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Isostasy and flexure of the lithosphere under the Hawaiian Islands

Previous flexural studies on Hawaii were done under the assumption that the amount of deflection caused by the loading of the islands is equivalent to the change in Moho depth. Crustal profiles based on seismic refraction suggest an alternative measurement of deflection as the change in depth of the boundary between the basement and oceanic layers. This alternative implies the existence of an unflexed portion of the crust that represents the difference between the basement-oceanic and Moho displacements and that effectively lessens the load due to its buoyancy. This material represents either a phase change of mantle to crustal material or intrusion into the lower crust. From the basic equations of flexure and Airy isostasy a more general form of equilibrium equation is developed and can be interpreted as an addition of an elastic flexure term to the isostatic equations or an addition of a term accounting for changes in the crustal column to the presently used flexure equations. The study of simple models indicates that the most reliable parameter in estimating flexural rigidity is the wavelength of the flexure. When elastic flexure and buoyancy of added crust are considered in relation to the Hawaiian Ridge at Oahu, a plausible case is made for the presence of both. A flexural rigidity of about 1.2×1023 N m for an unfractured lithosphere is derived for the Hawaiian Islands; this value agrees favorably with results of other studies.