Marine magnetic data from the northern Yucatan continental margin were used to obtain the radially-averaged power-density spectra (RAPDS) of 34 grids, each with dimensions of 128 by 128 km, overlapping 50 percent. Depths to the tops of three magnetic horizons were estimated from the slopes of linear segments in the spectra. The depth to the base of the magnetic crust was estimated using an implicit relationship between the deepest depth estimation and the spectral peak position. The depth determinations agree with the limited drillhole data available and show some consistency with seismic refraction interpretations for the study area. Results of similar studies, based on the spectral analysis of magnetic anomalies in different tectonic settings, seem to agree with other geophysical and geologic data. In this area however, the limited depth of resolution achieved by seismic methods, and the absence of deep drillhole data and heat-flow measurements, makes it difficult to assess the validity of this interpretation. I therefore examined the simplifications implicit in the data processing techniques commonly applied in the spectral analysis of magnetic anomalies, and the assumptions upon which the method is based. Finally, I tested the method itself by interpreting synthetic anomalies generated from model source bodies. I found that depth interpretations, based upon assuming that the slope of linear segments in the RAPDS are proportional to the average depth to the top of magnetic horizons, and thickness determinations using the position of the spectral peak, are unreliable. The problem has been oversimplified. I also show that the RAPDS is independent of the direction of both the geomagnetic field and the magnetization vector. It can be represented by a Functional in terms of depth to the top of the source, its thickness and its horizontal dimensions. The problem of interpreting the RAPDS was then formulated as an inverse problem. A solution was obtained through minimizing, iteratively, the sum of squares of residuals between a real-data spectrum and a synthetic spectrum. The minimization was based on a linearized model, using the ridge-regression algorithm. This technique provides acceptable solutions for synthetic anomalies produced by model source bodies. Depth and thickness determinations, obtained using this technique on the Yucatan spectra, were used to make contour maps of the average depths to the top and the base of the magnetic crust. The top of the magnetic crust is at an average depth of 1.8 km. The base of the magnetic crust is at an average depth of 25 km. Beneath the central Campeche Bank and Campeche Terrace the depth to the base is close to the depth to the crust-mantle boundary determined by gravity modeling and mass column analysis. This suggests that in those regions the mantle is the lower magnetic boundary. / Graduation date: 1991

Anomaly timescales for the last 90 million years, derived from marine magnetic profiles and published prior to mid-1979, are summarized, illustrated for comparison, and critically reviewed. A revised timescale is constructed using calibration points which fix the ages of anomalies 2.3', 5.5, 24, and 29. An equation is presented for converting K-Ar dates that is consistent with the recent adoption of new decay and abundance constants. The calibration points used in the revised timescale, named NLC-80, are so converted, as are the boundary ages of geologic epochs within the range of the timescale. NLC-80 is then used, along with recently acquired and rigorously navigated underway geophysical data from the region of the mouth of the Gulf of California, to prepare detailed bathymetric, gravimetric, and seismo-tectonic maps of the area. The basement ages at DSDP Leg 63 drilling sites 471, 472, and 473 are estimated from magnetic anomalies fit to timescale NLC-80. The estimates agree with biostratigraphically determined basement ages and support the proposal that an aborted ridge of about 14 MY age has left a small fragment of the Farallon Plate beneath the Magdalena Fan. Several large inactive faults are identified on the deep-sea floor west of the tip of the peninsula of Baja California. Additional magnetic anomaly profiles and bathymetric profiles across the Rivera Ridge are interpreted. These contradict the existence of a 3.5 MY old aborted spreading center on the Maria Magdalena Rise. Instead, it is proposed that an episode of subduction of the Pacific Plate beneath the southeastern tip of Baja California, concomitant with strike-slip faulting west of the peninsula, occurred and that this subduction may be responsible for the uncentered location of the Rivera Ridge within the mouth of the Gulf of California. A single magnetic anomaly profile obtained northeast of the Tamayo Fracture Zone is used to determine that the rate of Pacific/North American plate motion, for the last 3 MY is 68 km/MY at this location. This result, if correct, indicates that the peninsula of Baja California is separating from mainland Mexico faster than the Rivera Ridge is generating oceanic crust in the wake of opening in the gulf. This, in turn, requires that either slow diffuse extension is occurring presently across the Maria Magdalena Rise, or across the Cabo Corrientes-Colima region, or that the portion of North America south of the trans-Mexican volcanic belt is moving right-slip with respect to the North American Plate at a rate of 10-20 km/MY. Large horsts and many smaller continental fragments are found within the southern gulf. Several of them have active seismic boundaries, while others have apparently foundered. The gulf began to open approximately 14-15 MY ago with slow, diffuse block-faulting and the deposition of the Maria Magdalena Fan at the mouth of the gulf. Oceanic crust was exposed in the gulf by about 9-10 MY, at the same time that the Rivera Ridge began reorienting by clockwise rotation. Strike-slip motion along the Tosco-Abreojos Fault took up some of the Pacific/North American motion with the remainder occurring within the gulf itself. During this period the Pacific Plate forming within the gulf was slowly subducting beneath Baja California. By 4-5 MY subduction ceased and all of the Pacific/North American plate motion was shifted to the Gulf of California fault system. The gulf and peninsula of California are still in the process of adjusting to the change from Pacific/Farallon to Pacific/North American motion. / Graduation date: 1982

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

Thesis (M.Sc.) -- Memorial University of Newfoundland. / Typescript. Bibliography : leaves 145-149. Also available online.

Thesis (M.A.)--University of Wisconsin--Madison, 1963. / eContent provider-neutral record in process. Description based on print version record. Bibliography: l. 75-76.

