Pulsation tectonics as the control of continental breakup

Abstract

New data from the recent IPOD drilling of DSDP Site 534 in the Blake-Bahama Basin give a definitive age for the spreading-center shift involved in the breakup of the North American Atlantic margin. A basal Callovian age (~155 m.y.) is determined for the Blake Spur anomaly marking this spreading-center shift that signals the birth of the modern North Atlantic Ocean. This is some 20 m.y. younger than previously thought. One implication of this result is that this spreading-center shift starting North Atlantic breakup is now of an age which could be assigned to the spreading-center shift needed to end the rifting in the Gulf of Mexico. It is suggested that this might be one and the same event. Another implication of this younger age for the Blake Spur event is that relatively high spreading rates are now required for the Jurassic outer magnetic quiet zone along the North American margin. This association of a relatively high spreading rate with a magnetic quiet zone is similar to that for the mid-Cretaceous and implies a link between the processes controlling plate spreading, which are in the upper mantle, and the processes controlling the magnetic field, which are in the outer core. The cycles of fast and slow spreading and quiet and reversing magnetic field have a period of 60–100 m.y. A theory of pulsation tectonics involving the cyclic eruption of plumes of hot mantle material from the lowermost D″ layer of the mantle could explain the correlation. Plumes carry heat away from the core/mantle boundary and later reach the asthenosphere and lithosphere to induce faster spreading. The pulse of fast spreading in the Jurassic apparently caused the breakup of the North Atlantic. Other pulses of fast spreading appear to correlate with major ocean openings on various parts of the globe, implying that this might be a prevalent process. I suggest rifting of passive margins may be controlled by the more fundamental global processes described by the theory of pulsation tectonics. The term pulsation tectonics is appropriate for this new unifying theory, explaining variations in the magnetic field, plate motions, and paleoenvironment. The word pulsation was used by Grabau (1934) for a long term rhythm, nearly the length of a geological time period, in which there were widespread transgressions and regressions of seas over whole continents. Thus, pulsations are just the long term cycles of Vail et al. (1977). Grabau's pulsation theory prophetically invoked large-scale tectonic changes in the ocean basins as a cause for eustatic changes, in contrast to local continental tectonics, although he offered no explanation of the ocean changes. The cyclic character of crustal movements called for in the theory of pulsation tectonics revives many ideas of these past advocates of non-steady state tectonics and provides modern mechanisms that explain the observations on plate motions and the magnetic fields, data which were totally unknown to these past authors.