THE ROLE OF DIKE INTRUSION IN THE EARLY EVOLUTION OF THE SOUTH ALTANTIC CONTINENTAL MARGIN

Many models of continental break-up do not consider the mechanical role of dike intrusion in rift evolution. It has shown that the extensional force required to open dikes is much less than that required to extend the crust by normal faulting and consequently even small amounts of diking can significantly weaken non-volcanic margins. Despite clear evidence from the numerical models that dyke intrusion can weaken margins, it is difficult to test this hypothesis using geological observations.

South Atlantic margins display significant variation in structural style along their length. The reactivation of basement structures and/or fabrics is thought by many to have influenced the early breakup of Gondwana. The Santos basin is an obliquely rifted segment of the margin located offshore SE Brazil. The adjacent onshore region offers the chance to assess whether basement structures have played a significant role in influencing the initiation and development of syn-rift faults, dikes and fractures.

Field and remote sensing studies of the onshore region and on the conjugate margin in Africa show the strong geometric and temporal link between major 135Ma dike swarms and the development of faults and fractures. Three structural domains are defined, and correlated with lineament domains from the African margin. Faults, dikes and fractures in SE Brazil share consistent orientations with the large-scale morphotectonic lineaments. Brittle structures show orientations and kinematics that are consistent with regional E-W oriented transtension. Little evidence of basement influence can be seen at outcrop scale, although it is speculated that larger-scale structures may be reactivated.

Faults in the offshore basin share similar geometries to brittle structures onshore. As rifting progressed, and dike intrusion ceased, NNE-SSW trending structures to the north of the basin ceased to be active, while N-S trending extensional faults continued to deform. N-S trending faults and fractures are also observed to cut dikes in the onshore region. We suggest that the dikes were responsible for partitioning strain during the early oblique rifting phase. SE Brazil and other South Atlantic margins provide a suitable region to test the dike weakening hypothesis using geological observations.