The research shows a novel approach for the Magnetic Anomaly Differentiation and Localization Algorithm, which simultaneously localizes multiple magnetic anomalies with weak total field signatures (tens of nT). In particular, it focuses on the case where there are two homogeneous targets with known magnetic moments. This was done by analyzing the magnetic signals and adapting Independent Component Analysis (ICA) and Simulated Annealing (SA) to solve the problem statement. The results show the groundwork for using a combination of fastICA and SA to give localization errors of 3 meters or less per target in simulation and achieved a 58% success rate. Experimental results experienced additional errors due to the effects of magnetic background, unknown magnetic moments, and navigation error. While one target was localized within 3 meters, only the latest experimental run showed the second target approaching the localization specification. This highlighted the need for higher signal-to-noise ratio and equipment with better navigational accuracy. The data analysis was used to provide recommendations on the needed equipment to minimize observed errors and improve algorithm success. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2020. / FAU Electronic Theses and Dissertations Collection

No description available.

The continental margin off the coast of Pakistan between the Murray ridge and the Gulf of Cambay has been studied in this work using gravity, magnetic and bathymetric data. Two dimensional gravity and magnetic models based on free-air gravity and residual magnetic data are developed along a north-south profile off the coast of Karachi. The purpose was to interpret the gross crustal structure of the region. A magnetic map has also been developed for the region between latitudes 20°N and 27°N and between longitude 60°E and 70°E. The gravity model extends to a distance of about 1200 km seaward south of Karachi. The seaward end of the gravity model is constrained by seismic refraction data which suggest the presence of typical oceanic crust. The Moho depth at this end of the profile is about 12 km. At the landward end of the profile A-A' the Moho depth is not constrained by seismic data. The gravity model suggests 27 to 17 km as the possible range of the depth of the Moho and a gradual thinning of the crust from land to sea. In addition, the gravity models as interpreted in this study show grabens at the distances of 350 and 450 km along the profile. If the graben-like structures are rift grabens formed during the rifting of India from Africa then transitional crust can be expected to extend to the 500 km mark along the profile A-A'. Two dimensional models for the magnetic data along the profile were also developed. These anomalies can be interpreted as due to oceanic crust or magnetic bodies embedded in transitional crust. The possibility that the observed magnetic anomalies are due to oceanic crust has been studied in detail in this work. The location of the observed magnetic anomalies with respect to marine magnetic anomaly (28) observed earlier on the Indian Ocean floor, were compared to a marine magnetic time scale. To get a reasonable correlation between the observed and theoretical anomalies requires a considerable amount of adjustment in the spreading rate of individual magnetic blocks. Also on the magnetic map the trend of the lineation of these anomalies is perpendicular to the continental margin instead of being parallel to the continental margin as expected for a rifted continental margin. The presence of horst-and-graben structures in the inland region suggests the rifted nature for the continental margin off Karachi than the sheared nature. This indicates that the lineations should be parallel to the margin. But the magnetic lineations are perpendicular to the continental margin and if they are from oceanic crust then they would suggest that the margin is a sheared margin, which contradicts the extensional structures observed inland. This makes it very unlikely that the source of these anomalies is oceanic crust. However, it is quite possible that the magnetic lineations observed in the map were parallel to the continental margin initially but later on the continent rotated clockwise along a fault landward of the magnetic lineation. This rotation is perhaps responsible for making the lineation perpendicular to the continental margin. One objective of this study was to locate the continent-ocean boundary, but with the available amount of data it is not possible to decide on the most appropriate source for the observed magnetic anomalies. Hence it was not possible to decide exactly on the location of continent-ocean boundary. However, on the basis of gravity and magnetic data it can be said that the continent-ocean boundary lies at a distance of 500 km or greater along the profile. / Graduation date: 1992

Geophysical surveys in the Gulf of California provided the data to construct contour maps of bathymetry, free-air anomalies and total field magnetic anomalies for the area north of 27° N. Major faults such as the Ballenas- Salsipuedes, Tiburón, Guaymas, and the South Cerro Prieto are clearly observable on these maps. Spectral analysis, using 2-D Fast Fourier Transform methods, of the magnetic anomalies north of 29° N, allowed the identification of at least three distinct magnetic source horizons. The shallowest depth magnetic horizon, with an average depth to the top of 3.1 km below sea level, is interpreted as the top of the magnetic basement. The intermediate depth magnetic horizon, with an average depth to the top of 5.3 km below sea level, may represent either a lithological discontinuity in continental crust, or a transition zone characterized by the intrusion of igneous rocks, faulting, and fracturing associated with rifling processes. Some lineaments observed in the contour map on this horizon are oriented about 15° counterclockwise from the expected orientation of faults. Other lineaments are almost perpendicular to those faults. The deepest magnetic horizon is not apparent at all locations. Computed depths to the bottom of the magnetized crust average 11.5 km below sea level. The depth to the bottom of the magnetic crust is interpreted as the depth of the Curie-point isotherm. Assuming a Curie-point temperature of 580°C and a thermal conductivity of 2.2 W/m °C, the calculated heat flow averages 114 mW/m². Using a two-dimensional Maximum Entropy Method (2DMEM) to obtain the power spectrum of the magnetic anomalies increased the horizontal spatial resolution of the depth determinations by a factor of 4. This method when used to compute the depth to the top of the intermediate horizon, shows an improvement in the delineation of structures. However, the other magnetic horizons and the depth to the bottom of the magnetic crust were not clearly observable using this technique. / Graduation date: 1988

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