Graduate Thesis Or Dissertation
 

Gravity and crustal structure of the south-central Gulf of Mexico, the Yucatan Peninsula, and adjacent areas from 17°30ʹN to 26°N and from 84°W to 93°W

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  • About 9600 nautical miles of marine gravity, magnetic, and bathymetric data were collected by Oregon State University during the cruise Yucatan'85 in the north and eastern margins of the Yucatan Peninsula, using a research vessel of the Mexican Navy, the Altair. A compilation of Yucatan' 85 marine data and marine and terrestrial data provided by the Defense Mapping Agency produced Gravity Anomaly and Bathymetric/Topographic Maps of the south-central Gulf of Mexico, the Yucatan Peninsula, and adjacent areas from 17°30'N to 26°N and from 840W to 930W. The Gulf of Mexico is a small ocean basin underlain by crustal structures that are significantly different from normal continents or ocean basins. Seismic refraction data, in addition to the marine gravity, magnetic, and bathymetric data, constrain a crustal and subcrustal model across the Campeche Escarpment in the south-central Gulf of Mexico. The model presented here provides an overview of the transitional structure between oceanic and continental crust in the northern Yucatan Platform region. A mantle layer of density 3.46 g/cc in the southcentral Gulf of Mexico is found to be denser than expected based on the Ludwing, Nafe and'Drake velocity-density curve, and a compressional wave velocity of 8.0 km/sec. The presence of material denser than 3.32 g/cc beneath the crustal structure of the Gulf of Mexico may be attributable to thermal contraction associated with the great age of the basin (Triassic or older), and to a denser than average lithosphere-asthenosphere substratum as indicated by a regional geoid "high" in southern Mexico and Central America. The computed crustal cross-section suggests that the Campeche Escarpment has a tectonic origin due to faulting. According to the crustal model, no extent of an attenuated or rifted continental crust (transitional crust) is evident. On the contrary, the model suggests the existence of an abrupt oceanic-continental boundary. Mapping of a pre-rift crystalline-basement high overlain by a high-density "carbonate" block (older in age than the Challenger Unit) over the north-central Yucatan Platform, makes this area a promising place for a drilling site that would recover samples of basement rocks. Direct identification of the composition and age of the crystalline-basement and the overlying "carbonate" sequence would contribute to a better understanding of the origin and evolution of the Gulf of Mexico. Large tectonic-eustatic sea level changes occurred during the early rift phase of the Gulf of Mexico. A model for the sequence of sedimentary deposition, subsidence, and opening of the Gulf which influenced the present crustal architecture of the "carbonate" and crystalline basement suggests that the "carbonate" layer may be constituted of two materials: a) a mix of carbonate sediments and detrital sediments eroded from the continent above sea level that were deposited in shallow water on both sides of the crystalline-basement "high", and b) by carbonate sediments created once the basement block completely subsided below sea level. Computed mass columns east and northwest of the Yucatan Platform indicate that a lighter density material (3.40 g/cc), as compared with that existing beneath the northcentral Yucatan Platform, must form the mantle in these basinal areas. Comparison of mass columns for wells located in the central and northeastern Yucatan Peninsula with the crustal model, indicates a difference in density between the basements forming the lower crust beneath the drilled sedimentary sequence. These differences suggest that the basement in the eastern and northern Yucatan Platform is formed of denser material than the basement in the central Yucatan Peninsula. The differences also suggest that the Yucatan Block is formed of a series of "micro-continental" blocks which encircle the central Yucatan Platform.
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  • Master files scanned at 600 ppi (256 Grayscale) using Capture Perfect 3.0 on a Canon DR-9080C in TIF format. PDF derivative scanned at 300 ppi (256 B&W, 256 Grayscale), using Capture Perfect 3.0, on a Canon DR-9080C. CVista PdfCompressor 3.1 was used for pdf compression and textual OCR.
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