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

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

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.