Thesis (M. Phil.)--University of Hong Kong, 2008. / Includes bibliographical references (leaf 141-151) Also available in print.

Isostatic equilibrium is commonly defined as the state achieved when there are no lateral gradients in hydrostatic pressure, and thus no lateral flow, at depth within the lower viscosity mantle that underlies a planetary body's outer crust. In a constant-gravity Cartesian framework, this definition is equivalent to the requirement that columns of equal width contain equal masses. Here we show, however, that this equivalence breaks down when the spherical geometry of the problem is taken into account. Imposing the "equal masses" requirement in a spherical geometry, as is commonly done in the literature, leads to significant lateral pressure gradients along internal equipotential surfaces and thus corresponds to a state of disequilibrium. Compared with the "equal pressures" model we present here, the equal masses model always overestimates the compensation depth-by similar to 27% in the case of the lunar highlands and by nearly a factor of 2 in the case of Enceladus. Plain Language Summary "Isostasy" is the principle that, just as an iceberg floats on the water, crustal rocks effectively float on the underlying higher density mantle, which behaves essentially like a fluid on geologic timescales. Although there are subtle inconsistencies among the various ways isostasy can be defined, they have not been historically problematic for bodies like the Earth, where the crust is thin compared with the overall radius. When the thickness of the crust is a nonnegligible fraction of a planetary body's radius, however, it becomes important to take the spherical geometry into account. In this case, the inconsistencies in the definitions can lead to significant discrepancies. Here we argue that one of the most commonly used approaches, which requires equal width columns to contain equal masses, always results in overestimating the crustal thickness. In particular, we suggest that the lunar and Martian highlands crustal thickness may have been overestimated by similar to 27% and similar to 10%, respectively, and that the ice shell thickness for Saturn's small icy moon Enceladus may have been overestimated by nearly a factor of 2.

The research presented in this thesis seeks to address meaningful geodynamic problems related to the viscosity structure of the Earth’s interior. Isostatic adjustment is a process which is dependent upon the mechanical properties of the lithosphere and mantle. By performing computational simulations of the isostatic response for various surface-loading scenarios and numerous viscosity structures, insight can be gained into the mechanical structure of the Earth and geodynamic processes related to that structure. The modelled isostatic signal for a given set of Earth model parameters can be compared to real-world observational data in order to identify valid Earth model configurations. In Chapter 2, the “Transition Zone Water Filter” theory is tested by modelling the geophysical effects of a low-viscosity melt-rich layer atop the 410 km discontinuity. The thickness and viscosity of this layer, and the surrounding mantle, is constrained using observations of relative sea level and the geodetic J ̇_2 parameter, as well as multiple ice-loading scenarios by which the isostatic adjustment process is driven. The relative sea level data, being most sensitive to the upper mantle and the theorized melt-rich layer it contained, constrain layer properties more effectively than the J ̇_2 observation, which is strongly dependent on the lower mantle. Constraints on the viscosity of the melt-rich layer vary according to thickness, with thicker layers requiring stiffer viscosities to satisfy observations. For instance, a 20 km thick layer would require a viscosity of 10^17 Pas or greater, but any of the considered viscosities could be possible for a 1 km thick layer. Similarly, a broad range of upper mantle viscosities are possible, but they must be balanced by variations in the lower mantle. However, J ̇_2 results show a strong preference for a high-viscosity lower mantle (≥10^22 Pas). For every evaluated Earth model parameter, there is evidence of ice-model sensitivity in the inversion results. Although the results of this study demonstrate that observables related to glacial isostatic adjustment can provide constraints on the properties of this theorized melt-rich layer, the confounding effect of parameter trade-off prevents a more definitive test of this model of mantle geodynamics. The purpose of the study presented in Chapter 3 is to analyze the nature of solid-Earth deformation beneath the Lower Mississippi River, most crucially in the Mississippi Delta region where subsidence is an ongoing and costly problem. The study uses the displacement of the long profile of the Lower Mississippi River over the last 80 kyr to constrain isostatic deformation and determine constraints on the mechanical structure of both the mantle and lithosphere. Deformation recorded in the northern portion of the long profile is dominated by the effect of glacial isostatic adjustment, whereas the southern portion is governed by sediment isostatic adjustment. However, the southern portion is also potentially affected by past fault displacement, and to account for this the observational data are corrected using two distinct faulting scenarios. Displacement of the long profile is modelled using either an entirely elastic lithosphere or a lithosphere with internal viscoelastic structure, the latter of which is derived from two end-member geothermal profiles. Between the elastic and viscous lithosphere models, the viscous models are better able to replicate the observational data for each faulting scenario – both of which prefer a viscous lithosphere corresponding to the warmer geotherm. The chosen faulting scenario exerts no control over the optimal mantle model configuration, however the optimal mantle for the viscous lithosphere models is much stiffer than was determined for their elastic counterparts, reflecting significant parameter trade-off between mantle and lithosphere mechanical structure. These study results demonstrate the utility of the long profile displacement data set for constraining Earth viscosity structure, as well as the importance of considering more-complex models of lithosphere mechanical structure when addressing surface-loading problems similar to those encountered in the Mississippi Delta region.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 162-176).

