. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 1] GRAVITY AT SEA 159 mediate stages; all of the sections are closely in isostatic balance; and the M- discontinuity varies considerably in steepness but continental crustal thickness changes to ocean crustal thickness within 200 km. Drake et al. (1959) computed various structures for the Portland, Maine section on various assumptions of crustal layering and densities covering the gamut of seismological and geological possibilities. They found that none of the CJrOSS SECTION OF CRUST BASED ON DIRECT METHOD â 3i60 -3.
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 1] GRAVITY AT SEA 159 mediate stages; all of the sections are closely in isostatic balance; and the M- discontinuity varies considerably in steepness but continental crustal thickness changes to ocean crustal thickness within 200 km. Drake et al. (1959) computed various structures for the Portland, Maine section on various assumptions of crustal layering and densities covering the gamut of seismological and geological possibilities. They found that none of the CJrOSS SECTION OF CRUST BASED ON DIRECT METHOD â 3i60 Vertical Exaggeration 10 I. ' Theoretical. Mohorovtc'ic Dtscontinvity ZOO 400 600 ^ 9O0 DISTANCE IN KILOMETERS 1000 Fig. 17. Crustal section across continental margin and trench near Antofagasta, Chile, computed from gravity data. (After Wuenschel, vmpublished thesis.) possibihties that they considered reasonable appreciably modified the section from those computed in the earlier work of Worzel and Shurbet except for a different choice of density for the mantle. Wuenschel, in his unpublished thesis, derived three sections across the western continental margin of southern South America. Fig. 17 shows one of his jirofiles, which is quite similar to the other two. The analysis was complicated by the deep trench near the continental slope and the high mountains on the shore side, and the lack of detailed geological knowledge in the region. The densities he used are shown on the figure. Once again one sees the abrupt change from continental crustal thickness (mountainous here) to oceanic thicknesses in a distance of less than 200 km. Fig. 18 shows a first approximation crustal section along the west coast of the United States near San Diego. No seismic data were available so all the crust was considered to have a density of It is known that there are a number of basins with some sedimentary fill in the region but so far no estimate of the density or the sediment thi
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