STATIONARY AND NONSTATIONARY BEHAVIOR WITHIN THE GEOMAGNETIC POLARITYTIME-SCALE

We analyse the geomagnetic polarity time scale (GPTS) since the Upper Jurassic by displaying the successive lengths of polarity intervals as a function of their order of occurrence. The sequence consists of thr ee segments. Between the Upper Jurassic and the Lower Cretaceous, segm ent ''A'' comprises intervals of short duration, with a mean duration of about 0.29 My, and no clear long-term evolution. Segment ''B'' begi ns around 130 Ma, displays a sudden increase of the duration of the ma gnetic intervals, an interval of maximum duration, the normal Cretaceo us superchron, and a long and erratic sequence of intervals with decre asing average duration between 85 Ma and about 25 Ma. From 25 Ma to th e present, segment ''C'' consists of intervals of short duration with a mean value of 0.23 My. This description suggests that the Earth's ma gnetic field could have experienced a fairly stationary regime until s lightly before the onset of the Cretaceous superchron, when the regime has been rapidly and strongly perturbed before progressively returnin g to another stationary regime about 25 Ma ago. A geophysical explanat ion for this sequence of events could be that the geodynamo has been p erturbed by the arrival of some cold material at the core mantle bound ary. As this material would have heated up, the geodynamo would have b een brought back to its stationary regime.