员将在高温高压下从地壳下部挤压矿物，以确定其与其他性质的关系。这些结果将应用于西南印度脊的岩石样品，以研究该地区滑脱断裂的演化和起源。更广泛的影响包括与布朗大学和波士顿学院的研究人员建立新的合作关系，并为本科生和一名博士后研究员提供专业发展机会。拆离断层在海洋盆地的形成中起着中心作用，因此在全球板块构造的运作中也起着中心作用。尽管如此，关于海洋地壳在流变学上是强还是弱的基本问题仍然存在。该项目将揭示一个具有良好特征的海洋滑脱断层系统——亚特兰蒂斯海岸、西南印度海脊——的热力学状态，其最终目标是限制控制滑脱断层成核、生长、空间分布和寿命的流变过程。在此过程中，该项目将调和长期以来关于海洋下地壳强度的相互矛盾的观点。高温高压变形实验将使用布朗大学的三轴格里格斯仪器对斜长石和斜辉石的单矿物聚集体进行。变形的实验样品将通过电子背散射衍射进行分析，以测量亚晶粒尺寸和再结晶晶粒尺寸作为施加应力的函数，从而校准可用于估计亚特兰蒂斯滩（ODP孔735B）辉长岩糜棱岩岩心样品所支持的应力的压力测量关系。同时，通过主元素和稀土元素交换测温，分别利用电子探针和LA-ICP-MS分析估算了735B孔岩心样品的同步变形温度。流变学和地球化学结果最终将与波士顿学院的研究人员合作，用于基准测试和开发剥离断层成核和生长的有限元模型。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。

New crust forms at mid-ocean ridges. At slow-spreading ridges large detachment faults act as a geologic conveyor belt moving rocks from the lower crust and upper mantle upwards towards the seafloor. There is a long-standing debate about whether the oceanic lower crust is strong or weak. Understanding the strength of the lower crust is important for understanding how detachment faults form and evolve. This project will measure the strength of the lower crust using rock samples from a detachment fault at the Southwest Indian Ridge. Project investigators will squeeze minerals from the lower crust at high temperatures and high pressures to determine a relationship with other properties. These results will be applied to rock samples from the Southwest Indian Ridge to investigate the evolution and origin of detachment faulting at this location. Broader impacts include establishing new collaborations with researchers at Brown University and Boston College and providing professional growth opportunities for undergraduate guest students and one postdoctoral investigator.Detachment faults play a central role in the formation of ocean basins and, therefore, in the operation of global plate tectonics. Despite this, fundamental questions remain over whether the oceanic crust is rheologically strong or weak. This project will shed light on the thermomechanical state of a well-characterized oceanic detachment fault system—Atlantis Bank, Southwest Indian Ridge—with the ultimate goal of constraining the rheological processes that govern detachment fault nucleation, growth, spatial distribution, and longevity. Along the way, the project stands to reconcile long-conflicting views over the strength of the oceanic lower crust. High-temperature, high-pressure deformation experiments will be performed on monomineralic aggregates of plagioclase and clinopyroxene using a triaxial Griggs apparatus at Brown University. The deformed experimental samples will then be analyzed via electron backscatter diffraction to measure subgrain size and recrystallized grain sizes as a function of applied stress, and thereby calibrate piezometric relationships that can be used to estimate the stresses supported by gabbro mylonite core samples from Atlantis Bank (ODP Hole 735B). Meanwhile, syn-deformation temperatures will be estimated for the same Hole 735B core samples via major element and rare-Earth element exchange thermometry, using electron microprobe and LA-ICP-MS analyses, respectively. Rheological and geochemical results will ultimately be used to benchmark and develop finite-element models of detachment fault nucleation and growth in collaboration with researchers at Boston College.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.