Thesis (doctoral)--Westfälische Wilhelms-Universität Münster, 2003. / "Dezember 2003"--P. [2] of cover. Includes bibliographical references (p. 132-139). Also available via the World Wide Web.

The dynamic environmental history and relative sea level (RSL) changes experienced on the Pacific Northwest Coast of North America during the early post-glacial period and the early Holocene resulted in significant visibility challenges for prospection of early coastal archaeological sites. Archaeological visibility is the degree to which cultural material survives post-depositional processes and is detectable on the landscape today. It is influenced by environmental factors such as localized differences in relative sea level change, the rainforest canopy and dynamic post-glacial activity. This study offers an integrated methodological approach for locating palaeo-coastal sites by combining: i) geomorphic interpretation of landscape attributes captured by LIDAR (Light Detection and Ranging) mapping, ii) GIS-based archaeological site potential mapping, and iii) local RSL history. The RSL history for the study site (Quadra Island, British Columbia, Canada) shows notable regression over the past 14 500 years from a highstand of at least 195 m resulting from post-glacial isostatic rebound. Late Pleistocene and early Holocene palaeo-shorelines are found inland from, and elevated above, modern sea level and represent key areas for archaeological prospecting. Bare-earth Digital Terrain Models (DTMs) derived from the LIDAR dataset were interpreted to identify palaeo-shorelines at 10 m and 30 m above modern mean sea level. A GIS-derived map was created to identify regions of high archaeological potential using a decision tree method with variables including distance to palaeo-shoreline, low slope and a coastal complexity parameter. Select geoarchaeological sites were examined in terms of sedimentology, stratigraphy, microfossil content and geochronology as site-specific examples of sea level regression stillstands. Field validation results suggest that this integrated methodology provides a promising approach for archaeological prospection that could be applied to other post-glacial coastal settings. / Graduate

No description available.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987. / Vita. / Includes bibliographical references. / Gravity derived from Seasat altimetry has provided a means of estimating seafloor topography and its compensation, which in turn can be used to understand the evolution of oceanic lithosphere. In the first study, the correlation between the geoid, deflection of the vertical, and seafloor topography is investigated along a section of the Southwest Indian Ridge. Geoid anomalies computed from a simple thermal model fairly accurately predict the intermediate-wavelength anomalies across the fracture zones. The shorter wavelength anomalies are consistent with those calculated from topography using elastic plate compensation. The combined effect of the thermal offset and seafloor topography produces an anomaly which has a small-amplitude, short-wavelength depression directly over the fracture zone valley. Pronounced lineations in the horizontal geoid gradient do not coincide with the valley but have trends parallel to the fracture zones. In the second study, fracture zones along the Southwest Indian Ridge are identified using altimeter profiles and bathymetry. Finite poles of rotation are determined from the fracture zone locations and magnetic anomaly lineations for anomalies 6 (20 Ma), 13 (37 Ma), and 20 (45 Ma). The new poles are in general agreement with previously published poles and describe a fairly consistent direction of relative motion between Africa and Antarctica for the past 45 Myr. A present-day pole of rotation calculated from transform fault azimuths determined primarily from their geoid anomalies, agrees with published poles based on bathymetric data. In the third study, the rate of change of the geoid with age has been estimated as a function of age from geoid offsets across the Eltanin and Udintsev fracture zones and used to constrain thermal models of lithospheric cooling. Observed trends in the geoid slope versus age plots are similar on both branches of the Eltanin and the east limb of the Udintsev fracture zone. The similarity in trends argues against the effects of isolated thermal or bathymetric anomalies and appears instead to reflect a general feature of the geoid-slope versus average age relationship across fracture zones. Although the thermal plate cooling model is successful in predicting both seafloor depths and heat flow values out to ages of at least 80 m.y. B.P., it cannot explain the observed geoid slope values for these two fracture zones. It is not clear at this point whether this is due to inadequacies in the cooling model or to peculiarities in fracture zone evolution. / by Mavis Lynn Driscoll. / Ph.D.

No description available.

published_or_final_version / Earth Sciences / Master / Master of Philosophy