| Popis: |
A detailed study of the best understood portion of the Mid-Atlantic Ridge (MAR) between 0 and 80 N latitude, which involved a comparison of petrologic and geochemical parameters of magmatism against satellite altimetry, seismology, deep tomog- raphy, and geoid surface data, reveals a persistent correlation of these parameters along the MAR's zero-age axial zone. In this zone, coexistence of plume- and spreading-related basaltic associations (PB and SB, respectively), originating in two fundamentally con- trasting geodynamic settings, has been shown to occur. The way these associations are distributed within the ridge is in accordance with its tectono-magmatic segmentation and corresponds to the correlation of petrologic and geophysical parameters. Results thus obtained provided an incentive to reconstruct the tectono-magmatic conditions of genesis of the Atlantic lithosphere through time using along-isochron gravity variations overlain on magnetic lineations. It was at this angle that we approached the portion of the Atlantic between 15 N and 40 N latitude. Here, the PB/SB boundary runs near 30 N along the MAR, which corresponds to the south terminus of the Azores megaplume. Along the M5, M13, M21, and M30 (west and east) magnetic lineation pairs after (Sandwell and Smith, 1997), isochron profiles of free-air gravity anomalies were constructed with a 2 0 resolution. A detailed correlation of isochron profile pairs with the along-ridge axis profile bears the following implications: (1) The lithospheric tract between the Kane and Atlantis fracture zones has been generated over the past 65 m.y. in a steady spreading environment virtually outside the reach of any tectonic or magmatic overprint. (2) Regions corresponding to PB and SB provinces within the study area have existed since at least 67 Ma. (3) Symmetry perturbations in the gravity field on both sides of the ridge axis induced by subsequent magmatic or tectonic events cannot be timed using paleomagnetic data. (4) As suming that the correlation of petrologic and geophysical parameters, which has been ascertained for zero-age events along the MAR axis, has been maintained since 67 Ma, it can be inferred that over this time interval the geodynamic setting responsible for the formative conditions of the Azores megaplume migrated southward from 41 N to 30 N latitude at a rate of ca. 18 mm/yr. |
