Determining the Strength of the Oceanic Lower Crust: a Geochemical, Microstructural, and Rheological Investigation of ODP Hole 735B, Southwest Indian Ridge

At slow- and ultraslow-spreading ridges, detachment-style faulting, rather than magma upwelling, is the dominant mode of sea-floor spreading. Numerical models suggest that the strength of the oceanic lower crust exerts a key control over the nucleation and evolution of detachment faults during mid-ocean spreading. However, to date, few direct constraints have been placed on the strength of the oceanic lower crust. Thus, it is difficult to draw definitive conclusions about the behavior of these faults through time and space. To better understand the nature of detachment-style faulting at slow- and ultraslow-spreading ridges, quantitative constraints on the strength of gabbroic rocks, which make up the bulk of the oceanic lower crust, are needed.

In this study we investigate the geochemical, microstructural, and rheological properties of gabbros from Ocean Drilling Program hole 735B, a 1.5-km long, near-continuous section through the oceanic lower crust at the Atlantis Bank oceanic core complex in the Southwest Indian Ridge. The presence of numerous ductile shear zones along the core suggests that crystal plastic deformation aided detachment faulting and extension. Compositional data from LA-ICP-MS and electron probe microanalysis have been used to constrain syn-deformation temperatures, based on clinopyroxene-plagioclase rare-earth-element thermometry and amphibole-plagioclase major-element thermometry, respectively. Meanwhile, subgrain sizes-which inversely correlate with differential stress-have been measured via electron backscatter diffraction. Together, stress and temperature data are used in conjunction with experimentally-derived rheological "flow laws" to constrain the strength—that is, viscosity—of the Hole 735B gabbros. Our results place new, quantitative constraints on the strength of the oceanic lower crust, and provide a basis for benchmarking numerical models of detachment-fault nucleation and evolution during mid-ocean spreading.