Global Systematics of Mid-Ocean Ridge Morphology

Abstract

Global mid-ocean ridge morphology is characterized by a dichotomy in which slow spreading centers have discontinuous axial valleys and rugged flanking morphology while fast spreading centers have more continuous axial rises and smoother flanking morphology. This study investigates the relationship between axial and flanking morphology and the nature of the dichotomy. Both axial and flanking morphology show similar variations with spreading rate but the transition between axial valley and axial rise morphology suggests that the amplitude of the axial morphology is not the sole determinant of flanking roughness. At slower spreading rates, ridge flank roughness is moderately correlated with axial valley relief, but at intermediate spreading rates significant seafloor deformation can occur without the formation of an axial rise or axial valley. At faster spreading rates, well developed axial rises have smoother flanking morphology and show no correlation between axial relief and flanking roughness. This reflects a fundamental difference in the mechanisms of lithospheric deformation as well as the importance of extrusive volcanism at high spreading rates and thermal environments where an axial magma chamber can be maintained. Axial valley spreading centers exhibit a spreading rate dependence in both amplitude and variability of axial relief, asymmetry, flanking roughness and depth but spreading centers'with axial rises show much less variability and no spreading rate dependence for any observable above intermediate (∼70 km/Ma) spreading rates. The variability of axial valley environments at a given spreading rate results primarily from intrasegment structure but may also reflect temporal episodicity of magmatic emplacement and extension. The dichotomy, which is also manifest in the subsurface structure of spreading centers, suggests a transition from episodicity and rate dependence at slow spreading rates to a metastable mode of lithospheric accretion at faster spreading rates. The most plausible mechanism for the transition is a stabilization of mid-ocean ridge thermal structure above a critical spreading rate.