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. / Graduation date: 1974
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28305 |
| Date | 19 December 1973 |
| Creators | Barday, Robert James |
| Contributors | Couch, Richard W. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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