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SEAFLOOR EVOLUTION: CASE HISTORY OF ONE SMALL SEAFLOOR PATCH

Bathymetric contour figures A through I show how one small area of the world ocean (25-27.5S, 12-15.5W) has "evolved" as a function of different bathymetrists varying interpretations of mostly the same very limited sounding database (A, B, C, D -- all prepared during the 1970s and early 1980s). Note that although all four chartlets postdate the plate tectonic revolution, they differ strongly in the amount of  "plate tectonic fabric" conjecturally attributed to the ocean floor topography. The Heezen-Tharp interpretation (A) seems to have been based on experience from the East Pacific Rise and other fast-spreading parts of the Mid-Oceanic Ridge, where transform faults/fracture zones appear as distinctive troughs, but rift-parallel topography (abyssal hills) is subdued. Interpretation B (Emery and Uchupi, 1984) exhibits only a mild (fracture zone-parallel) plate tectonic fabric in the contours.  The contour interpretation C combines a strong fracture zone signal, with prominent rift-parallel topography, shown as transverse bands of egg-shaped highs. Interpretation D also combines the two types of topography, but is more loyal to the actual soundings, even when they appear not to fit the purported plate tectonic fabric. Note that these four interpretations of the same area are quite, some might say wildly different from each other and from what mid-1980's multibeam mapping revealed on this patch of ocean floor (Chartlets G, H, and I). Bathymetrists are not, as some claim to be, clairvoyant "bathymagicians", even when their pencil is guided by the invisible hand of a geological paradigm.

Chartlets E and F show the predictive power of satellite altimetry in revealing much of the coarse (down to ca. 20-25 km wavelength), largely plate-tectonic-determined bathymetric fabric over the Mid-Oceanic Ridge and whatever flank topography has not been largely buried under sediments. Note that the spatial resolution that could be obtained by a GOMaP sidescan image would be ca. 100 times sharper than what the gravity field can resolve, while a GOMaP multibeam swath bathymetry of this area would exhibit ca. 25 times better spatial resolution. The finer features resolved under GOMaP could not be shown at the scale of the chartlets presented on this web page without a zoom option, however.

Chartlets G, H, and I show APPROXIMATELY what the seafloor topography actually looks like at these scales, as a result of1985-1986 SEABEAM multibeam mapping from R/V Robert Conrad. Note that even chart I does not represent the 100% coverage that would be returned under GOMaP, and no sidescan sonar was obtained. In fact, only a relatively modest fraction of seafloor was topographically "illuminated" by SEABEAM, and particularly given the lack of sidescan sonar coverage, it would be most practical to remap the entire area under GOMaP, rather than attempting to fill the complex patterns of "holiday areas" with newer data, producing an inhomogeneous database. Even with the large amount of holiday area (Chartlet G), however, this area is one of the best-mapped patches of the global ocean floor!!

Note that multibeam bathymetry (nowadays including sidescan imagery from the same hullmounted system) resolves many classes of features too small to be resolved in chartlets G, H, and I. Thus, the appearance that survey with line track spacing as in G would suffice to provide a reasonably accurate picture of the plate-tectonic scale fabric misses the fact that most smaller-scale features would be missed entirely. Chartlet H illustrates just one category of "smaller-scale feature", viz. mini-seamounts. These are shown only where actually mapped, i.e. only a fraction of those that must be present, given the proportion of holiday area (H).