Thesis (M.A.)--United States International University, 1973. / Facsimile reproduction by microfilm-xerography. Includes bibliographical references (leaves [221]-226).

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.S.)--Oregon State University, 1982. / Typescript (photocopy). Includes bibliographical references (leaves 89-93). Also available on the World Wide Web.

This dissertation examines the shrimp industry in the Gulf of California from a political ecology perspective. The interaction between fishermen and their marine environment is explored, as well as the historical factors that led to vastly different types of fishermen in the communities of Guaymas and Empalme. Some have specialized in the harvesting of shrimp; others are diversified, multiple species fishermen. Some are highly industrialized offshore shrimpers; others are small-scale fishermen, more modest in their technology but more resilient when facing the current crisis in the shrimp industry. The underlaying causes of this crisis are explored by looking at state development policies, the assumptions behind fisheries management, and the configuration of markets. These have all emphasized specialization in the production of shrimp while ignoring the high interannual variability characteristic of shrimp populations. The end result: an overcapitalized, overexpanded industry and a possible overexploitation of shrimp stocks. Rather than addressing the root causes of the crisis, recent policies have instead transferred rights to the offshore fishery from cooperatives to private investors. At the same time there has been a concerted attack against small-scale producers. It is believed that by getting rid of this sector, catch per boat in the offshore sector will increase and overall "efficiency" will be improved. I compare industrialized trawlers and the small-scale sector and argue that the latter is currently producing high quality shrimp at lower monetary and ecological costs. But small-scale fishing is not equated with sustainability. Instead, differences among small-scale fishermen are analyzed. I contend that those who belong to traditional fishing families and have access to collective knowledge about the marine environment that has accumulated through generations, are better able to deal with a highly unpredictable environment and minimize risk. Those who do not have access to this knowledge have specialized in the harvesting of shrimp. I argue that a strategy of diversification is both more profitable in the short-term and sustainable in the long-run. Avoidance strategies among small-scale fishermen in response to externally imposed regulations are also examined. Fishermen are analyzed as individual profit maximizers and as community members who break the rules to serve collective interests. Just as individuals act collectively to deal with an unpredictable environment, they also act collectively to effectively challenge the institutions of rule-making.

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

No description available.

Graduation date: 1988

Graduation date: 1984

No description available.

Using gravity, magnetic, bathymetric and seismic refraction data, I have constructed a geophysical cross-section of the central part of the northern Gulf of California. The section exhibits a crustal thickness of 18 km and features an anomalous block of high density lower basement (3.15 g/cm³) which probably resulted from rifting processes during the opening of the Gulf. The magnetization of the upper basement ranges from 0.0005 to 0.0030 emu/cm³. Three different layers of sediments are modeled, ranging from unconsolidated (1.85 g/cm³) to compacted (2.50 g/cm³). I present a deconvolution method for automated interpretation of magnetic profiles based on Werner's (1953) simplified thin-dike assumption, leading to the linearization of complex nonlinear magnetic problems. The method is expanded by the fact that the horizontal gradient of the total field caused by the edge of a thick interface body is equivalent to the total field of a thin dike. Statistical decision making and a seven point operator are used to insure good approximations of susceptibility, dip, depth, and horizontal location of the source. After using synthetic models to test the inversion method, I applied it to the Northern Gulf of California using data collected in 1984 by the Continental Margins Study Group at Oregon State University. Fault traces, computed by the deconvolution, are plotted on a map. The faulting pattern obtained is in good agreement with that proposed by other workers using other methods. The depths to the top of the faults range from 4 to 5 km in the eastern part of the Gulf, where they may be interpreted as the top of the structural basement. Deeper estimates are obtained for the western part of the Gulf. / Graduation date: 1990