SEAFLOOR TECTONIC FABRIC FROM SATELLITE ALTIMETRY¹

Supplementary Data

• Supplemental Figures

  The color-shaded relief images shown at right illustrate the improved resolution of seafloor tectonic fabric in gravity (Figure 1) and topography (Figure 2), simple (Figure 3) and more sophisticated (Figure 4) methods for isolating tectonic fabric signals in altimeter data, and comparison of the correlation between fabric amplitude and seafloor spreading rate (Figure 5).

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  Figure 1. The resolution of the marine gravity field by spacecraft improved tremendously when new data became available in mid-1995. The top panel shows the resolution from Seasat (Haxby et al 1983, Haxby 1987), while the bottom panel shows the resolution from Geosat and ERS-1 (Sandwell & Smith 1997). The area shown here is on the Pacific-Antarctic Ridge and corresponds to that of Figures 2-5 in the hard copy of the review (Annual Review of Earth and Planetary Sciences 26:697-748); Figure 5 in the volume gives an index to the tectonic features in this area. Download Postscript file for printing (4MB).

  

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  Figure 2. When the new gravity information is used to estimate seafloor depth, a more correct picture of tectonic fabric emerges. The top panel shows the ETOPO-5 depths prepared by gridding data on contour charts drawn in the early 1980s and earlier (National Geophysical Data Center 1988). The bottom panel shows the depths estimated using the satellite gravity (Smith & Sandwell 1997a). The ETOPO-5 solution fails to resolve the Hollister Ridge, the ridge-and-trough structures along fracture zones, and the offsets in the Pacific-Antarctic Ridge between 55 and 60° S latitude. The ETOPO-5 solution also creates the illusion that the seafloor roughness increases south of the 60° parallel; this is due to the fact that the contour maps north and south of this boundary were drawn by different individuals with different interpretive styles. The advantage of the satellite data is that they provide a uniform view over all the oceans. Download Postscript file for printing (4MB).

  

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  Figure 3. Gravity anomalies caused by seafloor tectonic fabric are only one part of the spectrum of the total gravity field, and so some sort of filtering scheme is required to isolate the tectonic fabric band, as discussed in the review. Shown here is a simple method for detecting structural highs and lows in tectonic fabric using the vertical gravity gradient. Although this quantity varies continuously over a range of values, as shown in the top panel, values greater (lesser) than 5 (-5) Eötvös may be said to indicate structural highs (lows). This threshold has been used to make the black and white fabric maps in the review. Download Postscript file for printing (4MB).

  

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  Figure 4. A more sophisticated way of filtering the gravity field to isolate the tectonic fabric band is to use a band-pass filter with a "downward continuation" operation (Smith & Sandwell 1994a). This allows the amplitudes of anomalies to be compared between regions of different regional average water depths, such as ridge axes and ridge flanks. The top panel shows the complete gravity field, and the bottom panel the result of this special filter. Download Postscript file for printing (2.5MB).

  

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  Figure 5. The root-mean-square (rms) amplitudes of the band-pass--filtered and downward-continued gravity field show some correlation with seafloor spreading rate, in addition to being large over major tectonic lineaments. Very smooth seafloor (low rms gravity) occurs only in areas of seafloor generated at high spreading rates. Clear changes in rms associated with changes in spreading rate are visible along ridge flanks in all the ocean basins. Earlier workers (cited in the review) had noted that mid-ocean ridge axis morphology seems to depend on spreading rate. The data shown here indicate that rate-dependent tectonic fabric occurs throughout the ocean basins, not only along the ridge axes. This suggests that ridge-axis processes generate most of the band-scale (10-160 km) fabric of the seafloor, and that the rate-dependence of these processes has persisted over the history of the ocean basins (200 million years). Download Postscript file for printing (4MB).