Experimental and Observational Constraints on Deformation, Seismicity, and Fluid Transport at Oceanic Spreading Centers

Brian J. deMartin, Ph.D., 2007
J. Gregory Hirth, Advisor

Oceanic spreading centers are sites of magmatic, tectonic, and hydrothermal processes. In this thesis I present experimental and seismological constraints on the evolution of these complex regions of focused crustal accretion and extension. Experimental results from drained, triaxial deformation experiments on partially molten olivine reveal that melt extraction rates are linearly dependent on effective mean stress when the effective mean stress is low and non-linearly dependent on effective mean stress when it is high. Microearthquakes recorded along the TAG segment of the Mid-Atlantic Ridge delineate for the first time the subsurface structure of an active detachment fault. This fault penetrates the oceanic crust and forms the high-permeability pathway sustaining high-temperature hydrothermal venting. Long-lived detachment faulting exhumes lower crustal and mantle rocks. Residual stresses generated by thermal expansion anisotropy and mismatch in the uplifting, cooling rock can trigger grain-boundary microfractures. Experimental results coupled with geomechanical models indicate pervasive grain boundary cracking occurs in mantle peridotite when it is uplifted to within 4 km of the seafloor. Whereas faults provide high-permeability pathways necessary to maintain high-temperature hydrothermal circulation, grain boundary cracks form the interconnected network required for pervasive alteration. This thesis provides fundamental constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers.