Geology Reference
In-Depth Information
Figure 3.7. Ages of sediments immediately above the basaltic basement of the
sea floor of the South Atlantic, plotted against distance from the crest of the Mid-
Atlantic Ridge. The ages are inferred from micro-fossils. From Maxwell
et al.
[34].
Copyright American Association for the Advancement of Science. Reprinted with
permission.
nodal plane approximately parallel to the fracture zone, consistent with strike-
slip faulting (Figure 3.6). Most importantly, the sense of strike-slip motion was
consistent with that predicted by the transform fault hypothesis, and opposite to
that predicted by the simple transcurrent offset interpretation. This was another
kind of observation strongly supportive of seafloor spreading.
3.5 Evidence for motion - sediments
Ewing, during the same period, had used seismic refraction to determine the thick-
ness of sediments in the Atlantic. If seafloor spreading were occurring, the thickness
of sediments should increase with distance from the ridge crest. The results were
confusing, partly because of the rough seafloor topography of the Atlantic, and
Ewing was reluctant to come out in support of seafloor spreading.
Later, a different approach became possible through a deep-sea drilling pro-
gramme, which allowed the recovery of long sediment cores. An early cruise in
the South Atlantic Ocean was aimed specifically at testing seafloor spreading.
The results were spectacular. It was found that the age of the oldest sediment,
just above the basaltic basement, determined from micro-fossils, increased in sim-
ple proportion to distance from the ridge crest, exactly as predicted by assuming
seafloor spreading at a nearly constant rate (Figure 3.7). The results also provided
