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Earth deformation in response to surface loading : application to the formation of the Hawaiian ridge
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|Title:||Earth deformation in response to surface loading : application to the formation of the Hawaiian ridge|
|Authors:||Suyenaga, W (Wayne)|
Geophysics -- Hawaii
|Abstract:||The Hawaiian Ridge represents a load on the surface of the earth. According to a hypothesis, this load is sufficient to fracture the lithosphere and thereby set up a self-propagating mechanism for the extension of the ridge. The hypothesis is based on the theory of flexure of an elastic beam. In this dissertation, the hypothesis is examined in two ways. In the first study the application of flexure theory is investigated. Previous work on Hawaii was 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 which represents the difference between the basement-oceanic and Moho displacements and which effectively lessens the load due to its buoyancy The unflexed crust represents either a phase change of mantle to crustal material or intrusion into the lower crust. From the basic equations of flexure and 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. 1~en 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. The new interpretation results in a reduction in tensile stress at the base of the lithosphere but the stress still seems sufficient to cause fracture. On the other hand, second study results indicate that for long term surface loading, the lithosphere should not be modeled as a purely elastic structure. The study utilizes the finite element method and a non-linear steady state creep equation. It is assumed that any element in which creep strain is greater than the elastic strain is fluid and no longer contributes to the elastic support of the load. A set of tests on a model approximating a half space indicates that creep is initially concentrated under the load at depths of 80-250 km primarily because that is where the homologous temperature reaches its maximum, especially under wet conditions. With increasing time and load, fluid conditions spread outward revealing a lithosphere-asthenosphere structure. A second series of tests on an 80 km thick plate overlying a fluid showed that, over time periods on the order of 105-106 years, there is significant creep in the lower lithosphere. These tests lead to an alternative model of flexure of a viscous lithosphere. According to this model the flexural rigidity of the lithosphere decreases because a thinning portion of lithosphere elastically supports the load. The creep process continues until a final state is reached where the elastic portion of the lithosphere is about 10-30 km thick. Comparison with data around the Hawaiian Islands indicates that the flexural response of almost all of the load is in the final state and thus does not produce any stress at depths of 60 km, the depth of magma origin. This interpretation casts serious doubts on the hypothesis of island chain formation by fracture due to surface loading.|
Thesis (Ph. D.)--University of Hawaii at Manoa, 1977.
Bibliography: leaves 139-147.
x, 147 leaves ill., maps
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|Appears in Collections:||Ph.D. - Geology and Geophysics|