The sinking of cold, dense slabs into the mantle may influence true polar wander. Dense masses that find their way to the mantle, such as those that occur in subduction zones at the edge of some oceans, will affect the position of the rotation axis. The equator would tend to shift toward the dense masses. If high-density masses tend to concentrate near the equator, downwelling and cooler mantle spots are most likely to prevail in the equatorial upper mantle, explaining at least qualitatively the cold upper mantle belt and resulting lack of normal melting in the equatorial zone of the Atlantic.
Diving for deep-sea data
A COLDER-THAN-NORMAL equatorial mantle when the Atlantic first opened would imply a colder and thicker continental lithosphere along the equatorial belt. (The equator 100 million years ago crossed the future Atlantic coastlines of Africa and South America roughly along the same position as it does today.) The cold and thick equatorial lithosphere must have resisted the rift propagating from both the south and the north. The equatorial region may have behaved as a "locked zone" (in the sense used by French geologist Vincent E. Courtillot). As a result, the equatorial Atlantic opened sluggishly. This slow, difficult opening may have created the large equatorial fracture zones, visible today as east-west breaks that offset short segments of the mid-ocean ridge.
Now that we know that today's mantle upwelling below mid-ocean ridges is heterogeneous in terms of temperature and composition, the next question is: How do the properties of the mantle upwelling below a given segment of the ridge change over time? This information would enlighten us on an important issue, namely, how ocean basins evolve. But the research to obtain the necessary data would require sampling older oceanic lithosphere at various distances from the axis of the mid-ocean ridge. And unfortunately, the older lithosphere is normally buried deep below sediments.
We felt we might have a chance to reach old lithospheric material in the central Atlantic in the vicinity of the Vema Transform Fault. This fault offsets the crest of the Mid-Atlantic Ridge by 320 kilometers, cutting a deep valley through the oceanic crust. A long sliver of seafloor appears to have been uplifted on the southern side of the transform, and we hoped that this uplifted seafloor would expose a pristine section of lithosphere.
To test this hypothesis, in 1989 we organized an expedition in conjunction with Jean-Marie Auzende of the French oceanographic institution Ifremer. We planned to descend to the seafloor--more than five kilometers down--in the research submersible Nautile. Most of our colleagues viewed our task with skepticism: prevalent opinion held that the normal sequence of upper mantle and crust is completely disrupted near a transform fault.
Nevertheless, we pressed on. We began a series of dives that started at the base of the section and moved up the slope. Each dive lasted about 12 hours, about half of which was spent descending to the seafloor and returning to the surface. The cramped quarters of the Nautile, a sphere of titanium one meter and 80 centimeters in diameter, can accommodate two pilots and one scientist, who lies face down for the duration of the trip.
On our first dive we verified that the base of the section consists of mantle peridotite for a thickness of about one kilometer. On the second day we discovered a layer of gabbros--rocks that form below the seafloor when basaltic melts cool slowly--resting above the peridotite. According to widely accepted geophysical models, gabbros are the main component of the lower part of the oceanic crust. So in going upslope from mantle peridotites to crustal gabbros, we had crossed the Moho discontinuity.
The next day I took the Nautile on a dive that started from the level reached by the submersible the previous day. As I progressed along the slope, skimming the seafloor, a spectacular rock formation called a dike complex gradually revealed itself. Theory holds that dike complexes form where hot molten material, generated by partial melting of the mantle, squirts upward toward the seafloor through many narrow fissures in the crust. Never before had a dike complex been observed on the seafloor.