Pattern of opening rates along the axis of the Mid-Atlantic Ridge

Vine and Wilson1, using the Vine and Matthews2 hypothesis, were the first to try to relate the magnetic pattern over the crests of the mid-ocean ridges to the known geomagnetic time scale to determine the spreading rate over the past few million years. Many determinations of spreading rate at the axis of the ridge during Plio-Pleistocene times have been published: Le Pichon3 and Morgan4, for example, used spreading rates computed for the axial magnetic pattern (10 Myr: anomaly 5 of Heirtzler et al.5 or for the past 5 Myr respectively) even when trying to assess the properties of instantaneous plate kinematics. Likewise in the systematic inversion of Minster et al.6 anomalies 3 and 5 were generally used thus averaging plate speeds over the past 5–10 Myr. Minster and Jordan7 sought to improve this and redetermined the spreading rates using magnetic anomalies 2 and 2′ yielding a mean averaging interval of <3 Myr. Here we examine plate behaviour over a shorter time scale in the reasonably well-surveyed axial region of the Mid-Atlantic Ridge south of the Azores. The average opening rates we obtained for this region for magnetic anomalies 1 (Brunhes), 2 and 2′ using two different geomagnetic time scales indicate that (1) there was a relative slowing down of the opening between anomalies 1 and 2 which may be of global significance; (2) the opening is compatible with a rigid plate model when it is averaged over ∼2.5 Myr although for shorter averaging times (1.8 and 0.7 Myr) there is some evidence of non-rigidity of the plates. Thus different segments of the Mid-Atlantic Ridge may display an independent behaviour and, with respect to relatively short time intervals, may show departures from characteristics typically associated with slow accretion.