Crustal structure of the conjugate Equatorial Atlantic Margins, derived by gravity anomaly inversion

The crustal structure of the Equatorial Atlantic conjugate margins(South America and West Africa) has been investigated using 3D gravityanomaly inversion, which allows for (1) the elevated geothermal gradient ofthe lithosphere following rifting and break-up and (2) magmatic addition tothe crust during rifting and break-up. It is therefore particularly suitablefor the analysis of rifted margins and their associated ocean basins. Mapsof crustal thickness and conjugate-margin stretching, derived from gravityanomaly inversion, are used to illustrate how the Equatorial Atlantic openedas a set of stepped rift-transform segments, rather than as a simpleorthogonal rifted margin. The influence of the transform faults andassociated oceanic fracture zones is particularly clear when the results ofthe gravity anomaly inversion are combined with a shaded-relief display ofthe free-air gravity anomaly. A set of crustal cross-sections has beenextracted from the results of the gravity inversion along both equatorialmargins. These illustrate the crustal structure of both rifted-marginsegments and transform-margin segments. The maps and cross-sections are usedto delineate crustal type on the margins as (1) inboard, entirelycontinental, (2) outboard, entirely oceanic and (3) the ocean–continenttransition in between where mixed continental and magmatic crust is likelyto be present. For a given parameterization of melt generation the amount ofmagmatic addition within the ocean–continent transition is predicted by thegravity inversion. One of the strengths of the gravity-inversion techniqueis that these predictions can be made in the absence of any other directlyacquired data. On both margins anomalously thick crust is resolved close toa number of oceanic fracture zones. On the South American margin we believethat this thick crust is probably the result of post-break-up magmatismwithin what was originally normal-thickness oceanic crust. On the WestAfrican margin, however, three possible origins are discussed: (1)continental crust extended oceanwards along the fracture zones; (2) oceaniccrust magmatically thickened at the fracture zones; and (3) oceanic crustthickened by transpression along the fracture zones. Gravity inversion alonecannot discriminate between these possibilities. The cross-sections alsoshow that, while ‘normal thickness’ oceanic crust( c. 7 km) predominates regionally, localareas of thinner ( c. 5 km) and thicker( c. 10 km) oceanic crust are also presentalong both margins. Finally, using maps of crustal thickness and thinningfactor as input to plate reconstructions, the regional palaeogeography ofthe Equatorial Atlantic during and after break-up is displayed at 10 Maincrements.