A GLOBAL ANALYSIS OF MIDOCEAN RIDGE AXIAL TOPOGRAPHY

Current views of mid-ocean ridges are strongly influenced by extensive mapping of the Mid-Atlantic Ridge and East Pacific Rise. The global p icture of the mid-ocean ridge system, particularly in the sparsely sur veyed Southern Oceans, is still based primarily on underway bathymetry profiles collected over the past 40 years. This study presents a quan titative analysis of global mid-ocean ridge morphology based on 156 of these underway bathymetric profiles, thereby allowing commonly recogn ized features such as axial valleys and axial ridges to be compared on a global basis. An Empirical Orthogonal Function (EOF) analysis is us ed to separate deterministic and stochastic components of axial morpho logy and to quantify the dependence of each on parameters such as spre ading rate and axial depth. It is found that approximately 50 per cent of the variance in axial morphology may be described as a linear comb ination of five independent symmetric and anti-symmetric modes; the re mainder is considered stochastic. Maximum axial valley relief decrease s with spreading rate for rates less than 80 mm yr(-1) while axial rid ge relief remains relatively constant for all rates greater than 50 mm yr(-1). The stochastic component of the axial morphology, referred to as bathymetric roughness, also decreases with spreading rate for rate s less than 80 mm yr(-1) and remains relatively constant at higher rat es. Although both axial valley relief and bathymetric roughness near t he ridge axis show a similar spreading rate dependence, they are weakl y correlated at slow spreading centres. The distinct differences in mo rphologic variability of fast and slow spreading ridges may result fro m the episodicity of magmatic heat input which controls the lithospher ic theology at slow spreading ridges. These observations support the n otion of a critical threshold separating two dynamically distinct mode s of lithospheric accretion on mid-ocean ridges.