ASPECTS OF THE FORMATION AND EVOLUTION OF RIFTED MARGINS: SEGMENTATION, UPLIFT AND POST-BREAKUP DEFORMATION

In this study, aspects of the incipient stages of continental rifting, of rift zones around the time of continental breakup, and of mature rifted margins are evaluated. We focus on structural segmentation at incipient continental rift zones, and its subsequent effect on margin deformation. The uplift of rift zones at continental rupture is addressed as well. Using three-dimensional numerical crustal models and analogs from the Rio Grande Rift, and the East African Rift System, rift segmentation is studied. The models predict that rift-bounding faults nucleate due to a concentration of stresses above weaknesses in a heterogeneous lower crust. The orientation of the crustal weak zones with respect to the tensional stress field affects the structural development and orientation of rift basins, as well as the accommodation zones that connect the basins. The relative positioning of the rift basins and their border faults controls the nature and deformation of the accommodation and transfer zones.

In contrast with previous studies, we find that continental rupture is marked by uplift or reduced subsidence at both volcanic and magma-starved margins. Isostatic calculations suggest that this relative uplift is largely due to lithospheric mantle detachment around the time of rupture. The resulting uplift increases potential energy, contributing to tensional forces required for breakup.

The Somali Basin margin formed as a result of oblique rifting of east and west Gondwana in the Mesozoic. The northern part of the margin is oblique, transitioning southward into a sheared margin. An Eocene-Miocene unconformity in the basin is analyzed to study its relation to the change in margin structure, and the Afar mantle plume. Seismic lines of the Ocean Drilling Program (ODP) have been interpreted, and the tectonic subsidence history of seven wells in the Indian Ocean has been calculated. These analyses constrain the spatial extent and character of the unconformity that changes along the margin from north to south, corresponding to the structural changes along the margin. The timing and spatial extent of the unconformity are well explained by the arrival of the Afar plume in the Eocene.