Structure of the Panama Basin from marine gravity data Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/dv13zx61s

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  • In order to quantitatively examine the crustal structure of the Panama Basin without the benefit of local seismic refraction data, the following assumptions were made: (1) No significant lateral changes in density take place below a depth of 50 km. (2) The densities of the crustal layers are those of a 50-km standard section derived by averaging the results of 11 seismic refraction stations located in normal oceanic crust 10 to 40 million years (m. y. ) in age. (3) The density of the upper mantle is constant to a depth of SO km. (4) The thickness of the oceanic layer is normal in that region of the basin undergoing active spreading, exclusive of aseismic ridges. (5) The thickness of the transition layer is 1. 1 kin everywhere in the basin. Subject to these assumptions, the following conclusions are drawn from the available gravity, bathymetry, and sediment-thickness data: (1) Structurally, the aseismic ridges are surprisingly similar, characterized by a blocky, horst-like profile, an average depth of less than 2 km, an average depth to the Mohorovicic discontinuity of 17 km, and an average free-air anomaly of greater than +20 mgal. The fact that their associated free-air anomalies increase from near zero at their seaward ends to greater than +40 mgal at their landward ends suggests that the Cocos and Carnegie ridges are uplifted at their landward ends by lithospheric bending. (2) The centers of sea-floor spreading and fracture zones are characterized by a shoaling of the bottom and an apparent deepening of the Mohorovicic discontinuity. The only exception to this generalization is the northern end of the Panama fracture zone between the Cocos and Coiba ridges. (3) The Panama fracture zone and the fracture zone at 85°20'W longitude divide the Panama Basin into three provinces of different crustal thickness. Between these two fracture zones the crustal thickness is normal; west of 85°20W longitude it is greater than normal; and east of the Panama fracture zone it is less than normal. (4) In that part of the Panama Basin east of the Panama fracture zone there is a major discontinuity at 3°N latitude between a smooth, isostatically compensated crust to the south and an extremely rugged, uplifted crust to the north. An explanation for this discontinuity is the effect of the inflection in the shape of the continental margin at 3°N latitude on the eastward subductiori of the Nazca plate.
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