The nature and origin of fine-scale sea-floor relief
Shih, John Shai-Fu
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East Pacific Rise
Bathymetric profiles of mid-ocean ridges show that the morphology of ridge crests and the roughness of sea-floor relief created there is commonly related to the spreading rate. To study the creation of fine-scale sea-floor relief at mid-ocean ridges, detailed bathymetric profiles of mid-ocean rid£es collected with the deep tow instrument package were compiled. Tectonic features were identified in order to provide an estimate of the zone over which tectonic relief is created. The results suggest the age of the sea-floor to which tectonism is active is considerably greater at the slow spreading Mid-Atlantic Ridge though not noticeably variable among faster spreading centers. The roughness of sea-floor relief was measured using longer deep tow profiles. Surprisingly, the sea-floor relief created at slow spreading centers is not noticeably rougher than that created at faster spreading centers, contrary to the often noted inverse relationship between sea-floor roughness and spreading rate. It is postulated that the apparent smooth sea-floor relief created at fast spreading centers is due to the inability of typical surface ship profiling systems to resolve the small amplitude/ short wavelength relief created there. Surface ship bathymetric profiles of mid-ocean ridges were also compiled to better define the relationships between the dimensions of median rifts or central highs at spreading centers and the roughness of sea-floor relief as seen by surface ship profiling systems with spreading rate. The measurement of ridge crest dimensions shows that though the slow spreading Mid-Atlantic Ridge commonly does have medium rifts and the fast spreading East Pacific Rise has central highs, when ridge crest dimensions are plotted versus spreading rate, no clear correlation can be seen in the individual oceans. The roughness of sea-floor relief was measured using the compiled surface ship profiles. A good inverse correlation between roughness and spreading rate can be seen in the Atlantic but not in the Pacific. The hypothesis that the roughness of sea-floor relief created at spreading centers is related to the ability of the lithosphere to support relief within the zone of relief formation was considered. The strength of the lithosphere at spreading centers was estimated from measured strengths of rocks and theoretical thermal models of the lithosphere near spreading axes. The load imposed on the lithosphere by sea-floor relief was estimated using deep tow bathymetric profiles. The calculations show the lithosphere should achieve much higher strengths within the zone of relief formation at slow spreading centers compared to fast spreading centers. Furthermore, the calculated lithospheric strengths within the zone of relief formation increase exponentially for spreading centers of lower spreading rates. This can explain why an inverse correlation between sea-floor roughness and spreading rate could be seen along the slow spreading Mid-Atlantic Ridge but not along the fast spreading East Pacific Rise. Finally, the extent of tectonism, or the width of the zone of relief formation, at spreading centers is suggested to be controlled by the width of magma chambers at faster spreading centers and the extent of viscous forces at deeply rifted slow spreading centers.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1979
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